Solution Processed Al-Doped ZnO Nanoparticles/TiOx Composite for Highly Efficient Inverted Organic Solar Cells

We investigated the electrical properties of solution processed AI-doped ZnO (AZO) nanoparticles, stabilized by mixing with a TiOx complex. Thin solid films cast from the solution, of AZO-TiOx (AZOTi) (Ti/Zn similar to 0.4 in the bulk and similar to 0.8 on its surface) is processable in inert environment, without a need for either ambient air exposure for hydrolysis or high temperature thermal annealing commonly applied to buffer layers of most metal-oxides. It was found that the electronic structure of AZOTi matches the electronic structure of several electron acceptor and donor materials used in organic electronic devices, such as solar cells. Inverted solar cells employing a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester, cast on an indium tin-oxide/AZOTi electrode, and capped with a tungsten oxide/aluminum back electrode, give rise to a nearly 70% fill factor and an optimized open-circuit voltage as a result of efficient hole blocking behavior of AZOTi. The resulting electron collecting/blocking capability of this material solves crucial interfacial recombination issues commonly observed at the organic/metal-oxide interface in most inverted organic bulk heterojunction solar cells.