Spontaneous Passivation of Hybrid Perovskite by Sodium Ions from Glass Substrates: Mysterious Enhancement of Device Efficiency Revealed

The efficiency of a solar cell generally decreases over time due to degradation of the devices. Here we report a spontaneous increase of device efficiency for organic–inorganic hybrid perovskite (OIHP) solar cells after a period of storage that is speculated to be related to the sodium ions (Na+) diffused from the substrate. The efficiency of a p–i–n planar heterojunction structure device rises from 18.8 to 20.2% after 24 h of storage in nitrogen. The increased efficiency can be explained by the prolonged carrier lifetime and reduced trap density in the OIHP films. The expected contribution by Na+ in defect passivation has been evidenced by studying the evolution of the film's photoluminescence (PL) lifetime, trap density, and device efficiency over storage duration on varied substrates that either contain Na+ or do not. The passivation effect of Na+ is further identified by the improved PL lifetime observed in the OIHP film made on a silicon substrate with intentionally added Na+.