Fine tuning of terpolymer properties by incorporating electron-accepting difluorobenzene and diketopyrrolopyrrole units

A series of low band gap terpolymers (PBDT–DFBx–DPPy) incorporating various ratios of two acceptor units, difluorobenzene (DFB) and diketopyrrolo[3,4-c]pyrrole (DPP), were synthesized by Pd-catalyzed Stille polymerization. The effects of the DFB and DPP units on UV–Vis absorption, highest occupied molecular orbital (HOMO) energy levels, and terpolymer: PC₆₀BM solar cell performance, especially the short-circuit current (J _sc), were investigated. The polymer films exhibited broad and intense absorption spectra, covering the spectral range from 300 nm up to 900 nm. The optical properties of these random terpolymers can be easily controlled by tuning the ratio between DFB and DPP; an increase in DFB induced increased absorption between 300 and 550 nm and a lower HOMO energy level. The terpolymer PBDT–DFB50–DPP50 with the percentage of DFB up to 50 % in the bulk heterojunction polymer solar cell showed a high open-circuit voltage of 0.74 V and a J _sc of 7.83 mA cm⁻². This study demonstrated that significant changes in polymer electronic properties can be tuned by the strategic combination of simple electron-deficient monomers and their composition in the random terpolymers.