Significant Improved Performance of Photovoltaic Cells Made from a Partially Fluorinated Cyclopentadithiophene/Benzothiadiazole Conjugated Polymer

A partially fluorinated low bandgap polymer, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b']dithiophene)-alt-4,7-(5-fluoro-[2,1,3]-benzothiadiazole)] (PCPDTFBT) was synthesized through a microwave-assisted Stille polymerization. It was found that PCPDTFBT has better pi-pi stacking in solution than its nonfluorinated analogue, poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-([2,1,3]-benzothiadiazole)] (PCPDTBT), resulting in 2 times higher hole mobility. Power conversion efficiency (PCE) of the device using PCPDTEBT/PC71BM as active layer (5.51%) is much higher than the device using PCPDTBT/PC71BM (2.75%) that was fabricated under the same condition without using any solvent additive to modify the morphology. The significantly enhanced PCE is the result of improved open circuit voltage and short circuit current coming from the lower lying HOMO energy level and the appropriate morphology of PCPDTFBT. In addition, the device with PCPDTFBT/PC71BM could also be processed from nonchlorinated organic solvents such as o-xylene to obtain high PCE of 5.32% (which is the highest value for PCPDTBT type polymers processed without using chlorinated solvents). Further device optimization by inserting a thin layer of fullerene-containing surfactant between the active layer and Ag cathode resulted in even higher PCE of 5.81%. These encouraging results showed that PCPDTFBT has the potential to be used as a low bandgap polymer to provide complementary absorption in tandem solar cells.