The influence of secondary solvents on the morphology of a spiro-MeOTAD hole transport layer for lead halide perovskite solar cells

2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) has been widely employed as a hole transport layer (HTL) in perovskite-based solar cells. Despite high efficiencies, issues have been reported regarding solution processed spiro-MeOTAD HTL such as pinholes and the strong dependence of electrical properties upon air exposure, which poses challenges for solar cell stability and reproducibility. In this work, we perform a systematic study to unravel the fundamental mechanisms for the generation of pinholes in solution-processed spiro-MeOTAD films. The formation of pinholes is closely related to the presence of small amounts of secondary solvents (e.g. H₂O, 2-methyl-2-butene or amylene employed as a stabilizer, absorbed moisture from ambient, etc), which have low miscibility in the primary solvent generally used to dissolve spiro-MeOTAD (e.g. chlorobenzene). The above findings are not only applicable for spiro-MeOTAD (a small organic molecule), but also applicable to polystyrene (a polymer). The influence of secondary solvents in the primary solvents is the main cause for the generation of pinholes on film morphology. Our findings are of direct relevance for the reproducibility and stability in perovskite solar cells and can be extended to many other spin-coated or drop-casted thin films.