ZnO is a widely used metal-oxide semiconductor for photovoltaic application. In solar cell heterostructures they not only serve as a charge selective contact, but also act as electron acceptor. Although ZnO offers a suitable interface for exciton dissociation, charge separation efficiencies have stayed rather poor and conceptual differences to organic acceptors are rarely investigated. In this work, we employ Sn doping to ZnO nanowires in order to understand the role of defect and surface states in the charge separation process. Upon doping we are able to modify the metal-oxide work function and we show its direct correlation with the charge separation efficiency. For this purpose, we use the polymer poly(3-hexylthiophene) as donor and the squaraine dye SQ2 as interlayer. Interestingly, neither mobilities nor defects are prime performance limiting factor, but rather the density of available states around the conduction band is of crucial importance for hybrid interfaces. This work highlights crucial aspects to improve the charge generation process of metal-oxide based solar cells and reveals new strategies to improve the power conversion efficiency of hybrid solar cells.

In this contribution, we investigate the optoelectronic properties of a donor-acceptor poly(N-Dodecyl-2,7-carbazole-alt-benzothiadiazole (CBZ-BT) copolymer in solutions and thin films, by a combination of complementary optical and electronic spectroscopy techniques including stationary absorption and fluorescence, femtosecond time-resolved and ultraviolet/x-ray photoelectron spectroscopy. Absorption spectroscopy revealed two bands at 322/338 and 445/475 nm for CBZ-BT in solutions/films attributed to the carbazole and benzothiadiazole groups respectively. Photoexcitation either to the absorption band of the carbazole or benzothiadiazole group led to a broad and structureless fluorescence spectrum due to large torsional disorder in the excited state, originating from intramolecular energy transfer between carbazole and benzothiadiazole. Time resolved spectroscopy in solutions reveals a transient red-shift of the emission spectrum within less than 5 ps due to exciton migration and/or conformational relaxation of the polymer backbone. In films, this relaxation is faster accompanied by a quenching of the exciton lifetime.