Good quality polycrystalline thin films can be made at low temperature (< 500 °C) by thermocatalytic decomposition (better known as Hot Wire Chemical Vapor Deposition) of silane/hydrogen mixtures, with great prospects for stable thin film solar cells on cheap substrates utilizing absorber layers with an optical band gap of 1.1 eV. Some of the requirements of the poly-Si films for solar cell application (e.g. in n-i-p solar cells) are that they should be intrinsic and should have low oxygen incorporation. The extent of oxygen incorporation depends on the deposition rate, the gas mixture (hydrogen dilution) and the purity of the source gases. We have been able to achieve, by HWCVD, device quality poly-Si:H films with very intrinsic nature (activation energy of 0.54 eV and band gap of 1.1 eV). These films were made using low hydrogen dilution of the silane. SIMS analysis showed that the oxygen concentration in the bulk in the film is as low as 10¹⁸/cm³ and that oxygen content drops to this value within a depth of only 50 nm from the surface showing the low permeability to oxygen. Other films, made at higher hydrogen dilution, had a high and homogeneous oxygen content of more than 4 %. The advantage, however, of the films made using higher dilution is that they show immediate nucleation without any significant incubation phase. For solar cell applications, we investigated the feasibility of a double layer structure where the poly-silicon layer made at high hydrogen dilution is capped by a poly-silicon layer made at low hydrogen dilution. Indeed, the oxygen content of the bottom layer is significantly smaller than that of a bare film.

The profiled growth of compact poly-Si:H i-layer at a deposition rate of 0.5 nm/s has been employed in n-i-p solar cells deposited on plain stainless steel (without back reflector). We obtained an increase in the efficiency from 0.64 % to 4.41 %, primarily due to the FF of 0.607. Due to the native surface texture of these layers, a current density of 19.95 mA/cm² is generated in an i-layer that is only ~1.22 mm thick. This is a fundamentally different approach than the use of thick (20 - 100 mm) poly-Si:H made at 900 - 1200 °C, thus opening up new possibilities for low-cost thin polycrystalline film solar cells on a variety of substrates.