Effects of energy offsets and molecular packing on exciton and charge carrier dynamics in small-molecule donor-acceptor composites

Small-Molecule Bulk Heterojunctions: Distinguishing Between Effects of Energy Offsets and Molecular Packing on Optoelectronic Properties

“…Figure a,b show transient photocurrent dynamics in pristine ADT-TES-F and several D/A films with 5% acceptor concentration. In all samples, the rise time (defined here as the time during which the photocurrent increases from 10% to 90% of its peak value) of the photocurrents was below 0.6 ns, limited by the excitation laser pulse width and jitter, indicative of fast charge photogeneration, which is consistent with previous observations. ,,,, At all values of applied E-field, photocurrents measured in the D/A films were lower than those in pristine ADT-TES-F films, regardless of the acceptor (Figure ) . In ADT-TES-F/ADT-R-F films, no considerable effects on the transient photocurrent amplitude due to the sizes of R-groups or the different packing motifs of ADT-R-F acceptors were observed.…”

“…Figure a,b shows PL spectra of ADT-TES-F/ADT-TIPS-F and ADT-TES-F/ADT-TSBS-F films, respectively, normalized at the peak PL values, at several acceptor concentrations. At all concentrations, the PL emission was dominated by that of the ADT-TES-F donor . The PL intensity increased as the concentration of the ADT-R-F acceptor increased (Figure c).…”

“…Detailed analysis of optical absorption and PL emission spectra of ADT-R-F molecules in solution and in pristine thin films can be found elsewhere. ,− Briefly, it has been established that the spectra of isolated ADT-R-F molecules (i.e., in solutions or dispersed at low concentrations in solid matrices) are not affected by the side group R. In contrast, both the absorption and the PL spectra of pristine ADT-R-F films exhibit differences in the wavelengths of absorption and PL maxima (Table , Figure S1) and in Huang–Rhys factors, ,, dependent on the molecular packing in the film dictated by the side groups R . The absorption spectra of functionalized ADT and Pn derivatives are red-shifted in films as compared to those of solutions by an amount which depends on the molecular packing and is also related to the strength of intermolecular interactions which promote exciton delocalization. , Of all ADT-R-F derivatives used in our studies, the red shift was highest in pristine ADT-TES-F films, which suggests the highest exciton delocalization is in these films, followed by that in pristine ADT-TIPS-F and ADT-TSBS-F films (Table ).…”