Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A−π–D−π–A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (0T–5T) π-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about −3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around −5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from “outward” to “inward”, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the “inward” alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15–20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.

Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A−π–D−π–A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (0T–5T) π-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about −3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around −5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from “outward” to “inward”, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the “inward” alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15–20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.

•Two new small molecules DCAO3TF and DCAO3TCz were designed and synthesized.•Both molecules show deep HOMO levels and exhibit a high open-circuit voltage up to 1.07 V.•Power conversion efficiency of the OSCs based on DCAO3TCz as donor reached 3.63%.With the goal of increasing the open-circuit voltage, two new solution-processable A–D–A structure small molecule donor materials, named DCAO3TF and DCAO3TCz, using two weak electron-donating units, fluorene and carbazole as the central block have been designed and synthesized for photovoltaic applications. While bulk heterojunction photovoltaic devices based on DCAO3TF:PC61BM and DCAO3TCz:PC61BM as the active layers exhibit moderate power conversion efficiencies of 2.38% and 3.63%, respectively, devices based on DCAO3TF:PC61BM do exhibit an impressively high open-circuit voltage (Voc) up to 1.07 V, which is one of the highest Voc in organic solar cells based on donor:PCBM blend films.Graphical abstract

Two acceptor–donor–acceptor small molecules based on thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b′]dithiophene, DRBDT-TT with alkyl side chain and DRBDT-STT with alkylthio side chain, were designed and synthesized. Both molecules exhibit good thermal stability, suitable energy levels, and ordered molecular packing. Replacing the alkyl chain with alkylthio increases the dihedral angle between the thieno[3,2-b]thiophene (TT) and benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, and thus slightly decreases its intermolecular interactions leading to its blue-shift absorption in the solid state. The best devices based on DRBDT-TT and DRBDT-STT both exhibited power conversion efficiencies (PCEs) over 8% with high fill factors (FFs) over 0.70 under AM 1.5G irradiation (100 mW cm–2), which are attributed to their optimized morphologies with feature size of 20–30 nm and well-balanced charge transport properties. The devices based on DRBDT-STT exhibited relatively lower short-circuit current density (Jsc) and thus slightly lower PCE as compared to the devices of DRBDT-TT, mainly due to its relatively poorer absorption. These results demonstrate that thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b′]dithiophene derivatives could be promising donor materials for obtaining high efficiencies and fill factors.

•Two new small molecules DCAO3TF and DCAO3TCz were designed and synthesized.•Both molecules show deep HOMO levels and exhibit a high open-circuit voltage up to 1.07 V.•Power conversion efficiency of the OSCs based on DCAO3TCz as donor reached 3.63%.With the goal of increasing the open-circuit voltage, two new solution-processable A–D–A structure small molecule donor materials, named DCAO3TF and DCAO3TCz, using two weak electron-donating units, fluorene and carbazole as the central block have been designed and synthesized for photovoltaic applications. While bulk heterojunction photovoltaic devices based on DCAO3TF:PC61BM and DCAO3TCz:PC61BM as the active layers exhibit moderate power conversion efficiencies of 2.38% and 3.63%, respectively, devices based on DCAO3TF:PC61BM do exhibit an impressively high open-circuit voltage (Voc) up to 1.07 V, which is one of the highest Voc in organic solar cells based on donor:PCBM blend films.Graphical abstract

Two acceptor–donor–acceptor small molecules based on thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b′]dithiophene, DRBDT-TT with alkyl side chain and DRBDT-STT with alkylthio side chain, were designed and synthesized. Both molecules exhibit good thermal stability, suitable energy levels, and ordered molecular packing. Replacing the alkyl chain with alkylthio increases the dihedral angle between the thieno[3,2-b]thiophene (TT) and benzo[1,2-b:4,5-b′]dithiophene (BDT) unit, and thus slightly decreases its intermolecular interactions leading to its blue-shift absorption in the solid state. The best devices based on DRBDT-TT and DRBDT-STT both exhibited power conversion efficiencies (PCEs) over 8% with high fill factors (FFs) over 0.70 under AM 1.5G irradiation (100 mW cm–2), which are attributed to their optimized morphologies with feature size of 20–30 nm and well-balanced charge transport properties. The devices based on DRBDT-STT exhibited relatively lower short-circuit current density (Jsc) and thus slightly lower PCE as compared to the devices of DRBDT-TT, mainly due to its relatively poorer absorption. These results demonstrate that thieno[3,2-b]thiophene-substituted benzo[1,2-b:4,5-b′]dithiophene derivatives could be promising donor materials for obtaining high efficiencies and fill factors.