Halide perovskites have generated tremendous interest as low-cost semiconductors for optoelectronics, such as photovoltaics, lasers, and light emitting diodes due to their extraordinary optical and transport properties. Perovskite photovoltaics have demonstrated a meteoric rise in power conversion efficiencies and drawn considerable interest as a next-generation solar energy material. The rapid development has centered around lead-based derivatives, and concerns regarding the toxicity of lead has sparked interest in low toxicity and more environmentally friendly halide perovskites. In this regime, tin (Sn) is regarded as a prominent alternative owing to the ideal bandgap and reduced toxicity exhibited by Sn-halide perovskites. Not without its own shortcomings, tin perovskite photovoltaics currently suffer from lower efficiencies and stability than their lead counterparts. Here, interfaces within the device are of upmost importance as the surface chemistry and energetics significantly impact the performance and stability of photovoltaic devices.

To investigate the interfacial energetics in tin perovskites, low energy ultraviolet photoelectron spectroscopy (UPS) and low energy inverse photoelectron spectroscopy (IPES) systems were developed. For the former, a novel H Lyman-α vacuum ultraviolet photon source was introduced and characterized to show its ability to mitigate sample degradation and reduce background noise. Application of these systems to tin perovskites provided insight into the variation of reported ionization energies of neat films and revealed that both the inclusion of common additives and sustained air exposure can play a considerable role on measured energetics. UPS, IPES, and X-ray photoelectron spectroscopy (XPS) were also employed to perform the first direct measurements of frontier electronic energy levels at the formamidinium tin iodide (FASnI3)/C60 interface. We observe band bending in both materials and transport gap widening in FASnI3 at the interface with C60. XPS results show that this is due, in part, to iodide migration from the perovskite into C60, which results in a distortion of the perovskite surface structure and n-doping of C60. To further probe surface/interfacial chemistry and energetics, perovskite films were modified with small molecules. FASnI3 treated with carboxylic acids and bulky ammonium substituted surface ligands resulted in slight alterations of the interfacial energetics, which resulted in a more favorable energetic landscape and improved PV performance. Further, PV devices treated with a fluorinated carboxylic acid derivate showed greatly enhanced device stability. Finally, the ability of surface ligands to reduce iodide diffusion out of the perovskite layer was examined.