Mutual Destabilization of Wide Bandgap Perovskite and PbI2 Inclusions through Interface Carrier Trapping

Lead halide perovskites (LHPs) are promising for versatile optoelectronic applications due to their tunable bandgap, but their photoinstability impedes the development. Lead iodide (PbI₂) inclusions are commonly seen in LHPs, either formed during the fabrication of the thin film or during its degradation under external stimuli like heat and illumination. Here It is shown how the coexistence of these different phases mutually boost their photodegradation. It is demonstrated that the photodecomposition of PbI₂ to form I₂, and the photo-induced halide segregation and decomposition of the mixed I─Br perovskite are both accelerated when they are interfaced. Such a mutual destabilization originates from the nature of PbI₂, both because it is rich in hole traps whose energy level spreads within the bandgap of perovskite and because it has a loose layered structure. Thus it becomes a sink for both photocarriers (holes) and ions (A-site cations and halides) transferred from the perovskite, respectively. It is shown that consistently reducing the PbI₂ inclusions in the mixed I─Br perovskite thin film improves the solar cell stability substantially, extending, in the model devices, the operational time from ≈10 to 500 h under illumination.