Development of diketopyrrolopyrrole-based donor-acceptor oligomers using direct arylation polymerization: Non-linear and ultrafast optical analysis, and their application in ternary organic solar cells

A series of donor-acceptor (D-A) oligomers based on thieno[3,2-b]thiophene (TT), 2,2′-bithieno[3,2-b]thiophene (BTT), and diketopyrrolopyrrole (DPP) were synthesized through eco-friendly direct arylation polymerization using two different Fagnou conditions. These DPP-based oligomers, characterized on average by the chain of 4 units, presented notable bands of infrared photoluminescence with peaks in the wavelength range 712–753 nm. Time-resolved transient absorption spectroscopy was used for a basic understanding of the dynamics of excited states in these oligomers in solution, nanoparticles, and solid films. The optical nonlinearities were also evaluated through Z-scan experiments under femtosecond excitation, finding significant cross-sections of nonlinear absorption (∼920 GM) at 950 nm. Further, these DPP-based organic materials were loaded in different weight percentages, as an electro-donor material in ternary organic solar cells in combination with poly(3-hexylthiophene) P3HT and PC71BM. An improvement in conversion efficiency of 28% regarding a binary cell was achieved with the BTT derivative due to its complementary role to recollect light in the infrared region, better energy level matching, and improved quality in the formation of solid films compared with the TT derivative, which in combination increased Jsc from 9.55 to 13.5 mA/cm2. Theoretical calculations show that oligomers of 4 units produced optimized optical properties with reduced optical band gap and maximum dipole moments.