Towards high-efficiency thin-film silicon solar cells with the ''micromorph'' concept

Tandem solar cells with a microcrystalline silicon bottom cell (1 eV gap) and an amorphous-silicon top cell (1.7 eV gap) have recently been introduced by the authors; they were designated as ''micromorph'' tandem cells. As of now, stabilised efficiencies of 11.2% have been achieved for micromorph tandem cells, whereas a 10.7% cell is confirmed by ISE Freiburg. Micromorph cells show a rather low relative temperature coefficient of 0.27%/K. Applying the grain-boundary trapping model so far developed for CVD polysilicon to hydrogenated microcrystalline silicon deposited by VHF plasma, an upper limit for the average defect density of around 2 x 10(16)/cm(3) could be deduced; this fact suggests a rather effective hydrogen passivation of the grain-boundaries. First TEM investigations on mu c-Si:H p-i-n cells support earlier findings of a pronounced columnar grain structure. Using Ar dilution, deposition rates of up to 9 Angstrom/s for microcrystalline silicon could be achieved.