Microcrystalline Silicon n-i-p Solar Cells Deposited Entirely by the Hot-Wire Chemical Vapor Deposition Technique
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ABSTRACT We describe a series of microcrystalline (lic) silicon n-i-p solar cell devices fabricated entirely by the hot-wire chemical vapor deposition technique. These devices are deposited on flat stainless-steel at a substrate temperature below 250'C, and are evaluated using solar-cell performance and quantum-efficiency (QE) measurements. We explore the effect of crystallite size, as examined by X-ray diffraction, by varying the hydrogen-to-silane ratio from 5 to 40, while keeping the Jic-n and the gc-p layers the same. We find a significant blue shift of the QE peak and an enhancement of red response compared with a standard a-Si:H solar cell. The blue shift increases with increasing hydrogen-to-silane ratio. We attribute this shift to the i-layer becoming more n-type with increasing hydrogen dilution. We also use a hydrogen gas purifier and find a large improvement in device performance.
INTRODUCTION Hydrogenated microcrystalline silicon (ptc-Si:H) has recently attracted scientific and technological interest. This thin-film silicon material has been grown by various techniques, such as very-high-frequency (VHF) chemical vapor deposition (CVD), plasma-enhanced (PE) CVD, hot-wire (HW) CVD, and electron cyclotron resonance (ECR). This material differs from either crystalline or amorphous silicon. Therefore, the understanding of the growth, structural, electronic, and optical properties of this new type of material are of scientific interest. Technologically, the recent success of a microcrystalline silicon solar cell of 7.7% efficiency without light-induced degradation by the Neuchatel group [I] and the polysilicon solar cell of 9.2% efficiency by the Sanyo group [2] attracted the attention to the photovoltaic (PV) community. It may be an alternative material for thin-film silicon PV applications. Also, [tc-Si:H material has shown a relatively high mobility compared to a-Si:H [3] and is therefore a potential material for thin film transistors in flat-panel display applications. In this preliminary research, we focus on the PV application of lic-Si:H material. We intentionally select the direct device approach and choose a relatively low substrate temperature (below 250'C), aiming at a low-temperature process for jic-Si:H solar cell fabrication. The deposition parameters are not optimized. gc-Si:H n-i-p solar cells are deposited entirely by the HW-CVD technique. The advantages of the HW-CVD technique are a simple process, high deposition rates, good-quality material, and a wide range of deposition parameters to grow gc-Si:H material. We have three things in mind: can we make a reasonable jtc-Si:H n-i-p device entirely by HW-CVD? Can we make a high-quality ltc-Si:H 903 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society
Table 1. Deposition parameters of the i-layers and XRD results Sample
Substrate Temp. (°C)
Pressure (mT)
H2 Flow (sccm)
SiH 4 Flow (sccm)
(220) (nm)
THjtcl THjic2 Hnip3 Hnip4 Hnip5 Hnip6 THDnip398a
230-238 218-223 222-251 223-248 216-247 226-251 228-253
50 25 30 30 30 30 3
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