Impact of interfacial polymer morphology on photoexcitation dynamics and device performance in P3HT/ZnO heterojunctions

To understand the critical factor(s) that influence short-circuit current in poly(3-hexylthiophene) (P3HT)/ZnO solar cells, we investigate the morphology of the interfacial polymer layer and the photoexcitation dynamics in the picosecond regime. Thin (similar to 6 nm) films of P3HT deposited on bare ZnO and ZnO modified with an alkanethiol monolayer are used as model systems for the heterojunction interface. Results are compared with thin P3HT films on glass for the behavior of the polymer alone. Synchrotron grazing incidence X-ray diffraction spectra of P3HT thin films deposited on glass and on an alkanethiol-modified ZnO surface identify a crystalline P3HT interfacial layer, while an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay dynamics of initial photoexcited states, the samples are interrogated by pump-probe spectroscopy with sub-picosecond time resolution. Compared to P3HT/ZnO composite films, the decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly slower with alkanethiol modification, indicating a reduction in early-stage charge recombination. These experiments demonstrate how the interfacial polymer morphology has a critical role in determining device performance.