Query: : Effect of Stacking Order on Bandgap and Its Potential Applications in Thin-Film Solar Cells

Candidate papers:

abstract: Phosphorene, a monolayer of black phosphorus, is promising for nanoelectronic applications not only because it is a natural p type semiconductor but also because it possesses a layer number dependent direct bandgap (in the range of 0.3 to 1.5 eV) On basis of the density functional theory calculations, we investigate electronic properties of the bilayer phosphorene with different stacking orders. We find that the direct bandgap of the bilayers can vary from 0.78 to 1.04 eV with three different stacking orders. In addition, a vertical electric field can further reduce the bandgap to 0.56 eV (at the field strength 0.5 V/A) More importantly, we find that when a monolayer of MoS2 is superimposed with the p type AA or AB stacked bilayer phosphorene, the combined trilayer can be an effective solar cell material with type II heterojunction alignment. The power conversion efficiency is predicted to be ~18 or 16% with AA or AB stacked bilayer phosphorene, higher than reported efficiencies of the state of the art trilayer graphene/transition metal dichalcogenide solar cells.
author_names: Jun Dai

Xiao Cheng Zeng
title: Bilayer Phosphorene: Effect of Stacking Order on Bandgap and Its Potential Applications in Thin Film Solar Cells.
venue: The journal of physical chemistry letters
year: 2014

abstract: Already, several technologies of polycrystalline thin film photovoltaic materials have achieved certified record small cell power conversion efficiencies exceeding 22% They are CdTe, Cu(In,Ga)(S,Se)2 (CIGS) and metal halide perovskite (PSC) each named after the light absorbing semiconductor material. Thin film solar cells and modules require very little active material due to their very high absorption coefficient. Efficient production methods with low materials waste, moderate temperatures, attractive cost structures, and favorable energy payback times will play a strong role in market development as thin film technologies reach full maturity, including mass production and the standardization of production machineries. In fact, the first two technologies have already been developed up to the industrial scale with a market share of several GW. In this review article, we outline similarities and differences between these high efficiency thin film technologies from both the materials and the industrial point of view. We address the materials characteristics and device concepts for each technology, including a description of recent developments that have led to very high efficiency achievements. We provide an overview of the CIGS industry players and their current status. The newcomer PSC has demonstrated its potential in the laboratory, and initial efforts in industrial production are underway. A large number of laboratories are experimenting through a wide range of options in order to optimize not only the efficiency but also stability, environmental aspects, and manufacturability of PSC. Its high efficiency and its high bandgap make PSC particularly attractive for tandem applications. An overview of all these topics is included here along with a list of materials configurations.
author_names: Michael Powalla

Stefan Paetel

Erik Ahlswede

Roland Wuerz

Cordula D Wessendorf

Theresa Magorian Friedlmeier
title: Thin film solar cells exceeding 22% solar cell efficiency: An overview on CdTe Cu(In,Ga)Se2 and perovskite based materials
year: 2018

abstract: Abstract Silicon nitride (SixNy) and silicon oxynitride (SiOxNy) are materials that can find multifarious application in solar cells by tuning their microstructure and optical properties. In this work we have investigated a series of SixNy and SiOxNy films deposited by rf PECVD at a low temperature of 200 degC having different compositional and optical properties by changing the gas mixture ratio during deposition resulting in varying refractive index materials. The effect of varying gas ratio on the composition of these materials has been investigated comprehensively by X ray Photoelectron Spectroscopy (XPS) and the bond analysis using Fourier Transform Infrared Spectroscopy (FTIR) thereby creating a material library to select materials of desired properties and bond composition for specific applications in solar cells. The structural and optical properties were also analysed using Micro Raman spectroscopy, X ray Diffraction (XRD) UV VIS NIR spectrophotometer and ellipsometry. The films were found to be amorphous, hydrogenated, compact, presents high deposition rates and needs lesser thermal budget. In order to demonstrate their multifaceted application potential, we have fabricated multilayer anti reflection coatings (ARC) novel Rugate ARC, dielectric Bragg mirrors using SiOxNy/SixNy multi layers as back reflectors for thin crystalline silicon (c Si) wafers as well as the passivation of c Si. The novel bilayer passivation stack over CZ p type c Si wafers using SiOxNy/SixNy presented a minority carrier lifetime of 169 ms with an implied Voc of 673 mV.
author_names: Anishkumar Soman

Aldrin Antony
title: Broad range refractive index engineering of SixNy and SiOxNy thin films and exploring their potential applications in crystalline silicon solar cells
year: 2017

abstract: Abstract Semi transparent solar cells have been well developed in recent years, and can potentially be applied in many areas. Sb2Se3 has also received an increasing amount of attention because of its excellent optoelectronic properties. In order to combine these advantages, we attempted to fabricate a semi transparent thin film Sb2Se3 solar cell which meanwhile maintains the high efficiency. The effect of thickness of Sb2Se3 thin film on the optical and electrical properties were investigated. The optical absorption and power conversion efficiency were improved for Sb2Se3 thin film solar cells as the increase of the thickness of Sb2Se3 thin film, however, the obtained 120 nm thick Sb2Se3 film was completely opaque. The optimized device was achieved with the thickness of 80 nm which exhibits the semi transparent property and almost the same efficiency as the 120 nm Sb2Se3 thin film solar cell. This work demonstrates that the fabricated device has potential for applications in small wearable devices and may be extended to large area glass face applications.
author_names: Zhiwen Chen

Xiaohai Guo

Huafei Guo

Changhao Ma

J H Qiu

Ningyi Yuan

Jianning Ding
title: Fabrication of a semi transparent thin film Sb2Se3 solar cell
venue: Materials Letters
year: 2019

abstract: Abstract Cu2ZnSn(S,Se)4 (CZTSSe) has gained significant attention as a solar cell absorber material owing to its great long time application potential. In this study, the precursors deposited by single quaternary target sputtering were annealed in different S/S Se ratios atmosphere and CZTSSe solar cells were prepared. The mechanism of the S/Se replacement process and its effect on the composition, morphology, bandgap, and performance of the CZTSSe solar cells were systematically investigated. It was found that a decrease in the Se content in the annealing atmosphere showed an effect on the growth and properties of the thin films similar to that shown by an increase in the S/(S Se) ratio. However, this effect was much milder than that of the increase in the S/(S Se) ratio. The S/(S Se) ratio of the CZTSSe thin films could be controlled by varying their S and Se contents in the annealing atmosphere and then adjusting their bandgaps. An increase in the bandgap improved the open circuit voltage and efficiency of the resulting solar cells. The cell annealed with the S:Se molar ratio of 1:3 showed an efficiency of 8.41% without anti reflection coating.
author_names: Chunhong Zeng

Yunfeng Liang

Longlong Zeng

Linquan Zhang

Jian Zhou

Ruijiang Hong
title: Effect of S/(S+Se) ratio during the annealing process on the performance of Cu2ZnSn(S,Se)4 solar cells prepared by sputtering from a quaternary target
year: 2019

abstract: Abstract Increasing global energy consumption together with environmental concerns has led to much interest in alternative, cleaner sources of energy such as solar photovoltaic. Researchers in the solar cell community have been looking for ways to reduce costs while maintaining or increasing already high efficiencies. A fundamental understanding of the materials under consideration is essential to rapid development of new technologies. The I III VI2 thin films offer promising systems for achieving high efficiency solar cells at lower costs. In fact, by tailoring the chemistry of the compounds it is possible to change the bandgap of the material in order to collect sunlight more efficiently. First of all, this thesis focuses on absorber layer material preparation and characterization, especially nanocrystalline thin films and consideration of both structural and electrical characteristics of such main cell absorber layer.The thesis examines how different preparation techniques and material usage could affect the properties of the synthesized thin films (absorber layer) In this study CuInSe2 and CuInS2 thin films were deposited onto ITO glass substrate using the electrodeposition technique in aqueous solution. The electrodeposited films were characterized by X ray diffraction (XRD) scanning electron microscopy (SEM) and energy dispersive X ray analysis (EDS) The annealing effects on the electrodeposited precursors were investigated. The chalcopyrite structure of CuInSe2/CuInS2 showed an enhancement of crystallinity after subsequent selenization/sulfurization treatment in Se/S atmosphere, respectively. XRD and SEM studies revealed a dramatic improvement of the crystalline quality of CIS films after annealing treatments. The optical properties of annealed CuInSe2 Se and CuInSe2 S thin films have been studied in order to determine the effect of annealing process in different selenium and sulfur atmosphere. In the second step we modified copper indium CuInxCryGa1 x ySe2 where x=0.4, y= (0.0, 0.1, 0.2, 0.3)superstrate layer by spin coating process. CuInxCryGa1 x ySe2 where x=0.4, y= (0.0, 0.1, 0.2, 0.3) nanoparticles have been synthesized firstly using a wet chemical hydrothermal method that is based on a non vacuum thermal process without any additional selenization process. Introducing different metal sources in an autoclave with ethylenediamine as solvent, CIGS nanoparticles were obtained at different temperatures range 190 230degC. The X ray diffraction (XRD) results confirmed the formation of a tetragonal CuInxCryGa1 x ySe2 chalcopyrite structure. Finally, we turned again to the study of the annealing temperature effect onKesterite materials but this time in those of very low cost materials and environmentally friendly Cu2ZnSnS4. We studied the growth of quaternary Cu2ZnSnS4 (CZTS) kesterite thin films by a single step electrochemical deposition followed by annealing at low temperature. The influence of different annealing atmospheres at constant annealing times (t 45 min) and fixed preparation controlling parameters; i.e. starting materials (precursor metal salts) solution concentration, time of deposition and electrodeposition potential. Structural, compositional, morphological, and optical properties, as well as photoelectrochemical properties were studied.
author_names: Suzan Saber
title: Synthesis and characterization of some nano selenides and their applications in solar cells.
year: 2018

abstract: Abstract Since the absorption spectrum of CuO is well matched with the spectrum of the sunlight, it is considered to have potential applications in some optoelectronic devices such as solar cells etc. In order to obtain better performance of CuO based devices, it is necessary to deeply understand the effect of structural parameters of thin films such as thickness on the physical properties of CuO thin films. In this study, the CuO thin films with different thicknesses were prepared by a facile sol gel method. The structural, morphological, compositional and optical properties of the samples were analyzed by X ray diffraction (XRD) scanning electron microscopy (SEM) X ray photoelectron spectroscopy (XPS) UV vis absorption spectra and photoluminescence, respectively. The results show that all samples have a cubic structure regardless of film thickness. However, the photoluminescence behavior that strongly depends on the film thickness is found. When the film thickness is thin, CuO films show a strong violet, green and red co emission. However, as the thickness increases, the fluorescence of the CuO films is rapidly quenched. It is proposed that the photoluminescence behavior of CuO thin films is closely related to the change of optical bandgap caused by the variation of crystallite size and we gave a tentative explanation for the luminescence changes.
author_names: Linhua Xu

Gaige Zheng

Shi-xin Pei

Junfeng Wang
title: Investigation of optical bandgap variation and photoluminescence behavior in nanocrystalline CuO thin films
year: 2018

abstract: The work presented in this thesis focuses on the investigation and improvement of the window stack of layers for thin film CdTe solar cells fabricated in the Center for Renewable Energy Systems Technology (CREST) laboratories. In particular the aim was to change the standard structure including TCO, high resistive transparent (HRT)layer and CdS which is limited by the low transparency of the CdS layer, to a better performing one. The first result chapter of the thesis describes the study of ZnO HRT layers. ZnO thin films were deposited by radio frequency (RF) magnetron sputtering with different structural, optical and electrical properties which were characterized by X ray diffraction, electron microscopy, spectrophotometry, Hall Effect method and 4 point probe. ZnO films were then incorporated in CdTe solar cells with the structure: FTO/ZnO/CdS/CdTe/Au back contact and the performance of these devices were compared with the film properties to single out trends and identify optimal film characteristics. By varying the deposition pressure of ZnO films, it was possible to increase their transparency and significantly increase their resistivity. While better transparency positively affected the solar cell current density output and efficiency, the resistivity of ZnO films did not show any clear impact on device efficiency. By increasing the deposition temperature the ZnO film grain size was increased. Increased FF was observed in devices incorporating ZnO layers with bigger grains, although this gain was partially counterbalanced by the Voc degradation, leading to a limited efficiency improvement. Finally the addition of oxygen had the main effect of increasing the resistivity of ZnO films, similarly to what happened with the increase of the sputtering pressure. In this case however, an improvement of FF, Jsc and efficiency was observed, especially at an O2/Ar ratio of 1% By simulating the solar cells behavior with SCAPS 1D, it was found that these performance change can be explained by the variation of interface properties, precisely the amount of interface defects, rather than by bulk properties. The study presented in the second result chapter focuses on magnesium doped zinc oxide (MZO) and the variation of its energy band structure. MZO was initially used as the HRT layer within a solar cell structure: FTO/MZO/CdS/CdTe/Au back contact. Sputtering MZO films with a target containing MgO 11 weight% and ZnO 89 weight% allowed for and increased band gap from 3.3 eV of intrinsic ZnO to 3.65 eV for MZO deposited at room temperature. Increasing the superstrate deposition temperature allowed for a further band gap increase up to 3.95 eV at 400 degC due mainly to an conduction band minimum upward shift. It was highlighted the importance to create a positive conduction band offset with the MZO layer conduction band slightly above the CdS conduction band, with an optimum found in this case to be 0.3 eV (efficiency 10.6 By creating a positive conduction band offset all the performance parameters (Voc, FF, Jsc, efficiency) significantly increased. One of the reasons for this improvement was found to be a diminished interface recombination due to a more ideal MZO/CdS band alignment. In the second part of this investigation the MZO was used as a replacement for the CdS in a simplified structure: FTO/MZO/CdTe/Au back contact. The concepts used to optimise the performance of these devices also involved tuning the conduction band alignment between MZO/CdTe and efficiencies of 12.5 were achieved with a at conduction band offset. The efficiency increase was achieved mainly thanks to a better transparency of the MZO layer and a higher Jsc output, compared to devices using a CdS buffer layer. The MZO buffers have been tested in combination with different TCOs. Results are presented in the third result chapter and showed that AZO is a good alternative to FTO working effectively in combination with MZO. AZO/MZO efficiency thin film CdTe solar cells (12.6% compared to 12.5% with FTO) It was found that increasing the IR transparency of the TCOs leads to a potentially higher Jsc. Achieving a better transparency was obtained by using TCOs with high mobility and lower carrier concentration (AZO and ITiO) and also by using a boro aluminosilicate glass with low iron content. ITiO yielded the best opto electrical properties among all the TCO materials. Devices incorporating ITiO however, showed lower performance then those using FTO and AZO. ITO/MZO windows also yielded poor performance. In addition, the ITO films deposited had a high carrier concentration leading to a high NIR absorption by plasma resonance and resulted not ideal for application in thin film CdTe PV.
author_names: Francesco Bittau
title: Analysis and optimisation of window layers for thin film CDTE solar cells
year: 2017

abstract: An improved technique for synthesis of Cu(In,Al)Se2 is demonstrated, associated with sequential electrodeposition of a stacked layer in the order of Cu/Al/In followed by annealing in selenium vapor. Many advantages such as adjustable constitutes composition, good polycrystalline structure, pure chalcopyrite phase, uniform and compact surface morphology are obtained compared to the film fabricated by conventional electrodeposition process. The film thickness and the concentration of each metal deposited were controlled by the flexibility parameters of deposition time. The influence of Al content on the crystal structure, surface morphology, photoelectrochemical performance, optical and electronic properties of the films were investigated. The crystal size decrease and the energy bandgap of Cu(In,Al)Se2 thin film increase gradually with the increasing Al were revealed. Impedance potential test reveals the manufactured Cu(In,Al)Se2 thin films are all p type semiconductor and the carrier concentration increases with the Cu/(Al In) and Al/(In Al) ratio. Photoelectrochemical investigation of Cu(In,Al)Se2 films verified that a higher photocurrent was obtained with a relative lower Al content due to a narrower bandgap leading to lower energy photon absorption and a lower carrier density and a larger grain size both benefitting the transfer of photogenerated carriers and decrease the recombination of charge carriers. The obtained optimum Cu(In,Al)Se2 thin film based solar cell has been theoretical modeling and simulated. A high PCE of 17.08% was gained implying its potential application in photovoltaic devices.
author_names: Jiahui Luo

Jie Jiang

Yaguang Shi

Jie Yi

Sui Yang

Rui Shan

Hongxing Li

Jianxin Zhong
title: Cu(In,Al)Se2Photovoltaic Thin Film Solar Cell from Electrodeposited Stacked Metallic Layers
year: 2020

abstract: To thoroughly disclose the role of siloxane terminated side chain with different substituent positions, three difluorobenzotriazole dithienylbenzodithiophene (FTAZ BDTT) based polymers PBZ 1Si, PBZ 2Si and PBZ 3Si with the siloxane terminated side chain on the FTAZ unit (PBZ 1Si) on the BDTT unit (PBZ 2Si) and both on BDTT and FTAZ unit (PBZ 3Si) respectively, were synthesized. The different side chain substitutions have slight influences on absorption behavior, thermal stability, and frontier molecular orbitals, but showing great effect on the aggregation of the polymers. Grazing incidence wide angle X ray scattering (GIWAXS) measurements reveal that, relative to PBZ 1Si with branched alkyl on the BDTT unit, polymers PBZ 2Si and PBZ 3Si, bearing the siloxane terminated side chains on the BDTT unit, exhibit smaller p p stacking distances and larger crystal coherence lengths (CL) suggesting that adopting the siloxane terminated side chain on the BDTT unit can promote the interchain p p interaction and the ordering of molecular packing. With IT M as the non fullerene acceptor, among the three polymers, the PBZ 2Si based active layer possesses the highest ordered crystals for both polymer and IT M as well as the purest domain, which affords efficient exciton dissociation, the most balanced hole electron transport, and reduced recombination, leading to the highest short circuit current density (Jsc) and fill factor (FF) and then the highest power conversion efficiency (PCE) of 11.14% In contrast, PBZ 1Si and PBZ 3Si based devices show lower PCEs of 8.98% and 9.92% respectively. Moreover, PBZ 2Si:IT M also exhibits good thickness tolerance and its thick active layer of 240 nm shows the most limited decreasing of efficiency after a storage for 77 days, supplying good potential for mass farbication. Our work suggests that the fine pairing of a siloxane terminated side chain and an alkyl side chain is benifical for the opitmizing of a conjugated polymer donor towards high performance non fullerene polymer solar cells.
author_names: Haiying Jiang

Feilong Pan

Lianjie Zhang

Xiaobo Zhou

Zhen Wang

Yao Nian

Cang Liu

Wei Tang

Qiao Ma

Z -H Ni

Mingjun Chen

Wei Ma

Yong Cao

Junwu Chen
title: Impact of Siloxane Terminated Side Chain on Photovoltaic Performances of Dithienylbenzodithiophene Difluorobenzotriazole Based Wide Bandgap Polymer Donor in Non fullerene Polymer Solar Cells.
venue: ACS applied materials interfaces
year: 2019