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

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 (TiO₂) layers onto hydrogen-passivated surfaces of crystalline-silicon (c-Si). Energy level alignment and chemical composition at these abrupt, interfacial layer-free TiO₂/Si heterojunctions are investigated via ultraviolet, X-ray, and inverse photoemission spectroscopy, for c-Si doping ranging from p⁺⁺(10¹⁹) to n⁺⁺(10¹⁹). The interface Fermi level position and device-relevant TiO₂/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 TiO₂/Si-based diodes confirm the energy level alignment yielded by spectroscopic measurements and the hole-blocking properties of the TiO₂/Si heterojunction, exclude hole conduction in the TiO₂ as a transport mechanism, and show carrier recombination at the TiO₂/p-Si heterojunction.