Researchers from Karlsruhe Institute of Technology (KIT), Technical University of Munich (TUM), DESY (Deutsches Elektronen-Synchroton), Ludwig-Maximilians-Universität München (LMU), the KTH Royal Institute of Technology in Stockholm and additional institutes recently demonstrated how rapid temperature swings drive degradation in state-of-the-art wide-bandgap perovskite and tandem solar cells - and how molecular design can significantly improve their resilience.
The team investigated triple‑cation wide‑bandgap (WBG) perovskite solar cells with a bandgap of about 1.68 eV and a champion power conversion efficiency (PCE) of 24.31% in 0.05 cm² devices, employing dual passivation based on 3‑fluorophenethylammonium iodide (3‑F‑PEAI) and ethylenediamine diiodide (EDAI₂). Under rapid solar‑thermal cycling with a temperature change rate of around 10 °C/min, they found that device degradation is largely independent of the initial passivation strategy and instead follows a universal two‑regime pattern. The first regime is a pronounced burn‑in phase, during which the cells can lose about 60% of their relative performance over the first cycles, followed by a second regime of steadier degradation where photovoltaic parameters fluctuate with temperature but evolve more slowly.
Using operando grazing‑incidence wide‑angle X‑ray scattering (GIWAXS) and photoluminescence (PL) at synchrotron beamlines, the researchers were able to “watch” the WBG perovskite lattice expand and contract in real time as the temperature was ramped, effectively seeing the material “breathe” under thermal cycling. These measurements show that temperature‑induced strain and reversible phase transitions create and activate non‑radiative recombination centers both in the perovskite bulk and at the perovskite/charge‑transport layer interfaces, which in turn leads to pronounced losses in open‑circuit voltage (VOC) and fill factor (FF) during operation. The study further clarifies that ion migration and bandgap shifts are not the primary origin of the initial burn‑in loss under these rapid cycling conditions; instead, the accumulation of non‑radiative defects associated with structural distortions dominates the first degradation regime.
In addition to the in‑situ structural analysis, the group extracted temperature coefficients for single‑junction WBG perovskite cells (with and without dual passivation) and for perovskite/Si tandem solar cells (TSCs) under various constant‑temperature conditions. A key insight is that temperature coefficients derived from conventional J-V measurements at fixed temperatures do not capture the actual device behavior when the device is driven through repeated heating and cooling cycles, as in outdoor operation. Under real‑world‑like thermal cycling, the dynamic interplay of strain, phase transitions and defect formation induces performance losses and partial recoveries that cannot be predicted from static temperature sweeps alone, underscoring the need for dedicated cycling protocols when assessing long‑term stability.
The researchers extended their investigation to perovskite/Si tandem solar cells, where the high‑efficiency WBG perovskite acts as the top cell stacked on a silicon bottom cell to harvest more of the solar spectrum. Under the same rapid solar‑thermal cycling protocol, the tandem devices showed improved temperature resilience at lower temperatures and retained about 94% of their original PCE after more than 200 minutes of cycling, indicating that the tandem architecture can buffer some of the thermomechanical stress. These results highlight that enhancing interfacial robustness and suppressing the initial burn‑in phase are central design targets for both single‑junction WBG perovskite cells and tandems intended for decades‑long outdoor deployment, where daily thermal swings are unavoidable.
In a separate paper, the researchers reported how to stabilize the sensitive crystal material using special organic molecules that act as spacers, holding the structure together like a molecular scaffold. By comparing different spacers, they found that while common spacers led to structural breakdown under thermal cycling, the bulkier organic molecule PDMA acted as a superior anchor and yielded significantly more robust solar cells that remain stable even under the mechanical stress of rapid heating and cooling.
Together, the mechanistic understanding of burn‑in, the quantification of performance loss under realistic cycling, and the identification of effective molecular anchoring strategies could chart a route toward wide‑bandgap perovskite and tandem modules that combine high efficiency with the thermal durability needed for long‑term outdoor use.
