Alkali acetate-assisted enhanced electronic coupling in CsPbI₃ perovskite quantum dot solids for improved photovoltaics

Fully inorganic CsPbI₃ perovskite quantum dots (CsPbI₃-PQDs) are known as the best-performing photovoltaic absorber in colloidal quantum dot solar cells. This is achieved by improving the cubic-phase-stabilization and electronic-coupling in CsPbI₃-PQD solids. In conventional approaches, the hydrolysis of methyl acetate (MeOAc) resulting in acetic acid and methanol as intermediate substances plays a key role in replacing long-chain hydrocarbons with short-chain ligands, which improves charge transport in the CsPbI₃-PQD solids. However, CsPbI₃-PQDs suffer from lattice distortion and instability under acidic conditions including protons and polar media, leading to CsPbI₃-PQD fusion and poor photovoltaic performance. Herein, we report that electronic coupling and photovoltaic performance of CsPbI₃-PQD solids are improved by efficient removal of long-chain oleate ligands using a solution of sodium acetate (NaOAc) in MeOAc, which results in the direct generation of OAc ions without forming protons and methanol. NaOAc-based ligand exchange of CsPbI₃-PQDs enables preservation of their nanocrystal size without fusion and minimization of surface trap states originating from metal hydroxide formation on their surfaces. Consequently, the best solar cell comprising NaOAc-treated CsPbI₃-PQDs shows an improved device performance with a power conversion efficiency (PCE) of 13.3%, as compared with a lead nitrate-treated control device (12.4% PCE).