Abstract
As perovskite solar cells have yielded impressive efficiency at a low cost, the focus has shifted to increase their service life as they are plagued by rapid degradation. Refreshingly, CsPbBr3 solar cells built on a conductive ZnO nanowire electron transport layer with a graphite counter electrode not only avoided degradation but also showed some of the reverse trends under specific conditions, showing significant maturation over time. In this work, this phenomenon is first confirmed to be reproducible from a large sample size with on average a 40 ± 10% increase in efficiency after 2 weeks of storage. To explore the mechanisms of this positive maturing effect, samples were stored under different controlled conditions and tested regularly by using scanning electron microscopy, powder X-ray diffraction, current–voltage (IV) curves, and impedance spectroscopy. The samples stored in a methanol atmosphere presented a dramatic positive effect, giving a 4-fold increase in efficiency after 2 days of storage. However, in the saturated H2O environment, the device performance rapidly degraded. By observing the solar cell performance affected by various storage conditions, including various solvent vapors, light illumination, and an inert gas (N2), we suggest three possible complementary factors. First, solvents shifted the equilibrium of crystal phase ratio of CsPbBr3 to CsPb2Br5. Second, the CsPbBr3 grain size was reduced with improved electrical contact with the conductive ZnO nanowires. Finally, ion migration and accumulation lead to the formation of local p–n junctions at crystal grain boundaries with improved charge separation. This was evidenced by the increased kinetic relaxation times on ionic time scales. Rather than degrading, under appropriate conditions, these cells were able to increase in value/efficiency over storage time. By elucidating the underlying mechanisms for the CsPbBr3 solar cell stability, the work offers guidelines for improving perovskite solar cell long-term efficiency.