Impact of Film Thickness of Ultrathin Dip-Coated Compact TiO2 Layers on the Performance of Mesoscopic Perovskite Solar Cells

Uniform and pinhole-free electron-selective TiO₂ layers are of utmost importance for efficient perovskite solar cells. Here we used a scalable and low-cost dip-coating method to prepare uniform and ultrathin (5–50 nm) compact TiO₂ films on fluorine-doped tin oxide (FTO) glass substrates. The thickness of the film was tuned by changing the TiCl₄ precursor concentration. The formed TiO₂ follows the texture of the underlying FTO substrates, but at higher TiCl₄ concentrations, the surface roughness is substantially decreased. This change occurs at a film thickness close to 20–30 nm. A similar TiCl₄ concentration is needed to produce crystalline TiO₂ films. Furthermore, below this film thickness, the underlying FTO might be exposed resulting in pinholes in the compact TiO₂ layer. When integrated into mesoscopic perovskite solar cells there appears to be a similar critical compact TiO₂ layer thickness above which the devices perform more optimally. The power conversion efficiency was improved by more than 50% (from 5.5% to ∼8.6%) when inserting a compact TiO₂ layer. Devices without or with very thin compact TiO₂ layers display J–V curves with an “s-shaped” feature in the negative voltage range, which could be attributed to immobilized negative ions at the electron-extracting interface. A strong correlation between the magnitude of the s-shaped feature and the exposed FTO seen in the X-ray photoelectron spectroscopy measurements indicates that the s-shape is related to pinholes in the compact TiO₂ layer when it is too thin.