Deuterated N,N-dimethylformamide (DMF-d7) as an additive to enhance the CsPbI3 solar cell efficiency

Typically, DMSO and DMF are the two most commonly used co-solvents; however, their boiling points differ greatly and the volatilization rate is not consistent when the film is annealed and crystallized, which leads to the inhomogeneity of the components. Herein, the hydrogen isotope effect (HIE) is produced by introducing N,N-dimethylformamide-d7 (DMF-d7) into the CsPbI3 precursor solution for the first time. The introduction of deuterium affects the intermolecular interaction between the solvent molecules and Pb2+, and ameliorates the undesirable situation of excessive uncoordinated Pb2+ remaining on the surface of the film. As a result, the power conversion efficiency (PCE) of the optimized device reached 18.55%. The characterization of the film structure, morphology, and components demonstrates that the addition of DMF-d7 improves the quality of the film. More importantly, according to the apparent change in the atomic ratios of Cs and Pb (Cs/Pb), we speculate that different film crystallization rates eventually lead to two different crystallization termination surfaces, i.e., with PbI2 or CsI, and illustrate their effects on the properties of the film by ab initio simulations. Overall, this study developed a novel solvent system that provides a promising method for the fabrication of high-quality CsPbI3 films.