Development of high band gap protocrystalline material for p-i-n thin film silicon solar cells
High band gap thin film silicon solar cells are of high interest for the use as top cell in triple junctions solar cells. Protocrystalline silicon, in the transition phase between amorphous and microcrystalline silicon, is such a high band gap material. During this project, protocrystalline intrinsic layers have been developed and optimized in p-i-n single junction solar cells by plasma-enhanced chemical vapour deposition (PECVD) in a new multi-chamber deposition system. In the first series, cells and layers were deposited with different dilutions going from silane to hydrogen flux ratios of 1:1 to 1:64. At the dilutions 1:16 and 1:32, protocrystalline ma-terial was obtained which showed amorphous characteristics only up to a certain thickness, where crystallites started to evolve. With dilution 1:16, a second cell series has been deposited with variation of the intrinsic layer thick-ness. The best cells in terms of open-circuit voltage and fill factor product were obtained with an i-layer thickness of 100 nm. These cells have open-circuit voltages and fill factors up to 0.975 V and 75.8 % (0.932 V and 69.9 %) for the initial (degraded) state. These are promising values for the use of this material in top cells. However, the low current density means that the optimum i-layer thickness is probably higher for triple cell application. Further cells have been deposited at different temperatures. These results imply that there is still room for further optimization, when current limitations of the heating system will be overcome.