Influence of titanium dioxide surface activation on the performance of mesoscopic perovskite solar cells

Highlights: • The type of TiO₂ scaffold activation method greatly influences the PSC performance. • Poor wetting inside the scaffold promotes PbI₂-to-perovskite conversion. • Good wetting on the top of the film improves perovskite capping layer quality. • Plasma activates the entire scaffold, while UV only activates the topmost layer. • Device performance is improved with UV activation, but worsened with plasma. -- Abstract: Perovskite solar cells with record efficiencies already above 24% are a highly promising clean energy technology. However, the reproducibility in their fabrication has proven to be challenging and needs more attention. Here we demonstrate that surface activation of the mesoscopic titanium dioxide (TiO₂) scaffold, utilized in the two-step perovskite synthesis process, significantly affects the final device performance. Irradiating the mesostructured substrate with ultraviolet (UV) light prior to lead iodide (PbI₂) deposition has a positive effect on the short-circuit current density and on the overall device performance (leading to a >20% increase in efficiency in our devices). As most of the UV light is absorbed in the topmost TiO₂ layer, the interior of the scaffold remains less activated. This results in a sparsely packed PbI₂ structure that facilitates an efficient conversion to the perovskite, while the activated topmost surface improves the perovskite capping layer. On the contrary, plasma treatment of the scaffold also activates the interior parts of the scaffold, which leads to a dense PbI₂ structure that hampers the conversion and causing a >25% efficiency drop. We show that also minor changes in the surface properties of the mesoporous TiO₂ scaffold can affect the device performance, which could explain some of the large efficiency variations observed between laboratories.