Operando Trapped Carrier Dynamics in Perovskite Solar Cells Observed via Infrared Optical Activation Spectroscopy

Metal halide perovskites show promise for cost-effective and high-efficiency photovoltaics. Recent progress in perovskite solar cells (PeSCs) has achieved a certified power conversion efficiency (PCE) of approximately 26.0%. To push PeSCs closer to the Shockley-Queisser limit and enhance their performance, reducing non-radiative recombination caused by carrier 'traps' is crucial under standard sunlight conditions. Understanding these trap states and trapped carrier dynamics is vital to minimise performance losses.
To overcome the limitations of conventional spectroscopic techniques like time-resolved photoluminescence and transient absorption, which lack the necessary selectivity for detecting trapped carriers, we used a novel method called infrared optical activation spectroscopy, specifically optical pump-IR push-photocurrent (PPPc), to monitor trapped carriers in real-time during PeSC operation. PPPc involves generating band-edge carriers with a visible 'pump' beam, followed by carrier trapping. Subsequently, trapped carriers absorb IR 'push' beam photons, returning them to the band states. IR-detrapped carriers contribute to additional device photocurrent, allowing us to assess trapped carrier concentration and dynamics based on the amplitude and behaviour of IR-induced photocurrent.
Here, we employed PPPc (both its time-resolved and quasi-steady-state versions) to investigate how the surface passivation process and strain of perovskite make an impact on the trapped carrier dynamics, respectively:
1) To study surface passivation effects, we fabricated FA_0.99Cs_0.01PbI₃ PeSCs with and without surface passivation. Our device structure was ITO/SnO₂/FA_0.99Cs_0.01PbI₃/(OAI)/Spiro-OMeTAD/Au, where n-octylammonium iodide (OAI) served as a passivator for both cation and halide vacancies at the FA_0.99Cs_0.01PbI₃ surface.
We found that bulk traps filled rapidly (within 10 ns) due to nearby photocarrier trapping, while surface trap filling was slower (tens to hundreds of ns) and involved band-edge carrier drift/diffusion to the perovskite surface. The filling of surface trap states created an interfacial charge layer that screened the internal field and slowed carrier drift/diffusion. This process was also influenced by device temperature. The surface-passivated device exhibited faster saturation of trapped carrier concentration compared to the pristine device with higher trap density. Our kinetic model estimated a ~50 times reduction in trap states after surface passivation. Notably, the activation energy of trap state bands (~280 meV) remained nearly identical in both devices, indicating that surface passivation reduced trap numbers without changing trap types.
2) To investigate the impact of strain, we fabricated two perovskite films with varying strain levels: MA_0.95GA_0.05PbI₃ with tensile strain and MA_0.95GA_0.05Pb(I_0.95Br_0.05)₃ with free strain due to Br (with smaller size) compensation in the lattice.
We found that non-radiative recombination loss was suppressed in the strain-free perovskite, which resulted in better emission properties and higher device performance. Considering trap-assisted recombination is the main process accounting for the non-radiative recombination loss, again, the combination of both quasi-steady-state and time-resolved PPPc measurements revealed that strain relaxation reduced trap density, shallowed trap depth, as well as prolonged trapped carrier lifetimes. Hence, this mitigated trap-assisted recombination losses in the strain-free device. This study represents the first explicit correlation between strain engineering and its effects on overall trap-assisted recombination processes.