Latest developments of high-efficiency micromorph tandem silicon solar cells implementing innovative substrate materials and improved cell design

We report here on the latest research developments of tandem micromorph (amorphous/ microcrystalline) silicon solar cells in our laboratory. High conversion efficiency for micromorph cells requires both a dedicated light management to keep the absorber layers as thin as possible, and optimized growth conditions of the microcrystalline silicon (μc-Si:H) material. We will show that an improved cell design based on the use of silicon oxide doped layers permits to achieve high efficiencies on substrates that are usually considered as inappropriate for μc-Si:H because of their roughness. Furthermore, recently, new front contacts based e.g. on bi-layers or Ultraviolet nanoimprint lithography were developed, leading to very promising results in micromorph solar cells. While efficiencies of 12.7% initial and 11.3% stable could be achieved with only 1.1 μm of bottom cell on a front rough Low Pressure Chemical Vapor Deposited (LPCVD) ZnO, a remarkable 12% initial was also reached on textured replica (as on the master). Emphasis is also laid in our lab on increasing the deposition rate of mc-Si:H while maintaining high quality material. This is done by reducing the interelectrode gap while working at high deposition pressure, in "powder free" processes at 40 MHz. We could observe that high pressure-low hydrogen dilution process conditions lead to dense high quality material. So far, conversion efficiencies up to 8.5% have been achieved at 1 nm/s for single junction μc-Si:H solar cells with 1.8 μm thick absorber layer. We also report a promising micromorph tandem initial efficiency of 11.9% with the μc-Si:H i-layer at 0.9 nm/s. High efficiency micromorph solar cells could thus be fabricated under conditions that are favorable to industrial, low-cost, fabrication of micromorph modules. Recent results of tandems combining original substrates and improved deposition processes suggest that stabilised efficiencies close to 13% can be expected in a near future.