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 N+ 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 Nat or do not. The passivation effect of N+ is further identified by the improved PL lifetime observed in the OIHP film made on a silicon substrate with intentionally added Na+.