Tuning Excitonic Properties of Pure and Mixed Halide Perovskite Thin Films via Interfacial Engineering

The authors explore the potential of ZnO layers of different morphologies, including single crystalline, micro-structured, and nano-structured substrates, for tuning exciton binding energy and influencing charge extraction when interfaced with pure (CH₃NH₃PbI₃) and mixed (CH₃NH₃PbI_{3− x}Cl_x) halide hybrid perovskite (PVSK) thin films. Electron microscopy characterization of the PVSK/ZnO interfaces are correlated with charge transfer properties, probed by means of temperature, power, and time-resolved photoluminescence (PL) spectroscopy. The results show that at room temperature, the single crystalline ZnO film promotes PL quenching, and reduces recombination lifetime along with exciton density in the PVSK films, all indicative of efficient electron extraction. Nevertheless, the micro-structured ZnO layers exhibit a mild increase of the PVSK PL at room temperature, and the nano-structured ZnO enhances PL by up to several thousand-fold, while simultaneously enhancing recombination rates by 50%. These trends are temperature dependent, and the findings highlight two opposing aspects of how excitonic dissociation in PVSK thin films is affected by the morphology of the underlying ZnO layers. While the single crystalline ZnO can be leveraged as an efficient electron extraction layer for application in photovoltaic devices, the micro- and nano-structured ones offer potential new opportunities for utilization of high quantum yield hybrid perovskites in opto-electronic platforms.