Understanding the Charge Transfer at the Interface of Electron Donors

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Understanding the charge transfer at the interface of electron donors and acceptors: TTF-TCNQ as an example Changwon Park, Viktor Atalla, Sean C. Smith, and Mina Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b04148 • Publication Date (Web): 16 Jun 2017 Downloaded from http://pubs.acs.org on June 18, 2017

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ACS Applied Materials & Interfaces

Understanding the charge transfer at the interface of electron donors and acceptors: TTF-TCNQ as an example Changwon Park,1 Viktor Atalla,2,1 Sean Smith,3 and Mina Yoon1,* 1

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.

2

Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, 14195 Berlin, Germany

3

University of New South Wales, Sydney NSW 2052, Australia

*M. Yoon, [email protected]

Abstract Charge transfer between an electron donor and an electron acceptor is widely accepted as being independent of their relative configurations if the interaction between them is weak; however, the limit of this concept for an interacting system has not yet been well established. Our study of prototypical electron donor–acceptor molecules, Tetrathiafulvalene-Tetracyanoquinodimethane (TTF-TCNQ), using density functional theory based on an advanced functional, clearly demonstrates that for interacting molecules, their configurational arrangement is as important as their individual electronic properties in the asymptotic limit to determine the charge transfer direction. We for the first time demonstrate that by changing their relative orientation, one can reverse the charge transfer direction of the pair, causing the molecules to exchange roles as donor and acceptor. Our theory has important implications for understanding the interfacial chargetransfer mechanism of hybrid systems and related phenomena.

Keywords: density functional theory, hybrid functional, charge transfer, interface phenomena, TTF-TCNQ, electron donor–acceptor molecules

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Introduction Organic-based materials have physical and chemical properties useful for practical application: they are mechanically flexible, easy to modify through chemical interactions, flexible for processing, adaptable to various substrates, and environmentally friendly. Recent studies have demonstrated their application in devices such as optical sensors1, organic solar cells2, organic field-effect transistors3, and spin valves4. Among the interesting semiconducting organic materials, well-crystallized complexes of strong electron acceptors and donors have gained significant attention5,6 as prospective low-dimensional molecular conductors and charge-transfer compounds. The concept of a donor–acceptor complex is widely applied in the field of organic electronics. In particular, donor–acceptor interfaces offer a promising avenue for triggering decision phenomena in devices, such as exciton separation in organic photovoltaics, charge recombination in organic light-emitting devices and organic field-effect transistors, and work function engineering at the metal–organic junction. Metallic conductivity in the stacked interface of TTF and TCNQ, which individually have large energy gaps, suggests that the charge transfer into TCNQ remains stable7. Studies of TCNQ-based salts on the Au(111) surface have shown that supermolecular states along the 1D TCNQ chains are dependent on the geometry of neighboring molecules and on hybridization with the underlying metal substrate8. More recently, weak charge-transfer-induced magnetism has been reported for TCNQ adsorbed on graphene9. Although charge transfer is pivotal in such effects, as yet no detailed experimental or theoretical studies have investigated the dependence of charge-transfer phenomena on local configuration and the environment Understanding and designing donor–acceptor complexes may be regarded as a key challenge in the field. Electron donors and electron acceptors are defined in relation to the paired molecules; when interacting with other molecules, a typically electron-donating molecule can be a donor or an acceptor based on whether it has relatively lower ionization potential (IP) or higher electron affinity (EA), respectively. Typically, a donor–acceptor complex consists of an electron-donating


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molecule, which is characterized by its IP, and an electron-accepting molecule, which, in turn, is characterized by its EA. It is widely accepted that the charge transfer between conventional electron donor and acceptor molecules is independent of their relative configurations if the interaction between the molecules is weak; electrons are transferred from the lower IP molecule to the higher EA molecule. In this letter we show that the conventional picture of electron transfer is not valid for our example of a prototypical electron donor–acceptor molecular pair, the TTF-TCNQ dimer. Our study, using density functional theory (DFT) based on a hybrid functional, clearly demonstrates that for interacting molecules, the configuration of the molecules is as important as their individual electronic properties in the asymptotic limit to determine the charge transfer direction. DFT functional artifacts can lead to a qualitatively different and quantitatively incorrect conclusion. Using an improved functional, we for the first time demonstrate that by changing their relative orientation, one can reverse the charge-transfer direction for the same pair, with the conventional electron donor (TTF) becoming the electron acceptor and the acceptor (TCNQ) becoming the donor. The current understanding of donor–acceptor interaction and the charge-transfer mechanism has to be modified. While TTF and TCNQ as well as its derivatives are widely applied in experimental work, a theoretical description within DFT with standard approximations to the exchange-correlation (XC) functional is aggravated by the many electron self-interaction errors present in these functionals. In particular, widely used (semi)local functionals such as Perdew–Burke–Ernzerhof (PBE)10 and the local density approximation (LDA)11 as well as hybrid functionals such as PBE012 erroneously predict electron transfer from TCNQ to TTF, in seeming agreement with chemical intuition. Using an improved functional, we investigate the physical origin of electron transfer behavior in this prototypical electron donor–acceptor pair and reveal that the conventional understanding of its charge-transfer mechanism has been largely derived from standard XC functional artifacts.



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Methods Most calculations were performed with the plane-wave basis code Vienna Ab Initio Simulation Package (VASP)13. Ionic potentials were described by the projector augmented-wave method14, which accurately mimics the all-electron frozen core calculation. We considered PBE hybrid functionals (PBEh), with exchange energy Ex being given by a function of α, Ex(α) = αExHF + (1 − α)ExPBE, where ExHF and ExPBE denote Hartree-Fock and PBE exchange energy, respectively. The supercell was sized 30 Å × 30 Å × 30 Å to avoid intermolecular interactions between different supercells. A plane-wave cutoff of 400 eV was used for the electronic structures and geometric relaxations. The hybrid functionals were employed to investigate the electronic properties of PBE optimized molecular structures. All configurations were fully relaxed, unless specified, until the atomic forces were less than 0.02 eV/Å. G0W0 and fully self-consistent GW (scGW)15 calculations were performed with the FHI-aims code using numeric atom-centered orbital basis sets (NAOs)16,17. G0W0 and scGW results were obtained with the tier 3 and tier 1 hierarchy of the FHI-aims NOAs16, respectively.

Results and Discussion 1.Recovering the correct energy level hierarchy In approximate XC functionals the EA and IP of molecules can be accurately calculated by use of total energy differences (the ∆SCF approach) or the Slater–Janak transition state18. For example, PBE predicts an IP of TTF of 6.18 eV and an EA of TCNQ of 3.65 eV within the ∆SCF approach. Thus ∆SCF reproduces the experimential IP of TTF (6.7 eV) and the CCSD(T) EA of TCNQ (3.22 eV)19 within 0.5 eV. In particular, the IP of TTF is several electron-volts higher than the EA of TCNQ, in agreement with experiment and CCSD(T). However, this approach conceals the deficiency of PBE to favor partial electron transfer even if TTF and TCNQ are infinitely separated. The reason is that not only PBE but also standard hybrid functionals do not obey the piecewise linear total energy behavior of the exact XC functional20. This is also known as the many-electron self-interaction error21. An immediate consequence of this deficiency is that the highest occupied molecular orbital (HOMO) Kohn Sham (KS) level is too high in energy with respect to the negative of the IP obtained by the ∆SCF approach. Similarly, the lowest unoccupied molecular orbital (LUMO) KS level is too low in energy with 4 ACS Paragon Plus Environment

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respect to the negative of the ∆SCF EA. In the PBE case the KS HOMO of TTF (LUMO of TCNQ) is located at −1.97 (−7.01) eV and thus underestimates the corresponding ∆SCF values by several electron-volts. Most notably, PBE locates the LUMO of TCNQ below the HOMO of TTF by about 5 eV. As a result of the wrong KS level alignment and the Aufbau principle, TCNQ receives a certain amount of electrons from TTF even if the two molecules are in the interaction-free limit. Since the relative alignment of the KS frontier orbitals determines charge transfer for weakly interacting molecules, a qualitatively incorrect description of their locations could result in a completely erroneous description of electron transfer and interaction energies between different kinds of molecules, specifically molecules having large EA and IP differences. A typical misleading picture of standard DFT calculation is exemplified in the TTF-TCNQ cofacial dimer in Fig. 1(b), where all the calculations were performed by use of the PBE functional. Electronic properties of the dimer are also compared with an improved DFT functional shown in Fig. 1(c). The details of the functional will be explained later. The difference charge density of a dimer, defined as the changes in total charge density upon dimerization with respect to that of individual molecules, is depicted in the right panel of Fig. 1(b) and 1(c), where the red region indicates electron accumulation and the blue region indicates electron depletion with the isosurface of 1.0 × 10-4 e/Å3. The difference charge density plot calculated from PBE [Fig. 1(b)] clearly shows that overall, charges are transferred from the electron donor to the electron acceptor upon dimerization of the molecule, resulting in blue for TTF and red for TCNQ. The wrong level hierarchy for the key energy levels of TTF and TCNQ from the semilocal functional such as PBE accompanies the observed charge transfer from the electron donor to the electron acceptor. The HOMO level of the TTF molecule in gas phase is higher than the LUMO level of the noninteracting TCNQ molecule by 1.9 eV [see the data point in Fig. 1(a) at α = 0, corresponding to PBE]. In the weak interaction or weak orbital hybridization limit, this artificial charge transfer is alleviated by another artifact of the approximated exchangecorrelation functional, that is, the dependence of energy level on the amount of partial occupation. The HOMO level of TTF is being lowered as it loses its electron, whereas the LUMO level of TCNQ is being raised as it gains partial charge. The amount of charge transfer is limited by the cross point of these two frontier levels, and two frontier levels are pinned at the 5 ACS Paragon Plus Environment


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Fermi level (−5 eV from the vacuum level), which results in a high density of states at the Fermi level from the overlap between TTF-HOMO and TCNQ-LUMO, displayed in the projected density of states (PDOS) plot of Fig. 1(b). A scheme that solves this problem was proposed by22, which regarded the PBE hybrid functional (PBEh)12 as a function of the portion of exact exchange energy, α. In exact DFT the HOMO level of a finite system can be rigorously assigned to the IP23,24, making the self-energy correction to the HOMO strictly zero. Therefore, we chose α by minimizing the quasi-particle (QP) correction from the G0W0 approximation (single shot GW, Greens function G of the Coulomb interaction W) to the HOMO level. For TTF and TCNQ this requirement gives an optimum α = α* = 0.8, for which the quasi-particle correction vanishes22. This scheme, which mimics the self-consistent GW scheme, has been applied to small molecules and shows great enhancement over PBE for calculating IPs of the G2 data set and weak interaction energies of the S22 data set22. Figure 1(a) shows the energy level of HOMO (LUMO) for TTF (TNCQ), which is linearly decreased (increased) depending on the portion of exact exchange energy (α). To reproduce experimental molecular-level or higher-level benchmark theory from quantum chemistry calculations, about 80% of exact exchange energy should be considered. This is in good agreement with our previous results in obtaining α* by minimizing QP correction to the HOMO from GW. Thus, we adopted the PBE hybrid functional with α = α∗ = 0.8 for all calculations22 unless otherwise specified. We also compared the performance of PBEh(α∗) with GW and selfconsistent GW approaches for energy levels and total charge density. By introducing an 80% portion of exact exchange energy, the correct level of hierarchy was achieved between TTF-HOMO and TCNQ-LUMO [see Fig. 1(a)]. The difference charge density plot from PBEh(α∗) shown in Fig. 1(c) displays quite a different feature as compared to the results from the PBE functional [Fig.1(b)]: not a simple charge transfer from the electron donor to the electron acceptor, but, rather, charge redistribution upon molecular dimerization accompanied by strong hybridization at the interface region. In fact, the revised functional 6 ACS Paragon Plus Environment

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predicts significantly less charge transfer; specifically, the dipole moment of the cofacial dimer, for example, is about 4 times smaller than the PBE results. For the other dimer configuration, this difference can be as large as 25 times (see other configurations in Fig. 2). In blind DFT calculations with conventional XC functionals, a large amount of artificial charge transfer overwrites these effects and deceptively gives an incorrect result that seems to confirm the typical donor–acceptor picture. Thus, the seemingly physical phenomenon of charge transfer that is deduced from conventional local and semi-local functionals is purely an artifact of the DFT functional.

2. Orientation-dependent role of electron donor and acceptor Using PBEh(α∗), which solves the artificial charge-transfer problem of DFT, we investigated the intermolecular interactions between TTF and TCNQ as a molecular dimer of various geometries and studied the hybridization and charge redistribution effects on their charge transfer. Figure 2 shows the molecular dimers of three different configurations considered in this study: (a) a perpendicular geometry, in which the long and short axes of two molecules are perpendicular each other; (b) a cofacial geometry, in which two molecular planes face each other; and (c) a parallel geometry, in which the long and short axes of two molecules are parallel and two molecular planes share the same plane. Translation distance d measures the distance from the point at which the dimer has mirror symmetry, as indicated by the dotted lines in the right panel of Fig. 2. Optimum intermolecular distances of each configuration, defined as reference points (i.e., d = 0) and determined from PBE calculations, are 3.15 Å for perpendicular, 3.30 Å for cofacial, and 2.60 Å for parallel, depicted as vertical lines on the right panel of Fig. 2. The optimum distances are kept constant during the molecular translation (the directions of the translational trajectories are highlighted by the vertical dotted lines in the right panel of Fig. 2). The left panel of Fig. 2 displays the difference charge densities at the two points, A and B, indicated in the dipole moment plot presented in the middle panel. Rearrangement of the charges upon molecular interaction occurs mostly on the interface between the molecules. The amount of transferred charges using the Hirshfeld partitioning scheme25 was less than 0.05e for perpendicular and parallel geometries and 0.2e for cofacial geometry. However, such a small charge transfer has no significant physical implication, and more importantly, the estimation 7 ACS Paragon Plus Environment


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strongly depends on the charge partition scheme. Instead, dipole moment is a physically welldefined and meaningful quantity that is independent of charge analysis schemes. The dipole moments (D) of dimers are presented in the middle panel of Fig. 2. We found it interesting that the characteristics of dipole moments are strongly geometry dependent; thus the dipole moments change significantly and in nontrivial ways as the relative distances between the molecules change. For the perpendicular geometry, in which the nitrogen atoms at the edge of TCNQ are in close contact with TTF atoms, a prominent effect is from a charge overlap between the lone pair electrons of sulfur (yellow atoms) and nitrogen atoms (blue balls). Due to the exchange interaction between them, lone pair electrons are pushed back along each molecule [see the left panel of Fig. 2(a)], which, in turn, breaks the molecular symmetry of electron distribution and initiates local molecular polarization. Interestingly, the total dipole moment changes direction as the distance increases, leading to a rather surprising conclusion, that the prototypical electron donor and acceptor exchange their roles just by changing their relative configuration. This polarization reversal proves to be a complex interplay between local charge redistribution and induced molecular polarization. For the cofacial geometry, a dominant contribution to the total dipole comes from the π-stacking interaction between the two frontier orbitals and the molecular overlap. Such interaction results in partial charge transfer at a small d. As d increases, the interaction weakens; increasing the distance between the two charged molecules compensates for the dipole moment. As a result of the competing factors, total dipole moment reaches its maximum at d = 3 Å. A strong π-electron interaction results in a positive dipolar moment at all distances; that is, electron transfer from TTF to TCNQ proceeds in the conventional manner. For the parallel configuration, the direction of molecular polarization can be also reversed, but the origin of the dipole inversion differs from that in the perpendicular geometry. Positively charged hydrogen atoms in a neutral molecule tend to attract electrons from the facing sulfur or nitrogen atoms, which are originally negatively charged. By translating TTF along TCNQ, the facing atoms change nearest neighbors, causing the resulting total dipole to change in magnitude 8 ACS Paragon Plus Environment

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and direction. Polarization reversal in both perpendicular and parallel geometries is confirmed by independent self-consistent GW calculation (data points of solid squares) with a numeric atom centered basis set and is shown in the middle panels of Fig. 2(a) and (c). Our observation of strong orientation-dependent charge-transfer phenomena indicates that there exists a critical intermolecular distance (dC) below which interaction between molecules becomes as important as individual molecular properties, and that the overall charge-transfer phenomena are determined by their orientational arrangement. At distances shorter than dC, electron donors and acceptors defined in the interaction-free limit can revert, depending on the local chemical environment. Our results on the dimer system have further implications for extended systems. For example, at the solid interfaces of TTF and TCNQ molecular crystals, knowing the facing crystallographic directions may be critical for determining interfacial dipole moment. In contrast, for donor and acceptor molecules in a polar solvent where circumstantial dielectric screening helps to reduce or collapse the HOMO-LUMO gap, the typical donor– acceptor behavior is probably still valid. Figure 3 shows the energy level shifts of TTF-HOMO and TCNQ-LUMO of dimers as a function of their distance d from the high symmetry points of each configuration (denoted in Fig. 2). The level shifts are calculated by comparing the frontier energy eigenvalues of TTFTCNQ dimer with respect to those of individual molecules. Artificial charge transfer of the (semi)local functionals or hybrid functionals with an insufficient amount of exact exchange (α < 0.3) induces the pinning of frontier orbitals in each molecule. On the other hand, with improved functionals such as PBEh (α∗), which predicts the correct level of hierarchy, a typical secondorder energy level repulsion occurs between TTF-HOMO and TCNQ-LUMO levels; the HOMO level of TTF is downshifted and the LUMO level of TCNQ is upshifted in energy, with the HOMO-LUMO gap increasing in most cases. As the distance between two molecules increases, the gap increase lessens and eventually saturates to the ~3 eV of noninteracting systems. An asymmetry of level shift about zero implies the existence of a local polarization field and a change in orbital shape from self-consistently redistributed charge density.



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Deviation of the energy gap from isolated molecules is presented in Fig. 3. It is a good measure for estimating intermolecular interaction strength. We can roughly deduce the interaction strength by assuming two frontier levels with hybridization parameter t and HOMO-LUMO gap ∆ (~3 eV). This simplified model Hamiltonian H can be written as a 2×2 matrix of the form  ∆  − 2 H =   t 

 t  . ∆   2 

Due to the orbital hybridization, the HOMO-LUMO gap changes from ∆ to √ ∆2 + 4t2. From HOMO-LUMO gap changes of (a) 0.3, (b) 1.2, and (c) 0.5 eV at d = 0 (see Fig. 3), for perpendicular, cofacial, and planar configurations, t is estimated to be 0.7, 1.5, and 0.9 eV, respectively. If we neglect overlap of the basis, the resulting amount of charge transfer [(t/∆)2] can be estimated to be 0.05, 0.15, and 0.08 e (electrons), respectively. Especially in the cofacial geometry, there is meaningful partial charge transfer from TTF to TCNQ that is not affected much by the charge analysis scheme and is the dominant contributor to the total dipole. The picture of donor–acceptor molecules is based on the relative frontier molecular levels. But even in the case of a typical donor (acceptor) molecule such as TTF (TCNQ), the mechanism of charge transfer turns out to be not so trivial. The experimental HOMO-LUMO gap is so large (over 3 eV) that charge transfer is strongly suppressed owing to the large energy cost. The observed charge transfer in TTF-TCNQ systems from various experiments26 cannot be explained within this simplified molecular-level diagram. Moreover, the amount of transferred charges can be significantly small, as in the case of perpendicular and parallel geometries, which exhibit electronic couplings smaller than in cofacial geometry. Figure 4 shows intermolecular binding energy as a function of d. The binding energy is significantly overestimated by the PBE functional owing to the electrostatic interaction of the artificial charge transfer. The hybrid functional (α = 0.8) removes this artifact, thereby rigidly upshifting the binding energy of the two molecules from the PBE values and slightly increasing the dispersion. The increase in dispersion is most prominent for the cofacial geometry [Fig. 4(b)], and the binding energy becomes even larger in some other geometries (4–6Å). For the 10 ACS Paragon Plus Environment

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other cases [Fig. 4(a), 4(c)], electrostatic interactions are dominant and the hybrid calculations result in significantly weaker interactions than those from PBE. Increase in interaction energy as the proportion of exact exchange has also been reported27. This originates from the fact that the decaying behavior of the wave function depends on the flavor of the XC functionals. In a finite system such as a molecule, the PBE XC potential decays exponentially as distance from the nuclei increases. In contrast, Hartree–Fock exchange potential shows the correct 1/r behavior, indicating that the Hartree–Fock molecular orbital is somewhat more extended than the PBE molecular orbital. Typically, an extended molecular orbital increases the electronic coupling between two weakly overlapping electronic densities28. In practical applications29, TTF and TCNQ are interfaced in molecular film form and resulting bulk effects should be incorporated in our analysis of single molecule limit. From our band calculation on TTF (TNCQ) crystal, the broadening of HOMO (LUMO) level corresponds to 0.8 (0.6) eV. Also, depending on the surface crystallographic direction, the valence band maximum (conduction band minimum) also changes. Both can effectively change the energy gap between TTF HOMO and TCNQ LUMO but are not strong enough to invert them, and we expect the effect of energy level shift due to bulk formation to not be large. Rather, charge transfer tends to slightly decrease30 because of repulsive electrostatic interaction between neighboring charged molecule in the crystal.

Conclusion In summary, our study of prototypical electron donor–acceptor molecules clearly demonstrates that the conventional electronic picture is invalid for determining their charge transfer direction in the interaction regime; their configurational arrangement is as important as their individual electronic properties in the asymptotic limit. We for the first time demonstrate that by changing their relative orientation, one can reverse the charge-transfer direction for the same pair, with the conventional electron donor (TTF) becoming the electron acceptor and the acceptor (TCNQ) becoming the donor. Our theory has important implications for understanding the interfacial charge-transfer mechanism in hybrid systems and related phenomena.



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Acknowledgement We acknowledge M. Scheffler for comments on the manuscript. This work was supported by theme research at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory (ORNL) by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Also, this work used computing resources at the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. V.A. and M.Y. acknowledge support from a Laboratory Directed Research and Development award from ORNL.

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FIGURE 1 (Color online). (a) Location of TTF HOMO and TCNQ LUMO as a function of the amount of exact exchange (α), with the vacuum level set to zero. Projected density of states (PDOS) of the TTF-TCNQ cofacial dimer for (b) PBE (α = 0) and (c) α = 0.8. Gaussian broadening of 0.02 eV was applied to each level. Isosurfaces of charge redistributions (1.0 × 10-4 e/Å3 ) are drawn in red (charge accumulation) and blue (charge depletion).


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FIGURE 2 (Color online). Dipole moments (D) and difference charge densities of the TTF-TCNQ dimers on various geometries. TTF (top molecule) is translated by d along xdirection from (a) perpendicular, (b) cofacial, and (c) parallel geometry, and selected configurations (A and B) are indicated in the middle plots. Black double-headed arrows indicate the equilibrium distance of PBE calculation at d = 0 configuration. Blue arrows denote the relative magnitude and direction of dipoles. In the isosurface plots, red color indicates electron accumulation and blue indicates electron depletion; the isosurface values are (a) 1.0 × 10-4 e/Å3, (b) 5.0 × 10-4 e/Å3, and (c) 2.0 × 10-4 e/Å3.



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FIGURE 3. Energy level shifts induced by intermolecular interactions in (a) perpendicular, (b) cofacial, and (c) planar geometries. Solid lines are a LUMO level shift of TCNQ, and broken lines are a HOMO level shift of TTF. The definition of d is the same as in Fig. 2. The HOMOLUMO gap without interaction is 3 eV.


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FIGURE 4. Binding energy of TTF and TCNQ in (a) perpendicular, (b) cofacial, and (c) planar geometries. Solid lines with solid circles are results of the hybrid functional with α = 0.8, and dotted lines with empty circles are those of the PBE functional. The definition of d is the same as in Fig. 2, and positive (negative) energy denotes a repulsive (attractive) interaction.



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Table Of Contents


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Understanding the Charge Transfer at the Interface of Electron Donors

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