October 2003 • NREL/SR-520-34824 

P. Fauchet 

University of Rochester 

Rochester, New York

Porous Polycrystalline Silicon 

Thin Film Solar Cells 

Final Report 

24 May 1999–24 May 2002 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle • Bechtel 

Contract No. DE-AC36-99-GO10337 
October 2003 • NREL/SR-520-34824 

Porous Polycrystalline Silicon 

Thin Film Solar Cells 

Final Report 

24 May 1999–24 May 2002 

P. Fauchet 

University of Rochester 

Rochester, New York 

NREL Technical Monitor: R. Matson 

Prepared under Subcontract No. AAD-9-18668-06 

National Renewable Energy Laboratory 

1617 Cole Boulevard 

Golden, Colorado 80401-3393 

NREL is a U.S. Department of Energy Laboratory 

Operated by Midwest Research Institute • Battelle • Bechtel 

Contract No. DE-AC36-99-GO10337 
This publication was reproduced from the best available copy 

Submitted by the subcontractor and received no editorial review at NREL

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Table of Contents 

Statement of Purpose ........................................................................................................1 

Evolution of the Project Goals..........................................................................................1 

Core Concept – Electrochemical Etching of Silicon ........................................................2 

Graded Refractive Index Coatings....................................................................................3 

Major Accomplishments of This Project ..........................................................................5 

 1) Simulation..........................................................................................................5 

 2) Graded Index Etching Technique on P-Type Silicon Wafers............................7 

 3) Evergreen Solar Collaboration...........................................................................9 

 4) EBARA Solar Collaboration............................................................................11 

 5) Characterization Assistance from NREL.........................................................12 

Conclusions and Future Prospects for PSi Technology..................................................12 

References.......................................................................................................................13 

Appendix: List of Publications ......................................................................................15 

Journal Publications............................................................................................15 

Conference Presentations and Papers .................................................................15 

List of Figures 

Figure 1. The electrochemical etching mechanism in silicon ........................................3 

Figure 2. Four basic spatial refractive index profiles of thickness d..............................4 

Figure 3. Simulated reflectivity of 0% -100% porosity gradients with various 

thicknesses ......................................................................................................6 

Figure 4. Simulated reflectivity spectra of a complete and a partially graded PSi 

film of 100 nm thickness.................................................................................6 

Figure 5. Total transmission through the first 110 nm of silicon in solar cells with 

a PSi and a silicon nitride ARC ......................................................................7 

Figure 6. Reflectivity comparison of a 100 nm gradient index PSi film and an 80 nm 

homogeneous 75% porosity film formed on a polished p+ Si wafer..............9 

Figure 7. Substantial reflectivity reduction was achieved on a multicrystalline solar 

cell substrate after a dynamic PSi etching procedure .....................................9 

Figure 8. Light IV curves for a string ribbon solar cell with and without a graded 

index PSi ARC..............................................................................................10 

Figure 9. A refractive index profile determined using spectroscopic ellipsometry 

(500 nm excitation) is plotted next to a cross-sectional SEM image of the 

corresponding PSi film .................................................................................11 
1 

Porous Polycrystalline Silicon Thin Film Solar Cells 

Statement of Purpose 

Crystalline (including multicrystalline) silicon based devices dominate the present solar cell 

industry due to their durability, their relatively low cost, and the vast silicon knowledge base 

developed by the microelectronics industry. However, the high refractive index of crystalline 

silicon at solar wavelengths (approximately 3.5) creates large reflection losses that must be 

compensated for by applying antireflective (AR) coatings. Although highly efficient double and 

triple layer AR coatings are available, most manufactured crystalline silicon photovoltaics 

employ simple and inexpensive single layer AR coatings, with relatively poor AR properties. 

This project has focused on the development of a simple and low cost alternative to these 

deposited AR coatings, using an electrochemical etching technique to form AR layers of porous 

silicon (PSi). 

Evolution of the project goals

The initial focus of this project was to build on previous work performed in our group and 

investigate the use of PSi as an active material in a solar cell to take advantage of potential 

bandgap tuning, light trapping, and blue-harvesting photoluminescence effects. However, 

through our work on various substrate materials we have found that the electrical characteristics 

of thick (several microns) PSi layers needed for active devices are extremely sensitive to the 

substrate doping profiles and crystalline morphology. This sensitivity makes reproducibility 

difficult even in a well-controlled research environment, and therefore prospects for commercial 

thick-film PSi devices are limited. 

We have now focused our investigation on the use of thin PSi layers for AR coating applications. 

By isolating these layers in the heavily doped surface region of a solar cell, the electrical 

properties of the p-n junction are relatively unaffected. Provided that good electrical contacts 

can be made to the material under the porous layer, the electrical properties of the PSi film are 

not critical, and only the optical properties need to be optimized. 
2 

Core concept – electrochemical etching of silicon

PSi has been widely studied since the early 1990’s, due primarily to interest in its development 

as an efficient light emitting material for optoelectronic applications [1], but had been applied to 

solar cell research as early as 1982 [2]. The light trapping and anti-reflection properties of PSi, 

in addition to its simplicity of formation and broadly tunable morphology, make it particularly 

well suited for photovoltaic applications. It has been used as single or multilayer antireflective 

coatings (ARCs) by exploiting the tunability of the film’s effective index of refraction [3,4]. 

Demonstrated reflectivities tend to average between 10%-15% over the spectral range 350 nm - 

1000 nm, but average reflectivities as low as 2% have been shown with graded index layers [5]. 

Improvements of 50% in ISC have also been demonstrated while maintaining fill factor and 

VOC [6]. 

PSi results from the electrochemical dissolution of a silicon substrate in the presence of a 

hydrofluoric acid (HF) electrolyte and positive charge carriers (holes). Randomly formed pits in 

the surface form pores that propagate into the substrate when the dissolution reaction is favored 

at the pore tips. As illustrated in figure 1, this condition exists when current is passed through 

the electrochemical cell, where holes propagating toward the substrate surface first encounter 

fluorine ions at the pore tips. If all holes are consumed at the pore tips, the upper porous matrix 

is protected from dissolution, and a stable PSi layer can be formed [7]. The porosity (percentage 

of empty space) of the PSi formed at the pore tips is proportional to the current density through 

the cell at that time. Since the porosity at a given depth is controlled by the electrochemical 

current density, dynamic control of the current during the etch can produce porosity gradients or 

alternating layers of varying porosity within the film [8]. Porosity changes correspond to 

changes in the effective refractive index through the Looyenga effective medium approximation 

[9], and therefore the engineering of porous layers with desirable optical properties can be 

achieved. 
3 

The morphology of the PSi matrix that remains 

after electrochemical etching is not only 

dependent on current density and substrate 

doping, but is also affected by the HF electrolyte 

concentration, the duration of the etch, 

temperature, and ambient lighting conditions. 

Within this large parameter space, the resulting 

structure can have feature sizes that vary over a 

wide range, from nanometers to microns, 

depending on the nature of the etch. From the 

physical optics viewpoint, structures with sizes larger 

than the wavelength of incoming light enhance light 

trapping by reflecting incoming radiation in random 

directions [10]. Structures much smaller than the 

incoming wavelength alter the effective refractive 

index of the film and allow it to be tuned between that of crystalline silicon (c-Si) and air [9]. 

Direct control of the effective refractive index depth profile using the etching current density 

provides an excellent opportunity to form graded index ARCs with properties superior to discrete 

layer coatings [11]. Graded-index ARCs can eliminate or limit the large discontinuity in index 

of refraction at solar cell surfaces that is responsible for reflection losses. As mentioned earlier, 

wide bandwidth graded-index PSi ARCs with extremely low reflectivity have been 

demonstrated [5]. 

Graded refractive index coatings

Refractive index gradients in the direction of light propagation have been theoretically studied 

for more than 100 years [12]. Physical implementation of this concept for AR applications has 

also been investigated based on the evaporation of very thin (<<λ) alternating high/low index 

films that create an effective index gradient by varying the thickness ratios in these stacks 

[11,13]. Chemical dissolution of glass optical elements to produce a porous surface layer with 

Figure 1. The electrochemical etching 

mechanism in silicon. Holes drifting 

toward the substrate surface drive the 

electrochemical process that forms 

porous silicon. These holes are most 

likely to react with fluorine ions at the 

bottoms of pores or pits, deepening 

these features and creating a porous 

surface film. 
4 

broadband AR properties has also been demonstrated and simulated based on gradient index 

properties [14] and exhibit the durability necessary for commercial applications [15].

In past PSi studies, porosity gradients are mentioned primarily as artifacts in standard constant 

current etching [16], as a characterization tool for spectroscopic ellipsometry studies [17], and 

for rugate filter applications [18]. For silicon solar cells, one group has formed gradient index of 

refraction ARCs by gradually changing the porosity of discrete layers in a 20-layer stack. 

Although excellent broadband reflectivity <3% in the visible spectrum has been achieved, the 

overall thickness of these stacks (>1 micron) would considerably degrade the electrical 

characteristics of a solar cell [5].

As stated previously, the relatively high reflectivity of a silicon solar cell in the 350 nm – 1000 

nm spectral range is due to the large refractive index discontinuity that exists at the air-silicon 

interface (figure 2a). By placing a single layer ARC of intermediate refractive index on the 

silicon surface, this large index discontinuity is broken into two smaller steps (figure 2b), 

resulting in a lower broadband reflectivity. Very low narrowband reflectivity can also be 

achieved by exploiting the interference of light reflected from the two interfaces. Further 

reduction in broadband reflectivity can be 

achieved by adding additional intermediate 

index layers, thus breaking the air-silicon 

index discontinuity into smaller and smaller 

steps. Therefore, a gradient index ARC is 

the limit of this progression, where a single 

index discontinuity is replaced by a 

continuous transition from a low to a high 

index material (figure 2c). If this 

continuous index transition occurs over 

several wavelengths of optical path length, 

broadband reflectivity approaching zero 

can be achieved [12]. However, such films 

would be far too thick for practical solar 

cell applications. 

Figure 2. Four basic spatial refractive index 

profiles of thickness d. 

(a) No AR film 

(b) Standard single layer AR film (or constant 

 current PSi etching) 

(c) Theoretical 100% linear graded film (n(x) is 

 arbitrary) 

(d) PSi film with a partial gradient
5 

This present work has focused on PSi layers of ~100 nm thickness that would minimally affect 

the electrical characteristics of a typical industrial solar cell with a 300 nm junction depth. From 

our simulation, we have determined that for PSi layers of this thickness, the optimal solution 

combines a refractive index gradient with small discontinuities at both interfaces as shown in 

figure 2(d). This design incorporates a smaller magnitude index gradient that can be effective 

over shorter length scales and small discontinuities that can be optimized to create a region of 

destructive interference in the center of the broadband solar spectrum. In our dynamic etching 

procedure, this type of index profile is formed by initially applying a high current density (high 

porosity) to the cell and decreasing the current density (lowering porosity) to zero over the 

duration of the etch. These films are thin enough to be formed on silicon solar cells after emitter 

diffusion with minimal electrical degradation and offer superior AR characteristics relative to 

current commercially deposited ARCs. 

Major accomplishments of this project:

1) Simulation 

Although exact solutions to the problem of light propagation in a stratified medium (gradient 

index in the direction of propagation) exist, they are only available for a small number of 

specifically engineered gradient profiles [12]. Therefore, in our simulation, we slice arbitrary 

continuous profiles into many discreet homogenous layers, to approximate the index profile of 

interest. If the individual layer thicknesses are <<λ, the simulated film stack converges to the 

solution of the original continuous index profile. The reflectivity of thin film stacks can be 

easily calculated by multiplying the characteristic matrix of each layer with that of each adjacent 

layer until the resulting matrix, representing the characteristics of the entire stack, is obtained 

[19]. The reflectivity is then directly calculated from the elements of this resultant characteristic 

matrix. Our simulation calculates film reflectivity based on a specified spatial porosity profile 

and accounts for the dispersion of the silicon refractive index in the visible spectrum [20]. 

Absorption of light (at each wavelength) in the PSi film is also factored into the calculation. 

Using this technique, the characteristics of films with various thicknesses, porosity profiles, and 

interface discontinuities were investigated. 
6 

Using this simulation, the general behavior of 

graded refractive index films was 

demonstrated and an optimal thin film index 

profile (critical for PSi photovoltaic 

applications) was developed. Figure 3 

illustrates the simulated reflectivity of PSi 

films with a linear porosity gradient from 

100% at the air interface to 0% at the 

substrate. This complete porosity gradient 

corresponds to a refractive index profile that 

makes a continuous transition from 1.0 (air) 

to ~3.5 (silicon). The graphed data clearly demonstrates a continuous improvement in AR 

properties as film thickness increases. Thus, as film thickness becomes >>λ, broadband 

reflectivity can be almost completely suppressed as predicted by theory [12]. 

However, PSi films approaching 1 micron in 

thickness would consume the junction of most 

commercial solar cells during the etch, greatly 

degrading performance. For practical 

application in silicon solar cell manufacturing, 

a much thinner solution had to be found. This 

thickness constraint led to the discovery that 

small refractive index discontinuities at each 

interface will result in lower reflectivity for 

100 nm films. A plot of this effect is shown in 

figure 4, where an 80% - 20% gradient 

outperforms a 100% - 0% gradient. Although 

several concepts contribute to this observation, 

the introduction of two-interface interference 

effects is the primary factor in this lowered 

Figure 3. Simulated reflectivity of 0% -100% 

porosity gradients with various thicknesses.

The more gradually the porosity changes, the

lower the broadband reflectivity. 

Figure 4. Simulated reflectivity spectra of a

complete and a partially graded PSi film of

100 nm thickness. The broadband reflectivity

of the partially graded film is substantially

lower across the solar spectrum. At this film 

thickness, a graded index alone is less 

effective than a combined graded indexinterference coating design.
7 

reflectivity. Therefore this structure 

combines the features of gradient index and 

interference coatings to produce a high 

performance AR coating. This discovery 

has led to the development of efficient thin 

film PSi graded index AR films during this 

project. 

Another issue explored using this simulation 

is the surface absorption loss in cells with 

silicon nitride and PSi ARCs. This loss is 

considerable lower in devices with PSi films 

due to the nature of the etching process. 

Unlike deposited films, PSi AR technology 

consumes a portion of the surface material, 

converting it to a less absorbing material. 

Most commercial silicon solar cells have a 

diffused top junction and as a result of the diffusion process have a thin surface region with 

extremely high dopant (and defect) concentration. This surface region is not part of the space 

charge (active) region of the cell, and therefore any light absorbed near the surface is not 

converted into electrical energy, lowering the efficiency. However, by converting the surface 

region into a PSi ARC, this absorption loss is greatly reduced and AR benefits are also achieved. 

Figure 5 illustrates this point by simulating the transmission of light through the first 110 nm of a 

cell. The reduced surface absorption and enhanced AR properties greatly improve the 

transmission in the PSi covered cell. The transmission of a bare silicon substrate is also shown. 

This reduced surface absorption may prove to be the greatest benefit of PSi AR technology as the 

maximum efficiency limits for nitride coated cells is approached. 

2) Graded index etching technique on p-type silicon wafers 

Based on past experience within our group, heavily doped (0.01 ohm-cm) p-type silicon wafer 

substrates were used for the first proof of principle experiments with graded porosity etching. 

Figure 5. Total transmission through the first 

110 nm of silicon in solar cells with a PSi and

a silicon nitride ARC. The substantial 

improvement in blue response for the PSi 

coated sample is due to the conversion of the 

upper layer of silicon to PSi during the etch. 

PSi exhibits substantially lower absorption than 

silicon in this spectral range and therefore 

allows more light to pass through to the 

underlying junction. The transmittance of a

bare substrate is also shown.
8 

Such substrates have been previously etched to form PSi with a broad porosity range [8,10] and 

offer consistent material properties from sample to sample. The extremely planar surface of 

silicon wafers also enables the formation of optical quality PSi films that can be directly 

compared to simulation results. 

A computer controlled etching system was first designed that would be capable of applying a 

continuously changing current to the electrochemical etching cell. The input parameters 

consisted of the initial current density, the duration of the etch, and the mathematical function 

followed by the current density throughout the etch. The current functions were kept fairly 

simple, starting at the specified initial current value and continuously decreasing to zero 

following cosine or power law dependencies. Such functions would therefore form PSi films 

with high porosity (high current) near the surface and lower porosity near the substrate interface. 

This type of PSi etching has been termed “dynamic etching” by our group. 

After completion of the dynamic etching setup, a large set of experiments was designed to gain 

an understanding of how the current function parameters influence the characteristics of PSi 

films. Because many variables affect the PSi properties, we first decided to etch all samples in 

the dark to eliminate ambient lighting as a potential variable. After testing several different 

solution concentrations, it was determined that a 1:2 (HF:ethanol) mixture yielded the most 

uniform and repeatable films over a wide range of current densities. 

The remaining parameters involve the dynamics of the etch itself and were optimized by 

preparing a matrix of samples in which the initial current density, the etching time, and the 

current function were variables. The reflectivity of all samples was measured, and SEM images 

were taken of a critical subset of samples. This optimization focused on minimizing film 

reflectivity over the spectral range 350 nm to 1000 nm while maintaining thicknesses of ~100 

nm. The lowest average reflectivity in this experiment was 8.1% compared to 41.2% for a bare 

wafer. This result was for a PSi film etched for 5 seconds with an initial current density of 100 

mA/cm2

 with a parabolic (I(t)~(t-to)

2

) time dependence, where to is the duration of the etch. The 

measured and simulated reflectivity spectra of this film is plotted in figure 6. Also plotted is the 

measured reflectivity of a homogeneous (constant refractive index) PSi film. The simulation 
9 

closely matches the experimental spectra and 

indicates that the porosity varies from 94% to 

33%. A film thickness of 100 nm was 

determined from the simulation and was 

confirmed by SEM images. In comparison to a 

homogeneous PSi film, the graded index sample 

exhibits substantially lower reflectivity in the 

blue portion of the spectrum. 

3) Evergreen Solar collaboration 

Through a collaboration with Evergreen Solar 

(initiated through a discussion at an NCPV 

program review meeting), our group was 

provided with non-metallized string ribbon 

multicrystalline silicon solar cell substrates. The 

goal of this collaboration was to apply our PSi 

AR technology to commercial cells. Our 

experiments using silicon wafer substrates 

demonstrated the potential advantages of graded 

index films, however because this process is 

substrate dependent, a study of film properties on 

working cells was necessary. String ribbon is an 

inexpensive, thin (~200 microns) multicrystalline 

silicon substrate material formed directly from a 

silicon melt [21]. The surface of these substrates 

have some intrinsic roughening due to crystalline 

defects and grain boundaries, and are doped with 

a phosphosilicate glass diffusion process. As a 

low-cost process, PSi ARC etching is particularly 

well suited for this type of low cost 

multicrystalline silicon substrate. 

Figure 6. Reflectivity comparison of a 100 

nm gradient index PSi film and an 80 nm 

homogeneous 75% porosity film formed on a

polished p+ Si wafer. A simulated layer with 

a porosity gradient from 94% to 33% shows 

excellent agreement with the measured

reflectivity. 

Figure 7. Substantial reflectivity reduction 

was achieved on a multicrystalline solar cell 

substrate after a dynamic PSi etching 

procedure. The PSi film thickness was 107 

nm determined through SEM cross-sectional 

analysis. The reflectivity of a standard

silicon nitride ARC and the terrestrial solar

power spectrum are plotted for reference.
10 

Several test etches determined that uniform PSi films can be formed on these substrates using the 

same process used on the wafer substrates. A similar optimization process determined that the 

same etching conditions could be used, but a longer (7 seconds) etch duration achieved the 

lowest reflectivity. Figure 7 shows the reflectivity of an optimized graded index PSi film relative 

to a bare string ribbon substrate and an evaporated silicon nitride. The terrestrial AM1.5 solar 

spectrum is also plotted for reference. The PSi film had an average reflectivity of 3.7% across 

this spectral range, compared to 10.1% for the silicon nitride ARC and 38.1% for the bare string 

ribbon substrate. 

The reflectivity of graded index PSi films on string ribbon substrates is considerably lower than 

that obtained on the initial experiments on silicon wafer substrates. This result was expected 

because the reflectivity of the bare string ribbon substrates is lower than that of a silicon wafer 

due to its intrinsic surface structure. The surface structure and the dopant profile will also affect 

the resulting PSi films because the etch is sensitive to these material properties. 

After demonstrating the low reflectivity of 

these films, the electrical properties were 

examined to determine whether any 

degradation in the p-n junction characteristics 

resulted from the etching process. As 

illustrated by the light IV data in figure 8, both 

the short circuit current and the open circuit 

voltage of our test cell improved while 

maintaining the fill factor. This indicates that 

that etching of a thin (~100 nm) PSi films 

minimally affects the properties of the 

underlying junction material. Thicker films did 

exhibit degradation in the light I-V curves, 

stressing the need for a thin film solution. 

Figure 8. Light IV curves for a string ribbon 

solar cell with and without a graded index PSi 

ARC. Improvements in both the short circuit 

current density and the open circuit voltage 

were achieved by incorporating a PSi ARC.
11 

4) EBARA Solar collaboration 

Dendritic web multicrystalline silicon from EBARA Solar was also investigated for 

compatibility with the dynamic PSi etching treatment. This is another type of inexpensive thin 

(~ 100 microns) multicrystalline substrate formed directly from a silicon melt [22]. However, 

the appearance and material properties of these substrates are substantially different than string 

ribbon silicon. Dendritic web silicon has a very smooth surface with little roughening and all 

crystal grains exhibit a preferential (111) orientation. The EBARA cells also have a rear junction 

and the bulk of the cell material is n-type. EBARA Solar provided us with non-metallized 

sample cells to determine the effectiveness of the dynamic etching procedure on their substrates. 

These substrates were easily etched, but finding optimal etching conditions proved difficult 

because a 1:2 (HF:ethanol) solution did not produce low reflectivity films. Several other 

solutions were tried and a 1:1 (HF:ethanol) mixture yielded the best results. Unfortunately, we 

ran out of substrates before the current conditions could be completely optimized. The best films 

had an average reflectivity of 12%-15%, however we are confident that additional experiments 

could greatly improve upon these results. 

Figure 9. A refractive index profile determined using spectroscopic ellipsometry (500 nm

excitation) is plotted next to a cross-sectional SEM image of the corresponding PSi film. The

two distinct regions of this profile may indicate that a transition region exists, where individual

pores terminate at different depths, creating a large index gradient at the PSi-Si interface. The

general morphology of a graded porosity film can also be observed in the SEM image.
12 

5) Characterization assistance from NREL 

Several samples were tested at the NREL characterization laboratory to gain additional 

information about the material properties of the PSi films and substrates. Secondary Ion Mass 

Spectrometry (SIMS) analysis was used to determine the dopant profile in several solar cell 

substrates and to gain information about the residual elements that remain in the PSi film after 

etching. Spectroscopic ellipsometry was also employed to obtain the approximate shape of the 

refractive index gradient that exists in the PSi films. Figure 9 shows an index profile and an 

SEM cross-sectional image of a typical graded index PSi film. Two distinct regions are visible 

in the index profile that may correspond to a graded porosity region and a transition region where 

the porous material mixes with the solid substrate. This transition region also appears more 

dense relative to the surface region in the SEM image. 

Conclusions and future prospects for PSi technology

This project has successfully demonstrated the potential AR performance improvements that 

could be achieved through the application of graded index PSi technology to commercial 

multicrystalline silicon solar cells. The advantages of this process include: 

1) Lower reflectivity then single layer deposited AR films 

2) Reduced surface absorption relative to cells with deposited AR films 

3) No vacuum processes or hazardous gasses are necessary 

4) Film formation in >10 seconds with good repeatability 

5) PSi etching is relatively independent of crystalline orientation, unlike alkaline texturization 

that requires (100)-orientated substrates. 

6) Simultaneous removal of phosphosilicate glass remaining after dopant diffusion 

7) Potential usage as a substrate cleaning process 

8) Could be implemented after front side metallization, preventing any disruption of existing 

manufacturing processes 

Future work on PSi AR technology must be conducted in direct partnership with a solar cell 

manufacturer. Large volume availability of substrates and detailed knowledge of companyspecific manufacturing processes are needed for further progress toward the adaptation of this 
13 

technology for commercial use. In addition, the ability to customize manufacturing processes 

like dopant diffusion and screen print metallization would be helpful. A final recommendation 

would be to experiment with forming PSi after the front surface metal grids are deposited. 

During this project, all front contacts were formed photolithographically after PSi formation. 

We believe the PSi layer added series resistance to these contacts, and therefore improved 

electrical characteristics would be expected if etching followed metallization. Although the 

photovoltaic community does not consider wet electrochemistry a “standard” process, we believe 

that substantial efficiency improvements, particularly blue response, can be achieved with 

minimal investment, if this technology is implemented. 

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York 1997 (p. 249). 

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14 

[18] M.G. Berger, R. Arens-Fischer, M. Thonisson, M. Kruger, S. Billat, H. Luth, S. Hilbrich, 

W. Theiss, P. Grosse, Thin Solid Films 297, 237 (1997). 

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Solar Energy Materials and Solar Cells, 65, 621 (2001). 
15 

Appendix: List of Publications 

Journal Publications: 

C.C. Striemer and P.M. Fauchet, “Dynamic Etching of Silicon for Broadband Antireflection 

Applications,” Appl. Phys. Lett., to be published (October 2002). 

C.C. Striemer and P.M. Fauchet, “Dynamic Etching of Silicon for Solar Cell Applications,” 

Phys. Stat. Sol. A, to be published (September 2002). 

Conference Presentations and Papers: 

Porous Semiconductors Science and Technology Conference – March 2002 

C.C. Striemer and P.M. Fauchet, “Dynamic Etching of Silicon for Solar Cell Applications,” 

Phys. Stat. Sol. A, to be published (September 2002). 

National Center for Photovoltaics Program Review Meeting – October 2001 

C.C. Striemer, F. Shi, and P.M. Fauchet, “Electrochemical Etching of Silicon for Inexpensive 

and Effective ARCs,” National Center for Photovoltaics Program Review Meeting, October 

2001, Lakewood, CO, pp. 305 (2001). 

11th Workshop on Crystalline Silicon Solar Cell Materials and Processes – August 2001

C.C. Striemer, F. Shi, and P.M. Fauchet, “Rapid ARC Formation Using a Simple and Versatile 

Etching Process,” 11th Workshop on Crystalline Silicon Solar Cell Materials and Processes, pp. 

194 (2001). 

199th Meeting of the Electrochemical Society – March 2001 

C.C. Striemer, F. Shi, and P.M. Fauchet, “Electrochemical Etching of Silicon Substrates for 

Photovoltaic Applications”, Photovoltaics for the 21st Century II, edited by R.D. McConnell and 

V.K. Kapur, Vol. 2001-10, pp. 370 (2001). 

28th IEEE Photovoltaic Specialists Conference – September 2000 

C.C. Striemer, F. Shi, P.M. Fauchet, S.P. Duttagupta, “Porous Silicon Texturing of Polysilicon 

Substrates,” 28th IEEE Photovoltaic Specialists Conference, Anchorage, AK, September 2000, 

pp. 932 (2000). 

National Center for Photovoltaics Program Review Meeting – April 2000 

C.C. Striemer, F. Shi, P.M. Fauchet, “Porous Silicon Texturing of Polysilicon Substrates,” 

National Center for Photovoltaics Program Review Meeting, April 2000, Denver, CO, pg. 169 

(2000).
REPORT DOCUMENTATION PAGE Form Approved 

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE

October 2003 

3. REPORT TYPE AND DATES COVERED 

Subcontract Report 

24 May 1999–24 May 2002 

4. TITLE AND SUBTITLE

Porous Polycrystalline Silicon Thin Film Solar Cells: Final Report, 

 24 May 1999–24 May 2002 

6. AUTHOR(S) 

 P. Fauchet 

5. FUNDING NUMBERS

PVP32601 

AAD-9-18668-06 

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 

 University of Rochester 

 Rochester, New York 14627

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REPORT NUMBER 

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National Renewable Energy Laboratory 

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Golden, CO 80401-3393

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NREL/SR-520-34824 

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NREL Technical Monitor: Richard Matson 

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13. ABSTRACT (Maximum 200 words): The initial focus of this project was to build on previous work performed in our group and investigate 

the use of porous polycrystalline silicon (Psi) as an active material in a solar cell to take advantage of potential bandgap tuning, 

light trapping, and blue-harvesting photoluminescence effects. However, through our work on various substrate materials, we have 

found that the electrical characteristics of thick (several microns) PSi layers needed for active devices are extremely sensitive to 

the substrate doping profiles and crystalline morphology. This sensitivity makes reproducibility difficult even in a well-controlled 

research environment, and therefore, prospects for commercial thick-film PSi devices are limited. We have now focused our 

investigation on the use of thin PSi layers for antireflective coating applications. By isolating these layers in the heavily doped 

surface region of a solar cell, the electrical properties of the p-n junction are relatively unaffected. Provided that good electrical 

contacts can be made to the material under the porous layer, the electrical properties of the PSi film are not critical, and only the 

optical properties need to be optimized.

14. SUBJECT TERMS: PV; silicon thin films; devices; solar cells; antireflective (AR) coating; 15. NUMBER OF PAGES

electrochemical etching; porous silicon (PSi); photoluminescence; short-circuit current; 

open-circuit voltage 16. PRICE CODE

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