Power Dependence of the Magnetic Field Effect on Triplet Fusion: A Quantitative Model.

Two strategies for improving solar energy efficiencies, triplet fusion and singlet fission, rely on the details of triplet-triplet interactions. In triplet fusion, there are several steps, each of which is a possible loss mechanism. In solution, the parameters describing triplet fusion collisions are difficult to inspect. Here we show that these parameters can be determined by examining the magnetic field dependence of triplet fusion upconversion. We show that there is a reduction of the magnetic field effect for perylene triplet fusion as the system moves from the quadratic to linear annihilation regimes with an increase in laser power. Our data are modeled with a small set of parameters that characterize the triplet fusion dynamics. These parameters are cross-validated with molecular dynamics simulations. This approach can be applied to both solution and solid state materials, providing a tool for screening potential annihilators for photon upconversion.

Large, Tunable, and Reversible pH Changes by Merocyanine Photoacids.

Molecular photoswitches capable of generating precise pH changes will allow pH-dependent processes to be controlled remotely and noninvasively with light. We introduce a series of new merocyanine photoswitches, which deliver reversible bulk pH changes up to 3.2 pH units (pH 6.5 to pH 3.3) upon irradiation with 450 nm light, displaying tunable and predictable timescales for thermal recovery. We present models to show that the key parameters for optimizing the bulk pH changes are measurable: the solubility of the photoswitch, the acidity of the merocyanine form, the thermal equilibrium position between the spiropyran and the merocyanine isomers, and the increased acidity under visible light irradiation. Using ultrafast transient absorption spectroscopy, we determined the quantum yields for the ring-closing reaction and found that the lifetimes of the transient cis-merocyanine isomers ranged from 30 to 550 ns. Quantum yields did not appear to be a limitation for bulk pH switching. The models we present use experimentally determined parameters and are, in principle, able to predict the change in pH obtained for any related merocyanine photoacid.

Singlet Fission in Concentrated TIPS-Pentacene Solutions: The Role of Excimers and Aggregates.

The excited-state dynamics of 6,13-bis(triisopropylsilylethynyl)pentacene is investigated to determine the role of excimer and aggregate formation in singlet fission in high-concentration solutions. Photoluminescence spectra were measured by excitation with the evanescent wave in total internal reflection, in order to avoid reabsorption effects. The spectra over nearly two magnitudes of concentration were nearly identical, with no evidence for excimer emission. Time-correlated single-photon counting measurements confirm that the fluorescence lifetime shortens with concentration. The observed rate constant grows at high concentrations, and this effect is modeled in terms of the hard-sphere radial distribution function. NMR measurements confirm that aggregation takes place with a binding constant of between 0.14 and 0.43 M-1. Transient absorption measurements are consistent with a diffusive encounter mechanism for singlet fission, with hints of more rapid singlet fission in aggregates at the highest concentration measured. These data show that excimers do not play the role of an emissive intermediate in exothermic singlet fission in solution and that, while aggregation occurs at higher concentrations, the mechanism of singlet fission remains dominated by diffusive encounters.

Development of tethered dual catalysts: synergy between photo- and transition metal catalysts for enhanced catalysis.

While dual photocatalysis-transition metal catalysis strategies are extensively reported, the majority of systems feature two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we synthesised a series of tethered dual catalysts allowing us to investigate this underexplored area of dual catalysis. In particular, Ir(i) or Ir(iii) complexes were tethered to a BODIPY photocatalyst through different tethering modes. Extensive characterisation, including transient absorption spectroscopy, cyclic voltammetry and X-ray absorption spectroscopy, suggest that there are synergistic interactions between the catalysts. The tethered dual catalysts were more effective at promoting photocatalytic oxidation and Ir-catalysed dihydroalkoxylation, relative to the un-tethered species, highlighting that increases in both photocatalysis and Ir catalysis can be achieved. The potential of these catalysts was further demonstrated through novel sequential reactivity, and through switchable reactivity that is controlled by external stimuli (heat or light).

High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films.

Modest exciton diffusion lengths dictate the need for nanostructured bulk heterojunctions in organic photovoltaic (OPV) cells; however, this morphology compromises charge collection. Here, we reveal rapid exciton diffusion in films of a fused-ring electron acceptor that, when blended with a donor, already outperforms fullerene-based OPV cells. Temperature-dependent ultrafast exciton annihilation measurements are used to resolve a quasi-activationless exciton diffusion coefficient of at least 2 × 10-2 cm2/s, substantially exceeding typical organic semiconductors and consistent with the 20-50 nm domain sizes in optimized blends. Enhanced three-dimensional diffusion is shown to arise from molecular and packing factors; the rigid planar molecular structure is associated with low reorganization energy, good transition dipole moment alignment, high chromophore density, and low disorder, all enhancing long-range resonant energy transfer. Relieving exciton diffusion constraints has important implications for OPVs; large, ordered, and pure domains enhance charge separation and transport, and suppress recombination, thereby boosting fill factors. Further enhancements to diffusion lengths may even obviate the need for the bulk heterojunction morphology.

End-Functionalized Semiconducting Polymers as Reagents in the Synthesis of Hybrid II-VI Nanoparticles.

The functionalization of II-VI nanocrystals with semiconducting polymers is of fundamental interest for lightweight, solution-processed optoelectronics. The direct surface functionalization of nanocrystals is useful for facilitating charge transfer across the donor/acceptor interface, in addition to promoting good mixing properties and thereby helping prevent nanoparticle aggregation. In this work, we develop a new method for the direct attachment of semiconducting polymers to II-VI inorganic nanocrystals, where the polymer plays a dual role, acting as both the desired capping agent and a chalcogenide monomer during synthesis. The success of this hybridization procedure relies on the establishment of a new polymer end-functionalization scheme, where a route toward a thio-phosphonate polymer end-group is developed; this end-group resembles many chalcogenide precursor materials used in the synthesis of II-VI nanomaterials. We show the applicability of this hybrid functionalization procedure by attaching poly(3-hexylthiophene-2,5-diyl) to CdSe and CdS. We followed the progress of the reaction by NMR and used transmission electron microscopy to determine the morphology of the resulting materials, which we found to have narrow size distributions after hybridization. Polymer attachment to the nanocrystals was confirmed by examining the steady-state and time-resolved optical properties of the hybrid materials, which also provided an insight into excited-state processes occurring across the hybrid interface.

Balanced Partnership between Donor and Acceptor Components in Nonfullerene Organic Solar Cells with >12% Efficiency.

Relative to electron donors for bulk heterojunction organic solar cells (OSCs), electron acceptors that absorb strongly in the visible and even near-infrared region are less well developed, which hinders the further development of OSCs. Fullerenes as traditional electron acceptors have relatively weak visible absorption and limited electronic tunability, which constrains the optical and electronic properties required of the donor. Here, high-performance fullerene-free OSCs based on a combination of a medium-bandgap polymer donor (FTAZ) and a narrow-bandgap nonfullerene acceptor (IDIC), which exhibit complementary absorption, matched energy levels, and blend with pure phases on the exciton diffusion length scale, are reported. The single-junction OSCs based on the FTAZ:IDIC blend exhibit power conversion efficiencies up to 12.5% with a certified value of 12.14%. Transient absorption spectroscopy reveals that exciting either the donor or the acceptor component efficiently generates mobile charges, which do not suffer from recombination to triplet states. Balancing photocurrent generation between the donor and nonfullerene acceptor removes undesirable constraints on the donor imposed by fullerene derivatives, opening a new avenue toward even higher efficiency for OSCs.

Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells.

Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm2 as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is found not to affect the ultrafast charge-generation kinetics, but instead is found to improve charge-collection yield subsequent to charge generation, linked to improved electron mobility and improved phase separation. Fluorination also leads to improved light absorption, with the blue-shifted absorption profile of the fluorinated polymers complementing the absorption profile of the low-band gap PTB7-Th.

Mapping Polymer Donors toward High-Efficiency Fullerene Free Organic Solar Cells.

Five polymer donors with distinct chemical structures and different electronic properties are surveyed in a planar and narrow-bandgap fused-ring electron acceptor (IDIC)-based organic solar cells, which exhibit power conversion efficiencies of up to 11%.

Impact of Fullerene Mixing Behavior on the Microstructure, Photophysics, and Device Performance of Polymer/Fullerene Solar Cells.

Here, a comprehensive study of the influence of polymer:fullerene mixing behavior on the performance, thin-film microstructure, photophysics, and device physics of polymer solar cells is presented. In particular, blends of the donor polymer PBDTTT-EFT with the acceptor PC71BM that exhibit power conversion efficiencies over 9% are investigated. Through tuning of the fullerene concentration in PBDTTT-EFT:PC71BM blends, the impact of fullerene mixing behavior is systematically investigated via a combination of synchrotron-based X-ray scattering and spectroscopy techniques. The impact of fullerene loading on photophysics and device physics is further explored with steady-state photoluminescence measurements, ultrafast transient absorption spectroscopy, and transient photovoltage measurements. In the low fullerene concentration regime (<50 wt %), most fullerene molecules are dispersed in the polymer matrix, resulting in severe geminate and nongeminate recombination due to a lack of pure fullerene aggregates and percolating pathways for charge separation and transport. In the high fullerene concentration regime (>70 wt %), large fullerene domains result in incomplete PC71BM exciton harvesting with the presence of fullerene molecules also disrupting the molecular packing of polymer crystallites. The optimum fullerene concentration of ∼60-67 wt % balances the requirements of charge generation and charge collection. These findings demonstrate that controlling the fullerene concentration in the mixed phase and optimizing the balance between pure and mixed phases are critical for maximizing the efficiency of highly mixed polymer/fullerene solar cells.

Evolution of Nonmirror Image Fluorescence Spectra in Conjugated Polymers and Oligomers.

The nonmirror image relationship between absorption and fluorescence spectra of conjugated polymers contrasts with most organic chromophores and is widely considered a signature of interchromopohore energy funneling. We apply broad-band ultrafast fluorescence spectroscopy to resolve the evolution of fluorescence spectra for dilute solutions of conjugated oligothiophenes, where no energy transfer is possible. Fluorescence spectra evolve from a mirror image of absorption, which lacks vibronic structure, toward a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond time scale. Our analysis of this fluorescence spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films, which is supported by Raman spectroscopy and quantum chemical modeling. Our data have important implications for understanding different energy-transfer regimes that are delineated by structural relaxation.

Quantifying highly efficient incoherent energy transfer in perylene-based multichromophore arrays.

Multichromophore perylene arrays were designed and synthesized to have extremely efficient resonance energy transfer. Using broadband ultrafast photoluminescence and transient absorption spectroscopies, transfer timescales of approximately 1 picosecond were resolved, corresponding to efficiencies of up to 99.98%. The broadband measurements also revealed spectra corresponding to incoherent transfer between localized states. Polarization resolved spectroscopy was used to measure the dipolar angles between donor and acceptor chromophores, thereby enabling geometric factors to be fixed when assessing the validity of Förster theory in this regime. Förster theory was found to predict the correct magnitude of transfer rates, with measured ∼2-fold deviations consistent with the breakdown of the point-dipole approximation at close approach. The materials presented, along with the novel methods for quantifying ultrahigh energy transfer efficiencies, will be valuable for applications demanding extremely efficient energy transfer, including fluorescent solar concentrators, optical gain, and photonic logic devices.

An N-heterocyclic carbene phenanthroline ligand: synthesis, multi-metal coordination and spectroscopic studies.

Dimetal complexes of a new N-heterocyclic carbene/phenanthroline ligand have been synthesized. Coordination of both ruthenium and rhenium to the phenanthroline moiety in combination with platinum at the carbene moiety are reported. Steady-state and time-resolved optical absorption and photoluminescence spectra were obtained for the complexes. These results illustrate significant changes occur with the incorporation of the second metal, and that the specific metal bound to the phenanthroline moiety is important to the photophysical characteristics of this system.