A method for manufacturing membrane layers of organic solar cells by roll to roll coating utilizes a roll to roll process for manufacturing an electron transferring layer and an active layer of the organic solar cells is disclosed. The roll to roll process adopted by the method cooperates with a particular solvent and accompanies a parameter control such as temperature and processing time during the sintering and baking steps. The method utilizes a slot-die coating technique in the interim, whereby a membrane layer of the solar cells can be manufactured with a large area for reducing the cost, and the formed membrane layers can have a good efficiency.