Design and Synthesis of Chlorinated Benzothiadiazole-Based Polymers for Efficient Solar Energy Conversion

Chlorinated benzothiadiazole-based polymers with multiple chlorine atoms were designed and synthesized for polymer solar cells to achieve enhanced open-circuit voltage and improved power conversion efficiency. Chlorine substitution was found to affect molecular orientation, increase crystallinity, and thereby alter band gap and charge transport properties. The one-chlorine-substituted PCBT4T-2OD exhibited a larger portion of face-on orientation than that of other two polymers, nonchlorinated PBT4T-2OD and two-chlorine-substituted PCCBT4T-2OD, in the polymer:PC71BM blended films. PCBT4T-2OD also showed the largest crystallite sizes in those three polymers. The improved molecular orientation and larger crystallite sizes would definitely facilitate the charge transport in the active layer and enhance the performance of corresponding polymer solar cells. The highest power conversion efficiency of 8.21% with PC71BM was achieved in the photovoltaic device of PCBT4T-2OD. It is approximately 68% higher than that of the nonchlorine analog.