Precursor Engineering to Reduce Processing Temperature of ZnO Films for Flexible Organic Solar Cells

Sol-gel-derived ZnO is one of the most widely used electron-transport layers in inverted organic solar cells. The sol-gel ZnO precursor consists of zinc acetate dehydrate (ZAH) and ethanolamine dissolved in 2-methoxyethanol, where ethanolamine chelates with ZAH, which helps ZAH dissolve in the 2-methoxyethanol. However, an annealing temperature above 120 °C is required to convert the complexes into ZnO. High temperatures are incompatible with flexible plastic substrates such as polyethylene terephthalate. In this work, we report an amine-free recipe consisting of ZAH in methanol to prepare ZnO films. The complex formed in the amine-free precursor solution is methanol-solvated ZAH, which is simpler than that of the amine-containing precursor solution. The temperature required for converting the precursor complex into ZnO was reduced to 90 °C for the amine-free recipe. Low-temperature-processed ZnO can function efficiently as an electron-transport layer in both rigid and flexible inverted nonfullerene solar cells.