I just wanted to put another paper in this context: http://science.sciencemag.org/content/324/5923/63.short Solar cells based on Oxides, in particular BiFeO3. The key point here, is that while halide perovskites provide excellent efficiency, they only last a few hours because the material is very unstable. I learned this year about this oxide based materials exhibiting photovoltaic effect. The nice point of this materials is i) they are oxides, extremely stable and ii) are ferroelectrics, they have an internal polarization which would help in ionizing the photogenerated excitons even in the absence of pn junction.