Charge Carrier Dynamics in Organo Lead Halide Perovskites

In the last five years, organo lead halide perovskites have emerged as a new and efficient player in the field of photovoltaics, jumping in a short time to an advanced front position. This class of materials has also shown proficient light emission capabilities, and perovskite-based light emission devices have been demonstrated as well. Therefore, the optical characterization of these materials has been widely used to explain and evaluate their light absorption/emission characteristics. Despite their remarkable success, they also have shown severe intrinsic sensitivity to their surrounding conditions, e.g. UV light, oxygen, and moisture. This thesis is composed of three studies on the optical characterization of perovskites. The thesis starts with an introductory chapter on organo lead halide perovskites, which includes a literature review on the possible origins of perovskite phase instability. Next, the first study of this work presents a detailed investigation on the photovoltaic and spontaneous emission properties of FAPbBr3 perovskite films (FA = Formamidinium). The study first introduces the absorption/emission detailed balance equilibrium in a working perovskite solar cell, based on which the device characteristics are calculated under different ideal/non-ideal conditions. Time-resolved photoluminescence (TRPL) is shown to be a good measure for evaluating the maximum possible open-circuit voltage (Voc) of the device. Perovskite solar cells were fabricated, using two differently prepared FAPbBr3 perovskite films, and the results were correlated with TRPL measurements. A record Voc of 1.42 V that is higher than the value of the built-in potential (as reported in the literature) developed by the device contacts was achieved, and consequential insights about charge separation dynamics are discussed. Amplified spontaneous emission (ASE) was successfully demonstrated for the same perovskite films. Bandgap renormalization (BGR) was observed as a red-shift in the ASE peak, and the gain behavior is shown to allow the estimation of BGR values directly from the ASE peak position. A value of 5-7 × 10-9 was obtained for the BGR constant. The second study in this work addresses the UV-induced damage on MAPbI3 perovskite (MA = Methylammonium). The study reports that UV degradation can occur in the perovskite bulk, and is not only associated with the presence of oxygen or TiO2 surface, as reported by some recently published works. UV degradation is found to switch off when the sample is cooled below 200 K, which indicates the presence of a thermally activated process. Based on that, the direct bond dissociation in the thermally liberated MA cations is proposed to be a first UV-induced degradation step, which can be accompanied/followed by other degradation processes. The third study demonstrates cathodoluminescence (CL) mapping of MAPbI3-xBrx microcubes. An abnormal CL intensity contrast between the center and edge of single microcubes is observed, where the CL emission is highly reduced in the centers at room temperature, and almost vanishes at 4 K, while the edges have prominent CL emission. The observed behavior is correlated, with the help of STEM-EDX maps, with a noticeable deficiency of MA ions in the centers. The CL maps are also correlated with Monte Carlo simulations under different e-beam acceleration voltages, and the enhanced edge CL emission is explained as a result of diffusion assisted carrier recombination.


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