Is it possible for a perovskite p-n homojunction to persist in the presence of mobile ionic charge?

TL;DR

This paper critically examines the stability of p-n homojunctions in perovskite solar cells, arguing that mobile ionic charges undermine the junction's effectiveness, challenging prior claims of enhanced charge separation.

Contribution

The study demonstrates through simulations that mobile ionic defects in perovskites likely screen the p-n junction, questioning previous experimental interpretations of improved charge separation.

Findings

Mobile ionic defects are highly mobile at room temperature in methyl-ammonium lead iodide.

Large densities of ionic charge can screen the electric field of the p-n junction.

Experimental data does not support the existence of a stable p-n homojunction in perovskites.

Abstract

Recently Cui et al. reported on the fabrication a p-n homojunction perovskite solar cell (PSC) using stoichiometric control of sequentially-deposited perovskite layers. The authors propose that the junction leads to an enhanced electric field in the perovskite absorber resulting in improved charge separation. In this response to Cui et al. 2019 we show that the experimental data presented in the paper does not directly support this claim. Furthermore, Cui et al.'s thesis is not compatible with the large body of existing literature showing that mobile ionic defects present in methyl-ammonium lead iodide (MAPI) and its derivatives are highly mobile at room temperature. Using drift diffusion device simulations we show that large densities of mobile ionic charge in the system are likely to the screen any beneficial effects of a p-n homojunction.