The effect of concentration and post-deposition annealing on silica coated germanium quantum dot thin films grown by vertical deposition

Thin films have been grown using silica coated germanium quantum dot (Ge@SiO2) nanoparticles (NP) as well as their phosphorus-doped analogues (P-Ge@SiO2). The Ge quantum dots (QDs) were coated through the seeding of Stober particles. The film thickness and uniformity were investigated using aqueous solutions at a range of dilutions from the as-prepared solutions. The films have been characterized by SEM, XRD, and I/V measurements of test solar cells using doped n-type Si substrates. While the films were relatively compact they are actually made of large plaques of particles rather than a continuous layer, and the film thickness showed little significant variation with concentration for the Ge@SiO2 films; although a more usual trend was observed for the P-Ge@SiO2 films. Films grown using a solution 1/4 of the maximum concentration provided the highest solar cell efficiency. Thermal annealing of the films prior to deposition of the front and back contacts enabled a doubling in the cell efficiency, but did not show any marked increase in the density or crystallinity of the films.