Manipulating ionic configurations in perovskite solar cells

Metal–halide perovskites combine high photovoltaic efficiency with low-temperature processing, positioning this material class as a promising thin-film solar technology. A major open question concerns the role of mobile ionic species in setting device performance and stability under operating conditions. Ionic reconfiguration under internal and external fields alters the internal potential landscapes experienced by charge carriers, leading to interfacial band bending, hysteresis, and operational losses due to ion-induced field screening. These ionic effects can influence steady-state current collection, fill factor, and open-circuit voltage. Understanding how specific ionic distributions affect device operation may support strategies to mitigate losses and improve efficiency.

Under dark conditions the mobile ions respond to the net potential drop across the perovskite layer and redistribute to screen the electric field. At room temperature, ionic distributions under different bias conditions are difficult to probe because of high ionic mobility. Lowering the temperature reduces ionic motion. Here, ionic distributions are created by applying an electrical bias, a light bias, or both, and then “frozen in” by cooling the device under the applied conditions. The cells’ current–voltage behavior and absolute photoluminescence (APL) are measured at discrete temperatures after polarization. The quasi-Fermi level splitting (QFLS) is determined from APL as an internal voltage metric and compared with the open-circuit voltage (Voc). From the mismatch between QFLS and Voc, an effective potential barrier associated with the frozen ionic distribution is measured as a function of the polarization bias.

Below 180K, the performance of a polarized Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃  perovskite solar cell remains stable over time, suggesting a frozen ionic regime. Between 180K and 255K, the Voc, fill factor, and hysteresis suffer due to the low ion mobility. Above 255K, the ions are sufficiently mobile to respond to standard voltage sweeps at 0.25 V/s, indicating an increased mobility region. A general trend is observed when cells are polarized under a higher electric field, their low-temperature device performance improves significantly, with power conversion efficiency rising from about 5% to approximately 26% at 170 K for polarization biases of −1 V and 2 V. Cooling is accompanied by a higher integrated APL spectrum and increased QFLS, consistent with reduced non-radiative recombination. At cryogenic temperatures, the QFLS of various half-stacks and full devices even approaches the radiative limit. At room temperature, introducing a C60 layer on the perovskite results in an ~80 mV reduction in QFLS. This reduction diminishes at lower temperatures, where the QFLS of a complete device converges to the QFLS of an intrinsic perovskite, indicating a significant reduction in non-radiative losses at the interface. In these measurements, the QFLS at low temperatures appears largely independent of the polarization bias used.