ASYMMETRIC INTERMEDIATE REFLECTOR FOR N-I-P MULTI-JUNNCTION THIN FILM SILICON SOLAR CELLS
We investigate n-i-p/n-i-p micromorph tandem cells deposited at process temperature below 200°C in order to be compatible with low Tg (<180°C) plastic substrates. In this configuration, the thick microcrystalline cell is grown first and tends to smoothen the initial substrate texture. Hence, the amorphous cell is grown on an almost flat surface that yields low current density. To prevent this effect and to obtain a proper current matching, an asymmetric intermediate reflector, creating a structure favourable for a-Si:H cell, is developed and incorporated into the cells. The effective optical thickness of the amorphous intrinsic layer can be increased by a factor of more than three. Hence, the light induced degradation is reduced below 10% for 180 nm amorphous layer whereas the degradation for cells without intermediate reflector is between 15-20%, because 300 nm amorphous top cell is required for matching. We present a-Si:H/a-Si:H micromorph tandem cells with 10.1 % stable efficiency after 1000h light soaking and triple junction a-Si:H/a-Si:H/μc-Si:H solar cell with initial efficiency of 10.4%.