Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV23)
DOI: https://doi.org/10.29363/nanoge.hopv.2023.097
Publication date: 30th March 2023
Lead halide perovskites have been in the lime light of emerging photovoltaic materials the last decade, due to their high absorption coefficient, high defect tolerance and charge mobility, and power conversion efficiency exceeding 25% in solar cell devices. The photoexcited charge density response is here important to describe lead halide perovskites under operation, and is in turn related to the material structure [1,2], photoinduced response, and subsequent electronic and lattice relaxation in the system. In this contribution, we present investigations of the photoinduced ion migration mechanism and nature of the excited state [3-6] and their relation to pathways for electronic and lattice relaxations with both experimental and theoretical probes. In particular, we present how the A-site cation and type of halide affect the chemical bonding and photoinduced ion movement in the system. Experiments from photoinduced Stark-effects and Raman spectroscopy are presented as well as corroborating theoretical investigations using both ground state and time-dependent density-functional theory (TDDFT). We show that the excess energy after thermalization into phonons under blue-light illumination is large enough to overcome the activation energy for iodide displacement, and can thus trigger vacancy formation and ion movement in contrast to red-light illumination [4,5]. In addition, we briefly discuss the role of mixed (Cs, FA) monovalent A-site cations in the view of their excited state response and subsequently enhanced optoelectronic properties [6]. The results form a basis for a fundamental understanding of the excited-state properties of halide perovskite material to reveal the underlying mechanism for vacancy formation, photoinduced halide segregation, excitation energy dependent hysteresis effects, and reported defect tolerance when using organic or mixed A-site cations. In an extension, the results give rationale for using dipolar A-site cations and mixed halide perovskites to decrease halide migration, and the mechanistic origin of photoinduced vacancy formation, excitation energy dependent hysteresis, and reported stability issues under blue and UV-light illumination.
We acknowledge financial support from the Swedish Research Council (grant number 2019-05591), the Swedish Energy Agency (grant number 50667-1, P2020-90215), and the Swedish National Infrastructure for Computing (SNIC) for providing computational resources under project SNIC2020-5-294.
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