Delayed Electron Transfer through Interface States in Hybrid ZnO/Organic-Dye Nanostructures

Electron injection from photoexcited chemisorbed dyes into zinc oxide is known to proceed in a stepwise manner, yet the origin of the injection retardation remains controversial. Here we present a complementary time-resolved spectroscopy study on the electron injection dynamics from organic dyes into ZnO using model perylene derivatives with systematically lengthened bridge units to clarify the influence of the positively charged cation on the escape of the injected electron. The combination of transient absorption, optical-pump terahertz-probe, and time-resolved two-photon photoemission spectroscopy reveals that the delayed release of charges into ZnO is independent of Coulomb attraction between the dye cation and the injected electron. Rather, following dye photoexcitation the primary acceptor states of electron transfer into ZnO are interface states formed between the dye and the ZnO surface, which retard the formation of free charges.