Conference Paper

NREL/CP-520-45965

June 2009

Epitaxial Thin Film Silicon Solar 

Cells Fabricated by Hot Wire 

Chemical Vapor Deposition 

Below 750oC 

Preprint 

K. Alberi, I.T. Martin, M. Shub, C.W. Teplin, 

E. Iwaniczko, Y. Xu, A. Duda, P. Stradins, 

S.W. Johnston, M.J. Romero, H.M. Branz, and 

D.L. Young

To be presented at the 34th IEEE Photovoltaic Specialists Conference

Philadelphia, Pennsylvania

June 7–12, 2009
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1 

Epitaxial Film Silicon Solar Cells Fabricated by Hot Wire Chemical Vapor Deposition 

Below 750°C

Kirstin Alberi, Ina T. Martin, Maxim Shub, Charles W. Teplin, Eugene Iwaniczko, Yueqin Xu, Anna Duda, Paul Stradins, 

Steve W. Johnston, Manuel J. Romero, Howard M. Branz, and David L. Young

National Center for Photovoltaics, National Renewable Energy Laboratory, Golden, CO 80401

ABSTRACT

We report the fabrication of film c-Si solar cells on Si 

wafer templates by hot wire chemical vapor deposition. 

These devices, grown at glass-compatible temperatures 

below 750° C, demonstrate open-circuit voltages greater 

than 500 mV and efficiencies above 5%. Analysis of the 

device characteristics and quantum efficiency provides 

important information about the epitaxial c-Si absorber 

material quality as a function of growth temperature.

INTRODUCTION

Film crystal silicon solar cells produce energy at much 

lower costs than Si wafer-based photovoltaics due to 

reduced material and energy consumption. We are 

working to grow silicon films epitaxially on crystalline seed 

layers affixed to inexpensive foreign substrates [1-3]. 

Through the integration of light trapping schemes, cells 

with 2-10 μm absorber layers could realistically display 

efficiencies above 15%. Such efficiencies will require 

growth of high quality absorber layers on substrates that 

are constrained to low temperatures compared with those 

used in conventional c-Si epitaxy. Thus, the development 

of a scalable, low-T high-quality epitaxial silicon growth 

technique is critical. Here we report on the realization of 

single crystalline thin film Si solar cells with efficiencies 

over 5%, grown on Si wafer templates by hot wire 

chemical vapor deposition (HWCVD) at glass compatible 

temperatures ≤ 750°C. Our results yield a consistent 

picture of the relationship between the absorber material 

quality and device performance and suggest means of 

improvement. 

EXPERIMENTAL

Hot wire chemical vapor deposition is a scalable 

approach to low temperature Si epitaxy that has been 

shown to produce c-Si layers with controlled dopant 

concentrations and majority carrier mobilities that are 

nearly equal to those of bulk crystalline material [4]. The 

simple reactor design and high deposition rates (almost 

300 nm/min has been demonstrated), make HWCVD an 

economical route to film c-Si production. In order to 

demonstrate the feasibility of growing high quality c-Si 

films at display-glass compatible temperatures (620 - 

750°C), our initial efforts have focused on assessing the 

performance of devices fabricated on heavily-doped (100) 

single crystal wafer templates. Cells consisting of a 2 μm 

epitaxial n-type (1016 cm-3 P) base layer were fabricated 

on an electronically dead (1019 cm-3 As) n++ wafer and 

finished with a hydrogenated amorphous Si 

heterojunction, as displayed in Fig. 1. The area of each 

device was 0.05 cm2

, so no metal front grid was used. 

The substrate temperatures, Tsub, for the growth of the 

epitaxial layer ranged from 650°C to 750°C, and no postdeposition hydrogenation or annealing treatments were 

applied to these devices. There is no intentional texturing 

or other light-trapping structure.

Fig. 1. Device structures for n- epitaxial thin film c-Si cells 

fabricated on heavily-doped Si wafer and seed-on-glass 

substrates.

RESULTS AND DISCUSSION

Representative current-voltage (J-V) characteristics for 

two cells, one grown at 655°C and one at 710°C, are 

displayed in Fig. 2. Both the open circuit voltage (VOC) 

and short circuit current (JSC) exhibit enhancement with an 

increase in Tsub, indicating an improvement in the absorber 

crystalline quality. In fact, these results (VOC = 0.51 V, 

efficiency = 5.11%), demonstrate that the quality of 
2 

HWCVD films is competitive with other epitaxial 

technologies [5]. 

Fig. 2. Current-voltage characteristics for devices grown 

on Si wafer substrates at 710° C (green) and 655° C (red). 

Internal quantum efficiency (IQE) measurements, 

displayed in Fig. 3 for the same cells, provide additional 

insight into the absorber quality. The cell grown at 710° C 

demonstrates up to 70% conversion of photons in the 500-

800 nm range. Furthermore, the relatively high IQE in the 

600-1000 nm range compared to that expected from a 2 

μm cell of ideal material quality (no light trapping) 

suggests that most of the carriers generated at the back of 

the absorber are collected. We note here that in this 

device configuration it is possible to collect up to a 1 

mA/cm2 of current from the heavily-doped wafer substrate, 

however some degree of unintentional light trapping from 

a rough back interface is conceivable as well. Analysis [6] 

of the inverse IQE vs absorption depth (1/α) yields an 

effective diffusion length, Leff, of roughly 7 μm, which is 

more than three times the absorber thickness. In 

comparison, the device grown at 655° C exhibits lower

IQE at all wavelengths, and the calculated Leff =1.5 μm is 

less than the absorber thickness. 

The performance of these cells is consistent with the 

material quality ascertained by other means. Average 

lifetime values of approximately 10 ns were determined by 

microwave photoconductive decay in identically grown 40 

μm films. The measured device J-V parameters are also 

reproduced in PC1D simulations using nanosecond 

minority carrier lifetimes in the absorber layer. 

Considering the significant number of extended defects 

observed in these films by optical microscopy, post-growth 

rapid thermal annealing and hydrogenation steps are 

expected to substantially enhance the device 

performance. Enlargement of the cell area and the 

addition of a front metal grid should also improve the fill 

factor.

Fig. 3. Internal quantum efficiency spectra for devices 

grown on Si wafer substrates at 710° C (green) and 655°

C (red). 

While we have made significant progress in the 

fabrication of low-temperature epitaxial materials on single 

crystalline substrates, the transfer of an optimized 

HWCVD epitaxial growth routine to the deposition of c-Si 

on a seed layer is the true test of this technology. We 

have fabricated our first device on a layer-transferred Sion-glass substrate, the structure of which is also shown in 

Fig. 1. In this configuration, the back contact was made 

by etching down to an n+ epi layer (1018 cm-3 P) after all of 

the layers were deposited. The total absorber thickness 

including the n+ and n- epitaxial c-Si layers was 

approximately 5 μm. Current-voltage characteristics for 

this cell are displayed in Fig. 4. The low VOC = 0.22 V 

indicates that the absorber material quality did not 

approach that grown on Si wafers and is most likely limited 

by the lower temperature required to accommodate the 

glass substrate as well as the rough surface of the Si 

template. Overall, the device demonstrated an efficiency 

of 0.42 %. We anticipate significant improvement, 

however, with optimization of the cell structure and growth 

conditions. 

CONCLUSIONS

In summary, thin film c-Si solar cells grown by HWCVD 

show promise as an inexpensive alternative to waferbased PV. Model devices fabricated on Si wafer 

templates at glass-compatible growth temperatures 

demonstrate open circuit voltages above 0.5V and 

efficiencies greater than 5%. Initial devices were also 

demonstrated on seed-on-glass substrates. Postdeposition treatments are expected to lead to significant 

improvement in cell performance, as the epitaxial absorber 
3 

material quality is constrained by the glass-compatible 

deposition temperatures.

Fig. 4. Current-voltage characteristics for a 0.05 cm2

device grown on a seed-on-glass substrate (see Fig. 1 for 

cell structure).

We acknowledge Scott Ward, Lorenzo Roybal and Qi 

Wang for helpful suggestions and Steve Droes of Sharp 

Labs for providing layer transferred silicon on glass 

substrates. Financial support was provided by the U.S. 

Department of Energy under Contract No. DE-AC36-

08GO28308

REFERENCES

[1] W. Fuhs, et al., “A Novel Route to a Polycrystalline 

Silicon Thin-Film Solar Cell”, Sol. Energy 77 (6), 2004, pp.

961

[2] A.G. Aberle, et al., “Polycrystalline Silicon Thin-Film 

Solar Cells on Glass by Aluminum-Induced Crystallization 

and Subsequent ion-Assisited Deposition (ALICIA)”, Progr. 

Photovoltaics 13, 2005, pp. 37

[3] C.W. Teplin, D.S. Ginley, H.M. Branz, “A New 

Approach to Thin Film Crystal Silicon on Glass: BiaxiallyTextured Silicon on Foreign Template Layers”, J. NonCrystalline Solids, 352, 2006, pp. 984

[4] I.T. Martin, et al., “Doping of High-Quality Epitaxial 

Silicon Grown by Hot-Wire Chemical Vapor Deposition 

Near 700 C”, Thin Solid Films, 517, 2009, pp. 3496

[5] P. Dogan, et al., “Structural and Electrical Properties of 

Epitaxial Si Layers Prepared by E-beam Evaporation”,

Thin Solid Films, 516 , 2008, pp. 6989

[6] P.A. Basore, “Extended Spectral Analysis of Internal 

Quantum Efficiency”, Twenty Third IEEE PVSC, 1993, pp. 

147
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Epitaxial Thin Film Solar Cells Fabricated by Hot Wire Chemical 

Vapor Deposition Below 750°C: Preprint

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DE-AC36-08-GO28308

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6. AUTHOR(S)

K. Alberi, I.T. Martin, M. Shub, C.W. Teplin, E. Iwaniczko, Y. Xu, A. 

Duda, P. Stradins, S.W. Johnston, M.J. Romero, H.M. Branz, and 

D.L. Young

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NREL/CP-520-45965

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14. ABSTRACT (Maximum 200 Words)

We report the fabrication of film c-Si solar cells on Si wafer templates by hot wire chemical vapor deposition. These 

devices, grown at glass-compatible temperatures < 750°C, demonstrate open-circuit voltages > 500 mV and 

efficiencies upward of 5%. Analysis of the device characteristics and quantum efficiency provides important 

information about the epitaxial c-Si absorber material quality as a function of growth temperature.

15. SUBJECT TERMS

PV; hot-wire chemical vapor deposition; polycrystalline silicon; devices; solar cells; fabrication; thin film; epitaxial 

absorber; materials quality; growth temperature

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