Spectroscopic studies on intramolecular charge-transfer characteristics in small-molecule organic solar cell donors: A case study on Ada and dad triad donors

To explore the efficient way of assembling electron-donating (D) and -accepting (A) moieties in small-molecule donors for organic solar cells (OSCs), ADA- and DAD-type triad donor molecules were synthesized and investigated using indolo-[3,2-b]indole and diketopyrrolopyrrole (DPP) as D and A moieties, respectively. Designing D−A-type donor materials possessing intramolecular charge-transfer (ICT) characteristics is important to facilitate exciton dissociation and retard charge-carrier recombination at the donor and acceptor (PC₆₁BM) interface of bulk heterojunction OSCs. While ADA and DAD triad donors showed similar absorption spectra, their photoinduced ICT nature in the excited state monitored by the transient absorption spectroscopy was quite different. Both molecules exhibit strong electronegativity and abundance of electrons on DPP moieties, facilitating interaction with the neighboring molecules. However, ADA exhibits stronger ICT character than DAD because of the spatially more delocalized lowest unoccupied molecular orbital and abundant electron density at the end-capping DPP moieties. Owing to its stronger ICT character in the excited state, the ADA:PC₆₁BM blend showed more favorable charge separation and reduced charge-carrier recombination at the donor/PC₆₁BM interface. Consequently, ADA:PC₆₁BM devices exhibited higher J_SC than DAD:PC₆₁BM OSCs.