Solution-processable small molecules for organic solar cells have attracted intense attention for their advantages of definite molecular structures compared with their polymer counterparts. However, the device efficiencies based on small molecules are still lower than those of polymers, especially for inverted devices, the highest efficiency of which is <9%. Here we report three novel solution-processable small molecules, which contain π-bridges with gradient-decreased electron density and end acceptors substituted with various fluorine atoms (0F, 1F and 2F, respectively). Fluorination leads to an optimal active layer morphology, including an enhanced domain purity, the formation of hierarchical domain size and a directional vertical phase gradation. The optimal morphology balances charge separation and transfer, and facilitates charge collection. As a consequence, fluorinated molecules exhibit excellent inverted device performance, and an average power conversion efficiency of 11.08% is achieved for a two-fluorine atom substituted molecule.

The high efficiency all-small-molecule organic solar cells (OSCs) normally require optimized morphology in their bulk heterojunction active layers. Herein, a small-molecule donor is designed and synthesized, and single-crystal structural analyses reveal its explicit molecular planarity and compact intermolecular packing. A promising narrow bandgap small-molecule with absorption edge of more than 930 nm along with our home-designed small molecule is selected as electron acceptors. To the best of our knowledge, the binary all-small-molecule OSCs achieve the highest efficiency of 14.34% by optimizing their hierarchical morphologies, in which the donor or acceptor rich domains with size up to ca. 70 nm, and the donor crystals of tens of nanometers, together with the donor-acceptor blending, are proved coexisting in the hierarchical large domain. All-small-molecule photovoltaic system shows its promising for high performance OSCs, and our study is likely to lead to insights in relations between bulk heterojunction structure and photovoltaic performance.

A ternary blend system with two donors and one acceptor provides an effective route to improve the performance of organic solar cells. A synergistic effect of polymer and small molecules is observed in ternary solar cells, and the power conversion efficiency (PCE) of the ternary system (8.40%) is higher than those of binary systems based on small molecules (7.48%) or polymers (6.85%).

Two new star-shaped D-π-A molecules with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 4,4 0 -dihexyl-2,2 0 -bithiophene (bT) or 4, 4 0 -dihexyl-2,2 0 -bithiophene vinylene (bTV) as π bridge, S(TPA-bT-DCN) and S(TPA-bTV-DCN), were synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The two compounds are soluble in common organic solvents, because of the three-dimensional structure of the TPA unit and the two hexyl side chains on the bithiophene unit. S(TPA-bTV-DCN) film shows a broad absorption band from 360 to 750 nm. Absorption edge of S(TPA-bTV-DCN) film is red-shifted by ca. 78 nm than that of S(TPA-bT-DCN) film, benefitted from the vinylene bridges between TPA and bithiophene units in S(TPA-bTV-DCN). Power conversion efficiency (PCE) of the solution-processed bulk-heterojunction OSC based on a blend of S(TPA-bTV-DCN) and [6,6]-phenyl-C 71 -butyric acid methyl ester (1:2, w/w) reached 3.0% with a short circuit current density of 7.76 mA/cm 2 and an open circuit voltage of 0.88 V, under the illumination of AM.1.5, 100 mW/cm 2 . In comparison, PCE of the OSC based on S(TPA-bT-DCN) as donor is 1.4% under the same experimental conditions. The PCE of 3.0% for S(TPA-bTV-DCN) is among the top values for the solution-processed molecule-based OSCs reported so far.