Development of low band gap molecular donors with phthalimide terminal groups for use in solution processed organic solar cells

•Synthesis of phthalimide end-capped derivatives of DTS(FBT-Th-Pth-Hexyl)2.•Compounds exhibit higher solubility and deeper HOMO levels.•Solar cell devices can achieve higher open circuit voltages.•Performance limited by unfavorable morphology with large domain phase separation.

The synthesis and characterization of two novel narrow bandgap ‘donor’ small molecule semiconductors are reported. The new compounds are based upon the popular DTS(FBT-Th2-Hexyl)2 donor molecule which has a D2-A-D1-A-D2 architecture with D1 = dithienosilole (DTS), A = 2-fluorobenzothiadiazole (FBT), and D2 = hexyl-bithiophene (Th2-Hexyl). We have replaced the D2 hexyl-bithiophene unit with electron withdrawing phthalimide units. The new materials were characterized using a combination of theoretical calculations, UV-visible spectroscopy, cyclic voltammetry, and thermal analysis. The phthalimide substitution resulted in an overall stabilization of the highest occupied molecular orbital energy levels, and an increase in both dipole moment and organic solvent solubility. When paired with PC61BM, organic solar cells gave surprisingly low power conversion efficiencies. Investigation of the active layer morphologies revealed large phase segregated domains indicating that phthalimide substitution renders the donor molecule immiscible with fullerene acceptors.

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