444 research outputs found
Charge separation by photoexcitation in semicrystalline polymeric semiconductors: An intrinsic or extrinsic mechanism?
We probe charge photogeneration and subsequent recombination dynamics in neat
regioregular poly(3-hexylthiophene) films over six decades in time by means of
time-resolved photoluminescence spectroscopy. Exciton dissociation at 10K
occurs extrinsically at interfaces between molecularly ordered and disordered
domains. Polaron pairs thus produced recombine by tunnelling with distributed
rates governed by the distribution of electron-hole radii. Quantum-chemical
calculations suggest that hot-exciton dissociation at such interfaces results
from a high charge-transfer character.Comment: 10 pages, 3 figure
Multiresonant thermally activated delayed fluorescence emitters based on heteroatom doped nanographenes : recent advances and prospects for organic light-emitting diodes
We thank the Leverhulme Trust (RPG-2016-047) for the financial support. S.S. acknowledges sup-port from the Marie SkĆodowska-Curie Individual Fellowship (NarrowbandSSL EC Grant Agree-ment No: 838885). Computational resources have been provided by the Consortium des Ăquipements de Calcul Inten-sif (CĂCI), funded by the Fonds de la Recherche Scientifiques de Belgique (F.R.S.-FNRS) under Grant No. 2.5020.11, as well as the Tier-1 supercomputer of the FĂ©dĂ©ration Wallonie-Bruxelles, infrastructure funded by the Walloon Region under the grant agreement n1117545. DB is a FNRS Research Director.Since the first report in 2015, multiresonant thermally activated delayed fluorescent (MRâTADF) compounds, a subclass of TADF emitters based on a heteroatomâdoped nanographene material, have come to the fore as attractive hosts as well as emitters for organic lightâemitting diodes (OLEDs). MRâTADF compounds typically show very narrowâband emission, high photoluminescence quantum yields, and small ÎE ST values, typically around 200 meV, coupled with high chemical and thermal stabilities. These materials properties have translated into some of the best reported deepâblue TADF OLEDs. Here, a detailed review of MRâTADF compounds and their derivatives reported so far is presented. This review comprehensively documents all MRâTADF compounds, with a focus on the synthesis, optoelectronic behavior, and OLED performance. In addition, computational approaches are surveyed to accurately model the excited state properties of these compounds.Publisher PDFPeer reviewe
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Singlet exciton fission in solution.
Singlet exciton fission, the spin-conserving process that produces two triplet excited states from one photoexcited singlet state, is a means to circumvent the Shockley-Queisser limit in single-junction solar cells. Although the process through which singlet fission occurs is not well characterized, some local order is thought to be necessary for intermolecular coupling. Here, we report a triplet yield of 200% and triplet formation rates approaching the diffusion limit in solutions of bis(triisopropylsilylethynyl (TIPS)) pentacene. We observe a transient bound excimer intermediate, formed by the collision of one photoexcited and one ground-state TIPS-pentacene molecule. The intermediate breaks up when the two triplets separate to each TIPS-pentacene molecule. This efficient system is a model for future singlet-fission materials and for disordered device components that produce cascades of excited states from sunlight.B.J.W. was supported by a Herchel Smith Research Fellowship. A.J.M. received funding
from a Marie Curie Scholarship. D.B. is a FNRS Research Director. Both A.J.M and D.B.
acknowledge support from the European Communityâs Initial Training Network SUPERIOR (PITN-GA-2009-238177). Further funding for this project came from the Engineering and Physical Sciences Research Council (EPSRC) and a pump-prime grant from the Winton Programme for the Physics of Sustainability.This is the accepted version of an article originally published in Nature Chemistry 5, 1019â1024 and available online at http://www.nature.com/nchem/journal/v5/n12/full/nchem.1801.html. Nature Publishing Group's conditions for reuse are detailed at http://www.nature.com/authors/policies/license.html
Ultrafast Non-Forster Intramolecular Donor Acceptor Excitation Energy Transfer
Ultrafast intramolecular electronic energy transfer in a conjugated donor-acceptor system is simulated using nonadiabatic excited-state molecular dynamics. After initial site-selective photoexcitation of the donor, transition density localization is monitored throughout the S-2 -> S-1 internal conversion process, revealing an efficient unidirectional donor acceptor energy-transfer process. Detailed analysis of the excited state trajectories uncovers several salient features of the energy-transfer dynamics. While a weak temperature dependence is observed during the entire electronic energy relaxation, an ultrafast initially temperature-independent process allows the molecular system to approach the S-2-S-1 potential energy crossing seam within the first ten femtoseconds. Efficient energy transfer occurs in the absence of spectral overlap between the donor and acceptor units and is assisted by a transient delocalization phenomenon of the excited-state wave function acquiring Frenkel-exciton character at the moment of quantum transition.This project has received funding from the Universidad Carlos III de Madrid, the European Unionâs Seventh Framework Programme for research, technological development and demonstration under grant agreement n° 600371, el Ministerio de Economia y Competitividad (COFUND2014-51509), el Ministerio de EducacioÌn, cultura y Deporte (CEI-15-17) and Banco Santander. This work was partially supported by CONICET, UNQ, ANPCyT (PICT-2014-2662). We also acknowledge support of the Center for Integrated Nano-technology (CINT), a U.S. Department of Energy, Office of Basic Energy Sciences user facility, as well as additional funding from the Bavarian University Centre for Latin America (BAYLAT). The work in Mons is supported by BELSPO through the PAI P6/27 Functional Supramolecular Systems project and by the Belgian National Fund for Scientific Research FNRS/F.R.S. DB is a Research Director of FNRS
Comprehensive modelling study of singlet exciton diffusion in donor-acceptor dyads:When small changes in chemical structure matter
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Breakdown of the mirror image symmetry in the optical absorption/emission spectra of oligo(para-phenylene)s
The absorption and emission spectra of most luminescent, pi-conjugated,
organic molecules are the mirror image of each other. In some cases, however,
this symmetry is severely broken. In the present work, the asymmetry between
the absorption and fluorescence spectra in molecular systems consisting of
para-linked phenyl rings is studied. The vibronic structure of the emission and
absorption bands is calculated from ab-initio quantum chemical methods and a
subsequent, rigorous Franck-Condon treatment. Good agreement with experiment is
achieved. A clear relation can be established between the strongly anharmonic
double-well potential for the phenylene ring librations around the long
molecular axis and the observed deviation from the mirror image symmetry.
Consequences for related compounds and temperature dependent optical
measurements are also discussed.Comment: 12 pages, 13 Figure
Vacuum-deposited donors for low-voltage-loss nonfullerene organic solar cells
The advent of nonfullerene acceptors (NFAs) enabled records of organic photovoltaics (OPVs) exceeding 19% power conversion efficiency in the laboratory. However, high-efficiency NFAs have so far only been realized in solution-processed blends. Due to its proven track record in upscaled industrial production, vacuum thermal evaporation (VTE) is of prime interest for real-world OPV commercialization. Here, we combine the benchmark solution-processed NFA Y6 with three different evaporated donors in a bilayer (planar heterojunction) architecture. We find that voltage losses decrease by hundreds of millivolts when VTE donors are paired with the NFA instead of the fullerene C60, the current standard acceptor in VTE OPVs. By showing that evaporated small-molecule donors behave much like solution-processed donor polymers in terms of voltage loss when combined with NFAs, we highlight the immense potential for evaporable NFAs and the urgent need to direct synthesis efforts toward making smaller, evaporable compounds
Charge-transfer excitons in strongly coupled organic semiconductors
Time-resolved and temperature-dependent photoluminescence measurements on
one-dimensional sexithiophene lattices reveal intrinsic branching of
photoexcitations to two distinct species: self-trapped excitons and dark
charge-transfer excitons (CTX; > 5% yield), with radii spanning 2-3 sites. The
significant CTX yield results from the strong charge-transfer character of the
Frenkel exciton band due to the large free exciton bandwidth (~400 meV) in
these supramolecular nanostructures.Comment: Physical Review B Rapid Communications (In Press
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