The influence of alkyl side chains on molecular packing and solar cell performance of dithienopyrrole-based oligothiophenes

The synthesis of a series of dithieno[3,2-b:2',3'-d]pyrrole (DTP) containing A-D-A oligomers for use in solution-processed bulk heterojunction solar cells is presented. This series allows investigation of the effect of alkyl chains attached to the thiophene moieties or the nitrogen of the DTP unit on the thermal and physical properties of the oligomers. The photoactive layers were probed using absorption spectroscopy, grazing incident X-ray diffraction, and atomic force microscopy. From these experiments, it was found that the hexyl chains attached to the thiophene units are instrumental in obtaining good packing and high fill factors in the solar cell devices. For the fabrication of solar cells, both PEDOT:PSS and V2O5 were investigated as hole transport layers and PDMS was employed as a solvent additive. High open-circuit voltages of up to 1.1 V, moderate short-circuit current densities, and high fill factors of up to 0.63 were obtained yielding power conversion efficiencies as high as 5.3%.