Existence of Ligands within Sol–Gel-Derived ZnO Films and Their Effect on Perovskite Solar Cells

The sol–gel (SG) method has been well-documented as one useful way to produce ZnO films as an excellent electron transport material (ETM) for efficient perovskite solar cells (PSCs). Generally, the precursor films containing zinc acetate dihydrate and a stabilizing ligand monoethanolamine (EA) were annealed to obtain ZnO films. A noteworthy issue is the commonly reported annealing temperature (Ta) in a wide range of 150–600 °C. In this work, we investigated the effect of the annealing temperature on the film composition and first confirmed the co-existence of acetate and EA species when Ta is below 380 °C. EA still survived within the ZnO films when Ta was between 380 and 450 °C. When Ta was over 450 °C, pure ZnO films can be obtained. The presence of ligands also remarkably altered the work function of the corresponding ZnO samples, thereby resulting in the remarkably different effects on the efficiency and stability of PSCs with the ZnO samples as ETMs. This work affords a clearer understanding of ZnO films prepared by the SG method at molecular insights, promoting their application in photoelectric fields.