Solution processable low bandgap small molecule donors with naphthalene end-groups for organic solar cells

Three new low bandgap donor–acceptor–donor (D–A–D) type small molecules (M1–M3) containing naphthalene, thiophene, and diketopyrrolopyrrole were synthesized by Suzuki and Stille coupling reactions. The thermal, optical, electrochemical and photovoltaic properties of these molecules were investigated. These materials possess strong absorption at 475–750 nm and low-lying HOMO levels (−5.28 to −5.35 eV). Solution-processed bulk heterojunction solar cells based on M1–M3/PC71BM blends (1:1, w/w) demonstrate power conversion efficiencies (PCEs) as high as 2.59%, with a short-circuit current density (Jsc) of 6.83 mA/cm2, and an open-circuit voltage (Voc) of 0.88 V under AM1.5G irradiation (100 mW/cm2). Naphthalene-ended M1 and M3 show higher photovoltaic performance than M2 due to broad absorption and better morphology of the devices.

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► Three new low bandgap donor–acceptor–donor type small molecules were developed.
► These molecules possess low-lying HOMO energy levels (−5.28 to −5.35 eV).
► Power conversion efficiencies of solar cells based on these molecules reaches 2.59%.
► Naphthalene-ended molecules show higher photovoltaic performance.