Researchers at the University of Oxford, Philipps-University Marburg, Swansea University, the Fraunhofer Institute for Solar Energy Systems ISE, the University of Surrey and the Chinese University of Hong Kong have tackled a fundamental barrier to commercializing perovskite photovoltaics: how to make highly efficient devices durable without relying on expensive or scarce electrode materials.
The team shows that a 5-nanometer chromium interlayer beneath a cheap aluminum electrode can match, and in some cases beat, the stability of a gold electrode, while cutting the electrode's raw material cost by more than 100,000 times. In a related finding from the same study, the team also identifies indium migrating out of the standard transparent front electrode and into the perovskite as a degradation pathway that is already active during device processing, and shows that a simple substitution fixes it.
Gold rear electrodes are the reliable choice for stable perovskite solar cells, since more reactive metals like silver, aluminum and copper react with halides escaping the perovskite layer and accelerate its breakdown. But gold is far too expensive for terawatt-scale manufacturing, and the alternative - carbon electrodes - is difficult to integrate into the higher-efficiency p-i-n device architecture. Front transparent electrodes carry their own overlooked risk: indium tin oxide (ITO), the industry-standard transparent conductor, has known chemical and thermal stability limits, but how that translates into device-level degradation had not been thoroughly studied.
The researchers fabricated otherwise-identical devices with silver, aluminum, copper and gold rear electrodes, with and without the chromium underlayer, then aged them under heat and simulated sunlight for over 1,000 hours. Unprotected copper and aluminum degraded even faster than silver, but adding the chromium nanolayer roughly doubled the time each metal's champion device took to lose half its efficiency, with chromium/aluminum ultimately matching or exceeding gold's stability - a reversal, since aluminum alone was the fastest-degrading electrode of all four metals tested. X-ray photoelectron spectroscopy on aged devices showed the chromium layer forms a mixed metallic/oxidized, likely carbide-containing barrier that suppresses halide transport to the aluminum beneath, blocking the aluminum-bromide formation seen in unprotected electrodes.
On the front-contact side, depth-resolved mass spectrometry revealed indium ions distributed throughout the perovskite layer in both fresh and aged ITO-based devices, with evidence that leaching begins during fabrication itself, not just under later heat or light stress. Photoluminescence measurements confirmed the damage: perovskite films on ITO collapsed from over 1% to below 0.01% quantum yield during aging, while identical films on fluorine-doped tin oxide (FTO) held between 1% and 5% throughout. Deliberately doping perovskite with indium iodide reproduced the same accelerated degradation, confirming indium itself, not just the ITO interface, as the culprit.
Combining both fixes (FTO in place of ITO, plus the chromium/aluminum rear electrode) and reproducing the result in a second laboratory in Oxford, the team's best device retained 66% of its initial efficiency after more than 1,000 hours of aging at 75°C under simulated sunlight, and reached 24.7% power conversion efficiency in a narrower-bandgap composition. The chromium/aluminum electrode also proved more resistant to reverse-bias breakdown than gold and recovered performance more readily afterward. The authors frame the combined result as a practical, terawatt-compatible route to durable perovskite photovoltaics that avoids both gold and scarce indium.
