Near-Surface Composition, Structure, and Energetics of TiO2 Thin Films: Characterization of Stress-Induced Defect States in Oxides Prepared via Chemical Vapor Deposition versus Solution Deposition from Sol–Gel Precursors

We present here a comprehensive comparison of near-surface composition, structure, and energetics of compact ultrathin TiO2 thin films deposited by two approaches: (i) chemical vapor deposition (CVD) and (ii) spin-casting of solution sol–gel (SG) precursors, on either indium tin oxide (ITO) or fluorine-doped tin oxide (FTO). These deposition methods are representative of approaches that have been widely used to create electron-selective charge harvesting and injecting electrical contacts in a variety of scalable optoelectronic platforms. We show that "strain" at atomic length scales in as-deposited device relevant 30–50 nm TiO2 films can be uniquely probed by combinations of grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), and X-ray and UV-photoemission (XPS/UPS). Temperature-dependent changes in crystallization, morphology, band edge energies, and defect density of states (DOS) above the valence band maximum (VBM) show that release of this strain ultimately lowers the concentration of electron-rich midgap defects which may be sites for chemical reactions with active layer precursors in perovskite solar cells and recombination centers in a broad array of optoelectronic devices. The average (101) lattice spacing (d) for as-deposited CVD-TiO2 crystallites is considerably smaller than the bulk crystal value and indicates enhanced lattice strain that is greater for CVD oxide films, just below annealing temperatures where crystallization begins, versus SG films and is released at different threshold temperatures and with different coherence lengths of the resultant crystalline grains. AFM studies show significant differences in organization of the anatase crystallites after annealing on ITO versus FTO substrates: CVD TiO2 films on ITO ultimately produce arrays of monodisperse and anisotropic anatase crystallites, whereas annealed TiO2 films on FTO produce anisotropic, polydisperse, and weakly coupled anatase crystallites. XPS studies show that binding energy differences in the principal core levels (O 1s–Ti 2p3/2 peaks) show abrupt decreases when the nearly amorphous TiO2 films are converted to crystalline arrays. High-sensitivity quantitative UPS studies (dynamic range >105 cts/s) show that both CVD and SG films exhibit defect states above VBM with relative concentrations between 0.05% and 1.0% within the ca. 2 nm probe depth of that experiment, correlated to the degree of microscopic stress in the oxide film. These electron-rich defect states demonstrate broad energetic distributions that extend nearly to the conduction band minimum (CBM). These near-surface defects are relevant to device efficiency and stability of perovskite solar cells and to conventional photovoltaic energy conversion platforms and dye-sensitized solar cells, where energetic alignment with charge transport layers and surface recombination brought about by midgap states can be efficiency and stability determining.