Three solution-processable small molecules of DPPT, DPPSe and DPPTT were synthesized by Stille coupling through attaching donor units of thiophene (T), selenophene (Se) and thieno[3,2-b]thiophene (TT) to the diketopyrrolopyrrole (DPP) core, respectively. Replacement of the T donors with the more polarized Se units results in a balance between the a and b direction packing and an obvious increase of the power conversion efficiency (PCE) from 1.90% to 2.33% with the increase of the short-circuit current (Isc) from 5.59 to 5.81 mA cm−2 and the open-circuit voltage (Voc) from 0.78 V to 0.86 under the small molecule/acceptor ratio of 3 : 1. However, introduction of the conjugation-enlarged TT groups (versus the T units) leads to a decrease of the PCE, down to 1.70%, with a significant decrease of the fill factor (FF) (38% versus 44%), due to its poor film-forming characteristics.
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