Design of medium band gap random terpolymers containing fluorene linked diketopyrrolopyrrole and thiophene co-monomers: an experimental and theoretical study

In this study, new polymers PTFDPP, PBTFDPP, PTTFDPP, PTCNEFDPP, PTDCNFDPP and PTPTFDPP were designed and synthesized by employing a D-A random copolymer strategy involving 9,9-dihexylfluorene (electron donor) linked to diketopyrrolopyrrole (electron acceptor) and thiophene comonomer (as donor as well as acceptor). Their optical band gap and HOMO and LUMO energy levels were estimated from ultraviolet-visible spectroscopy and cyclic voltammetry. By changing the nature of thiophene from donor to acceptor state, the HOMO and LUMO energy levels and optical band gaps were modified, which varied from 1.75 to 1.65 eV. A series of systematic density functional theory (DFT) calculations have been performed on model co-polymers to gain insights into the structural, electronic, optical and charge transport properties. The experimental values are in close agreement with calculated electronic properties. Salient findings from both experimental and calculated values indicate that these new polymers can be efficiently exploited for the development of new donor materials for OPV applications.