The present invention relates to: a method of preparing a coating ink for forming a zinc oxide layer, which method comprises the steps of: a) mixing zinc acetate and AlOH (OAc)2 in water or methanol and b) filtering out solids; a coating ink comprising zinc acetate and AlOH (OAc)2 in aqueous or methanolic solution; a method of preparing a layer comprising zinc oxide, which method comprises: i) coating a substrate with the coating ink of the invention to form a film, ii) drying the film, and iii) heating the dry film to convert the zinc acetate substantially to ZnO; and a method of preparing an organic photovoltaic device or an organic LED having a polymer/fullerene active layer, the method comprising, in this order: a) providing a substrate bearing a first electrode layer, b) forming an electron transport layer according to the method of preparing a layer comprising zinc oxide according to the invention, c) forming an active layer, d) forming a hole transport layer, and e) forming a second electrode layer.