Zn₂SnO₄-based photoelectrodes for organolead halide perovskite solar cells

We report a new ternary Zn₂SnO₄ (ZSO) electron-transporting electrode of a CH₃NH₃PbI₃ perovskite solar cell as an alternative to the conventional TiO₂ electrode. The ZSO-based perovskite solar cells have been prepared following a conventional procedure known as a sequential (or two-step) process with ZSO compact/mesoscopic layers instead of the conventional TiO₂ counterparts, and their solar cell properties have been investigated as a function of the thickness of either the ZSO compact layer or the ZSO mesoscopic layer. The presence of the ZSO compact layer has a negligible influence on the transmittance of the incident light regardless of its thickness, whereas the thickest compact layer blocks the back-electron transfer most efficiently. The open-circuit voltage and fill factor increase with the increasing thickness of the mesoscopic ZSO layer, whereas the short-circuit current density decreases with the increasing thickness except for the thinnest one (∼100 nm). As a result, the device with a 300 nm-thick mesoscopic ZSO layer shows the highest conversion efficiency of 7%. In addition, time-resolved and frequency-resolved measurements reveal that the ZSO-based perovskite solar cell exhibits faster electron transport (∼10 times) and superior charge-collection capability compared to the TiO₂-based counterpart with similar thickness and conversion efficiency.