Structural design of asymmetric diketopyrrolopyrrole polymers for organic solar cells processed from a non-halogenated solvent

Diketopyrrolopyrrole (DPP) polymers possess narrow optical bandgaps and high charge carrier mobilities which make them attractive for solar cell applications. DPP polymers are generally only soluble in chlorinated solvents, which is a drawback for commercial application. Solubility in non-halogenated solvents can be achieved by reducing the translational symmetry of the chain by employing two different aromatic moieties on either side of the DPP units. Here a series of polymers in which thiophene (T) and pyridine (Py) sandwich the DPP units in the main chain is reported. These asymmetric T-DPP-Py units increase the solubility dramatically. The side chain length, nature of the co-monomer, and regioregularity of the main chain are varied to investigate their effect on the solubility in toluene, the active layer morphology and the performance of organic solar cells. We demonstrate that polymers processed from both chloroform and toluene reach very similar power conversion efficiencies and blend morphologies. In general, a small co-monomer, short side chains, a regioregular main chain, and a high molecular weight give the best performance for solar cells processed from toluene.