Structure and dynamics correlations of photoinduced charge separation in rigid conjugated linear donor-acceptor dyads towards photovoltaic applications

The rates of photoinduced charge separation (CS) and charge recombination (CR) of four rigidly linked donor (D)-acceptor (A) dyads that were synthesized as intramolecular p/n junctions were obtained by optical transient absorption spectroscopy in solutions and self-assembled films. The CS processes were much more efficient than CR processes for all dyads in both solutions and films, which is preferable for photovoltaic applications. The rates of CR processes in covalently linked dyad films were slower than those in non-covalently linked D (i.e. oligothiophene (OTH)) and A (i.e. perylenediimide (PDI)) mixtures, demonstrating that the covalent linker may have reduced the probability of geminate recombination of charges due to direct face-to-face packing between D and A. Although both the CS and CR rates of the four dyads were similar with respect to one another in solution, they differed from one another in films, with the slowest CR rate observed in the dyad connected to a bulky alkyl group that hindered packing. After annealing, the CR rates of all films increased non-uniformly, and the extent of increase was correlated to structural changes as evidenced by the grazing-incident X-ray scattering (GIXS).