Overcoming efficiency challenges in organic solar cells: rational development of conjugated polymers

In this perspective article, we discuss the development of organic photovoltaic (OPVs) solar cells. Our focus will be on discussing the development of new donor polymers and device technologies, which resulted in enormous progress in OPV performances with power conversion efficiencies (PCEs) of 8-9%. However, for the wide spread usage of OPVs, high module efficiencies (>10%) and lifetimes suitable for commercial applications are required. To achieve such goals, interdisciplinary advances in the development of new light-harvesting materials, the improvement of device structures, and the development of cost effective device processing methods are crucial. In particular, new donor polymers overcoming the drawbacks of current polymer solar cells can play an important role to further improve the PCEs and device stability. This perspective article addresses the polymer design criteria that have been distilled out from the research of the past 20 years: energy level matching, nano-morphology of polymer/acceptor blend films, local dipole moments of the polymer chains, and stability. Also, we introduce representative donor polymers and describe the research progress in the polymers development to move beyond certain milestones. We emphasize the importance of the synergetic research efforts in developing new materials, such as the design of new polymers with improved physical properties, the development of device technologies and a fundamental understanding of OPV mechanisms, which will help to continuously enhance the performance of OPVs.