Researchers at the Chinese Academy of Sciences, working with the University of Science and Technology of China (USTC) and Finland's Abo Akademi University, have identified a previously overlooked degradation pathway in copper-electrode n-i-p perovskite solar cells and shown that a thin bismuth interlayer can largely eliminate it.
Copper is an attractive low-cost alternative to gold and silver top electrodes, offering comparable conductivity, but it is prone to halide-driven corrosion and has mostly been confined to p-i-n device architectures, where energy-level alignment is more favorable. In this study, the team built n-i-p cells with a MoO3 buffer layer between the Spiro-OMeTAD hole transport layer and a copper top electrode, which resolved the band-alignment mismatch and brought copper-electrode devices to efficiencies comparable to silver-electrode controls (17.48% vs. 17.74% for fresh devices). Under continuous light soaking at the maximum power point, however, the copper-electrode cells degraded rapidly, losing about 40% of their initial efficiency within 100 hours, while the silver-electrode devices held up far better.
Microscopy and TOF-SIMS mapping traced the failure to the electrode's exposed edges rather than the active area: on the sides of the device where copper sits directly on MoO3 without underlying ITO, the researchers found pronounced corrosion, while the region where copper overlaps the bottom ITO electrode stayed largely intact. Photoluminescence and transient photovoltage/photocurrent measurements showed a roughly elevenfold collapse in carrier lifetime in the corroded edge regions after aging, consistent with severe trap-assisted recombination. The team proposes that under illumination, iodide released by perovskite decomposition is oxidized at the MoO3 surface into molecular iodine, which then reacts with the copper electrode to form corrosive copper iodide (CuI) - a reaction confirmed both by TOF-SIMS iodine mapping and by a simple starch-based iodine test on MoO3/iodide precursor mixtures. Light-beam-induced current mapping showed that the ITO/copper overlap region acts as an effective "electrical sink," extracting charge efficiently enough to suppress the local ion migration that drives corrosion elsewhere.
To address the unprotected edges, the researchers inserted a 50-angstrom bismuth interlayer between the MoO3 and copper layers. Because bismuth iodide is thermodynamically less favorable to form than copper iodide, the interlayer acts as a chemical and physical barrier against iodine diffusion, provided it fully covers the copper electrode with no exposed overlap edges. Bismuth-modified devices retained 80% of their initial efficiency after 100 hours of continuous illumination with a small-area MoO3 layer, and 88% after 350 hours when the MoO3/bismuth coverage was extended beyond the copper electrode's footprint, compared with roughly 60% retention for unprotected reference devices at 100 hours. A comparison against a chromium interlayer showed bismuth delivering the best stability of the interlayer materials tested. The team also found that simply redepositing a fresh copper electrode on an aged, corroded device recovered part of the lost performance, further pointing to electrode-interface failure rather than bulk perovskite decomposition as the dominant degradation mechanism.
