J. Phys. Chem. 1995,99, 16597-16601
16597
A Delta Self-Consistent-Field Study of Core Electron Binding Energies of Model Molecules for the AluminumLPolythiophene Interface M. Boman, H. Aigren, and S. Stafstr;dm* Department of Physics, Linkoping Universio, $581 83 Linkoping, Sweden Received: June 6,1995; In Final Form: August 28, 1995@
Interactions at the interface between aluminum and polythiophene are investigated by means of model molecules. The model systems are studied with ab initio Hartree-Fock calculations, where core electron binding energies are determined by the delta self-consistent-field method. The theoretical results are compared to reported experimental core-level X-ray photoelectron spectroscopy data of the interface. A model system in which two aluminum atoms are bound to the a-carbons of a thiophene ring is found to give chemical shifts in agreement with those observed upon interface formation.
I. Introduction The interface between an organic conjugated polymer and a metal has attracted an increasing interest during the past few years from both a fundamental and a technological point of view.’ This is partly due to the novel use of conjugated polymers as an electroactive material in microelectronicdevices such as light emitting diodes (LEDs), Schottky diodes, and field effect transistors (FETs).*-~Polymers with a thiophene backbone have been used for this purpose (see Figure 1). These components have almost the same characteristics as those made out of inorganic semiconductors, but with new properties such as mechanical flexibility and ease of pro~essing.~ Recently, LEDs with variable colors, covering the whole visible range, was fabricated out of polymers with thiophene in the main chain.5 The high anisotropy of oriented polymers gives the possibility of LEDs emitting polarized light.6 Most of these components have an interface between the polymer and a metal. The metal is often aluminum, working as an electron-injectingele~trode.~~’ This work presents a theoretical study of the interactions at the aluminudpolythiophene interface and contributes to the understanding of such an interface by means of a quantum chemical approach. Experimentally, the aluminudpolythiophene interface has been studied with surface sensitive photoelectron spectroscopy; both core-level X-ray photoelectron spectroscopy (XPS) and valence-level ultraviolet photoelectron spectroscopy ( U P S ) have been The evolution of the spectra was monitored as the polymer surface was gradually covered with aluminum. X P S , also known as electron spectroscopy for chemical analysis (ESCA), gives together with calculations of core electron binding energies the possibility to analyze the chemical structure at the interface since the core levels of an atom are sensitive to the chemical environment. We will therefore focus on the XPS data in the following (UPS spectra have been analysed elsewherelo). Measurements have been performed for two different polyhophene systems: poly(3-octylthiophene) (P30T), and a-sexithiophene oligomer (a-6T). The C(1s) and S(2p) core levels in pristine a-6T system are located at 284.9 and 164.2 eV, relative to the Fermi level, and the work function is determined to 4.6 eVS9 The S(2p) peak has a shoulder on the high binding energy side, since each S(2p) peak is a composite of two spin-orbit split contributions. The corresponding binding energies for P30T are almost the same. The changes @
Abstract published in Advance ACS Abstracts, October 15, 1995.
0022-365419512099-16597$09.0010
Figure 1. A schematic structure of polythiophene (above) and a-3T (below).
in the spectra induced by deposition of aluminum also agree for the two systems showing that the interaction takes place on the thiophene backbone and not on the alkyl side chains in the case of P30T. Upon aluminum deposition, a new component gradually grows on the low binding energy side of the S(2p) peak. This new component has 1.6 eV lower binding energy than the main line. The intensity of the C(1s) peak also increases at the low binding energy side. This new feature of the C(1s) peak is not separated from the main peak as in the case of S(2p) and it is hard to give definite values for the chemical shifts since it has merged with the main peak. The change in the C( 1s) peak is less pronounced for P30T compared to a-6T since the unaffected carbons in the side chains represent the major contribution of the main peak. Finally, as the amount of deposited aluminum increases a new component with higher binding energy than the main peak grows in the Al(2p) spectra. Theoretical investigations of the interactions at the interface have involved semiempirical, ab initio Hartree-Fock, and density functional methods.I0.” It was found in the HartreeFock-based study that the most stable interaction is an aluminum dimer bound to a single thiophene monomer.’O The aluminum atoms are bound to the a-carbon atoms and the interaction results in a transfer of electronic charge (of about half an electron) from each aluminum atom to the sulfur and a-carbon atoms of the thiophene ring. The changes of the atomic charges indicate that the chemical shifts of this interaction could be in agreement with those observed. However, such a simple consideration can lead to even qualitative wrong conclusions, since it does not take the full initial intramolecular potential into account and neglects all final state effects upon ionization. The A1 dimer bound to a single thiophene monomer was also found to be stable in the density-functional study.“ Another conformation was also proposed where an aluminum atom lies approximately above the center of the ring. It was also found 0 1995 American Chemical Society
Boman et al.
16598 J. Phys. Chem., Vol. 99, No. 45, 1995 that aluminum clusters can exert a significant role on the electron density distribution. To the best of our knowledge, the present study of the aluminudpolythiophene interface is the first ab initio study of core electron binding energies, quantitative predictions of chemical shifts, and direct comparison with experimental XPS data of the interface. The methodology used for these studies is described in section 11, the results are presented and discussed in section 111, and the conclusion summarized in section IV.
TABLE 1: Binding Energies of C(1s) and S(2p) in a-3P atomb
ASCF
Koopmans
exp‘
SI(2Pr) s 1 (2Pd sI (2PJ SZ(2PJ SZ(2PJ S2(2P:)
171.99 171.95 171.83 172.07 172.03 171.92 292.12 292.98 293.24 292.02 292.57 292.49
181.63 181.60 181.52 181.59 181.55 181.48 306.25 307.51 307.46 306.23 306.20 306.96
168.8
CI
11. Methodology
The interaction chemistry of the aluminudpolythiophene has been studied using model systems consisting a thiophene oligomer (up to a-trithiophene) interacting with a few aluminum atoms. In particular, this model is valid at the early stages of metal deposition before clustering effects become important. The geometry of each model systems has been optimized by minimization of the total energy with respect to all geometrical parameters. In a few cases, explicitly stated on appearance, geometrical parameters have been kept fixed in order to investigate their importance. The calculations have been performed at the ab initio Hartree-Fock level, using a split valence basis set of contracted Gaussian functions (6-31G). In some calculations the basis set has been extended with polarization functions. The core electron binding energy, Et,, is defined as the energy difference between the ground state of the neutral unionized system and the core hole state of the ionic system. In the delta self-consistent-field method (ASCF) both the initial and final state are determined as SCF wave functions.I2
c2
c3
c4 cs c6
289.5
a Calculated values refer to the ASCF and Koopmans energies. Atom labels are shown in Figure 1. Experimental values (ref 9) are relative to the vacuum level; i.e., a work function of 4.6 eV is included.
Concerning the spin part of the wave function the restricted Hartree-Fock approximation is used. In the case of a closedshell neutral system, Le., a singlet, the core hole state of the ionic system is a doublet. The core hole state and the outgoing photoelectron combine to form a singlet state, which is spin allowed. In the case of a single open-shell neutral system, Le., a doublet, both the singlet and triplet core hole states of the ionic system are spin allowed. This is the case for the studied systems that contain a single aluminum atom, and consequently chemical shifts based on both the singlet and triplet ionic state are calculated for those systems. 111. Results and Discussion
A. The Pristine Thiophene System. Core electron binding energies have been calculated for systems consisting of up to three thiophene rings. The ASCF results for a-3T (see Figure For ,)$[:IV the electrons respond to and are relaxed in the 1) is given in Table 1, together with Koopmans and experimental presence of the core hole. The relaxation in the final state is in values for comparison. The molecular field splitting of the 2p general quite large and must be included in order to obtain orbitals of a sulphur atom is less than 0.16 eV. This is too reasonable values for core level binding energies.13 The ASCF small to be resolved experimentally and we will hereafter only method takes this into account in contrast to calculations based refer to the mean values, 171.92 eV for SI and 172.01 eV for upon Koopman’s theorem where only the initial state contribuS 2 . The core binding energy of the carbon atoms varies by 1.2 tions are considered. eV. This leads in the XPS spectra to a broad C( 1s) peak where The initial state, YrGF, can be determined by straightforthe major contribution to the high binding energy side comes ward minimization of the energy expectation value. However, from the a-carbons, while the P-carbons mainly contribute to this cannot be done for the final state since the core hole state the low-energy side. The maximal separation of two consecuis an excited state and subject to a variational collapse; Le., all tive C 1s binding energies is 0.4 eV, but apparently this is also core orbitals become doubly occupied. We have employed a too small to be resolved in the experimental XPS spectra. The two-step procedure implemented in the SIRIUS SCFMCSCF difference between ASCF and Koopmans’ binding energies program package in order to determine the core hole ~ t a t e . ’ ~ . ’ ~ shows that the relaxation energy is about 10 eV for sulfur and In the first step the opened core orbital is kept frozen while the 14 eV for carbon. other orbitals are optimized such that a minimum is obtained The effect of using a limited size of the thiophene system is in this limited variational space. This step takes the wave investigated by studying the binding energies in systems function to a local region in the full variational space of the consisting of one (a-lT), two (a-2T), or three (a-3T) thiophene final core hole state. In the second step, the optimization is rings. The binding energies decrease as the size of system performed in the full variational space relaxing all orbitals, using increases. The 1s binding energy of the carbon atom at the the Newton-Raphson method. Since the Newton-Raphson end of the oligomers (e.g., Cg in Figure 1) decreases by 0.33 optimization always converges to the stationary point in the local eV in going from a-1T to a-2T, and by 0.09 eV in going from region, the final state Y&;) is found without a variational a-2T to a-3T. Similar changes are found for atoms in the middle of the oligomer. Apparently, there is little improvement collapse. The orbitals of the initial state, YrJF,are used as a in using larger model systems than a-3T. starting guess in the first step. However, if the initial state has delocalized core orbitals, a calculation with perturbed nuclei A comparison between the absolute values calculated by charges is carried out in order to ensure that the ionized core ASCF and those found experimentally shows that the calculated orbital is localized to single atom.13 The method has, when binding energies lie approximately 3 eV above the measured applied to organic systems, given an accuracy of 1 eV for values both for carbon and sulfur. A significant part of the binding energies, and a few tenths of an electronvolt for difference can be attributed to intermolecular relaxation present chemical shifts.I6 in the solid state samples but absent in our model systems. A
Interactions at the Alumi~umPolythiopheneInterface
Figure 2. Optimized structure of the AI2/(a-3T) complex. TABLE 2: Chemical Shifts of Ald(a-3T) Relative to a-3T chemical shifts (eV)b atoma
ASCF
Koopmans
SI
- 1.65 -0.48 0.07 -0.81 -0.5 1 -0.02 -0.26 -0.15
-0.66 -0.34 0.18 -0.69 -0.30 -0.16 -0.21 -0.24
s2
CI
CZ c3 c 4
C5 c 6
expC -1.6 small negative shifts
Atom labels are shown in Figure 1. Negative shifts correspond to lower binding energies when aluminum is present. Experimental values from refs 8 and 9. a
rough estimate by a simple continuum model" gives in our case an intermolecular relaxation of the order of 1-2 eV. The effect of the finite basis set has been tested by adding polarization functions to an ionization site. This led to an improvement of only 0.06 eV in the case of C2 in a-3T. Thus, for the absolute values of the core binding energies of the model system considered here, we do not claim a better accuracy for the ASCF method than 1-2 eV. However, more important in our analysis of the thiophene system before and after A1 deposition is the accuracy in the differences between binding energies, the chemical shifts. The errors in the chemical shifts are considered smaller due to cancellation, a few tenths of an electronvolt only. For instance, the difference between the C(1s) peak and S(2p) peak is 120.7 eV in the experimental spectra, while ASCF yields 120.6 eV calculated by taking the difference between the mean values of the of the binding energies for C(1s) and S(2p). B. The Ald(a-3T) Complex. In the early studies of the interaction between aluminum and polythiophene using Hartree-Fock theory, it was found that the most stable interaction corresponds to two aluminum atoms interacting with a single thiophene unit.Io The aluminum atoms form bonds to the a-carbons by sp3 hybridization of these carbons. The optimised AI-C, bond length is 2.1 A. This was later confirmed to be a stable interaction geometry also from density-functionalcalculations." To investigate this type of interaction we have performed ASCF calculations on a model system, the Al2/(a3T) complex shown in Figure 2 (see ref 10 for more details about the geometry and the nature of the bonds). The calculated chemical shifts of the complex relative to the pristine a-3T oligomer are listed in Table 2. These chemical shifts should be compared to the changes in the experimental XPS spectra upon aluminum deposition. The chemical shift of S1(2p), the sulfur atom in the thiophene ring to which the aluminum atoms are bound, is found to be - 1.65 eV in the ASCF calculation. This is in perfect agreement with experiments, where a shift of -1.6 eV is observed for the sulfur atoms that are affected by the aluminum A comparison with the Koopmans shift shows that final state
J. Phys. Chem., Vol. 99, No. 45, 1995 16599 effects give the major contribution to the Sl(2p) shift and it is therefore necessary to include these in order to get a quantitative correct description. The sulfur atom S2 in the neighboring ring shifts only -0.48 eV, this is too small to be distinguished clearly from the main S(2p) peak in the experimental spectra (see discussion in section IIIA). The calculated C(1s) shifts of A12/(a-3T) are between +0.07 and -0.81 eV. This will not give rise to a new peak or distinct shoulder in the spectra, since the shifts are small and more or less spread over this energy region. The atoms that correspond to the largest shifts, the a-carbons, are shifted from the high binding energy side of the C( 1s) peak toward the low binding energy side, which makes it hard to distinguish these from the main peak. Experimental investigations report a weak increase of the intensity at the low binding energy side of the C( 1s) peak, or even absence of changes in the C(1s) These observations are consistent with the calculated shifts. Despite the fact that only a small model molecule is used, where a few metal aoms interact with a short oligomer, the calculated chemical shifts and those observed in experiments are in full agreement. This can be understood as follows. Firstly, the chemical shift is a locally determined property; Le., it is primarily determined by the absolute neighborhood of the ionization site, and a model molecule is therefore sufficient. In section IIIA it was found that the binding energy had converged within -0.1 eV for an oligomer of three monomers. Due to cancellation effects the chemical shift can be expected to converge at least as rapid. Secondly, the experiments are performed such that aluminum is evaporated onto a the surface of a polymer film. Thus, at the early stages, interactions between the polymer and single metal atoms (or groups of atoms) are expected, rather than an interaction with a bulk metal. The geometry of the Ald(a-3T) complex is a fully relaxed ground state geometry, where the a-carbons bind to the aluminum atoms by sp3 hybridizations. As a consequence, the a-3T oligomer loses its planarity such that a delocalization of the n-electron system is prevented.I0 This could have important effects on device properties, e.g., transport across the interface. To investigate whether this occurs in the a real sample where interchain interactions are present, we performed a calculation on Ald(a-3T) where the thiophene oligomer was forced to be planar. The consequences for the chemical shifts are only a few tenths of an electronvolt for C(1s) but large for S(2p): the maximum shift for S(2p) reduces to -0.70 eV. This should be compared to the experimental value and the value obtained from the fully optimized structure, which both are equal to - 1.6 eV. The results indicate that a loss of the planarity of the polymer upon interaction with aluminum does occur. C. Alternative Interaction Structures. In this section a number of interaction models are studied in order the investigate if there are other structures that reproduce the experimental data (see Figure 3). Some of the structures were proposed from density-functionalcalculations.' For computationalreasons we restrict the thiophene system to a single ring. The chemical shifts of system A in Figure 3 relative to a pristine thiophene ring is shown in Table 3. System A is essentially obtained from the larger Ald(a-3T) complex by substitution of the outer thiophene rings with hydrogen atoms. The effect of using the smaller model systems on the chemical shifts is negligible for the sulfur atom and a few tenths of an electronvolt for the carbon atoms. The conformation of system A was also found to be stable in the density-functional calculations. The most striking difference between the optimized geometries in Hartree-Fock and density-functional calculations is the distance between the aluminum atoms: 3.91
Boman et al.
16600 J. Phys. Chem., Vol. 99, No. 45,1995
A
B
C
D
E
F
Figure 3. Optimized structures of aluminum atoms interacting with a single thiophene ring.
TABLE 3: ASCF Chemical Shifts of the Systems Shown in Figure 3 Relative to a-1T chemical shifts (eV). System atomb A B C' D' E F
~
~~
S
C
-1.61 -1.07
-1.62 -1.09
-0.95 -0.84
-0.13 -0.57
-1.37 -1.32
-1.85 -2.04
Negative shifts correspond to lower binding energies when aluminum is present. Only the largest shift among the carbon atoms. The average value of the singlet and the triplet ionic states.
8, (Hartree-Fock) and 2.87 8, (density functional). To estimate the importance of this, we have performed a separate optimization in which the distance between the aluminum atoms was fixed to 2.87 8,. The chemical shifts with this geometry are -1.38 eV for S(2p) and -0.94 eV for C(1s). Thus, the magnitude of the shifts are slightly reduced compared to those of the fully optimized system A shown in Table 3. In system B two aluminum atoms are bound to the a-carbon atoms as in system A but now in a trans formation, Le., on opposite sides of the thiophene ring. Systems A and B have almost the same total energy (they differ only by 0.3 kcal/mol), and their chemical shifts, shown in Table 3, are nearly identical. The two systems should therefore be equally good models for the interaction at the interface. In the density-functional study it was found that a conformation where an aluminum atom lies approximately above the center of the thiophene ring is stable'' (system D in Figure 3). This is not the case in the Hartree-Fock calculations. Starting with the aluminum atom in the central position above the thiophene, the geometry optimization leads instead to a conformation where the aluminum atom binds to one of the a-carbon atoms as shown in Figure 3 (system C). The chemical shift of the sulfur atom is substantially reduced when going
from system A or B to system C as seen in Table 3, Le., when going from two to one aluminum atoms. The smaller shifts for system C, -0.92 eV (-0.98 eV) for the singlet (triplet) ionic state, is in clear disagreement with experimental data of the interface. In order to investigate the chemical shifts of the conformation found in density-functional calculations, system D, the equilibrium geometry from such calculations" was imposed. The total energy of system D is in the Hartree-Fock calculation found to be 15 kcal/mol higher than that of system C. This is in contrast to the density-functional calculations where system D was found to be slightly more stable (1.4 kcaumol) than system C. The chemical shifts for system D are -0.50 eV (-0.64 eV) for the a-carbon 1s singlet (triplet) ionic state; -0.08 eV (-0.18 eV) for S(2p) singlet (triplet) ionic state. In particular, the very small shifts for S(2p) are in contradiction with the experimental shift of -1.6 eV and give no support for the conformation proposed from density-functional calculations. However, it should be noted that, although we have used the same geometry as in the density-functional study, the electronic structure can still be different in the Hartree-Fock and densityfunctional calculations. We have not been able to find a stable structure where an aluminum atom binds directly to the sulfur atom. Geometry optimization leads to a complete separation of those atoms when brought close to each other at the initial step. A similar tendency was also seen in density-functional calculations. In summary, we conclude that the fundamental interaction unit of the interface must contain at least two aluminum atoms. A single aluminum atoms does not give a consistent picture of experimental and theoretical results. In system E one of the aluminum atoms is attached to a /?-carbon atom. The agreement between the calculated and observed shifts is somewhat poorer for this system than for systems A and B. The total energy increases by 10.4 kcaVmol in going from system B to system E. In system F a second aluminum atom was bound to the one in system C, and the system settles such that the A12 moiety is located above the center of the C,-Cp bond. This has a dramatic effect on the carbon shift which becomes roughly twice as large (-2.04 eV) than for the other systems in Table 3. It is interesting to note that in one of the experiments the sample is heated such that diffusion of aluminum into the bulk of the thiophene sample occurs. As a consequence a new feature appears in the C(1s) spectrum, with a large shift, -2.5 eV, relative to the main peak.9 Apparently, this corresponds partly to a new type of interaction in the interface region. The result for system F indicates that very small groups of aluminum atoms can give rise to large C(1s) shifts. However, the total energy of the system F is much higher than those of systems A and B, 33.5 kcal/mol. In agreement with previous studies this shows that the system gains energy by letting the aluminum atoms interact directly with the thiophene system instead of forming The interaction picture can be different in the bulk and at the surface. Interchain interactions are not taken into account in our calculations which therefore resembles best the outermost polymer layer at the early stages of interface formation. The results of system F are also interesting since it can give the tendency of the chemical shifts as the amount of aluminum is increased toward a cluster, in fact, a similar system has previously been used to investigate the influence of a second atomic layer.' I However, investigations using an aluminum cluster interacting with polythiophene remain to be done and this motivates future work.
Interactions at the AluminumPolythiophene Interface
J. Phys. Chem., Vol. 99, No. 45, 1995 16601
IV. Conclusions
References and Notes
The ASCF method with a split valence basis set is a sufficient level of theory to calculate chemical shifts of the aluminum/ polythiophene interface with an accuracy of a few tenths of an electronvolt. The energy of final state relaxation upon core ionization is around 10 eV for the sulfur atoms and 14 eV for the carbon atoms and it is necessary to include it in order to get quantitatively correct predictions for the chemical shifts. The preferred interaction site of A1 on polythiophene is the a-carbon atoms. The fundamental interaction unit contains at least two aluminum atoms. The Ald(a-3T) complex in Figure 2 is found to give an S(2p) shift of - 1.6 eV in perfect agreement with experimental data. The C(1s) shifts of the complex are fully consistent with the observed weak increase at the low binding energy side of the C(1s) peak. The cis and trans attachments of the aluminum atoms are found to be equally good models. These results combined with earlier studies establish the AW(a-3T) complex as the prime model of the interaction at the early stages of the interface formation. It is also found that even very small groups of aluminum atoms can give rise to large C(1s) shifts.
(1) Polymer-Solid Interfaces; Pireaux, J. J., Bertrand, P., Brkdas, J. L., Eds.; IOP Publishing: Bristol, U.K., 1992. (2) Burroughes, J. H.; Jones, C. A.; Friend, R. H. Nature 1988, 335, 137. (3) Braun, D.; Gustafsson, G.; McBranch, D.; Heeger, A. J. J. Appl. Phys. 1992, 72, 564. (4) Gamier, F.; Horowitz, G.; Ping, X.; Fichou, D. Adu. Mater. 1990, 2 , 592. ( 5 ) Berggren, M.; Inganas, 0.;Gustafsson, G.; Rasmusson, J.; Andersson, M. R.; Hiertberz. T.: Wennerstrom. 0. Nature 1994. 372. 444. (6) Hagle; T. Wy; Pakbaz, K.; Voss, K. F.; Heeger, A. J. Phys. Rev. B 1991,44, 8652. (7) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Bums, P. L.; Holmes, A. B. Nature 1990,347, 539. (8) Lazzaroni, R.; Bredas, J. L.; Dannetun, P.; Logdlund, M.; Uvdal, K.; Salaneck, W. R. Synth. Met. 1991, 41-43, 3323. (9) Dannetun, P.;Boman, M.; Stafstrom, S.; Salaneck, W. R.; Lazzaroni, R.; Fredriksson, C.; Brtdas, J. L.; Zamboni, R.; Taliani, C. J. Chem. Phys. 1993, 99, 664. (10) Boman, M.; Stafstrom, S . ; Brtdas, J. L. J. Chem. Phys. 1992, 97, 9144. ( 1 1) ParantB, V.; Lazzaroni, R.; Selmani, A,; Brtdas, J. L. Synth. Met. 1994, 67, 147. (12) Bagus, P. S. Phys. Rev. 1965, 139, A619. (13) Bagus, P.; Coulbaugh, D.; Kowalczyk, S. P.; Pacchioni, G.; Parmigiani, F. J. Electron Spectrosc. Rela!. Phenom. 1990, 51, 69. (14) Jensen, H. J. Aa.; Jorgensen, P.; Agren, H. J. Chem. Phys. 1987, 87, 45 1. (15) Jensen, H. J. Aa.; &en, H. Chem. Phys. 1986, 104, 229. (16) Naves de Brito, A.; Correria, N.; Svensson, S.; Agren, H. J . Chem. Phvs. 1991. 95. 2965. (17) Himpsel, F.-J.; Schwenter, N.; Koch, E. E. Phys. Status Solidi ( b ) 1975, 71, 615.
Acknowledgment. The authors thank Dr. 0.Vahtras for help with the adaptation of the programs to the Cray Y-MPl464 computer used at the National Supercomputer Center (NSC), and Dr. Logdlund and Dr. Lazzaroni for discussions about XPS experiments and density-functional calculations. The work was partly supported by the Swedish Research Council for Engineering Sciences (TFR).
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