Optical spectroscopic studies of charge carrier dynamics in lead halide perovskite solar cells
Creators: name: Xu, Weidong
Publisher: name: Imperial College London
Year: 2022

description: This thesis focuses on using various optical spectroscopic methodologies to study the impact of different materials and device processing on charge carrier dynamics that correlate to device performance in perovskite solar cells (PSCs). The underlying dynamics, including recombination, transport and transfer, are mainly investigated by voltage-dependent photoluminescence (PL), electroluminescence (EL), time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy. Different-processed perovskite layers with controlled morphology and defect densities, and their contacted charge transport layers (CTLs) with varied energetics and mobilities are studied. At open circuit (OC), for high-quality perovskites with lower defect densities, as fewer charges recombined non-radiatively, the PSC is expected to show larger PL emission. Likewise, perovskite films with interlayers or their complete cells with suppressed surface/interface recombination are also supposed to show high PL at OC. When a PSC is switched to short circuit (SC), most of the charges should be extracted to the external circuit, such that the device PL is expected to be significantly reduced. Thus, this thesis introduces a figure of merit, named device OC to SC PL quenching efficiency (PLQ OC−SC), for estimating the charge extraction efficiency in different-processed PSCs. The results show that efficient PSCs have high values of (PLQ OC−SC), while low values are observed in the cells with either severe non-radiative recombination or impeded charge extraction. In addition to the device OC to SC PL measurement, a more quantitative analysis based on operando PL measurements, which converts the absolute PL emission into quasi-fermilevel-splitting (QFLS), is implemented on PSCs with varied energetic alignment at the perovskite/electron transport layer (ETL) interface. This measurement system allows a real-time PL spectrum acquisition while doing a J-V scan. The results show that a mismatch of the interface energetic alignment will reduce device performance either by accumulating charges in bulk or by introducing surface recombination. More importantly, high QFLS values indicative of substantial charge accumulation are observed in all devices, even at SC. This phenomenon can be attributed to the screening of the internal electric field by ion migration, which is evidenced by drift-diffusion-based simulation results. In the final chapter, a simple TRPL method is demonstrated to characterise the kinetics of charge transport across the bulk perovskite and charge transfer from the perovskite layer to the CTLs. This work elucidates the dependence of these dynamics on film thickness, grain boundaries (GBs), and CTLs. Using asymmetric laser excitation, charge transport and transfer can be selectively probed by generating charges close to and far from the heterojunction interface. These results are further correlated with device performance. The finding suggests that both film thickness and GBs affect the asymmetry between electron and hole charge transport across the bulk perovskite and charge transfer from the bulk perovskite to the respective CTLs.