Star-shaped organic semiconductors with planar triazine core and diketopyrrolopyrrole branches for solution-processed small-molecule organic solar cells

• A series of star-shaped molecules have been synthesized for organic solar cells.
• Excellent charge transfer resulted in a low energy band-gap for the molecules.
• The molecules with lower band gap energy led to a larger efficiency of solar cells.

A series of novel triazine-cored, star-shaped, conjugated molecules have been synthesized as donor materials for small molecule based organic solar cells. The structural features of these star-shaped molecules include a planarized triazine as the central core, 2,5-thienyl diketopyrrolopyrrole and 1,4-phenylene diketopyrrolopyrrole as the π-conjugated bridge, and tert-butyl-substituted triphenylamine and tert-butyl-substituted carbazole as the role of end groups and donor units. Photovoltaic properties of the solar cells based on the star-shaped molecule with 2,5-thienyl diketopyrrolopyrrole branches were much better than those of the solar cells based on the one with 1,4-phenylene diketopyrrolopyrrole branches. A power conversion efficiency of 1.57%, a short-circuit current density of 6.34 mA/cm2, an open-circuit voltage of 0.73 V, and a fill factor of 0.34 were observed for the organic solar cell based on the star-shaped molecule with 2,5-thienyl diketopyrrolopyrrole branches.

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