Copper corrosion began mainly at the sides of the electrode
Iodine released from the perovskite reacted with copper to form copper iodide
Stabilized cells incorporating a 5 nm bismuth interlayer retained 88% of their initial efficiency after 350 hours
Copper is being explored as a lower-cost alternative to silver electrodes in perovskite solar cells due to its high electrical conductivity. However, iodide ions released from the perovskite absorber can migrate toward the copper electrode and ultimately cause corrosion, affecting device performance and operating life.
Researchers from the Chinese Academy of Sciences (CAS) and partner institutions investigated how this corrosion develops in n-i-p perovskite solar cells. In the study, titled ‘Unexpected side Cu corrosion at Cu/MoO₃ interface in Cu-based n-i-p type perovskite solar cells’, they also evaluated a thin bismuth interlayer as a barrier against iodine migration.
The cells used a copper rear electrode with a molybdenum trioxide (MoO₃) buffer layer between the copper and the hole transport layer. Adding MoO‚ÇÉ improved charge transfer and increased the cell's maximum efficiency from 9.24% to 17.48%. A comparable cell using a silver electrode with the same buffer layer reached 17.74%.
However, the Cu/MoO₃ cells retained only about 60% of their initial efficiency after 100 hours of continuous illumination. Microscopic analysis showed that the copper did not corrode uniformly. Damage began at the sides of the copper electrode that extended beyond the underlying indium tin oxide (ITO) electrode and gradually progressed inward. The central active area, where the copper and ITO electrodes overlapped, was comparatively less affected.
The researchers attributed the difference to charge collection. According to their proposed explanation, the ITO electrode supported more effective charge extraction in the overlap area. At the sides without underlying ITO, charge accumulation promoted the movement of iodine-containing ions toward the copper.
The proposed degradation mechanism begins with the release of iodide ions from the perovskite absorber under continuous illumination. These ions migrate toward the rear electrode and are converted into iodine at the MoO‚ÇÉ layer. The iodine then reacts with copper to form copper iodide (CuI), resulting in electrode corrosion. Thus, while MoO₃ improved the electrical contact, the researchers found that it also contributed to the corrosion pathway.
Chemical analysis detected iodine near the copper electrode in aged cells and confirmed CuI formation in the corroded regions.
The researchers also examined how much electrode corrosion contributed to the performance loss. A representative cell used for this test had an initial efficiency of 16.40%, which fell to 10.16% after aging. Removing the damaged electrode and depositing a new copper layer increased the efficiency to 12.41%. The partial recovery indicated that copper corrosion contributed to the degradation, while irreversible damage had also occurred elsewhere in the cell.
To suppress corrosion, the researchers deposited a 5 nm bismuth interlayer between MoO₃ and copper. The interlayer acted as a physical barrier that limited iodine migration toward the copper electrode.
The protection depended on the deposition area. When the Bi/MoO₃ layers did not fully cover the area beneath the copper electrode, iodine could still reach the copper through the edges. Extending the Bi/MoO₃ deposition area beyond the copper electrode removed these pathways and prevented visible side corrosion.
The stabilized cells also included a PCBM layer at the SnO₂/perovskite interface to reduce the initial performance loss under illumination. With the PCBM layer and extended Bi/MoO₃ configuration, the cells retained 88% of their initial efficiency after 350 hours. Chemical analysis found less iodine near the copper electrode and reduced CuI formation compared with the reference cells.
The study was limited to small-area cells under continuous white LED illumination in a nitrogen-filled glovebox.
