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:

Metal halide perovskite solar cells have considerable potential for next-generation solar power production. However, if not controlled, the migration of mobile ions can hamper the stability of perovskite solar cells. Intensive research efforts have devised methods of suppressing ion migration and degradation in perovskite materials, resulting in solar cells that are stable over thousands of hours during accelerated ageing testing. Here, we review the chemical origins of ion migration, its effect on material and device performance and stability, and strategies to mitigate its impact. Ion migration originates in the soft lattice of the halide perovskite framework and its low defect-formation energy, but there are many different strategies to reduce its effects, from compositional engineering of materials and device architecture changes to additives and strain engineering. The field has made great progress in understanding the origin and properties of mobile ions in halide perovskites and has improved operational stability beyond expectations. Nonetheless, there are still ample opportunities to further improve the long-term durability of perovskite solar cells, either by reducing ion migration or its effect on solar cell efficiency.

Abstract:

Mobile ions play a key role in the degradation of perovskite solar cells, making their quantification essential for enhancing device stability. Various electrical measurements have been applied to characterize mobile ions. However, discerning between different ionic migration processes can be difficult. Furthermore, multiple measurements at different temperatures are usually required to probe different ions and their activation energies. Here, we demonstrate a new characterization technique based on measuring the thermally activated ion current (TAIC) of perovskite solar cells. The method reveals density, diffusion coefficient, and activation energy of mobile ions within a single temperature sweep and offers an intuitive way to distinguish mobile ion species. We apply the TAIC technique to quantify mobile ions of MAPbI₃ and triple-cation perovskite solar cells. We find a higher activation energy and a lower diffusion coefficient in the triple-cation devices. TAIC measurements are a simple yet powerful tool to better understand ion migration in perovskite solar cells.

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.