Interface Formation Between ZnO Nanorod Arrays and Polymers (PCBM and P3HT) for Organic Solar Cells

We investigated the interface formation between a ZnO nanorod array and active layers of [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) and poly[3-hexylthiophen] (P3HT) in organic solar cells (OSC). We measured the interfacial electronic structures with in situ photoemission spectroscopy combined with an electrospray deposition system. Different interfacial electronic structures were observed on the ZnO nanorod array, which were compared to those of a two-dimensional ZnO film. Comparing the interfacial orbital line-ups of the active layers on the nanorod array and the film, PCBM shows Fermi level pinning behavior, but P3HT does not. These induce nearly identical orbital line-ups at the interfaces of PCBM/film and PCBM/nanorod but different line-ups at the interfaces of P3HT/film and P3HT/nanorod. These differences are understood with the integer charge transfer model with the different thresholds of Fermi level pinning of PCBM and P3HT. These results give insight into the design not only of OSCs but also of any organic electronic devices with nanostructures: changes in electronic structure due to the nanostructure formation should be considered thoroughly.