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metal interfaces
Organic Electronics 10 (2009) 990–993
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Role of interfacial dipole layer for energy-level alignment at organic/metal interfaces Yusuke Tanaka a,*, Kaname Kanai b, Yukio Ouchi a, Kazuhiko Seki a a b
Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Research Core for Interdisciplinary Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 15 May 2009 Accepted 15 May 2009 Available online 22 May 2009
PACS: 71.20.Rv 73.30.+y 68.35.bm
a b s t r a c t The energy-level alignment at Cu-phthalocyanine/metal interfaces has been studied using the Kelvin probe method. The organic layer on a metal surface plays two important roles in the energy-level alignment: the formation of the interfacial dipole (ID) and the passivation of the metal surface. The ID layer determines the injection barrier between the metal and the organic semiconductor. In cases where the lowest unoccupied molecular orbital level of the organic layer on the ID layer is below the Fermi level of the passivated metal substrate, the spontaneous charge transfer from the passivated metal substrate to several organic layers leads to the Fermi-level pinning. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Organic semiconductor interface Energy-level alignment Slope (S) parameter Vacuum-level shift Spontaneous charge transfer Fermi-level pinning
1. Introduction Organic semiconductors have recently attracted much interest for their potential application to organic devices such as light-emitting diodes, solar cells, and field-effect transistors. One of the key issues for organic devices is the energy-level alignment at organic/electrode interfaces. It is well known that the Schottky–Mott Model that presupposes a vacuum-level (VL) alignment cannot be applied to organic/metal interfaces because of the interfacial dipole (ID) formation [1]. The discovery of this dipole layer opened the door to the research on organic/electrode interfaces. Recently, several models for the energy-level alignment have been proposed and discussed in relation to * Corresponding author. Tel.: +81 52 789 2945; fax: +81 52 789 2944. E-mail address: [email protected] (Y. Tanaka). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.05.014
the slope (S) parameter, which is defined as the rate of change of the Schottky barrier versus the metal work function (Um) and has a value from 0 to 1 [2]. At organic/clean metal interfaces, Flores and Kahn proposed the induced density of interface states (IDIS) model [3,4], Lee proposed the electronegativity model [5,6], and Mönch proposed the metal-induced gap state (MIGS) model [7]. At organic/passivated substrate interfaces, Fahlman and Salaneck proposed the integer charge transfer (ICT) model [8,9], and other groups have published related reports [10,11]. Although many experimental and theoretical efforts have been exerted, more systematic investigation of the energy-level alignment is needed to obtain a full explanation of the organic/electrode interfaces. Only a few studies have reported the detailed Um dependence of the S parameter throughout a wide range of Um, including thickness dependence of organic films. In particular, studies of the system
Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
with low Um metal are needed to understand clearly how the interaction between the metal and the organic layer affects the energy-level alignment at organic/metal interfaces. In this paper, we present the systematic study of the VL shifts at Cu-phthalocyanine (CuPc)/metal interfaces over a wide range of Um (=2.5–5.3 eV) using the Kelvin probe (KP) method. We observed the transition of the energy-level alignment from Um dependence to Um independence by decreasing Um. Our results show that the injection barrier cannot be shorter than a threshold value. This transition can be explained by considering that the ID layer on the metal surface plays two major roles in the energy-level alignment: the formation of the ID and the passivation of the metal surface. Considering the roles of the ID layer, we propose a model of the energy-level alignment at organic/electrode interfaces that includes clean and passivated metal substrates. 2. Experimental CuPc with a quoted purity of 99% and sublimation grade was purchased from Sigma–Aldrich. We used Au, Ag, Mg, Ca, and Sm films deposited by vacuum evaporation on silicon wafers as metal substrates. The CuPc films were vacuum deposited onto these substrates in ultra-high vacuum (UHV) (1 106 Pa). The thickness and the rate of deposition (0.1 nm/min) of the CuPc film were monitored with a quartz crystal microbalance. The deposition was performed in a stepwise manner with the KP measurements at each step. The KP measurements were also performed in UHV (8 108 Pa) using an apparatus described elsewhere [12]. 3. Results and discussion
991
substrates as a function of the film thickness (d) using the KP method. The large shifts of eFvac observed at the interface (d < 5 nm) of all the substrates were indicated by the charge redistribution at the interface. The eFvac sharply shifted right at the interfaces when the thickness was less than 1 nm. In the thicker region than 1 nm, the eFvac were almost independent of the thickness on Au and Ag substrates. The little VL shifts observed on these substrates within 1–2 nm may be induced by the changing of the molecular orientation [13,14]. It should be noted that such VL shifts are not caused by the final-state screening because the KP measurement does not use the photoionization process [12]. These shifts were caused by the ID formation [1]. The eFvac ðIDÞ was defined as
eFvac ðIDÞ ¼ Um þ DID
ð1Þ
where the DID is the VL shift caused by the ID layer. On the other hand, gentle downward shifts on the Mg substrate and sharp upward shifts on Ca and Sm substrates were observed when the thickness increased up to 5 nm. It should be stressed that such energy-level shifts at the interface were responsible for the energy-level alignment at the thick film region because little VL shifts occurred in the thicker region than 5 nm on all the substrates. Fig. 2 shows the eFvac ðbulkÞ, which was defined as the eFvac of the thick films (d = 14 nm for Ca and d = 20 nm for others) against Um of the various metal substrates based on the results in Fig. 1. Open circles identify results in this work. Closed circles represent the results taken from Refs. [14–16]. The eFvac ðbulkÞ depended on Um at the higher Um above Mg with S = 0.5 ± 0.1 (S = deFvac ðbulkÞ/dUm). On the other hand, the eFvac ðbulkÞ no longer depended on Um at the lower Um below Mg with S = 0.0 ± 0.1. The Mg substrate is supposed to remain around the critical value of Um. In this paper, we refer to the regions with S = 0 and
Fig. 1 shows the VL energy of the CuPc film (eFvac ) relative to the substrate Fermi level (EF) deposited on metal
Fig. 1. Variation in the eFvac on Au, Ag, Mg, Ca, and Sm substrates as a function of the CuPc thickness d. The eFvac sharply shifts right at the interfaces when the thickness is less than 1 nm. In the thicker region than 1 nm, the eFvac were almost independent of the thickness on Au and Ag substrates. The little VL shifts observed on these substrates within 1– 2 nm may be induced by the changing of the molecular orientation [13,14]. These shifts are caused by the ID formation; the DID for the CuPc film on Au substrate is shown.
Fig. 2. The eFvac ðbulkÞ are plotted as a function of Um; the eFvac ðbulkÞ stands for the eFvac at 20 nm (Au, Ag, Mg, and Sm) and 14 nm (Ca). Open circles identify results in this work. Closed circles represent the results taken from Refs. [14–16]. The dotted lines show the results of fitting analysis by the least squares method with linear function S. The estimated S parameters are given in the figure.
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Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
Fig. 3. The schematics of the energy-level diagrams in: (a) the noninteger S regime and (b) the EF pinning regime. The DID is caused by the ID formation. The eFvac ðIDÞ can be regarded as the work function of the passivated metal substrates. In (a) the noninteger S regime, little VL shift occurred in the thicker region than the ID layer. Therefore the energy-level alignment is mainly determined by the DID, and the eFvac ðbulkÞ satisfies Eq. (2). In (b) the EF pinning regime, the spontaneous CT from the metal to the CuPc film (CT layer) should occur via tunneling through the ID layer until Eq. (4) is satisfied. The d is the maxima barrier height for the occurrence of the spontaneous CT.
S = 0.5 as the ‘‘EF pinning regime” and the ‘‘noninteger S regime,” respectively. The energy-level alignment models for organic/metal interfaces such as the IDIS model, electronegativity model, and MIGS model cannot explain the separated S regimes and the VL shift in the case of the CuPc/Ca interface. The energy-level alignment at CuPc/metal interfaces is discussed with the simple model depicted in Fig. 3. Fig. 3a and b represents the energy-level alignments at the CuPc/ Au interface as an example of the noninteger S regime and the CuPc/Ca interface as an example of the EF pinning regime, respectively. When the CuPc molecule adsorbs on a metal surface, the ID layer is formed at the interface. It is still difficult to predict the estimation of the DID quantitatively, because the ID layer should be formed from multiple origins, as shown in Ref. [1]. In this discussion, we consider the ID layer as a role of passivating the metal surface. Such passivation of the metal surface by the ID layer prevents the molecular orbital of the CuPc layer from overlapping the metal wave functions. It means that the ID layer/metals are the passivated metal substrates, and the eFvac ðIDÞ is the work function of the passivated substrates. Consequently, the energy-level alignment at the thicker film region than the ID layer/metal interfaces can be described as the system at organic/passivated substrate interfaces [8–11]. In the noninteger S regime with high Um, little VL shift occurred in the thicker region than the ID layer as shown in Fig. 1. Therefore the energy-level alignment is mainly determined by the DID, and the eFvac ðbulkÞ satisfies the equation:
eFvac ðbulkÞ eFvac ðIDÞ ¼ Um þ DID
ð2Þ
On the other hand, in the EF pinning regime, the lowest unoccupied molecular orbital (LUMO) level of the CuPc layer on the passivated metal substrate stayed below the substrate EF; for instance, in the case of the CuPc/Ca interface, the LUMO level of the CuPc (d > 1 nm) layer should be
located below the EF by 0.66 eV because eFvac ðIDÞ and the electron affinity (EA) of CuPc are 2.5 and 3.16 eV [17], respectively; and the spontaneous charge transfer (CT) from the metal to the CuPc (d > 1 nm) layer occurred via tunneling through the ID layer to establish the thermodynamic equilibrium. The spontaneous CT will take place if the following condition is found:
eFvac ðIDÞ < EA þ d
ð3Þ
where the d is the relaxation energy which is the maxima barrier height for the occurrence of the spontaneous CT. It means that the spontaneous CT will occur until the LUMO level of the CuPc layer is located above the EF by the d. From the results of the inverse photoemission spectroscopy at CuPc/Mg, Ca, and Sm interfaces (not presented here), the d was estimated to be about 0.4 eV [18]. According to the ICT model, it is supposed to be existed the ICT states just below the LUMO level of molecules. Although the origin of the ICT states in small molecules such as the CuPc molecule is still unclear, energy differences between LUMO levels of molecules (or polymers) and ICT states are good agreement with the d at CuPc/metals interfaces [8,9]. The complicated behaviors of the VL shifts at low Um substrates such as Mg, Ca, and Sm, as observed in Fig. 1, must be caused by the charge redistribution with the spontaneous CT from the passivated metal substrates into CuPc layers, and this induced the VL shifts (DCT) from a 1 nm to 5 nm thickness, i.e., DCT = 0.9 eV and 0.5 eV for Ca and Sm, respectively. The spontaneous CT no longer occurs and the VL shift is not observed if the following condition is satisfied:
eFvac ðIDÞ þ DCT ¼ EA þ d
ð4Þ
here, the ‘‘eFvac ðIDÞ þ DCT ” corresponded to the eFvac ðbulkÞ, and we can rewrite Eq. (4) as a simpler equation:
eFvac ðbulkÞ ¼ EA þ d
ð5Þ
Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
The eFvac ðbulkÞ is mainly determined by the EA of organic molecule. We suppose that little VL shifts in the thicker region than 5 nm at CuPc/Mg, Ca, and Sm interfaces are observed, and the spontaneous CT no longer occurs because Eq. (5) is satisfied. 4. Conclusion In conclusion, we investigated the energy-level alignment at CuPc/metal interfaces. It was found that there are separated S regimes with the noninteger S regime and the EF pinning regime. The organic layer on a metal surface plays two important roles in the energy-level alignment: the ID formation and the passivation of the metal surface. In the noninteger S regime, the energy-level alignment is mainly determined by the DID because little VL shifts occurred in the thicker region than the ID layer. This shows that the organic layer on the ID layer is little influenced by the passivated metal substrate. On the other hand, in the EF pinning regime, the energy-level alignment is determined not only by the DID but also by the spontaneous CT between the organic film and the passivated metal substrate. In this regime, while the DID scattered in various metal substrates, the same energy-level alignments were realized for the thick film region. In the case of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA)/metal interfaces, a similar trend has been reported [19]. Although the obvious differences in interface states depending on the metals were observed, the same energy-level alignments for the thick film region were also realized. The model we suggested can also explain the energy-level alignments at PTCDA/metal interfaces and other reported results [20,21]. Finally, this model, which deals with the energy-level alignment between the thicker region than the ID layer and the passivated metal substrate, provides a new perspective on the energy-level alignment at organic/electrode interfaces that include clean and passivated metal substrates.
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Acknowledgments This work was supported by the Grant-in-Aid for Scientific Research (S) (Grant No. 19105005), the Research Fellowship for Young Scientists, and Grant for Basic Science Research Projects (No. 060816). We thank the Japan Society for the Promotion of Science and the Sumitomo Foundation. This paper is dedicated to the late Prof. K. Seki. References [1] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [2] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. [3] H. Vázquez, R. Oszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 802. [4] H. Vázquez, W. Gao, F. Flores, A. Kahn, Phys. Rev. B 71 (2005) 041306. [5] J.X. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 87 (2005) 252110. [6] Y.C. Zhou, J.X. Tang, Z.T. Liu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 93 (2008) 093502. [7] W. Mönch, Appl. Phys. Lett. 88 (2006) 112116. [8] C. Tengstedt, W. Osikowicz, W.R. Salaneck, I.D. Parker, C.-H. Hsu, M. Fahlman, Appl. Phys. Lett. 88 (2006) 053502. [9] M. Fahlman, A. Crispin, X. Crispin, S.K.M. Henze, M.P. de Jong, W. Osikowicz, C. Tengstedt, W.R. Salaneck, J. Phys.: Condens. Matter. 19 (2007) 183202. [10] H. Fukagawa, S. Kera, T. Kataoka, S. Hosoumi, Y. Watanabe, K. Kudo, N. Ueno, Adv. Mater. 19 (2007) 665. [11] N. Koch, A. Vollmer, Appl. Phys. Lett. 89 (2006) 162107. [12] N. Hayashi, H. Ishii, Y. Ouchi, K. Seki, J. Appl. Phys. 92 (2002) 3784. [13] H. Yamane, Y. Yabuuchi, H. Fukagawa, S. Kera, K.K. Okudaira, N. Ueno, J. Appl. Phys. 99 (2006) 093705. [14] H. Peisert, M. Knupfer, T. Schwieger, J.M. Auerhammer, M.S. Golden, J. Fink, J. Appl. Phys. 91 (2002) 15. [15] J.X. Tang, C.S. Lee, S.T. Lee, Appl. Surf. Sci. 252 (2006) 3948. [16] F. Song, H. Huang, W. Dou, H. Zhang, Y. Hu, H. Qian, H. Li, P. He, S. Bao, Q. Chen, W. Zhou, J. Phys.: Condens. Matter 19 (2007) 136002. [17] R. Murdey, N. Sato, M. Bouvet, Mol. Cryst. Liq. Cryst. 455 (2006) 211. [18] Y. Tanaka, K. Kanai, Y. Ouchi, K. Seki, Mater. Res. Soc. Symp. Proc., submitted to publication. [19] S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R.L. Johnson, F. Schreiber, N. Koch, Org. Electron. 9 (2008) 111. [20] J.X. Tang, C.S. Lee, S.T. Lee, Y.B. Xu, Chem. Phys. Lett. 396 (2004) 92. [21] S. Toyoshima, K. Kuwabara, T. Sakurai, T. Taima, K. Sato, H. Kato, K. Akimoto, J. Appl. Phys. 46 (2007) 2692.
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Role of interfacial dipole layer for energy-level alignment at organic/metal interfaces Yusuke Tanaka a,*, Kaname Kanai b, Yukio Ouchi a, Kazuhiko Seki a a b
Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan Research Core for Interdisciplinary Sciences, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2009 Received in revised form 15 May 2009 Accepted 15 May 2009 Available online 22 May 2009
PACS: 71.20.Rv 73.30.+y 68.35.bm
a b s t r a c t The energy-level alignment at Cu-phthalocyanine/metal interfaces has been studied using the Kelvin probe method. The organic layer on a metal surface plays two important roles in the energy-level alignment: the formation of the interfacial dipole (ID) and the passivation of the metal surface. The ID layer determines the injection barrier between the metal and the organic semiconductor. In cases where the lowest unoccupied molecular orbital level of the organic layer on the ID layer is below the Fermi level of the passivated metal substrate, the spontaneous charge transfer from the passivated metal substrate to several organic layers leads to the Fermi-level pinning. Ó 2009 Elsevier B.V. All rights reserved.
Keywords: Organic semiconductor interface Energy-level alignment Slope (S) parameter Vacuum-level shift Spontaneous charge transfer Fermi-level pinning
1. Introduction Organic semiconductors have recently attracted much interest for their potential application to organic devices such as light-emitting diodes, solar cells, and field-effect transistors. One of the key issues for organic devices is the energy-level alignment at organic/electrode interfaces. It is well known that the Schottky–Mott Model that presupposes a vacuum-level (VL) alignment cannot be applied to organic/metal interfaces because of the interfacial dipole (ID) formation [1]. The discovery of this dipole layer opened the door to the research on organic/electrode interfaces. Recently, several models for the energy-level alignment have been proposed and discussed in relation to * Corresponding author. Tel.: +81 52 789 2945; fax: +81 52 789 2944. E-mail address: [email protected] (Y. Tanaka). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.05.014
the slope (S) parameter, which is defined as the rate of change of the Schottky barrier versus the metal work function (Um) and has a value from 0 to 1 [2]. At organic/clean metal interfaces, Flores and Kahn proposed the induced density of interface states (IDIS) model [3,4], Lee proposed the electronegativity model [5,6], and Mönch proposed the metal-induced gap state (MIGS) model [7]. At organic/passivated substrate interfaces, Fahlman and Salaneck proposed the integer charge transfer (ICT) model [8,9], and other groups have published related reports [10,11]. Although many experimental and theoretical efforts have been exerted, more systematic investigation of the energy-level alignment is needed to obtain a full explanation of the organic/electrode interfaces. Only a few studies have reported the detailed Um dependence of the S parameter throughout a wide range of Um, including thickness dependence of organic films. In particular, studies of the system
Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
with low Um metal are needed to understand clearly how the interaction between the metal and the organic layer affects the energy-level alignment at organic/metal interfaces. In this paper, we present the systematic study of the VL shifts at Cu-phthalocyanine (CuPc)/metal interfaces over a wide range of Um (=2.5–5.3 eV) using the Kelvin probe (KP) method. We observed the transition of the energy-level alignment from Um dependence to Um independence by decreasing Um. Our results show that the injection barrier cannot be shorter than a threshold value. This transition can be explained by considering that the ID layer on the metal surface plays two major roles in the energy-level alignment: the formation of the ID and the passivation of the metal surface. Considering the roles of the ID layer, we propose a model of the energy-level alignment at organic/electrode interfaces that includes clean and passivated metal substrates. 2. Experimental CuPc with a quoted purity of 99% and sublimation grade was purchased from Sigma–Aldrich. We used Au, Ag, Mg, Ca, and Sm films deposited by vacuum evaporation on silicon wafers as metal substrates. The CuPc films were vacuum deposited onto these substrates in ultra-high vacuum (UHV) (1 106 Pa). The thickness and the rate of deposition (0.1 nm/min) of the CuPc film were monitored with a quartz crystal microbalance. The deposition was performed in a stepwise manner with the KP measurements at each step. The KP measurements were also performed in UHV (8 108 Pa) using an apparatus described elsewhere [12]. 3. Results and discussion
991
substrates as a function of the film thickness (d) using the KP method. The large shifts of eFvac observed at the interface (d < 5 nm) of all the substrates were indicated by the charge redistribution at the interface. The eFvac sharply shifted right at the interfaces when the thickness was less than 1 nm. In the thicker region than 1 nm, the eFvac were almost independent of the thickness on Au and Ag substrates. The little VL shifts observed on these substrates within 1–2 nm may be induced by the changing of the molecular orientation [13,14]. It should be noted that such VL shifts are not caused by the final-state screening because the KP measurement does not use the photoionization process [12]. These shifts were caused by the ID formation [1]. The eFvac ðIDÞ was defined as
eFvac ðIDÞ ¼ Um þ DID
ð1Þ
where the DID is the VL shift caused by the ID layer. On the other hand, gentle downward shifts on the Mg substrate and sharp upward shifts on Ca and Sm substrates were observed when the thickness increased up to 5 nm. It should be stressed that such energy-level shifts at the interface were responsible for the energy-level alignment at the thick film region because little VL shifts occurred in the thicker region than 5 nm on all the substrates. Fig. 2 shows the eFvac ðbulkÞ, which was defined as the eFvac of the thick films (d = 14 nm for Ca and d = 20 nm for others) against Um of the various metal substrates based on the results in Fig. 1. Open circles identify results in this work. Closed circles represent the results taken from Refs. [14–16]. The eFvac ðbulkÞ depended on Um at the higher Um above Mg with S = 0.5 ± 0.1 (S = deFvac ðbulkÞ/dUm). On the other hand, the eFvac ðbulkÞ no longer depended on Um at the lower Um below Mg with S = 0.0 ± 0.1. The Mg substrate is supposed to remain around the critical value of Um. In this paper, we refer to the regions with S = 0 and
Fig. 1 shows the VL energy of the CuPc film (eFvac ) relative to the substrate Fermi level (EF) deposited on metal
Fig. 1. Variation in the eFvac on Au, Ag, Mg, Ca, and Sm substrates as a function of the CuPc thickness d. The eFvac sharply shifts right at the interfaces when the thickness is less than 1 nm. In the thicker region than 1 nm, the eFvac were almost independent of the thickness on Au and Ag substrates. The little VL shifts observed on these substrates within 1– 2 nm may be induced by the changing of the molecular orientation [13,14]. These shifts are caused by the ID formation; the DID for the CuPc film on Au substrate is shown.
Fig. 2. The eFvac ðbulkÞ are plotted as a function of Um; the eFvac ðbulkÞ stands for the eFvac at 20 nm (Au, Ag, Mg, and Sm) and 14 nm (Ca). Open circles identify results in this work. Closed circles represent the results taken from Refs. [14–16]. The dotted lines show the results of fitting analysis by the least squares method with linear function S. The estimated S parameters are given in the figure.
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Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
Fig. 3. The schematics of the energy-level diagrams in: (a) the noninteger S regime and (b) the EF pinning regime. The DID is caused by the ID formation. The eFvac ðIDÞ can be regarded as the work function of the passivated metal substrates. In (a) the noninteger S regime, little VL shift occurred in the thicker region than the ID layer. Therefore the energy-level alignment is mainly determined by the DID, and the eFvac ðbulkÞ satisfies Eq. (2). In (b) the EF pinning regime, the spontaneous CT from the metal to the CuPc film (CT layer) should occur via tunneling through the ID layer until Eq. (4) is satisfied. The d is the maxima barrier height for the occurrence of the spontaneous CT.
S = 0.5 as the ‘‘EF pinning regime” and the ‘‘noninteger S regime,” respectively. The energy-level alignment models for organic/metal interfaces such as the IDIS model, electronegativity model, and MIGS model cannot explain the separated S regimes and the VL shift in the case of the CuPc/Ca interface. The energy-level alignment at CuPc/metal interfaces is discussed with the simple model depicted in Fig. 3. Fig. 3a and b represents the energy-level alignments at the CuPc/ Au interface as an example of the noninteger S regime and the CuPc/Ca interface as an example of the EF pinning regime, respectively. When the CuPc molecule adsorbs on a metal surface, the ID layer is formed at the interface. It is still difficult to predict the estimation of the DID quantitatively, because the ID layer should be formed from multiple origins, as shown in Ref. [1]. In this discussion, we consider the ID layer as a role of passivating the metal surface. Such passivation of the metal surface by the ID layer prevents the molecular orbital of the CuPc layer from overlapping the metal wave functions. It means that the ID layer/metals are the passivated metal substrates, and the eFvac ðIDÞ is the work function of the passivated substrates. Consequently, the energy-level alignment at the thicker film region than the ID layer/metal interfaces can be described as the system at organic/passivated substrate interfaces [8–11]. In the noninteger S regime with high Um, little VL shift occurred in the thicker region than the ID layer as shown in Fig. 1. Therefore the energy-level alignment is mainly determined by the DID, and the eFvac ðbulkÞ satisfies the equation:
eFvac ðbulkÞ eFvac ðIDÞ ¼ Um þ DID
ð2Þ
On the other hand, in the EF pinning regime, the lowest unoccupied molecular orbital (LUMO) level of the CuPc layer on the passivated metal substrate stayed below the substrate EF; for instance, in the case of the CuPc/Ca interface, the LUMO level of the CuPc (d > 1 nm) layer should be
located below the EF by 0.66 eV because eFvac ðIDÞ and the electron affinity (EA) of CuPc are 2.5 and 3.16 eV [17], respectively; and the spontaneous charge transfer (CT) from the metal to the CuPc (d > 1 nm) layer occurred via tunneling through the ID layer to establish the thermodynamic equilibrium. The spontaneous CT will take place if the following condition is found:
eFvac ðIDÞ < EA þ d
ð3Þ
where the d is the relaxation energy which is the maxima barrier height for the occurrence of the spontaneous CT. It means that the spontaneous CT will occur until the LUMO level of the CuPc layer is located above the EF by the d. From the results of the inverse photoemission spectroscopy at CuPc/Mg, Ca, and Sm interfaces (not presented here), the d was estimated to be about 0.4 eV [18]. According to the ICT model, it is supposed to be existed the ICT states just below the LUMO level of molecules. Although the origin of the ICT states in small molecules such as the CuPc molecule is still unclear, energy differences between LUMO levels of molecules (or polymers) and ICT states are good agreement with the d at CuPc/metals interfaces [8,9]. The complicated behaviors of the VL shifts at low Um substrates such as Mg, Ca, and Sm, as observed in Fig. 1, must be caused by the charge redistribution with the spontaneous CT from the passivated metal substrates into CuPc layers, and this induced the VL shifts (DCT) from a 1 nm to 5 nm thickness, i.e., DCT = 0.9 eV and 0.5 eV for Ca and Sm, respectively. The spontaneous CT no longer occurs and the VL shift is not observed if the following condition is satisfied:
eFvac ðIDÞ þ DCT ¼ EA þ d
ð4Þ
here, the ‘‘eFvac ðIDÞ þ DCT ” corresponded to the eFvac ðbulkÞ, and we can rewrite Eq. (4) as a simpler equation:
eFvac ðbulkÞ ¼ EA þ d
ð5Þ
Y. Tanaka et al. / Organic Electronics 10 (2009) 990–993
The eFvac ðbulkÞ is mainly determined by the EA of organic molecule. We suppose that little VL shifts in the thicker region than 5 nm at CuPc/Mg, Ca, and Sm interfaces are observed, and the spontaneous CT no longer occurs because Eq. (5) is satisfied. 4. Conclusion In conclusion, we investigated the energy-level alignment at CuPc/metal interfaces. It was found that there are separated S regimes with the noninteger S regime and the EF pinning regime. The organic layer on a metal surface plays two important roles in the energy-level alignment: the ID formation and the passivation of the metal surface. In the noninteger S regime, the energy-level alignment is mainly determined by the DID because little VL shifts occurred in the thicker region than the ID layer. This shows that the organic layer on the ID layer is little influenced by the passivated metal substrate. On the other hand, in the EF pinning regime, the energy-level alignment is determined not only by the DID but also by the spontaneous CT between the organic film and the passivated metal substrate. In this regime, while the DID scattered in various metal substrates, the same energy-level alignments were realized for the thick film region. In the case of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA)/metal interfaces, a similar trend has been reported [19]. Although the obvious differences in interface states depending on the metals were observed, the same energy-level alignments for the thick film region were also realized. The model we suggested can also explain the energy-level alignments at PTCDA/metal interfaces and other reported results [20,21]. Finally, this model, which deals with the energy-level alignment between the thicker region than the ID layer and the passivated metal substrate, provides a new perspective on the energy-level alignment at organic/electrode interfaces that include clean and passivated metal substrates.
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Acknowledgments This work was supported by the Grant-in-Aid for Scientific Research (S) (Grant No. 19105005), the Research Fellowship for Young Scientists, and Grant for Basic Science Research Projects (No. 060816). We thank the Japan Society for the Promotion of Science and the Sumitomo Foundation. This paper is dedicated to the late Prof. K. Seki. References [1] H. Ishii, K. Sugiyama, E. Ito, K. Seki, Adv. Mater. 11 (1999) 605. [2] S.M. Sze, Physics of Semiconductor Devices, second ed., Wiley, New York, 1981. [3] H. Vázquez, R. Oszwaldowski, P. Pou, J. Ortega, R. Pérez, F. Flores, A. Kahn, Europhys. Lett. 65 (2004) 802. [4] H. Vázquez, W. Gao, F. Flores, A. Kahn, Phys. Rev. B 71 (2005) 041306. [5] J.X. Tang, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 87 (2005) 252110. [6] Y.C. Zhou, J.X. Tang, Z.T. Liu, C.S. Lee, S.T. Lee, Appl. Phys. Lett. 93 (2008) 093502. [7] W. Mönch, Appl. Phys. Lett. 88 (2006) 112116. [8] C. Tengstedt, W. Osikowicz, W.R. Salaneck, I.D. Parker, C.-H. Hsu, M. Fahlman, Appl. Phys. Lett. 88 (2006) 053502. [9] M. Fahlman, A. Crispin, X. Crispin, S.K.M. Henze, M.P. de Jong, W. Osikowicz, C. Tengstedt, W.R. Salaneck, J. Phys.: Condens. Matter. 19 (2007) 183202. [10] H. Fukagawa, S. Kera, T. Kataoka, S. Hosoumi, Y. Watanabe, K. Kudo, N. Ueno, Adv. Mater. 19 (2007) 665. [11] N. Koch, A. Vollmer, Appl. Phys. Lett. 89 (2006) 162107. [12] N. Hayashi, H. Ishii, Y. Ouchi, K. Seki, J. Appl. Phys. 92 (2002) 3784. [13] H. Yamane, Y. Yabuuchi, H. Fukagawa, S. Kera, K.K. Okudaira, N. Ueno, J. Appl. Phys. 99 (2006) 093705. [14] H. Peisert, M. Knupfer, T. Schwieger, J.M. Auerhammer, M.S. Golden, J. Fink, J. Appl. Phys. 91 (2002) 15. [15] J.X. Tang, C.S. Lee, S.T. Lee, Appl. Surf. Sci. 252 (2006) 3948. [16] F. Song, H. Huang, W. Dou, H. Zhang, Y. Hu, H. Qian, H. Li, P. He, S. Bao, Q. Chen, W. Zhou, J. Phys.: Condens. Matter 19 (2007) 136002. [17] R. Murdey, N. Sato, M. Bouvet, Mol. Cryst. Liq. Cryst. 455 (2006) 211. [18] Y. Tanaka, K. Kanai, Y. Ouchi, K. Seki, Mater. Res. Soc. Symp. Proc., submitted to publication. [19] S. Duhm, A. Gerlach, I. Salzmann, B. Bröker, R.L. Johnson, F. Schreiber, N. Koch, Org. Electron. 9 (2008) 111. [20] J.X. Tang, C.S. Lee, S.T. Lee, Y.B. Xu, Chem. Phys. Lett. 396 (2004) 92. [21] S. Toyoshima, K. Kuwabara, T. Sakurai, T. Taima, K. Sato, H. Kato, K. Akimoto, J. Appl. Phys. 46 (2007) 2692.