Surface Ligand Effects on Energetics, Charge Transfer, and Stability at Interfaces Between Metal Halide Perovskites and Organic Semiconductors

Organic-inorganic interfaces are ubiquitous in electronically active materials and devices, from all types of organic electronic devices, to inorganic-organic nanocomposite thermoelectrics, to metal halide perovskite based photovoltaics (PVs). An increased fundamental understanding of how surface chemistry impacts interfacial energetics, charge transfer processes, and material and interface stability will greatly benefit the development of many electronic devices and materials. In this work we will focus on metal halide and organometal halide perovskites, as they are extremely promising materials for solar cells, lasers, and light emitting diodes, and provide several advantages for fundamental studies. The big picture objective of this work is to understand how mixed surface modifiers consisting of extended ?-conjugated organic surface ligands (COL) and small molecular or atomic ligands influence energetics, charge transfer, and stability at perovskite-organic interfaces. Here, it is predicted that the small ligands can be utilized to provide precise control over interfacial energetics, passivate surface defect states, and stabilize the inorganic material, while the COLs can be utilized to enhance electronic coupling and compatibility with ?-conjugated organic materials. Metal halide perovskites are chosen as the inorganic material due to their high relevance to a number of emerging applications,1 the ability to finely tune energetics through varying the ratio of the halide atoms (e.g. APbBr(3-x)Ix),2 and their ability to be easily synthesized in thin film form, nanoparticles, nanowires, and nanoplatelets.3 The first aim of the work is to quantify ligand binding strengths and compare to theoretical predictions for varying binding groups with perovskites of varying A and X site ions (APbX3, where A=Cs or CH3NH3+ and X=Cl, Br, or I). The second aim is to determine how mixed surface ligands can be utilized to manipulate energetics, as probed through our custom built ultraviolet and inverse photoelectron spectroscopy systems (UPS and IPES, respectively), and charge transfer rates at planar perovskite-organic interfaces and perovskite nanoparticle-organic interfaces. The third aim is to determine how mixed surface ligands can be utilized to improve the stability of perovskite thin films and perovskite nanoparticles. The final aim is to test how these interface modifications can be utilized to control charge recombination, stability, and morphology in perovskite based PVs; and charge transport, electrical conductivity, and electroluminescence in perovskite nanoparticle-organic composites.