Hydrophilic Surface-Driven Crystalline Grain Growth of Perovskites on Metal Oxides

Metal oxide charge-transport layers (CTLs) are known to influence the properties and performance of organometal halide perovskite solar cells (PSCs). Accordingly, this work demonstrates, in detail, the crystalline grain growth mechanism of CH3NH3PbI3 (MAPbI3) depending on the hydrophilicity of CTLs such as compact (c)-TiO2, mesoporous (mp)-TiO2, SnO2, and NiOx. Importantly, smaller water contact angles of CTLs (11.5° for SnO2; 21.4° for mp-TiO2; 27.8° for NiOx; and 30.7° for c-TiO2) were linked to larger average grain sizes of a top-layered perovskite film (308.2 nm for SnO2; 266.4 nm for mp-TiO2; 209.7 nm for NiOx; and 185.4 nm for c-TiO2), indicating 'hydrophilic surface-driven crystalline grain growth' of MAPbI3 on metal oxides. Furthermore, by estimating the solubility parameter (δ) of CTLs, we explain that, when Δδ = δCTL – δsolvent is large, the MAPbI3 grain size increases because of a limited chance of nucleation during the antisolvent-assisted one-step coating process. However, it is notable that the hydrophilic surface of CTLs may induce instability of MAPbI3 under humidity. Finally, the highest power conversion efficiency (∼19.03%) was obtained when SnO2 served as an electron-transport layer for the planar heterojunction PSCs.