Donor–acceptor–acceptor (D–A–A) type 1,8-naphthalimide based small molecules SM1 and SM2 functionalized with tetracyanobutadiene (TCBD) and dicyanoquino-dimethane (DCNQ) modules, showing strong absorption in the visible and near-infrared (NIR) region are reported. TCBD and DCNQ linked SM1 and SM2 exhibit multi-redox waves. The electrochemical and optical HOMO–LUMO gaps show similar trends. These SMs exhibit a broad absorption profile which is complementary to the D–A copolymer P donor and also possess an appropriate lowest unoccupied molecular orbital (LUMO) to serve as an acceptor with P with a LUMO level of −3.33 eV. The organic solar cells based on P:SM1 and P:SM2 exhibit a PCE of 4.94% and 6.11%, respectively. The higher value of the PCE for the SM2 based organic solar cells has been attributed to the broader absorption profile, more balanced charge transport and lower photon energy loss. The values of V_oc of the organic solar cells for the SM1 acceptor (1.06 V and 1.02 V without and with solvent additive) are the highest values reported for devices based on non-fullerene acceptors to the best of our knowledge. The energy loss (E_loss) of 0.56 eV and 0.48 eV for SM1 and SM2 based devices, respectively is one of the smallest reported for BHJ organic solar cells.

Two D–A copolymers, F1 and F2, with fluorene and thiazole units were substituted, respectively, on a thiadiazoloquinoxaline (TDQ) unit to enhance the electron-accepting strength of TDQ. The copolymers were synthesized by a cross-coupling Stille reaction and their optical and electrochemical properties were examined, which revealed that they have ultra-low band gaps and absorption in the near-infrared. These copolymers were employed as donors along with PC₇₁BM as an electron acceptor for the fabrication of solution-processed bulk heterojunction (BHJ) polymer solar cells. After the optimization of the donor-to-acceptor weight ratio and the solvent additive (4 v% DIO as solvent additive), devices with F1:PC₇₁BM and F2:PC₇₁BM displayed power conversion efficiencies (PCEs) of 5.80% and 3.32%, respectively. Although F2 possesses a broader absorption profile compared with F1, the lower value of PCE for the F2-based device was attributed to the low LUMO offset between F2 and PC₇₁BM, which limited the exciton dissociation. The abovementioned results indicate that these copolymers can be utilized for ternary BHJ and tandem solar cells to achieve a high PCE.

We synthesized two novel ultra low bandgap donor–acceptor (D–A) copolymers (E_g ≤ 1.2 eV), containing the thiadiazoloquinoxaline unit as the main electron accepting unit (A) and benzodithiophene (BDT) and dithienosilole (DTS) as different donor units (D), denoted as P1 and P2, respectively, using the cross-coupling Stille reaction. The copolymers possess light absorption ranging from UV (350 nm) to near-IR (1300 nm) with optical bandgaps of 1.16 eV and 1.08 eV, respectively. Quantum-chemical calculations and experimental data were compared for proposing a more detailed concept for the optical and electronic properties of these copolymers which can be used as donors for polymer solar cells (PSCs). The PSCs based on optimized P1:PC₇₁BM and P2:PC₇₁BM showed overall power conversion efficiencies (PCEs) of 4.32% and 3.48%, respectively. Although P2 possesses a broad absorption coverage of up to 1300 nm, the lower PCE may be attributed to the low J_sc, due to the poor driving force for exciton dissociation, since the LUMO offset with PC₇₁BM is less than 0.3 eV. The PCE has been significantly increased to 7.27% and 6.68% for solvent vapor annealing (SVA) treated P1:PC₇₁BM and P2:PC₇₁BM active layers, respectively. This improvement arises from the appropriate nanoscale morphology and an increase in hole mobility, induced by the SVA treatment of the active layers.