Interaction of bathocuproine with metals (Ca, Mg, Al, Ag, and Au) studied by density functional theory

Applied Surface Science 256 (2010) 2661–2667

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Review

Interaction of bathocuproine with metals (Ca, Mg, Al, Ag, and Au) studied by density functional theory Mahesh Datt Bhatt a,*, Shugo Suzuki b, Takeaki Sakurai a, Katsuhiro Akimoto a a b

Institute of Applied Physics, University of Tsukuba, Tsukuba-shi, Tennoudai 1-1-1, Tsukuba, Ibaraki 305-8577, Japan Institute of Material Science, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 October 2009 Received in revised form 2 November 2009 Accepted 12 November 2009 Available online 24 November 2009

Electronic structure of the interface between a bathocuproine (BCP) molecule and metal surface was studied by density functional theory. The bond energy of BCP with metal surface increased with decreasing work function of metals, that is, Au, Ag, Al, Mg, and Ca in this order. The charge transfer also increased with decreasing metal work function. It was analyzed that the Fermi level and the lowest unoccupied molecular orbital of BCP are composed of both BCP and metal orbital component, and electron transport properties across the BCP and metal is discussed based on these analyses. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Bathocuproine Density functional theory Interface states Organic solar cell

Contents 1. 2. 3. 4.

Introduction . . . . . . . . . . . . . . Computational methodology . Results and discussion . . . . . . Conclusions . . . . . . . . . . . . . . References . . . . . . . . . . . . . . .

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1. Introduction Organic semiconductors are attracting attention for application to solar cells from the view point of low cost and simple fabrication process for large area devices [1–5]. The efficiency of such devices is, however, relatively lower than inorganic devices. One of the important issues to govern the conversion efficiency of organic solar cells is charge transport process between organic layer and metal electrode [6]. It has been pointed out that the electronic structure at organic/metal interfaces play an important role in the process of charge transport at these interfaces [7]. The electronic structures at the interface between organic layer and metal electrode have been investigated theoretically and experimentally [8–19], however, the transport properties at the interface are not well understood yet. It is well known that the electron transport

* Corresponding author. Tel.: +81 9083497682; fax: +81 298536177. E-mail address: [email protected] (M.D. Bhatt). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.11.044

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properties at the interface between organic layer and metal electrode are remarkably improved by depositing bathocuproine (BCP) thin layer on metal electrode. Such improvement is, however, effective when relatively lower work function metal like Ca was used. Despite the technological importance of such transport properties, the mechanisms for the improvement in the transport properties related with work function are not well understood yet. In this paper, electronic structure of a BCP molecule on Ca (1 1 1), Mg (1 1 1), Al (1 1 1), Ag (1 1 1), and Au (1 1 1) surfaces which are composed of 13 atoms was studied by density functional theory (DFT). The simulation results were compared with those of experimental results and the electron transport properties are discussed based on the electronic structure at the interface. The chemical structure of BCP is shown in Fig. 1. The highest occupied molecular orbital (HOMO) of BCP is situated at 6.4 eV below the vacuum level and the energy gap between HOMO and the lowest unoccupied molecular orbital (LUMO) has been reported to be 4.2 eV [20]. Then the BCP is used as a hole (or

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2. Computational methodology

Fig. 1. Chemical structure of BCP.

exciton) blocking and electron transport material. Various metal substrates were chosen to understand the chemical trend of the interaction between BCP and metals. In principle, organic–organic molecule interaction induces small broadening of the electronic levels of each individual molecule, but does not create an electron density of states in the molecular energy gap. The interaction between BCP molecule and the topmost layer of the metal surface is supposed to be strong. Therefore, we can safely neglect the molecule–molecule interaction and consider only a single molecule on metal monolayer surface interaction to understand the interface properties qualitatively.

Fig. 2. Atomic geometry of metal (1 1 1) surface (a) and top view of BCP molecule on metal substrate in case of BCP on Ca (b).

In our model, Ca (1 1 1), Mg (1 1 1), Al (1 1 1), Ag (1 1 1), and Au (1 1 1) surfaces were constructed using 13 metal atoms having the same arrangement for all metals in x–y-axes plane as shown in Fig. 2(a). The atomic distances were kept constant with 0.394, 0.319, 0.286, 0.289, and 0.288 nm for Ca, Mg, Al, Ag, and Au, respectively, which are the same as those of the bulk values. The small metal clusters were chosen in our calculations due to the fact that their work functions were found to be close to their experimental values and our purpose of this research is to investigate the chemical trend in the interaction of BCP with various kinds of metals qualitatively. A single BCP molecule was basically kept in parallel with a metal (1 1 1) surface in a flat-lying manner because there are several works reporting energetically stable configuration as flat-lying geometry in polycyclic molecules on metal surface [19]. The atomic arrangement for the case of BCP on Ca in x–y-axes plane is shown in Fig. 2(b). In the first step of our calculations, we have fixed the metal atoms and moved rigidly the BCP molecule in x–y-axes plane, keeping it at a constant distance from the metal surface and also twisted around at the same time to find the adsorption configuration which minimizes the total energy for the system and provide a starting point for a more accurate analysis [19] with PC general atomic and molecular electronic structure system (PC GAMESS). In the next step, we performed our calculations with varying inter-planar distance between BCP and metal surface, that is, z-axis, within the density functional theory, where the exchange-correlation function has been parameterized according to the generalized gradient approximation (GGA) in the Becke [21] and Lee-Yang-Parr [22] (B3LYP5) flavor using PC GAMESS/Firefly version 7.1C and Facio 11.4.1 programs. The hybrid parameter consists of B3LYP as implemented in GAMESS (US), using VWN formula 5 [23] correlations. The basis set 6-31+G** was chosen for C, N, and H atoms of isolated BCP molecule as well as for metals Ca, Mg, Al and core frozen basis set SBKJC+** for Ag, Au junctions with one BCP molecule to give the result in consistent with previous ultra-violet photoemission spectroscopy (UPS) results [24]. These calculations were performed in Restricted Hartree-Fock (RHF) mode for closed shell metals such as Ca and Mg; while in Restricted Open Hartree-Fock (ROHF) mode for open shell metals such as Al, Ag and Au. The approximate basis set superposition error (BSSE) [25] for all BCP/metal systems using both basis sets 6-31+G** and

Fig. 3. Work function of Aln clusters, where n is number of Al atoms.

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Fig. 5. Bond energy and square of charge transfer versus work function of metals.

where Etotal(BCP + metal), Etotal(BCP) and Etotal(metal) denote the

Fig. 4. Bond energy versus planar distance of BCP on Ca (a) and Au substrate (b).

SBKJC+**, was calculated by counter poise (CP) method and was observed to be negligibly small. 3. Results and discussion In order to discuss the validity of metal (1 1 1) surface model composed of 13 atoms, the work function of Aln clusters, where n was chosen as 3, 5, 7, 9, 11, 13, 15, and 17, was examined. It was found that the work function increased with increasing number of atom till n = 11 and then the work function showed saturation as shown in Fig. 3. The saturated value of the Al work function is about 3.92 eV, and this value is in roughly agreement with the experimental value of 4.10 eV. It seems to be possible, therefore, to use the 13-atom cluster as surface model for the discussion of the characteristics of the interaction with BCP. The structure of BCP on metal was obtained by evaluating bond energy, Eb(BCP/metal), using the equation: Eb ðBCP=metalÞ ¼ Etotal ðBCP þ metalÞ  Etotal ðBCPÞ  Etotal ðmetalÞ;

total energies of the adsorbed system, of the isolated BCP molecule, and of the metal substrate, respectively [19]. The bond energy versus inter-planar distance, d, was plotted around the equilibrium distances for BCP on Ca and Au, and the equilibrium distances were found to be 0.40 and 0.39 nm for Ca and Au as shown in Fig. 4(a) and (b), respectively. In the same way, the equilibrium distances for Mg, Al and Ag were obtained to be 0.36, 0.36, and 0.41 nm, respectively. The bond energies for all BCP/metal systems, corrected by basis set superposition error [25] using counter poise method, found to be almost similar to the bond energy calculated initially with basis sets 6-31+G** for metals Ca, Mg, Al and SBKJC+** for metals Ag, Au. The experimental metal–metal distances, calculated equilibrium distances between BCP and metal (1 1 1) surfaces, optimized bond energies, magnitude of charge transfer calculated according to Mulliken population analysis, and calculated and experimental metal work functions, are given in Table 1. The calculated work functions of metals were found to be roughly consistent with their experimental values [24]. Therefore, the cluster model used in this study may be effective to discuss the chemical trend of the electronic structure qualitatively. The calculated bond energy, Eb, at the equilibrium distance for BCP on Ca is about 0.63 eV; the negative value suggests that the process is exothermic and therefore energetically more stable than the two separate constituents. The Eb value decreased with increasing work function of metals and the bond energy for BCP on Au is about 0.17 eV. Such small bond energy for BCP on Au indicates the less stable system. The bond energy and the square of transferred charge versus work functions of metals for BCP/metal systems are shown in

Table 1 Metal atomic distances, BCP–metal equilibrium distances, optimized bond energies, magnitude of charge transfer according to Mulliken population analysis, calculated and experimental values of metal work functions are listed. Substrate metals

Metal atomic distance (nm)

BCP–metal equilibrium distance (nm)

Bond energy (eV)

Charge transfer (jej)

Ca Mg Al Ag Au

0.394 0.319 0.286 0.289 0.288

0.40 0.36 0.36 0.41 0.39

0.63 0.49 0.36 0.26 0.17

0.64 0.31 0.21 0.17 0.02

All experimental work function values in brackets are taken from Ref. [24].

Work function Calculated (eV)

(Experimental) (eV)

2.58 3.08 3.92 4.04 5.33

(2.90) (3.70) (4.10) (4.73) (5.20)

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Fig. 5. The bond energy decreases gradually with increasing metal work function. The square of transferred charge decreases gradually with increasing metal work function similar to the binding energy. Since the square of charge is proportional to the Coulomb interaction and since the equilibrium distances between BCP and metal surfaces are roughly the same, therefore, contribution of ionic interaction in the bond energy between BCP and metals seems to be relatively large. Fig. 6(a)–(f) shows the DFT density of states of Ca (1 1 1) surface (a), Mg (1 1 1) surface (b), Al (1 1 1) surface (c), Ag (1 1 1) surface (d), Au (1 1 1) surface (e), isolated BCP molecule (f) obtained according to the molecular levels broadened by Gaussian broadening with the full width at half maximum of 0.5 eV for each molecular level. The occupied orbitals in DOS shown in Fig. 6 are indicated by black arrows. The density of states of Ca, Mg, Al, Ag, and Au obtained by DFT are almost the same as those obtained by UPS in the energy range below the Fermi level. The experimental HOMO–LUMO gap of BCP is reported to be about 4.2 eV [20] and

the calculated value in this work is 4.2 eV as shown in Fig. 6(f). As mentioned above, calculated metal work functions in this work are roughly consistent with the experimental values, and the calculated HOMO–LUMO gap is also consistent with the experimental one, therefore, the simulation models used in this study can be considered to be good enough to investigate the interaction of BCP with metals qualitatively. Fig. 6(g)–(k) shows the simulation results of BCP on metal systems for BCP on Ca (1 1 1) surface (g), BCP on Mg (1 1 1) surface (h), BCP on Al (1 1 1) surface (i), BCP on Ag (1 1 1) surface (j), and BCP on Au (1 1 1) surface (k). It should be noted that peaks indicated as IS1 and IS2 in the figures appeared in the HOMO–LUMO gap for BCP on metals other than Au. To confirm the appearance of the new peaks, difference spectra between BCP on metal system and metal surfaces are shown in Fig. 7(a)–(e). The new peaks were clearly observed near the Fermi level for BCP on Ca, Mg, Al, and Ag systems; however, no peak was observed for BCP on Au system. These results are fairly consistent with the UPS results in which new peaks

Fig. 6. Density of states obtained by DFT for Ca (a), Mg (b), Al (c), Ag (d), Au (e), BCP (f), BCP on Ca (g), BCP on Mg (h), BCP on Al (i), BCP on Ag (j), and BCP on Au (k).

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Fig. 6. (Continued ).

denoted as ‘‘interface states’’ were observed for BCP deposited on Ca, Mg, Al, and Ag substrate and no ‘‘interface state’’ was observed for BCP on Au substrate [24]. The positions of the ‘‘interface states’’ measured from the Fermi level obtained in this work and UPS results reported previously are listed in Table 2. The major atomic components for BCP on metal system for the molecular orbitals of HOMO, LUMO, interface states and the Fermi level were examined by analyzing atomic orbital coefficient, and atomic component ratio Ai:Zi derived from BCP and metal, respectively, was extracted, where Ai was defined as follows: Ai ¼

X m

ðC mi Þ2 ;

Table 2 Positions of the ‘‘interface states (IS)’’ measured from the Fermi level obtained by DFT and UPS measurements from Ref. [24]. BCP/metal cluster system

BCP/Ca BCP/Mg BCP/Al BCP/Ag BCP/Au

Calculated (DFT) peak positions

Experimental (UPS) peak positions

IS1 (eV)

IS2 (eV)

IS1 (eV)

IS2 (eV)

1.2 0.3 0.0 1.4 –

2.1 2.2 0.8 2.1 –

1.5 1.0 0.8 0.5 –

2.8 2.2 2.0 1.9 –

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Fig. 7. Difference spectra for (a) BCP on Ca, (b) BCP on Mg, (c) BCP on Al, (d) BCP on Ag, and (e) BCP on Au.

and Cmi is the coefficients of the mth basis function in the ith molecular orbital, and the mth basis function is considered only for carbon 2s and 2p orbitals of BCP. Similarly, Zi was defined as follows: X Zi ¼ ðC mi Þ2 ; m

and the mth basis function is considered only for s and p orbitals of metal. The atomic orbital component ratios derived from BCP and metal, Ai:Zi, for the molecular orbitals of HOMO, interface states (IS1 and IS2), Fermi level, and LUMO are listed in Table 3. As shown in Table 3, for the BCP on Au system, the HOMO and LUMO are composed mainly of BCP derived atomic orbitals,

however, a small contribution from BCP to the Fermi level is observed. Such less orbital mixing is consistent with the estimation of small binding energy as shown in Table 1. For BCP on Ca, Mg, Al and Ag system, the orbital mixing becomes significant, especially in the Fermi level and LUMO. The mixing of carbon atomic orbitals into the Fermi level increased with decreasing metal work function. The contribution of carbon atomic orbitals and metal atomic orbitals for LUMO is almost even. When we consider the electron transport between BCP and metal electrode, we should remark the continuity of the wave function at the interface. For BCP on Ca, Mg, Al, Ag system, the LUMO and Fermi level are composed of both BCP and metal derived orbitals, therefore, the wave function at the interface is connected continuously between BCP

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the Fermi level of metal may play an important role in determining the barrier height. 4. Conclusions

Fig. 8. Plot of barrier height of electrons versus work function of metals.

The electronic structures of the interface between BCP and metal surfaces; Ca (1 1 1), Mg (1 1 1), Al (1 1 1), Ag (1 1 1), and Au (1 1 1) which is composed of 13 atoms, were investigated by DFT. In the case of BCP on Au, the bond energy and charge transfer were found to be small, however, for BCP on Ca, Mg, Al, and Ag, the bond energy and charge transfer increased with decreasing metal work function. Molecular orbitals of LUMO and the Fermi level in BCP on Ca, Mg, Al, and Ag systems are composed of both BCP and metal derived atomic orbitals, that is, orbital mixing occurred. On the other hand, molecular orbitals of LUMO and the Fermi level in BCP on Au system is composed mainly of BCP and Au, respectively, that is orbital mixing does not occur. Such orbital mixing may play an important role in electron transport at the interface between BCP and metal. References

Table 3 P Atomic component ratio Ai:Zi, where Ai and Zi are defined as Ai ¼ m ðC mi Þ2 , and Cmi is the coefficients of the mth basis function in the ith molecular orbital, and the mth basis function is considered only for carbon 2s and 2p orbitals in BCP. P Z i ¼ m ðC mi Þ2 , and the mth basis function is considered only for s and p orbitals in metal. BCP/metal

BCP/Ca BCP/Mg BCP/Al BCP/Ag BCP/Au

HOMO

0.93:0.07 0.91:0.09 0.90:0.10 0.89:0.11 0.98:0.02

Interface states IS1

IS2

0.18:0.82 0.25:0.75 0.24:0.76 0.21:0.79 –

0.17:0.83 0.24:0.76 0.23:0.77 0.19:0.81 –

EF

LUMO

0.42:0.58 0.45:0.55 0.24:0.76 0.23:0.77 0.09:0.91

0.51:0.49 0.50:0.50 0.49:0.51 0.43:0.57 0.93:0.07

and metal. On the other hand, for BCP on Au, the wave function at the interface is hardly connected continuously. Such continuity of the wave function at the interface between BCP and metal is in good agreement with the results of electron transport properties obtained experimentally [24]. Fig. 8 shows the relationship between the experimental metal work function and the energy difference of [LUMO-Fermi level] which corresponds to a barrier height for electrons. The barrier heights for BCP on Ca, Mg, Al system are roughly 1 eV, although the metal work function are different each other. The reason for such low and constant barrier height for low work function metals is not clear at present; however, interaction between LUMO of BCP and

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