Positive Feedback Mechanism of Probe Sonication for the Perovskite Films in Solar Cells

The performance of perovskite solar cells (PSCs) is governed by the quality of perovskite films, whereby compact, pinhole-free perovskite films are desired, in addition to its composition. We have demonstrated probe sonication as a processing technique to provide positive feedback for enhancing the perovskite film quality and photovoltaic parameters, with two systems, CH₃NH₃PbI₃ (MAPbI₃) and Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃. In probe sonication, the ultrasound results in the formation, growth, and collapse of the bubbles through shock wave inside the gas phase of the collapsing bubble. This phenomenon has a chemical impact on the nucleation of the perovskite phases and interconnectivity of the grains. The 60 min sonicated films with stronger hydrogen bonding network are devoid of unwanted Pb⁰, δ-FAPbI₃, and PbI₂ phases, having tightly packed homogeneous grains, minimum electron-hole recombination pathways, and improved light absorption. The surface potential remains mostly unaltered across the grains and grain boundaries, and the realignment of the Fermi energy (E_F) favors facile carrier transport. The photoconversion efficiency (PCE) of the MAPbI₃ and Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃ devices is improved by 28.1 and 17.2% in comparison to the pristine perovskites, respectively. The 60 min sonicated Cs_0.17FA_0.83Pb(I_0.83Br_0.17)₃ PSC has 20.20 ± 0.40% PCE with 1000 h ambient stability having >60% retention of the original PCE.