Diketopyrrolopyrrole/perylene-diimide and thiophene based D-π-A low bandgap polymer sensitizers for application in dye sensitized solar cells

Abstract

In this study, diketopyrrolopyrrole-alt-thiophene (P1 and P2) and perylene-diimide-alt-thiophene (P3 and P4) based donor-π-acceptor (D-π-A) copolymers have been synthesized from the corresponding monomers through Pd-catalyzed Sonogashira polymerization protocol. The well defined and soluble π-conjugated copolymers having alkyl and fluoroalkyl substituents (P1-P4) have been characterized by multinuclear NMR spectra as well as by tetradetector GPC studies showing molecular weight (Mn) in the range of 18–20 kDa with good polydispersity indices of 1.31–1.48. The donor-acceptor based copolymers absorb broadly throughout the visible region. Notably, perylene diimide-thiophene based copolymers (P3 and P4) exhibits an absorption onset at ca. 800 nm corresponding to a bandgap of 1.63 and 1.61 eV (Egopt). DFT computational studies of the model π-conjugated units have also been investigated to understand the molecular geometries and electronic properties of the polymeric unit. The synthesized D-π-A polymers have been utilized as active materials for polymer-sensitized solar cells (PSSCs). The copolymers are effectively adsorbed onto the surface of nanostructured TiO2 photoanode as a result of facile interaction of the anchored –CO units with the metal oxide surface. The spectral profile of the polymer films on mesoporous oxide surface approximately similar to the solution absorption spectra of the polymer. Interestingly, the polymers featuring perylene diimide unit (P3 and P4) exhibit promising power conversion efficiency (PCE) of 2.71 and 2.96% with a short circuit current (JSC) of 7.54 and 7.85 mA cm−2 respectively, and IPCE of 42–45% under 1.5 AM illumination.