Determining Optimal Crystallinity of Diketopyrrolopyrrole-Based
Terpolymers for Highly Efficient Polymer Solar Cells and Transistors
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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 cm<sup>2</sup> V<sup>โ1</sup> s<sup>โ1</sup>, 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