Novel polymer donors based on simple A1-π-A2 structure for non-halogen solvent-processed organic solar cells

Abstract

Two low band gap conjugated polymers based on acceptor1-π-acceptor2 (A1-π-A2) and donor-π-acceptor (D-π-A) type architecture were synthesized for organic solar cells. The novel A1-π-A2 type polymer contains a fluorine substituted benzotriazole (FTAZ) as acceptor1 block unit (A1), tetrazine (TTz) as acceptor2 unit (A2), and thiophene as the π-bridge, namely PBTZ-TTz. The compared D-π-A type polymer contains a benzodithiophene (BDT) as rich electronic unit, TTz as electron-deficient unit and thiophene as the π-bridge, namely PBDT-TTz. Although mounting more thiophene π bridge units for better solubility and backbone geometry, the novel “A1-π-A2” type copolymer PBTZ-TTz achieved a deep HOMO energy level of −5.55 eV, just slightly higher than that of D-π-A type polymer PBDT-TTz. Moreover, the polymer PBTZ-TTz obtained a wider and stronger UV–vis absorption, better coplanarity and π-π stacking than PBDT-TTz. After processed by non-halogen solvent 1,2,4- trimethylbenzene (TMB), the PBTZ-TTz:IT-4F based device achieved a decent open circuit voltage (Voc) of 0.88 V, a lower power conversion efficiency (PCE) of 5.9%, while PBDT-TTz based device reached the PCE of 7.4% with Voc of 0.95 V. The unexpected lower PCE of PBTZ-TTz:IT-4F device is mainly due to the holes cannot transfer efficiently from acceptor to donor, the lower external quantum efficiency (EQE) spectra responding, as well as the poor phase separation morphology of the blend film. It is worth noting that both the Voc achieved are in outstanding high levels among IT-4F based devices. These results reveal that the A1-π-A2 type copolymers can enrich non-halogen solvent-processed donor materials through simple synthesis, which is beneficial for future environmentally friendly processing.