Substituents Effect in Magnesium Tetraethynylporphyrin for Bulk Heterojunction Organic Solar Cells

Abstract

Porphyrin-based small-molecule electron donors have recently emerged as a new option for use in bulk heterojunction organic solar cells (OSCs) and dye-sensitized solar cells. High power conversion efficiencies (PCEs) in OSCs have been realized with these electron-donating materials by using high light absorption efficiency of porphyrins and their wide absorption range that extends to the near-IR region when the porphyrin units are conjugated with electron-deficient units such as diketopyrrolopyrrole (DPP). 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 highest occupied molecular orbital (HOMO) because the localized HOMO on the porphyrin core was conjugated with the Ar groups through the 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 formation of phase-separated structures with a high fill factor, yielding a respectable power conversion efficiency of 5.73%. Use of the electron-withdrawing CF3 group gave a high open-circuit voltage of 0.79 V because of the lowered HOMO level, while 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. Figure 1