RESUMEN
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.
RESUMEN
In this paper the three new narrow bandgap DA conjugated copolymers P1, P2 and P3 based on different weak donor fused thiophene-imidazole containing derivatives and the same benzothiadiazole acceptor unit were synthesized by Stille cross-coupling polymerization and characterized by 1H NMR, elemental analysis, GPC, TGA, DSC. These copolymers exhibit intensive absorbance in the range 350900 nm and the optical bandgap lies in the range of 1.501.61 eV, which corresponds to the maximum photon flux of the solar spectrum. The electrochemical bandgap derived from cyclic voltammetry varies within the limits 1.471.65 eV and is approximately very close to the optical bandgap. The highest occupied molecular orbital (HOMO) energy level of all copolymers is deep lying (−5.24 eV and −5.37 eV and −5.25 eV for P1, P2 and P2, respectively) which shows that copolymers have good stability in the air and assured a higher open circuit voltage (Voc) for polymer BHJ solar cells. These copolymers were used as donors along with PC71BM and the BHJ polymer solar cells based on P1:PC71BM, P2:PC71BM and P3:PC71BM processed from chloroform (CF) solvent with 3 v% DIO as an additive showed an overall PCE of 4.55%, 6.76% and 5.16%, respectively.
RESUMEN
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 Voc 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 (Eloss) 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.