Wide bandgap p-type nanocrystalline silicon oxide as window layer for high performance thin-film silicon multi-junction solar cells

High efficiency thin-film silicon multi-junction solar cells require both high open-circuit voltage (Voc) and high blue spectral response in the top amorphous silicon (a-Si:H) cell. Here we investigated the mixed-phase p-type nanocrystalline silicon oxide (p-SiOx) films and used this material as window layer in high Voc a-Si:H p-i-n solar cells. The introduction of oxygen suppresses the nucleation of Si nanocrystallites. Therefore, p-SiOx film with low oxygen content should be used for the contact layer, to guarantee growth of highly conductive Si nanocrystallites in the initial few nanometers. With p-SiOx as p-layer, the optimal p-SiOx film has high oxygen content and thus high bandgap, resulting higher Voc and better spectral response than the standard p-type amorphous silicon carbide alloys (p-SiC) based window layer. Although the optimal p-SiOx film has very low planar conductivity (in the order of 10-12 S/cm), the filament-like Si nanocrystallites which grow perpendicular to the substrate enable the adequate transverse conduction for the solar cells. Consequently, a-Si:H solar cells with Voc>1 V and FF>70% have been obtained. Finally, the p-SiOx window layers were successfully applied to thin-film silicon multi-junction solar cells. A high initial efficiency of 14.4% has been achieved in a-Si:H/nc-Si:H tandem solar cells.