2, University of Cologne, Cologne, , Germany
We elucidate how the surface chemistry of compact TiO2 electron-selective contacts dramatically affects the nucleation, growth, bulk composition and energetics of device-relevant hybrid perovskite (PVSK) thin films that are processed via solution- and vacuum-based deposition methods. The surface chemistry and energy level alignment of compact TiO2 thin films, which are grown by either chemical vapor deposition (CVD) or sol-gel methods, is systematically modified by combinations of annealing temperature/environment, plasma activation and end-functional silane modification. The TiO2 surface chemical composition (e.g., stoichiometry, hydroxyl concentration, monolayer composition, etc.) is quantified by high-resolution, monochromatic X-ray photoelectron spectroscopy (XPS), and the frontier orbital energetics are determined using ultraviolet photoelectron spectroscopy (UPS), which shows that the work function and energy level alignment of the TiO2 contact can be tuned by approximately 1 V depending on the treatment conditions. In situ XPS and UPS measurements of incrementally co-evaporated methylammonium lead triiodide (MAPbI3) films reveal that initial nucleation and subsequent growth of MAPbI3 PVSK films strongly depends on the chemical functionality of the TiO2 surface, where silane-modified surfaces surpress chemical reactions between the PVSK precursors (e.g., methylammonium iodide) and TiO2 surface species (e.g., hydroxyls). X-ray diffraction (XRD) studies of solution-processed PVSK films based on methylammonium (MA) and formamidinium (FA) organic cations reveal that the bulk film composition and crystallinity are controlled by the TiO2 work function and surface energy, where low work function and low surface energy TiO2 contacts show enhanced conversion of the precursors to the PVSK phase and higher crystallinity. In addition, scanning electron microsope (SEM) images show that the TiO2 surface free energy, which is tailored by end-functional silane monolayers and plasma activation, and processing conditions have a strong influence on the morphology (e.g., grain size) of both vacuum- and solution-processed PVSK films based on MA and FA organic cations. These combined studies show how the formation mechanism, interfacial/bulk energetics, chemical composition, crystallinity and morphology of device-relevant PVSK active layers is critically dependent on the surface chemistry and energetics of TiO2 contacts, which have significant consequences related to the processing, stability and operation of next-generation optoelectronic device platforms.