Organic solar cells (OSCs) have received a special attention over the past years due to their solution processability, low cost, flexibility and capability of role-to-role production. The power conversion efficiency of these devices has been significantly increased over the past decades from 1% in 1986 to 5% in 2005 and to up to 13% in 2017 thanks to the molecular optimization and the use of non-fullerene acceptors in their active materials. Despite such interesting efficiency, their applications remain limited so far because of instability and short life time of their active layers. It is expected that these obstacles will be surmounted in a foreseeable future upon rigorous research studies performed in the field. This paper is devoted to reviewing the operating principle, characterization parameters and the most important approaches that are considered aiming at improving the overall performance of these devices.