Structural and chemical investigation of CdSe crystals deposited in a nanoporous sol-gel: Effect of chemistry and defects on photovoltaic properties

Nanoporous cadmium selenide films were prepared by electro crystallization inside a nanoporous SiO₂ network, prepared by a sol-gel process on an electrically conducting indium tin oxide (ITO) substrate. Subsequent removal of the SiO₂ template should, according to literature reports, give a nanoporous network of CdSe in electronic contact with the substrate. The structure/morphology of the film was explored during the preparative steps by grazing incidence small/wide angle X-ray scattering, scanning electron microscopy and atomic force microscopy. The chemistry was studied by time of flight secondary ion mass spectrometry methodologies, such as imaging and depth profiling that maps the in-plane and in-depth distribution of chemistry, i.e. full 3-dimensional chemical imaging of the nanoporous CdSe layer. Finally, the nanoporous CdSe films were tested as electron conductors in hybrid organic/inorganic solar cells employing CdSe as the electron conductor and poly-3-hexylthiophene as the hole conductor. ITO and gold served as the electrodes. It was found that removal of the SiO₂ from the micellar SiO₂-CdSe nanostructure led to a collapse of the resulting CdSe structure with a poor structural conservation. Furthermore, a high pinhole density in the sol-gel resulted in a high degree of shunting paths that led to poor photovoltaic properties.