Theoretical characterisation and design of D–π–A star-shaped molecules with triphenylamine as core and diketopyrrolopyrroles as arms for organic solar cells

A series of novel D–π–A star-shaped molecules with triphenylamine (TPA) fragments as cores, diketopyrrolopyrrole (DPP) fragments as arms, and different conjugate π-bridges those connect TPA and DPP fragments have been designed for small molecules based organic solar cells (OSCs) applications. The optical, electronic, and charge transporting properties have been systematically investigated by means of density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The parameters such as energetic driving force ΔE_L-L, adiabatic ionisation potentials (AIP), and adiabatic electron affinities (AEA) were also calculated at the same level to preliminarily evaluate the performance of the OSCs. The results of frontier molecular orbitals (FMOs) show that the designed molecules provide the best match matched energy levels with typical acceptors PCBM, bisPCBM, and PC70BM except for molecule containing benzo[c][1,2,5]thiadiazole unit because of its low energetic driving force ΔE_L-L. It turned out that the FMO energy levels, the band gaps, AIP, AEA, and absorption spectrum can be tuned effectively by the introduction of different groups. Additionally, our results suggest that the designed molecules can act as promising candidates for donor materials and ambipolar charge transport materials for OSCs.