We report on the formation of charge-transfer (CT) excitons and their effect on photocurrent dynamics in composites with a fluorinated anthradithiophene (ADT-TES-F) donor (D) and acceptors (A) with (i) various LUMO energy offsets with respect to that of the donor (ΔLUMO) and (ii) several different side groups that modify spatial D/A separation at the D/A interface. Exciplexes and nonemissive CT excitons were formed in composites with ΔLUMO <0.6 and >0.6 eV, respectively. A competition between fast charge carrier photogeneration and CT exciton formation was observed, with outcomes depending on the applied electric field (E) and on the D/A separation. At low E fields, CT formation was dominant, and up to a factor of 2 increase in charge photogeneration efficiency due to CT exciton dissociation was observed in composites with large spatial D/A separation compared with that in pristine D films. At high E fields, fast charge carrier photogeneration was dominant in all composites, and no improvement in charge photogeneration efficiency with respect to that in pristine D films was observed. Dramatic changes in charge recombination dynamics were observed depending on the spatial D/A separation. These contributed to a factor of 5−10 improvement in continuous-wave photocurrents in composites with large spatial D/A separation as compared with those in pristine D films.

We report on physical mechanisms behind the temperature-dependent optical absorption, photoluminescence (PL), and photoconductivity in spin-coated films of a functionalized anthradithiophene (ADT) derivative, ADT-TES-F, and its composites with C 60 and another ADT derivative, ADT-TIPS-CN. Measurements of absorption and PL spectra, PL lifetimes, and transient photocurrent were performed at temperatures between 98 K and 300 K as a function of applied electric field. In pristine ADT-TES-F films, absorptive and emissive species were identified to be disordered H-aggregates whose properties are affected by static and dynamic disorder. The exciton bandwidths were ≤0.06 eV and ~0.115 eV for absorptive and emissive aggregates, respectively, indicative of higher disorder in the emissive species. The exciton in the latter was found to be delocalized over ~4-5 molecules. The PL properties were significantly modified upon adding a guest molecule to the ADT-TES-F host. In ADT-TES-F/C 60 composites, the PL was 2 considerably quenched due to photoinduced electron transfer from ADT-TES-F to C 60 , while in ADT-TES-F/ADT-TIPS-CN blends, the PL was dominated by emission from an exciplex formed between ADT-TES-F and ADT-TIPS-CN molecules. In all materials, PL quantum yield dramatically decreased as the temperature increased due to thermally activated nonradiative recombination. Considerable electric-field-induced PL quenching was observed at low temperatures at electric fields above ~10 5 V/cm due to tunneling into dark states. No significant contribution of ADT-TES-F emissive exciton dissociation to transient photocurrent was observed.In all materials, charge carriers were photogenerated at sub-500 ps time scales, limited by the laser pulse width, with temperature-and electric field-independent photogeneration efficiency. In ADT-TES-F/C 60 (2%) composites, the photogeneration efficiency was a factor of 2-3 higher than that in pristine ADT-TES-F films. In ADT-TES-F/ADT-TIPS-CN (2%) blends, an additional charge carrier photogeneration component was observed at room temperature at time scales of ~20 ns due to exciplex dissociation. At ~0.5-5 ns after photoexcitation, the carriers propagated via thermally-and electric field-activated hopping with an activation energy of ~0.025 eV. At time scales longer than ~5 ns, charge transport of carriers that are not frozen in traps proceeded through tunneling via isoenergetic sites.3

We present a model that describes nanosecond (ns) time-scale photocurrent dynamics in functionalized anthradithiophene (ADT) films and ADT-based donor-acceptor (D/A) composites. By fitting numerically simulated photocurrents to experimental data, we quantify contributions of multiple pathways of charge carrier photogeneration to the photocurrent, as well as extract parameters that characterize charge transport (CT) in organic films including charge carrier mobilities, trap densities, hole trap depth, and trapping and recombination rates. In pristine ADT films, simulations revealed two competing charge photogeneration pathways: fast, occurring on picosecond (ps) or sub-ps time scales with efficiencies below 10%, and slow, which proceeds at the time scale of tens of nanoseconds, with efficiencies of about 11%–12%, at the applied electric fields of 40–80 kV/cm. The relative contribution of these pathways to the photocurrent was electric field dependent, with the contribution of the fast process increasing with applied electric field. However, the total charge photogeneration efficiency was weakly electric field dependent exhibiting values of 14%–20% of the absorbed photons. The remaining 80%–86% of the photoexcitation did not contribute to charge carrier generation at these time scales. In ADT-based D/A composites with 2 wt.% acceptor concentration, an additional pathway of charge photogeneration that proceeds via CT exciton dissociation contributed to the total charge photogeneration. In the composite with the functionalized pentacene (Pn) acceptor, which exhibits strong exciplex emission from a tightly bound D/A CT exciton, the contribution of the CT state to charge generation was small, ∼8%–12% of the total number of photogenerated charge carriers, dependent on the electric field. In contrast, in the composite with PCBM acceptor, the CT state contributed about a half of all photogenerated charge carriers. In both D/A composites, the charge carrier mobilities were reduced and trap densities and average trap depths were increased, as compared to a pristine ADT donor film. A considerably slower recombination of free holes with trapped electrons was found in the composite with the PCBM acceptor, which led to slower decays of the transient photocurrent and considerably higher charge retention, as compared to a pristine ADT donor film and the composite with the functionalized Pn acceptor.

We report Förster resonant energy transfer (FRET) with a Förster radius R 0 of 4.8 nm and exciplex formation in composites containing two functionalized anthradithiophene (ADT) derivatives, ADT-TES-F (donor, D) and ADT-TIPS-CN (acceptor, A) depending on the D−A distance. In composites containing bulk D/A heterojunctions, exciplex photoluminescence (PL) emission peaked at ∼668 nm dominated the PL spectra. The exciplex contributed to charge carrier photogeneration on nanosecond time scales in composites, in contrast to sub-500 ps carrier photogeneration observed in ADT-TES-F pristine films. Finally, significantly slower charge carrier recombination was obtained in composites, as compared to that of pristine ADT-TES-F films, due to electron trapping at the ADT-TIPS-CN enabling the hole to propagate in the ADT-TES-F host.

We present an experimental and numerical study of the wavelength dependence, near resonance, of the optical tweezer trap stiffness on three different dye-doped 1 m polystyrene spheres with peak absorptions at = 625, 775, and 840 nm. Experimentally, an increase in the trap stiffness of ϳ35% on the red side of resonance was observed for the dye-doped spheres relative to polystyrene spheres without dye. Numerical simulations for spheres of different sizes, between 20 nm and 1 m, and for absorption strengths corresponding to peak extinction coefficient values between 0.0027 and 0.081 were also conducted. Numerical results showed a maximum increase in the trap stiffness of ϳ35%, which is consistent with experimental results.

Acenes larger than pentacene are predicted to possess enticing electronic properties, but are insoluble and prone to rapid decomposition. Utilizing a combination of functionalization strategies, we present stable, solution-processable hexacenes and an evaluation of their hole and electron transport properties.

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We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentafluorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline films with considerably larger grain sizes in the case of the spin cast film. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast film. As a result, the spin cast film exhibited higher photocurrents despite a considerably lower film thickness (and thus reduced optical absorption and cross section of the current flow).