CuI passivation layer for stabilizing BiI₃-sensitized QDSSCs: long-term performance stability assessment

Poor performance stability in quantum dot-sensitized solar cells (QDSSCs), mainly arising from electrolyte-induced degradation at the ETM/QD interface, has not been widely investigated. We addressed this problem by using PbI₂ and CuI passivation layers for BiI₃-sensitized QD-sensitized solar cells with solid-state electrolyte species due to their higher conduction band and valence band than that of the BiI₃ sensitizer and TiO₂ electron transport material (ETM), and good thermal stability. We improved the performance stability of BiI₃-sensitized solar cells over 30 days using the CuI passivation layer and reached a maximum efficiency of 6.55% (degraded to 4.82%, which was 73.58% of the initial day), which was superior to the PbI₂-passivated device. The CuI passivation layer produced the least defective surface of the photoanode (as confirmed by field emission scanning electron microscopy), which restricted direct contact between the ETM and electrolyte, thereby reducing leakage of the electrolyte. As a result, the sensitizer did not undergo degradation under prolonged illumination. Furthermore, J-V traces under dark conditions confirmed a very small leakage current, which was a sign of better performance stability of the device. These findings suggested that the CuI passivation layer offered good performance stability of the BiI₃-sensitized QDSSC as compared with that of the PbI₂-passivated device.