Synthesis of star-shaped non-fullerene acceptors and their applications in organic solar cells

Star-shaped planar acceptors have many advantages than linear planar ones such as stronger absorption, higher solubility and better film-forming properties, which are expected to have better solar cell performance. So far, most of star-shaped small molecule acceptors have a bandgap larger than 1.8 eV which cannot make use of near-infrared sunlight. In this work, three star-shaped small-molecular acceptors (TPA-ID, TPA-IC and TPA-ICF) with triphenylamine as core, 1,1-dicyanomethylene-3-indanone derivative as terminal group and 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b']-dithiophene as bridge group were synthesized. These materials exhibit excellent solubility, high thermal stability and high extinction coefficient. TPA-IC and TPA-ICF have narrow bandgap (1.48 and 1.52 eV) and high HOMO energy levels (-5.31 eV and -5.36 eV) that matches well with that of typical donor PBDB-T. A PCE of 4.61% was achieved from photovoltaic device with a structure of ITO/ZnO/PBDB-T:TPA-IC(1:1.5)/MoO3/Ag with a high Voc of 0.90 V. The Voc loss of this device is only 0.64 eV. The current device performance is limited by the low electron mobility of the acceptor which restricts absorber layer thickness.