Dynamic Reversible Oxidation-Reduction of Iodide Ions for Operationally Stable Perovskite Solar Cells under ISOS-L-3 Protocol

Despite rapid advancements in the photovoltaic efficiencies of perovskite solar cells (PSCs), their operational stability remains a significant challenge for commercialization. This instability mainly arises from light-induced halide ion migration and subsequent oxidation into iodine (I2). The situation is exacerbated when considering the heat effects at elevated temperatures, leading to the volatilization of I2 and resulting in irreversible device degradation. Mercaptoethylammonium iodide (ESAI) is thus incorporated into perovskite as an additive to inhibit the oxidation of iodide anion (I-) and  the light-induced degradation pathway of FAPbI3→FAI+PbI2. Additionally, the formation of a thiol-disulfide/I--I2 redox pair within the perovskite film provides a dynamic mechanism for the continuous reduction of I2 under light and thermal stresses, facilitating the healing of iodine-induced degradations. This approach significantly enhances the operational stability of PSCs. Under the ISOS-L-3 testing protocol (maximum power point (MPP) tracking in an environment with relative humidity of ≈50% at ≈65 °C), the treated PSCs maintain 97% of their original power conversion efficieney (PCE) after 300 h of aging. In contrast, control devices exhibit almost complete degradation, primarily due to rapid thermal-induced I2 volatilization. These results demonstrate a promising strategy to overcome critical stability challenges in PSCs, particularly in scenarios involving thermal effects.

Mixed-Halide Perovskites with Stabilized Bandgaps

One merit of organic-inorganic hybrid perovskites is their tunable bandgap by adjusting the halide stoichiometry, an aspect critical to their application in tandem solar cells, wavelength-tunable light emitting diodes (LEDs), and lasers. However, the phase separation of mixed-halide perovskites caused by light or applied bias results in undesirable recombination at iodide-rich domains, meaning open-circuit voltage (V_OC) pinning in solar cells and infrared emission in LEDs. Here, we report an approach to suppress halide redistribution by self-assembled long-chain organic ammonium capping layers at nanometer-sized grain surfaces. Using the stable mixed-halide perovskite films, we are able to fabricate efficient and wavelength-tunable perovskite LEDs from infrared to green with high external quantum efficiencies of up to 5%, as well as linearly tuned V_OC from 1.05 to 1.45 V in solar cells.

Direct Observation of Halide Migration and its Effect on the Photoluminescence of Methylammonium Lead Bromide Perovskite Single Crystals

Optoelectronic devices based on hybrid perovskites have demonstrated outstanding performance within a few years of intense study. However, commercialization of these devices requires barriers to their development to be overcome, such as their chemical instability under operating conditions. To investigate this instability and its consequences, the electric field applied to single crystals of methylammonium lead bromide (CH₃ NH₃ PbBr₃ ) is varied, and changes are mapped in both their elemental composition and photoluminescence. Synchrotron-based nanoprobe X-ray fluorescence (nano-XRF) with 250 nm resolution reveals quasi-reversible field-assisted halide migration, with corresponding changes in photoluminescence. It is observed that higher local bromide concentration is correlated to superior optoelectronic performance in CH₃ NH₃ PbBr₃ . A lower limit on the electromigration rate is calculated from these experiments and the motion is interpreted as vacancy-mediated migration based on nudged elastic band density functional theory (DFT) simulations. The XRF mapping data provide direct evidence of field-assisted ionic migration in a model hybrid-perovskite thin single crystal, while the link with photoluminescence proves that the halide stoichiometry plays a key role in the optoelectronic properties of the perovskite.