A solid state issue. Photons entering the doped semiconductor junction have sufficient energy to promote charge into the conduction band.
With pin diodes the doping can be tailored to make the semiconductor sensitive in specific bands from the near IR into the UV.
Can't do that in insulators. Conductors like cesium make good photo-injectors but that is a surface work function issue.
Are you interested in organic photovoltaics? I wonder how that research is going? Promising technology there. Wouldn't it be cool if some clever gent or lady could design and market an inexpensive roof shingle with a flexible integrated organic photovoltaic, which interconnect in some simple way like LEGO, and still keeps the weather out.
Every house in America goes green. Entrepreneurial windfall $$$$