Coupling of Ionic and Electronic Processes in Lead Halide Perovskite Devices and Nanostructures




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Lead Halide Perovskites (LHPs) such as Methylammonium Lead Iodide (MAPbI3) or Cesium Lead Bromide (CsPbBr3) have garnered massive attention from researchers in photovoltaic, light emitting, and other optoelectronic fields as an interesting class of materials with attractive semiconducting and optical properties. Fabrication processes of LHPs are relatively simple compared to conventional inorganic semiconductors and don’t require powerful or expensive equipment which is promising for commercial development. However, they have shown dynamic performance behaviors and instabilities that have yet to be fully understood. These dynamic behaviors on the time scale of seconds have been attributed to their mobile charged point defects redistributing themselves during device operation. The ions (or charged defects) act as mobile intrinsic donors and acceptors that can be utilized in device operations. This thesis discusses the physical origin that strongly couples the electronic and ionic transport of LHPs and demonstrates devices that exploit and utilize this coupling. This coupling enables the dual functionality of mixed halide solar cells to also act as effective light emitting devices. We also demonstrate that utilizing extrinsic ionic salts such as lithium hexafluorophosphate (LiPF6) can serve as sacrificial ions that help protect the bulk of the perovskite from fast degradation. We studied the dynamic performance of LHP optoelectronic devices configured as perovskite lightemitting electrochemical cells (PeLECs) under various temperatures and conditions. We develop novel equivalent circuit models to extract the diffusion coefficients and concentrations of dominant ionic species at play in PeLEC devices in vacuum and dry air, in the dark or under illumination, and at different temperatures. Activation energies of ionic species in PeLECs were calculated by the temperature dependence of fitted parameters from diffusion elements in the equivalent circuits used to model impedance spectroscopy measurements. This thesis advances the knowledge and understanding of ion migration (or charged point defect transport) in LHP devices and how it affects their performance.



Physics, Condensed Matter, Chemistry, Physical, Chemistry, General