# Micro/Nanostructures for Light Trapping in Monocrystalline Silicon Solar Cells

Authors: Huan Liu; YuXuan Du; XingTian Yin; MinYu Bai; WeiGuo Liu
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

Due to the ongoing depletion of fossil energy, alternative energy-sources and their respective conversion technologies have become very essential. An inexhaustible and clean energy form, which is already widely used, is solar energy. However, despite much progress in recent decades, due to the limitations of materials and manufacturing technology, the ability of solar cells to convert light into electrical energy is still not very high. Enhancing light absorption and reducing electric loss are the keys to improving the overall conversion efficiency of solar cells and reducing raw material costs. The former can be achieved by using micro/nanostructures and other surface features, while the latter can be realized by surface passivation. Researchers have developed different silicon-surface texturing methods to fabricate random or periodic micro/nanostructures on the surface of silicon wafers. Thanks to the special and efficient light-trapping effects of silicon micro/nanostructures, both full angle and wideband light absorption can be achieved. Different passivation methods and materials have also been widely studied, which helps to improve the surface recombination of photogenerated carriers caused by light trapping structure and significantly enhance the power conversion efficiency of Si solar cells. In this work, theoretical studies of enhanced light-trapping in micro/nanostructures are introduced. In addition, several advanced methods for preparing micro/nanostructures on the surface of monocrystalline silicon are discussed. These can be classified as top-down and bottom-up approaches. Furthermore, passivation methods for micro/nanostructures on the surface of monocrystalline silicon solar cells are reviewed, including chemical passivation and field-effect passivation. Finally, advantages and disadvantages of the micro/nanostructure preparation technologies, and light-trapping effects of the micro/nanostructures, which were fabricated using these manufacturing technologies were summarized. Moreover, the effects of different passivation technologies on the optical properties and electrical properties of these micro/nanostructures are studied. An outlook of expected and emerging research directions for monocrystalline silicon solar cells concludes this study.


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## Body

## 1. Introduction

Given the threat of global warming and fossil energy depletion, the development of renewable energy has become a high priority for research and development groups around the world. The Paris Agreement took effect on November 4, 2016, which highlighted a global commitment to advance renewable energy [1]. According to a prediction in the BP Energy Outlook report 2020, the share of renewable energy among primary energy uses is expected to increase from around 5% in 2018 to 45% by 2050 [2]. According to the forecast of the International Energy Agency (IEA), the total installed capacity of global photovoltaics (PV) is expected to reach 1721 GW by 2030 and further increases to 4670 GW by 2050 with great potential. To achieve the above goals, continuous development of renewable energy is imperative. Among all renewable energy types, solar energy developed the fastest because Si is the second highest content element in the earth’s crust [3]. With the progress of crystalline silicon refining and growth technology, high-quality silicon wafers have been very easy to obtain and have been widely used in the IC industry. This advantage in output and technological maturity has helped reduce the cost of silicon photovoltaic modules to 0.4US$/WP [4]. Consistent technological progress is essential to modify the global energy structure and facilitate a paradigm change with respect to global energy consumption [5].Over the past decades, researchers have been trying to improve the conversion efficiency of single/polycrystalline silicon solar cells using several methods. Chapin et al. [6] first developed practical monocrystalline silicon solar cells in 1954. The initial efficiency of silicon-based solar cells was below 10%. By 2022, the maximum power conversion efficiency (PCE) of monocrystalline silicon cells and polycrystalline cells produced on a large scale is 26.1% and 24.4%, respectively [7]. Based on the concept of carrier selectivity, ISFH (Institute for Solar Energy Research, Hamelin) performed a detailed analysis on the theoretical PCE limit of solar cells with different structures in the report of the silicon PV conference in 2019. The conclusion is that tunnel oxide passivated contact cells have higher PCE limit (28.2% and 28.7%). It is higher than the 27.5% PCE limit of HIT (heterojunction with intrinsic thin layer) cells [8], but also much higher than PERC (passive emitter and rear contact) cells (24.5%) [9, 10], which is closest to the 30% theoretical PCE limit of crystalline silicon solar cells [11]. Although the current research shows that TOPCON cells have the highest theoretical PCE limit, the highest certified PCE of HIT cells has reached 26.3%, exceeding 26.0% of p-type TOPCON cells. The highest certified efficiency of PERC cells is 24-25%, which has reached the theoretical PCE limit [12]. As part of the continuous improvement in the efficiency of crystalline silicon cells, researchers are still looking for new solar cells types with higher theoretical limits for power conversion efficiency [13, 14].Second-generation solar cells include amorphous silicon thin films and compound semiconductor cells. Although amorphous silicon cells have low requirements for raw material quality and are inexpensive, the conversion efficiency is much lower for monocrystalline silicon cells. This is because of the high number of material defects. Hence, second-generation solar cells are predominantly represented by CdTe [15, 16], GaAs [17, 18], and CIGS [19, 20] cells. However, both CdTe and GaAs cells contain toxic elements, which can pollute the environment. Moreover, the manufacturing costs of GaAs thin-film cells are high. These disadvantages hinder their application significantly. The third-generation solar cell utilizes a new concept based on high efficiency, environmental protection, and advanced nanotechnology. It mainly includes dye-sensitized solar cells (DSSCs) [21–23], perovskite solar cells (PSCs) [24–26], and quantum dot solar cells (QDSCs) [27–29]. At present, the main problem of these new solar cells is poor stability, PCE is lower than commercial crystalline silicon cells, and the preparation process and theoretical research are not mature enough, so their practical application still has a long distance.The second and third generation solar cells have higher theoretical conversion efficiency and considerable development potential. However, crystalline silicon cells are likely to maintain their dominant position in the solar photovoltaic industry given the low manufacturing cost, technical maturity, reliable raw material supply, and environmental friendliness. Moreover, the power conversion efficiency of crystalline silicon solar cells still has room for improvement. This is mainly achieved by enhancing the light-capture characteristics of the cells and reducing electric losses. A conventional way to increase the light absorption efficiency is to use double- or multilayer antireflection films, where films with different refractive index (from low to high) are stacked on the surface of silicon wafers. Furthermore, reflectivity can be reduced by lowering the refractive index mismatch between air and the silicon substrate. However, for an increasing number of layers, the manufacturing costs increase significantly. In addition, the thermal expansion and stability of films should also be considered. Moreover, an antireflection film shows wavelength- and angle-selectivity, and it can only create a good antireflection effect for a certain range of wavelengths and angles [30–33].Another way to improve the absorption efficiency of incident light is to use antireflective micron/nanostructures on the surface of crystalline silicon. At the same time, the volume of silicon wafer is reduced, and the thickness of thin-film silicon solar cell is reduced, which is more conducive to the transport of photogenerated carriers; therefore, it not only reduces the cost of raw materials but also improves the carrier collection efficiency. Using micro/nanostructure to reduce light reflection was first inspired by moth eyes [34, 35]. This protrusion array widely existing on the surface of insect eyes is considered to enhance the sensitivity of visual system and reduce the probability of predator discovery by reducing reflection [36]. Similarly, in order to improve the efficiency of photosynthesis, plants have developed the strategy of using micron/nanostructure to improve light absorption in the long evolutionary process [37, 38]. Inspired by the wisdom of nature, human beings continue to improve the theoretical research, design methods of antireflection micron/nanostructures, and develop various micro- and nanoprocessing technologies to manufacture micron/nanostructures on different material surfaces. At present, rigorous coupled wave analysis (RCWA) and finite difference time domain (FDTD) are the two most commonly used methods to design and study the optical properties of periodic micron/nanostructure arrays. They both belong to differential-based methods. RCWA uses eigenvalue numerical technique to calculate the order efficiency of diffraction grating [36, 39], while FDTD transforms Maxwell’s spinor equation into difference equation and discretizes it in space and time, so as to calculate the distribution of electromagnetic field [40, 41]. Both methods have been widely used to simulate the optical properties of artificial subwavelength nanostructures and are in good agreement with the experiment [42–44]. In addition to the bionic moth eye structure, researchers have designed a series of artificial nanostructures with good antireflection effects, including nanowires [45, 46], nanocones [47, 48], nanoholes [49, 50], nanopyramids [51, 52], and some complex composite structures [53, 54]. Micronanomanufacturing technology is the key to realizing that artificial antireflection nanostructures can be applied to silicon solar cells. Because Si has an appropriate hardness and is easy to react with acids and alkaline, almost all kinds of nanomanufacturing technologies we know can be used to manufacture antireflection micron/nanostructures on the silicon surface, including wet etching [55–57], dry etchings [58, 59], nanoimprinting [60, 61], and laser interference lithography [62, 63]. Because these nanomanufacturing technologies have different characteristics, appropriate manufacturing technologies should be selected according to the shape, size, and pattern of nanostructures, and a variety of nanomanufacturing technologies are usually required to be combined to manufacture nanostructures with complex shapes and periodic arrangements. In addition, Si solar cell is a large-scale industrial product, so whether the nanomanufacturing technology for texturing silicon surface is practical or not must also consider the cost, stability, compatibility, and environmental protection. Therefore, the research and improvement of existing nanomanufacturing technology and the exploration of new nanomanufacturing technology are keys to accelerate the development of Si solar cells with antireflective nanostructures. The pyramid-shaped surface texture fabricated by wet etching has been widely used in commercial silicon solar cells and achieved excellent results. Only one-third of standard 160 μm commercial wafers is required to reach the same equivalent open circuit voltage and efficiency. This represents a substantial reduction of the manufacturing costs [64].In this review, first, several theoretical studies dealing with light capture are summarized according to the surface texture size (from microstructure to nanostructure). Light trapping which can be produced by surface plasmons is also included in studies. Second, several advanced methods to prepare nanostructures on monocrystalline silicon are introduced. These include top-down alkaline etching, metal-assisted etching, and some new dry-etching technologies. Third, the passivation of nanostructures on a monocrystalline silicon surface is discussed, and the effect of different passivation technologies on the performance of monocrystalline silicon solar cells are analyzed. Finally, the state of the art of theoretical research, preparation technology, and surface passivation of single crystal silicon surface nanostructures are summarized. Based on micro/nanostructures to reduce reflection, an outlook of the development of high-performance monocrystalline silicon solar cells is provided.


## 2. Theoretical Investigations of Light Trapping

Theoretical studies of the light-trapping properties of nanostructures are the basis for structure design and optimization. During propagation, light reflects and refracts according to the rules of geometrical optics, when it encounters an object much larger than its wavelength. When the size of the object is close to the wavelength of the incident light, scattering and diffraction occur. Therefore, different theories are needed to describe and analyse the light-trapping properties of micro/nanostructures with diverse feature dimensions such as the micron pyramid texture and subwavelength nanostructure, which are inspired by the compound eyes of moths. Some of the most important theories of light capture are reviewed below.


### 2.1. Geometrical Optics Method

The geometrical optics theory is useful to extend the optical path by causing the incident light to refract and reflect in the media using microstructure, which ultimately enhances light trapping. This theory has been used to design thin-film silicon solar cells for a long time. A reflection layer is created on the back of the solar cell. The reflector forms a certain angle with the front surface of the cell, with which the incident light is reflected to the front surface. If the incident angle is greater than the critical angle, complete internal reflection occurs. In this way, light can pass through the silicon layer many times, and the light-absorption efficiency of the thin-film silicon solar cell is enhanced. For practical applications, a grooved back reflector is more suitable. One advantage of the grooved mirror is that the gain of the optical path is almost independent of the incident angle (within 40° of the vertical angle) [65].According to literature, the combination of a back reflector with a front surface texture can achieve better light trapping. As can be seen in Figure1(a), the incident light first scatters randomly due to the primary effect of the textured surface. Then, it is reflected into the silicon layer via an aluminum back reflector, thus increasing the optical path of the incident light in the silicon layer [66]. For commercial solar cells, the most common surface texture is the pyramid structure. The light-trapping mechanism for pyramid textures larger than several microns is shown in Figure 1(b).Figure 1
(a) Design of commercial solar cells with enhanced light capture performance based on geometrical optics [66]. (b) Ray schematic of a textured silicon wafer based on geometric optics [67].
(a)(b)The incident light I vertically irradiates the surface of the left pyramid with an incident angle ofθ1. One part of the refracted light enters the silicon layer, and the reflected light R1 irradiates the surface of the adjacent pyramid and refracts again and again. Light tilting in the wafer leads to enhanced absorption near the front collecting junction. Directional randomization of weakly absorbed rays causes total internal reflection on the surface, which enables effective light capture. Ultimately, surface reflectivity could be reduced to below 10% [67, 68].Using computer-based ray tracing, Campbell and Green systematically studied the light-capture characteristics of pyramid arrangements. Examples include regular pyramids, random pyramids, staggered layouts, and double-sided texturing. Their performances were compared with a randomly textured (Lambertian) surface [69]. The group found that solar cells with flat back reflectors and pyramid-textured front surfaces show inferior light capturing than a Lambertian front surface. Because a considerable proportion of light passes through the cell for the first time. It is then reflected to the pyramid surface (by the back reflector) and directly coupled out. This is especially true if the pyramids are of the same size and positioned within a regular square matrix. Furthermore, randomly distributed pyramids perform even better. On the other hand, staggered pyramids showed a much better performance but at the expense of increased geometric sensitivity. For a double-sided texture, both the regular pyramid distribution and the random pyramid distribution permit almost all incident light to pass through the cell at least four times. The light capture performance is very close to that of the Lambertian surface. Han and Chen designed a solar cell with a planar back reflector and optimized the front-surface pyramid texture. The light absorption characteristics calculated by the transfer matrix method show that light capture performance with a single-side texture can also approach the Lambertian limit [70].Geometrical optics theory can well explain the light-trapping effect of microstructures whose dimensions are much larger than the wavelength of incident light. Nonetheless, microstructures that can exceed the Lambertian limit are difficult to design. This limit represents the thermodynamic limit for maximum absorption in a slab using geometrical optics. Compared with a flat surface, the length of the optical path can be increased4n2 times, where n is the refractive index of the medium [71, 72].


### 2.2. Effective Media Theory

When the nanostructure size is much smaller than the wavelength of the incident light, effective media theory is needed to study its light-trapping characteristics. Because in this case, only zero-order reflection waves and zero-order transmission waves are generated by zero-order diffraction after the interaction between the incident light and the nanostructure. The other higher-order diffraction waves are evanescent waves [73]. For the micron-rough surface, the average irregularity and the corresponding correlation length are below the incident wavelength. As a result, the incident light can interact with the whole surface but not each rough point.Effective media theory describes the interaction between light and a subwavelength structure. It uses a homogeneous material with effective optical constants to represent the subwavelength heterogeneous region: dielectric constantε, permeability μ, and conductivity σ. The optical properties of these effective media depend on the specific structural mixing between the incident light and the substrate materials. Generally, the more different substrate materials are used, the closer the optical properties of the region are to the substrate. In other words, when light interacts with subwavelength structures, it reacts to them if it encounters an effective medium, whose optical properties are the weighted spatial average of the optical properties in the profile region [74].The abrupt change of the refractive index between air and silicon solar cells can cause high reflected-light loss. Therefore, multilayer antireflection coating, with a gradually-changing refractive index, can be used to reduce the abrupt change of refractive index and cause light-reflection restraining [75]. On the basis of effective media theory, a nanostructure can realize the antireflection using similar principles and even exhibit light capturing that is better than that of multilayer antireflection coating. Assuming two-layer effective media, the relationship between the refractive index n of the effective medium and the refractive index n1 and n2 of the two effective media can be expressed using the Maxwell Garnett (MG) model [76].
(1)n2−n12n2+2n12=1−f1n22−n12n22+2n12.Here,f1 and f2=1−f1 are the corresponding volume fractions. In this model, Equation (1) holds only, when the layer with the refractive index n2 is surrounded by a layer with a refractive index n1. If n1 and n2 exchange, that is, the layer with refractive index n1 is surrounded by a layer with refractive index n2, the final effective medium n can also change. If the two layers are assumed to be uniformly mixed, according to Bruggeman’s effective medium approximation [77], the deformation is as follows:
(2)f1n12−n2n12+2n2+f2n22−n2n22+2n2=0.The uniqueness of the effective medium approximation allows it to be extended to multiple layers:(3)∑i=1nfini2−n2ni2+2n2=0.As shown in Figure2(a), the effective medium with a multilevel surface-relief profile is equivalent to an antireflection film made of multilayer films with different refractive index. Furthermore, ni is the refractive index of the incident medium, and ns is the refractive index of the nanostructures. It is assumed that ni<ns. The refractive indices of the first, second, and third level are n1, n2 and n3, and n1<n2<n3. In Figure 2(b), an effective medium with continuous profile is equivalent to an antireflective film with a gradual refractive index, where the refractive index gradually decreases along the z-axis and ni<nz<ns. Because effective medium is obtained by the weighted spatial average of the optical properties in the profile region, the optical properties of the effective medium must be constrained by the properties of the incident medium and the substrate medium [74].Figure 2
(a) A multilevel profile can be equivalent to an effective medium of multilayer film. (b) A continuous profile can be equivalent to an effective medium of gradient-index film [74]. (c) Silicon thin film solar cells with periodic submicron pattern on the surface [78].
(a)(b)(c)Theoretically, the effective medium with the continuous profile has a better light-trapping effect than a multilevel-relief surface. This is because the refractive index of the former changes more smoothly. According to the theory of effective media, nearly-perfect continuous-gradient index-profiles can be designed, such as a quintic profile and half-period profile for the exponential sine function [33, 79]. Although they have excellent antireflection properties, because these complex profiles are difficult to manufacture, they are not suitable for mass production. Nonideal profiles such as the linear gradient profile and the Gaussian profile were found either theoretically or experimentally to have extremely low reflectivity using RCWA and FDTD studies. With such profiles, the reflectivity from 300 nm to 1200 nm is generally below 10%. Compared with multilayer antireflection coatings, these nonideal profiles show smaller angle dependencies and a wider antireflection wavelength-range [76, 80, 81]. Zanotto et al. [78] analyzed the effect of submicron patterns and antireflection coatings on the surface of thin-film solar cells using the RCWA method as shown in Figure 2(c). By changing the structural parameters, they found that the AR (antireflection) coating significantly increased the short-circuit current jsc. Compared with the structure with the optimized AR coating but no pattern, the efficiency of a-Si cells with a submicron pattern increased by 12.4%. When monocrystalline silicon was used as the absorbing material, the efficiency of the solar cell increased by 36.5%. The group also found an expression for the effective refractive index of the dielectric-slices-arrangement:
(4)εefff=12fS+1−fεSO2+1f/εS+1−f/εSO2.To match the air impedance with the effective layer impedance when the antireflective coating is applied on the patterned structure, the relationship between the refractive indexn of the antireflective coating and the silicon filling factor f under the optimal matching conditions is given as
(5)nf=nefff1/2=εefff1/4.


### 2.3. Scatters

If the size of the microstructure and the wavelength of the incident light are of the same order of magnitude, the interaction between them can produce strong scattering, and the intensity of the scattered light can be stronger in the forward direction. If the photosensitive material is located below the scattering microstructure, the scattered light can be coupled into the photosensitive material effectively. As early as 1908, Gustav Mie proposed the concept of Mie scattering. According to Maxwell’s electromagnetic theory, a rigorous mathematical solution of particle scattering was derived for spherical particles in a uniform medium for monochromatic plane-wave illumination. The light-field distribution, which was calculated from this solution, is called Mie scattering. Calculation results of Mie scattering for the particle limit are basically consistent with both Rayleigh scattering and Fraunhofer diffraction theory. Nevertheless, Rayleigh scattering and Fraunhofer diffraction theory cannot be used to calculate scattering effects when the particle diameter is close to the wave length [82]. Since Mie scattering particles consist of dielectric materials, there is no electric impedance for metal-ion resonance, and its optical loss is minimal [83]. The limitation of Mie-scattering theory is that it can only deal with the scattering characteristics of a single spherical particle, although it can be applied to particles of any size and material [84–86].Poruba et al. analyzed the surface- and bulk-scattering effects of microcrystalline silicon thin-film solar cells, which were deposited by very high-frequency glow discharge (VHF-GD). This was done using the constant photocurrent method (CPM). The group found that light-absorption enhancement in microcrystalline silicon films and solar cells is mainly due to the long light-path, which is generated by effective diffuse light scattering on the surface of textured films. This light-scattering effect is a key characteristic of high-efficiency thin-film silicon solar cells. Additionally, surface scattering is of great significance for thin-film silicon solar cells. As a result of the indirect optical transition in crystalline silicon, light absorption in the film is very low. To absorb most of the solar spectrum in the film with a thickness of only several microns, light absorption must be increased. Therefore, a complex optical capture scheme is proposed. Although no nanostructures were made on the surface of microcrystalline silicon films, most of the high-quality films do not show mirror-like free surfaces. These “milky white” layers have rough surfaces with a typical root-mean-square (RMS) value of 15-35 nm, which causes surface scattering. Finally, they experimentally demonstrated that the infrared absorption enhancement of nanotextured microcrystalline silicon films and solar cells is mainly due to surface scattering. In the subgap region, scattering in the material contributes significantly to the CPM results [87].Nanostructures, which are designed according to the scattering principle, were numerically shown to have very good light trapping. The mixed texture consisted of a diffraction grating with a random rough surface and a type of quasirandom binary grating pattern. The absorptivity was shown to be very close to the Lambertian limit [88]. The optimized diffraction grating structure leads to both scattering and higher-order diffraction, which contributes to the enhanced optical absorptivity of silicon thin-film solar cells. Compared with the planar case, the overall reflectivity decreases from 76% to 45.1%, and the quantum efficiency increases from 0.2 to 0.47 for 700 nm monochromatic illumination [89]. A scattering layer with periodic patterns was introduced at the top of the cladding layer, and each cell consisted of several air slots. These grooves were oriented in different directions to ensure that the scattering intensity was not strongly dependent on the angle and polarization of the incident light. Finally, the numerical results show that the enhancement factor of this structure exceeds twice the Lambertian limit [90]. Yu et al. fabricated hexagonal silicon nanopillar arrays on the surface of monocrystalline silicon. The group revealed the (quantitative) role of the light-trapping structure (LTS) using a three-dimensional FDTD method and experimental tests. The results show that the experimental measurement and simulation results are consistent. As shown in Figures 3(c) and 3(d), the average absorptivity of planar silicon from 550 nm to 1100 nm is 27.6%, while the absorptivity of silicon with the surface light-trapping structure increased to 69%. The simulation results show that the average absorption rate increased from 28% to 77%. This suggests that diffraction/scattering of the top hexagonal pattern increases the optical interaction length, which increases the absorption. Some high-order diffracted/scattered light was trapped in the silicon through total internal reflection at the top surface [91].Figure 3
(a) Schematic diagrams of ordered and random Si NW arrays. (b) Reflectivity and absorptivity of ordered and random Si NW arrays from FDTD simulation [91]; absorbed photon flux were from (c) experimental data and (d) results of calculation under the AM 1.5D standard solar spectrum as a function of wavelength for bare Si and bare Si with ARC+LTS+BSR [93].
(a)(b)(c)(d)Gjessing et al. presented a comprehensive comparison of light-trapping properties among various biperiodic structures with a square lattice. The geometric shapes they studied were cylinders, cones, inverted pyramids, dimples (hemispheres), and three more advanced structures (roof mosaics, roses, and zigzag) as shown in Figure4. The simulated photogenerated current Jph of the zigzag structure was 37.3 mA/cm2, which performed best. Its light-capture efficiency is equivalent to that of Lambertian. The enhanced performance can be explained as follows. On one hand, the number of sharp edges and angles in these structures increases scattering. On the other hand, the reduced symmetry of the diffraction patterns of rose shaped and zigzag structures reduces the possibility of any subsequent out-coupled interaction with diffraction gratings [92].Figure 4
Schematic representation of each of the investigated periodic light trapping structures [92].
(a)
Cylinder(b)
Dimple(c)
Cone(d)
Inverted pyramid(e)
Roof-mosaic(f)
Rose(g)
ZigzagSilicon-nanowire arrays have excellent light-trapping properties and are popular topics among surface nanostructures of silicon solar cells due to their easy fabrication and controllable size. Researchers have already numerically studied the optical properties of silicon nanowire arrays [93–95]. Bao and Ruan analyzed the light-trapping characteristics of ordered nanowire arrays and nanowire arrays with random positions, diameters, and lengths, using the FDTD method. As shown in Figure 3(a), the absorption and reflectivity curves of ordered nanowire arrays (and nanowire arrays with random position and length) are shown in Figure 3(b). The absorption peak for random-position nanowire arrays is consistent with that for ordered nanowire arrays because the diameters of the nanowires are the same. The absorption of random position nanowires is slightly higher than that of ordered nanowire arrays. This indicates that enhanced multiple scattering increases the optical path. In the case of random diameter arrays, no obvious absorption peak can be found as diverse diameters correspond to different resonance frequencies and absorption peaks. The clearly broadened and enhanced absorption of random diameter arrays can be attributed to the strengthened multiple interwire scattering and/or inner-wire resonance [93].


### 2.4. Plasmonics

Many free electrons are present on the surface and inside of metals. Under certain conditions, these electrons can interact with the incident light wave to generate a collective resonance. Correspondingly, the collective resonance of this charge can react with the electromagnetic field of the light wave. According to the dispersion characteristics, surface plasmon resonance can be divided into local surface-plasmon resonance (LSPR) and surface-plasmon polarization resonance (SPPR). The phenomenon of surface-plasmon resonance (SPR) was first discovered by Wood in a spectroscopic experiment of a metal diffraction grating in 1902 [96]. Since then, Fano, Ritchie, and Powell and Swan further developed the SPR theory through both experiments and theoretical analysis [97–99]. In 1960, Stern and Ferrell studied the conditions to generate surface electromagnetic waves. In addition, the concept of surface-plasmon polaritons (SPPs) was proposed for the first time, and the dispersion relation was derived [100]. Although both types of SPPs show local characteristics, they are different excited states. LSPR is a nonpropagation mode, which can radiate electromagnetic waves. SPPR is one excitation element that propagates along the interface between metal and medium.Three different plasmon models are shown in Figure5. A schematic for metal-nanoparticle scattering is shown in Figure 5(a). The scattering principle of such nanoparticles is similar to the one of the previous section. However, because of the collective resonance of free electrons in metal particles, light near the vibration frequency can be scattered strongly. Moreover, when the metal nanoparticles are located at the interface of two different media, the scattered light can preferentially enter the medium with the higher dielectric coefficient [101]. However, there are also disadvantages when using metal-particle scattering. Both the separated nanoparticles and the connected nanostructures can lead to ion absorption loss. If the nanoparticles are in direct contact with the semiconductor absorption layer, additional surface recombination (caused by metal particles) can occur. Fortunately, these negative effects can be reduced by introducing a dielectric layer [102, 103]. Figure 5(b) shows LSPR (near-field enhancement), where diameters of the metal nanoparticles are very small, mostly concentrated in the range of 5-20 nm. The dimension is much smaller than the wavelength of visible and near-infrared light, and, thus, the scattering effect is very limited. Therefore, the metal particles act as optical antennas to store the incident energy in the localized surface-plasmon mode. The SPPR mode is shown in Figure 5(c). It is caused by a light incidence at the interface between metal and semiconductor. The propagation direction of an electromagnetic wave can change from perpendicular to horizontal (relative to the interface). The benefit of this mode can easily be utilized in existing planar thin-film solar cells [104].Figure 5
Three different plasmonic modes of thin-film solar cells: (a) scattering caused by metal nanoparticles on the surface of the semiconductor, (b) excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor, and (c) excitation of surface plasmon polaritons at the interface between metal and semiconductor [104].
(a)(b)(c)The history of using plasmon-structure to enhance light absorption of thin-film solar cells is not long. However, with the continuous development of surface-plasmon theory, more researchers began to study it in thin-film solar cells. This initiated a new research direction to increase the efficiency of thin-film solar cells. SPPR can be obtained by placing a metallic nanograting on the back of thin-film silicon solar cells. Compared with that without metallic nanograting, the absorption of certain wavelengths is enhanced by a factor of 14. While maintaining similar absorption in the short wavelength range, large, broadband, and polarization-insensitive absorption enhancement can be achieved in the long wavelength range (from visible to infrared) [105]. The spectral scattering response depends on the object geometry, the absorbed film’s thickness, and the selection of materials. As validated by numerical simulation, the coupling of sunlight is insensitive with respect to the incident angle. Accordingly, a 100 nm wide groove was created below the 200 nm Si film. For the solar spectrum near the Si bandgap, the absorption could be enhanced 2.5 times in the 10 μm region [106].8Several studies focused on the localized surface-plasmon resonance of noble metal particles. Schaadt et al. showed a mechanism to increase the optical absorption of semiconductors by exciting the surface-plasmon resonance in proximate metal nanoparticles. At or near the resonance wavelength of the plasma, the photocurrent response of the diode increased by 50%-80% or more, compared with the Si PN-junction diode. The response can increase even more with a higher density of nanoparticles [108]. In addition, the energy, which is transmitted to the amorphous silicon layer, can be increased. The light absorption, which is related to metal nanoparticles, can be reduced by exciting higher-order surface-plasmon resonance in larger nanoparticles. Therefore, the overall broadband light-absorption of the photoactive layer can be improved significantly [109]. To solve the substantial transmission loss of silicon for long wavelength light, Beck et al. fabricated silver nanoparticles using self-assembly technology. They showed that surface-plasmon resonance can be optimized by changing the local dielectric environment. The proportion of light, which is absorbed in solar cells, can be increased within a specific spectral region [110]. Despite metal particles, as observed in Figure 6(a), the silver strip array on the surface of Si/SiO2 can also excite local surface plasmon resonance. The near-field concentration, which is caused by LSPR with a silver strip array, greatly improves the normalized and time-averaged field intensity in the silicon film. The corresponding enhancement factor was 5 as shown in Figures 6(b) and 6(c) [107].Figure 6
(a) The schematic diagram of the designed plasmon-enhanced cell structure. Normalized and time-averaged optical field intensity plots under normal incidence, TM illumination of (b) bareSi/SiO2 structure and (c) and (d) the same structure with a periodic structure of narrow metal strips spaced at ρ=312nm, a thickness of spacer layer equal to s=10nm, and a thickness of the absorbent Si film of a=50nm. The incident wavelengths (energy) of (b) and (c) λ=650nm (1.91 eV) and (d) λ=505nm (2.46 eV) were selected to demonstrate the strong near-field light concentration or excitation of waveguide modes caused by the metal strips [107].
(a)(b)(c)(d)One-dimensional silicon particle chains, which are embedded in SiO2, can also excite local surface plasma resonance. The spatial distribution and interaction of silicon particles can be directly observed using electron energy loss spectroscopy (EELS). The diameter of silicon nanoparticles was about 12 nm, and the distance between them was in the range of several nanometers to tens of nanometers. The near-field interaction between adjacent nanoparticles led to splitting of the surface plasmon resonance into transverse and longitudinal polarization with different spatial distributions. Moreover, when the distance between particles was below 10 nm, spatial redistribution and enhancement of the SPR intensity in longitudinal mode were observed [111].


## 2.1. Geometrical Optics Method

The geometrical optics theory is useful to extend the optical path by causing the incident light to refract and reflect in the media using microstructure, which ultimately enhances light trapping. This theory has been used to design thin-film silicon solar cells for a long time. A reflection layer is created on the back of the solar cell. The reflector forms a certain angle with the front surface of the cell, with which the incident light is reflected to the front surface. If the incident angle is greater than the critical angle, complete internal reflection occurs. In this way, light can pass through the silicon layer many times, and the light-absorption efficiency of the thin-film silicon solar cell is enhanced. For practical applications, a grooved back reflector is more suitable. One advantage of the grooved mirror is that the gain of the optical path is almost independent of the incident angle (within 40° of the vertical angle) [65].According to literature, the combination of a back reflector with a front surface texture can achieve better light trapping. As can be seen in Figure1(a), the incident light first scatters randomly due to the primary effect of the textured surface. Then, it is reflected into the silicon layer via an aluminum back reflector, thus increasing the optical path of the incident light in the silicon layer [66]. For commercial solar cells, the most common surface texture is the pyramid structure. The light-trapping mechanism for pyramid textures larger than several microns is shown in Figure 1(b).Figure 1
(a) Design of commercial solar cells with enhanced light capture performance based on geometrical optics [66]. (b) Ray schematic of a textured silicon wafer based on geometric optics [67].
(a)(b)The incident light I vertically irradiates the surface of the left pyramid with an incident angle ofθ1. One part of the refracted light enters the silicon layer, and the reflected light R1 irradiates the surface of the adjacent pyramid and refracts again and again. Light tilting in the wafer leads to enhanced absorption near the front collecting junction. Directional randomization of weakly absorbed rays causes total internal reflection on the surface, which enables effective light capture. Ultimately, surface reflectivity could be reduced to below 10% [67, 68].Using computer-based ray tracing, Campbell and Green systematically studied the light-capture characteristics of pyramid arrangements. Examples include regular pyramids, random pyramids, staggered layouts, and double-sided texturing. Their performances were compared with a randomly textured (Lambertian) surface [69]. The group found that solar cells with flat back reflectors and pyramid-textured front surfaces show inferior light capturing than a Lambertian front surface. Because a considerable proportion of light passes through the cell for the first time. It is then reflected to the pyramid surface (by the back reflector) and directly coupled out. This is especially true if the pyramids are of the same size and positioned within a regular square matrix. Furthermore, randomly distributed pyramids perform even better. On the other hand, staggered pyramids showed a much better performance but at the expense of increased geometric sensitivity. For a double-sided texture, both the regular pyramid distribution and the random pyramid distribution permit almost all incident light to pass through the cell at least four times. The light capture performance is very close to that of the Lambertian surface. Han and Chen designed a solar cell with a planar back reflector and optimized the front-surface pyramid texture. The light absorption characteristics calculated by the transfer matrix method show that light capture performance with a single-side texture can also approach the Lambertian limit [70].Geometrical optics theory can well explain the light-trapping effect of microstructures whose dimensions are much larger than the wavelength of incident light. Nonetheless, microstructures that can exceed the Lambertian limit are difficult to design. This limit represents the thermodynamic limit for maximum absorption in a slab using geometrical optics. Compared with a flat surface, the length of the optical path can be increased4n2 times, where n is the refractive index of the medium [71, 72].


## 2.2. Effective Media Theory

When the nanostructure size is much smaller than the wavelength of the incident light, effective media theory is needed to study its light-trapping characteristics. Because in this case, only zero-order reflection waves and zero-order transmission waves are generated by zero-order diffraction after the interaction between the incident light and the nanostructure. The other higher-order diffraction waves are evanescent waves [73]. For the micron-rough surface, the average irregularity and the corresponding correlation length are below the incident wavelength. As a result, the incident light can interact with the whole surface but not each rough point.Effective media theory describes the interaction between light and a subwavelength structure. It uses a homogeneous material with effective optical constants to represent the subwavelength heterogeneous region: dielectric constantε, permeability μ, and conductivity σ. The optical properties of these effective media depend on the specific structural mixing between the incident light and the substrate materials. Generally, the more different substrate materials are used, the closer the optical properties of the region are to the substrate. In other words, when light interacts with subwavelength structures, it reacts to them if it encounters an effective medium, whose optical properties are the weighted spatial average of the optical properties in the profile region [74].The abrupt change of the refractive index between air and silicon solar cells can cause high reflected-light loss. Therefore, multilayer antireflection coating, with a gradually-changing refractive index, can be used to reduce the abrupt change of refractive index and cause light-reflection restraining [75]. On the basis of effective media theory, a nanostructure can realize the antireflection using similar principles and even exhibit light capturing that is better than that of multilayer antireflection coating. Assuming two-layer effective media, the relationship between the refractive index n of the effective medium and the refractive index n1 and n2 of the two effective media can be expressed using the Maxwell Garnett (MG) model [76].
(1)n2−n12n2+2n12=1−f1n22−n12n22+2n12.Here,f1 and f2=1−f1 are the corresponding volume fractions. In this model, Equation (1) holds only, when the layer with the refractive index n2 is surrounded by a layer with a refractive index n1. If n1 and n2 exchange, that is, the layer with refractive index n1 is surrounded by a layer with refractive index n2, the final effective medium n can also change. If the two layers are assumed to be uniformly mixed, according to Bruggeman’s effective medium approximation [77], the deformation is as follows:
(2)f1n12−n2n12+2n2+f2n22−n2n22+2n2=0.The uniqueness of the effective medium approximation allows it to be extended to multiple layers:(3)∑i=1nfini2−n2ni2+2n2=0.As shown in Figure2(a), the effective medium with a multilevel surface-relief profile is equivalent to an antireflection film made of multilayer films with different refractive index. Furthermore, ni is the refractive index of the incident medium, and ns is the refractive index of the nanostructures. It is assumed that ni<ns. The refractive indices of the first, second, and third level are n1, n2 and n3, and n1<n2<n3. In Figure 2(b), an effective medium with continuous profile is equivalent to an antireflective film with a gradual refractive index, where the refractive index gradually decreases along the z-axis and ni<nz<ns. Because effective medium is obtained by the weighted spatial average of the optical properties in the profile region, the optical properties of the effective medium must be constrained by the properties of the incident medium and the substrate medium [74].Figure 2
(a) A multilevel profile can be equivalent to an effective medium of multilayer film. (b) A continuous profile can be equivalent to an effective medium of gradient-index film [74]. (c) Silicon thin film solar cells with periodic submicron pattern on the surface [78].
(a)(b)(c)Theoretically, the effective medium with the continuous profile has a better light-trapping effect than a multilevel-relief surface. This is because the refractive index of the former changes more smoothly. According to the theory of effective media, nearly-perfect continuous-gradient index-profiles can be designed, such as a quintic profile and half-period profile for the exponential sine function [33, 79]. Although they have excellent antireflection properties, because these complex profiles are difficult to manufacture, they are not suitable for mass production. Nonideal profiles such as the linear gradient profile and the Gaussian profile were found either theoretically or experimentally to have extremely low reflectivity using RCWA and FDTD studies. With such profiles, the reflectivity from 300 nm to 1200 nm is generally below 10%. Compared with multilayer antireflection coatings, these nonideal profiles show smaller angle dependencies and a wider antireflection wavelength-range [76, 80, 81]. Zanotto et al. [78] analyzed the effect of submicron patterns and antireflection coatings on the surface of thin-film solar cells using the RCWA method as shown in Figure 2(c). By changing the structural parameters, they found that the AR (antireflection) coating significantly increased the short-circuit current jsc. Compared with the structure with the optimized AR coating but no pattern, the efficiency of a-Si cells with a submicron pattern increased by 12.4%. When monocrystalline silicon was used as the absorbing material, the efficiency of the solar cell increased by 36.5%. The group also found an expression for the effective refractive index of the dielectric-slices-arrangement:
(4)εefff=12fS+1−fεSO2+1f/εS+1−f/εSO2.To match the air impedance with the effective layer impedance when the antireflective coating is applied on the patterned structure, the relationship between the refractive indexn of the antireflective coating and the silicon filling factor f under the optimal matching conditions is given as
(5)nf=nefff1/2=εefff1/4.


## 2.3. Scatters

If the size of the microstructure and the wavelength of the incident light are of the same order of magnitude, the interaction between them can produce strong scattering, and the intensity of the scattered light can be stronger in the forward direction. If the photosensitive material is located below the scattering microstructure, the scattered light can be coupled into the photosensitive material effectively. As early as 1908, Gustav Mie proposed the concept of Mie scattering. According to Maxwell’s electromagnetic theory, a rigorous mathematical solution of particle scattering was derived for spherical particles in a uniform medium for monochromatic plane-wave illumination. The light-field distribution, which was calculated from this solution, is called Mie scattering. Calculation results of Mie scattering for the particle limit are basically consistent with both Rayleigh scattering and Fraunhofer diffraction theory. Nevertheless, Rayleigh scattering and Fraunhofer diffraction theory cannot be used to calculate scattering effects when the particle diameter is close to the wave length [82]. Since Mie scattering particles consist of dielectric materials, there is no electric impedance for metal-ion resonance, and its optical loss is minimal [83]. The limitation of Mie-scattering theory is that it can only deal with the scattering characteristics of a single spherical particle, although it can be applied to particles of any size and material [84–86].Poruba et al. analyzed the surface- and bulk-scattering effects of microcrystalline silicon thin-film solar cells, which were deposited by very high-frequency glow discharge (VHF-GD). This was done using the constant photocurrent method (CPM). The group found that light-absorption enhancement in microcrystalline silicon films and solar cells is mainly due to the long light-path, which is generated by effective diffuse light scattering on the surface of textured films. This light-scattering effect is a key characteristic of high-efficiency thin-film silicon solar cells. Additionally, surface scattering is of great significance for thin-film silicon solar cells. As a result of the indirect optical transition in crystalline silicon, light absorption in the film is very low. To absorb most of the solar spectrum in the film with a thickness of only several microns, light absorption must be increased. Therefore, a complex optical capture scheme is proposed. Although no nanostructures were made on the surface of microcrystalline silicon films, most of the high-quality films do not show mirror-like free surfaces. These “milky white” layers have rough surfaces with a typical root-mean-square (RMS) value of 15-35 nm, which causes surface scattering. Finally, they experimentally demonstrated that the infrared absorption enhancement of nanotextured microcrystalline silicon films and solar cells is mainly due to surface scattering. In the subgap region, scattering in the material contributes significantly to the CPM results [87].Nanostructures, which are designed according to the scattering principle, were numerically shown to have very good light trapping. The mixed texture consisted of a diffraction grating with a random rough surface and a type of quasirandom binary grating pattern. The absorptivity was shown to be very close to the Lambertian limit [88]. The optimized diffraction grating structure leads to both scattering and higher-order diffraction, which contributes to the enhanced optical absorptivity of silicon thin-film solar cells. Compared with the planar case, the overall reflectivity decreases from 76% to 45.1%, and the quantum efficiency increases from 0.2 to 0.47 for 700 nm monochromatic illumination [89]. A scattering layer with periodic patterns was introduced at the top of the cladding layer, and each cell consisted of several air slots. These grooves were oriented in different directions to ensure that the scattering intensity was not strongly dependent on the angle and polarization of the incident light. Finally, the numerical results show that the enhancement factor of this structure exceeds twice the Lambertian limit [90]. Yu et al. fabricated hexagonal silicon nanopillar arrays on the surface of monocrystalline silicon. The group revealed the (quantitative) role of the light-trapping structure (LTS) using a three-dimensional FDTD method and experimental tests. The results show that the experimental measurement and simulation results are consistent. As shown in Figures 3(c) and 3(d), the average absorptivity of planar silicon from 550 nm to 1100 nm is 27.6%, while the absorptivity of silicon with the surface light-trapping structure increased to 69%. The simulation results show that the average absorption rate increased from 28% to 77%. This suggests that diffraction/scattering of the top hexagonal pattern increases the optical interaction length, which increases the absorption. Some high-order diffracted/scattered light was trapped in the silicon through total internal reflection at the top surface [91].Figure 3
(a) Schematic diagrams of ordered and random Si NW arrays. (b) Reflectivity and absorptivity of ordered and random Si NW arrays from FDTD simulation [91]; absorbed photon flux were from (c) experimental data and (d) results of calculation under the AM 1.5D standard solar spectrum as a function of wavelength for bare Si and bare Si with ARC+LTS+BSR [93].
(a)(b)(c)(d)Gjessing et al. presented a comprehensive comparison of light-trapping properties among various biperiodic structures with a square lattice. The geometric shapes they studied were cylinders, cones, inverted pyramids, dimples (hemispheres), and three more advanced structures (roof mosaics, roses, and zigzag) as shown in Figure4. The simulated photogenerated current Jph of the zigzag structure was 37.3 mA/cm2, which performed best. Its light-capture efficiency is equivalent to that of Lambertian. The enhanced performance can be explained as follows. On one hand, the number of sharp edges and angles in these structures increases scattering. On the other hand, the reduced symmetry of the diffraction patterns of rose shaped and zigzag structures reduces the possibility of any subsequent out-coupled interaction with diffraction gratings [92].Figure 4
Schematic representation of each of the investigated periodic light trapping structures [92].
(a)
Cylinder(b)
Dimple(c)
Cone(d)
Inverted pyramid(e)
Roof-mosaic(f)
Rose(g)
ZigzagSilicon-nanowire arrays have excellent light-trapping properties and are popular topics among surface nanostructures of silicon solar cells due to their easy fabrication and controllable size. Researchers have already numerically studied the optical properties of silicon nanowire arrays [93–95]. Bao and Ruan analyzed the light-trapping characteristics of ordered nanowire arrays and nanowire arrays with random positions, diameters, and lengths, using the FDTD method. As shown in Figure 3(a), the absorption and reflectivity curves of ordered nanowire arrays (and nanowire arrays with random position and length) are shown in Figure 3(b). The absorption peak for random-position nanowire arrays is consistent with that for ordered nanowire arrays because the diameters of the nanowires are the same. The absorption of random position nanowires is slightly higher than that of ordered nanowire arrays. This indicates that enhanced multiple scattering increases the optical path. In the case of random diameter arrays, no obvious absorption peak can be found as diverse diameters correspond to different resonance frequencies and absorption peaks. The clearly broadened and enhanced absorption of random diameter arrays can be attributed to the strengthened multiple interwire scattering and/or inner-wire resonance [93].


## 2.4. Plasmonics

Many free electrons are present on the surface and inside of metals. Under certain conditions, these electrons can interact with the incident light wave to generate a collective resonance. Correspondingly, the collective resonance of this charge can react with the electromagnetic field of the light wave. According to the dispersion characteristics, surface plasmon resonance can be divided into local surface-plasmon resonance (LSPR) and surface-plasmon polarization resonance (SPPR). The phenomenon of surface-plasmon resonance (SPR) was first discovered by Wood in a spectroscopic experiment of a metal diffraction grating in 1902 [96]. Since then, Fano, Ritchie, and Powell and Swan further developed the SPR theory through both experiments and theoretical analysis [97–99]. In 1960, Stern and Ferrell studied the conditions to generate surface electromagnetic waves. In addition, the concept of surface-plasmon polaritons (SPPs) was proposed for the first time, and the dispersion relation was derived [100]. Although both types of SPPs show local characteristics, they are different excited states. LSPR is a nonpropagation mode, which can radiate electromagnetic waves. SPPR is one excitation element that propagates along the interface between metal and medium.Three different plasmon models are shown in Figure5. A schematic for metal-nanoparticle scattering is shown in Figure 5(a). The scattering principle of such nanoparticles is similar to the one of the previous section. However, because of the collective resonance of free electrons in metal particles, light near the vibration frequency can be scattered strongly. Moreover, when the metal nanoparticles are located at the interface of two different media, the scattered light can preferentially enter the medium with the higher dielectric coefficient [101]. However, there are also disadvantages when using metal-particle scattering. Both the separated nanoparticles and the connected nanostructures can lead to ion absorption loss. If the nanoparticles are in direct contact with the semiconductor absorption layer, additional surface recombination (caused by metal particles) can occur. Fortunately, these negative effects can be reduced by introducing a dielectric layer [102, 103]. Figure 5(b) shows LSPR (near-field enhancement), where diameters of the metal nanoparticles are very small, mostly concentrated in the range of 5-20 nm. The dimension is much smaller than the wavelength of visible and near-infrared light, and, thus, the scattering effect is very limited. Therefore, the metal particles act as optical antennas to store the incident energy in the localized surface-plasmon mode. The SPPR mode is shown in Figure 5(c). It is caused by a light incidence at the interface between metal and semiconductor. The propagation direction of an electromagnetic wave can change from perpendicular to horizontal (relative to the interface). The benefit of this mode can easily be utilized in existing planar thin-film solar cells [104].Figure 5
Three different plasmonic modes of thin-film solar cells: (a) scattering caused by metal nanoparticles on the surface of the semiconductor, (b) excitation of localized surface plasmons in metal nanoparticles embedded in the semiconductor, and (c) excitation of surface plasmon polaritons at the interface between metal and semiconductor [104].
(a)(b)(c)The history of using plasmon-structure to enhance light absorption of thin-film solar cells is not long. However, with the continuous development of surface-plasmon theory, more researchers began to study it in thin-film solar cells. This initiated a new research direction to increase the efficiency of thin-film solar cells. SPPR can be obtained by placing a metallic nanograting on the back of thin-film silicon solar cells. Compared with that without metallic nanograting, the absorption of certain wavelengths is enhanced by a factor of 14. While maintaining similar absorption in the short wavelength range, large, broadband, and polarization-insensitive absorption enhancement can be achieved in the long wavelength range (from visible to infrared) [105]. The spectral scattering response depends on the object geometry, the absorbed film’s thickness, and the selection of materials. As validated by numerical simulation, the coupling of sunlight is insensitive with respect to the incident angle. Accordingly, a 100 nm wide groove was created below the 200 nm Si film. For the solar spectrum near the Si bandgap, the absorption could be enhanced 2.5 times in the 10 μm region [106].8Several studies focused on the localized surface-plasmon resonance of noble metal particles. Schaadt et al. showed a mechanism to increase the optical absorption of semiconductors by exciting the surface-plasmon resonance in proximate metal nanoparticles. At or near the resonance wavelength of the plasma, the photocurrent response of the diode increased by 50%-80% or more, compared with the Si PN-junction diode. The response can increase even more with a higher density of nanoparticles [108]. In addition, the energy, which is transmitted to the amorphous silicon layer, can be increased. The light absorption, which is related to metal nanoparticles, can be reduced by exciting higher-order surface-plasmon resonance in larger nanoparticles. Therefore, the overall broadband light-absorption of the photoactive layer can be improved significantly [109]. To solve the substantial transmission loss of silicon for long wavelength light, Beck et al. fabricated silver nanoparticles using self-assembly technology. They showed that surface-plasmon resonance can be optimized by changing the local dielectric environment. The proportion of light, which is absorbed in solar cells, can be increased within a specific spectral region [110]. Despite metal particles, as observed in Figure 6(a), the silver strip array on the surface of Si/SiO2 can also excite local surface plasmon resonance. The near-field concentration, which is caused by LSPR with a silver strip array, greatly improves the normalized and time-averaged field intensity in the silicon film. The corresponding enhancement factor was 5 as shown in Figures 6(b) and 6(c) [107].Figure 6
(a) The schematic diagram of the designed plasmon-enhanced cell structure. Normalized and time-averaged optical field intensity plots under normal incidence, TM illumination of (b) bareSi/SiO2 structure and (c) and (d) the same structure with a periodic structure of narrow metal strips spaced at ρ=312nm, a thickness of spacer layer equal to s=10nm, and a thickness of the absorbent Si film of a=50nm. The incident wavelengths (energy) of (b) and (c) λ=650nm (1.91 eV) and (d) λ=505nm (2.46 eV) were selected to demonstrate the strong near-field light concentration or excitation of waveguide modes caused by the metal strips [107].
(a)(b)(c)(d)One-dimensional silicon particle chains, which are embedded in SiO2, can also excite local surface plasma resonance. The spatial distribution and interaction of silicon particles can be directly observed using electron energy loss spectroscopy (EELS). The diameter of silicon nanoparticles was about 12 nm, and the distance between them was in the range of several nanometers to tens of nanometers. The near-field interaction between adjacent nanoparticles led to splitting of the surface plasmon resonance into transverse and longitudinal polarization with different spatial distributions. Moreover, when the distance between particles was below 10 nm, spatial redistribution and enhancement of the SPR intensity in longitudinal mode were observed [111].


## 3. Fabrication of Nanostructures on Monocrystalline Silicon

On the basis of the chemical and physical properties of monocrystalline silicon, many methods can be used to prepare a nanostructure on the surface. These methods can be divided into two categories: top-down and bottom-up. Top-down methods remove part of the original material, and the remaining part represents the desired nanostructure. Typical representatives are MACE and reactive ion etching (RIE), which belong to wet etching and dry etching respectively. The bottom-up method is mainly realized by the deposition or chemical reaction to grow new materials. Typical processes include chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and molecular beam epitaxy (MBE). These methods have their own advantages and can be used separately or combined to realize different nanostructures. More details are discussed in the following.


### 3.1. Top-Down Method

#### 3.1.1. Alkaline Etching

Due to the anisotropy of monocrystalline silicon in alkaline etching solutions, a method to prepare pyramid- or inverted-pyramid structures on its surface, using NaOH, KOH, and other corrosive solutions, was developed [112]. Today, this method is widely used for commercial silicon solar cells. The homogeneity of the pyramid nanostructure can be effectively improved by optimizing the corrosion time with a specific concentration of alkaline corrosive solution [113, 114]. The etching rate of silicon in solution and the anisotropy factor (quotient of the etching rate in the <100> to <111> directions) affect both the density and size of the pyramid in the <100> direction. Seidel found that the maximum etching rate of 0.9 μm/min can be achieved at 72°C for KOH solution with concentration of 15% [115]. However, the concentration of KOH less than 20% usually leads to the increased surface roughness and the formation of insoluble precipitates [116]. Higher concentration of KOH solution and higher temperature can increase the etching rate and make the etching surface smoother. For example, some studies have confirmed that the etching depth per minute of a 50% KOH solution at 80°C increased by 0.5 μm in the direction (100) [117]. The high concentration NaOH solution has a high etching rate. The work of Green et al. shows that the etching rate of NaOH solution with a concentration of 20-30% can reach 4-6 μm/min at 90°C [118, 119]. However this rate is too high for pyramid texturing of silicon. When texturing the surface of monocrystalline silicon with strong alkaline corrosive solutions such as NaOH and KOH, the addition of can effectively improve the uniformity of texture [120, 121].Based on a kinetics study of anisotropic etching, pyramid growth begins at the preferred position caused by crystal defects or wetting. At the beginning of the etching process, the inhomogeneity of the pyramid nucleation can be avoided by mixing additives into the etching solution. Vazsonyi et al. found the best texture condition at 80°C,where a relatively high etching rate (0.6μm/min) can be reached in the <100> direction. The anisotropy factor was 10. The pyramid size can be adjusted by changing the etching time to obtain low reflectivity from the textured silicon wafer [122]. Regular and well distributed pyramids can be obtained by etching in NaOH and isopropanol solution for 25 to 45 minutes. Reflectance ranges from 450 nm to 1000 nm between 11% and 14% [114]. However, IPA has also some disadvantages: high cost, volatile when heated, and low corrosion-rate at high concentrations. These disadvantages can be overcome by using metal grids with periodic openings, which improves the pyramid-texture uniformity. Chu et al. showed that when the metal mesh is placed near the wafer surface, hydrogen bubbles, which are produced by the KOH Si reaction, are limited by opening of the mesh because of the surface tension of the bubbles. Because hydrogen bubbles adhere to the wafer surface and inhibit the chemical reaction, they can act as a “pseudo mask.” Accordingly, both the position and size of the bubbles are relatively stable given the limitation to metal grids. Stable and uniform pyramid structures can be obtained without adding IPA [120].To avoid the inherent shortcomings of KOH and NaOH solutions during the texturing process of monocrystalline silicon, the application of other alkaline solutions such as tribasic sodium phosphate (Na3PO4·12H2O), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), and sodium acetate (CH3COONa) were studied [123–125]. These solutions show the benefits of lower cost and reduced environmental pollution. On further research, the tetramethyl ammonium hydroxide (TMAH) solution was found to show the best overall effect. It overcomes the problem of mobile ion pollution, which is caused by the KOH and NaOH alkaline etching solutions that are commonly used in IC processes. The advantages of this solution include good controllability and a stable etching rate. Consequently, the texture of the inverted pyramid, which can be etched using the TMAH solution, is smooth but the top of the pyramid is sharp [126]. In addition, the texture, which was obtained using TMAH, can be passivated more easily than with KOH [127]. TMAH solution can be reused many times, which can reduce the cost of the pyramid texturing process [128]. Choi et al. studied the effect of the etchant concentration, ambient temperature, and silicon wafer thermal history on the formation of the pyramid in TMAH. The number and size of the pyramids were used as parameters in the study. Their results show that pH theory can well explain the effects of TMAH concentration and ambient temperature on silicon surface changes. However, the results of rapid thermal annealing, Czochralski wafers, and floating zone wafers seem to favor the defect theory [129]. To better explain the mechanism of silicon corrosion by TMAH, it is necessary to combine pH with the defect theory. When using the KOH and TMAH solution to texture the surface of monocrystalline silicon, the addition of IPA can alter the corrosion rate and roughness of the silicon surface <100>. When the IPA is adsorbed on the etched surface, it can hinder the reaction between Si and OH− and reduce the etching rate of the specific crystal face. Then, the equilibrium between oxidation and diffusion of the reaction products establishes the necessary conditions to generate smooth surfaces. The reduced reaction rate, which is caused by IPA adsorption, enables uniform surface etching. During the etching process, the IPA adsorption can be isotropic or anisotropic—depending on the composition of the solution. When TMAH solution (2-2.5 m) is mixed with IPA (10-30%), the decreased rates for (100) and (hkl) crystal planes are almost the same—independent of the crystal orientation. The anisotropic adsorption of IPA for different crystal orientations can be observed by mixing medium concentrated KOH solution (3-7 m) with low concentrated IPA (few to several percent). When the highly concentrated KOH solution was mixed with the extremely low concentrated IPA (2%), the IPA almost had no effect, and the etching rates for the different crystal orientations were almost the same [130].The electrochemical corrosion mechanism for monocrystalline silicon in alkaline solution was also studied. According to the model proposed by Seide et al., the total reaction process is summarized as the following equation:(6)Si+2OH−+2H2O⟶SiO2OH2−+2H2↑.In the oxidation reaction stage, due to the high pH environment, the surface atoms of bulk silicon react with fourOH− to produce SiO2OH2−.At the same time, four electrons are injected into the conduction band of silicon, two of them are transferred through the surface state of dangling bond, and the remaining two electrons are transferred through the surface state of back bond. These four electrons will be bound near the silicon surface due to the potential well generated by the downward bending energy band. In this space charge region, the recombination probability of injected electrons is very small. In the reduction process, implanted electrons will leave the solid and react with water molecules to produceOH− and H2. These OH− are generated directly on the Si surface and participate in the oxidation reaction. The orbital electrons in their respective surface states are thermally excited into the conduction band, resulting in the fracture of the back bond, which is considered to be the rate limit. Different crystallographic back bond surface states have different energy levels, which eventually leads to the anisotropy of etching, and the energy level of (111) plane is the lowest [115].According to the theoretical analysis in Section2.1, pyramid array textures with specific periodic arrangements show a better light-trapping effect than random-pyramid arrays. Therefore, it is important to study the preparation of periodic nanostructures. In the aforementioned method of texturing a monocrystalline silicon surface with anisotropic alkaline solution, the etching process is difficult to control because there are no masks. Only random pyramid textures can be obtained. A mask with periodic patterns can precisely control the etching position and produce the desired periodic nanostructures. The most commonly used mask is a photoresist with periodic patterns that were formed after the photolithography. The periodic inverted-pyramid pattern, shown in Figure 7, is formed by interference lithography and subsequent etching in KOH solution. The periodicity of the inverted pyramid array is 700 nm, and the average separation is 100 nm [131]. Patterned SiO2 can also be used as a mask for anisotropic alkaline etching. Using photolithography and dry etching methods, a silica rectangular window can be formed to expose silicon under the SiO2 layer. The periodically arranged inverted pyramidal array is obtained by etching in TMAH solution [132]. Sivasubramaniam and Alkaisi employed interference lithography combined with dry etching and KOH wet etching to fabricate inverted pyramid nanoarrays with a characteristic size of 700 nm on the surface of monocrystalline silicon. The vertical incident reflectivity in the whole visible light range was suppressed to less than 10%, and the overall efficiency of solar cells was improved by 67% [133]. Han et al. adopted almost the same method to fabricate symmetry breaking inverted nanopyramid arrays on the surface of monocrystalline silicon thin films. Their experimental and calculation results show that symmetry breaking can improve the photovoltaic efficiency of thin film silicon solar cells. For the active layer with a thickness of 10μm, the photovoltaic efficiency of the traditional symmetrical inverted nanopyramid array is 27%, and the introduction of a symmetry breaking ring improves the photovoltaic efficiency to 27.9% [134].Figure 7
(a) Photograph, (b) SEM vertical view, and (c) SEM cross-sectional view images of the 10μm c-Si film illustrating periodical reverted nanopyramids with a period of 700 nm. A SiNx layer with a thickness of 90 nm deposited on the surface of nanopyramids, and a SiO2 (1 μm) and a silver (200 nm) layer are at the reverse side of the c-Si film. The right bottom insets are enlarged drawings of the blue and red boxed areas. Scale bars:1 μm for (b) and (c) and 200 nm for insets in (c) [131].
(a)(b)(c)Periodic arrays of upright pyramids can be fabricated using a similar method. As shown in Figure8, photoresist is spin-coated on the surface of SiO2. Two-dimensional periodic photoresist masks can be obtained using laser interference lithography. RIE was used to transfer photoresist patterns to the SiO2 layer. Hydrofluoric acid (HF) is used to further reduce the size of the SiO2 mask. After anisotropic alkaline etching in a KOH solution, the periodic array of positive pyramids is obtained. The results show that the reflectivity of the front surface of the array is lower than that of the conventional micrometer pyramid texture. The excellent antireflection performance facilitates an absorption photocurrent density for a 40μm silicon absorption layer reaching 40.8 mA/cm2. This value is 0.7 mA/cm2 higher than for the periodic inverted-pyramid array, 0.8 mA/cm2 higher than the randomly arranged inverted-pyramid array, and 1 mA/cm2 higher than the randomly arranged vertical pyramid. The reason for the reduction in reflectivity is provided in the theoretical analysis in Sections 2.2 and 2.3 [135].Figure 8
(a) A schematic outline of the periodic upright nanopyramid preparation workflow based on laser interference lithography and anisotropic etching. (b) Corresponding 3D schematic diagram. (c) SEM images ofSiO2 template [135].
(a)(b)(c)The mask selection is mainly determined by the fabrication process. Furthermore, whether the photoresist with the periodic pattern is used as a mask orSiO2 with a periodic pattern (via photolithography) is used, which is determined by the photolithography step. It inevitably increases the cost of texturing the monocrystalline silicon surface. Nanoimprinting has lower cost than lithography, so it can be a good substitute. Gaucher et al. chose soft UV nanoimprint lithography to manufacture Cr etching mask on the surface of monocrystalline silicon film. After KOH solution etching, a nanoinverted pyramid array was formed. Finally, the short-circuit current density of thin-film crystalline silicon solar cells with a thickness of only 2.75 μm reached 25.3 mA/cm2[137]. Sun et al. demonstrated a simpler and cheaper nonphotolithographic method to fabricate wafer-level subwavelength antireflection gratings on monocrystalline silicon as shown in Figure 9. First, a monolayer of colloidal silica particles was spin-coated on the surface of <100> silicon for UV curing. Then, RIE was used to remove any SiO2 particles left behind by the colloid mask. Subsequently, electron-beam evaporation was used to deposit a 30 nm thick Cr layer. SiO2 particles were then removed to form a periodic nanopore array, whose diameter was determined by the size of the silica sphere template. These circular nanoholes were used as masks for the anisotropic KOH etching process, and a wafer-level inverted pyramid array was formed with a staggered layout in silicon [136].Figure 9
Fabrication of inverted nanopyramid arrays using 320 nm diameter silica spheres as templates. (a) 300 nm size nanopyramids and (b) 360 nm size nanopyramids both are etched at 60°C, etching times are 90 s and 210 s, respectively. (c) Schematic flow of the templating procedures for fabricating subwavelength-scale antireflection nanopyramid structures on monocrystalline silicon [136].
(a)(b)(c)This method greatly reduces the cost and allows for good controllability because it does not require lithography steps. The morphology of the final inverted pyramid can be controlled by the size of theSiO2 particles, the etching time, and the etching temperature. The reflection of the 360 nm inverted-pyramid array is reduced to 10% for a long wavelength (600 nm). When the wavelength is about 400 nm, the reflection is further reduced to 2%, which indicates good antireflection characteristics. Similarly, Zhou et al. used hexagonally close-packed polystyrene sphere (PS) as mask, ion etching was used to control the diameter of PS, and then, magnetron sputtered 50 nm Ti film. After PS removal, the front and rear surfaces of crystalline silicon films were etched with a NaOH and IPA mixed solution. The periodic inverted pyramid arrays of different sizes were obtained by controlling etching time. When the period of the inverted pyramid array is 1400 nm, the best reflection reduction effect is achieved. The current density of solar cells can reach 39.86 mA/cm2, and the contrast plane (22.63 mA/cm2) is increased by 70% [138].


#### 3.1.2. Metal-Assisted Chemical Etching (MACE)

MACE uses metal catalysis so that both monocrystalline silicon and polycrystalline silicon can be anisotropically etched in acid solution [139–141]. Unlike the anisotropic etching solution, which can reduce cost [142]. In the process of preparing porous silicon from HF, HNO3, and H2O mixed solution, Dimova-Malinovska et al. found that the Al film, which was deposited on silicon surface, had a catalytic effect [143]. Subsequently, Li and Bohn studied the preparation of porous silicon in HF/H2O2 solution with noble metals (Au, Pt, and Au/Pt alloys) as catalysts. The group proposed the MACE mechanism for the first time [144]. As shown in Figure 10, a key feature of this reaction scheme is the formation of holes from H2O2 and the reduction of H+ to H2, both of which are facilitated by metal particles. The MACE of Si can be considered to be a local electrochemical process. The noble metal, which is deposited on the surface of Si sheet, is used as a local cathode catalytic oxidant to generate holes. It is then injected into the valence band of Si to oxidize and form silicon-ions dissolved in HF solution. The porous silicon, which was fabricated using this method, has a reflectivity of 2% in the range of 200-1000 nm [145]. Au nanoparticles were used to catalyze subwavelength structure formation after 15 min etching. The reflectivity of the subwavelength structure was below 5% for the range 300 nm to 800 nm [146].Figure 10
Catalytic-induced electrodynamics for autonomic motion of Ag nanoparticle in silicon substrate. (a) A auto genetic electric field caused by a hydrated proton gradient passing through the Ag particles. (b) Self electro-phoresis-driven movement of Ag particles. (c) Collective and extensive tunnelling motility of Ag (or metal) particles in a silicon substrate that forms the nanostructures of silicon [147].
(a)(b)(c)The detailed reaction mechanism for silicon MACE is as follows [143, 144]:Cathodic reaction (metal):(7)H2O2+2H+⟶2H2O+2H+2H++2e−⟶H2↑.Anodic reaction:(8)Si+4H++4HF⟶SiF4+4H+SiF4+2HF⟶H2SiF6.Total reaction:(9)Si+H2O2+HF↔2H2O+H2SiF6+H2↑.Electrochemical etching is a type of conventional wet isotropic acid etching [149–154] in which high aspect-ratio holes can be created on the surface of a silicon substrate. However, the uniformity and layer thickness of the silicon wafer surface texture are difficult to control using this method. Furthermore, the repeatability is poor, and this process is mostly used for surface texturing of polycrystalline silicon. In addition, electrochemical etching needs to connect the silicon wafer to an external power supply, which increases both the process complexity and energy consumption. Some studies showed that isotropic acid etching can achieve better antireflection properties than alkaline etching [155, 156]. However, the obtained rough surface greatly increases the surface carrier recombination and is not conductive to the subsequent surface passivation. When the poor controllability of the process and surface roughness issues are considered, the benefits of electrochemical etching for solar cells are very limited.Compared with the surface texture of polysilicon etched usingHF/HNO3 aqueous solution without metal catalyst, the efficiency of the solar cell with a bowl-like surface texture fabricated with the MACE method and acid postmodification increased by 1.2% (reaching 19.1% efficiency). This is mainly attributed to the fact that the lowest average reflectivity of the bowl-like structure is only 5.3% between 500 and 1100 nm. For comparison, the average reflectivity of a conventional micro texture is 14.5%. In addition, the surface area was reduced after acid modification, and the efficiency of the solar cells improved. The bowl-like textured solar cell shows a good broadband spectral response. This is because it has excellent antireflection properties at short wavelengths and long wavelengths. Moreover, the texture enables special light absorption at wide angles of incidence (AoI) from 0° to 70°, and it shows an all-round characteristics [157]. Recently, Zhang et al. also used CuSO4 etchant to manufacture high-efficiency inverted pyramid texture monocrystalline silicon solar cells. They compared the etching mechanisms of Cu (NO3)2 and CuSO4. CuSO4 produced fewer bubbles in the etching process. Because SO4−2 did not participate in the reaction, the etching rate of CuSO4 was significantly lower than that of Cu(NO3)2. The finally deposited Cu particles would be distributed on the surface of monocrystalline silicon, and a uniformly distributed random inverted pyramid structure could be obtained. They also confirmed that the PCE of solar cells etched with CuSO4 reached 22.63%, which was 0.6% and 1.56% higher than that etched with Cu(NO3)2 and NaOH, respectively [158].As can be seen from Figure11, Fang et al. deposited a 10-50 nm thick network film of Ag nanoparticles on the cleaned surface of silicon (111). Then, a large area of tilted nanowire arrays was fabricated by etching using mixtures of deionized water, HF, and H2O2, ranging from room temperature to 80°C. Compared with solar cells fabricated using a vertically-aligned silicon nanowire array (VA SiNW array), the reflectivity of the tilted nanowire arrays from 300 to 1000 nm is 5%, which is slightly higher than for a VA SiNW array 2.5%. Due to the inclined arrangement, the device has better electric contact. The power conversion efficiency of a slantingly-aligned SiNW array (SA SiNW array) solar cells is 11.37%, which is the highest among the devices that they studied [148].Figure 11
SEM figures of SA-SiNW arrays on p-Si (111) substrate: (a) planar figure without tilt angle, (b) cross-sectional figure, (c) SA-SiNW arrays with no metal electrodes, (d) SA-SiNW arrays covered with metal mesh electrodes, (e) magnified planar figure of the electrode regions, and (f) tilted cross-sectional BSE figure of the electrode regions [148].
(a)(b)(c)(d)(e)(f)Furthermore, MACE can be combined with anisotropic alkaline etching to create composite nanostructures with better light-trapping properties on the surface of monocrystalline silicon [159, 160]. He et al. fabricated a pyramidal texture on the surface of monocrystalline silicon using a mixture of KOH and isopropanol. The group then used a mixed solution of HF and AgNO3 to etch the pyramid-texture surface to obtain the silicon nanowire/pyramid binary structure. The measured results showed that the reflectivity of all textured samples was lower than for planar Si samples. The samples with pyramid texture showed 6-18% reflectivity over a wide spectrum (350-1100 nm). Compared with planar Si and pyramid-textured Si, the reflectance of all SiNWs and binary samples was much lower. It was below 5% for the entire 350-1100 nm range [161]. According to the theoretical analysis in Section 2.3, this is because the diameter of the SiNWs is equivalent to that of light at these wavelengths. This improved light scattering and increased the length of the optical path. This particular geometry has several advantages for hybrid solar cell applications. First, it increases the area of SiNW/PEDOT heterojunctions, not just the SiNW array on planar Si. Second, excellent optical capture can be achieved only using short SiNWs with low aspect ratio. Their aggregation is below that of longer SiNWs and allows for lower carrier recombination [162, 163]. In addition, shorter SiNWs are more robust than longer ones, which leads to more mechanically robust devices.The controllable parameters in MACE determine the final morphology of the etched nanostructures and affect the reflectivity directly. Furthermore, the shape of the metal-particle is an important factor in controlling the direction of holes [140]. Black silicon (B-Si) nanopore morphology is highly dependent on the volume ratio HF:H2O2:H2O and etching time. When the concentration of Ag+ is 50 μM, the lowest relative effective reflectivity (0.17%) can be obtained when the ratio of HF:H2O2:H2O is 1:5:2 [164]. During the study of MACE on non (100) Si substrates at different temperatures and oxidant concentrations, the etching direction of (110) and (111) Si substrates is affected by the concentration of the oxidant in the solution. In the solution with low oxidant concentration, etching was carried out along the preferred (100) direction of the crystal. On the other hand, in the solution with high oxidant concentration, etching was carried out along the vertical direction relative to the substrate surface. This was done because of the lattice structure of the Si crystal. The atoms of the plane have two back bonds with the atoms below, while the atoms of the <111> plane have three back bonds. Therefore, compared with the atoms of the <111> plane, the atoms of the plane are easier to dissolve [165].Yeo et al. studied the effects of HNO3, HF, deionized water concentration, and etching temperature on the morphology and optical properties of the silicon nanostructures in MACE. As a result, a practical application for antireflective silicon nanostructures with self-cleaning capability was found. Furthermore, the optical properties of Si nanostructures are closely related to the Si-MACE conditions. The solar weighted reflectance (SWR) of the Si nanostructures under optimal MACE conditions was 1.96%, and an angle-dependent SWR of < 4%, up to an AoI of 60°, was obtained. Compared with bulk silicon (SWR, 35.91%; angle-dependent SWR, 37.11%) for the wavelength range 300-1100 nm, the contact angle for water was about 102°. These results add to the understanding of Si MACE and provide valuable guidance for the realization of ideal antireflective Si nanostructures with self-cleaning properties to be used in high-efficiency c-Si solar cells [166].Moreover, catalytic etching with different metals was already done experimentally for Pd, Au, Pt, and Ag. The metal type can affect both the silicon etching rate and the porous structure in MACE [146, 167, 168]. Peng et al. and Liu et al. studied the motion of metal particles in Si. They found that the autonomous motion of the metal particles (Ag and Au) is highly uniform. However, it shows directionality and selectivity along the (100) direction of Si, rather than always being perpendicular to the surface of silicon [147, 169]. This feature can be used to fabricate inclined silicon nanowire arrays in single-crystalline silicon. With increasing HF and H2O2 concentration, the velocity of Ag particles in Si increases. To explain these observations, an electro kinetic model (self electrophoresis) was proposed. As shown in Figure 10, the movement of Ag and other metal particles in Si is related to the catalytic conversion of chemical free energy into propulsive mechanical energy.In an electrochemical system like Ag-Si-HF-H2O2, the reaction in two half cells, including the cathode and the anode, can be greatly enhanced for different positions on the surface of metal particles. The side of Ag particles facing the etching solution can act as a cathode to catalyze the reduction of H2O2 (or protons, H+) and consume H+ and electrons in the process. While the other side of Ag particles facing Si can act as a catalyst for Si oxidation and, thus, produce H+ and electrons. Because the resistivity of Si is much higher than Ag, a great potential difference can exist between cathode field and anode field. Therefore, a local corrosion-current can flow from the cathode to the anode. This leads to a net flux of electrons passing through the silver particles.Furthermore, the electron transfer of Ag particles is coupled with proton translocation in the fluid that surrounds the particles. The dependence of the metal-particle movement on the crystal orientation can be attributed to different removal rates for the Si atoms on the surface of different crystal orientations. The presence of Ag particles on the Si surface leads to the formation of early shallow pits and (ultimately) long holes in Si. Finally, the characteristics of the metal/Si interface can affect movement of the corresponding metal-particles in the substrate. Most metals interact strongly with silicon and form silicides at the metal-Si interface. Ag is an exception, it does not form silicides at the interface with silicon and maintains atomically sharp. There are also compounds similar to silicides at Si-Cu interface, and its bonding characteristics are close to those of near noble-metal-silicides. In reactive metal/Si systems such as Ag/Si with atomically sharp interface will help to promote uniform and straight movement of metal particles in bulk silicon. While active metal/Si systems such as Pt/Si with an alloy interface will make the motion of metal particles unstable in Si and lead to a curved trajectories of metal particles in bulk Si, as shown in Figure12 [147].Figure 12
(a) Transverse-sectional SEM view of an etched (100) silicon wafer, showing parallel straight tunnels perpendicular to the silicon surface. Ag particles at the bottom of the channels. (b) SEM figures of the etched silicon substrate precoated with Pt nanoparticles in an aqueous solution of HF/H2O2 [147].
(a)(b)With alkaline etching, highly ordered nanopore or nanowire-arrays can be fabricated by combining masks with MACE. Peng et al. used deep ultraviolet lithography (UVL) to form periodic array masks for circular holes on the surface of monocrystalline silicon (100). The group then evaporated the silver film and removed the photoresist to obtain a periodically arranged disk-pattern silver film. The films were etched in aqueous HF solution containing oxidized H2O2 at room temperature. The depth of the nanopores is determined by the etching time. Moreover, nanoporous silicon solar cells with three-dimensional radial PN junctions were formed using phosphorus diffusion. Under the illumination of 1 sun AM1.5G, the Voc, Jsc, and power conversion efficiency of nanoporous solar cells were 566.6 mV, 32.2 mA/cm2, and 9.51%, respectively. These numbers are much higher than planar Si and even higher than some pyramid-textured and nanowire Si solar cells [170]. In most current studies, MACE is still limited for nanowire arrays with low density, high surface area ratio, and arbitrary spatial pattern. Kang et al. overcame the inherent disadvantage of MACE by using two-step nanosphere lithography. They introduced large and small PS spheres as masks in the etching process. After etching, large-diameter and small-diameter silicon nanowire arrays will appear. After appropriate thermal oxidation and wet etching, small-diameter nanowires can be completely removed, leaving nanowire arrays with low density and high surface area ratio. According to their research, small diameter nanowires are the key to achieve this result. They act as a channel for mass transport of MACE chemicals to the narrow interface between silicon and metal. The density of the final silicon nanowire array is 0.1-1 per square micron adjustable, and the surface area ratio can reach 30 [171]. This research is more helpful to fabricate silicon nanowire arrays with specific patterns, so that the patterns of actually manufactured nanowire arrays can be more consistent with the simulation design.Thiyagu et al. spin coated poly (styrene-b-methacrylate) (PS-b-PMMA) blocked copolymer solution on the surface of silicon substrate, annealed, soaked in acetic acid, and then washed with deionized water. PMMA was removed to obtain polystyrene spheres template. Subsequently, the silver film was deposited using thermal evaporation, and then, MACE started in the HF/H2O2 solution. The required length of silicon nanowires was obtained by controlling the etching time. Finally, polystyrene spheres and silver films were removed to obtain silicon nanowires with uniform diameter and height. The reflectivity of these nanowire arrays was close to 0.1% for a wavelength range of 300-800 nm. This method does not require the photolithography step, which can further reduce the cost of preparing uniform nanowire arrays via MACE [172].


#### 3.1.3. Reactive Ion Etching

Reactive ion etching (RIE) is the most widely used and mature dry-etching technology. Dry etching usually combines predefined templates to control the nanostructures accurately. As a result of the high cost associated with photolithography, etching masks, which were fabricated using silica spheres and self-assembly technology, have been studied widely. This method was first proposed by Deckman and Dunsmuir in 1982, when it was referred to as “natural lithography” [173]. The advantage of this method is that nanospheres can be assembled into tightly-aligned arrays. The lateral periodicity of the mask is controlled by the particle size. Another benefit is that standard etching processes such a KOH etching can be used. There is no need to modify the industrial etching program to benefit from automatic masking. Under certain etching conditions, nanowires, nanopillars, or nanocone arrays can be formed [174, 175].A typical process is shown in Figure13(a). The colloidal silica crystal with a single layer and not close-packed particles was spin-coated on a silicon wafer. The polymer was etched with O2 ions. Then, the not close-packed silica particles are used as the etching mask in the SF6 RIE process to form a nipple-pattern array directly on the wafer surface [176]. Similarly, the Langmuir Blodgett method was used to make a mask with a diameter of 500 nm on the silicon substrate surface. First, using the RIE process, the silica spheres were etched with oxygen and a CHF3 gas mixture. The diameter of the spheres was reduced to 200 nm. Then, chlorine (Cl2) and hydrogen bromide (HBr) were used as etching gases. Due to the selectivity, Si nanocones, with a height of 400 nm and a diameter of 450 nm, formed [177]. In addition, RIE can be easily combined with wet etching. Li et al. first used a single-layer not closed-packed PS microsphere as a mask to form a silver-film grid with hexagonal-hole arrays. Then, the silver film was used as a catalyst to form silicon nanopillar arrays (via MACE) in HF and H2O2 solutions. The PS microspheres were burned, and the silver membrane was dissolved in aqua regia [178]. The hollow tip array (with excellent antireflection and waterproofness) was fabricated using short-time RIE. The reflectivity of the structure was below 1% for 250-1600 nm. Garnett and Yang demonstrated a simple and scalable method to fabricate large area silicon nanowire radial p-n junction solar cells. By measuring the optical transmission and photocurrent of the films, it was shown that the ordered silicon nanowire arrays increased the path length for incident solar radiation by 73 times. The length-enhancement factor for the special light-capture path is higher than for the random scattering limit (2n225 without back reflector). The preparation process consisting of four simple steps is shown in Figure 13(b): Synthesis of silica beads, dip coating on the surface of monocrystalline silicon wafers to form self-assembled monolayer silica beads, deep RIE to form nanowire arrays, and diffusion-forming of a radial p-n junction. The distance and diameter of the silicon nanowires were controlled via diameter of the silica beads, while the length was determined by the reaction time. Figure 13(c) shows that large-area silicon nanowires have a periodic arrangement, a smooth surface, and good uniformity (10 cm2) [179].Figure 13
(a) Process flow diagram for the fabrication of moth-eye antireflective nanostructures on Si wafer [176]. (b) Schematic diagram for fabrication of a periodic Si nanowire array. There are three main steps in the preparation process illustrated above: a layer of silica sphere was coated on the surface of silicon wafer; silicon nanowires were obtained by DRIE etching silicon wafer and silica sphere as mask; removal of silica spheres by HF and boron doped silicon nanowires to form the radial p-n junction. (c) Si nanowires fabrication and structure morphology, SEM view of a close-packed monolayer of silica spheres is in the top left corner, SEM top view of a completed periodic silicon nanowire array is in the top right corner, tilted cross-sectional SEM of the silicon nanowire arrays is in the bottom left corner, and tilted optical image of silicon nanowire arrays is in the lower right section of image [179].
(a)(b)(c)By changing the RIE conditions, both nanowire- and nanocone-structures can be obtained. According to the theoretical analysis in Section2.3, the nanocone structure provides almost perfect impedance matching between Si and air by gradually reducing the effective refractive index. This enables better antireflection than the nanowire structure. As shown in Figure 14, for the wavelength range of 400-650 nm, the absorption of the nanocone array exceeded 93%, which is much better than the nanowire array (75%) and the thin-film (64%). Furthermore, the sensitivity to the incident light angle is also much lower than that for the nanowire array and the antireflection coating [180]. The key to produce a nanocone structure is to combine isotropic RIE with anisotropic RIE. First, Si nanorods were formed using anisotropic RIE based on Cl2. Second, C2ClF5/SF6 was used to further etch, isotropically, the preformed nanocolumns to create undercut and sharpened nanopillars. The formation mechanism of the tip can be described as follows: first, Cl and Br radicals reach the silicon surface from all directions during the RIE process, and they induce isotropic etching or undercutting of Si in the assumed anisotropic step. Second, the etching selectivity of Si to SiO2 is about 26. Therefore, when the SiO2 mask is used, the degree of lateral etching can increase due to mask erosion. Third, when the etching products are deposited, the deposition rate from the bottom to the top of the column decreases, and a conical sidewall will appear [181].Figure 14
(a) Absorptivity of planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm was measured at normal incidence. (b) Simulation results of absorption on planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm by normal incidence. (c) and (d) are the experimental and calculated results of the absorptivity in the range of incident angle 0-100° respectively. The incident light wavelength is 488 nm [180].
(a)(b)(c)(d)To improve the side-wall-profile controllability of the nanostructures in the RIE process, Zhao et al. proposed an innovative strategy to create silicon nanostructures. The group used fluorine-enhanced oxidation to achieve segment by segment wall-control. This method is based on fluoride RIE. Abundant fluorine species accumulate on the surface greatly enhancing the spontaneous growth of a passive oxide layer on the side wall, thus effectively controlling the profile. In the subsequent etching process, the oxide layer is selectively removed to form a new sidewall section with the required geometry. In this way, three different nanostructures are eventually achieved, including upstanding nanopillars with vertical sidewalls, narrowed necks, and suspending cantilever beams with controllable thickness [182].The RIE process is a combination of the chemical activity of active substances produced in plasma and physical effects caused by ion bombardment. Therefore, by changing the type and flow rate of the etching gas, working pressure, RF power, etching time, and other key parameters, a random texture with good antireflection can be fabricated on the surface of monocrystalline silicon or polycrystalline silicon [183]. Moreno et al. found that RF power has a crucial effect on the c-Si texture during the process of forming normal and inverted pyramids using the optimized gas ratio. At low RF power, pyramid-like texture is formed, and the texture is controlled via the anisotropic process. On the other hand, at high RF power, the inverted pyramid structure is obtained, and the texture is mainly controlled by the isotropic ion-assisted process. The upright pyramid structure on the c-Si surface results in an average reflectivity of about 18%. In contrast, the inverted pyramid structure results in an average reflectivity of as low as 6% without an antireflective coating [184].These results represent potential alternatives to low-cost manufacturing of c-Si solar cells because the process is completely dry. No deionized water, wet chemicals, or photolithography are required. Yoo fabricated a crater-like pyramid structure on c-Si surface using RIE without a mask. According to the extensive research of the RIE parameters, the optimized process conditions require a gas ratio SF6/O2 of 1.5, an operating pressure of 182 mTorr and power of 100 W. Compared with conventional saw damage removal (SDR) and alkaline solution texturing, the silicon loss of wafers that were cleaned by acid or alkaline solution followed by RIE texturing is reduced significantly. The crystalline silicon solar cells, which were fabricated by acid etching, were textured using RIE. The conversion efficiency was 16.7%, and the silicon loss was 67% lower than for conventional silicon solar cells [185].As an advanced nanomanufacturing technology, RIE has shown many advantages for the preparation of nanostructures on the surface of monocrystalline silicon solar cells. However, early RIE usually had a disadvantage that the nanostructured surface is relatively rough, which greatly increases the surface area [186]. An increased number of surface defects and dangling bonds can seriously affect the final PCE of solar cells. With the in-depth study of RIE technology, there are many methods to basically solve this problem. Schaefer and Lüdemann found that appropriate RIE etching parameters can greatly reduce the surface damage of silicon during etching. Then, through simple annealing, the carrier lifetime of RIE etched silicon can be restored to the level of the wet etched reference sample, and a high etching rate can be obtained. Their main conclusions are (1) RF power and DC bias should be as low as possible, (2) the etching time should be kept within a few minutes, but it should not be too short, and (3) pure SF6 should be used as the etching gas as far as possible, and oxygen shall not be introduced [187]. In addition, some studies showed that fluoride-enhanced oxidative passivation can improve the surface-profile control of RIE nanostructures and complete the surface passivation simultaneously [182]. Alternatively, one could use Cl2 to make the nanostructured surface smoother [188]. These works provide an important reference for reducing or eliminating the surface damage introduced in the process of etching crystalline silicon by RIE technology and help RIE show more advantages in the surface texturing of crystalline silicon solar cells.


#### 3.1.4. Other Dry Etching Methods

Other dry etching methods, including femtosecond-laser etching, focused ion beam etching (FIB), and mechanical etching are not widely used for monocrystalline silicon solar cells. However, they still need to be considered/studied because of their unique advantages.Mechanical texturing usually uses motorized diamond tips to treat the monocrystalline silicon surface, which can form grooves that are tens of microns deep. After being modified by alkaline or acidic solutions, this simple and low-cost method can also greatly reduce the total reflectivity in the wavelength range 300-1200 nm. Ultrasonic nanocrystal surface modification (UNSM) is another method for mechanical texturing. Micro/nanopits are produced on the monocrystalline silicon surface using ultrasound. The increased surface roughness not only reduces reflectivity but also improves wear resistance. Although mechanical texturing is simple and cheap, the size, morphology, and pattern of nanostructures, which are obtained using this method, are greatly limited. Light trapping is not as good as that obtained using other etching methods. Furthermore, the preparation efficiency is low, and it is difficult to use for large area fabrication [189–191].In 1997, Mazur et al. discovered for the first time that, in an environment filled with SF6 or Cl2, a series of high current, femtosecond-laser pulses irradiated the surface of monocrystalline silicon <100> to form a sharp conical array of micrometer size. However, in vacuum, N2, or He, sharp tips were not formed. This indicates that the chemical reaction between SF6 or Cl2 and silicon contributed to the formation of the tips [192]. Further study revealed that the absorption of the sharp conical array, which was fabricated in SF6 atmosphere, was 90% for a wavelength range of 1.2-2.5 μm. This is due to the light-trapping effect produced by the conical spike array. Nevertheless, when a femtosecond-laser is used to ablate a silicon wafer in SF6 atmosphere, SF6 can be decomposed and mixed into the silicon surface using laser ablation. The impurity energy level can reduce the bandgap width of silicon and lead to strong subbandgap infrared absorption. The results show that the addition of 0.6% sulfur to the microstructure sample is enough to produce a wide absorption band [193–195].There are a few advantages to fabricating nanostructures via femtosecond-laser etching. Doping and structure-production can be carried out in a single process step without a predeposited layer-doping source. In addition, the morphology and optical absorption properties of the laser-treated Si surface can be easily tuned from UV light to infrared (≈2.5μm). Both properties depend on the number of laser pulses, irradiation intensity, atmosphere, and pulse duration. Another advantage is that laser etching is independent of the crystal structure. Femtosecond-laser etching, in particular, is suitable for polysilicon [196, 197]. Combined with anisotropic alkaline etching, large-area ordered silicon nanopillar arrays can be fabricated without photolithography [198]. However, femtosecond-laser etching has many inherent limits. High price of femtosecond-laser devices can greatly increase the preparation cost for the nanostructures. After femtosecond-laser ablation, the surface roughness of the nanostructures is large, which results in many defects and dangling bonds. If not passivated, a carrier lifetime that is too short effective can greatly reduce the efficiency of solar cells. Moreover, femtosecond-laser etching cannot achieve accurate control of nanostructure and morphology. These shortcomings make it difficult to use for the fabrication of surface microstructures of monocrystalline silicon solar cells on a large scale.The FIB system can focus a high-intensity ion beam via an electrostatic lens. Liquid metal ion sources (LMIs) are used in commercial systems, and gallium (Ga) is mostly used because of its low melting point, low vapor pressure, and good oxidation resistance. When the high-energy focused ion beam bombards the sample, its kinetic energy can be transferred to the atoms and molecules inside, which produces a sputtering effect that enables continuous etching, i.e., cutting of the sample. Its cutting-positioning accuracy can reach 5 nm. Furthermore, it can be used to prepare nanostructures on various materials including metals [199, 200]. Recently, Garg et al. fabricated Gaussian subwavelength 3D nanostructures using one-step rapid prototyping on a silicon surface via FIB milling. Highly ordered Gaussian columnar subwavelength nanostructures were fabricated on a Si surface using accelerated ions of the focused ion beam system. The height and period intervals of the Gaussian column were 250 nm and 500 nm, respectively. The fabricated columns could be tuned via the FIB dwell time. The diameter and periodicity of these columns are controlled using the beam current and step size, respectively. This provides additional degrees of freedom for the required functions. The reflectance spectrum of the optimized Gaussian column shows that the maximum reflectivity was 10-20% for the wide wavelength range of 400-1000 nm, which has clear improvements in light-capture [201].FIB etching has clear advantages for the preparation of nanostructures. For example, most of the surface texturing processes mentioned above either involve complex multiple steps and require posttreatment of contamination or they cause problems with inhomogeneity and uncontrollability. The combination of inaccuracy and contamination causes poor optical performance. Furthermore, FIB provides a simpler method and design for controlling the 3D surface texture. FIB etching can be done by rapid prototyping in one step and patterned without mask [202, 203]. This makes the preparation process simple and maintains the optical properties of the nanostructures. However, before FIB etching can be used for nanostructure fabrication on the surface of large-scale monocrystalline silicon solar cells, problems such as FIB-induced damage, ion contamination, redeposition, and the beam-tail effect need to be resolved [202, 204, 205].


### 3.2. Bottom-Up Method

The bottom-up method mainly involves chemical reaction growth or the deposition to introduce new materials and produce the required nanostructures on the surface of monocrystalline silicon. The bottom-up approach has more advantages in terms of cost and material selection than the top-down method, which requires removal of excess material. Main bottom-up methods include chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and molecular beam epitaxy (MBE) based on the growth mechanism of vapor-liquid-solid (VLS) [206–208]. This mechanism was first proposed by Wanger and Eills at Bell Laboratories in 1964 [209]. Among these methods, CVD is most commonly used to grow high-quality silicon nanowires. Large area Si nanowire arrays can be grown on commonly used (e.g., glass, Si) or reusable substrates. In addition, the different operating modes of a CVD system can greatly affect the VLS growth of silicon nanowires.Sanchar et al. studied the effects of two operating modes of a cold wall catalytic CVD system on the growth of VLS silicon nanowires: hot filament chemical vapor deposition (HWCVD) and chemical vapor deposition (CVD). They found that the HWCVD mode has a higher growth rate. However, it also has disadvantages such as small length, tapered nanowires, and a large amount of uncatalyzed silicon deposition on the substrate. This is because using the HWCVD mode, the precursor gas can decompose directly after the tungsten wire is heated. Heating furthermore, there is no “catalytic effect” on the surface of the silicon substrate. Any anisotropic growth is due to the fact that the solubility and adhesion coefficient of Si atoms in liquid Au/Si eutectic droplets are much higher than on solid Si substrates. The heating effects of both the filament and the substrate contribute to a much higher temperature in the spherical chamber than the minimum temperature required for the catalytic decomposition of SiH4. This makes the growth rate of silicon nanowires extremely high. Another important factor for the appearance of a conical structure is the high diffusion rate of Au/Si eutectic droplets along the side wall of the grown nanowires at high temperature. This is why the catalyst of the nanowires is lost soon after the start of VLS growth, and the tip appears at the end of the VLS growth. In the CVD mode with optimized conditions, the nanowires are not conical, and the length can reach several microns. No uncatalyzed silicon deposition was observed on the substrate. Therefore, the CVD mode is more suitable for the VLS growth of silicon nanowires [210].A typical process of CVD VLS growth is shown in Figure15. First, Au is deposited on the surface of silicon substrate via magnetron sputtering, electron beam evaporation, or thermal evaporation. Then, the diameter and density of the nanowires are determined by the size and density of the catalytic metal particles. The melting point of Au-Si alloy is lower than that of Au or Si, so Au-Si alloy (4:1) is liquid at temperatures exceeding 363°C. The mixed gas of SiCl4 and H2 is adsorbed on the surface of this liquid, and the mixture reacts during the catalysis of the metal. The reaction formula is as follows:
(10)SiCl4+2H2⟶Si+4HCl.Figure 15
(a) The growth process of silicon nanowires is illustrated: (a) gold thin film is deposited on the silicon substrate by PVD; (b) Au-Si droplets are formed by thermal annealing; and (c) SiCl4 reacts with H2 and grows Si nanowires from the substrate under the catalysis reaction of gold [211].
(a)(b)(c)Generally, the reaction occurs above 800°C. When the temperature exceeds 363°C, the liquid Au-Si alloy can absorb the Si formed in the reaction until the Si in Au is supersaturated. The melting point of silicon is 1414°C, which is much higher than that of the Au-Si alloy. Therefore, Si atoms can precipitate from the supersaturated liquid alloy at the interface of Au-Si liquid alloy/Si substrate. Furthermore, one-dimensional silicon nanowires can grow from liquid metal alloy droplets[211–213]. Similar methods are vapor-solid-solid (VSS), solid-liquid-solid (SLS), supercritical fluid-liquid-solid (FLS), and supercritical fluid-solid-solid (FSS) [214–218]. In the CVD system, the diameter, length, and morphology of nanowires grown using VLS are mainly affected by the metal catalysts. Temperature and pressure are also important conditions that determine the geometric characteristics of nanowires [219, 220].The diffusion properties of Au fundamentally limit length, shape, and the side-wall morphology of the Si nanowires. There is a key difference between low pressure CVD in ultrahigh vacuum on a clean substrate and conventional CVD, when it comes to the prevention of Au migration under high pressure. An inevitable difference is cleanliness, which may be due to the reduction of Au migration by adsorbing contaminants from the precursor gas. These results indicate that the size of the catalyst droplets changes during nanowire growth at 600°C. During growth, Au preferentially migrates, from smaller catalyst droplets to larger ones, to reduce the total droplet-surface-area. In this process, some droplets coarsen completely, forming shorter tapered nanowires. In the process of VLS growth, the side walls of the nanowires are relatively smooth, when catalyst droplets are present. This indicates that the growth rate of VLS is faster than that of CVD for the side-walls covered with Au. Once the droplets are consumed, the growth of VLS stops. However, direct CVD growth on the side-wall of Si nanowires continues, greatly changing the side-wall morphology [221].In the field of nanoelectronics, platinum nanoparticles provide a feasible alternative to conventional gold catalysts to control the single-crystal size of SiNWs, which were synthesized using epitaxial vapor phase CVD. Due to the middle gap trap state of Au in silicon, it can “poison” the performance of devices. Therefore, it is generally not allowed in electronic manufacturing laboratories and clean rooms. The average diameter of undoped wires, grown from 9.3 nm Pt nanoparticles, is 11.3 nm, while that of boron doped wires is larger (17.9 nm). SiNWs are mainly grown in the (110) direction, which results in the partially vertical alignment of SiNWs on the monocrystalline Si <110> wafer. Despite the strong surface effect due to the small diameter, the electric properties of boron-doped wires are similar to those of other nonfunctionalized SiNWs. The optimal mobility and switching ratio are 28 cm2V−1s-1 and 7400, respectively. In addition to reducing gold-related microelectronic processing problems, the Pt catalyst also allows 9 nm particles to be used in high-temperature SiCl4-CVD reactors without significant aggregation. This enables small-diameter wires and simple integration [223]. The density of silicon nanowires, which are grown using VLS, has an important effect on the performance of radial junction solar cells. A suitable growth density can produce silicon nanowires with a more-uniform diameter. This helps form a uniform and high-quality radial junction around the SiNWs. The radial PN solar cell with optimized nanowire-density reached an open circuit voltage of 0.80 V, a short circuit current density of 16.1 mA/cm2, and a conversion efficiency of 8.14% [206].To ensure good light-trapping of the nanowire array, the silicon nanowire must be perpendicular to the substrate surface as much as possible. This is because Si nanowires are epitaxially grown from binary droplet to substrate, i.e., Si <111> wafer. In addition, only by keeping the substrate surface clean and preventing oxide or other impurities from interfering the VLS growth process can vertically-grown Si nanowires be obtained. However, gaseous HCl is a by-product of the SiCl4 decomposition in reaction tubes. It corrodes the oxide layer on the surface and presents a clean Si-crystal surface, which makes Si continuously precipitated from binary droplets. At this interface, the epitaxial deposition of Si induces growth of nanowires on the crystal face of the Si wafer. Therefore, substrate treatment and precursor selection are essential to keep the substrate vertically aligned during growth [220, 224]. If other CVD precursors, such as SiH4, are used without adding HCl or taking special preventive measures to remove the oxide layer, it is impossible to realize large-scale vertical growth of nanowire arrays [219, 225]. As shown in Figure 16, photolithography is still the best way to precisely control the growth position and diameter of nanowires in VLS growth to form various periodic arrays of nanowires, although it can increase the preparation cost [222, 226].Figure 16
The arrangement pattern and optical properties of each nanowire array. (a) SEM top view of nanowire arrays. (a) SEM images viewed at 20° angle. (c) Transmitted diffraction patterns of polymer-embedded nanowire arrays on a quartz substrate, the incident wavelength is 488 nm. (d) Transmission intensity of each nanowire array as a function of the incident light angle (θx, θy) [222].According to the theoretical analysis of Section2.3, the antireflection properties of nanowire arrays are affected by the nanowire array arrangement. Compared with quasiperiodic or random arrays, the aligned lines, which are produced by periodic arrays, lead to a larger-area stacking fraction (thus improving the overall optical absorptivity). However, the periodic arrangement shows a strong anisotropic angular absorption-profile, which can produce a low absorption “dead point” in photovoltaic devices [222]. Using photolithography, Kayes has grown periodically vertically-aligned silicon nanowire arrays with a diameter of 1.5 μm and a length of 75 μm in the region>1cm2. They first thermally grew 300 nm oxides on the surface of Si (111) and then fabricated periodic patterns using photolithography. After development, the surface of silicon dioxide was exposed, and the sample subsequently was immersed in HF solution to remove the SiO2 layer to expose the silicon surface. Thermal evaporation of 500 nm gold or copper on the surface of silicon wafer was used as a catalyst for VLS growth, and the remaining photoresist was removed by lift off. Next, VLS was grown at the optimal temperature and precursor gas ratio. The process is the same as the typical VLS growth process described above. This method produces an array with almost no defects, which suggest an extremely narrow diameter, low length distributions, and highly controllable wire positions. The key to realize a large area, highly uniform, and periodic array of silicon nanowires lies in the application of a silica buffer layer. The group also carried out experiments without the use of a silica buffer layer. As a result of the migration effect of the gold catalyst mentioned above during the annealing process, a large amount of agglomeration of the catalyst occurred without the restriction of the silica buffer layer, which seriously worsened the fidelity of the periodic pattern. Therefore, to obtain periodic aligned and homogeneous silicon nanowire arrays via VLS, it is necessary to limit the catalyst to the required position using a buffer layer to prevent the migration of Au and other metal catalysts during annealing and growth [224].In addition to photolithography combined with a silica buffer layer, the method of using self-organizing anodic alumina film (AAM) as a template to grow ordered monocrystalline nanopillar arrays has great potential. Furthermore, AAM templates can control both the morphology and size of the nanopillars and even form double-diameter nanostructures, which can further improve light-capture performance [227].


## 3.1. Top-Down Method

### 3.1.1. Alkaline Etching

Due to the anisotropy of monocrystalline silicon in alkaline etching solutions, a method to prepare pyramid- or inverted-pyramid structures on its surface, using NaOH, KOH, and other corrosive solutions, was developed [112]. Today, this method is widely used for commercial silicon solar cells. The homogeneity of the pyramid nanostructure can be effectively improved by optimizing the corrosion time with a specific concentration of alkaline corrosive solution [113, 114]. The etching rate of silicon in solution and the anisotropy factor (quotient of the etching rate in the <100> to <111> directions) affect both the density and size of the pyramid in the <100> direction. Seidel found that the maximum etching rate of 0.9 μm/min can be achieved at 72°C for KOH solution with concentration of 15% [115]. However, the concentration of KOH less than 20% usually leads to the increased surface roughness and the formation of insoluble precipitates [116]. Higher concentration of KOH solution and higher temperature can increase the etching rate and make the etching surface smoother. For example, some studies have confirmed that the etching depth per minute of a 50% KOH solution at 80°C increased by 0.5 μm in the direction (100) [117]. The high concentration NaOH solution has a high etching rate. The work of Green et al. shows that the etching rate of NaOH solution with a concentration of 20-30% can reach 4-6 μm/min at 90°C [118, 119]. However this rate is too high for pyramid texturing of silicon. When texturing the surface of monocrystalline silicon with strong alkaline corrosive solutions such as NaOH and KOH, the addition of can effectively improve the uniformity of texture [120, 121].Based on a kinetics study of anisotropic etching, pyramid growth begins at the preferred position caused by crystal defects or wetting. At the beginning of the etching process, the inhomogeneity of the pyramid nucleation can be avoided by mixing additives into the etching solution. Vazsonyi et al. found the best texture condition at 80°C,where a relatively high etching rate (0.6μm/min) can be reached in the <100> direction. The anisotropy factor was 10. The pyramid size can be adjusted by changing the etching time to obtain low reflectivity from the textured silicon wafer [122]. Regular and well distributed pyramids can be obtained by etching in NaOH and isopropanol solution for 25 to 45 minutes. Reflectance ranges from 450 nm to 1000 nm between 11% and 14% [114]. However, IPA has also some disadvantages: high cost, volatile when heated, and low corrosion-rate at high concentrations. These disadvantages can be overcome by using metal grids with periodic openings, which improves the pyramid-texture uniformity. Chu et al. showed that when the metal mesh is placed near the wafer surface, hydrogen bubbles, which are produced by the KOH Si reaction, are limited by opening of the mesh because of the surface tension of the bubbles. Because hydrogen bubbles adhere to the wafer surface and inhibit the chemical reaction, they can act as a “pseudo mask.” Accordingly, both the position and size of the bubbles are relatively stable given the limitation to metal grids. Stable and uniform pyramid structures can be obtained without adding IPA [120].To avoid the inherent shortcomings of KOH and NaOH solutions during the texturing process of monocrystalline silicon, the application of other alkaline solutions such as tribasic sodium phosphate (Na3PO4·12H2O), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), and sodium acetate (CH3COONa) were studied [123–125]. These solutions show the benefits of lower cost and reduced environmental pollution. On further research, the tetramethyl ammonium hydroxide (TMAH) solution was found to show the best overall effect. It overcomes the problem of mobile ion pollution, which is caused by the KOH and NaOH alkaline etching solutions that are commonly used in IC processes. The advantages of this solution include good controllability and a stable etching rate. Consequently, the texture of the inverted pyramid, which can be etched using the TMAH solution, is smooth but the top of the pyramid is sharp [126]. In addition, the texture, which was obtained using TMAH, can be passivated more easily than with KOH [127]. TMAH solution can be reused many times, which can reduce the cost of the pyramid texturing process [128]. Choi et al. studied the effect of the etchant concentration, ambient temperature, and silicon wafer thermal history on the formation of the pyramid in TMAH. The number and size of the pyramids were used as parameters in the study. Their results show that pH theory can well explain the effects of TMAH concentration and ambient temperature on silicon surface changes. However, the results of rapid thermal annealing, Czochralski wafers, and floating zone wafers seem to favor the defect theory [129]. To better explain the mechanism of silicon corrosion by TMAH, it is necessary to combine pH with the defect theory. When using the KOH and TMAH solution to texture the surface of monocrystalline silicon, the addition of IPA can alter the corrosion rate and roughness of the silicon surface <100>. When the IPA is adsorbed on the etched surface, it can hinder the reaction between Si and OH− and reduce the etching rate of the specific crystal face. Then, the equilibrium between oxidation and diffusion of the reaction products establishes the necessary conditions to generate smooth surfaces. The reduced reaction rate, which is caused by IPA adsorption, enables uniform surface etching. During the etching process, the IPA adsorption can be isotropic or anisotropic—depending on the composition of the solution. When TMAH solution (2-2.5 m) is mixed with IPA (10-30%), the decreased rates for (100) and (hkl) crystal planes are almost the same—independent of the crystal orientation. The anisotropic adsorption of IPA for different crystal orientations can be observed by mixing medium concentrated KOH solution (3-7 m) with low concentrated IPA (few to several percent). When the highly concentrated KOH solution was mixed with the extremely low concentrated IPA (2%), the IPA almost had no effect, and the etching rates for the different crystal orientations were almost the same [130].The electrochemical corrosion mechanism for monocrystalline silicon in alkaline solution was also studied. According to the model proposed by Seide et al., the total reaction process is summarized as the following equation:(6)Si+2OH−+2H2O⟶SiO2OH2−+2H2↑.In the oxidation reaction stage, due to the high pH environment, the surface atoms of bulk silicon react with fourOH− to produce SiO2OH2−.At the same time, four electrons are injected into the conduction band of silicon, two of them are transferred through the surface state of dangling bond, and the remaining two electrons are transferred through the surface state of back bond. These four electrons will be bound near the silicon surface due to the potential well generated by the downward bending energy band. In this space charge region, the recombination probability of injected electrons is very small. In the reduction process, implanted electrons will leave the solid and react with water molecules to produceOH− and H2. These OH− are generated directly on the Si surface and participate in the oxidation reaction. The orbital electrons in their respective surface states are thermally excited into the conduction band, resulting in the fracture of the back bond, which is considered to be the rate limit. Different crystallographic back bond surface states have different energy levels, which eventually leads to the anisotropy of etching, and the energy level of (111) plane is the lowest [115].According to the theoretical analysis in Section2.1, pyramid array textures with specific periodic arrangements show a better light-trapping effect than random-pyramid arrays. Therefore, it is important to study the preparation of periodic nanostructures. In the aforementioned method of texturing a monocrystalline silicon surface with anisotropic alkaline solution, the etching process is difficult to control because there are no masks. Only random pyramid textures can be obtained. A mask with periodic patterns can precisely control the etching position and produce the desired periodic nanostructures. The most commonly used mask is a photoresist with periodic patterns that were formed after the photolithography. The periodic inverted-pyramid pattern, shown in Figure 7, is formed by interference lithography and subsequent etching in KOH solution. The periodicity of the inverted pyramid array is 700 nm, and the average separation is 100 nm [131]. Patterned SiO2 can also be used as a mask for anisotropic alkaline etching. Using photolithography and dry etching methods, a silica rectangular window can be formed to expose silicon under the SiO2 layer. The periodically arranged inverted pyramidal array is obtained by etching in TMAH solution [132]. Sivasubramaniam and Alkaisi employed interference lithography combined with dry etching and KOH wet etching to fabricate inverted pyramid nanoarrays with a characteristic size of 700 nm on the surface of monocrystalline silicon. The vertical incident reflectivity in the whole visible light range was suppressed to less than 10%, and the overall efficiency of solar cells was improved by 67% [133]. Han et al. adopted almost the same method to fabricate symmetry breaking inverted nanopyramid arrays on the surface of monocrystalline silicon thin films. Their experimental and calculation results show that symmetry breaking can improve the photovoltaic efficiency of thin film silicon solar cells. For the active layer with a thickness of 10μm, the photovoltaic efficiency of the traditional symmetrical inverted nanopyramid array is 27%, and the introduction of a symmetry breaking ring improves the photovoltaic efficiency to 27.9% [134].Figure 7
(a) Photograph, (b) SEM vertical view, and (c) SEM cross-sectional view images of the 10μm c-Si film illustrating periodical reverted nanopyramids with a period of 700 nm. A SiNx layer with a thickness of 90 nm deposited on the surface of nanopyramids, and a SiO2 (1 μm) and a silver (200 nm) layer are at the reverse side of the c-Si film. The right bottom insets are enlarged drawings of the blue and red boxed areas. Scale bars:1 μm for (b) and (c) and 200 nm for insets in (c) [131].
(a)(b)(c)Periodic arrays of upright pyramids can be fabricated using a similar method. As shown in Figure8, photoresist is spin-coated on the surface of SiO2. Two-dimensional periodic photoresist masks can be obtained using laser interference lithography. RIE was used to transfer photoresist patterns to the SiO2 layer. Hydrofluoric acid (HF) is used to further reduce the size of the SiO2 mask. After anisotropic alkaline etching in a KOH solution, the periodic array of positive pyramids is obtained. The results show that the reflectivity of the front surface of the array is lower than that of the conventional micrometer pyramid texture. The excellent antireflection performance facilitates an absorption photocurrent density for a 40μm silicon absorption layer reaching 40.8 mA/cm2. This value is 0.7 mA/cm2 higher than for the periodic inverted-pyramid array, 0.8 mA/cm2 higher than the randomly arranged inverted-pyramid array, and 1 mA/cm2 higher than the randomly arranged vertical pyramid. The reason for the reduction in reflectivity is provided in the theoretical analysis in Sections 2.2 and 2.3 [135].Figure 8
(a) A schematic outline of the periodic upright nanopyramid preparation workflow based on laser interference lithography and anisotropic etching. (b) Corresponding 3D schematic diagram. (c) SEM images ofSiO2 template [135].
(a)(b)(c)The mask selection is mainly determined by the fabrication process. Furthermore, whether the photoresist with the periodic pattern is used as a mask orSiO2 with a periodic pattern (via photolithography) is used, which is determined by the photolithography step. It inevitably increases the cost of texturing the monocrystalline silicon surface. Nanoimprinting has lower cost than lithography, so it can be a good substitute. Gaucher et al. chose soft UV nanoimprint lithography to manufacture Cr etching mask on the surface of monocrystalline silicon film. After KOH solution etching, a nanoinverted pyramid array was formed. Finally, the short-circuit current density of thin-film crystalline silicon solar cells with a thickness of only 2.75 μm reached 25.3 mA/cm2[137]. Sun et al. demonstrated a simpler and cheaper nonphotolithographic method to fabricate wafer-level subwavelength antireflection gratings on monocrystalline silicon as shown in Figure 9. First, a monolayer of colloidal silica particles was spin-coated on the surface of <100> silicon for UV curing. Then, RIE was used to remove any SiO2 particles left behind by the colloid mask. Subsequently, electron-beam evaporation was used to deposit a 30 nm thick Cr layer. SiO2 particles were then removed to form a periodic nanopore array, whose diameter was determined by the size of the silica sphere template. These circular nanoholes were used as masks for the anisotropic KOH etching process, and a wafer-level inverted pyramid array was formed with a staggered layout in silicon [136].Figure 9
Fabrication of inverted nanopyramid arrays using 320 nm diameter silica spheres as templates. (a) 300 nm size nanopyramids and (b) 360 nm size nanopyramids both are etched at 60°C, etching times are 90 s and 210 s, respectively. (c) Schematic flow of the templating procedures for fabricating subwavelength-scale antireflection nanopyramid structures on monocrystalline silicon [136].
(a)(b)(c)This method greatly reduces the cost and allows for good controllability because it does not require lithography steps. The morphology of the final inverted pyramid can be controlled by the size of theSiO2 particles, the etching time, and the etching temperature. The reflection of the 360 nm inverted-pyramid array is reduced to 10% for a long wavelength (600 nm). When the wavelength is about 400 nm, the reflection is further reduced to 2%, which indicates good antireflection characteristics. Similarly, Zhou et al. used hexagonally close-packed polystyrene sphere (PS) as mask, ion etching was used to control the diameter of PS, and then, magnetron sputtered 50 nm Ti film. After PS removal, the front and rear surfaces of crystalline silicon films were etched with a NaOH and IPA mixed solution. The periodic inverted pyramid arrays of different sizes were obtained by controlling etching time. When the period of the inverted pyramid array is 1400 nm, the best reflection reduction effect is achieved. The current density of solar cells can reach 39.86 mA/cm2, and the contrast plane (22.63 mA/cm2) is increased by 70% [138].


### 3.1.2. Metal-Assisted Chemical Etching (MACE)

MACE uses metal catalysis so that both monocrystalline silicon and polycrystalline silicon can be anisotropically etched in acid solution [139–141]. Unlike the anisotropic etching solution, which can reduce cost [142]. In the process of preparing porous silicon from HF, HNO3, and H2O mixed solution, Dimova-Malinovska et al. found that the Al film, which was deposited on silicon surface, had a catalytic effect [143]. Subsequently, Li and Bohn studied the preparation of porous silicon in HF/H2O2 solution with noble metals (Au, Pt, and Au/Pt alloys) as catalysts. The group proposed the MACE mechanism for the first time [144]. As shown in Figure 10, a key feature of this reaction scheme is the formation of holes from H2O2 and the reduction of H+ to H2, both of which are facilitated by metal particles. The MACE of Si can be considered to be a local electrochemical process. The noble metal, which is deposited on the surface of Si sheet, is used as a local cathode catalytic oxidant to generate holes. It is then injected into the valence band of Si to oxidize and form silicon-ions dissolved in HF solution. The porous silicon, which was fabricated using this method, has a reflectivity of 2% in the range of 200-1000 nm [145]. Au nanoparticles were used to catalyze subwavelength structure formation after 15 min etching. The reflectivity of the subwavelength structure was below 5% for the range 300 nm to 800 nm [146].Figure 10
Catalytic-induced electrodynamics for autonomic motion of Ag nanoparticle in silicon substrate. (a) A auto genetic electric field caused by a hydrated proton gradient passing through the Ag particles. (b) Self electro-phoresis-driven movement of Ag particles. (c) Collective and extensive tunnelling motility of Ag (or metal) particles in a silicon substrate that forms the nanostructures of silicon [147].
(a)(b)(c)The detailed reaction mechanism for silicon MACE is as follows [143, 144]:Cathodic reaction (metal):(7)H2O2+2H+⟶2H2O+2H+2H++2e−⟶H2↑.Anodic reaction:(8)Si+4H++4HF⟶SiF4+4H+SiF4+2HF⟶H2SiF6.Total reaction:(9)Si+H2O2+HF↔2H2O+H2SiF6+H2↑.Electrochemical etching is a type of conventional wet isotropic acid etching [149–154] in which high aspect-ratio holes can be created on the surface of a silicon substrate. However, the uniformity and layer thickness of the silicon wafer surface texture are difficult to control using this method. Furthermore, the repeatability is poor, and this process is mostly used for surface texturing of polycrystalline silicon. In addition, electrochemical etching needs to connect the silicon wafer to an external power supply, which increases both the process complexity and energy consumption. Some studies showed that isotropic acid etching can achieve better antireflection properties than alkaline etching [155, 156]. However, the obtained rough surface greatly increases the surface carrier recombination and is not conductive to the subsequent surface passivation. When the poor controllability of the process and surface roughness issues are considered, the benefits of electrochemical etching for solar cells are very limited.Compared with the surface texture of polysilicon etched usingHF/HNO3 aqueous solution without metal catalyst, the efficiency of the solar cell with a bowl-like surface texture fabricated with the MACE method and acid postmodification increased by 1.2% (reaching 19.1% efficiency). This is mainly attributed to the fact that the lowest average reflectivity of the bowl-like structure is only 5.3% between 500 and 1100 nm. For comparison, the average reflectivity of a conventional micro texture is 14.5%. In addition, the surface area was reduced after acid modification, and the efficiency of the solar cells improved. The bowl-like textured solar cell shows a good broadband spectral response. This is because it has excellent antireflection properties at short wavelengths and long wavelengths. Moreover, the texture enables special light absorption at wide angles of incidence (AoI) from 0° to 70°, and it shows an all-round characteristics [157]. Recently, Zhang et al. also used CuSO4 etchant to manufacture high-efficiency inverted pyramid texture monocrystalline silicon solar cells. They compared the etching mechanisms of Cu (NO3)2 and CuSO4. CuSO4 produced fewer bubbles in the etching process. Because SO4−2 did not participate in the reaction, the etching rate of CuSO4 was significantly lower than that of Cu(NO3)2. The finally deposited Cu particles would be distributed on the surface of monocrystalline silicon, and a uniformly distributed random inverted pyramid structure could be obtained. They also confirmed that the PCE of solar cells etched with CuSO4 reached 22.63%, which was 0.6% and 1.56% higher than that etched with Cu(NO3)2 and NaOH, respectively [158].As can be seen from Figure11, Fang et al. deposited a 10-50 nm thick network film of Ag nanoparticles on the cleaned surface of silicon (111). Then, a large area of tilted nanowire arrays was fabricated by etching using mixtures of deionized water, HF, and H2O2, ranging from room temperature to 80°C. Compared with solar cells fabricated using a vertically-aligned silicon nanowire array (VA SiNW array), the reflectivity of the tilted nanowire arrays from 300 to 1000 nm is 5%, which is slightly higher than for a VA SiNW array 2.5%. Due to the inclined arrangement, the device has better electric contact. The power conversion efficiency of a slantingly-aligned SiNW array (SA SiNW array) solar cells is 11.37%, which is the highest among the devices that they studied [148].Figure 11
SEM figures of SA-SiNW arrays on p-Si (111) substrate: (a) planar figure without tilt angle, (b) cross-sectional figure, (c) SA-SiNW arrays with no metal electrodes, (d) SA-SiNW arrays covered with metal mesh electrodes, (e) magnified planar figure of the electrode regions, and (f) tilted cross-sectional BSE figure of the electrode regions [148].
(a)(b)(c)(d)(e)(f)Furthermore, MACE can be combined with anisotropic alkaline etching to create composite nanostructures with better light-trapping properties on the surface of monocrystalline silicon [159, 160]. He et al. fabricated a pyramidal texture on the surface of monocrystalline silicon using a mixture of KOH and isopropanol. The group then used a mixed solution of HF and AgNO3 to etch the pyramid-texture surface to obtain the silicon nanowire/pyramid binary structure. The measured results showed that the reflectivity of all textured samples was lower than for planar Si samples. The samples with pyramid texture showed 6-18% reflectivity over a wide spectrum (350-1100 nm). Compared with planar Si and pyramid-textured Si, the reflectance of all SiNWs and binary samples was much lower. It was below 5% for the entire 350-1100 nm range [161]. According to the theoretical analysis in Section 2.3, this is because the diameter of the SiNWs is equivalent to that of light at these wavelengths. This improved light scattering and increased the length of the optical path. This particular geometry has several advantages for hybrid solar cell applications. First, it increases the area of SiNW/PEDOT heterojunctions, not just the SiNW array on planar Si. Second, excellent optical capture can be achieved only using short SiNWs with low aspect ratio. Their aggregation is below that of longer SiNWs and allows for lower carrier recombination [162, 163]. In addition, shorter SiNWs are more robust than longer ones, which leads to more mechanically robust devices.The controllable parameters in MACE determine the final morphology of the etched nanostructures and affect the reflectivity directly. Furthermore, the shape of the metal-particle is an important factor in controlling the direction of holes [140]. Black silicon (B-Si) nanopore morphology is highly dependent on the volume ratio HF:H2O2:H2O and etching time. When the concentration of Ag+ is 50 μM, the lowest relative effective reflectivity (0.17%) can be obtained when the ratio of HF:H2O2:H2O is 1:5:2 [164]. During the study of MACE on non (100) Si substrates at different temperatures and oxidant concentrations, the etching direction of (110) and (111) Si substrates is affected by the concentration of the oxidant in the solution. In the solution with low oxidant concentration, etching was carried out along the preferred (100) direction of the crystal. On the other hand, in the solution with high oxidant concentration, etching was carried out along the vertical direction relative to the substrate surface. This was done because of the lattice structure of the Si crystal. The atoms of the plane have two back bonds with the atoms below, while the atoms of the <111> plane have three back bonds. Therefore, compared with the atoms of the <111> plane, the atoms of the plane are easier to dissolve [165].Yeo et al. studied the effects of HNO3, HF, deionized water concentration, and etching temperature on the morphology and optical properties of the silicon nanostructures in MACE. As a result, a practical application for antireflective silicon nanostructures with self-cleaning capability was found. Furthermore, the optical properties of Si nanostructures are closely related to the Si-MACE conditions. The solar weighted reflectance (SWR) of the Si nanostructures under optimal MACE conditions was 1.96%, and an angle-dependent SWR of < 4%, up to an AoI of 60°, was obtained. Compared with bulk silicon (SWR, 35.91%; angle-dependent SWR, 37.11%) for the wavelength range 300-1100 nm, the contact angle for water was about 102°. These results add to the understanding of Si MACE and provide valuable guidance for the realization of ideal antireflective Si nanostructures with self-cleaning properties to be used in high-efficiency c-Si solar cells [166].Moreover, catalytic etching with different metals was already done experimentally for Pd, Au, Pt, and Ag. The metal type can affect both the silicon etching rate and the porous structure in MACE [146, 167, 168]. Peng et al. and Liu et al. studied the motion of metal particles in Si. They found that the autonomous motion of the metal particles (Ag and Au) is highly uniform. However, it shows directionality and selectivity along the (100) direction of Si, rather than always being perpendicular to the surface of silicon [147, 169]. This feature can be used to fabricate inclined silicon nanowire arrays in single-crystalline silicon. With increasing HF and H2O2 concentration, the velocity of Ag particles in Si increases. To explain these observations, an electro kinetic model (self electrophoresis) was proposed. As shown in Figure 10, the movement of Ag and other metal particles in Si is related to the catalytic conversion of chemical free energy into propulsive mechanical energy.In an electrochemical system like Ag-Si-HF-H2O2, the reaction in two half cells, including the cathode and the anode, can be greatly enhanced for different positions on the surface of metal particles. The side of Ag particles facing the etching solution can act as a cathode to catalyze the reduction of H2O2 (or protons, H+) and consume H+ and electrons in the process. While the other side of Ag particles facing Si can act as a catalyst for Si oxidation and, thus, produce H+ and electrons. Because the resistivity of Si is much higher than Ag, a great potential difference can exist between cathode field and anode field. Therefore, a local corrosion-current can flow from the cathode to the anode. This leads to a net flux of electrons passing through the silver particles.Furthermore, the electron transfer of Ag particles is coupled with proton translocation in the fluid that surrounds the particles. The dependence of the metal-particle movement on the crystal orientation can be attributed to different removal rates for the Si atoms on the surface of different crystal orientations. The presence of Ag particles on the Si surface leads to the formation of early shallow pits and (ultimately) long holes in Si. Finally, the characteristics of the metal/Si interface can affect movement of the corresponding metal-particles in the substrate. Most metals interact strongly with silicon and form silicides at the metal-Si interface. Ag is an exception, it does not form silicides at the interface with silicon and maintains atomically sharp. There are also compounds similar to silicides at Si-Cu interface, and its bonding characteristics are close to those of near noble-metal-silicides. In reactive metal/Si systems such as Ag/Si with atomically sharp interface will help to promote uniform and straight movement of metal particles in bulk silicon. While active metal/Si systems such as Pt/Si with an alloy interface will make the motion of metal particles unstable in Si and lead to a curved trajectories of metal particles in bulk Si, as shown in Figure12 [147].Figure 12
(a) Transverse-sectional SEM view of an etched (100) silicon wafer, showing parallel straight tunnels perpendicular to the silicon surface. Ag particles at the bottom of the channels. (b) SEM figures of the etched silicon substrate precoated with Pt nanoparticles in an aqueous solution of HF/H2O2 [147].
(a)(b)With alkaline etching, highly ordered nanopore or nanowire-arrays can be fabricated by combining masks with MACE. Peng et al. used deep ultraviolet lithography (UVL) to form periodic array masks for circular holes on the surface of monocrystalline silicon (100). The group then evaporated the silver film and removed the photoresist to obtain a periodically arranged disk-pattern silver film. The films were etched in aqueous HF solution containing oxidized H2O2 at room temperature. The depth of the nanopores is determined by the etching time. Moreover, nanoporous silicon solar cells with three-dimensional radial PN junctions were formed using phosphorus diffusion. Under the illumination of 1 sun AM1.5G, the Voc, Jsc, and power conversion efficiency of nanoporous solar cells were 566.6 mV, 32.2 mA/cm2, and 9.51%, respectively. These numbers are much higher than planar Si and even higher than some pyramid-textured and nanowire Si solar cells [170]. In most current studies, MACE is still limited for nanowire arrays with low density, high surface area ratio, and arbitrary spatial pattern. Kang et al. overcame the inherent disadvantage of MACE by using two-step nanosphere lithography. They introduced large and small PS spheres as masks in the etching process. After etching, large-diameter and small-diameter silicon nanowire arrays will appear. After appropriate thermal oxidation and wet etching, small-diameter nanowires can be completely removed, leaving nanowire arrays with low density and high surface area ratio. According to their research, small diameter nanowires are the key to achieve this result. They act as a channel for mass transport of MACE chemicals to the narrow interface between silicon and metal. The density of the final silicon nanowire array is 0.1-1 per square micron adjustable, and the surface area ratio can reach 30 [171]. This research is more helpful to fabricate silicon nanowire arrays with specific patterns, so that the patterns of actually manufactured nanowire arrays can be more consistent with the simulation design.Thiyagu et al. spin coated poly (styrene-b-methacrylate) (PS-b-PMMA) blocked copolymer solution on the surface of silicon substrate, annealed, soaked in acetic acid, and then washed with deionized water. PMMA was removed to obtain polystyrene spheres template. Subsequently, the silver film was deposited using thermal evaporation, and then, MACE started in the HF/H2O2 solution. The required length of silicon nanowires was obtained by controlling the etching time. Finally, polystyrene spheres and silver films were removed to obtain silicon nanowires with uniform diameter and height. The reflectivity of these nanowire arrays was close to 0.1% for a wavelength range of 300-800 nm. This method does not require the photolithography step, which can further reduce the cost of preparing uniform nanowire arrays via MACE [172].


### 3.1.3. Reactive Ion Etching

Reactive ion etching (RIE) is the most widely used and mature dry-etching technology. Dry etching usually combines predefined templates to control the nanostructures accurately. As a result of the high cost associated with photolithography, etching masks, which were fabricated using silica spheres and self-assembly technology, have been studied widely. This method was first proposed by Deckman and Dunsmuir in 1982, when it was referred to as “natural lithography” [173]. The advantage of this method is that nanospheres can be assembled into tightly-aligned arrays. The lateral periodicity of the mask is controlled by the particle size. Another benefit is that standard etching processes such a KOH etching can be used. There is no need to modify the industrial etching program to benefit from automatic masking. Under certain etching conditions, nanowires, nanopillars, or nanocone arrays can be formed [174, 175].A typical process is shown in Figure13(a). The colloidal silica crystal with a single layer and not close-packed particles was spin-coated on a silicon wafer. The polymer was etched with O2 ions. Then, the not close-packed silica particles are used as the etching mask in the SF6 RIE process to form a nipple-pattern array directly on the wafer surface [176]. Similarly, the Langmuir Blodgett method was used to make a mask with a diameter of 500 nm on the silicon substrate surface. First, using the RIE process, the silica spheres were etched with oxygen and a CHF3 gas mixture. The diameter of the spheres was reduced to 200 nm. Then, chlorine (Cl2) and hydrogen bromide (HBr) were used as etching gases. Due to the selectivity, Si nanocones, with a height of 400 nm and a diameter of 450 nm, formed [177]. In addition, RIE can be easily combined with wet etching. Li et al. first used a single-layer not closed-packed PS microsphere as a mask to form a silver-film grid with hexagonal-hole arrays. Then, the silver film was used as a catalyst to form silicon nanopillar arrays (via MACE) in HF and H2O2 solutions. The PS microspheres were burned, and the silver membrane was dissolved in aqua regia [178]. The hollow tip array (with excellent antireflection and waterproofness) was fabricated using short-time RIE. The reflectivity of the structure was below 1% for 250-1600 nm. Garnett and Yang demonstrated a simple and scalable method to fabricate large area silicon nanowire radial p-n junction solar cells. By measuring the optical transmission and photocurrent of the films, it was shown that the ordered silicon nanowire arrays increased the path length for incident solar radiation by 73 times. The length-enhancement factor for the special light-capture path is higher than for the random scattering limit (2n225 without back reflector). The preparation process consisting of four simple steps is shown in Figure 13(b): Synthesis of silica beads, dip coating on the surface of monocrystalline silicon wafers to form self-assembled monolayer silica beads, deep RIE to form nanowire arrays, and diffusion-forming of a radial p-n junction. The distance and diameter of the silicon nanowires were controlled via diameter of the silica beads, while the length was determined by the reaction time. Figure 13(c) shows that large-area silicon nanowires have a periodic arrangement, a smooth surface, and good uniformity (10 cm2) [179].Figure 13
(a) Process flow diagram for the fabrication of moth-eye antireflective nanostructures on Si wafer [176]. (b) Schematic diagram for fabrication of a periodic Si nanowire array. There are three main steps in the preparation process illustrated above: a layer of silica sphere was coated on the surface of silicon wafer; silicon nanowires were obtained by DRIE etching silicon wafer and silica sphere as mask; removal of silica spheres by HF and boron doped silicon nanowires to form the radial p-n junction. (c) Si nanowires fabrication and structure morphology, SEM view of a close-packed monolayer of silica spheres is in the top left corner, SEM top view of a completed periodic silicon nanowire array is in the top right corner, tilted cross-sectional SEM of the silicon nanowire arrays is in the bottom left corner, and tilted optical image of silicon nanowire arrays is in the lower right section of image [179].
(a)(b)(c)By changing the RIE conditions, both nanowire- and nanocone-structures can be obtained. According to the theoretical analysis in Section2.3, the nanocone structure provides almost perfect impedance matching between Si and air by gradually reducing the effective refractive index. This enables better antireflection than the nanowire structure. As shown in Figure 14, for the wavelength range of 400-650 nm, the absorption of the nanocone array exceeded 93%, which is much better than the nanowire array (75%) and the thin-film (64%). Furthermore, the sensitivity to the incident light angle is also much lower than that for the nanowire array and the antireflection coating [180]. The key to produce a nanocone structure is to combine isotropic RIE with anisotropic RIE. First, Si nanorods were formed using anisotropic RIE based on Cl2. Second, C2ClF5/SF6 was used to further etch, isotropically, the preformed nanocolumns to create undercut and sharpened nanopillars. The formation mechanism of the tip can be described as follows: first, Cl and Br radicals reach the silicon surface from all directions during the RIE process, and they induce isotropic etching or undercutting of Si in the assumed anisotropic step. Second, the etching selectivity of Si to SiO2 is about 26. Therefore, when the SiO2 mask is used, the degree of lateral etching can increase due to mask erosion. Third, when the etching products are deposited, the deposition rate from the bottom to the top of the column decreases, and a conical sidewall will appear [181].Figure 14
(a) Absorptivity of planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm was measured at normal incidence. (b) Simulation results of absorption on planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm by normal incidence. (c) and (d) are the experimental and calculated results of the absorptivity in the range of incident angle 0-100° respectively. The incident light wavelength is 488 nm [180].
(a)(b)(c)(d)To improve the side-wall-profile controllability of the nanostructures in the RIE process, Zhao et al. proposed an innovative strategy to create silicon nanostructures. The group used fluorine-enhanced oxidation to achieve segment by segment wall-control. This method is based on fluoride RIE. Abundant fluorine species accumulate on the surface greatly enhancing the spontaneous growth of a passive oxide layer on the side wall, thus effectively controlling the profile. In the subsequent etching process, the oxide layer is selectively removed to form a new sidewall section with the required geometry. In this way, three different nanostructures are eventually achieved, including upstanding nanopillars with vertical sidewalls, narrowed necks, and suspending cantilever beams with controllable thickness [182].The RIE process is a combination of the chemical activity of active substances produced in plasma and physical effects caused by ion bombardment. Therefore, by changing the type and flow rate of the etching gas, working pressure, RF power, etching time, and other key parameters, a random texture with good antireflection can be fabricated on the surface of monocrystalline silicon or polycrystalline silicon [183]. Moreno et al. found that RF power has a crucial effect on the c-Si texture during the process of forming normal and inverted pyramids using the optimized gas ratio. At low RF power, pyramid-like texture is formed, and the texture is controlled via the anisotropic process. On the other hand, at high RF power, the inverted pyramid structure is obtained, and the texture is mainly controlled by the isotropic ion-assisted process. The upright pyramid structure on the c-Si surface results in an average reflectivity of about 18%. In contrast, the inverted pyramid structure results in an average reflectivity of as low as 6% without an antireflective coating [184].These results represent potential alternatives to low-cost manufacturing of c-Si solar cells because the process is completely dry. No deionized water, wet chemicals, or photolithography are required. Yoo fabricated a crater-like pyramid structure on c-Si surface using RIE without a mask. According to the extensive research of the RIE parameters, the optimized process conditions require a gas ratio SF6/O2 of 1.5, an operating pressure of 182 mTorr and power of 100 W. Compared with conventional saw damage removal (SDR) and alkaline solution texturing, the silicon loss of wafers that were cleaned by acid or alkaline solution followed by RIE texturing is reduced significantly. The crystalline silicon solar cells, which were fabricated by acid etching, were textured using RIE. The conversion efficiency was 16.7%, and the silicon loss was 67% lower than for conventional silicon solar cells [185].As an advanced nanomanufacturing technology, RIE has shown many advantages for the preparation of nanostructures on the surface of monocrystalline silicon solar cells. However, early RIE usually had a disadvantage that the nanostructured surface is relatively rough, which greatly increases the surface area [186]. An increased number of surface defects and dangling bonds can seriously affect the final PCE of solar cells. With the in-depth study of RIE technology, there are many methods to basically solve this problem. Schaefer and Lüdemann found that appropriate RIE etching parameters can greatly reduce the surface damage of silicon during etching. Then, through simple annealing, the carrier lifetime of RIE etched silicon can be restored to the level of the wet etched reference sample, and a high etching rate can be obtained. Their main conclusions are (1) RF power and DC bias should be as low as possible, (2) the etching time should be kept within a few minutes, but it should not be too short, and (3) pure SF6 should be used as the etching gas as far as possible, and oxygen shall not be introduced [187]. In addition, some studies showed that fluoride-enhanced oxidative passivation can improve the surface-profile control of RIE nanostructures and complete the surface passivation simultaneously [182]. Alternatively, one could use Cl2 to make the nanostructured surface smoother [188]. These works provide an important reference for reducing or eliminating the surface damage introduced in the process of etching crystalline silicon by RIE technology and help RIE show more advantages in the surface texturing of crystalline silicon solar cells.


### 3.1.4. Other Dry Etching Methods

Other dry etching methods, including femtosecond-laser etching, focused ion beam etching (FIB), and mechanical etching are not widely used for monocrystalline silicon solar cells. However, they still need to be considered/studied because of their unique advantages.Mechanical texturing usually uses motorized diamond tips to treat the monocrystalline silicon surface, which can form grooves that are tens of microns deep. After being modified by alkaline or acidic solutions, this simple and low-cost method can also greatly reduce the total reflectivity in the wavelength range 300-1200 nm. Ultrasonic nanocrystal surface modification (UNSM) is another method for mechanical texturing. Micro/nanopits are produced on the monocrystalline silicon surface using ultrasound. The increased surface roughness not only reduces reflectivity but also improves wear resistance. Although mechanical texturing is simple and cheap, the size, morphology, and pattern of nanostructures, which are obtained using this method, are greatly limited. Light trapping is not as good as that obtained using other etching methods. Furthermore, the preparation efficiency is low, and it is difficult to use for large area fabrication [189–191].In 1997, Mazur et al. discovered for the first time that, in an environment filled with SF6 or Cl2, a series of high current, femtosecond-laser pulses irradiated the surface of monocrystalline silicon <100> to form a sharp conical array of micrometer size. However, in vacuum, N2, or He, sharp tips were not formed. This indicates that the chemical reaction between SF6 or Cl2 and silicon contributed to the formation of the tips [192]. Further study revealed that the absorption of the sharp conical array, which was fabricated in SF6 atmosphere, was 90% for a wavelength range of 1.2-2.5 μm. This is due to the light-trapping effect produced by the conical spike array. Nevertheless, when a femtosecond-laser is used to ablate a silicon wafer in SF6 atmosphere, SF6 can be decomposed and mixed into the silicon surface using laser ablation. The impurity energy level can reduce the bandgap width of silicon and lead to strong subbandgap infrared absorption. The results show that the addition of 0.6% sulfur to the microstructure sample is enough to produce a wide absorption band [193–195].There are a few advantages to fabricating nanostructures via femtosecond-laser etching. Doping and structure-production can be carried out in a single process step without a predeposited layer-doping source. In addition, the morphology and optical absorption properties of the laser-treated Si surface can be easily tuned from UV light to infrared (≈2.5μm). Both properties depend on the number of laser pulses, irradiation intensity, atmosphere, and pulse duration. Another advantage is that laser etching is independent of the crystal structure. Femtosecond-laser etching, in particular, is suitable for polysilicon [196, 197]. Combined with anisotropic alkaline etching, large-area ordered silicon nanopillar arrays can be fabricated without photolithography [198]. However, femtosecond-laser etching has many inherent limits. High price of femtosecond-laser devices can greatly increase the preparation cost for the nanostructures. After femtosecond-laser ablation, the surface roughness of the nanostructures is large, which results in many defects and dangling bonds. If not passivated, a carrier lifetime that is too short effective can greatly reduce the efficiency of solar cells. Moreover, femtosecond-laser etching cannot achieve accurate control of nanostructure and morphology. These shortcomings make it difficult to use for the fabrication of surface microstructures of monocrystalline silicon solar cells on a large scale.The FIB system can focus a high-intensity ion beam via an electrostatic lens. Liquid metal ion sources (LMIs) are used in commercial systems, and gallium (Ga) is mostly used because of its low melting point, low vapor pressure, and good oxidation resistance. When the high-energy focused ion beam bombards the sample, its kinetic energy can be transferred to the atoms and molecules inside, which produces a sputtering effect that enables continuous etching, i.e., cutting of the sample. Its cutting-positioning accuracy can reach 5 nm. Furthermore, it can be used to prepare nanostructures on various materials including metals [199, 200]. Recently, Garg et al. fabricated Gaussian subwavelength 3D nanostructures using one-step rapid prototyping on a silicon surface via FIB milling. Highly ordered Gaussian columnar subwavelength nanostructures were fabricated on a Si surface using accelerated ions of the focused ion beam system. The height and period intervals of the Gaussian column were 250 nm and 500 nm, respectively. The fabricated columns could be tuned via the FIB dwell time. The diameter and periodicity of these columns are controlled using the beam current and step size, respectively. This provides additional degrees of freedom for the required functions. The reflectance spectrum of the optimized Gaussian column shows that the maximum reflectivity was 10-20% for the wide wavelength range of 400-1000 nm, which has clear improvements in light-capture [201].FIB etching has clear advantages for the preparation of nanostructures. For example, most of the surface texturing processes mentioned above either involve complex multiple steps and require posttreatment of contamination or they cause problems with inhomogeneity and uncontrollability. The combination of inaccuracy and contamination causes poor optical performance. Furthermore, FIB provides a simpler method and design for controlling the 3D surface texture. FIB etching can be done by rapid prototyping in one step and patterned without mask [202, 203]. This makes the preparation process simple and maintains the optical properties of the nanostructures. However, before FIB etching can be used for nanostructure fabrication on the surface of large-scale monocrystalline silicon solar cells, problems such as FIB-induced damage, ion contamination, redeposition, and the beam-tail effect need to be resolved [202, 204, 205].


## 3.1.1. Alkaline Etching

Due to the anisotropy of monocrystalline silicon in alkaline etching solutions, a method to prepare pyramid- or inverted-pyramid structures on its surface, using NaOH, KOH, and other corrosive solutions, was developed [112]. Today, this method is widely used for commercial silicon solar cells. The homogeneity of the pyramid nanostructure can be effectively improved by optimizing the corrosion time with a specific concentration of alkaline corrosive solution [113, 114]. The etching rate of silicon in solution and the anisotropy factor (quotient of the etching rate in the <100> to <111> directions) affect both the density and size of the pyramid in the <100> direction. Seidel found that the maximum etching rate of 0.9 μm/min can be achieved at 72°C for KOH solution with concentration of 15% [115]. However, the concentration of KOH less than 20% usually leads to the increased surface roughness and the formation of insoluble precipitates [116]. Higher concentration of KOH solution and higher temperature can increase the etching rate and make the etching surface smoother. For example, some studies have confirmed that the etching depth per minute of a 50% KOH solution at 80°C increased by 0.5 μm in the direction (100) [117]. The high concentration NaOH solution has a high etching rate. The work of Green et al. shows that the etching rate of NaOH solution with a concentration of 20-30% can reach 4-6 μm/min at 90°C [118, 119]. However this rate is too high for pyramid texturing of silicon. When texturing the surface of monocrystalline silicon with strong alkaline corrosive solutions such as NaOH and KOH, the addition of can effectively improve the uniformity of texture [120, 121].Based on a kinetics study of anisotropic etching, pyramid growth begins at the preferred position caused by crystal defects or wetting. At the beginning of the etching process, the inhomogeneity of the pyramid nucleation can be avoided by mixing additives into the etching solution. Vazsonyi et al. found the best texture condition at 80°C,where a relatively high etching rate (0.6μm/min) can be reached in the <100> direction. The anisotropy factor was 10. The pyramid size can be adjusted by changing the etching time to obtain low reflectivity from the textured silicon wafer [122]. Regular and well distributed pyramids can be obtained by etching in NaOH and isopropanol solution for 25 to 45 minutes. Reflectance ranges from 450 nm to 1000 nm between 11% and 14% [114]. However, IPA has also some disadvantages: high cost, volatile when heated, and low corrosion-rate at high concentrations. These disadvantages can be overcome by using metal grids with periodic openings, which improves the pyramid-texture uniformity. Chu et al. showed that when the metal mesh is placed near the wafer surface, hydrogen bubbles, which are produced by the KOH Si reaction, are limited by opening of the mesh because of the surface tension of the bubbles. Because hydrogen bubbles adhere to the wafer surface and inhibit the chemical reaction, they can act as a “pseudo mask.” Accordingly, both the position and size of the bubbles are relatively stable given the limitation to metal grids. Stable and uniform pyramid structures can be obtained without adding IPA [120].To avoid the inherent shortcomings of KOH and NaOH solutions during the texturing process of monocrystalline silicon, the application of other alkaline solutions such as tribasic sodium phosphate (Na3PO4·12H2O), sodium carbonate (Na2CO3), sodium hydroxide (NaOH), and sodium acetate (CH3COONa) were studied [123–125]. These solutions show the benefits of lower cost and reduced environmental pollution. On further research, the tetramethyl ammonium hydroxide (TMAH) solution was found to show the best overall effect. It overcomes the problem of mobile ion pollution, which is caused by the KOH and NaOH alkaline etching solutions that are commonly used in IC processes. The advantages of this solution include good controllability and a stable etching rate. Consequently, the texture of the inverted pyramid, which can be etched using the TMAH solution, is smooth but the top of the pyramid is sharp [126]. In addition, the texture, which was obtained using TMAH, can be passivated more easily than with KOH [127]. TMAH solution can be reused many times, which can reduce the cost of the pyramid texturing process [128]. Choi et al. studied the effect of the etchant concentration, ambient temperature, and silicon wafer thermal history on the formation of the pyramid in TMAH. The number and size of the pyramids were used as parameters in the study. Their results show that pH theory can well explain the effects of TMAH concentration and ambient temperature on silicon surface changes. However, the results of rapid thermal annealing, Czochralski wafers, and floating zone wafers seem to favor the defect theory [129]. To better explain the mechanism of silicon corrosion by TMAH, it is necessary to combine pH with the defect theory. When using the KOH and TMAH solution to texture the surface of monocrystalline silicon, the addition of IPA can alter the corrosion rate and roughness of the silicon surface <100>. When the IPA is adsorbed on the etched surface, it can hinder the reaction between Si and OH− and reduce the etching rate of the specific crystal face. Then, the equilibrium between oxidation and diffusion of the reaction products establishes the necessary conditions to generate smooth surfaces. The reduced reaction rate, which is caused by IPA adsorption, enables uniform surface etching. During the etching process, the IPA adsorption can be isotropic or anisotropic—depending on the composition of the solution. When TMAH solution (2-2.5 m) is mixed with IPA (10-30%), the decreased rates for (100) and (hkl) crystal planes are almost the same—independent of the crystal orientation. The anisotropic adsorption of IPA for different crystal orientations can be observed by mixing medium concentrated KOH solution (3-7 m) with low concentrated IPA (few to several percent). When the highly concentrated KOH solution was mixed with the extremely low concentrated IPA (2%), the IPA almost had no effect, and the etching rates for the different crystal orientations were almost the same [130].The electrochemical corrosion mechanism for monocrystalline silicon in alkaline solution was also studied. According to the model proposed by Seide et al., the total reaction process is summarized as the following equation:(6)Si+2OH−+2H2O⟶SiO2OH2−+2H2↑.In the oxidation reaction stage, due to the high pH environment, the surface atoms of bulk silicon react with fourOH− to produce SiO2OH2−.At the same time, four electrons are injected into the conduction band of silicon, two of them are transferred through the surface state of dangling bond, and the remaining two electrons are transferred through the surface state of back bond. These four electrons will be bound near the silicon surface due to the potential well generated by the downward bending energy band. In this space charge region, the recombination probability of injected electrons is very small. In the reduction process, implanted electrons will leave the solid and react with water molecules to produceOH− and H2. These OH− are generated directly on the Si surface and participate in the oxidation reaction. The orbital electrons in their respective surface states are thermally excited into the conduction band, resulting in the fracture of the back bond, which is considered to be the rate limit. Different crystallographic back bond surface states have different energy levels, which eventually leads to the anisotropy of etching, and the energy level of (111) plane is the lowest [115].According to the theoretical analysis in Section2.1, pyramid array textures with specific periodic arrangements show a better light-trapping effect than random-pyramid arrays. Therefore, it is important to study the preparation of periodic nanostructures. In the aforementioned method of texturing a monocrystalline silicon surface with anisotropic alkaline solution, the etching process is difficult to control because there are no masks. Only random pyramid textures can be obtained. A mask with periodic patterns can precisely control the etching position and produce the desired periodic nanostructures. The most commonly used mask is a photoresist with periodic patterns that were formed after the photolithography. The periodic inverted-pyramid pattern, shown in Figure 7, is formed by interference lithography and subsequent etching in KOH solution. The periodicity of the inverted pyramid array is 700 nm, and the average separation is 100 nm [131]. Patterned SiO2 can also be used as a mask for anisotropic alkaline etching. Using photolithography and dry etching methods, a silica rectangular window can be formed to expose silicon under the SiO2 layer. The periodically arranged inverted pyramidal array is obtained by etching in TMAH solution [132]. Sivasubramaniam and Alkaisi employed interference lithography combined with dry etching and KOH wet etching to fabricate inverted pyramid nanoarrays with a characteristic size of 700 nm on the surface of monocrystalline silicon. The vertical incident reflectivity in the whole visible light range was suppressed to less than 10%, and the overall efficiency of solar cells was improved by 67% [133]. Han et al. adopted almost the same method to fabricate symmetry breaking inverted nanopyramid arrays on the surface of monocrystalline silicon thin films. Their experimental and calculation results show that symmetry breaking can improve the photovoltaic efficiency of thin film silicon solar cells. For the active layer with a thickness of 10μm, the photovoltaic efficiency of the traditional symmetrical inverted nanopyramid array is 27%, and the introduction of a symmetry breaking ring improves the photovoltaic efficiency to 27.9% [134].Figure 7
(a) Photograph, (b) SEM vertical view, and (c) SEM cross-sectional view images of the 10μm c-Si film illustrating periodical reverted nanopyramids with a period of 700 nm. A SiNx layer with a thickness of 90 nm deposited on the surface of nanopyramids, and a SiO2 (1 μm) and a silver (200 nm) layer are at the reverse side of the c-Si film. The right bottom insets are enlarged drawings of the blue and red boxed areas. Scale bars:1 μm for (b) and (c) and 200 nm for insets in (c) [131].
(a)(b)(c)Periodic arrays of upright pyramids can be fabricated using a similar method. As shown in Figure8, photoresist is spin-coated on the surface of SiO2. Two-dimensional periodic photoresist masks can be obtained using laser interference lithography. RIE was used to transfer photoresist patterns to the SiO2 layer. Hydrofluoric acid (HF) is used to further reduce the size of the SiO2 mask. After anisotropic alkaline etching in a KOH solution, the periodic array of positive pyramids is obtained. The results show that the reflectivity of the front surface of the array is lower than that of the conventional micrometer pyramid texture. The excellent antireflection performance facilitates an absorption photocurrent density for a 40μm silicon absorption layer reaching 40.8 mA/cm2. This value is 0.7 mA/cm2 higher than for the periodic inverted-pyramid array, 0.8 mA/cm2 higher than the randomly arranged inverted-pyramid array, and 1 mA/cm2 higher than the randomly arranged vertical pyramid. The reason for the reduction in reflectivity is provided in the theoretical analysis in Sections 2.2 and 2.3 [135].Figure 8
(a) A schematic outline of the periodic upright nanopyramid preparation workflow based on laser interference lithography and anisotropic etching. (b) Corresponding 3D schematic diagram. (c) SEM images ofSiO2 template [135].
(a)(b)(c)The mask selection is mainly determined by the fabrication process. Furthermore, whether the photoresist with the periodic pattern is used as a mask orSiO2 with a periodic pattern (via photolithography) is used, which is determined by the photolithography step. It inevitably increases the cost of texturing the monocrystalline silicon surface. Nanoimprinting has lower cost than lithography, so it can be a good substitute. Gaucher et al. chose soft UV nanoimprint lithography to manufacture Cr etching mask on the surface of monocrystalline silicon film. After KOH solution etching, a nanoinverted pyramid array was formed. Finally, the short-circuit current density of thin-film crystalline silicon solar cells with a thickness of only 2.75 μm reached 25.3 mA/cm2[137]. Sun et al. demonstrated a simpler and cheaper nonphotolithographic method to fabricate wafer-level subwavelength antireflection gratings on monocrystalline silicon as shown in Figure 9. First, a monolayer of colloidal silica particles was spin-coated on the surface of <100> silicon for UV curing. Then, RIE was used to remove any SiO2 particles left behind by the colloid mask. Subsequently, electron-beam evaporation was used to deposit a 30 nm thick Cr layer. SiO2 particles were then removed to form a periodic nanopore array, whose diameter was determined by the size of the silica sphere template. These circular nanoholes were used as masks for the anisotropic KOH etching process, and a wafer-level inverted pyramid array was formed with a staggered layout in silicon [136].Figure 9
Fabrication of inverted nanopyramid arrays using 320 nm diameter silica spheres as templates. (a) 300 nm size nanopyramids and (b) 360 nm size nanopyramids both are etched at 60°C, etching times are 90 s and 210 s, respectively. (c) Schematic flow of the templating procedures for fabricating subwavelength-scale antireflection nanopyramid structures on monocrystalline silicon [136].
(a)(b)(c)This method greatly reduces the cost and allows for good controllability because it does not require lithography steps. The morphology of the final inverted pyramid can be controlled by the size of theSiO2 particles, the etching time, and the etching temperature. The reflection of the 360 nm inverted-pyramid array is reduced to 10% for a long wavelength (600 nm). When the wavelength is about 400 nm, the reflection is further reduced to 2%, which indicates good antireflection characteristics. Similarly, Zhou et al. used hexagonally close-packed polystyrene sphere (PS) as mask, ion etching was used to control the diameter of PS, and then, magnetron sputtered 50 nm Ti film. After PS removal, the front and rear surfaces of crystalline silicon films were etched with a NaOH and IPA mixed solution. The periodic inverted pyramid arrays of different sizes were obtained by controlling etching time. When the period of the inverted pyramid array is 1400 nm, the best reflection reduction effect is achieved. The current density of solar cells can reach 39.86 mA/cm2, and the contrast plane (22.63 mA/cm2) is increased by 70% [138].


## 3.1.2. Metal-Assisted Chemical Etching (MACE)

MACE uses metal catalysis so that both monocrystalline silicon and polycrystalline silicon can be anisotropically etched in acid solution [139–141]. Unlike the anisotropic etching solution, which can reduce cost [142]. In the process of preparing porous silicon from HF, HNO3, and H2O mixed solution, Dimova-Malinovska et al. found that the Al film, which was deposited on silicon surface, had a catalytic effect [143]. Subsequently, Li and Bohn studied the preparation of porous silicon in HF/H2O2 solution with noble metals (Au, Pt, and Au/Pt alloys) as catalysts. The group proposed the MACE mechanism for the first time [144]. As shown in Figure 10, a key feature of this reaction scheme is the formation of holes from H2O2 and the reduction of H+ to H2, both of which are facilitated by metal particles. The MACE of Si can be considered to be a local electrochemical process. The noble metal, which is deposited on the surface of Si sheet, is used as a local cathode catalytic oxidant to generate holes. It is then injected into the valence band of Si to oxidize and form silicon-ions dissolved in HF solution. The porous silicon, which was fabricated using this method, has a reflectivity of 2% in the range of 200-1000 nm [145]. Au nanoparticles were used to catalyze subwavelength structure formation after 15 min etching. The reflectivity of the subwavelength structure was below 5% for the range 300 nm to 800 nm [146].Figure 10
Catalytic-induced electrodynamics for autonomic motion of Ag nanoparticle in silicon substrate. (a) A auto genetic electric field caused by a hydrated proton gradient passing through the Ag particles. (b) Self electro-phoresis-driven movement of Ag particles. (c) Collective and extensive tunnelling motility of Ag (or metal) particles in a silicon substrate that forms the nanostructures of silicon [147].
(a)(b)(c)The detailed reaction mechanism for silicon MACE is as follows [143, 144]:Cathodic reaction (metal):(7)H2O2+2H+⟶2H2O+2H+2H++2e−⟶H2↑.Anodic reaction:(8)Si+4H++4HF⟶SiF4+4H+SiF4+2HF⟶H2SiF6.Total reaction:(9)Si+H2O2+HF↔2H2O+H2SiF6+H2↑.Electrochemical etching is a type of conventional wet isotropic acid etching [149–154] in which high aspect-ratio holes can be created on the surface of a silicon substrate. However, the uniformity and layer thickness of the silicon wafer surface texture are difficult to control using this method. Furthermore, the repeatability is poor, and this process is mostly used for surface texturing of polycrystalline silicon. In addition, electrochemical etching needs to connect the silicon wafer to an external power supply, which increases both the process complexity and energy consumption. Some studies showed that isotropic acid etching can achieve better antireflection properties than alkaline etching [155, 156]. However, the obtained rough surface greatly increases the surface carrier recombination and is not conductive to the subsequent surface passivation. When the poor controllability of the process and surface roughness issues are considered, the benefits of electrochemical etching for solar cells are very limited.Compared with the surface texture of polysilicon etched usingHF/HNO3 aqueous solution without metal catalyst, the efficiency of the solar cell with a bowl-like surface texture fabricated with the MACE method and acid postmodification increased by 1.2% (reaching 19.1% efficiency). This is mainly attributed to the fact that the lowest average reflectivity of the bowl-like structure is only 5.3% between 500 and 1100 nm. For comparison, the average reflectivity of a conventional micro texture is 14.5%. In addition, the surface area was reduced after acid modification, and the efficiency of the solar cells improved. The bowl-like textured solar cell shows a good broadband spectral response. This is because it has excellent antireflection properties at short wavelengths and long wavelengths. Moreover, the texture enables special light absorption at wide angles of incidence (AoI) from 0° to 70°, and it shows an all-round characteristics [157]. Recently, Zhang et al. also used CuSO4 etchant to manufacture high-efficiency inverted pyramid texture monocrystalline silicon solar cells. They compared the etching mechanisms of Cu (NO3)2 and CuSO4. CuSO4 produced fewer bubbles in the etching process. Because SO4−2 did not participate in the reaction, the etching rate of CuSO4 was significantly lower than that of Cu(NO3)2. The finally deposited Cu particles would be distributed on the surface of monocrystalline silicon, and a uniformly distributed random inverted pyramid structure could be obtained. They also confirmed that the PCE of solar cells etched with CuSO4 reached 22.63%, which was 0.6% and 1.56% higher than that etched with Cu(NO3)2 and NaOH, respectively [158].As can be seen from Figure11, Fang et al. deposited a 10-50 nm thick network film of Ag nanoparticles on the cleaned surface of silicon (111). Then, a large area of tilted nanowire arrays was fabricated by etching using mixtures of deionized water, HF, and H2O2, ranging from room temperature to 80°C. Compared with solar cells fabricated using a vertically-aligned silicon nanowire array (VA SiNW array), the reflectivity of the tilted nanowire arrays from 300 to 1000 nm is 5%, which is slightly higher than for a VA SiNW array 2.5%. Due to the inclined arrangement, the device has better electric contact. The power conversion efficiency of a slantingly-aligned SiNW array (SA SiNW array) solar cells is 11.37%, which is the highest among the devices that they studied [148].Figure 11
SEM figures of SA-SiNW arrays on p-Si (111) substrate: (a) planar figure without tilt angle, (b) cross-sectional figure, (c) SA-SiNW arrays with no metal electrodes, (d) SA-SiNW arrays covered with metal mesh electrodes, (e) magnified planar figure of the electrode regions, and (f) tilted cross-sectional BSE figure of the electrode regions [148].
(a)(b)(c)(d)(e)(f)Furthermore, MACE can be combined with anisotropic alkaline etching to create composite nanostructures with better light-trapping properties on the surface of monocrystalline silicon [159, 160]. He et al. fabricated a pyramidal texture on the surface of monocrystalline silicon using a mixture of KOH and isopropanol. The group then used a mixed solution of HF and AgNO3 to etch the pyramid-texture surface to obtain the silicon nanowire/pyramid binary structure. The measured results showed that the reflectivity of all textured samples was lower than for planar Si samples. The samples with pyramid texture showed 6-18% reflectivity over a wide spectrum (350-1100 nm). Compared with planar Si and pyramid-textured Si, the reflectance of all SiNWs and binary samples was much lower. It was below 5% for the entire 350-1100 nm range [161]. According to the theoretical analysis in Section 2.3, this is because the diameter of the SiNWs is equivalent to that of light at these wavelengths. This improved light scattering and increased the length of the optical path. This particular geometry has several advantages for hybrid solar cell applications. First, it increases the area of SiNW/PEDOT heterojunctions, not just the SiNW array on planar Si. Second, excellent optical capture can be achieved only using short SiNWs with low aspect ratio. Their aggregation is below that of longer SiNWs and allows for lower carrier recombination [162, 163]. In addition, shorter SiNWs are more robust than longer ones, which leads to more mechanically robust devices.The controllable parameters in MACE determine the final morphology of the etched nanostructures and affect the reflectivity directly. Furthermore, the shape of the metal-particle is an important factor in controlling the direction of holes [140]. Black silicon (B-Si) nanopore morphology is highly dependent on the volume ratio HF:H2O2:H2O and etching time. When the concentration of Ag+ is 50 μM, the lowest relative effective reflectivity (0.17%) can be obtained when the ratio of HF:H2O2:H2O is 1:5:2 [164]. During the study of MACE on non (100) Si substrates at different temperatures and oxidant concentrations, the etching direction of (110) and (111) Si substrates is affected by the concentration of the oxidant in the solution. In the solution with low oxidant concentration, etching was carried out along the preferred (100) direction of the crystal. On the other hand, in the solution with high oxidant concentration, etching was carried out along the vertical direction relative to the substrate surface. This was done because of the lattice structure of the Si crystal. The atoms of the plane have two back bonds with the atoms below, while the atoms of the <111> plane have three back bonds. Therefore, compared with the atoms of the <111> plane, the atoms of the plane are easier to dissolve [165].Yeo et al. studied the effects of HNO3, HF, deionized water concentration, and etching temperature on the morphology and optical properties of the silicon nanostructures in MACE. As a result, a practical application for antireflective silicon nanostructures with self-cleaning capability was found. Furthermore, the optical properties of Si nanostructures are closely related to the Si-MACE conditions. The solar weighted reflectance (SWR) of the Si nanostructures under optimal MACE conditions was 1.96%, and an angle-dependent SWR of < 4%, up to an AoI of 60°, was obtained. Compared with bulk silicon (SWR, 35.91%; angle-dependent SWR, 37.11%) for the wavelength range 300-1100 nm, the contact angle for water was about 102°. These results add to the understanding of Si MACE and provide valuable guidance for the realization of ideal antireflective Si nanostructures with self-cleaning properties to be used in high-efficiency c-Si solar cells [166].Moreover, catalytic etching with different metals was already done experimentally for Pd, Au, Pt, and Ag. The metal type can affect both the silicon etching rate and the porous structure in MACE [146, 167, 168]. Peng et al. and Liu et al. studied the motion of metal particles in Si. They found that the autonomous motion of the metal particles (Ag and Au) is highly uniform. However, it shows directionality and selectivity along the (100) direction of Si, rather than always being perpendicular to the surface of silicon [147, 169]. This feature can be used to fabricate inclined silicon nanowire arrays in single-crystalline silicon. With increasing HF and H2O2 concentration, the velocity of Ag particles in Si increases. To explain these observations, an electro kinetic model (self electrophoresis) was proposed. As shown in Figure 10, the movement of Ag and other metal particles in Si is related to the catalytic conversion of chemical free energy into propulsive mechanical energy.In an electrochemical system like Ag-Si-HF-H2O2, the reaction in two half cells, including the cathode and the anode, can be greatly enhanced for different positions on the surface of metal particles. The side of Ag particles facing the etching solution can act as a cathode to catalyze the reduction of H2O2 (or protons, H+) and consume H+ and electrons in the process. While the other side of Ag particles facing Si can act as a catalyst for Si oxidation and, thus, produce H+ and electrons. Because the resistivity of Si is much higher than Ag, a great potential difference can exist between cathode field and anode field. Therefore, a local corrosion-current can flow from the cathode to the anode. This leads to a net flux of electrons passing through the silver particles.Furthermore, the electron transfer of Ag particles is coupled with proton translocation in the fluid that surrounds the particles. The dependence of the metal-particle movement on the crystal orientation can be attributed to different removal rates for the Si atoms on the surface of different crystal orientations. The presence of Ag particles on the Si surface leads to the formation of early shallow pits and (ultimately) long holes in Si. Finally, the characteristics of the metal/Si interface can affect movement of the corresponding metal-particles in the substrate. Most metals interact strongly with silicon and form silicides at the metal-Si interface. Ag is an exception, it does not form silicides at the interface with silicon and maintains atomically sharp. There are also compounds similar to silicides at Si-Cu interface, and its bonding characteristics are close to those of near noble-metal-silicides. In reactive metal/Si systems such as Ag/Si with atomically sharp interface will help to promote uniform and straight movement of metal particles in bulk silicon. While active metal/Si systems such as Pt/Si with an alloy interface will make the motion of metal particles unstable in Si and lead to a curved trajectories of metal particles in bulk Si, as shown in Figure12 [147].Figure 12
(a) Transverse-sectional SEM view of an etched (100) silicon wafer, showing parallel straight tunnels perpendicular to the silicon surface. Ag particles at the bottom of the channels. (b) SEM figures of the etched silicon substrate precoated with Pt nanoparticles in an aqueous solution of HF/H2O2 [147].
(a)(b)With alkaline etching, highly ordered nanopore or nanowire-arrays can be fabricated by combining masks with MACE. Peng et al. used deep ultraviolet lithography (UVL) to form periodic array masks for circular holes on the surface of monocrystalline silicon (100). The group then evaporated the silver film and removed the photoresist to obtain a periodically arranged disk-pattern silver film. The films were etched in aqueous HF solution containing oxidized H2O2 at room temperature. The depth of the nanopores is determined by the etching time. Moreover, nanoporous silicon solar cells with three-dimensional radial PN junctions were formed using phosphorus diffusion. Under the illumination of 1 sun AM1.5G, the Voc, Jsc, and power conversion efficiency of nanoporous solar cells were 566.6 mV, 32.2 mA/cm2, and 9.51%, respectively. These numbers are much higher than planar Si and even higher than some pyramid-textured and nanowire Si solar cells [170]. In most current studies, MACE is still limited for nanowire arrays with low density, high surface area ratio, and arbitrary spatial pattern. Kang et al. overcame the inherent disadvantage of MACE by using two-step nanosphere lithography. They introduced large and small PS spheres as masks in the etching process. After etching, large-diameter and small-diameter silicon nanowire arrays will appear. After appropriate thermal oxidation and wet etching, small-diameter nanowires can be completely removed, leaving nanowire arrays with low density and high surface area ratio. According to their research, small diameter nanowires are the key to achieve this result. They act as a channel for mass transport of MACE chemicals to the narrow interface between silicon and metal. The density of the final silicon nanowire array is 0.1-1 per square micron adjustable, and the surface area ratio can reach 30 [171]. This research is more helpful to fabricate silicon nanowire arrays with specific patterns, so that the patterns of actually manufactured nanowire arrays can be more consistent with the simulation design.Thiyagu et al. spin coated poly (styrene-b-methacrylate) (PS-b-PMMA) blocked copolymer solution on the surface of silicon substrate, annealed, soaked in acetic acid, and then washed with deionized water. PMMA was removed to obtain polystyrene spheres template. Subsequently, the silver film was deposited using thermal evaporation, and then, MACE started in the HF/H2O2 solution. The required length of silicon nanowires was obtained by controlling the etching time. Finally, polystyrene spheres and silver films were removed to obtain silicon nanowires with uniform diameter and height. The reflectivity of these nanowire arrays was close to 0.1% for a wavelength range of 300-800 nm. This method does not require the photolithography step, which can further reduce the cost of preparing uniform nanowire arrays via MACE [172].


## 3.1.3. Reactive Ion Etching

Reactive ion etching (RIE) is the most widely used and mature dry-etching technology. Dry etching usually combines predefined templates to control the nanostructures accurately. As a result of the high cost associated with photolithography, etching masks, which were fabricated using silica spheres and self-assembly technology, have been studied widely. This method was first proposed by Deckman and Dunsmuir in 1982, when it was referred to as “natural lithography” [173]. The advantage of this method is that nanospheres can be assembled into tightly-aligned arrays. The lateral periodicity of the mask is controlled by the particle size. Another benefit is that standard etching processes such a KOH etching can be used. There is no need to modify the industrial etching program to benefit from automatic masking. Under certain etching conditions, nanowires, nanopillars, or nanocone arrays can be formed [174, 175].A typical process is shown in Figure13(a). The colloidal silica crystal with a single layer and not close-packed particles was spin-coated on a silicon wafer. The polymer was etched with O2 ions. Then, the not close-packed silica particles are used as the etching mask in the SF6 RIE process to form a nipple-pattern array directly on the wafer surface [176]. Similarly, the Langmuir Blodgett method was used to make a mask with a diameter of 500 nm on the silicon substrate surface. First, using the RIE process, the silica spheres were etched with oxygen and a CHF3 gas mixture. The diameter of the spheres was reduced to 200 nm. Then, chlorine (Cl2) and hydrogen bromide (HBr) were used as etching gases. Due to the selectivity, Si nanocones, with a height of 400 nm and a diameter of 450 nm, formed [177]. In addition, RIE can be easily combined with wet etching. Li et al. first used a single-layer not closed-packed PS microsphere as a mask to form a silver-film grid with hexagonal-hole arrays. Then, the silver film was used as a catalyst to form silicon nanopillar arrays (via MACE) in HF and H2O2 solutions. The PS microspheres were burned, and the silver membrane was dissolved in aqua regia [178]. The hollow tip array (with excellent antireflection and waterproofness) was fabricated using short-time RIE. The reflectivity of the structure was below 1% for 250-1600 nm. Garnett and Yang demonstrated a simple and scalable method to fabricate large area silicon nanowire radial p-n junction solar cells. By measuring the optical transmission and photocurrent of the films, it was shown that the ordered silicon nanowire arrays increased the path length for incident solar radiation by 73 times. The length-enhancement factor for the special light-capture path is higher than for the random scattering limit (2n225 without back reflector). The preparation process consisting of four simple steps is shown in Figure 13(b): Synthesis of silica beads, dip coating on the surface of monocrystalline silicon wafers to form self-assembled monolayer silica beads, deep RIE to form nanowire arrays, and diffusion-forming of a radial p-n junction. The distance and diameter of the silicon nanowires were controlled via diameter of the silica beads, while the length was determined by the reaction time. Figure 13(c) shows that large-area silicon nanowires have a periodic arrangement, a smooth surface, and good uniformity (10 cm2) [179].Figure 13
(a) Process flow diagram for the fabrication of moth-eye antireflective nanostructures on Si wafer [176]. (b) Schematic diagram for fabrication of a periodic Si nanowire array. There are three main steps in the preparation process illustrated above: a layer of silica sphere was coated on the surface of silicon wafer; silicon nanowires were obtained by DRIE etching silicon wafer and silica sphere as mask; removal of silica spheres by HF and boron doped silicon nanowires to form the radial p-n junction. (c) Si nanowires fabrication and structure morphology, SEM view of a close-packed monolayer of silica spheres is in the top left corner, SEM top view of a completed periodic silicon nanowire array is in the top right corner, tilted cross-sectional SEM of the silicon nanowire arrays is in the bottom left corner, and tilted optical image of silicon nanowire arrays is in the lower right section of image [179].
(a)(b)(c)By changing the RIE conditions, both nanowire- and nanocone-structures can be obtained. According to the theoretical analysis in Section2.3, the nanocone structure provides almost perfect impedance matching between Si and air by gradually reducing the effective refractive index. This enables better antireflection than the nanowire structure. As shown in Figure 14, for the wavelength range of 400-650 nm, the absorption of the nanocone array exceeded 93%, which is much better than the nanowire array (75%) and the thin-film (64%). Furthermore, the sensitivity to the incident light angle is also much lower than that for the nanowire array and the antireflection coating [180]. The key to produce a nanocone structure is to combine isotropic RIE with anisotropic RIE. First, Si nanorods were formed using anisotropic RIE based on Cl2. Second, C2ClF5/SF6 was used to further etch, isotropically, the preformed nanocolumns to create undercut and sharpened nanopillars. The formation mechanism of the tip can be described as follows: first, Cl and Br radicals reach the silicon surface from all directions during the RIE process, and they induce isotropic etching or undercutting of Si in the assumed anisotropic step. Second, the etching selectivity of Si to SiO2 is about 26. Therefore, when the SiO2 mask is used, the degree of lateral etching can increase due to mask erosion. Third, when the etching products are deposited, the deposition rate from the bottom to the top of the column decreases, and a conical sidewall will appear [181].Figure 14
(a) Absorptivity of planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm was measured at normal incidence. (b) Simulation results of absorption on planar a-Si:H film, NW arrays, and NC arrays in the wavelength range of 400-800 nm by normal incidence. (c) and (d) are the experimental and calculated results of the absorptivity in the range of incident angle 0-100° respectively. The incident light wavelength is 488 nm [180].
(a)(b)(c)(d)To improve the side-wall-profile controllability of the nanostructures in the RIE process, Zhao et al. proposed an innovative strategy to create silicon nanostructures. The group used fluorine-enhanced oxidation to achieve segment by segment wall-control. This method is based on fluoride RIE. Abundant fluorine species accumulate on the surface greatly enhancing the spontaneous growth of a passive oxide layer on the side wall, thus effectively controlling the profile. In the subsequent etching process, the oxide layer is selectively removed to form a new sidewall section with the required geometry. In this way, three different nanostructures are eventually achieved, including upstanding nanopillars with vertical sidewalls, narrowed necks, and suspending cantilever beams with controllable thickness [182].The RIE process is a combination of the chemical activity of active substances produced in plasma and physical effects caused by ion bombardment. Therefore, by changing the type and flow rate of the etching gas, working pressure, RF power, etching time, and other key parameters, a random texture with good antireflection can be fabricated on the surface of monocrystalline silicon or polycrystalline silicon [183]. Moreno et al. found that RF power has a crucial effect on the c-Si texture during the process of forming normal and inverted pyramids using the optimized gas ratio. At low RF power, pyramid-like texture is formed, and the texture is controlled via the anisotropic process. On the other hand, at high RF power, the inverted pyramid structure is obtained, and the texture is mainly controlled by the isotropic ion-assisted process. The upright pyramid structure on the c-Si surface results in an average reflectivity of about 18%. In contrast, the inverted pyramid structure results in an average reflectivity of as low as 6% without an antireflective coating [184].These results represent potential alternatives to low-cost manufacturing of c-Si solar cells because the process is completely dry. No deionized water, wet chemicals, or photolithography are required. Yoo fabricated a crater-like pyramid structure on c-Si surface using RIE without a mask. According to the extensive research of the RIE parameters, the optimized process conditions require a gas ratio SF6/O2 of 1.5, an operating pressure of 182 mTorr and power of 100 W. Compared with conventional saw damage removal (SDR) and alkaline solution texturing, the silicon loss of wafers that were cleaned by acid or alkaline solution followed by RIE texturing is reduced significantly. The crystalline silicon solar cells, which were fabricated by acid etching, were textured using RIE. The conversion efficiency was 16.7%, and the silicon loss was 67% lower than for conventional silicon solar cells [185].As an advanced nanomanufacturing technology, RIE has shown many advantages for the preparation of nanostructures on the surface of monocrystalline silicon solar cells. However, early RIE usually had a disadvantage that the nanostructured surface is relatively rough, which greatly increases the surface area [186]. An increased number of surface defects and dangling bonds can seriously affect the final PCE of solar cells. With the in-depth study of RIE technology, there are many methods to basically solve this problem. Schaefer and Lüdemann found that appropriate RIE etching parameters can greatly reduce the surface damage of silicon during etching. Then, through simple annealing, the carrier lifetime of RIE etched silicon can be restored to the level of the wet etched reference sample, and a high etching rate can be obtained. Their main conclusions are (1) RF power and DC bias should be as low as possible, (2) the etching time should be kept within a few minutes, but it should not be too short, and (3) pure SF6 should be used as the etching gas as far as possible, and oxygen shall not be introduced [187]. In addition, some studies showed that fluoride-enhanced oxidative passivation can improve the surface-profile control of RIE nanostructures and complete the surface passivation simultaneously [182]. Alternatively, one could use Cl2 to make the nanostructured surface smoother [188]. These works provide an important reference for reducing or eliminating the surface damage introduced in the process of etching crystalline silicon by RIE technology and help RIE show more advantages in the surface texturing of crystalline silicon solar cells.


## 3.1.4. Other Dry Etching Methods

Other dry etching methods, including femtosecond-laser etching, focused ion beam etching (FIB), and mechanical etching are not widely used for monocrystalline silicon solar cells. However, they still need to be considered/studied because of their unique advantages.Mechanical texturing usually uses motorized diamond tips to treat the monocrystalline silicon surface, which can form grooves that are tens of microns deep. After being modified by alkaline or acidic solutions, this simple and low-cost method can also greatly reduce the total reflectivity in the wavelength range 300-1200 nm. Ultrasonic nanocrystal surface modification (UNSM) is another method for mechanical texturing. Micro/nanopits are produced on the monocrystalline silicon surface using ultrasound. The increased surface roughness not only reduces reflectivity but also improves wear resistance. Although mechanical texturing is simple and cheap, the size, morphology, and pattern of nanostructures, which are obtained using this method, are greatly limited. Light trapping is not as good as that obtained using other etching methods. Furthermore, the preparation efficiency is low, and it is difficult to use for large area fabrication [189–191].In 1997, Mazur et al. discovered for the first time that, in an environment filled with SF6 or Cl2, a series of high current, femtosecond-laser pulses irradiated the surface of monocrystalline silicon <100> to form a sharp conical array of micrometer size. However, in vacuum, N2, or He, sharp tips were not formed. This indicates that the chemical reaction between SF6 or Cl2 and silicon contributed to the formation of the tips [192]. Further study revealed that the absorption of the sharp conical array, which was fabricated in SF6 atmosphere, was 90% for a wavelength range of 1.2-2.5 μm. This is due to the light-trapping effect produced by the conical spike array. Nevertheless, when a femtosecond-laser is used to ablate a silicon wafer in SF6 atmosphere, SF6 can be decomposed and mixed into the silicon surface using laser ablation. The impurity energy level can reduce the bandgap width of silicon and lead to strong subbandgap infrared absorption. The results show that the addition of 0.6% sulfur to the microstructure sample is enough to produce a wide absorption band [193–195].There are a few advantages to fabricating nanostructures via femtosecond-laser etching. Doping and structure-production can be carried out in a single process step without a predeposited layer-doping source. In addition, the morphology and optical absorption properties of the laser-treated Si surface can be easily tuned from UV light to infrared (≈2.5μm). Both properties depend on the number of laser pulses, irradiation intensity, atmosphere, and pulse duration. Another advantage is that laser etching is independent of the crystal structure. Femtosecond-laser etching, in particular, is suitable for polysilicon [196, 197]. Combined with anisotropic alkaline etching, large-area ordered silicon nanopillar arrays can be fabricated without photolithography [198]. However, femtosecond-laser etching has many inherent limits. High price of femtosecond-laser devices can greatly increase the preparation cost for the nanostructures. After femtosecond-laser ablation, the surface roughness of the nanostructures is large, which results in many defects and dangling bonds. If not passivated, a carrier lifetime that is too short effective can greatly reduce the efficiency of solar cells. Moreover, femtosecond-laser etching cannot achieve accurate control of nanostructure and morphology. These shortcomings make it difficult to use for the fabrication of surface microstructures of monocrystalline silicon solar cells on a large scale.The FIB system can focus a high-intensity ion beam via an electrostatic lens. Liquid metal ion sources (LMIs) are used in commercial systems, and gallium (Ga) is mostly used because of its low melting point, low vapor pressure, and good oxidation resistance. When the high-energy focused ion beam bombards the sample, its kinetic energy can be transferred to the atoms and molecules inside, which produces a sputtering effect that enables continuous etching, i.e., cutting of the sample. Its cutting-positioning accuracy can reach 5 nm. Furthermore, it can be used to prepare nanostructures on various materials including metals [199, 200]. Recently, Garg et al. fabricated Gaussian subwavelength 3D nanostructures using one-step rapid prototyping on a silicon surface via FIB milling. Highly ordered Gaussian columnar subwavelength nanostructures were fabricated on a Si surface using accelerated ions of the focused ion beam system. The height and period intervals of the Gaussian column were 250 nm and 500 nm, respectively. The fabricated columns could be tuned via the FIB dwell time. The diameter and periodicity of these columns are controlled using the beam current and step size, respectively. This provides additional degrees of freedom for the required functions. The reflectance spectrum of the optimized Gaussian column shows that the maximum reflectivity was 10-20% for the wide wavelength range of 400-1000 nm, which has clear improvements in light-capture [201].FIB etching has clear advantages for the preparation of nanostructures. For example, most of the surface texturing processes mentioned above either involve complex multiple steps and require posttreatment of contamination or they cause problems with inhomogeneity and uncontrollability. The combination of inaccuracy and contamination causes poor optical performance. Furthermore, FIB provides a simpler method and design for controlling the 3D surface texture. FIB etching can be done by rapid prototyping in one step and patterned without mask [202, 203]. This makes the preparation process simple and maintains the optical properties of the nanostructures. However, before FIB etching can be used for nanostructure fabrication on the surface of large-scale monocrystalline silicon solar cells, problems such as FIB-induced damage, ion contamination, redeposition, and the beam-tail effect need to be resolved [202, 204, 205].


## 3.2. Bottom-Up Method

The bottom-up method mainly involves chemical reaction growth or the deposition to introduce new materials and produce the required nanostructures on the surface of monocrystalline silicon. The bottom-up approach has more advantages in terms of cost and material selection than the top-down method, which requires removal of excess material. Main bottom-up methods include chemical vapor deposition (CVD), plasma-enhanced chemical vapor deposition (PECVD), and molecular beam epitaxy (MBE) based on the growth mechanism of vapor-liquid-solid (VLS) [206–208]. This mechanism was first proposed by Wanger and Eills at Bell Laboratories in 1964 [209]. Among these methods, CVD is most commonly used to grow high-quality silicon nanowires. Large area Si nanowire arrays can be grown on commonly used (e.g., glass, Si) or reusable substrates. In addition, the different operating modes of a CVD system can greatly affect the VLS growth of silicon nanowires.Sanchar et al. studied the effects of two operating modes of a cold wall catalytic CVD system on the growth of VLS silicon nanowires: hot filament chemical vapor deposition (HWCVD) and chemical vapor deposition (CVD). They found that the HWCVD mode has a higher growth rate. However, it also has disadvantages such as small length, tapered nanowires, and a large amount of uncatalyzed silicon deposition on the substrate. This is because using the HWCVD mode, the precursor gas can decompose directly after the tungsten wire is heated. Heating furthermore, there is no “catalytic effect” on the surface of the silicon substrate. Any anisotropic growth is due to the fact that the solubility and adhesion coefficient of Si atoms in liquid Au/Si eutectic droplets are much higher than on solid Si substrates. The heating effects of both the filament and the substrate contribute to a much higher temperature in the spherical chamber than the minimum temperature required for the catalytic decomposition of SiH4. This makes the growth rate of silicon nanowires extremely high. Another important factor for the appearance of a conical structure is the high diffusion rate of Au/Si eutectic droplets along the side wall of the grown nanowires at high temperature. This is why the catalyst of the nanowires is lost soon after the start of VLS growth, and the tip appears at the end of the VLS growth. In the CVD mode with optimized conditions, the nanowires are not conical, and the length can reach several microns. No uncatalyzed silicon deposition was observed on the substrate. Therefore, the CVD mode is more suitable for the VLS growth of silicon nanowires [210].A typical process of CVD VLS growth is shown in Figure15. First, Au is deposited on the surface of silicon substrate via magnetron sputtering, electron beam evaporation, or thermal evaporation. Then, the diameter and density of the nanowires are determined by the size and density of the catalytic metal particles. The melting point of Au-Si alloy is lower than that of Au or Si, so Au-Si alloy (4:1) is liquid at temperatures exceeding 363°C. The mixed gas of SiCl4 and H2 is adsorbed on the surface of this liquid, and the mixture reacts during the catalysis of the metal. The reaction formula is as follows:
(10)SiCl4+2H2⟶Si+4HCl.Figure 15
(a) The growth process of silicon nanowires is illustrated: (a) gold thin film is deposited on the silicon substrate by PVD; (b) Au-Si droplets are formed by thermal annealing; and (c) SiCl4 reacts with H2 and grows Si nanowires from the substrate under the catalysis reaction of gold [211].
(a)(b)(c)Generally, the reaction occurs above 800°C. When the temperature exceeds 363°C, the liquid Au-Si alloy can absorb the Si formed in the reaction until the Si in Au is supersaturated. The melting point of silicon is 1414°C, which is much higher than that of the Au-Si alloy. Therefore, Si atoms can precipitate from the supersaturated liquid alloy at the interface of Au-Si liquid alloy/Si substrate. Furthermore, one-dimensional silicon nanowires can grow from liquid metal alloy droplets[211–213]. Similar methods are vapor-solid-solid (VSS), solid-liquid-solid (SLS), supercritical fluid-liquid-solid (FLS), and supercritical fluid-solid-solid (FSS) [214–218]. In the CVD system, the diameter, length, and morphology of nanowires grown using VLS are mainly affected by the metal catalysts. Temperature and pressure are also important conditions that determine the geometric characteristics of nanowires [219, 220].The diffusion properties of Au fundamentally limit length, shape, and the side-wall morphology of the Si nanowires. There is a key difference between low pressure CVD in ultrahigh vacuum on a clean substrate and conventional CVD, when it comes to the prevention of Au migration under high pressure. An inevitable difference is cleanliness, which may be due to the reduction of Au migration by adsorbing contaminants from the precursor gas. These results indicate that the size of the catalyst droplets changes during nanowire growth at 600°C. During growth, Au preferentially migrates, from smaller catalyst droplets to larger ones, to reduce the total droplet-surface-area. In this process, some droplets coarsen completely, forming shorter tapered nanowires. In the process of VLS growth, the side walls of the nanowires are relatively smooth, when catalyst droplets are present. This indicates that the growth rate of VLS is faster than that of CVD for the side-walls covered with Au. Once the droplets are consumed, the growth of VLS stops. However, direct CVD growth on the side-wall of Si nanowires continues, greatly changing the side-wall morphology [221].In the field of nanoelectronics, platinum nanoparticles provide a feasible alternative to conventional gold catalysts to control the single-crystal size of SiNWs, which were synthesized using epitaxial vapor phase CVD. Due to the middle gap trap state of Au in silicon, it can “poison” the performance of devices. Therefore, it is generally not allowed in electronic manufacturing laboratories and clean rooms. The average diameter of undoped wires, grown from 9.3 nm Pt nanoparticles, is 11.3 nm, while that of boron doped wires is larger (17.9 nm). SiNWs are mainly grown in the (110) direction, which results in the partially vertical alignment of SiNWs on the monocrystalline Si <110> wafer. Despite the strong surface effect due to the small diameter, the electric properties of boron-doped wires are similar to those of other nonfunctionalized SiNWs. The optimal mobility and switching ratio are 28 cm2V−1s-1 and 7400, respectively. In addition to reducing gold-related microelectronic processing problems, the Pt catalyst also allows 9 nm particles to be used in high-temperature SiCl4-CVD reactors without significant aggregation. This enables small-diameter wires and simple integration [223]. The density of silicon nanowires, which are grown using VLS, has an important effect on the performance of radial junction solar cells. A suitable growth density can produce silicon nanowires with a more-uniform diameter. This helps form a uniform and high-quality radial junction around the SiNWs. The radial PN solar cell with optimized nanowire-density reached an open circuit voltage of 0.80 V, a short circuit current density of 16.1 mA/cm2, and a conversion efficiency of 8.14% [206].To ensure good light-trapping of the nanowire array, the silicon nanowire must be perpendicular to the substrate surface as much as possible. This is because Si nanowires are epitaxially grown from binary droplet to substrate, i.e., Si <111> wafer. In addition, only by keeping the substrate surface clean and preventing oxide or other impurities from interfering the VLS growth process can vertically-grown Si nanowires be obtained. However, gaseous HCl is a by-product of the SiCl4 decomposition in reaction tubes. It corrodes the oxide layer on the surface and presents a clean Si-crystal surface, which makes Si continuously precipitated from binary droplets. At this interface, the epitaxial deposition of Si induces growth of nanowires on the crystal face of the Si wafer. Therefore, substrate treatment and precursor selection are essential to keep the substrate vertically aligned during growth [220, 224]. If other CVD precursors, such as SiH4, are used without adding HCl or taking special preventive measures to remove the oxide layer, it is impossible to realize large-scale vertical growth of nanowire arrays [219, 225]. As shown in Figure 16, photolithography is still the best way to precisely control the growth position and diameter of nanowires in VLS growth to form various periodic arrays of nanowires, although it can increase the preparation cost [222, 226].Figure 16
The arrangement pattern and optical properties of each nanowire array. (a) SEM top view of nanowire arrays. (a) SEM images viewed at 20° angle. (c) Transmitted diffraction patterns of polymer-embedded nanowire arrays on a quartz substrate, the incident wavelength is 488 nm. (d) Transmission intensity of each nanowire array as a function of the incident light angle (θx, θy) [222].According to the theoretical analysis of Section2.3, the antireflection properties of nanowire arrays are affected by the nanowire array arrangement. Compared with quasiperiodic or random arrays, the aligned lines, which are produced by periodic arrays, lead to a larger-area stacking fraction (thus improving the overall optical absorptivity). However, the periodic arrangement shows a strong anisotropic angular absorption-profile, which can produce a low absorption “dead point” in photovoltaic devices [222]. Using photolithography, Kayes has grown periodically vertically-aligned silicon nanowire arrays with a diameter of 1.5 μm and a length of 75 μm in the region>1cm2. They first thermally grew 300 nm oxides on the surface of Si (111) and then fabricated periodic patterns using photolithography. After development, the surface of silicon dioxide was exposed, and the sample subsequently was immersed in HF solution to remove the SiO2 layer to expose the silicon surface. Thermal evaporation of 500 nm gold or copper on the surface of silicon wafer was used as a catalyst for VLS growth, and the remaining photoresist was removed by lift off. Next, VLS was grown at the optimal temperature and precursor gas ratio. The process is the same as the typical VLS growth process described above. This method produces an array with almost no defects, which suggest an extremely narrow diameter, low length distributions, and highly controllable wire positions. The key to realize a large area, highly uniform, and periodic array of silicon nanowires lies in the application of a silica buffer layer. The group also carried out experiments without the use of a silica buffer layer. As a result of the migration effect of the gold catalyst mentioned above during the annealing process, a large amount of agglomeration of the catalyst occurred without the restriction of the silica buffer layer, which seriously worsened the fidelity of the periodic pattern. Therefore, to obtain periodic aligned and homogeneous silicon nanowire arrays via VLS, it is necessary to limit the catalyst to the required position using a buffer layer to prevent the migration of Au and other metal catalysts during annealing and growth [224].In addition to photolithography combined with a silica buffer layer, the method of using self-organizing anodic alumina film (AAM) as a template to grow ordered monocrystalline nanopillar arrays has great potential. Furthermore, AAM templates can control both the morphology and size of the nanopillars and even form double-diameter nanostructures, which can further improve light-capture performance [227].


## 4. Surface Passivation of Nanostructures

Optical loss and electric loss are the two main factors that affect the photovoltaic conversion efficiency. The former is mainly attributed to reflection loss of the incident light, the shadow loss of the front gate electrode, and the long-wave loss of the silicon. The latter generally results from carrier recombination loss and resistance losses. According to the nanostructure preparation in Section3, both wet etching and dry etching can increase the surface area of silicon solar cells. Especially dry etching can produce more defects and dangling bonds on the nanostructure surface. Furthermore, nanostructures not only enhance the light capture capacity of solar cells but also greatly increase the surface recombination of carriers. The overall efficiency of solar cells may not be improved or even decreased.According to the first principle calculations of Hong et al., the surface dangling bonds of Si nanostructures are the fundamental obstacle for nanodoping, and surface passivation is very important to prevent the deactivation of dopants [229]. Li and Mintmire also studied the effect of H, OH, and CH3 group passivation on the electronic band structure of silicon nanowires using first principles. Their results show that appropriate surface-passivation selection can tune the band structure and carrier mobility to specific applications. Supplement options such as diameter, orientation, and cross section can be used to modify the SiNW characteristics [230]. Therefore, compared with conventional planar solar cells, surface passivation is even more needed to prevent performance degradation of monocrystalline silicon solar cells with nanostructures. On the other hand, the passivation of nanostructures must minimize the negative effect with respect to the optical properties.Solar cell passivation technology is mainly divided into two groups: chemical passivation and field-effect passivation. Chemical passivation means to passivate the surface by reducing the number of dangling bonds by creating new stable bonds. The dangling bonds on the silicon surface are saturated either by direct chemical or electrochemical corrosion. Passivation is achieved by reducing the surface density of states [228, 231, 232]. This is why the surface damage of crystalline silicon usually etched by the wet method is much lower than that etched by the dry method. Trompoukis et al. found that compared with plasma etching periodic nanostructures, the inverted nanopyramid obtained by wet etching has higher carrier lifetime and lower surface recombination velocity. This trend appears for three different substrates: 280 μm FZP wafers, 180 μm CZP wafers, and 40 μm silicon epioils bonded to glass carrier substrates. In particular, the minority carrier lifetime (τeff=250μs) and surface recombination velocity (8 cm/s) of the third substrate are similar to those of the reference plane film [233].The acidic mixture of the HF-HNO3-H2O solution has been used for damage removal etching (DRE) on the surface of the nanocone with a diameter of 300 nm due to control of the etching rate of the silicon. As can be seen from Table 1, the effective lifetime of the carrier increases from 24.4μs to 1092μs after chemical passivation [228]. Recently, Sutter et al. also used HF-HNO3-H2O solution to passivate the surface damage of sinusoidal nanostructures fabricated by RIE on the surface of monocrystalline silicon. The minority carrier life increases with the increase of etching time. After etching for 35 s, the minority carrier life reaches the maximum value of 11.3 ms. It is equivalent to that of a planar reference wafer (13.4 ms). SiNx, SiOx, and a-Si:H thin film via PECVD or thermally grown silica (SiO2) layers are usually used for passivation of polished or micro textured surfaces. The SiOx passivation layer on the surface of nanopyramid textures was found to show a good passivation, and it has little effect on the antireflection properties of the nanopyramids. Compared with the nanopyramid textured solar cell without SiOx passivation layer, the conversion efficiency increased from 12.2% to 13.3% [80]. The SiOx antireflection passivation layer was deposited on the surface of porous silicon (PS) using PECVD. Refractive-index adaptation can be realized between the PS layer and the air, which can improve the antireflection performance. Furthermore, the presence of Si-O bonds passivates the dangling bonds in PS. About 4% hydrogen is contained in SiO2 and diffuses in the PS Si-layer, which increases the number of Si-H bonds that initially formed during PS formation, thus, enhances the surface passivation of pores [234].Table 1
Influence of DRE on the weighted reflection (RW) and effective carrier lifetime (τeff) of nanotextured surface. All τeff values were measured on samples deposited with 30 nm thick SiO2 layer at an excess carrier density of 1×1015cm−2 [228].
Surface typeDRE(s)DRE sequenceτeffRw%Polished--418334.79--24.42.2640143.82.35601102.22.64Nano-textured1201120.84.28240178.28.9160210922.49120246112.25SiNx is another commonly used dielectric material for passivation layer, and its passivation quality is related to its thickness. As shown in Figure 17, Kao et al. deposited SiNx with different thicknesses on the surface of submicron trapezoidal cone arrays (TCAs) as a passivation layer. They found that SiNx with thickness of 80 nm has the lowest reflectivity λ<600nm, while TCAS with 120 nm SiNx has the highest power conversion efficiency. This study shows that thicker SiNx will have better passivation, but may reduce the antireflection effect of light capture structure. If SiNx is used as the passivation layer, the thickness of the film needs to be optimized to balance the relationship between antireflection and passivation to improve the overall efficiency of Si solar cells [235]. The scheme of using SiNx as passivation layer can also be further enhanced by some methods. For example, Duan et al. deposited SiNx passivation layer on the surface of Si nanowires and then performed rapid thermal annealing in the mixed atmosphere of H2 and N2. H2 molecules were transformed into atomic hydrogen and diffused into silicon, reducing the defect density on the surface of Si. Not only does it increases the carrier lifetime but also it reduces the series resistance. Finally, the overall efficiency of Si nanowire solar cells is relatively improved by more than 30% [236]. SiNx can not only passivate the surface defects of nanostructures caused by dry etching but also passivate the structural defects caused by the uniform distribution of some metal catalysts in silicon nanowires in VLS process, such as stacking errors. Chen et al. first passivated the c-Si surface with a thin SiNx thin film, then grown Si nanowires with VLS, and finally electrically isolated Si nanowires with SiNx film. This passivation scheme increases the effective carrier lifetime of silicon nanowires to 1.7 ms, while the effective carrier lifetime of nanowires grown directly from the surface of c-Si is only 50 μs [237].Figure 17
The measured reflectivity (a) and EQE (b) of different light trapping structures in the range of 400-1000 nm. The inset is the structural diagram of TCAS solar cell [235].
(a)(b)Many studies have confirmed that a-Si: H thin film has a satisfactory passivation quality in Si heterojunction solar cells. Dan et al. used a thin layer of a-Si for in situ surface passivation, reduced the surface recombination of SiNWs by nearly two orders of magnitude, and increased the carrier diffusion length and lifetime by one order of magnitude and two orders of magnitude respectively [238]. In fact, different intrinsic a-Si:H thin film stacking will significantly affect its passivation quality. According to the research of Luderer et al., void rich i-layer (i1)+(hydrogen plasma treatment) HPT+dense a-Si:H layer (i2) has the best passivation quality. Because such a stacking scheme allows hydrogen to penetrate the i1 layer and saturate the defects on the a-Si: H / c-Si interface, so as to further reduce the interface defect density. In addition, the role of i2 in the i-layer stack may also used as a “spacer” to shield a-Si:H(i1)/ c-Si defects from minority carriers at the well passivated a-Si:H (i1) / c-Si interface [239].Mayet et al. studied and compared several effective micron/nanopore (Figure18) surface-passivation schemes with a high surface to volume ratio, including SiO2 and Si3N4 deposited by PECVD, HF treatment, and thermal oxidation. As shown in Figure 18(c), the passivation results show that, although PECVD oxide–nitride–oxide (ONO) passivation does not cover the holes completely, the dark current of silicon photodiode (PD) is reduced by more than three orders of magnitude. In addition, the quantum efficiency was also improved. This is because the refractive index of SiO2 and Si3N4 thin films is lower than that of Si, which can reduce surface reflection. However, this passivation method is not conformal for micro/nanopores with high surface to volume ratios. Thermal oxidation passivation is very effective in reducing the dark-current by more than four orders of magnitude. As shown in Figure 18(e), the thermal oxide layer covers the whole surface of the pore conformally and hardly changes the pore size. The conformal oxide on the sidewall of the hole may change the interaction between light and hole. The thermal oxidation process may cause the dopant to diffuse into the intrinsic region, which results in the contraction of the effective width of the depletion layer and decrease of the external quantum efficiency (EQE). HF treatment can saturate the dangling bonds of silicon with hydrogen atoms and reduce the dark current by more than four orders of magnitude. Among these passivation methods, HF treatment is relatively simple and cost-effective, while other technologies may change the light-capture characteristics of silicon PDS slightly, which can affect the external quantum efficiency. The characteristics of these passivation technologies are compared using the passivation for a silicon photodiode. Because the operating principle of the photodiode is very similar to solar cells, a study of photodiode-passivation methods has direct relevance for solar cell nanostructures [240]. Although chemical passivation of acid solution looks very simple, the effect is excellent, and it has great advantages in cost and process compatibility, and it needs to be further study whether the method of chemical passivation is still applicable to solar cells with complex and high surface to volume ratio nanostructures. Field effect passivation by introducing Al2O3 has attracted much attention recently. Kim et al. prepared Si nanopillar arrays structure by MACE and then compared the passivation quality of intrinsic a-Si:H, thermal oxide, wet oxide, organic layer, and Al2O3. Finally, it was proved that Al2O3 passivation treatment showed the highest effective carrier life and open circuit voltage (Voc), which were 31.9 μs and 595 mV, respectively [241]. Because Al2O3 thin film can improve charge-transfer effectively in Si solar cells, regardless of whether the Si surface is planar or textured. SiO2 can passivate a Si surface in a similar way. In fact, the thicker the passivation layer is, the stronger is the effect of field effect passivation until it reaches a saturation state. The stacked Si/SiO2 multiple quantum wells can provide bandgap tunability to adapt to the solar spectrum using the quantum size effect. It is also possible to reduce the loss caused by the mismatch between the bandgap energy of the absorber material and the solar spectrum. Because these passivation layers are insulators, thick layers can significantly increase the series resistance of solar cells. A compromise needs to be considered for field-effect passivation [242–244].Figure 18
(a) SEM images of an actual PD device. (b) Enlarged SEM images of the deep hole arrays, in (a) the diameter and period of deep hole are 1.5μm and 2 μm, respectively. (c) Cross-section SEM images of the ONO passivation tri-layers on the deep hole area. (d) Cross-section view SEM images of deep hole sidewall. (e) Cross-sectional SEM of the oxide layer formed on side wall of deep hole by hydrothermal oxidation. (f) Enlarged SEM image of (e), the oxide layer thickness of the deep hole upper surface and sidewall is about 120 nm [240].Besides a-Si and SiO2 layers, Al2O3 is another rear-surface passivation layer with an efficiency exceeding 21% in p-Si solar cells. The highest efficiency reached 21.5% after AlOx passivation. Atomic layer deposition (ALD) enables the deposit of ultrathin Al2O3 layers. However, the PECVD deposition rate is much higher, which means it is more suitable for industrial preparation. Both methods are valuable substitutes for SiO2 and thick Al2O3 [246].Thin Al2O3 layers, which were deposited using ALD, are very promising because of their conformal growth on high aspect ratio surfaces and their ability to passivate silicon. Furthermore, there is a high concentration of fixed negative charges in these thin Al2O3 layers, which generates a strong back surface field. Repo et al. used RIE process to fabricate silicon nanowire texture on the surface of high and low resistance monocrystalline silicon. After Al2O3 passivation layer was deposited on the nanowire textured surface by ALD, it was found that the passivation quality of Al2O3 on the silicon nanowire surface was equivalent to that on the polished surface, and the effective carrier lifetime measured in all samples was in the order of milliseconds. The reflectivity measurement results show that the thin and conformal ALD layer even further reduces the reflectivity of the silicon nanowire array, and the reflectivity in the wavelength range of 250-1100 nm is less than 0.85% [247].Otto et al. compared the passivation properties of ALD-Al2O3 on the surfaces of micron pyramids, nanowires, and pyramid nanowire binary nanostructures prepared by ICP, MACE, electrochemical, and alkaline etching processes. They found that all nanostructures can be passivated well. The surface recombination velocity of ICP etched silicon nanowires is less than 8 cm/s, while the recombination velocity of nontextured silicon as a reference is 4.2 cm/s [248]. Other studies have also proved that Al2O3 coating has little effect on antireflection performance and has a good passivation under a wide range of doping density. The effective surface recombination velocity of B-Si structure with medium morphology passivated by Al2O3 is very close to that of polished planar silicon [249, 250]. Gemmel et al. fabricated a surface texture containing a blind hole on monocrystalline silicon using anodic etching. The group then passivated the surface with AlOx, which was deposited using ALD. The results show that the carrier recombination velocity of blind hole texture is between 1.72 and 2.2 cm/s. On the other hand, that of planar silicon, with a SiNx passivation layer is (4.66A^±0.68) cm/s. This suggests that AlOx indeed has an excellent surface passivation [251].Dou et al. studied an Al2O3 layer, which was deposited using ALD, as passivation layer on a silicon nanotexture surface is shown in Figure 19. The advantage of using ALD to deposit the Al2O3 passivation layer is that the size of Al2O3 is much smaller than that of nanostructure. Furthermore, Al2O3 can be deposited in the gap between nanostructures to ensure that the whole surface is covered. By comparison, the passivation of 80 nm SiNx is equivalent to that of 20 nm Al2O3. The effective lifetime of the carriers increased to 20μs. For 60 nm and 100 nm Al2O3, the effective carrier lifetimes increased to 70 and 110μs, respectively. On the other hand, the effective carrier lifetimes of the nonpassivated nanostructures are only 3μs. The group found that the deposited with Al2O3 thermal atomic layer can conformally cover the rough surface and reduce the defect density of nanostructures. In addition, the negative charge in the Al2O3 layer leads to surface field-effect passivation. The results of lifetime measurements show that the passivation of Al2O3 layer is much better than in the SiNx layer. This is due to the introduction of H and O during the deposition of Al2O3 by ALD, which saturates the defects on the nanostructured surface and produces a chemical passivation. Furthermore, the deposited Al2O3 layer has a fixed negative charge. The formed electric field can drive away the electrons on the surface, which enables field-effect passivation. The combination of the two passivation effects further enhances the surface passivation [245, 251].Figure 19
Transverse-sectional SEM view of the silicon nanotextured surface deposited with (a) 80 nmSiNx, (b) 30 nm Al2O3, (c) 60 nm Al2O3, and (d) 100 nm Al2O3 films. The deposited Al2O3 film conformally covers the surface of the nanowires and fills the gap between the nanowires. (e) Effective lifetimes (τeff) of Si nanowires with different passivation layers and without passivation [245].
(a)(b)(c)(d)(e)Although the passivation performance of Al2O3 is better than SiNx, it may be a better choice if Al2O3 and SiNx are combined [252]. Xu et al. passivated the serious Auger recombination and surface recombination on the surface of nanopore-micron pyramid (NPP) binary light trapping structure with ALD-Al2O3/PECVD SiNx. Thanks to the excellent chemical passivation and field effect passivation of Al2O3/SiNx double passivation layer on the surface of p-type silicon, the average PCE of NPP TOPCON solar cell is 23.55%, the short-circuit current density is 41.44mA/cm2, and the average effective carrier lifetime is 2667 μs [253].In addition, both the front and rear surfaces of silicon solar cells can be passivated by field effect. Nevertheless, the interface field direction of the passivation layer with different doping-types needs to be consistent with the built-in electric field of solar cells to enable a better passivation. To passivate the front surface, a 20 nm SiO2 layer was deposited on the front surface by evaporation of the electron beam. For the passivation of the back surface, a 20 nm layer of Al2O3 was evaporated on the back side of a p-type Si substrate using the same method. By introducing the oxide film for field-effect passivation, the internal electric field is strengthened, charge-transfer is likely, and the current leakage at the edge is well blocked. This helps improve the overall efficiency of the solar cells. The samples, where only SiO2 passivation was applied on the front surface and passivation of the front and rear surfaces was applied simultaneously are compared in Table 2. The open circuit voltage Voc, short circuit current density Jsc, and fill factor FF improved. The overall efficiency increased from 13.27% to 15.47%. The efficiency of Si solar cells increased from 9.81% to 15.04%, which means that the relative efficiency increased by more than 50% [244]. For the structure similar to monocrystalline silicon nanowires, due to the large surface fluctuation, it was difficult to achieve full coverage of the nanostructure surface using PECVD and other means. Hence, double passivation can be considered. First, a thin layer of SiO2 (approximately 10 nm) was grown on the surface of silicon nanowires using thermal oxidation before deposition of the passivation layer (e.g., SiNx). Thermal oxidation can produce a self-growing layer of SiO2 on the whole surface of the Si wafer, regardless of how it fluctuates. A double passivation layer for SiNx was deposited by PECVD on the surface of the silicon nanowires. Compared with conventional silicon nanowires, which were only covered with SiNx, the open circuit voltage Voc and short circuit current Isc of double-passivation silicon nanowire-texture solar cells increased by 2% and 10%, respectively. The effective carrier lifetime increased by 28% [254].Table 2
Photovoltaic parameters of Si thin film solar cells with different passivation methods [244].
SampleV (mV)JsmA/cm2FFη%Model52527.440.6939.81Textured53628.800.71611.08Textured+SiO254233.880.72313.27Model+SiO2+Al2O354732.340.77113.94Textured+SiO2+Al2O355234.740.78615.04In addition to the conventional passivation materials discussed above, the use of metals such as Ag, Cu, and Tl to achieve surface- or even bulk-passivation of monocrystalline silicon or polycrystalline silicon has attracted worldwide attention[255–257]. The mechanism for metal passivation is still chemical. Based on research on the passivation of porous polysilicon surfaces with Al, a 5-10μm aluminum layer was obtained by screen printing on the front surface of monocrystalline Si. The aluminum was then annealed at 500°C for 30 minutes to diffuse into Si and bond with the dangling bonds to improve the passivation quality. To eliminate the solid Al layer and obtain a p-doped monocrystalline Si surface, the sample was dissolved in a solution of HF:HNO3:H2SO4 with a ratio of (1:3:5). The results of the infrared spectra show that Si-Ox, Al-O-Si, and Si-Hx lead to surface defects and the passivation of dangling bonds. Finally, the effective carrier lifetime increased by 98%, and the internal quantum efficiency increased from 20% to 60% [258]. In general, the passivation of silicon solar cells with nanostructures is essential to improve the conversion efficiency. The use of field-effect passivation by introducing thin Al2O3 layers is promising. HF and HF-HNO3-H2O solution treatments have advantages with respect to maintaining conformity, low-cost, and the passivation quality. However, whether acid solution treatment is suitable for passivation of small-sized nanostructures needs further studies.


## 5. Conclusions and Outlook

As presented, based on the theoretical analysis, geometrical optics theory is applicable when the size of the micro/nanostructure is much larger than wavelength of the incident light. The effective medium theory is applicable to the case where the nanostructure size is much smaller than the wavelength of the incident light. When the size of the subwavelength structure is close to the incident light wavelength, the scattering theory is more suitable. SPR mainly depends on free electrons on the surface and inside the metal, which interact with the incident light wave under certain conditions. The electrons produce LSPR or SPPR, which confines light propagation within a certain region. When nanomanufacturing technology had not yet been developed, researchers mainly designed light-trapping structures for solar cells using the geometrical optics theory framework. Although a good antireflection effect could be achieved this way, the Lambertian limit could not be exceeded. With the development of effective medium theory, scattering theory, and surface-plasmon theory, researchers designed nanostructures with an average reflectivity of less than 10% in the visible and near-infrared bands. Some nanostructures were shown to exceed the Lambertian limit for a wide range of incident angles and wavelengths.For nanostructure fabrication, anisotropic alkaline etching, MACE, RIE-based dry etching, and VLS growth were introduced. Generally, because of the low cost and simple process, anisotropic alkaline etching and MACE are particularly suitable for the large-scale production of nanostructures on a monocrystalline silicon surface. Moreover, surface defects and damage of the nanostructures are relatively small, which lowers the requirements for passivation. By combining photolithography and low-cost SiO2 self-assembly mask technology, it is possible to optimize the composition and concentration of the etching solution and select the appropriate catalytic metal to prepare periodic arrays of nanopyramids and nanowire. The reflectivity of nanopyramid arrays is below 10% for the visible light range and even below 1% for the nanowire array. However, the types of nanostructures that are fabricated using these two methods are relatively simple, mainly nanopyramids, nanowires, and porous structures. RIE represents a more advanced nanofabrication method. Combined with predefined templates, RIE can accurately control the arrangement and morphology and produce nanostructures with better antireflection properties and even more-complex nanostructures. Although there are still some problems with RIE such as high cost and large surface roughness, some studies have shown that the surface passivation of silicon nanostructures can be integrated into the process of etching. RIE appears to be an ideal method to fabricate surface nanostructures of monocrystalline silicon solar cells in the future. Other etching methods, such as femtosecond-laser and focused ion beam, cannot be applied to large-scale nanostructure manufacturing due to their inherent drawbacks such as high cost. Nevertheless, the unique advantages of these methods and the capability to manufacture some special nanostructure morphologies deserve further detailed future studies. CVD-based VLS growth is the most important bottom-up method. Compared with the etching method, such methods consume less material and can grow on substrate surfaces other than silicon. The diameter, length, and morphology of nanowires, which are grown using VLS, are mainly affected by the metal catalysts. Therefore, attention should be paid to metal catalyst annealing and migration during the growth process. Specifically, this should be done to ensure that the silicon nanowires are uniform and perpendicular to the substrate to allow for effective application in solar cells. In addition, the ordered growth control of silicon nanowires, using low-cost AAM as templates, is a promising development direction. The diffusion of Au catalysts, which are commonly used in VLS, can deteriorate the electric properties of Si nanowires. Therefore, it is necessary to study the catalytic properties of Ag, Pt, and other metals to find suitable substitutes.Although the cost of chemical passivation is low and the process is simple, considering the integrity and hydrophobicity of nanostructures, its application is limited. The deposition of a field-effect passivation layer on the surface of nanostructures using ALD and PECVD is a promising passivation method. Field-effect passivation by introducing Al2O3 can effectively promote charge transfer in Si solar cells, while it hardly affects the optical properties of the nanostructures. When Al2O3 is deposited using ALD, the introduction of H and O can saturate the defects on the nanostructured surface leading to a desired chemical passivation. The highest efficiency of p-Si solar cells, which were passivated using AlOx, is 21.5%. Due to the deposition rate problem and cost of ALD, PECVD is more suitable for the large-scale deposition of a surface field-effect passivation layer. However, the coverage of nanostructures using PECVD is worse than that of ALD. To ensure a sufficient passivation, double passivation can be implemented.Based on this review, we can foresee that surface nanostructures of single-crystal silicon solar cells can be designed using effective medium theory, scattering theory, and surface plasmon theory. The reflectivity can be one to two orders of magnitude lower than the pyramid structure on the surface of commercial crystalline silicon solar cells. MACE, RIE, and VLS are currently the most important nanomanufacturing technologies, and the manufacturing cost has the potential to be reduced further. The nanomanufacturing methods can also be combined to produce composite nanostructures with even better light-trapping properties. Field-effect passivation may become the most important surface passivation method for nanostructures. Double passivation and simultaneous passivation of front and rear surfaces can greatly reduce electric losses associated with nanostructures. These measures will push the PCE of monocrystalline silicon as close to its theoretical limit as possible.



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*Source: 8139174-2022-07-25.xml*