Low bandgap small molecules based on 2,2-bithiophene-3,3-dicarboximide for soluble-processed solar cells

• The bithiophene-imide was prepared and utilized for the synthesis of the small molecules.
• The conjugated materials with BTI group showed lower HOMO level to lead to large VOC.
• The device based on the SM:PCBM blend had PCE of 1.36%.

Small molecules, SM1 and SM2, with bithiophene-imide as the central electron deficient unit and bridged thiophene and terminal triphenyl amine as electron rich units, have been designed and synthesized. The dithienoazepinedione, bithiophene imide (BTI) group, is an attractive electron deficient unit since it demonstrates strong electron-withdrawing character, a planar architecture, and good solubility. The spectra of SM1 and SM2 in the solid thin films show absorption bands with maximum peaks at 368, 495 and 397, 517 nm, and the absorption onsets at 598 and 631 nm. The electrochemical band gaps are about 1.45 and 1.43 eV, somewhat lower than the optical band gaps. The best device with SM1: PC71BM in chloroform with thermal treatment at 130 °C showed VOC of 0.91 V, JSC of 3.93 mA/cm2, and FF of 0.38, which yielded PCE of 1.36%.

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