Requirements for efficient hole extraction in transition metal oxide-based silicon heterojunction solar cells

Transition metal oxides (TMOs) are of increasing importance for many applications reaching from thin-film transistors and non-volatile memory to novel contact layers in photovoltaics. Due to their tunable electrical properties and high transparency, TMOs are also promising candidates as contact layers in silicon heterojunction solar cells already leading to cell efficiencies of about 22%. However, the current extraction of charge carriers via these thin contact layers is still not fully understood. To assist the engineering of novel silicon heterojunctions, numerical device simulations are used to improve knowledge regarding relevant heterojunction and thin film properties. The efficient current extraction from a silicon absorber is investigated with Sentaurus TCAD for a TMO-based hole contact. It is shown that for an ideal hole extraction from the induced crystalline silicon pn-junction via the amorphous silicon buffer and the TMO into the external metal electrode, two requirements have to be fulfilled: (A) A sufficiently high TMO work function is needed to ensure a high hole conductivity (via a high charge carrier ratio p/n) in the induced pn-junction within the silicon absorber. (B) Extraction of those holes into the TMO calls for efficient trap-assisted tunneling. Experimental evidence for a limitation of hole extraction by (A) and (B) is given for a variety of TMO based hole contacts using molybdenum oxide.