Nature of Proton and Light induced Defects in Lead Halide Perovskites

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As record efficiencies of hybrid perovskite-based single and perovskite/silicon tandem solar cells approach those of conventional silicon solar cells, their stability becomes the most important issue. Although research on the stabilization of hybrid perovskite-based devices progresses, radiation-induced defect states remain under-investigated. Under operation, however, any solar cell is exposed to the UV/VIS and NIR part of the electromagnetic spectrum. Especially the high energetic part of the spectrum evidently forms trap states that act as non-radiative recombination centers. The same phenomena render spectroscopic methods that require UV, electron, or X-Ray radiation unreliable.This project proposal aims at a fundamental understanding of radiation-induced localized defects. Hence, defects will be generated in a controlled way employing light as well as high energetic proton irradiation. High energetic protons are an ideal choice for defect generation experiments, since they are capable of creating defects by dissociation of bonds in the organic cations and by displacing individual nuclei of the inorganic lattice. Defects generated by light and proton irradiation will be characterized with the aim to identify their electronic, structural, and optical properties and their impact on charge transport. For this purpose, a unique combination of electron paramagnetic resonance (EPR), vibrational spectroscopies, and surface photovoltage spectroscopy will be employed. To identify the nature of the defect states, we will not follow the concept of compositional engineering, which is widely used for high-efficiency solar cells. Instead, analysis will begin with one of the simplest hybrid perovskites: CH3NH3PbI3. Then the halide anions, as well as the organic cations, will be interchanged one by one. Additionally, lead, carbon, nitrogen and hydrogen isotopes will be incorporated into the perovskites to unambiguously identify the microscopy structure of localized defects and defect complexes. With this approach we will establish a sound understanding of the formation and the nature of radiation-induced defect states. This is an important step that will lay the foundation for developing stable hybrid perovskites.