CsF additive enables the efficiency and stability promotion of carbon-based hole-transport-layer-free CsPbI2Br perovskite solar cells

The inorganic perovskite solar cells (PSCs) exhibit superior thermal stability to the organic-inorganic hybrid PSCs. However, halide defects with low formation energy are often present at grain boundaries of the inorganic perovskite films. This results in many defects of Pb²⁺ uncoordinated with halides, causing in non-radiative recombination in the films. In this work, cesium fluoride (CsF) was chosen as an additive in the CsPbI₂Br precursor solution, in which Cs⁺ can passivate the A-site vacancy defects in CsPbI₂Br perovskite films; fluoride ion (F⁻) has a smaller ionic radius and is more electronegative than chloride ion (Cl⁻), iodide ion (I⁻), and bromide ion (Br⁻), which may allow it to fit in the smaller spaces in the host lattice, as well as weaken the lattice strain and improve the stability of the desired phase. Based on this strategy, CsF-treated carbon-based hole-transport-layer-free CsPbI₂Br PSCs were obtained with a champion photovoltaic conversion efficiency of 13.45 %, short-circuit current density of 15.15 mA/cm², open-circuit voltage of 1.18 V, and fill factor of 75 %. Meanwhile, the CsF-treated CsPbI₂Br PSCs possessed better environmental stability compared to the un-treated counterpart due to the introduction of the more hydrophobic F⁻. This strategy provides a simple and feasible strategy for the development of efficient and stable inorganic PSCs.