Design and structural modification of narrow-bandgap small molecules based on asymmetric porphyrin-diketopyrrolopyrrole backbone for solution-processed organic solar cells

Three novel organic small molecules based on asymmetric porphyrin-diketopyrrolopyrrole (DPP) backbone, namely ZnPTDPP, ZnPTDPPCN and ZnPTDPPO, have been designed and synthesized for solution-processed organic solar cells (OSCs). The terminal units of 4-cyanophenyl and 4-methoxyphenyl were introduced into ZnPTDPPCN and ZnPTDPPO respectively, and the structural modification effects on photoelectric and photovoltaic performance were fully investigated. Depending on the characteristics of porphyrin and DPP, all three molecules possess narrow bandgaps of lower than 1.50 eV, showing excellent absorption properties. Structural modification of terminal groups can effectively regulate energy levels of ZnPTDPPCN and ZnPTDPPO. As expected, extended molecular structure from D-A type to D-A-A(1) or D-A-D-1 type, was conducive to enlarging the pi-conjugation and affecting the push-pull effect of intramolecular electrons, thus affecting the photoelectric character of materials. Solution-processed OSCs employing these molecules as the donor materials and PC61BM as the acceptor material were fabricated. It was worth mentioning that devices based on materials ZnPTDPPCN and ZnPTDPPO achieved moderate power conversion efficiencies (PCEs) of 4.26% and 4.24%, respectively, while ZnPTDPP showed an inferior PCE of 1.46%. To our knowledge, the work based on asymmetric porphyrin-DPP backbone for solution-processed OSCs, is first reported. The work will provide valuable hints for rational design of asymmetric porphyrin-based donor materials.