Silicon solar cells: Titania lets electrons through

Sol. Energy Mater. Sol. Cells 150, 32–38 (2016)

Silicon solar cells have by far the largest market share of the photovoltaics (PV) industry. Although silicon PV is a relatively mature technology, continuous advances in materials, device architecture and processing keep pushing their power conversion efficiency (PCE) up and driving costs down. A strategy for increasing the PCE of the best performing silicon cells, based on a heterojunction of crystalline and amorphous silicon, is to use carrier-selective contacts, that is, contacts that only let through either electrons or holes. Now, Xinbo Yang and colleagues at the Australian National University have reported a silicon heterojunction cell with an ultrathin TiO_x layer acting as an electron-selective contact that achieves a PCE of about 20% with simplified, cheaper processing than in the case of amorphous silicon contacts.

A layer of TiO_x is deposited by atomic layer deposition on the back surface of a 2 × 2 cm² n-type silicon solar cell. The TiO_x layer passivates the silicon surface, thus reducing carrier recombination, while allowing a relatively low contact resistance of the TiO_x/n-Si heterojunction. The insertion of a 1.2 nm SiO₂ layer between a 3.5 nm TiO_x layer and the silicon improves the thermal stability up to 350 °C, while maintaining good passivation properties. The researchers demonstrate a maximum PCE of 20.5%. Although this value is lower than those reported with selective contacts based on amorphous silicon, the improved thermal stability and simplicity in the fabrication process might make the TiO_x contacts an appealing alternative.