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.

Herein, we investigated the photovoltaic properties of carbazole-based diketopyrrolopyrroles with tetracyanobutadiene acceptor units as highly efficient non-fullerene acceptors together with a D–A conjugated polymer, P, as a donor for polymer solar cells. After optimization, i.e. donor to acceptor weight ratio and solvent vapour annealing, the polymer solar cells exhibited power conversion efficiencies of up to 4.86% and 7.19% for DPP7 and DPP8 as acceptors, respectively. These results indicate that slight changes in the chemical structure of the small acceptor molecule significantly increase the efficiency of the device. The design and synthesis of these non-fullerene acceptors with broader absorption spectra extended towards the near infrared region may be key for the further development of high-performance and cost-effective solution-processed organic solar cells.

Two new alternating low band gap D–A copolymers with different acceptor structures of 4,8-bis-(5-bromothiophene-2-yl)-benzo[1,2,5]thiadiazole (P1) and 4,8-dithiophene-2-yl-benzo[1,2-c;4,5-c′]-bis-[1,2,5]thiadiazole (P2) and a common BDT donor segment have been synthesized under Stille reaction conditions and characterized. The polymers showed good solubility, broad absorption bands and optical band gaps of 1.62 eV and 1.16 eV for P1 and P2, respectively. Bulk heterojunction (BHJ) polymer solar cells based on P1 and P2 as electron donors and fullerene derivatives (PC₆₀BM and PC₇₀BM) as acceptor were fabricated and their photovoltaic response was investigated. The overall power conversion efficieny (PCE) achieved for BHJ solar cells based on P1:PC₆₀BM, P2:PC₆₀BM, P1:PC₇₀BM and P2:PC₇₀BM blends cast from THF solvent is about 2.17%, 0.80%, 3.45% and 1.19%, respectively. The higher PCE for the device based on P1 has been attributed to the high value of hole mobility for P1 as compared to P2 and a larger driving force i.e. LUMO–LUMO offset, for photo-induced charge transfer for P1:PCBM BHJ active layer. The PCE has been further increased up to 5.30% and 1.58% for P1:PC₇₀BM and P2:PC₇₀BM blends cast from DIO/THF solvent, which is attributed to the improved crystallinity and a more balanced charge transport in the device.