Determining optimal crystallinity of diketopyrrolopyrrole-based terpolymers for highly efficient polymer solar cells and transistors

Abstract

A new series of conjugated random terpolymers (PDPP2T-Se-Th) was synthesized from an electron-deficient diketopyrrolopyrrole (DPP)-based unit in conjugation with two electron-rich selenophene (Se) and thiophene (Th) species, with a view to inducing different crystalline behaviors of the polymers. The crystallinity of the polymers can be systematically controlled by tuning the ratio between Se and Th; an increase in Se content induced a remarkable increase in the melting and crystallization temperatures as well as the crystallinity of the PDPP2T-Se-Th terpolymers. These changes in the crystalline properties of polymers had a dramatic effect on the performances of organic field-effect transistors (OFETs) and polymer solar cells (PSCs). However, their effect on each type of devices was very different. The charge carrier mobilities of the PDPP2T-Se-Th terpolymers in OFET devices increased remarkably as the Se content increased in the polymers, showing that PDPP2T-Se100 with Se/Th ratio = 100/0 had very high hole and electron mobilities (4.72 and 5.54 cm2 V-1 s-1, respectively) with well-balanced ambipolar property. In contrast, the best power conversion efficiency (PCE) of 7.2% was observed for the PDPP2T-Se10-Th90 polymers that had Se/Th ratio of 10/90 due to the synergistic contributions from high charge mobility and optimized bulk-heterojunction (BHJ) morphology with fullerene acceptors. To understand the effects of the crystallinity of random terpolymers on their performances in OTFTs and PSCs, we systematically investigated the effects of the Se/Th compositions on their optical, electrical, and structural properties.