Electronically Passivated Hole-Blocking Titanium Dioxide/ Silicon Heterojunction for Hybrid Silicon Photovoltaics

摘要

Carrier-selective heterojunctions are important for low-cost silicon-based photovoltaic applications. A low temperature (<100 °C) chemical vapor deposition technique is used here to deposit ultrathin (n-type) titanium dioxide (TiO_2 ) layers onto hydrogen-passivated surfaces of crystalline-silicon (c-Si). Energy level alignment and chemical composition at these abrupt, interfacial layer-free TiO_2 /Si heterojunctions are investigated via ultraviolet, X-ray, and inverse photoemission spectroscopy, for c-Si doping ranging from p~++ (10~(19) ) to n ~++ (10~(19) ). The interface Fermi level position and device-relevant TiO_2 /Si band offsets are found to shift monotonically as a function of the Si doping, revealing the absence of Fermi level pinning at the c-Si interface and pointing to simple Fermi level equilibration as the driving mechanism behind the interface energy level alignment. Electrical transport measurements performed on TiO_2 /Si-based diodes confirm the energy level alignment yielded by spectroscopic measurements and the hole-blocking properties of the TiO_2 /Si heterojunction, exclude hole conduction in the TiO_2 as a transport mechanism, and show carrier recombination at the TiO_2 /p-Si heterojunction.