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Nonlinear Impedance Spectroscopy For Investigating Transient Dynamics In Perovskite Solar Cells
Paramadam, Sanish
Paramadam, Sanish
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Metal halide perovskite solar cells (PSCs) have emerged as promising next generation photovoltaicdevices due to their remarkable power conversion efficiencies, tunable bandgaps, and low cost solution processing. Despite these advantages, their long term operational stability remains a significant challenge compared to conventional silicon based technologies. This instability is primarily attributed to structural issues within the perovskite layer, often triggered by external stimuli such as moisture, heat, and light. While effective encapsulation mitigates degradation due to moisture, heat and illumination continue to activate mobile ions, leading to complex interactions with charge carriers. These coupled ionic and electronic processes give rise to nonlinear phenomena such as hysteresis and trap assisted recombination. Understanding such nonlinear behavior is therefore essential for improving device reliability and accelerating the commercialization of perovskite photovoltaics. Impedance spectroscopy is a standard and powerful diagnostic method for probing charge transport and interfacial properties in PSCs. However, conventional impedance analysis assumes a linear response to small perturbations, whereas the current voltage behavior of a solar cell is inherently nonlinear. To maintain linearity, traditional impedance spectroscopy uses small AC perturbations superimposed on a DC bias, restricting analysis to the near linear regime and excluding valuable nonlinear information. Major process such as ionic migration effects, charge accumulation, hysteresis, and recombination are fundamentally nonlinear, and their omission limits the understanding of device physics. In this work, nonlinear impedance spectroscopy (NLIS) is employed to resolve these nonlinear mechanisms by analyzing higher order harmonics generated under large AC excitation. The measured current is Fourier transformed to extract harmonic coefficients, from which higher order admittances are analytically derived. These higher order components isolate the nonlinear contributions to the device response, enabling clear differentiation between linear processes and nonlinear mechanisms. This dissertation presents a systematic study of nonlinear processes in PSCs using NLIS under three distinct conditions. The first investigates temperature dependence, revealing how ionic mobility and interfacial relaxation evolve from room temperature to cryogenic regimes. The second examines the effects of short term light exposure on ionic redistribution and interfacial band bending, highlighting reversible changes in higher order admittance spectra. The third explores the influence of interfacial passivation, demonstrating how surface treatments alter the magnitude and symmetry of nonlinear responses associated with recombination and ion accumulation.
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2026
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