Panchromatic Conjugated Polymers Containing Alternating Donor/Acceptor Units for Photovoltaic Applications

We designed and synthesized a series of conjugated polymers containing alternating electron-donating and electron-accepting units based on (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene), 4,7-(2,1,3)-benzothiadiazole, and 5,5‘-[2,2‘]bithiophene. These polymers possess an optical band gap as low as 1.4 eV (i.e., in the case of poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)]), and their absorption characteristics can be tuned by adjusting the ratio of the two electron-donating units: (4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b‘]dithiophene) and 5,5‘-[2,2‘]bithiophene. The desirable absorption attributes of these materials qualify them as excellent candidates for light-harvesting materials in organic photovoltaic applications allowing for high short-circuit current. Electrochemical studies indicate sufficiently deep HOMO/LUMO levels that enable a high photovoltaic device open-circuit voltage when fullerene derivatives are used as electron transporters. Field-effect transistors made of these materials show hole mobility in the range of 5 × 10-4−3 × 10-3 cm2/(V s), which promises good device fill factor. Because of the combination of these characteristics, power conversion efficiencies up to 3.5% and an external quantum efficiency of at least 25% between 400 and 800 nm with a maximum of 38% around 700 nm were achieved on devices made of bulk heterojunction composites of these materials with soluble fullerene derivatives. Further improvement of the materials will include the modification of both the side chains and the backbone to effect change to the active layer morphology to maintain good charge carrier mobility in the composite.