Publication date: 17th February 2025
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 high power conversion efficiency in solar cell devices. The photoexcited charge density response is here important for understanding lead halide perovskites during 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. 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]. Here, a dipolar A-site cation would decrease the energy of defect formation, but instead impede defect migration [6]. We highlight the high thermal expansion of the system compared to common substrate materials [7], an effect that recently has been found to be crucial to mitigate in order to achieve high-efficiency solar cells. [8]. Recent collaborative work has exploited this knowledge to fabricate solar cells with certified efficiencies above 26% [9, 10] with high tolerance to thermal stress [10]. We will briefly also report how the type of halide and number of monolayers affect the excitonic and vibrational properties in 2D perovskites. By addressing ion migration, interfacial strain management, and quantum tuning through halide selection and layer control, the findings contribute to the development of high-performance, stable perovskite-based technologies with tailored optical and electronic properties.
We acknowledge financial support from the Swedish Research Council (grant number 2023-05244), the Swedish Energy Agency (grant number 50667-1, P2020-90215), and the National Academic Infrastructure for Supercomputing in Sweden (NAISS) for providing computational resources under project NAISS 2024/5-372.
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