4
~ ,', ,4, ELSEVIER Synthetic Metals 67 (1994) 147-150 Sq QTIHI[TIIC D|TRLS A density-functional theory study of the aluminum/polythiophene interface V. Parent&'*, R. Lazzaroni a'l, A. Selmani b, J.L. Brddas a ~Service de Chimie des Matiriaux Nouveaux, Centre de Recherche en Electronique et Photonique Mol6culaires, Universit6 de Mons-Hainaut, place du Pare 20, 7000 Mons, Belgium UD6partement de G~nie Chirnique, Ecole Polytechnique de Montreal, Montreal, Que., H3C 3A7, Canada Abstract This work deals with the quantum-mechanical modeling of the interactions occurring at the molecular level during the formation of the interface between aluminum and polythiophene. Density-functional theory calculations are performed on a molecular model system consisting of a thiophene molecule surrounded by one or two aluminum atoms. The geometric structure of the organometallic complex is fully optimized and the possibilities of aluminum bonding to various sites of the molecule are evaluated. The evolution of the charge density distribution upon metal bonding is followed using a Mulliken population analysis. Keywords: Interface; Polythiophene; Aluminum; Density-functional theory 1. Introduction Conjugated polymers constitute a promising new class of organic semiconductors. Their potential has been clearly demonstrated by the fabrication of semicon- ducting devices (field-effect transistors, light-emitting diodes (LEDs)) in which polyphenylenevinylene or polythiophene constitute the active component [1,2]. A major aspect of these devices is the presence of interfaces between the polymer thin film and the metal contacts. Since these interfaces play a prominent role in the operation of the devices, it is of prime importance to determine their chemical nature in order to un- derstand how the interactions between the metal and the polymer can affect the polymer electronic structure. In this context, we have investigated the chemical and electronic structure of the interface between alu- minum and potythiophene. Recent studies of the alu- minum/polythiophene interface, combining experimen- tal data and theoretical calculations, indicate that the aluminum atoms interact strongly with polythiophene by forming new A1-C covalent bonds; this dramatically affects the ~r electronic structure [3,4]. In this work, we apply a theoretical approach based on the density- functional theory (DFT) to the study of the aluminum/ *Corresponding author• ~Chercheur qualifid du Fonds National de la Recherche Scientifique. polythiophene interface. With respect to the Har- tree-Fock-based calculations performed previously [3], the DFT formalism used here takes electron correlation into account, which should provide a better description of interacting species. This method is therefore of particular interest for studies of metal/polymer inter- faces. Since we are interested in the local interactions between aluminum atoms and polythiophene chains, we have first considered small molecular model systems which are best suited for sophisticated quantum-chem- ical treatments. In this paper, we present the results of DFT calculations on molecular complexes consisting of one thiophene molecule interacting with one or two aluminum atoms, as basic model systems for the chemical interactions occurring when aluminum is deposited onto the polythiophene surface. 2. Methodology The calculations presented in this work have been performed using the deMon [5] and DMol [6] programs; both are based on DFT. Full geometry optimizations have been carried out on different starting conformations in order to find the different local energy minima of the systems. The Vosko-Wilk-Nusair (VWN) analytic expression [7] has been chosen for the local exchange- 0379-6779/94/$07.00 ~3 1994 Elsevier Science S.A. All rights reserved SSDI 0379-6779(94)02227-P

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Page 1: A density-functional theory study of the aluminum/polythiophene interface

~ , ' , , 4 ,

E L S E V I E R Synthetic Metals 67 (1994) 147-150

Sq QTIHI[TIIC D|TRLS

A density-functional theory study of the aluminum/polythiophene interface

V. Parent&'*, R. Lazzaroni a'l, A. Selmani b, J.L. B r d d a s a

~Service de Chimie des Matiriaux Nouveaux, Centre de Recherche en Electronique et Photonique Mol6culaires, Universit6 de Mons-Hainaut, place du Pare 20, 7000 Mons, Belgium

UD6partement de G~nie Chirnique, Ecole Polytechnique de Montreal, Montreal, Que., H3C 3A7, Canada

Abstract

This work deals with the quantum-mechanical modeling of the interactions occurring at the molecular level during the formation of the interface between aluminum and polythiophene. Density-functional theory calculations are performed on a molecular model system consisting of a thiophene molecule surrounded by one or two aluminum atoms. The geometric structure of the organometallic complex is fully optimized and the possibilities of aluminum bonding to various sites of the molecule are evaluated. The evolution of the charge density distribution upon metal bonding is followed using a Mulliken population analysis.

Keywords: Interface; Polythiophene; Aluminum; Density-functional theory

1. Introduction

Conjugated polymers constitute a promising new class of organic semiconductors. Their potential has been clearly demonstrated by the fabrication of semicon- ducting devices (field-effect transistors, light-emitting diodes (LEDs)) in which polyphenylenevinylene or polythiophene constitute the active component [1,2]. A major aspect of these devices is the presence of interfaces between the polymer thin film and the metal contacts. Since these interfaces play a prominent role in the operation of the devices, it is of prime importance to determine their chemical nature in order to un- derstand how the interactions between the metal and the polymer can affect the polymer electronic structure.

In this context, we have investigated the chemical and electronic structure of the interface between alu- minum and potythiophene. Recent studies of the alu- minum/polythiophene interface, combining experimen- tal data and theoretical calculations, indicate that the aluminum atoms interact strongly with polythiophene by forming new A1-C covalent bonds; this dramatically affects the ~r electronic structure [3,4]. In this work, we apply a theoretical approach based on the density- functional theory (DFT) to the study of the aluminum/

*Corresponding author• ~Chercheur qualifid du Fonds National de la Recherche Scientifique.

polythiophene interface. With respect to the Har- tree-Fock-based calculations performed previously [3], the DFT formalism used here takes electron correlation into account, which should provide a better description of interacting species. This method is therefore of particular interest for studies of metal/polymer inter- faces.

Since we are interested in the local interactions between aluminum atoms and polythiophene chains, we have first considered small molecular model systems which are best suited for sophisticated quantum-chem- ical treatments. In this paper, we present the results of D F T calculations on molecular complexes consisting of one thiophene molecule interacting with one or two aluminum atoms, as basic model systems for the chemical interactions occurring when aluminum is deposited onto the polythiophene surface.

2. Methodology

The calculations presented in this work have been performed using the deMon [5] and DMol [6] programs; both are based on DFT. Full geometry optimizations have been carried out on different starting conformations in order to find the different local energy minima of the systems. The Vosko-Wilk-Nusair (VWN) analytic expression [7] has been chosen for the local exchange-

0379-6779/94/$07.00 ~3 1994 Elsevier Science S.A. All rights reserved SSDI 0 3 7 9 - 6 7 7 9 ( 9 4 ) 0 2 2 2 7 - P

Page 2: A density-functional theory study of the aluminum/polythiophene interface

148 V. Parentd et al. / Synthetic Metals 67 (1994) 147-150

correlation potential. Total energies have been cal- culated on each optimized geometry at both local (LSD) and nonlocal spin density levels (NLSD; only with DMol) to determine the most stable configurations. The LSD potential is described using the VWN potential while the NLSD calculations use the Lee-Yang-Parr (LYP) gradient-corrected expression [8] for the correlation term and the Becke (B88) formalism [9] for the nonlocal exchange energy. In the deMon program, a Gaussian basis set of split-valence quality, optimized for density functional purposes and including d-type polarization functions on the metallic atom, has been chosen with a model core potential on the sulfur atom. In the DMol program, calculations have been performed numerically, with the medium-sized grid defined in the program; a numerical atomic basis set of split-valence quality in- cluding polarization functions has been considered for all the atoms. The charge distribution for each con- figuration has been obtained via Mulliken population analysis.

3. Results and discussion

3.1. Thiophene interacting with one aluminum atom

The optimization of the system consisting of one aluminum atom interacting with the thiophene molecule leads to two stable configurations, Fig. 1 (both with deMon and DMol). Starting from conformations where aluminum is placed either above the sulfur atom, above the carbon in the/3 position (C~) or above the middle of the C,,-Ct3 bond, the geometry optimization leads to a situation where the aluminum atom lies approx- imately above the center of the thiophene ring, forming covalent bonds with the two Ca atoms (system 1A). If the aluminum atom is initially located above one the C,, atoms, it simply gets closer to that carbon atom and forms a covalent bond (system 1B).

The energy of eomplexation is calculated as the difference between the complex total energy and the sum of the energies of the separate partners. The gain in energy upon formation of complex 1A is estimated to be 29.5 kcal/mol with deMon and 32.4 kcal/mol with DMol, using the LSD approach. However, it is well established that LSD overestimates the binding energy [10]; it then comes as no surprise that the energy of complexation decreases to 10.7 kcal/mol when the NLSD potentials are incorporated in the Hamiltonian (note that these small binding energies are consistent with the long A1-C bonds that are calculated; in comparison, the length of the A1-C bond calculated at the Har- tree-Fock/CI level in methylaluminum is 1.99/~ [11]). In the LSD results, system IA is more stable than system 1B by about 4.4 kcal/mol with deMon and 5.8 kcal/mol with DMol. The energy difference between

A1-C~: deMon: 2.357 ] - - ' , ,AI-C~: deMon: 2.356 DMoh 2.320 r' ~ DMoI: 2.317

t

p

w

/ \ f -

/

s

1A

,, ) i f A]-C.: demon: 2.153 ! DMoh 2.157

\

/

\\ /J

1B Fig. 1. Optimized geometries of the aluminum/thiophene complex. The AI-C~ bond lengths (in ~ ) are reported for both deMon and DMol calculations.

these two conformations decreases to 1.4 kcal/mol when LYP and B88 nonlocal corrections are included in the calculations, with system 1A remaining the most stable situation.

In system 1A, the aluminum bonding forces the sulfur atom to move out of the thiophene plane (by about 20 ° ) in the opposite direction relative to aluminum. In system IB, sulfur remains in the same plane but the hydrogen atom linked to the C, connected to aluminum bends 35 ° out of the plane; this indicates that the Ca atom becomes sp3-hybridized upon aluminum bonding.

The interaction with the metal atom leads to a reversal in the single/double bond character of the C-C bonds in the thiophene molecule (Table 1). In conformation IA, the C,,-Ct3 bonds, which present a double-bond character in isolated thiophene, adopt a single-bond character while the Ct3-C ~ bond shortens to a value typical of a double bond. The evolution is not sym- metrical in conformation 1B due to the presence of only one aluminum atom above one C,, atom: only the C,~-Ct3 bond closer to aluminum evolves toward a single- bond character, the other one remaining unaffected. The Ct,-Ct3 bond adopts a length intermediate between single- and double-bond character.

The analysis of the electronic density distribution shows that only a small charge transfer takes place from aluminum towards the thiophene molecule. With

Page 3: A density-functional theory study of the aluminum/polythiophene interface

V. Parentd et al. / Synthetic Metals 67 (1994) 147-150

Table 1 Optimized bond lengths (in ~) for the thiophene molecule, isolated and interacting with aluminum

149

Thiophene Structure 1A Structure 1B Structure 2A Structure 2B

DMol deMon DMol deMon DMol deMon DMol deMon DMol

C~-S 1.713 1.721 1.766 1.759 1.785" 1.776" 1.836 1.829 1.809 C,-S 1.713 1.722 1.766 1.759 1.752 1.746 1.833 1.824 1.809 C~-Ct~ 1.370 1.373 1.421 1.426 1.433" 1.444 ~ 1.443 1.452 1.451 C~-C~ 1.369 1.373 1.420 1.428 1.375 1.378 1.443 1.451 1.450 Ct3-C ~ 1.413 1.418 1.387 1.399 1.400 1.410 1.364 1.369 1.390

"The bond closest to the metal atom.

deMon, this transfer is found to be 0.09[e] for system 1A and 0.14]e I for system 1B; in the latter, the atomic charge of the Ca atom linked to aluminum increases by -0.141e I. Other changes in electronic distribution due to the complexation remain small, the most im- portant modifications being for both C, (gaining -0.07]el) and for sulfur (losing -0.06[el) in complex 1A. With DMol, the charge lost by the aluminum atom is even smaller: -0.031e[ in system 1A and -0.071e [ in system lB. However, the electronic density on the C~ atom increase by -0.161e I and -0.261e I in complexes 1A and IB, respectively, while the change in the elec- tronic population of sulfur is almost zero. The increase in electronic density on the C,, atom is consistent with core-level photoelectron spectroscopy (XPS) data [4], which show the appearance of a new carbon species, with increased electronic population, upon interaction with aluminum. The use of LYP and B88 nonlocal corrections also affects the estimate of Mulliken pop- ulations. When using the NLSD potentials, the charge transfer from aluminum increases to 0.07[e[ and 0.1lie I in conformations 1A and IB, respectively. The electronic gain on C, upon complexation remains the same (-0.171e I and -0.271el), but a slight increase of the electronic density on the sulfur atom is now observed in complex lB. We note that the Mulliken charges are highly basis set dependent; in particular, if very diffuse basis set is used, the values turn out to be very small, which may explain the small charge transfers observed here.

3.2. Thiophene interacting with two aluminum atoms

Following previous theoretical studies [3], the first conformation we considered consists of one aluminum atom located above each Ca atom. This also appears to be a stable geometry at the DFT level; both aluminum atoms form covalent bond with one C, atom and the sulfur atom; this structure (2A) is presented in Fig. 2. As in the case of one aluminum interacting with the thiophene molecule, the complexation leads to sp 3- hybridized C~ atoms, with bending of hydrogen atoms out of plane (by about 50°). The distance calculated

AI-S: deMon: 2.640 AI-S: deMon: 2.65l DMoI: 2.622 DMoI: 2.625

( AI )

AI-Co: deMon: 2.082, • dMol: 2.(" '

(

2A

' AI )

p l ! f

, ,AI-C : deMon: 2.090 " ' ~ DMoI: 2.091

)

AI

A,C 2224 ,?',,i,, A,c 2224 AI-Cu: 2.108 / , ~ ', AI-C,~: 2.107

I S 2B

Fig. 2. Optimized geometries of the thiophene molecule interacting with two aluminum atoms. The covalent bond lengths (in ~ ) are reported for demon and DMol calculations in configuration 2A and for the DMol calculation in configuration 2B.

here between the two aluminum atoms (2.89 A with deMon and 2.87 ~ with DMol) is much shorter than the one obtained at the Hartree-Fock level for a similar configuration (3.91 ~) [3], a feature which might be due to the inclusion of electron correlation effects (note that the optimal bond length in an aluminum dimer is somewhat shorter: 2.65 and 2.47/~. with deMon and DMol, respectively). The reversal in the single/double- bond character in the thiophene ring is also observed (Table 1). At the electronic level, the behavior is similar to the previous system, with a charge transfer from

Page 4: A density-functional theory study of the aluminum/polythiophene interface

150 V. Parent~ et al. / Synthetic Metals 67 (1994) 147-150

each aluminum of 0.131e[ and 0.08le [ with deMon and DMol, respectively, leading to an increase in the elec- tronic densities of both Ca atoms (about -0.091e I with deMon and -0.34[e I with DMol). This feature is thus once again in good agreement with experimental data [4].

In order to investigate the influence of a second metallic layer, a conformation with two aluminum atoms located above the center of the thiophene ring, the A1-A1 axis being perpendicular to the thiophene plane, has been investigated. The optimization of this system (2B in Fig. 2) leads to a geometry showing again a reversal of the single- and double-bond characters in thiophene (Table 1). Note that the double-bond char- acter between the two C~ atoms is less marked here than in conformation 2A. Compared to conformation 1A (which is similar to this one without the second aluminum), the single character of both C~-C~ bonds is stronger in system 2B. Moreover, the second aluminum forces the first one closer to the organic molecule, leading to the formation of four covalent bonds (one with each carbon atom) and pushing the sulfur atom strongly out of the thiophene plane (about 30°). The A1-A1 bond length is 2.74 ~, i.e. shorter than in system A but still larger than in the aluminum dimer.

The presence of a second aluminum atom above the first one also significantly affects the LSD Mulliken population analysis. The aluminum closest to thiophene loses -0.12[e[ while each C~ gains -0.19[e] and the sulfur atom gains -0.04[@ Most important, when we perform a similar calculation without the second alu- minum, we observe that the electronic density on sulfur decreases by -0.05[@ Thus, it seems that a second metallic layer plays a significant role by affecting both the geometry and the electronic density of the polymer.

The aluminum bonding on the thiophene molecule leads, in system 2A, to a binding energy (versus isolated thiophene and aluminum dimer) of 51.5 kcal/mol with deMon and 48.7 kcal/mol with DMol. Using the LYP and B88 nonlocal potentials, the binding energy comes down to 25.9 kcal/mol. Comparing the two configurations in Fig. 2, the LSD calculations result in a larger stability for system 2A relative to system 2B, by 25.4 kcal/mol (DMol calculation); we are currently evaluating the relative stabilities at the NLSD level.

Other systems modeling the interaction between alu- minum and polythiophene are currently under study; the results will be reported elsewhere.

4. Conclusions

This DFT theoretical study on the aluminum/poly- thiophene interface confirms that the metal atoms tend to form covalent bonds with thiophene systems. Upon complexation, the alpha carbons linked covalently to aluminum become sp3-hybridized and we observe a reversal in single/double character of the C-C bond of the thiophene ring, in agreement with previous results [3]. This indicates that the 7r-conjugated system, re- sponsible for the semiconducting properties of the polymer, is strongly affected by the formation of the interface. Another important feature arising from this study is the influence of the second metallic layer; it appears that the aluminum atoms located in the second layer can exert a significant role on both the geometry and electronic density distribution of the conjugated system.

Acknowledgements

The work in Mons on metal/polymer interfaces is partly supported by the European Commission SCI- ENCE Program (POLYSURF 0661 Project), the Belgian Prime Minister Office of Science Policy 'Programme d'Impulsion en Technologie de l'Information/IT.SC.22' and 'P61e d'Attraction Interuniversitaire en Chimie Supramol6culaire et Catalyse' and an IBM Academic Joint Study. V.P. is holder of an IRS1A Ph.D. Grant. The Mons-Montreal collaboration is supported by a Quebec/French Community of Belgium (CGRI) grant.

References

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