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

Synopsis

There is considerable interest in titanium oxide/dioxide (TiO_X/TiO₂) based electron-selective, hole-blocking heterojunctions due to their applications in organic ¹ and inorganic photovoltaics ², and organic light-emitting diodes. In this work, a low temperature (< 100 °C) chemical vapor deposition (CVD) technique is used to deposit ultra-thin (n-type) 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 ultra-violet, X-ray and inverse photoemission spectroscopy (UPS, XPS, IPES), 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.


1. Kim, H. et al. Investigation of ultra-thin titania films as hole-blocking contacts for organic photovoltaics. J. Mater. Chem. A 18–28 (2015).

2. Nagamatsu, K. A. et al. Titanium dioxide/silicon hole-blocking selective contact to enable double-heterojunction crystalline silicon-based solar cell. Appl. Phys. Lett. 106, 123906 (2015).

Bio
Gabriel Man is a Ph.D. candidate in Electrical Engineering at Princeton University, under the supervision of Professor Antoine Kahn.  He is passionate about solving global-scale problems – such as generating energy sustainably and transmitting/storing it efficiently – through fundamental and applied research in electronic materials.  Gabriel received his B.A.Sc. and M.A.Sc. (co-advised by Professors Konrad Walus and Boris Stoeber) from UBC in Electrical & Computer Engineering.   He is a former NSERC Ph.D. fellow.