Molecular design strategies for voltage modulation in highly efficient polymer solar cells

Open-circuit voltage (V-oc) is one of the key parameters in determining the photovoltaic performance of polymer solar cells (PSCs). Although significant advances in materials and device physics of PSCs have been achieved in the past decade, the low V-oc values still hamper the enhancement of power conversion efficiencies (PCEs) of PSCs based on the widely known polymers like polythiophenes (PTs) and poly(benzo[1,2-b:4,5-b]dithiophene-co-thieno[3,4-b]thiophene) (PBDTTT) polymers. In order to pursue high PCE, more efforts should be directed towards improving V-oc through molecular design of conjugated polymers, i.e. to reduce the highest occupied molecular orbital levels without sacrificing optical absorption properties. In this mini-review, some feasible and effective strategies, such as inserting conjugated side groups with various electron-withdrawing effects, manipulating alkyl chains and introducing functional substituents, to improve V-oc of PSCs based on some highly efficient photovoltaic polymers, especially PTs and PBDTTT polymers, are summarized and discussed. Owing to these strategies, PCEs of PSCs based on PTs and PBDTTT polymers can be further boosted to ca 7% and ca 10%, respectively. Apparently, these strategies offer opportunities for achieving new breakthroughs in other -conjugated photovoltaic materials. (c) 2015 Society of Chemical Industry