We introduce lead ions adsorbed to TiO2 as a surface modification, which serves as a model system to begin understanding how the TiO2/perovskite interface influences nucleation and growth of mixed-halide cesium perovskites upon this metal oxide surface. Understanding the fundamental science at this interface is critical for elucidating how tuning atomic features of the chemically-diverse TiO2 surface can affect larger-scale properties, such as the charge collection efficiency when used as an electron selective interlayer. A combination of photoemission spectroscopies, x-ray diffraction, and atomic force microscopy are used to investigate the physical properties of both the modified TiO2 and the perovskite active layer on TiO2. By photoemission spectroscopy, we show that lead adsorbed from PbI2 preferentially binds to TiO2 at surface hydroxyl sites with a surface coverage of ca. 2% and that this bonding event lowers the work function of TiO2 by ca. 0.4 eV. Grazing-incidence wide-angle X-ray scattering (GIWAXS) data reveals that perovskites on lead-adsorbed TiO2 exhibit crystal growth with greater preferential orientation of the (100) axis perpendicular to the surface normal when compared to perovskites grown on the unmodified substrate. Moreover, perovskite films grown on lead-modified TiO2 show larger grains and more layer-by-layer film growth by atomic force microscopy. These results demonstrate that atomic-scale changes to the interfacial region of this system result in changes visible at the top surface of the perovskite film and that lead-modification offers a unique method to tune both the nucleation and growth and the energetic properties of these active layer films. This work also highlights the importance of understanding the reactivity of TiO2 and how surface passivation by lead adsorption can enhance the quality of perovskite thin films.

Enhancement of perovskite crystallinity by interfacial lead adsorption on TiO2

255th National Meeting (2018)

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