Incorporation of p-type microcrystalline silicon films in amorphous silicon based solar cells in a superstrate structure

Thin (< 20 nm) p-type microcrystalline silicon films have been deposited by plasma enhanced chemical vapor deposition in a parameter regime, specifically optimized for extremely thin films. High conductivity (> 10(-2) Ohm(-1) cm(-1)) and low activation energy (< 0.08 eV) have been achieved for thin films on various oxide substrates i.e., Corning 7059 glass, SnO2: F, TiO2 and Ta2O5. Deposition of thin p-mu c-Si : H is possible on void rich films (a-SiC : H and low-temperature deposited a-Si:H) but not on device quality a-Si : H. Single junction p-i-n cells were made in a superstrate structure using p-mu c-Si : H as the window-layer directly on top of SnO2 : F coated glass. For the first time an efficiency of 9.63% could be achieved for st single junction cell with a truly microcrystalline silicon p-layer in a superstrate configuration. There is an improvement in the blue spectral response compared to the cell made with a-SiC : H(B) as window layer. However, open circuit voltage and fill factor were critically dependent on the choice of buffer layer at the p/i interface. Computer simulations point out that this can be attributed to the valence band offset between the amorphous i-layer and the microcrystalline p-layer. The buffer acts as a barrier to electron back-diffusion and reduces the recombination in the p-layer. Tandem cells (a-Si : H/a-Si: H) incorporating p-mu c-Si : H along with n-mu c-Si : H in the tunnel junction showed an efficiency of 9.9% and FF of 0.73. The tunnel junction n-mu c-Si : H/p-mu c-Si : H needed an oxide interface layer for a good performance. The role of the interface layer may be to increase the tunnel recombination as well as to act as a diffusion barrier to dopants. (C) 1998 Elsevier Science B.V. All rights reserved.