Transparent hole-collecting and buffer layers for heterojunction solar cells based on n-type-doped silicon

Hole-collecting and buffer layers more transparent than amorphous silicon layers are regarded as one of the possible improvements needed in amorphous/crystalline silicon heterojunctions to help reach the limit efficiency of silicon-based solar cells. Recent studies have demonstrated the suitability of sub-stoichiometric molybdenum oxide (MoOx) as hole-extracting layer in heterojunction solar cells based on n-type crystalline silicon. However, up to now MoOx has mainly been used in combination with amorphous silicon, so limiting its potential as a transparent emitter. This work examines a hole-selective contact based on the combination of a passivating hydrogenated amorphous silicon oxide (a-SiOx:H) and MoOx. The use of these two high-band gap materials (1.9 and 3.5 eV, respectively) should lead to best cell performances if compared to heterojunction devices based only on amorphous silicon. Solar cells were produced and optimized in terms of layers thicknesses. The widely reported problem of s-shape in the light current–voltage characteristics, due to undesired barrier formation against photo-carrier collection, was addressed and solved. The temperature response of the devices was studied and the cell stability under thermal treatment up to 130 °C was demonstrated.