Operando dynamics of trapped carriers in perovskite solar cells observed via infrared optical activation spectroscopy.

Conventional spectroscopies are not sufficiently selective to comprehensively understand the behaviour of trapped carriers in perovskite solar cells, particularly under their working conditions. Here we use infrared optical activation spectroscopy (i.e., pump-push-photocurrent), to observe the properties and real-time dynamics of trapped carriers within operando perovskite solar cells. We compare behaviour differences of trapped holes in pristine and surface-passivated FACsPbI devices using a combination of quasi-steady-state and nanosecond time-resolved pump-push-photocurrent, as well as kinetic and drift-diffusion models. We find a two-step trap-filling process: the rapid filling (10 ns) of low-density traps in the bulk of perovskite, followed by the slower filling (100 ns) of high-density traps at the perovskite/hole transport material interface. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (50 times), improving the device performance substantially. Moreover, the activation energy (280 meV) of the dominant hole traps remains similar with and without surface passivation.