Synthesis and photovoltaic properties of new donor-acceptor (D-A) copolymers based on benzo[1,2-b:3,4-b′:6,5-b"] trithiophene donor and different acceptor units (P1 and P2)

Two new conjugated D-A copolymers P1 (PBTT-TQ) and P2 (PBTT-TPz) based on the same benzo[1,2-b:3,4-b':6,5-b ''] trithiophene (BTT) donor and different acceptor monomers 4,9-bis(5-bromithiophen-2-yl)-6,7-bis(2-ethylhexyl)[1,2,5] thiadiazolo[3,4-g] quinoxaline (TQ) and 5,7-bis(5-bromothiophen-2-yl)-2,3-bis(5-octylthiophen-2-yl) thieno[3,4-b] pyrazine (TPy), respectively, were synthesized by Stille cross-coupling reaction and characterized by gel permeation chromatography (GPC), H-1 NMR, UV-Vis absorption, thermal analysis and electrochemical cyclic voltammetry (CV) tests. The optical bandgaps of P1 and P2 measured from the onset of optical absorption of these copolymers in thin films were 1.14 eV and 1.38 eV, respectively. Photovoltaic properties of the bulk heterojunction (BHJ) devices were studied using P1 and P2 as the donor and PC61BM or PC71BM as the acceptor with weight ratios of polymer : PC61BM of 1 : 1, 1 : 2 and 1 : 3. The optimized photovoltaic devices fabricated with active blend layers of P1:PC71BM (1 : 2) and P2: PC71BM (1 : 2) cast from o-diclorobenzene (o-DCB) showed PCEs of 2.05% and 3.14%, respectively, whereas P2: PC61BM and P2: PC71BM yielded PCEs of 1.44% and 2.11%, respectively. The PCEs of BHJ solar cells based on P1: PC71BM and P2: PC71BM processed from 1-chloronapthanene (CN)/o-DCB solvent were further improved up to 5.28% and 2.6%, respectively.