Substituent effects in magnesium tetraethynylporphyrin with two diketopyrrolopyrrole units for bulk heterojunction organic solar cells

Abstract

Magnesium tetraethynylporphyrin complexes with two aryl groups (Ar = Ph, C₆H₄-n-hexyl-4, C₆H₄-CF₃-4, and C₆H₄-NMe₂-4) and two thienyl-substituted diketopyrrolopyrrole units were synthesized by Sonogashira coupling reactions. Electrochemical measurements and photoelectron yield spectroscopy showed that the energy levels of the compounds could be effectively tuned by electron-withdrawing and -donating substituents on the aryl groups. Theoretical calculation revealed that the electronic effects of the Ar group substituents strongly affected the energy level of the highest occupied molecular orbital (HOMO) because the localized HOMO on the porphyrin core was conjugated with the Ar groups through ethynyl linkers. Bulk heterojunction small-molecule organic solar cells with these magnesium porphyrin electron donors were fabricated to evaluate the morphological and electronic effects of the Ar group substituents. The n-hexyl groups promoted the formation of phase-separated structures with a high fill factor, yielding a respectable power conversion efficiency of 5.73%. The use of the electron-withdrawing CF₃ group gave a high open-circuit voltage of 0.79 V because of the lowered HOMO level, while the use of the electron-donating NMe₂ group produced a low band gap that extended the incident photon-to-current conversion efficiency spectrum to 1100 nm.