Toward high efficiency organic solar cells: Rational design, synthesis and photovoltaic effect in low bandgap polymers

Harvesting energy from sunlight using photovoltaic technology is one of the most important ways to address growing global energy needs. Alternating copolymers of benzo[1,2-b:4,5-b']dithiophene(BD) and thieno[3,4-b]thiophene(TT) were developed for polymer bulk heterojunction (BHJ) solar cells. Introduction of a fluorine atom into the thieno[3,4-b]thiophene ring substantially improved the open-circuit voltage (Voc), and the device achieved the highest power conversion efficiency (PCE) over 7%. To better understand the reasons behind the high performances of these polymers, we synthesized a series of PTB derivatives fluorinated in the polymer backbone and investigated the effects of fluorination on the properties of PTB polymers, especially the performance in BHJ solar cells. Incorporation of fluorine into the various positions of the polymer backbone significantly affected the solar cells' PCE from 2.3% to 7.2%. Detailed studies revealed that the polymer containing mono-fluorinated thienothiophene gave the best solar cell performance. Perfluorination of the polymer backbone led to poor compatibility with PC71BM molecules, thus poor solar energy conversion efficiency. Furthermore, it was found that perfluorination of the polymer backbone exhibited poor photochemical stability against singlet oxygen attack. In this research, several important points are deduced. First, fluorination gives the driving force for the phase separation of the polymer/PC71BM blend film, resulting in a big difference in solar cell efficiency among the polymers. Next, internal polarization is important to stabilize polymers against singlet oxygen attack, which is related to the charge density distribution at specific thiophene rings. We also developed other polymer system with a extended pi-conjugated unit of dithieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene (DBD). Herein, we present the synthesis of DBD derivatives and their alternating copolymers PTDBD1-PTDBD4, and their photovoltaic properties in polymer/PCBM bulk heterojunction (BHJ) solar cell devices. Fine-tuning of side alkyl chains on the polymer allows optimization of solar cell efficiency among the polymers by affecting the miscibility of the polymer with PCBMs as well as the long-range ordering of the polymer chains in the polymer/PCBM blend film. A power conversion efficiency of close to 7% has been achieved in solar cells based on PTDBD4/PC71BM composite films. The result shows that DBD containing copolymers are promising candidates for achieving BHJ solar cells with high conversion efficiencies.