A method of forming a thin film includes preparing a mixture including a light emitting layer material that forms a light emitting layer and a material that forms a layer adjacent to the light emitting layer, and sequentially laminating the light emitting layer and the layer adjacent to the light emitting layer onto a substrate inside a chamber by heating the mixture to a process temperature of the light emitting layer material and a process temperature of the additional material. The light emitting layer material and the additional material have process temperatures that are different from each other. The light emitting layer material and the additional material are laminated onto the substrate sequentially from one of the light emitting layer material and the additional material having a lower process temperature to one of the light emitting layer material and the additional material having a higher process temperature.