Effect of methyl substitution on the diketopyrrolopyrrole-based semiconducting polymers for organic thin film transistors

Abstract

Two semiconducting polymers, poly[2,5-bis(5-octylpentadecyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4( 2H,5H)-dione-selenophene] (PDPP-Se) and poly[3,6-bis(4-methylthiophen-2-yl)-2,5-bis(5-octylpentadecyl)pyrrolo[3,4-c] pyrrole-1,4(2H, 5H)-dione-selenophene] (PMDPP-Se) were designed and synthesized. These polymers contain diketopyrrolopyrrole (DPP) and methyl-substituted DPP (MDPP) units as the acceptor building blocks, and selenophene as a counter donor unit. The introduction of methyl groups on the thiophene units flanked by the DPP core substantially affected its optical, electrochemical, and charge-transporting properties. Experimental results revealed that the introduction of electron-donating methyl groups raised the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PMDPP-Se. Interestingly, the PMDPP-Se showed better co-planarity with the adjacent aromatic groups compared to PDPP-Se. Furthermore, the methyl-substituted polymer showed a transition behavior in its thin film morphology and crystal orientation with an increase in the annealing temperature, which induced an M-shape dependence of the field-effect mobility on the annealing temperature. PMDPP-Se recorded a maximum hole mobility mu(h) = 1.31 cm(2) V-1 s(-1) at optimum phase, which is higher than that of PDPP-Se (mu(h) = 0.86 cm(2) V-1 s(-1)).