Well-Defined\nNanostructured, Single-Crystalline TiO₂ Electron Transport\nLayer for Efficient Planar Perovskite\nSolar Cells

Abstract

An\nelectron transporting layer (ETL) plays an important role in\nextracting electrons from a perovskite layer and blocking recombination\nbetween electrons in the fluorine-doped tin oxide (FTO) and holes\nin the perovskite layers, especially in planar perovskite solar cells.\nDense TiO₂ ETLs prepared by a solution-processed spin-coating\nmethod (S-TiO₂) are mainly used in devices due to their\nease of fabrication. Herein, we found that fatal morphological defects\nat the S-TiO₂ interface due to a rough FTO surface, including\nan irregular film thickness, discontinuous areas, and poor physical\ncontact between the S-TiO₂ and the FTO layers, were inevitable\nand lowered the charge transport properties through the planar perovskite\nsolar cells. The effects of the morphological defects were mitigated\nin this work using a TiO₂ ETL produced from sputtering\nand anodization. This method produced a well-defined nanostructured\nTiO₂ ETL with an excellent transmittance, single-crystalline\nproperties, a uniform film thickness, a large effective area, and\ndefect-free physical contact with a rough substrate that provided\noutstanding electron extraction and hole blocking in a planar perovskite\nsolar cell. In planar perovskite devices, anodized TiO₂ ETL (A-TiO₂) increased the power conversion efficiency\nby 22% (from 12.5 to 15.2%), and the stabilized maximum power output\nefficiency increased by 44% (from 8.9 to 12.8%) compared with S-TiO₂. This work highlights the importance of the ETL geometry\nfor maximizing device performance and provides insights into achieving\nideal ETL morphologies that remedy the drawbacks observed in conventional\nspin-coated ETLs.