Latest Developments of High-Efficiency Micromorph Tandem Silicon Solar Cells Implementing Innovative Substrate Materials and Improved Cell Design

We report on the latest research developments of micromorph (amorphous/microcrystalline) tandem silicon solar cells in our laboratory. We show that an improved cell design based on the use of silicon-oxide-doped layers permits high efficiencies on substrates that are usually considered as inappropriate for microcrystalline silicon (mu c-Si:H) growth. Furthermore, advanced superstrates have recently been developed based on, e.g., multi-scales textures, ultraviolet nanoimprint lithography, and bilayers, leading to very promising results. While efficiencies of 12.7% initial and 11.3% stable were achieved with a bottom cell that is only 1.1 mu m thick on a rough front zinc oxide electrode, a high 12% initial efficiency was also reached on a textured replica. Our lab also placed emphasis on increasing the deposition rate of mu c-Si:H, and we observed that high depletion conditions lead to dense, high-quality material. So far, conversion efficiencies up to 8.5% have been achieved with single-junction 1.8-mu m-thick mu c-Si:H solar cells deposited at 1 nm/s. We also report a promising initial efficiency of 12.1% for a micromorph cell with a 1-mu m-thick bottom cell, for which the absorber layer was grown at 1 nm/s.