Exciton dissociation and charge transport properties at a modified donor/acceptor interface: Poly(3-hexylthiophene)/Thiol-ZnO bulk heterojunction interfaces

Engineering the electron donor/acceptor interface provides not only flexibility in designing optoelectronic devices but also new opportunities to optimize device performance. However, significant challenges arise from the difficulty of examining the interface, which is buried in a composite film. In this paper, we probed exciton dissociation at a poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction interface treated with thiol molecules and charge-transport properties in P3HT of the composite film using field effect transistor (FET) devices. The role of thiol modification of ZnO in charge transfer and charge transport within the bulk heterojunction interface was addressed through comparison of the threshold voltage and the FET mobility between P3HT/thiol-ZnO and P3HT/ZnO FETs. Attachment of thiol molecules onto the ZnO surface induced a larger photoinduced threshold voltage shift than that for an FET with pristine ZnO, indicating that more excitons were dissociated within the P3HT/ZnO composite film. The origin of this effect is discussed through the relationship between the increased interfacial area between P3HT and thiol-ZnO, inferred from surface morphology, and structural ordering of P3HT at the interface. The charge carrier transport properties of P3HT in the composite film are separated from charge transfer, and found to depend on the presence of thiol, offering information on carrier scattering/trapping at the interface. The integrated information of charge transfer and charge transport properties probed with an FET device would benefit efforts aimed at the optimization of solar cell devices.