Star-shaped magnesium tetraethynylporphyrin bearing four peripheral electron-accepting diketopyrrolopyrrole functionalities for organic solar cells†
Magnesium tetraethynylporphyrins bearing four electron-deficient diketopyrrolopyrrole (DPP; Mg-TEP-DPP4) units were synthesized by Sonogashira coupling, and their optical and electrochemical properties were investigated. The energy levels were effectively tuned by DPP with or without electron-withdrawing end-caps. Importantly, all the Mg-TEP-DPP4 molecules have broad absorption bands across the visible and near-infrared regions. The time-dependent excited state features of Mg-TEP-DPP4 and their kinetics were investigated by femtosecond transient absorption spectroscopy in combination with multiwavelength and global/target analyses of time-resolved spectra. Importantly, the charge-separated state was demonstrated to undergo recombination within 253 ps in benzonitrile. Bulk heterojunction organic solar cells fabricated with Mg-TEP-DPP4:PC61BM exhibited a power conversion efficiency of 7.40% after solvent vapor annealing, which enabled balanced charge carrier transport, reduced charge recombination, and favorable charge separation and collection.