Organic photovoltaic cells (OPVs) that produce electricity from sunlight are remarkable clean and renewable energy sources. OPVs also have many advantages such as their light weight, flexibility, and processability. Due to these advantages, significant effort has been devoted to OPV research. Recently, the power conversion efficiency (PCE) of several OPVs has been improved exceeding 10%. The effects to increase the PCE of OPVs include the application of donor-acceptor (D?A) type copolymer that reduces the band-gap energies of semiconducting donor polymers. Benzo[1,2-b:4,5-b'']dithiophene (BDT) is a popular donor building block for D?A copolymers. Also, the introduction of heteroatoms has attracted significant research interest to control the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels.
In this study, we designed and synthesized two conjugated semiconducting copolymers consisting of 4,7-bis(4-(2-ethylhexyl)-2-thiophene)-2,1,3-benzothiadiazole (DTBT) and benzo[1,2-b:4,5-b’]dithiophene with 5-(2-ethylhexyl)-2,2''-bithiophene (BDTBT) or 5-(2-ethylhexylthio)-2,2''-bithiophene (BDTBT-S) as a new donor building block for D?A type copolymers for OPVs. The alkylthio-substituted bithienyl group was introduced into BDT in order to tune the HOMO energy level and also extend the conjugation length of the resulting 2D-conjugated polymer for enhanced light absorption. Alkylthio-substituted PBDTBT-S-DTBT showed a higher hole mobility and lower highest HOMO energy level (by 0.08 eV) than the corresponding alkyl-substituted PBDTBT-DTBT. An OPV fabricated using PBDTBT-S-DTBT showed higher VOC and JSC values of 0.83 V and 7.56 mA/cm2, respectively, than those of a device fabricated using PBDTBT-DTBT (0.74 V) leading to a power conversion efficiency of 2.05% under AM 1.5G 100 mW/cm2 illumination.
Secondly, 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(octylthio)phenyl)quinoxaline (DTQx-S) and
5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-(octylthio)phenyl)quinoxaline (DFDTQx-S) were synthesized and copolymerized with benzo[1,2-b:4,5-b’]dithiophene containing 2-(2-ethylhexyl)thiophene and 2-(2-ethylhexylthio)thiophene, respectively. We compared the synthesized polymers with PBDTT-DFDTQx-O and PBDTT-S-DFDTQx-O. The polymers as thin film induced a broad absorption from 300 to 900nm. The optical band gap energies of the polymers were 1.74-1.80 eV calculated from the UV-visible absorption edge of the polymer films. All polymers showed deep HOMO energy level less than -5.45 eV. Especially, PBDTT-S-DFDTQx-S introduced sulfur and fluorine atoms in the polymeric backbone indicated the deepest HOMO energy level as -5.62 eV. These synthesized polymers could expect to have high VOC and JSC values and enhance the efficiency of OPV, consequently.