  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.
