The physics of ion migration in perovskite solar cells: Insights into hysteresis, device performance, and characterization

Perovskite solar cells promise to reach the highest efficiencies at the lowest costs, but the mobile ions create challenges in both cell measurements and performance improvement. Being able to understand the ion migration mechanism and its impacts would help to solve relevant issues and may open up opportunities for further development. Although some efforts have been made here, explanations are often too exotic resulting in not only some controversies but the neglect of some important aspects addressable by familiar concepts. This paper uses familiar knowledge in solar cell device physics to explain the ion migration mechanism and its impacts under conditions of interests. It is shown that, when depletion layers contract such as in a forward current-voltage scan, tardy ionic charges can distort the perovskite bands hence reduce the net currents before they move to new equilibriums, causing the hysteresis effect. As ion migration impacts the electron and hole distributions, it can affect the Shockley-Read-Hall and Auger processes differently, hence may either reduce or increase the ratio of radiative to nonradiative recombination. This previously unaddressed mechanism for variation in the electron-hole recombination can explain photoluminescence quenching and enhancement as well as performance degradation and improvement during ion migration.