At C209 [PhD Thesis Presentation] ‐ Mr. Collin Stecker – “Scanning tunneling microscopy and photoelectron spectroscopy studies of lead halide perovskite surfaces, defect dynamics and the perovskite-CuPc hole transport material interface”

Over the past decade, lead halide perovskites (PVKs) have emerged as a promising new light absorber material for thin film solar cells. Lab-scale perovskite-based photovoltaic devices have made impressive gains in power conversion efficiency (PCE) and are nearing the same efficiency as silicon-based solar cells. However, perovskite solar cells lack stability, and this is a major obstacle preventing commercialization. The interfaces between the different layers in a device have been implicated as potential areas of charge recombination and material degradation. Understanding the perovskite surface is crucial because it is involved in these interfaces and also because it is the layer that is in first contact with extrinsic species that may cause degradation. Defects in the perovskite material have also been identified as a potential cause of sub-optimal performance. Additionally, some strategies for improving stability have included using mixed halide perovskites, or perovskites containing cesium instead of or mixed with organic cations such as methylammonium (MA). Reports at the device engineering level are plentiful, but fundamental, atomic-scale understanding of the perovskite surface is scarce, especially from an experimental perspective. This thesis uses scanning tunneling microscopy (STM) to examine the perovskite surfaces of CsPbBr3 and mixed halide perovskites MAPbBr3-yIy and MAPbBr3-zClz, the surface defects of MAPbBr3 and their dynamics, as well a device-relevant perovskite/hole transport material (HTM) interface comprised of MAPbX3/CuPc, where X=I or Br. Furthermore, X-ray photoelectron spectroscopy (XPS) is used to characterize the sample composition and stability, and electronic properties are investigated by ultraviolet (UPS) and inverse photoemission spectroscopies (UPS and IPES). Where feasible, these experimental results are corroborated by density functional theory (DFT) calculations performed by collaborators. The goal of this thesis is to provide fundamental insight regarding perovskite surfaces, their defects and their dynamics, and their interfaces with other materials, which may help guide applied research toward creating devices with better performance and stability.