Growth of device quality p-type μc-Si:H films by hot-wire CVD for a-Si pin and c-Si heterojunction solar cells

We report on the growth of highly conducting p-pc-Si:H films by hot-wire CVD. A systematic variation of the doping gas (trimethylboron) ratio was first carried out. High dark conductivity, sigma (D) approximate to 1.0 Ohm (-1) cm(-1) and low activation energy of dark conductivity, E(a)approximate to 62 meV have been achieved for thick films. For thin films (d approximate to 20 nm), sigma (D)approximate to4.7X10(-2) Ohm (-1) cm(-1) and E(a)approximate to 80 meV were obtained. The annealing characteristics of thick films show thermally assisted dopant activation. Microcrystalline growth in these films was verified by XRD. A very thin p-a-Si:H seed layer (d approximate to2 nm) was grown for preparation on TCO-coated substrates prior to the deposition of the p-muc-Si:H layer. The incorporation of a p-muc-Si:H layer into a-Si:H p-i-n solar cells has improved the open circuit voltage (870-900 mV) relative to the use of a p-a-SiC:H layers. At present the best I-V parameters for p-i-n and muc-Si/c-Si heterjunction solar cells are J(sc)=12.3 mA cm(-2), V-oc=873 mV, FF=72%, eta =7.8% and J(sc=)26.6 mA cm(-2), V-oc=532 mV, FF=74%, and eta =10.6%, respectively. To the best of our knowledge, these are the first heterojunction solar cells with the p-muc-Si:H emitter grown by HWCVD. The necessity of a seed layer has been found to be very much important in both types of solar cells. Two different types of seed layers, and their thickness variation, were also investigated. (C) 2001 Elsevier Science B.V. All rights reserved.