Solvothermal synthesis of ZnO nanoparticles at low temperatures as cathode buffer layers for polymer solar cells with an inverted device structure

Inverted polymer solar cells of the conventional poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61butyric acid methyl ester (PC61BM) blend on indium tin oxide substrates were fabricated. By using mixed-solvent dispersed ZnO nanoparticles (NPs) as cathode buffer layer, device performances are improved obviously. ZnO NPs with a size of 3-5 nm synthesized by solvothermal synthesis at 65 A degrees C show relatively wide photoluminescence peak of 300-650 nm. Based on Hansen solubility parameter theory, a mathematical method was applied to calculate the Hansen solubility parameters of bi-solvent system to disperse ZnO NPs and the proportion of each component in the mixed solvent. This excellent dispersion for ZnO NPs in the bi-solvent system has a important influence on the performance of the device. Compared to other methods of ZnO nanofilm fabrication, this method reveals a simple, convenient, moderate and effective way to manufacture the favorable buffer layer in organic solar cells.