Loosdrecht, P.H.M. van

Orekhova, T.V.

Paraschuk, D.Yu.

Parashchuk, O.D.

Pavelyev, V.G.

Perepichka, I.F.

Pshenichnikov, M.S.

English

We study charge recombination rates in blends of MEH-PPV and a comprehensive set of low molecular-weight fluorine-based acceptors, using visible-pump – IR-probe photoinduced absorption technique. The positions of acceptor’s HOMO-LUMO levels are engineered by addition of different chemical substituents. The results clearly show the exponential scaling of the charge recombination rate with the acceptor LUMO energy. The minimal donor-acceptor LUMO energy difference required to form a CTC state is estimated as ~0.25 eV

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   University of GroningenAcceptor Energy Offset Manages Ultrafast Recombination Dynamics in Donor-AcceptorMixturesPavelyev, V.G.; Parashchuk, O.D.; Orekhova, T.V.; Perepichka, I.F.; Paraschuk, D.Yu.;Loosdrecht, P.H.M. van; Pshenichnikov, M.S.Published in:Proceedings of the XVIIIth International Conference on Ultrafast PhenomenaIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2013Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Pavelyev, V. G., Parashchuk, O. D., Orekhova, T. V., Perepichka, I. F., Paraschuk, D. Y., Loosdrecht, P. H.M. V., & Pshenichnikov, M. S. (2013). Acceptor Energy Offset Manages Ultrafast Recombination Dynamicsin Donor-Acceptor Mixtures. In M. Chergui, S. Cundiff, A. Taylor, R. D. Vivie-Riedle, & K. Yamanouchi(Eds.), Proceedings of the XVIIIth International Conference on Ultrafast Phenomena (41 ed., Vol. 41, pp.05036-p.1-05036-p.3). University of Groningen, The Zernike Institute for Advanced Materials.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum. Acceptor Energy Offset Manages Ultrafast Recombination Dynamics in Donor-Acceptor Mixtures V. G. Pavelyev,1 O. D. Parashchuk,2 T. V. Orekhova,3 I. F. Perepichka,4 D. Yu. Paraschuk,2 P. H. M. van Loosdrecht,1 and M. S. Pshenichnikov1 1 Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands 2 Faculty of Physics and International Laser Center, Lomonosov Moscow State University, Leninskie Gory, 119991 Moscow, Russia 3L. M. Litvinenko Institute of Physical Organic and Coal Chemistry, national Academy of Sciences of Ukraine, R. Luxemburg Street 70, Donetsk 83122, Ukraine 4 School of Chemistry, Bangor University, Deiniol Road, Bangor LL57 2UW, United Kingdom Abstract. We study charge recombination rates in blends of MEH-PPV and a comprehensive set of low molecular-weight fluorine-based acceptors, using visible-pump – IR-probe photoinduced absorption technique. The positions of acceptor’s HOMO-LUMO levels are engineered by addition of different chemical substituents. The results clearly show the exponential scaling of the charge recombination rate with the acceptor LUMO energy. The minimal donor-acceptor LUMO energy difference required to form a CTC state is estimated as ~0.25 eV. Ground-state charge transfer complexes (CTCs) lay in heart of many photoinduced processes in physics, chemistry, and biology. The ground-state CTCs are characterized by a partial shift of electronic density from the donor to the acceptor molecule already in the ground state. Upon optical excitation, the electron is transferred to the acceptor almost instantaneously, while back recombination occurs in the finite time. The CTC photophysical properties strongly depend on the molecular structure of the donor-acceptor pair. For instance, in polymer-fullerene blends the CTC absorption is extremely weak and hardly detectable [1]. In contrast, in blends of soluble poly-para-phenylene vinylenes (MEH-PPV) with fluorene-type acceptors, the CTC formation can be readily detected visually as a color change. In the latter type of materials, the CTC band is so intense that it could enhance the solar energy harvesting in the bandgap of both the donor and acceptor [2], similarly to push-pull copolymers [3]. Furthermore, the polymer photoxidation stability is drastically increased by the presence of the CTCs [4]. As was recognized in early studies of small-molecule CTCs [5], the acceptor electron affinity is a key parameter that determines the back electron transfer rates. Here we demonstrate that the acceptor electron affinity governs the CTC recombination rate in full accord with the Markus theory [6]. Donor-acceptor CTCs formed in blends between the conjugated polymer MEH-PPV and a family of low-molecular weight fluorene acceptors was used for their known property to form stable CTCs already in solution. To unravel recombination dynamics of the photogenerated charges, ultrafast visible-pump -- IR-probe photo-induced absorption (PIA) spectroscopy [7] was applied. EPJ Web of ConferencesDOI: 10.1051/C© Owned by the authors, published by EDP Sciences, 2013,epjconf 201/05036 (2013)4134105036 This  is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20134105036EPJ Web of Conferences The acceptor electron affinity engineering was achieved by attaching a number of electron-negative functional groups (cyano, nitro, and carbonyl) to the fluorene core (Fig.1a), which predictably modify the HOMO-LUMO energies of the acceptors. Both energies were calculated by DFT method at the B3LYP/6-31g(d,p) level of theory.                   Fig.1. (a) Chemical structures of MEH-PPV polymer (left) and three representative acceptors (named as NFon, DNF and DTNF) with the LUMO energies of -2.8, -3.3, and -4.5 eV respectively. (b) Absorption spectra of CTCs formed by polymer with the three representative acceptors with 1:0.3 weight concentrations (in films). Absorption spectra of the polymer-acceptor films (Fig.1b) exhibit a wing in the red region which is a clear signature of the CTC formation. Isotropic PIA transients (Fig.2a) demonstrate that charge Fig. 2. Normalized to the unity isotropic PIA (a) and anisotropy (b) transients for CTCs of MEH-PPV with three representative acceptors. The excitation and probe wavelengths are 640 nm and 3 µm, respectively. In (a), decay lifetimes are shown next to the transients. Anisotropy transients are depicted within the recombination lifetime. recombination dynamics in different CTCs are markedly different. Here we monitor the time evolution of the IR polaron absorption band associated with generation of the photoinduced charges and their subsequent recombination. In all blends, the PIA transients show exponential decay in the 1-100 ps range that is assigned to charge recombination. Transient anisotropy does not change in time (Fig.2b) which strongly suggests that the charges do not migrate at the time scale of the charge recombination, pointing at high degree of localization of the charge-transfer state. More importantly, 0.00.10.20.30.40.1 1 10 1000.00.10.20.30.4 DTNF DNF NFonAnisotropyDelay, ps  0.1 1 10 100 NFon DNF DTNF Normalized T/T (arb.units)Delay, ps0.1 1 10 100  NFon MEH-PPV a) DNF 400 500 600 700 8000,00,20,40,60,81,0 MEH-PPV NFon DNF TNFNormalized OD, a.u.Wavelength, nm400 500 600 700 8000,00,20,40,60,81,0  b) DTNF a) b) ~170 ps ~8 ps ~1 ps 05036-p.2-5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0110100 Time, psLUMO of Acceptor, eV110100the amplitudes of PIA anisotropy reflect different packing of the polymer and acceptors in the blend, resulting in different orientations of the excitation and polaron transient dipole moments.              Fig. 3. Charge recombination time versus position of the acceptors LUMO (points). The solid line shows a linear (in the logarithmic scale) fit to the data points where CTCs with the polymer are formed. Figure 3 shows the relation between the charge recombination time and the acceptor LUMO energy position. The relation between them is clear: the higher position of the LUMO, the slower the recombination dynamics. Furthermore, the experimental points cluster around a straight line in full agreement with the Marcus model [8]. The linear dependence extends until a point where the intramolecular recombination channel with a characteristic time of ~150 ps for neat MEH-PPV begins to prevail. The CTC formation energy can be estimated from the offset of the crossing point as 0.25 eV which is consistent with the slope value of 0.26 eV. From the ultrafast studies presented here, we arrive at the following conclusion, directly concerning organic photovoltaics. It is well established that increasing the acceptor electron affinity increases the driving force for efficient exciton dissociation of donor-acceptor pair. At the same time, this inevitably increases the unwanted back transfer electron rate unless the blend design provides the efficient charge delocalization [9]. References 1. L. Goris et al., Appl. Phys. Lett., 88, 052113 (2006) 2. A. A. Bakulin et al., Synth. Met. 147, 221 (2004) 3. T.-Y. Chu et al., J. Am. Chem. Soc., 133, 4250 (2011) 4. I. V. Golovnin et al., Appl. Phys. Lett. 92, 243311 (2008) 5. A. A. Bakulin et al., Chem. Phys. Lett. 482, 99 (2009) 6. R. A. Marcus, J. Chem. Phys. 24 (5), 966 (1956) 7. A. A. Bakulin et al., J. of Phys. Chem. B 112, 13730 (2008) 8. I. R. Gould et al., Chem. Phys. 176, 439 (1993) 9. A. A. Bakulin et al., Science 335, 1340 (2012)  ~0.25 eV MEH-PPV XVIIIth International Conference on Ultrafast Phenomena05036-p.3

Acceptor Energy Offset Manages Ultrafast Recombination Dynamics in Donor-Acceptor Mixtures

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Acceptor Energy Offset Manages Ultrafast Recombination Dynamics in Donor-Acceptor Mixtures

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