Insights Into the Local Heating Effects in Triple-Cation Mixed-Halide Perovskite Cells: Charge Dynamics, Coherent Phonons, Anion Segregation

The behavior of triple-cation mixed halide perovskite solar cells (PSCs) under ultrashort laser pulse irradiation at varying fluences is investigated, with a focus on local heating effects observed in femtosecond transient absorption (TA) studies. The carrier cooling time constant is found to increase from 230 fs at 2 µJ cm⁻ to 1.3 ps at 2 mJ cm⁻ while the charge population decay accelerates from tens of nanoseconds to the picosecond range within the same fluence range. At fluences between 0.5 and 5 mJ cm, distinct oscillations in the TA signal (≈1.1 MHz) reveal the presence of coherent longitudinal acoustic phonons (CLAP). These phonons induce lattice strain propagating at the speed of sound through the perovskite layer and exhibit relatively long damping times. TA spectra further reveal a partially reversible segregation of iodide and bromide anions under pulsed excitation. Interestingly, higher local heating at increased pump fluences slows the segregation process, with time constants extending from 40 min at low fluences to 110 min at high fluences. However, the continuous irradiation results in significantly smaller segregation effects compared to ultrashort pulse irradiation.