Jarla Thiesbrummel

Abstract:

The presence of mobile ions in metal halide perovskites has been shown to adversely affect the intrinsic stability of perovskite solar cells (PSCs). However, the actual contribution of mobile ions to the total degradation loss compared with other factors such as trap-assisted recombination remains poorly understood. Here we reveal that mobile ion-induced internal field screening is the dominant factor in the degradation of PSCs under operational conditions. The increased field screening leads to a decrease in the steady-state efficiency, often owing to a large reduction in the current density. Instead, the efficiency at high scan speeds (>1,000 V s−1), where the ions are immobilized, is much less affected. We also show that the bulk and interface quality do not degrade upon ageing, yet the open-circuit voltage decreases owing to an increase in the mobile ion density. This work reveals the importance of ionic losses for intrinsic PSC degradation before chemical or extrinsic mechanical effects manifest.

Abstract:

Halide perovskites face intrinsic stability challenges primarily due to light- and bias-induced ion migration. To mitigate ion-mediated degradation on operationally relevant time scales, this work investigates how introducing brief periodic intervals of light and darkness (LD cycling) can stabilize the average efficiency of perovskite films and devices. Systematic photoluminescence (PL) studies reveal that dark intervals on the order of seconds significantly suppress nonradiative recombination and slow degradation. The extent of PL enhancement depends on the duration of the dark time, the material composition, and critically, the sample's age. Remarkably, LD cycling increases PL by more than 7-fold even in aged samples that would otherwise undergo photodarkening under continuous illumination. Moreover, the PL kinetics under LD cycling mirror the corresponding open-circuit voltage dynamics in full solar cells, showing that local emission changes provide a direct measure of device-level behavior. Device measurements similarly show that LD cycling enhances the power conversion efficiency compared to continuous illumination and mitigates deterioration over extended operation. This strategy highlights a potential pathway to dynamically preserve or even improve perovskite performance in future optoelectronic applications.

Abstract:

Mobile ions limit halide perovskite device performance, yet quantifying ionic properties remains challenging. Frequency-domain electrical techniques are restricted to operational devices, and the resulting signals are often dominated by interfacial recombination which obscures ionic contributions. Here, we introduce intensity-modulated photoluminescence spectroscopy (IMPLS) as a fully optical alternative, where the amplitude and phase of the photoluminescence intensity is measured as a function of excitation modulation frequency. IMPLS is demonstrated on a Cs_0.07(FA_0.83MA_0.17)_0.93Pb-(I_0.83Br_0.17)₃ film. Fitting the data with an optical equivalent circuit model reveals two characteristic lifetimes: τ_char = 2.1 ms and 77 s, likely corresponding to defect formation and ionic diffusion, respectively. The diffusion feature is consistent with intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements on corresponding full devices. Importantly, IMPLS enables contact-free characterization of slow processes for all perovskite sample types, including films and devices, significantly expanding the techniques available for understanding mobile ions in these materials.