Intragap states are among the critical factors that limit the performance and stability of perovskite solar cells (PeSCs). They not only serve as trapping sites for photogenerated carriers and open the dominant non-radiative loss mechanism in PeSCs under sunlight illumination conditions, but also become original sites (such as defects) where degradation starts from. Further PeSCs development requires a comprehensive understanding of trap state properties, including how they are filled and depopulated in a working device. Conventional spectroscopic techniques are not sufficiently selective to specifically follow the dynamics of trapped carriers, particularly at actual PeSC working conditions. Here we apply novel infrared optical activation spectroscopy [i.e., optical pump-IR push-photocurrent (PPPc)], to observe in real time the evolution and properties of trapped carriers in operando PeSCs. In these techniques, band-edge carriers are generated by an optical visible "pump" beam, followed by the carrier trapping processes. Then the photons of IR "push" beam absorbed by the trapped carriers excite them back to the band states. The IR de-trapped carriers contribute to the device photocurrent, therefore the amplitude and behaviour of IR-induced current help to evaluate the concentration and dynamics of trapped carriers in the device.

    We compared the behaviour difference due to trapped holes in pristine and surface-passivated FA_0.99Cs_0.01PbI₃ PeSCs using a combination of temperature-dependent steady-state PPPc, ns time-resolved PPPc, and kinetic models. We found that the trap-filling process occurred in two steps: first, in a few-ns timescale, low-concentration trap states are filled in the bulk of perovskite material; then, in a much longer (~100 ns) timescale, high density of traps at material interfaces is populated. Surface passivation by n-octylammonium iodide dramatically reduces the number of trap states (~10 times) and hence substantially improves the device performance. The activation energy of the dominant hole traps was measured to be in the order of ~280 meV and was not affected by the surface-passivation process.

    Our results successfully demonstrate that PPPc techniques are powerful and highly sensitive to reveal the dynamic, concentration, and activation energy of trapped carriers, facilitating a comprehensive understanding of the role of trap states in PeSCs. We expect that the in-situ measuring of PPPc signals in working PeSCs under different ageing conditions (e.g., heat, illumination, humidity) allows us to trace the change of trapped carriers’ dynamics and properties. Therefore, it becomes another starting point for material scientists to further develop the existing materials and devices for the next-generation PeSCs with excellent stabilities.