Triphenylamine cored electron-donors for solution-processed organic solar cells: From tri-armed molecules to tetra-armed molecules

A novel tetra-armed D-A type small molecule 4BTF-DTPA-T was designed here by utilizing the thiophene connected ditriphenylamine (DTPA) core and fluorinated benzothiadiazole (BTF) arms. To make a better comparison, tri-armed (star-shaped) 3BTF-TPA was also synthesized. Although a similar absorption spectra covering from 300 to 600 nm was observed for 3BTF-TPA and 4BTF-DTPA-T, much higher extinction coefficient was achieved for tetra-armed 4BTF-DTPA-T in both solution and thin films. Bulk hetero-junction (BHJ) organic solar cells were also fabricated to investigate their photovoltaic performance and as anticipated, a moderate PCE value of 2.61% with a V_oc of 1.00 eV was observed in 3BTF-TPA-based solar cells. However, when the tetra-armed 4BTF-DTPA-T was employed as the electron-donors, much enhanced J_sc and FF values up to 9.33 mA/cm² and 44.80% were achieved for its homogeneous morphology in solar cells, finally leading to the maximum PCE value of 3.68%, indicating that the tetra-armed small molecules could be the promising electron-donors for solution-processed organic solar cells.