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Electron spectroscopy and the study of metal halide perovskite surfaces and interfaces 

Abstract The formidable promises of the “re-discovered” class of organic/inorganic metal halide perovskites (MHP) such as methylamonium lead tri-iodide (MAPbI3), and the rapid and steady rise in device performance achieved with these materials over the past seven years, have triggered a flurry of research all over the world. This talk reviews our efforts to understand key surfaces and interfaces of these materials. We first report a combined ultra-violet/inverse photoemission spectroscopy (UPS/IPES) - DFT study of the surface electronic structure of several 3D-MHPs, e.g., MAPbI3, MAPbBr3, and CsPbBr3, which yield valence and conduction band edge positions (VBM, CBM), ionization energy and electron affinity (IE, EA), energy gap. An unusually low density of states is found at the VBM of these materials, with potential consequences on device physics. We then look at recent measurements of the electronic structure of MHP interfaces with hole and electron transport layers used for carrier extraction/blocking in photovoltaic devices. Specifically, the role of p-doping is investigated in the case of interfaces between the HTL poly(triarylamine) (PTAA) and CsPbBr3.  We then turn to two-dimensional metal halide perovskites (2D-MHP) and present electronic structure measurements on several 2D butylammonium methylammonium lead iodide and bromide compounds, BA2MAn-1PbnI3n+1, n=1 - 4. XRD, AFM, UV-vis absorption, and UPS/IPES spectroscopies are used to investigate these compounds.  Their single-particle gap is obtained from UPS/IPES results, and compared with optical absorption measurements to deduce an exciton binding energy (EB). In agreement with previous results, EB is found to be large for n=1 and 2 (390 and 110 meV, respectively), but drops rapidly for n=3 and above. Finally, a simple model is presented to justify the electron and hole levels and the single particle gap in these quantum wells structures.   

inquires please contact Rosa Vargas rvargas@sas.upenn.edu