Competitive charge transport processes in inverted polymer solar cells based on ZnO thin films

Polymer solar cells (PSCs) with an inverted architecture are promising photovoltaic devices. The inverted device has clearly distinguishable electron and hole transport layers (ETL and HTL) compared to traditional PSCs with a direct structure. ETL and HTL ensure effective charge separation and transport, and their properties significantly affect the photovoltaic performance and stability of PSCs. In this research work, we studied the effect of the thickness and quality of zinc oxide (ZnO) ETL on dynamics of charge transport in FTO/ZnO/P3HT:IC60MA/MoO x /Ag devices and their photovoltaic performance. We observe that, when ZnO layer thickness is decreased, the power conversion efficiency (PCE) of the device sharply boosts reaching peak value and then steadily decreases. According to the impedance spectroscopy study, there are two competitive charge transfer processes in PSCs: electron transport from the photoactive layer to FTO through ZnO and charge recombination at the ZnO/P3HT:IC60MA interface. The kinetics of these charge transfer processes depends on ZnO thickness and quality. Our study indicates that there is a balance between charge transport and recombination rate at the thickness of the ZnO layer of approximately 50 nm. The champion device with ZnO ETL thickness of 54 nm has generated a short-circuit current of 9.5 mA/cm 2 , an open-circuit voltage of 0.6 V, and PCE of 3.2% under the light illumination with the intensity of 100 mW/cm 2 .