Nanostructure of the Interfaces Between ZnO, ZnO:Ga and ZnO:Al Films and Silicon

Abstract

ZnO is a promising material for use in solar cell applications which require antireflective coatings and transparent conducting materials in front contacts. Its resistivity can be reduced by appropriate doping with different group III elements, which act as donors, without sacrificing optical transmission. Besides, it constitutes a non-toxic and cheap alternative to ITO (In₂O₃:SnO₂), which is nowadays commonly used as transparent conducting oxide (TCO) in optoelectronic devices. Al(III) is one of the cheapest elements on nature, and has been therefore one of the first candidates to be used as a dopant. However recent studies demonstrate that for similar amounts of dopant the use of Ga largely improves both the optical and electrical properties of the films. The interface between Si (used as substrate) and the corresponding TCO is a critical part of Si-based solar cells, since the conversion efficiency may be profoundly altered by the presence of recombination centers formed at the interface. In this work we present a comparison of Si/ZnO, Si/ZnO:Ga and Si/ZnO:Al interfaces (with [dopant]/[Zn] = 2%) by using HRTEM and associated spectroscopies. Resistivity values of the films are: ρ(ZnO) = 4.4 × 10^-2 Ω × cm; ρ(Ga:ZnO) = 6.3 × 10^-4 Ω × cm; ρ(Al:ZnO) = 2.9 × 10^-3 Ω × cm. and the films optical transmitance is larger than 80% along the visible spectrum.