On the Interface Dipole at the Pentacene ... - ACS Publications

J. Phys. Chem. C 2010, 114, 3215–3224

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On the Interface Dipole at the Pentacene-Fullerene Heterojunction: A Theoretical Study Mathieu Linares,† David Beljonne, and Je´roˆme Cornil Laboratory for Chemistry of NoVel Materials, UniVersity of Mons, Place du Parc 20, B-7000 Mons, Belgium

Kelly Lancaster and Jean-Luc Bre´das School of Chemistry and Biochemistry, and Center for Organic Photonics and Electronics Georgia Institute of Technology, Atlanta, Georgia 30332-0400

Stijn Verlaak, Alexander Mityashin, and Paul Heremans IMEC, Kapeldreef 75, B-3001 LeuVen, Belgium Katholieke UniVersiteit LeuVen, Arenbergpark, B3001 LeuVen, Belgium

Andreas Fuchs and Christian Lennartz InnoVation Lab (iL) Heidelberg, Germany and BASF SE, D-67056 Ludwigshafen, Germany

Julien Ide´, Raphae¨l Me´reau, Philippe Aurel, Laurent Ducasse, and Fre´de´ric Castet* UniVersite´ de Bordeaux, Institut des Sciences Mole´culaires, UMR 5255 CNRS, 351 Cours de la Libe´ration, 33405 Talence, France ReceiVed: October 19, 2009; ReVised Manuscript ReceiVed: January 14, 2010

The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C60 molecules (and of the resulting local electrical fields sensed by the molecular units). 1. Introduction The emerging fields of organic electronics and spintronics rely on the use of organic conjugated molecules and polymers as active components in multilayered devices such as lightemitting diodes (OLEDs and, by extension, displays and lighting panels), solar cells, field-effect transistors (OFETs and, by extension, integrated circuits), (bio)chemical sensors, and storage devices.1 Because all organics-based devices are fabricated via deposition of successive layers (metal, oxide, insulating, or semiconducting layers), key electronic processes, such as charge injection from metallic electrodes, charge recombination into light or light conversion into charges, or spin injection, occur at interfaces.2 In the case of photovoltaic cells, the fraction of free charges produced upon light absorption depends on a subtle trade-off between the charge-separation and charge-recombination rates of the itinerant electron-hole pairs.3,4 These rates are governed by the nature of the electronic interactions at the donor-acceptor interface and the energetic profile in the vicinity of the interfacial dissociation zone. The formation of dipoles at metal/organic interfaces is well documented in many experimental and theoretical studies.5 In contrast, a comprehensive description of the electronic processes * To whom correspondence should be addressed. † Present address: Department of Theoretical Chemistry, Royal Institute of Technology, Roslagstullsbacken 15, S-106 91, Stockholm, Sweden.

occurring at organic/organic interfaces is currently missing. Strikingly, the choice for the donor (D) and acceptor (A) components in organic solar cells is generally based on the electronic properties of the isolated units, which neglects the impact of the interfacial electronic interactions on the energy level alignment. Ultraviolet photoelectron spectroscopy (UPS) measurements have provided clear evidence that such electronic interactions can translate into the appearance of a significant dipole at the interface between donor and acceptor materials.6-10 Thus, the interfacial dipole plays a major role in multilayered devices as it determines the actual offset between the frontier electronic levels of the adjacent layers. Because the origin of this interface dipole is still unclear, structure-property relationships for the design of materials with tailored interfacial properties remain to be established. Theoretical studies on donor/ acceptor complexes can thus prove very useful to shed light on the origin and magnitude of the interface dipole and to better understand the relationships between the electronic structure of molecules at D/A heterojunctions and the power conversion efficiencies of solar cells.4 By extending their work on metal/organic interfaces, Flores, Kahn, and co-workers introduced a model in which the formation of the interface dipole between two organic semiconductors is related to the alignment of the charge neutrality levels of both materials (these levels can be seen as the

10.1021/jp910005g  2010 American Chemical Society Published on Web 02/02/2010

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equivalent of chemical potentials).11,12 Though this model ignores the exact contact geometry at interfaces and the extreme sensitivity of electronic interactions to molecular packing, it was able to rationalize the vacuum level shift measured at the interface between several pairs of amorphous materials; however, this assignment was most probably due to some averaging effects. More recently, the factors contributing to the formation of an interface dipole at TTF/TCNQ interfaces were investigated by means of density functional theory (DFT) calculations carried out on simple dimers.13 The results show that the dipole is governed by a partial charge transfer between donor and acceptor units as well as by a mutual polarization of the electronic clouds of the two compounds. A small charge transfer has been also reported in the ground state of P3HT/C60 heterojunctions via DFT calculations.14 In the present work we present a detailed analysis of the interfacial dipole moment at the interface between pentacene and C60, which involves two prototypical molecules in the field of organic photovoltaics.15 This interface has also been the focus of several UPS studies.16,17 Our results indicate that the interface dipole is predominantly governed by the electronic polarization of the molecules and is strongly sensitive to the nature of the contact geometry; this suggests that UPS measurements can only probe an averaged dipole and hence provide an averaged value for the vacuum level shift. It is worth stressing that a specificity of the pentacene/C60 junction is the discontinuity of the quadrupolar field, which varies abruptly across the interface (because pentacene molecules possess a large quadrupole moment and spherical C60 molecules lack one). This discontinuity was recently shown to favor geminate pair dissociation at the interface for specific orientations of the pentacene π-systems relative to adjacent C60 molecules.18 We have developed a multiscale theoretical approach where molecular aggregates of increasing size and dimensionality are investigated by means of complementary theoretical methods. For small clusters, quantum-chemical methods are used to evaluate the impact of the molecular surrounding on the induced dipoles as well as the respective contributions of charge transfer vs electronic polarization to the dipole formation. Large clusters are investigated using a classical microelectrostatic (ME) model, in which molecules are coarse-grained into electrical multipoles. While a quantum-chemical description is more prone to provide a quantitative picture of the electronic structure at the interface without adjustable parameters, the parametrized ME approach is useful to study large-scale clusters, which is particularly important as electrostatic interactions are long-range effects. 2. Computational Methodology Molecular clusters of increasing complexity have been considered. First, the induced molecular dipole is investigated in pentacene/C60 complexes to assess the suitability of various levels of theory. In particular, the performance of microelectrostatic approaches is verified against corresponding quantumchemical calculations. In a second step, a system formed by a C60 probe molecule floating over the (01-1) pentacene surface is analyzed to gain insight into the magnitude and orientation of the induced dipole moment as a function of its position over the pentacene surface. These calculations are then extended to aggregates formed by a few molecules in a 2D arrangement and finally to the complete pentacene(01-1)/C60(001) interface obtained by bringing into contact two rigid half-crystals (see Supporting Information for details on the construction of the interface). The quantum-chemical investigations performed on smalland medium-size systems take advantage of the valence bond/

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Figure 1. Side and top views of the C60/pentacene cofacial dimer and Cartesian frame used in the calculations.

Hartree-Fock (VB/HF) model to characterize the electronic structure of molecules at interfaces. This semiempirical model is based on a fragment orbital formalism, which allows specific assignments of the electrons over the various molecular fragments. Therefore, it provides an efficient framework to evaluate the electronic properties of given molecular units embedded in a medium, including the polarization effects induced by intermolecular electrostatic interactions. The details of the VB/ HF formalism can be found in ref 19 and references therein. The classical microelectrostatic (ME) method used to analyze the large systems has also been described earlier.20,21 In this approach, molecules are described as an ensemble of polarizable points (inducible dipoles) and permanent quadrupoles distributed equally over all π-carbon atoms in every molecule. Several coarse-graining representations, differing by the number of polarizable points used to microelectrostatically represent the molecular units, have been tested against VB/HF calculations. Pentacene molecules have been represented either by 5 polarizable points (located at the center of each fused ring) or by 22 polarizable points (located on each carbon atom). Similarly, C60 molecules have been represented by 12 polarizable points (located at the center of each five-membered carbon ring) or by 60 points. The electric field due to quadrupoles is calculated at each polarizable point in the 3D cluster. All ‘atomic’ quadrupoles within the aggregate, as well as outside the aggregate within a radius of 10 nm with respect to its center (to reduce the edge effects), are taken into account to calculate the field (see ref 18 for more details). 3. Interface Dipole in Pentacene/C60 Complexes 3.1. Cofacial Pentacene/C60 Complex. 3.1.1. Origin of the Induced Dipole Moment. We first consider a cofacial pentacene/ C60 complex in which a six-membered ring of the fullerene lies above the central ring of pentacene at a distance of 3.7 Å within a Cs symmetry configuration (Figure 1). Although not representative of the real morphology at the interface, this simple system is expected to provide useful qualitative information on the magnitude and orientation of the induced molecular dipole moment, as well as on the relative contributions from chargetransfer and polarization interactions between the two compounds. Table 1 reports the z-component (i.e., along the stacking direction) of the total dipole moment calculated using semiempirical, ab initio, and DFT methods, as well as the total net charge carried by the fullerene molecule. By convention, the dipole vector is oriented from the negative to the positive pole. Two computational approaches providing atomic charge populations are used, namely the Mulliken and NPA (natural population analysis) schemes. As seen in Table 1, all theoretical approaches provide the same qualitative results: a significant dipole moment is found oriented from C60 toward pentacene, together with a negligible

Pentacene-Fullerene Heterojunction

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TABLE 1: z-Component of the Total Dipole Moment in the Model Dimer (µz, in Debye), as well as Total Net Charge on the C60 Molecule (QC60, in |e|) Calculated Using the Mulliken and NPA Schemesa

AM1 RHF/6-31G(d) RHF/TZVP B3LYP/6-31G(d) BH&HLYP/6-31G(d) B3LYP/TZVP BP86/TZVP MP2/TZVP VB/HF-AM1 VB/HF-AM1 (C60)b AM1 + point chargesc ME (5 + 12)d ME (5 + 60)d ME (22 + 12)d ME (22 + 60)d

µz

QC60 (Mulliken)

QC60 (NPA)

-0.525 -1.048 -1.180 -0.997 -1.008 -1.080 -1.181 -1.094 -0.499/-0.016 -0.481/0.000 -0.803 -1.589/-0.338 -1.509/-0.388 -1.186/-0.182 -1.167/-0.208

-0.0003 -0.0067 -0.0102 -0.0106 -0.0086 -0.0035 -0.0075 -0.0090 / / / / / / /

/ -0.0046 -0.0060 -0.0076 -0.0059 -0.0090 -0.0023 / / / / / / / /

a For VB/HF and ME calculations, the induced dipoles on both the C60 and pentacene molecules are provided [µz(C60)/µz(pentacene)]. b Only the C60 MOs are relaxed. c The pentacene molecule is replaced by a point charge distribution. d (n + m) refers to the partitioning scheme used in the coarse-grained calculations: the pentacene molecule is represented by n polarizable points and the C60 molecule by m polarizable points.

charge transfer between the two molecules (the net charge on C60 is slightly negative). This indicates that the major part of the interface dipole in this complex originates from polarization effects rather than from a partial charge transfer between the two fragments, in contrast to the results recently obtained for TTF/TCNQ dimers at similar intermolecular distances.13 Moreover, the similar values obtained at the RHF level indicate that the magnitude of the induced dipole does not depend much on the size of the basis set (6-31G(d) versus TZVP22) and hence that the dipole moments and charges already converge at a double-ζ level. DFT (using either the B3LYP, BH&HLYP, or BP86 functional) and RHF give accurate dipole moments and charges when compared to the corresponding MP2 values. AM1 is found to provide dipole moments in good qualitative agreement, although underestimated, with ab initio and DFT results. The induced dipole value obtained using the VB/HF-AM1 model, in which the charge neutrality on each fragment is strictly imposed, is very similar to that obtained with the ‘conventional’ AM1 scheme (without constraint on orbital localization). This confirms that polarization effects, instead of charge-transfer processes, are mostly responsible for the interface dipole at the pentacene/C60 heterojunction. Note that the same conclusion has been reached by performing similar calculations at the DFT level: when imposing the charge neutrality on each fragment within a constrained-B3LYP/6-31G approach,23 the resulting dipole changes by ∼15% compared to that obtained using standard (unconstrained) DFT calculations. It follows that the remaining 85% are induced by polarization effects. Alternatively, the polarization contribution to the total induced dipole can also be evaluated from the difference between the total dipole of the complex and the dipole arising exclusively from the charge-transfer contribution that can be obtained by summing the Coulomb interactions between the total net atomic charges on each molecule. Such estimates also demonstrate that the induced dipole is quasi exclusively driven by polarization effects (see Table S1 and Figure S1c, Supporting Information).

VB/HF calculations also indicate that the interface dipole primarily originates from the C60 polarization, and that the contribution of the pentacene polarization to the total interface dipole µz is negligible. The reorganization of the electronic density over the fullerene promotes a sizable intramolecular charge transfer, as supported by the net charge of +0.0188|e| on the C60 six-membered ring close to pentacene. That the interface dipole is governed by the deformation of the C60 electronic density is also evidenced by the very similar values obtained when the MOs of the two fragments are relaxed and when only those of the fullerene moiety are relaxed. The difference between these two approaches also demonstrates that the back-polarization effects induced by the electronic reorganization of the pentacene molecule are very weak. Table 1 also reports the value of the induced dipole moment on C60 calculated at the AM1 level when replacing the pentacene unit by corresponding point charges located at the atomic coordinates. The charge distribution is provided here by a Mulliken population analysis on the isolated pentacene molecule. Compared to the VB/HF calculations, the use of point charges instead of the actual electron density of pentacene leads to an overestimation of the dipole by about a factor of 1.5 due to the absence of back polarization and Pauli repulsion effects. Finally, the z-components of the total induced dipole calculated using various microelectrostatic representations are reported at the bottom of Table 1. Whatever the representation used, the induced dipole on the C60 molecule is 1 order of magnitude larger than on pentacene, in agreement with the VB/ HF results. However, the magnitude of the dipoles strongly depends on the partitioning scheme used in the coarse-grained calculations, with the closest match obtained for the finest graining (22 + 60). The induced dipoles are particularly sensitive to the microelectrostatic representation of the pentacene unit (5 vs 22 polarizable points) while they are less impacted by the representation of the C60 unit (12 vs 60 polarizable points). This might be related to the difference in the dimensionality of the ME representation of pentacene, which is onedimensional when using 5 polarizable points located at the center of each ring, and two-dimensional when using 22 polarizable points located on each carbon atom. 3.1.2. Energy LeWel Alignment at the Interface. The alignment of the frontier electronic levels of the donor and acceptor units in the pentacene/C60 complex is found to be affected by the interface dipole when compared to the energy diagram established for the isolated compounds. As illustrated in Figure 2, both the HOMO and LUMO levels of pentacene experience a shift to higher energy with respect to the MOs of the isolated molecule, the shift being of similar amplitude (-0.04 eV at the AM1 level) for the two orbitals. On the contrary, the energies of the frontier MOs of C60 are shifted to lower energy by 0.09 eV in the presence of the pentacene molecule. Similar results are obtained at the RHF/6-31G(d) and B3LYP/6-31G(d) levels (see Table 2); however, the shifts of the electronic levels are larger than those computed at the AM1 level due to the use of a more diffuse basis set. The HOMO level offset in the cofacial dimer (∆HOMO, defined as the energy difference between the HOMOs of the two fragments) is equal to 1.76 eV at the AM1 level while it amounts to 1.34 and 1.12 eV at the RHF and DFT levels. The corresponding values for the isolated systems are about 0.20 eV larger (Table 2). These values are consistent with the experimental value of 1.45 eV obtained from UPS measurements on layered C60/pentacene arrangements on PEDOT:PSS16 and polycrystalline Au17 substrates. Such a high ∆HOMO value ensures efficient exciton dissociation at the

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Figure 2. Energy diagram of the evolution of the frontier electronic levels going from the isolated pentacene and C60 molecules (left) to the pentacene/C60 cofacial complex (right), as calculated at the AM1 level. The numbers indicate the shift (in eV) in the energies of the MOs when going from the isolated molecules to the complex.

TABLE 2: Energies of the Frontier MOs (in eV) in the Isolated Molecules and in the Pentacene/C60 Cofacial Complexa AM1 HOMO pent LUMO pentacene HOMO C60 LUMO C60 ∆HOMO HOMO pent LUMO pentacene HOMO C60 LUMO C60 ∆HOMO

-7.650 -1.606 -9.533 -3.000 1.883

RHF/6-31G(d)

Isolated Molecules -5.927 0.785 -7.465 -0.306 1.538

B3LYP/6-31G(d) -4.608 -2.259 -5.902 -3.124 1.294

Pentacene/C60 Complex -7.687 (-0.037) -5.976 (-0.049) -4.660 (-0.052) -1.645 (-0.039) 0.713 (-0.072) -2.324 (-0.065) -9.444 (+0.089) -7.312 (+0.153) -5.776 (+0.126) -2.909 (+0.091) -0.164 (+0.142) -3.008 (+0.116) 1.757 1.336 1.116

a The values between parentheses indicate the shift in the electronic levels when going from the isolated molecules to the complex.

interface upon photoinduced hole transfer processes. Note that a fully quantitative comparison between the experimental and theoretical offsets is out of reach since solid-state effects are not taken into account in the calculations, which would require a precise knowledge of the interface morphology. Several recent studies have indeed demonstrated the crucial role of the supramolecular organization at the interface on the energy level offsets in organic/organic heterostructures.24 3.1.3. Influence of the RelatiWe Position of the Two Molecules. In order to further address the relative magnitude of the charge transfer versus polarization contributions to the interface dipole, the evolution of µz has been calculated as a function of the relative position of the two molecules. The induced dipole values as well as the amount of charge transfer between the two molecules have been first calculated at different quantum-chemical levels for intermolecular distances ranging from 2.5 Å to 4.5 Å along the z-axis. All results are gathered in Table S1 and Figures S1 and S2 in the Supporting Information. Whatever the level of theory, the results show that the previous conclusion (i.e., that polarization effects dominate

Linares et al. the dipole formation) holds true when reducing the intermolecular distance. When considering an intermolecular distance of 3 Å, the polarization contribution (evaluated as the difference between the total dipole and its charge-transfer contribution) represents 60% of the total induced dipole when using correlated approaches, with the amount of charge transferred between the two fragments remaining lower than 0.1 |e| (Table S1 and Figure S1c). A further decrease in the intermolecular distance down to 2.5 Å reduces the polarization contribution; in this geometry, the charge transfer processes dominate the dipole moment formation. On the other hand, increasing the intermolecular distance beyond 3.7 Å drastically reduces the charge transfer contribution to the dipole, so that polarization effects are only responsible for the dipole formation at large distances (see Figure S1c). We have then compared the evolution of the interface dipole µz as a function of the degree of translation between the two molecules along the y-axis, as obtained at the AM1, RHF/631G(d), and B3LYP/6-31G(d) levels (Figure 3). Although underestimated, the dipole moment calculated at the AM1 level follows the same trends as those predicted by the RHF and B3LYP calculations. In particular, a change in the direction of the dipole is observed for a translation ∆y of about 9 Å with the three approaches. In this geometric configuration, the amount of charge transfer between the two molecular fragments is essentially zero whatever the level of theory, which indicates that the change in the sign of the dipole is driven exclusively by polarization effects. The induced dipole moments change their orientation depending on whether the C60 center of mass is located on top of the pentacene molecular backbone, thus interacting mostly with the carbon atoms, or at the edge of the pentacene molecule with stronger interactions with the hydrogens atoms. This effect can be traced back to the uncompensated quadrupolar field at the pentacene/C60 interface. The pentacene quadrupole results from the high π-electronic density above and below the molecular plane combined with the compensating positive charges born by the nuclei. Alternatively, it can be viewed as the result of a collection of 14 CH units that are polarized with negative partial charges on the inner carbon atoms and positive charges on the outer hydrogen atoms. The variation of the z-component of the induced dipole moment when translating the C60 molecule in the (x, y) plane above the pentacene molecule, as calculated at the AM1 and VB/HF-AM1 levels (Figures S3a and S3b, Supporting Information), further highlights the dependence of the magnitude and orientation of µz on the relative positions of the two molecules. Moreover, the similarity between these two maps demonstrates that the charge transfer between the two fragments remains negligible for all investigated geometric arrangements (see also Figure S4, Supporting Information). 3.2. Tilted Pentacene/C60 Complex. We now consider a pentacene/C60 complex extracted from the pentacene(01-1)/ C60(001) interface. In this configuration, the pentacene molecule is tilted with respect to C60 in such a way that the pentacene molecular plane forms an angle of about 20° with respect to the (x,y) plane of the interface (Figure 4). We show in Figure 5a the variations in the z-component of the induced dipole moment when the C60 molecule is shifted in the (x, y) plane, as calculated using the VB/HF-AM1 method. The corresponding maps obtained from microelectrostatic coarse-grained calculations using the (5 + 60) and (12 + 60) partitioning schemes are displayed in Figures 5b and 5c, respectively. Interestingly, in contrast to the situation for the cofacial complex, the use of 5 polarizable points appears to be



Figure 3. Induced dipole moment (in Debye) in the pentacene/C60 complex and Mulliken net charge on the C60 molecule as a function of the degree of lateral translation along the y-axis.

Figure 4. Structure of the C60/pentacene tilted dimer, viewed along the y axis (left) and along the long axis of the pentacene molecule (right).

suitable to represent the pentacene molecule in the presence of a tilted structure and provides results in agreement with the VB/ HF results. This suggests that the choice of the best partitioning scheme is conditioned by the type of geometric arrangement under study. As discussed above, the orientation of the interface dipole depends on whether the C60 mainly interacts with the electronrich carbon backbone or with the electron-poor hydrogen atoms of pentacene. The VB/HF dipole values range from -0.5 to 0.2 D while the dipoles provided by ME (5 + 60) calculations span a broader range of values (from -1.2 to 0.4 D). According to the comparison made with ab initio and DFT calculations in Table 1, realistic values of the quadrupole-induced dipoles should lie between these two sets of values. 3.3. One-Dimensional Pentacene/C60 Stack. In order to assess the impact of the molecules in the inner layers on the induced dipole at the interface, we have considered a model 1D array made of 18 fullerenes and 18 pentacene molecules stacked along the z-axis (Figure 6a). As in the case for the cofacial complex investigated in Section 3.1, the intermolecular distance between the two fragments at the interface is set to 3.7 Å, while the distances between the closest atoms of two adjacent molecules in the inner layers are all equal to 3.5 Å. The z-components of the induced dipole moment on the pentacene and C60 molecules calculated at the VB/HF-AM1 level are compared in Figure 6b to the results obtained using the ME approach with the (5 + 60) and (12 + 60) partitioning schemes.

The results show a substantial evolution of the molecular induced dipoles with respect to the distance from the interface. Whatever the level of theory, the C60 and pentacene units acquire a negative induced dipole that goes asymptotically to zero as the distance from the interface increases. Since the molecular dipoles are induced by the unbalanced quadrupolar field of the pentacene molecules, they are not only generated in the monolayer next to the interface but as well to a certain depth into the bulk. The absence of a balancing field in the C60 part is felt up to a distance estimated at the VB/HF level to be ∼15 Å on the pentacene side and ∼70 Å on the C60 side. Therefore, the dipole at the pentacene/C60 interface should be visualized as a set of parallel sheets with diminishing dipole strengths when moving toward the bulk of pentacene or C60. The ME results provide another illustration of the impact of the partition scheme and confirm in the case of a cofacial stacking that the coarse-graining of the pentacene unit with only 5 polarizable points is less reliable than a more refined graining based on 22 points (which leads to a better agreement with the VB/HF results). In particular, both the VB/HF and ME (22 + 12) theoretical calculations predict that the magnitude of the dipole moment at a given distance from the interface is slightly larger on fullerenes than on the pentacene units, a feature not reproduced when using the simple (5 + 12) ME coarse-graining. Note also that the magnitude of the dipole on the pentacene units at the interface is ten times larger than the value found for the isolated pentacene/C60 complex (-0.165 D vs -0.016 D at the VB/HF level, -1.879 D vs -0.182 D at the ME (22 + 12) level); this indicates that local field effects induced by molecules in the bulk strongly impact the electric field felt by the molecules at the interface. Although the molecular stack considered here is not representative of the actual interface morphology, the results clearly demonstrate that inner-shell molecules have to be taken into account in the calculations in order to obtain a quantitative estimate of the magnitude of the interface dipole. The ME calculations lead to dipoles of similar magnitude on the C60 and pentacene molecules at the interface while the dipole on the pentacene

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Figure 5. Induced dipole moment (in Debye) on the C60 molecule as a function of its position in the (x, y) plane (see Figure 3), as calculated at the VB/HF-AM1 (a) and microelectrostatic levels using the (5 + 12) (b) and (22 + 12) (c) partitioning schemes.

Figure 6. (a) Structure of the model 1D pentacene/C60 array and molecular labeling. (b) z-components of the induced dipole (in Debye) on the individual molecules of the 1D stack as a function of the separation between their center of mass and the interface, as calculated at the VB/HF-AM1 and microelectrostatic levels.

provided by VB/HF calculations is 5 times smaller than that on C60. This discrepancy most likely originates in the fact that AM1 underestimates the perpendicular polarizability component with respect to the in-plane components in π-conjugated systems, though reproducing fairly well molecular quadrupoles.19 4. Interface Dipole at the Pentacene/C60 Interface 4.1. Interaction between a C60 Probe and a Pentacene Plane. We now turn to the case of a single C60 molecule interacting with a surface containing 49 pentacene units extracted from the (01-1) plane of the single crystal. Figure 7 illustrates the variation of the induced dipole moment on the C60 molecule as a function of its location on the pentacene plane, as calculated at the VB/HF-AM1 level. As previously observed for the pentacene/C60 complexes, the induced dipole moment changes sign when the fullerene is translated parallel to the long axis of the pentacene molecules. The amplitude of the dipole is weaker for an embedded complex compared to the corresponding isolated complex (with maximum absolute values reaching to only 0.15 D), due to the opposite electric fields originating from the individual pentacene

molecules. The dipole map obtained using the ME (5 + 12) approach is rather similar to that calculated at the VB/HF level (Figure S5, Supporting Information), demonstrating once again the suitability of the (5 + 12) coarse-grained representation in the presence of tilted pentacene molecules. The VBHF approach further allows us to assess the magnitude of ‘dynamic’ electronic polarization effects induced by the pentacene molecules on the local dipole sitting on the C60 molecule by considering the difference between the dipole values obtained with and without relaxation of the pentacene molecular orbitals. As shown in Figure S6, Supporting Information, such back-polarization effects can reduce by up to 50% the dipole induced on the fullerene moieties. 4.2. Interaction between Pentacene and C60 Single Layers. In this section, the interactions between pentacene and C60 molecular layers are investigated by building aggregates in which C60 units are progressively added on top of a pentacene plane containing 55 molecular units (Figure 8). The evolution of the z-component of the total interface dipole (µz), as well



Figure 7. C60 molecule above a plane of pentacene units (left) and amplitude of the z-component of the induced dipole on the C60 molecule as a function of its position on the (x, y) plane, as calculated using the VB/HF-AM1 model (right).

Figure 8. Structure of the molecular aggregates containing an increasing number N of C60 molecules interacting with a pentacene surface.

as of the averaged induced dipole per fullerene unit µz/N, as a function of the number of fullerenes (N) are reported in Figure 9. The nonmonotonic evolution of the total induced dipole moment µz is intimately linked to the way the C60 molecules are progressively deposited on the pentacene surface. As previously discussed, when a C60 molecule is added above the carbon backbone of a pentacene molecule, a positive partial charge appears on the section of C60 close to the interface, inducing a dipole moment toward the pentacene plane. On the contrary, the local contribution to µz points away from the interface when the additional C60 molecules are located above interstices between pentacene units, i.e., when the π-electrons of C60 are attracted by the positively charged hydrogen atoms. These two types of local induced dipoles compensate one another in aggregates of increasing size, which results in an averaged induced dipole per fullerene unit, µz/N, that saturates with N toward a small asymptotic value of ∼ -0.05 D. These calculations demonstrate that the measurement of the interfacial dipole averaged over the interface is not representative of the local dipoles induced by the quadrupoles of the pentacene molecules at the interface. The quadrupoleinduced dipoles (QID) are extremely sensitive to the topology of the interface and hence to the nature of the environment around each C60 molecule. This is illustrated in Figure 10, which shows the position-dependent QID on the C60 mol-

Figure 9. Evolution of the z-component of the total induced dipole moment (filled squares) and of the averaged induced dipole (open squares) in Debye as a function of the number N of C60 molecules at the interface, as calculated at the AM1 level. The value for N ) 0 corresponds to the residual dipole moment of the pentacene surface in the absence of C60.

ecule, as calculated at the VB/HF level for the largest cluster of Figure 8.

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Figure 10. Dispersion of the induced dipole moments on the C60 molecules at the pentacene/C60 heterojunction.

Figure 11. Quadrupole-induced dipoles at the pentacene(01-1)/C60(001) interface: individual induced dipoles (blue) and layer-averaged induced dipoles (red). All values are given in Debye.

5. Microelectrostatic Model for the Pentacene/C60 Interface Finally, the molecular dipoles generated at the pentacene (01-1)/C60(001) interface have been calculated with the ME model using the (5 + 12) coarsed-grained representation in a disk-like molecular cluster around the interface having a radius of 10 nm and a thickness of roughly 6 nm (3 nm of pentacene and 3 nm of C60). The quadrupolar field that is felt by those induced dipoles inside this disk comes from a shell of molecules (with a quadrupole associated to each π-atom) that is 5 nm larger in thickness and radius. The right part of Figure 11 illustrates the distribution of the dipole moments on the pentacene and C60 molecules in the vicinity of the junction between the two molecular materials while the red arrows on the left part

represent the dipoles averaged over each monolayer. The numerical values of the z-component of the monolayer-averaged dipoles are also reported in Figure 11. Figure 11 reveals that the dipole averaged in the pentacene monolayer next to the interface (µz ) 0.520 D) is larger and opposite in sign compared to the dipole averaged in the first C60 monolayer (µz ) -0.111 D). Each molecule in the first pentacene monolayer feels the electron-poor (hydrogen-rich) edge of the pentacene molecules in the second monolayer. This generates dipole moment vectors pointing toward the C60 side. On the other hand, molecules in the first C60 monolayer are not lattice-matched with the pentacene molecules and feel both electron-poor and electron-rich regions within the first pentacene monolayer; this results in a fluctuating orientation of the dipole



moment vectors, with the dominant contribution pointing toward the pentacene side (since the C60 molecules view more electronrich regions than electron-poor regions over the surface area). In addition, the separation between the first C60 monolayer and the first pentacene monolayer is larger than that between the first and second pentacene monolayers. This contributes to increase the amplitude of the dipole moment in the first pentacene monolayer compared to the first C60 monolayer. These results are qualitatively different from those reported in Figure 6b and thus highlight the impact of morphological differences. Such trends are not recovered when considering two monolayers in interaction. In particular, the QIDs on the pentacene molecules at the interface are underestimated when the quadrupolar field of bulk pentacene molecules is not taken into account, as discussed in Section 3.3. These results also point to the fact that the averaged dipoles do not provide a suitable description of the local electrical fields felt by the molecules, as discussed in Section 4.2. This is particularly visible in Figure 11 when comparing the averaged interface dipole over the first monolayer of C60 molecules (which points toward the pentacene side with µz ) -0.111 D) to the individual molecular induced dipoles that display alternations in their orientation at the interface (with µz ranging from -0.630 to 0.240 D). Another implication of these results is that UPS measurements reflect only the average value of the dipole at the interface and overlook the nanoscale variation of the electronic interactions; the same considerations apply to the vacuum level shift (VLS), which is directly connected to the µz component at the interface. The latter can be evaluated by summing the VLS contributions due to each pentacene and C60 monolayer separately:

(

VLS[eV] ≈ -

C60 sheets

i µz,C60 [D]

i

εC60SC60[Å2]

∑

+

pent sheets

i µz,pent [D]

i

εpentSpent[Å2]

∑

)

× 37.6

with Spent (SC60) the surface area occupied by one pentacene (C60) molecule within a monolayer, εpent (εC60) the dielectric i i (µz,C60 ) the averaged constant of the material, and µz,pent z-component of the molecular induced dipole in sheet i. The surface areas of the pentacene and C60 molecules have been estimated from crystal parameters25,26 to be 93 Å2 and 99 Å2, respectively. Moreover, since ME calculations implicitly include all screening effects, we have assumed εpent ) εC60 ) 1. By summing the contributions of seven monolayers on the pentacene side and of five monolayers on the C60 side (which corresponds to a distance of about 30 Å from the interface on both sides), the previous expression yields an estimate of VLS of ∼ +0.11 eV. Note that, due to the finite size of the aggregates, significant residual dipoles (∼0.02 D) are still present on monolayers further away from the interface, which might slightly affect the resulting VLS. Yet, the value calculated using this simple formalism is in very good agreement with the measured VLS for interfaces between pentacene and C60 (0.11 and 0.07 eV depending on the deposition sequence)17 and with the estimate of 0.1 eV obtained directly from microelectrostatic calculations.18 6. Conclusions We have reported complementary microelectrostatic and quantum-chemical calculations on pentacene/C60 aggregates of

different sizes and shapes in order to characterize the formation of a quadrupole-induced dipole moment at the interface between the two materials. We have shown that the interface dipole mostly originates in polarization effects rather than a partial charge transfer between the two compounds. Our work further demonstrates that a macroscopic description of the interfacial induced dipole moment (provided for instance by UPS measurements) does not capture the local electrical fields sensed by the molecular units at the interface since the orientation and magnitude of these local fields intimately depend on the atomistic details of the structural organization. This study complements our recent work on the modeling of the energetic barrier for dissociation of geminate pairs at the pentacene/C60 interface and possible band bending effects.18 In the long term, coupling the energetic profile at the interface to the calculation of rates for charge separation and recombination should allow us to better connect the microscopic description of electronic processes to the measured efficiencies of photovoltaic cells and hence provide guidelines for a more rational design of tailored interfaces for organics-based devices. Because the interface structures used here were obtained by optimizing the intermolecular distance between two rigid half crystals, additional work in progress aims at evaluating the impact of surface reconstruction on the energetic landscape at the interface. Acknowledgment. The authors acknowledge the European projects MINOTOR (FP7-NMP-228424), MODECOM (NMP3CT-2006-016434), and ONE-P (NMP3-LA-2008-212311) for financial support. The work in Mons is partly supported by the Interuniversity Attraction Pole IAP 6/27 of the Belgian Federal Governement, and the Belgian National Fund for Scientific Research (FNRS/FRFC). The work at Georgia Tech is partly supported by the STC program of the National Science Foundation under award DMR-0120967 and by the Center for Advanced Molecular Photovoltaics (Award No KUS-C1-01521 made by King Abdullah University of Science and Technology, KAUST). J.C. and D.B. are FNRS Research Fellows. J.I. is grateful to the “Advanced Material in Aquitaine” program (www.ama-materials.com) for his Ph.D. grant. J.C. thanks University Bordeaux I for a visiting professorship. Calculations were carried out on mainframe computers of the “M3PECMESOCENTRE” of University Bordeaux I financed by Conseil Re´gional d’Aquitaine and the French Ministry of Research and Technology, as well as on the Interuniversity Scientific Calculation Facility (ISCF) installed at Faculte´s Universitaires NotreDame de la Paix (Namur, Belgium), for which the authors gratefully acknowledge the financial support of FNRS-FRFC. Supporting Information Available: Details on the interface construction and complementary tables and figures. This information is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Chemical ReViews; American Chemical Society: Washington, D.C., 2007; Vol. 107, special issue. (b) Handbook of conducting polymers; Skotheim, T. A., Reynolds, J. R., Elesenbaumer, R. L., Eds.; Marcel Dekker: New York, 1997. (c) Organic PhotoVoltaics: Mechanisms, Materials and DeVices; Sun, S. S., Sariciftci, N. S., Eds.; Taylor & Francis Group LLC: New York, 2005. (d) Organic Electronics: An Industrial PerspectiVe; Klauk, H., Ed.; Wiley-VCH: Weinheim, 2006. (2) Conjugated Polymers and Molecular Interfaces: Science and Technology for Photonic and Optoelectronic Applications; Salaneck, W. R., Seki, K., Kahn, A., Pireaux, J.-J., Eds.; M. Dekker Inc.: New York, 2002. (3) Lenes, M.; Morana, M.; Brabec, C. J.; Blom, P. W. M. AdV. Funct. Mater. 2009, 19, 1106.

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On the Interface Dipole at the Pentacene ... - ACS Publications

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