Mediating solar cell performance by controlling the internal dipole change in organic photovoltaic polymers

We report synthesis and characterizations of two novel series of polymers, namely the PBTZ and PBIT series. The PBTZ1 polymer was synthesized as a copolymer of 4,8-bis(2-butyloctyl)benzo[1,2-b:4,5-b′]dithiophene (BDT) along with 2,5-bis(2-ethylhexyl)-3,6-bisthiazol-2-yl-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione (TzDPP), while PBTZ2 was a copolymer of TzDPP and 2-(1-butylheptyl)thieno[3,4-d]thiazole (TTz). The PBIT series based on dithienopyrrolobenzothiadiazole (DPBT), and BDT was also synthesized. The PBIT series of polymers showed enhanced ground and excited state dipole moments (μ _g and μ _e) when compared to the previously reported PBB3 polymer, while PBTZ1 showed the largest dipole change (1.52 D) from ground to excited state (Δμ _ge) in respective single polymer units. It was found that the power conversion efficiencies of the polymer series were strongly correlated to Δμ _ge. The results reported demonstrate the utility of the calculated parameter Δμ _ge of single units of the polymers to predict the performance of donor-acceptor copolymers in photovoltaic devices. We rationalize this result based on the large degree of polarization in the excited state, which effectively lowers the Coulomb binding energy of the exciton in the excited state and leads to faster charge separation kinetics, thus facilitating the full separation of electron and hole.