Bulk heterojunction organic solar cells fabricated using low-band-gap semiconducting polymers

Semiconducting copolymers composed of 10-(2'-hexylphenothiazine) (PTZ), 9,9-dioctyl-dithienylfluorene (DTF8), {(2E,2'E)-3,3'-[2,5-bis(octyloxy)-1,4-phenylene]-bis[2-(thiophen-2-yl)acrylonitrile]} (OPTAN), and benzo-[1,2,5]-thiadiazole (BT) units, i.e., poly(PTZ-co-OPTAN-co-BT) and poly(DTF8-co-OPTAN-co-BT), were synthesized through Pd(0)-catalyzed Suzuki coupling with appropriate control of the monomer ratio. The optical band gap energies of the polymers were in the range of 1.95-1.77 eV. The energy levels of the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals of the copolymers were determined via cyclic voltammetry and the optical band gap energies. Photovoltaic devices were fabricated using the polymers as p-type donors and [6,6]-phenyl C61-butyric acid methyl ester (PC60BM) or [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) as electron acceptors.