Perovskite solar cells (PSCs) are a promising technology for use in next generation photovoltaics. A unique aspect of PSCs is the presence of mobile ionic species in the active layer, which can significantly impact device performance in ways that are not yet fully understood [1-3]. Furthermore, the efficiency of PSCs is also strongly affected by the choice of electron transport material (ETM), with recombination at the perovskite/ETM interface often thought to limit device performance [4]. Thus, understanding the combined effects of mobile ionic charge and ETM properties on the carrier recombination and extraction dynamics in PSCs under operation is crucial to the further development of these devices. In this talk, we demonstrate the use of operando photoluminescence (PL) to investigate these dynamics in typical p-i-n PSCs with four different ETMs. Operando PL is a method which allows for the measurement of real-time PL spectra during current density-voltage (J-V) scans under 1-sun equivalent illumination. This allows direct comparison between the internal performance (recombination currents and quasi-Fermi-level-splitting (QFLS)) and the external performance (J-V) of a PSC [5]. When combined with the results of device simulations, this technique provides novel insights into the processes occuring during PSC operation.

Under short circuit conditions, significant charge accumulation was observed in all four devices, including our ~20% efficient champion device. That this was the case in all four PSCs, regardless of the properties of the ETM, suggests that this phenomenon is linked to an intrinsic property of the perovskite. By combining the results of operando PL with device simulations, we conclude that short-circuit charge accumulation is mainly due to field screening caused by ion migration in the perovskite layer. Moreover, by comparing different ETMs, we quantify the impact of energetic alignment at the perovskite/ETM interface on extraction and recombination kinetics and thus device efficiency. Whilst a deep LUMO is found to enhance electron transfer from the perovskite to the ETM, it results in a higher electron density on the ETM. This is observed to increase non-radiative recombination at the perovskite/ETM interface, limiting the PSC’s open circuit voltage (V_OC) and fill factor. On the contrary, a shallow ETM LUMO is observed to impede electron extraction which leads to a larger QFLS in the bulk perovskite. Although this is beneficial for the V_OC, the retarded charge extraction under low voltage conditions (when V < V_OC) results in a device with a low fill factor and short circuit current density (J_SC). Additionally, our simulation results suggest that the presence of mobile ions increases V_OC relative to the case without mobile ions. However, the reduction to both J_SC and fill factor due to ionic field screening outweigh this postive effect, meaning that the net effect mobile ions is to lower device efficiency.

Overall, our results demonstrate that, in addition to optimising the energetic alignment at the perovskite/ETM interface, mitigating the effects of ion migration in the perovskite layer is necessary to maximise the efficiency of charge extraction and J_SC in PSCs.