metal interfaces

Organic Electronics 12 (2011) 295–299

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Density-functional theoretical study of fluorination effect on organic/metal interfaces Kenji Toyoda a,⇑, Ikutaro Hamada b, Susumu Yanagisawa c, Yoshitada Morikawa c a

Advanced Technology Research Laboratories, Panasonic Corporation, 3-4 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0237, Japan WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan c Department of Precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan b

a r t i c l e

i n f o

Article history: Received 15 September 2010 Received in revised form 10 November 2010 Accepted 14 November 2010 Available online 25 November 2010 Keywords: Density-functional theory Organic/metal interface van der Waals interactions Vacuum level shift Fluorination Carrier injection

a b s t r a c t Density-functional theory with a semi-empirical dispersion correction was used to systematically examine how the number of fluorine (F) atoms affects atomic and electronic structures of fluorinated pentacene ðC22 Fn H14n Þ adsorbed on Cu(1 1 1) surfaces. The fluorination effect on the carrier injection efficiency at organic/metal interfaces was investigated. We found that as the number of F atoms decreases, the electron affinity of isolated molecules decreases, suggesting that the molecule becomes less reactive. However, for adsorbed systems, as the number of F atoms decreases, molecular orbitals of C22 Fn H14n strongly hybridize with the substrate states while retaining the n-type energy level alignment, resulting in lowering the barrier height of the carrier injection. Based on the calculation results, we propose using C22 Fn H14n ðn 6 8Þ with Cu electrodes for efficient electron injection. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Understanding the electronic structure at organic/metal interfaces is of great importance in developing organic devices [1,2]. The interface dipole at organic/metal interfaces induces vacuum level shift that modifies the barrier height of the carrier injection, which plays a decisive role in organic devices [1,2]. Therefore, accurate prediction and control of the interface dipole is crucial to designing the electrodes of organic devices. Pentacene (C22 H14 , Pen) and perfluoropentacene (C22 F14 , PFP) are prototypical p-type [3] and n-type [4] organic semiconductors, respectively. Accordingly, the interactions of Pen and PFP with metal substrate have been studied extensively both experimentally and theoretically [5–20]. In particular, elucidation of the electronic properties of PFP on metal is crucial, because n-type organic/metal interfaces are not as well understood as p-type organic/ ⇑ Corresponding author. E-mail address: [email protected] (K. Toyoda). 1566-1199/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.11.010

metal interfaces [21,22]. Efficient n-type interfaces must be designed to fabricate complementary integrated circuits. PFP is expected to lower the electron injection barrier height, because larger electron affinity makes the lowest unoccupied molecular orbital (LUMO) level closer to the Fermi energy of the electrode than that of Pen [23]. However, experimental reports have shown that the highest occupied molecular orbital (HOMO) state relative to the Fermi energy for PFP/Cu(1 1 1) is almost the same as that for Pen/Cu(1 1 1) [11], and moreover, that the PFP– substrate distance is larger than that for Pen/Cu(1 1 1) [16]. These indicate that the electron injection for PFP/ Cu(1 1 1) is less efficient than that for Pen/Cu(1 1 1), because the overlap between the molecular orbitals and the substrate for PFP/Cu(1 1 1) becomes smaller than that for Pen/ Cu(1 1 1). Our previous paper pointed out that the large PFP–Cu(1 1 1) distance originates from repulsion of F atoms by the substrate [10]. Therefore, it might be possible to modify the adsorption distance and electron injection barrier, by changing the number of F atoms in PFP.

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F

In this work, we have examined systematically how the number of F atoms affects the atomic and electronic structures of fluorinated pentacene ðC22 Hn F14n Þ adsorbed on Cu(1 1 1) using density-functional theory (DFT) within the generalized gradient approximation (GGA) with a semiempirical dispersion correction. We show that adsorption distance and hybridization of molecular orbitals of C22 Hn F14n on Cu(1 1 1) are tunable by changing the number of F atoms, and we propose an optimal organic/metal interface for efficient electron injection.

F

F

F

F

F

F F

F

F

F

(a) F12 F

F

2. Calculation method Calculations were carried out using STATE [24], which implements a plane wave basis set and pseudopotentials [25,26]. The plane-wave kinetic-energy cutoff for the wave functions and augmented charge density were set to 25 and 225 Ry, respectively. The Perdew–Burke–Ernzerhof (PBE)-GGA [27] was adopted to describe the exchange– correlation functional. Because a semilocal GGA functional cannot correctly describe van der Waals (vdW) forces, which dominate the interaction between p conjugated molecules and metal surfaces [5,7–10,18,19,28–31], we used a semi-empirical dispersion correction proposed by Grimme [32] (DFT-D) for adsorption systems. DFT-D was shown to yield accurate adsorption distances of organic molecules on metal surfaces as well as the work-function changes [7,9,10]. In this work, we examined the following three C22 Fn H14n molecules as shown in Fig. 1: 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 14-dodecanefluoropentacene (F12), 1, 2, 3, 4, 8, 9, 10, 11-octafluoropentacene (F8), and 2, 3, 9, 10-tetrafluoropentance (F4). The numbers of F atoms in F12, F8, and F4 were 12, 8, and 4, respectively. Isolated molecules were calculated in a 2  1  1 nm3 rectangular unit cell using the C point. The Cu(1 1 1) surface was represented by a repeated slab model, in which one slab consisted of four Cu atomic layers. A vacuum region of 2 nm was inserted in between slabs. A C22 Fn H14n molecule was adsorbed on only one surface of the slab its molecular plane parallel to pffiffiffiffiffiffiwithpffiffiffi the surface in a 43  2 3 surface unit cell, which is the same as that used in our previous calculation of PFP/metal systems [10]. A 2  4 k-point mesh was used to sample the surface Brillouin zone. The center of C22 Fn H14n was assumed to be located at an hcp-hollow site on the Cu(1 1 1) surface with the long molecular axis aligned with close-packed metal atom rows as shown in Fig. 2. In the geometry optimization, we fixed the bottom layer of the substrate slabs at their respective bulk positions. The remaining degrees of freedom were optimized with the DFT-D method until the maximum force dropped below a threshold value of 0.2 nN. The work-function difference between the two surfaces of a slab was compensated for by using a dipole correction [33]. Work-functions were calculated from the difference between the Fermi energy of the system and the average electrostatic potential at the center of the vacuum region, and the vacuum level shifts were calculated from the work-function changes induced by the adsorption of C22 Fn H14n .

F

F

F

F

F F

(b) F8

F

F

F

F

F

(c) F4 Fig. 1. The structural formula of (a) F12, (b) F8, and (c) F4.

Fig. 2. Plan view of a C22 Fn H14n molecule on Cu(1 1 1).

3. Results and discussion 3.1. Isolated molecules First, we calculated the atomic and the electronic structures of isolated PFP, F12, F8, F4, and Pen molecules. The optimized molecular structures of F12, F8, and F4 are planar, similar to Pen and PFP. The ionization potential (IP) and electron affinity (EA) of PFP, F12, F8, F4, and Pen are summarized in Table 1. The IP and EA were estimated from

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K. Toyoda et al. / Organic Electronics 12 (2011) 295–299 Table 1 IP and EA of PFP, F12, F8, F4, and Pen.

IP/eV EA/eV

PFP

F12

F8

F4

Pen

5.25 4.28

5.27 4.30

5.17 4.04

4.90 3.72

4.50 3.36

the Kohn–Sham energy levels of the HOMO and LUMO relative to the vacuum level, respectively. We found that as the number of F atoms in C22 Fn H14n decreases, the IP and EA of each molecule become small, suggesting that the molecule becomes less reactive. We also found that the IP and EA of F12 are almost the same as those of PFP. 3.2. Adsorbed systems Next, we calculated the atomic geometries and the vacuum level shifts of PFP, F12, F8, F4, and Pen adsorbed on Cu(1 1 1) and then inspected the electronic structures of the adsorption systems. 3.2.1. Atomic geometries and vacuum level shifts Fig. 3(a)–(c) depict a cross-sectional view of the atomic geometries of F12, F8, and F4 adsorbed on Cu(1 1 1). Table 2 summarizes the adsorption distances of C atoms depicted in Fig. 3 relative to the surface and the work-function change ðD/Þ for PFP, F12, F8, F4, and Pen adsorbed on Cu(1 1 1). It turns out that as the number of F atoms decreases, the average adsorption distance of the C atoms becomes smaller (Fig. 3 and Table 2), even though decreasing the number of F atoms makes EA smaller, i.e., chemically less reactive. The results suggest that the adsorption distance is mainly determined by the number of F atoms rather than the reactivity of the molecule, because of the repulsion between F 2p and substrate states [10]. As seen in Fig. 3(a)–(c), the C22 Fn Hn14 molecule is distorted when adsorbed on the surface, and the adsorption distance of an end C atom bonded to F (C1) is larger than that of the center C atom bonded to H (C2) (Table 2). The differences in adsorption distances of C1 and C2 for F12, F8, and F4 are almost the same (0.03–0.04 nm). The molecular distortion is attributed to (i) repulsion between F 2p and Cu substrate states and (ii) hybridization of the molecular orbitals with the substrate states [7]. When the number of F atoms is large (i.e., large distance), repulsion dominates and accordingly hybridization is small. On the other hand, when the number of F atoms is small, the adsorption distance becomes small (hence strong hybridization) because of the reduction of repulsion by F 2p. These effects compensate for each other depending on the distance, resulting in similar molecular distortion. Note that distortion caused by electron transfer is not significant [5,7].

Table 2 The average C adsorption distance ðZ ave C Þ, the adsorption distance of C1 and C2 in Fig. 3 (Z C1 and Z C2 , respectively), and the work-function change ðD/Þ for PFP, F12, F8, F4, and Pen adsorbed on Cu(1 1 1). Adsorption distances of C atoms are measured from the Cu surface. PFP

a

Fig. 3. Cross-sectional view of F12 (a), F8 (b), and F4 (c) on Cu(1 1 1).

b

Z ave C =nm Z C1 =nm Z C2 =nm

0.29a – –

D/=eV

0.30a

Ref. [10]. Ref. [7].

F12

F8

F4

0.29 0.27 0.30

0.27 0.25 0.29

0.26 0.24 0.27

0.29

0.41

0.62

Pen 0.24b – – 0.97b

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Fig. 4. The density of states projected (PDOS) onto molecular LUMO and HOMO for PFP, F12, F8, F4, and Pen on Cu(1 1 1). The arrows represent the peak positions of LUMO and HOMO states.

D/ becomes smaller (i.e., larger j D/ j) as the number of F atoms decreases. Because the work-function changes caused by the molecular distortion are small (0.04, 0.01, and 0.1 eV for F12, F8, and F4, respectively), we ascribe the change in D/ mainly to the change in the adsorption distance. 3.2.2. Electronic structures Fig. 4 shows the density of states projected (PDOS) onto molecular LUMO and HOMO for PFP, F12, F8, F4, and Pen on Cu(1 1 1). As shown in Fig. 4, the broadening of the molecular states (LUMO and HOMO) becomes larger as the number of F atoms decreases, as a result of stronger hybridization at a short distance. On the other hand, the PDOS for the molecular states of PFP and F12 are sharp, indicating weaker interaction of the adsorbate with the substrate. Although hybridization of the HOMO and LUMO states with the substrate depends significantly on the number of F atoms, their peak positions are almost unchanged, suggesting that n-type operation would be retained regardless of the number of F atoms. This is because the larger absolute value of D/ cancels out the smaller EA of the isolated molecule as the number of F atoms decreases. Our results indicate that the energy level alignment of C22 Fn H14n ðn P 0Þ adsorbed on Cu(1 1 1) is n-type, because a part of the LUMO state is below the Fermi energy. It should be noted that PBE-GGA has the well-known underestimation of the band gap [34–37] but can describe the LUMO and HOMO levels of an organic molecule adsorbed on a metal surface qualitatively [5,7,10].1 Our results suggest that reducing the number of F atoms makes it possible to fabricate an optimal interface for carrier injection because of the strong hybridization. On the basis of our systematic 1 The gap underestimation tends to be compensated for by the error in the present GGA to describe the polarization of the metal surface, which reduces the band gap [38].

study, we propose using C22 Fn H14n ð0 6 n 6 8Þ for efficient electron injection. 4. Conclusions We have studied the fluorination effect on C22 Fn H14n = Cuð1 1 1Þ interfaces using density-functional calculations with the semi-empirical van der Waals (vdW) correction. We found that although reducing the number of F atoms decreases the ionization potential and the electron affinity of the C22 Fn H14n molecule (i.e., the molecule became less reactive), the adsorption distance of the molecule on the Cu(1 1 1) surface becomes smaller due to the reduction of repulsion between F 2p and the substrate states. Calculated electronic structures show that the hybridization of the molecular orbitals with the substrate states becomes stronger, whereas the peak positions of the molecular states remains almost unchanged, retaining n-type interface as obtained with PFP, as the number of F atoms decreases. This suggests that electron injection is made efficient, and we therefore propose using C22 Hn F14n ðn 6 8Þ to achieve efficient electron injection. Our results indicate that controlling only the ionization potential and the electron affinity of isolated molecules is insufficient for designing an optimal organic/metal interface. We conclude that the energy level alignment at organic/metal interfaces is crucially governed by adsorption geometries which are quite flexible and are affected by a subtle balance between repulsive interaction due to Pauli repulsion and attractive interaction due to hybridization and vdW interactions. Acknowledgments The work was partly supported by Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sports, and Technology, Japan (Grant Nos.

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19054013 and 22241026). Numerical calculations were carried out at computer centers of Osaka University, the Institute for Solid State Physics of the University of Tokyo, and Tohoku University.

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