Microcrystalline Silicon for Solar Cells at High Deposition Rates by Hot Wire Cvd
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MICROCRYSTALLINE SILICON FOR SOLAR CELLS AT HIGH DEPOSITION RATES BY HOT WIRE CVD R. E. I. Schroppa, Y. Xu, E. Iwaniczko, G. A. Zahariasb, and A. H. Mahan a
National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401, USA Utrecht University, Debye Institute, Physics of Devices, 3508 TA Utrecht, The Netherlands b Chem. Engineering Dept., Stanford University, Stanford, CA 94305, USA
ABSTRACT We have explored which deposition parameters in Hot Wire CVD have the largest impact on the quality of microcrystalline silicon (µc-Si) made at deposition rates (Rd) > 10 Å/s for use in thin film solar cells. Among all parameters, the filament temperature (Tfil) appears to be crucial for making device quality films. Using two filaments and a filament-substrate spacing of 3.2 cm, µc-Si films, using seed layers, can be deposited at high Tfil (~2000°C) with a crystalline volume fraction > 70-80 % at Rd's > 30 Å/s. Although the photoresponse of these layers is high (> 100), they appear not to be suitable for incorporation into solar cells, due to their porous nature. n-i-p cells fabricated on stainless steel with these i-layers suffer from large resistive effects or barriers, most likely due to the oxidation of interconnected pores in the silicon layer. The porosity is evident from FTIR measurements showing a large oxygen concentration at ~1050 cm-1, and is correlated with the 2100 cm-1 signature of most of the Si-H stretching bonds. Using a Tfil of 1750°C, however, the films are more compact, as seen from the absence of the 2100 cm-1 SiH mode and the disappearance of the FTIR Si-O signal, while the high crystalline volume fraction (> 70-80 %) is maintained. Using this Tfil and a substrate temperature of 400°C, we obtain an efficiency of 4.9 % for cells with a Ag/ZnO back reflector, with an i-layer thickness of only ~0.7 µm. High values for the quantum efficiency extend to very long wavelengths, with values of 33 % at 800 nm and 15 % at 900 nm, which are unequalled by a-SiGe:H alloys. Further, by varying the substrate temperature to enable deposition near the microcrystalline to amorphous transition (‘edge’) and incorporating variations in H2 dilution during deposition of the bulk, efficiencies of 6.0 % have been obtained. The Rd’s of these i-layers are 8-10 Å/s, and are the highest to date obtained with HWCVD for microcrystalline layers used in cells with efficiencies of ~6 %. INTRODUCTION The recently demonstrated high efficiency and stability of solar cells based upon hydrogenated microcrystalline silicon (µc-Si) has led to an extensive research effort in many laboratories. Indeed, efficiencies for single junction µc-Si cells made by PECVD have now exceeded 10% [1], and a variety of different deposition methods have been explored, including the PECVD, VHFCVD [2], HWCVD [3], gas jet [4], and pulsed plasma [5] techniques. However, due to the thick absorber layers needed for efficient light collection, deposition rate (Rd) issues assume an increased importance. The highest cell efficiencies (near 10 %) have been obtained
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