Current status of the thermo-catalytic (hot-wire) CVD of thin silicon films for photovoltaic applications

Thermo-catalytic (TC) or hot-wire (HW) chemical vapor deposition (CVD) is a promising technique for growing amorphous and microcrystalline silicon films with improved stability and high rates. In this paper we report on the photovoltaic (PV) applications of thin silicon films deposited by this method. After a short review of the history of PV applications of TCCVD, from the beginning in 1993, the main part of the paper deals with our research and development of films and interfaces needed for the fabrication of different solar cell structures entirely by TCCVD. So far, our highest conversion efficiency is η=10.2% for a pin structure, containing only an intrinsic a-Si:H film deposited by TCCVD. Depositing the whole pin structure entirely by TCCVD, we have obtained η=8.8% until now. After development of a tunnel junction, the first tandem solar cell device, a stacked pin–pin structure has been recently produced showing η=7%. First attempts have been made for large-area deposition. In a 30×30 cm2 batch system, a-Si:H films with device grade photoelectronical properties and high thickness uniformity can be produced at a high rate of 6 Å/s. When incorporating i-layers from this system into pin solar cells, a conversion efficiency of η=(6.4±0.8)% was obtained on an area of 20×20 cm2. Finally, we report on silicon wafer-based solar cells, where a-Si, μc-Si and epi-Si film emitters were deposited by TCCVD showing conversion efficiencies up to η=15.2%.