Exploring the Impact of TiO2 Surface Chemistry on Nucleation and Growth of Perovskite Active Layers for Photovoltaic Applications

Abstract

We introduce lead ions adsorbed to TiO2 as a surface modification, which serves as a model system to begin understanding how the chemistry at the TiO2/perovskite interface influences nucleation and growth of mixed-halide cesium perovskites. The surface chemistry of TiO2 was incrementally changed by subjecting the thin films to both oxygen and argon plasma treatment and lead adsorption thereafter. A combination of x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), grazing-incidence wide-angle x-ray scattering (GIWAXS), and atomic force microscopy (AFM) were used to evaluate the surface chemistry, crystallinity, and morphology of both the modified TiO2 and the perovskite active layer on TiO2 with the hypothesis that lead adsorption on TiO2 would aid in the initial nucleation of the perovskite film by decreasing interfacial disorder by titrating away the reactive hydroxyl sites on the surface. By photoemission spectroscopy, we show that lead adsorbed from PbI2 preferentially binds to TiO2 at surface hydroxyl sites with a surface coverage ranging from 26-68% of a monolayer depending on the initial surface treatment. GIWAXS data reveals that perovskites on TiO2 exhibit crystal growth with greater preferential orientation of the (100) axis perpendicular to the surface normal and that the degree of preferential orientation depends on the availability of surface hydroxyl sites for the perovskite precursor materials to bind to. Moreover, perovskite films exhibited greater crystallinity and coherence lengths on substrates that have more available hydroxyl groups, such as as-deposited TiO2. AFM images evaluating the morphology of the perovskite films are consistent with findings acquired by XPS, XRD, and GIWAXS, demonstrating that atomic-scale changes to the interfacial region of this system result in changes visible at the top surface of the perovskite film. Although the data does not support the initial hypothesis, this work highlights the critical importance that adjacent hydroxyl groups have in the nucleation and growth of perovskite films. Passivation of these reactive sites by lead adsorption inhibits the initial crystal growth. Ultimately, understanding the importance of the reactive sites on TiO2 paves the way for future work on controlling hydroxyl density with the intent of controlling the nucleation and growth of perovskite active layers on TiO2 for photovoltaic applications.