pubs.acs.org/JPCL
Charge-Transfer Localization in Molecularly Doped Thiophene-Based Donor Polymers €rn-Oliver Vogel,† Silvia Janietz,z Patrick Pingel,†,‡ Lingyun Zhu,§ Kue Surk Park,† Jo § † €rgen P. Rabe, Jean-Luc Br Eung-Gun Kim, Ju edas,§ and Norbert Koch*,† †
Institut f€ ur Physik, Humboldt University Berlin, Newtonstrasse 15, 12489 Berlin, Germany, ‡Institute of Physics and Astronomy, University of Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany, §School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, and zFraunhofer Institute for Applied Polymer Research, Geiselbergstrasse 69, 14476 Potsdam, Germany
ABSTRACT We provide evidence for highly localized charge-transfer complex formation between a series of thiophenetetrafluorobenzene donor copolymers and the molecular acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ). Infrared absorption spectra of diagnostic vibrational bands in conjunction with theoretical modeling show that one acceptor molecule undergoes charge transfer with a quaterthiophene segment of the polymers, irrespective of the macroscopic polymer ionization energy and acceptor concentration in thin films. The interaction is thus determined by the “local ionization potential” of quaterthiophene. Consequently, using materials parameters determined on a macroscopic length scale as a guideline for making new charge-transfer complex materials for high electrical conductivity turns out to be oversimplified, and a reliable material design must take into account property variations on the nm scale as well. SECTION Macromolecules, Soft Matter
acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ; see Scheme 1).9 F4TCNQ doping was also applied to other conjugated polymers; however, to date, the combination of F4TCNQ with P3HT has resulted in the highest conductivities.5,10-13 In order to achieve further improvements of highly conductive organic materials, it is important to understand the fundamental mechanisms of charge transfer and its relationship to individual material properties. In ref 9, we pointed out that the formation of chargetransfer complexes between F4TCNQ and P3HT involves hybridization of donor and acceptor molecular orbitals. Moreover, we found that only one specific charge-transfer species is predominant in these samples, despite the potentially large number of different local conformations that could be expected due to the flexibility of the polymer chains and the manifold of possible interchain interactions. Hence, it appears that charge transfer is spatially restricted such that F4TCNQ interacts with only a few connected thiophene repeat units. In order to test the hypothesis of localization, we report here on F4TCNQ-doped layers of a series of soluble thiophenebased donor copolymers (3HT-TFB; see Scheme 1), which comprise various amounts of tetrafluorobenzene (TFB) units. TFB units are introduced into the main chain in order to interrupt the thiophene sequence and to provide a variation in effective conjugation length of the thiophene blocks, which
T
he field of organic electronics has made tremendous progress within the last two decades, which has led to electroluminescent, photovoltaic, and transistor devices in the stage of (or close to) commercialization.1 However, there is still a lack of comprehensive understanding of some fundamental processes in conjugated organic materials, such as the nature of charge transfer and electrical conductivity associated with oxidation and reduction (doping) of organic molecular compounds. From an application standpoint, the realization of printed all-organic circuits requires highly conductive organic layers that can be processed from solution, potentially even to serve as conductive wires.2,3 The materials presently available for that purpose almost exclusively comprise aqueous dispersions of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT/PSS); however, controlling thin-film properties is challenging because of the presence of residual water.4 Replacing PEDOT/PSS in devices is moreover desirable to avoid luminescence quenching and possible acidic etching of indium tin oxide due to excess PSS.5 A promising approach to achieving highly conductive polymer films free from water is the intentional molecular doping of conjugated polymers with organic compounds. An appropriate combination of soluble acceptor and donor systems can lead to ground-state charge transfer and a considerable increase in electrical conductivity with respect to the pure materials.6-8 Recently, we reported that a high conductivity of 1 S/cm can be reached in thin films when oxidizing (p-type doping) poly(3-hexylthiophene) (P3HT) in solution with the strong
r 2010 American Chemical Society
Received Date: April 16, 2010 Accepted Date: June 9, 2010 Published on Web Date: June 16, 2010
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DOI: 10.1021/jz100492c |J. Phys. Chem. Lett. 2010, 1, 2037–2041
pubs.acs.org/JPCL
Scheme 1. Chemical Structures of F4TCNQ (1) and 3HT-TFB (2a-c)
Table 1. Properties of the P3HTand 3HT-TFB (2a-c) Copolymers compound
Mna [g/mol]
ATFB/Thb [mol %]
IPc [eV]
P3HT
19 200
0
4.80 ( 0.05
2a 2b
19 800 14 800
3.0 5.1
5.00 ( 0.05 5.05 ( 0.05
2c
14 500
9.5
5.10 ( 0.05
a
b
Number-average molecular weight. Relative amount of TFB and thiophene units in the main chain. c Ionization potential from photoemission measurements.14
are intended to interact with F4TCNQ. We find that despite this variation and the presence of TFB, only a single chargetransfer species is formed, that is, a highly localized complex between F4TCNQ and a thiophene segment of the main chain. The synthesis of the donor copolymers followed the McCullough-Grignard metathesis method and is described in detail in ref 14. 1,4-Dithienyl-2,3,5,6-tetrafluorobenzene (TFB) units were introduced along the 3-hexylthiophene main chain, resulting in regioregularly linked random 3HT-TFB copolymers (2a-c) with similar molecular weights and a moderate polydispersity index of 1.5. The relevant materials parameters of 3HT-TFB are summarized in Table 1. Notably, the variation of the TFB amount along the main chain leads to a systematic change of the polymer ionization potential (IP) due to the electron-accepting character of TFB, that is, the IP varies from 4.8 to 5.1 eV in going from P3HT to compound 2c, as determined by ultraviolet photoelectron spectroscopy (see Supporting Information). We recall that the electron affinity (EA) of F4TCNQ is 5.24 eV.15 Thus, the acceptor EA is larger than the donor polymer IP in all cases. Nonetheless, differences in the amount of charge transfer could be expected as the charge redistribution along the polymer chain should be impacted by the various amounts of TFB units. Blend films of F4TCNQ and the various 3HT-TFB copolymers were prepared by drop-casting from chloroform solution [F4TCNQ/(thiophene monomer) ratios of 1:10 and 1:3 were used, typically 1 mg/mL] onto Si wafers with native oxide surfaces. We used infrared (IR) spectroscopy as a direct diagnostic tool to investigate the degree of charge transfer. Upon (partial) ionization, for example, following charge transfer, F4TCNQ changes its geometry from a quinoidal to a more aromatic structure.16,17 This is accompanied by a red shift of specific vibrational modes that serve as an indicator of charge transfer, in particular, the IR bands that are assigned to asymmetric CdC stretching (at 1547 and 1598 cm-1) and asymmetric CtN stretching (at 2214 and 2227 cm-1 for neutral F4TCNQ). The red shift of these IR bands was found to increase linearly with increasing degree of charge transfer.18-21
r 2010 American Chemical Society
Figure 1. IR spectra of neutral F4TCNQ (1) and the F4TCNQ/ (co)polymer blends [F4TCNQ/(thiophene monomer) ratios of 1:10 (a) and 1:3 (b)] in the CtN stretching region, indicative of the degree of charge transfer.
Full-range IR spectra of the pure and blended compounds [1:10 ratio of F4TCNQ/(thiophene monomer)] are provided in the Supporting Information, including the vibrational bands of the thiophene-based polymers in the CdC stretching region. In the following, we focus on the variations of the CtN stretching modes of the F4TCNQ cyano groups. Figure 1 shows the corresponding region of the IR spectra of neutral F4TCNQ thin films and of F4TCNQ/polymer blends. In neutral F4TCNQ films, a strong band is centered at 2227 cm-1 (labeled A), and a weak band appears at 2214 cm-1 (labeled B). It is known that F4TCNQ crystallizes in an orthorhombic structure when deposited from solution;22 following the assignment of Meneghetti et al.,17 these bands can be assigned to vibrational modes derived from molecular modes with symmetry labels b1u and b2u, respectively. Our density functional theory (DFT) calculations of the vibrational frequencies (at the B3LYP/6-31G(d) level) show that the b1u- and b2u-derived modes (the only ones with significant IR intensity in this energy region) are separated by ∼18 cm-1 in the F4TCNQ crystal (calculated at 2252-2255 and 2235 cm-1; see Supporting Information). We note that an isolated F4TCNQ molecule in the gas phase has b1u and b2u transitions with reversed intensities (the calculated intensity of A for isolated F4TCNQ is ∼10 times lower than that of B). The asymmetry in the shapes of the A and B bands with respect to their centers suggests a superposition of features due to crystallized and nonaggregated molecular species. In the blend layers, the bands are considerably red-shifted, indicative of strong charge transfer. According to our model calculations on F4TCNQ/quaterthiophene (4T) complexes (at the B3LYP-D/6-31þG(d) level, which includes dispersion
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DOI: 10.1021/jz100492c |J. Phys. Chem. Lett. 2010, 1, 2037–2041
pubs.acs.org/JPCL
Figure 2. Sketch of charge-transfer complex formation of F4TCNQ (green) with the oligothiophene building blocks (red) of P3HT and 3HT-TFB. In the latter, the thiophene segments are interrupted by TFB units (black).
corrections), features C and E can be assigned to the b1u and b2u modes, respectively (red-shifted features with respect to A and B of neutral F4TCNQ). In addition, feature E contains a contribution from a formerly IR-forbidden mode with symmetry b3g. The new intense feature D is related to the in-plane ag mode of symmetric CtN stretching, which is also IRinactive in neutral F4TCNQ. The appearance of the formerly IR-inactive modes in the charge-transfer complexes points toward a change of the F4TCNQ geometry to a nonplanar conformation, as suggested previously9 (see also Supporting Information, Figure S3). Note that all charge-transfer complexes, including doped P3HTand 3HT-TFB, exhibit IR bands at exactly the same positions. (The slight shift (
