Ziming Chen1
Imperial College London1
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.<br/>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.<br/>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:<br/>1) To study surface passivation effects, we fabricated FA<sub>0.99</sub>Cs<sub>0.01</sub>PbI<sub>3</sub> PeSCs with and without surface passivation. Our device structure was ITO/SnO<sub>2</sub>/FA<sub>0.99</sub>Cs<sub>0.01</sub>PbI<sub>3</sub>/(OAI)/Spiro-OMeTAD/Au, where n-octylammonium iodide (OAI) served as a passivator for both cation and halide vacancies at the FA<sub>0.99</sub>Cs<sub>0.01</sub>PbI<sub>3</sub> surface.<br/>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.<br/>2) To investigate the impact of strain, we fabricated two perovskite films with varying strain levels: MA<sub>0.95</sub>GA<sub>0.05</sub>PbI<sub>3</sub> with tensile strain and MA<sub>0.95</sub>GA<sub>0.05</sub>Pb(I<sub>0.95</sub>Br<sub>0.05</sub>)<sub>3</sub> with free strain due to Br (with smaller size) compensation in the lattice.<br/>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.