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ITO film using surface photovoltage technique
Materials Chemistry and Physics 100 (2006) 230–235
The electronic and optoelectronic properties study of N,N-dimethylperylene-3,4,9,10-dicarboximide/ITO film using surface photovoltage technique Qinglin Zhang, Dejun Wang ∗ , Xiao Wei, Qidong Zhao, Yanhong Lin, Min Yang College of Chemistry, Jilin University, Changchun 130023, PR China Received 13 July 2005; received in revised form 24 November 2005; accepted 19 December 2005
Abstract The electronic structure and photo-induced charge carriers transport have been studied in N,N-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) films evaporated on ITO substrate using surface photovoltage (SPV) technique. For the Me-PTCDI film with the thickness of 456 nm, with changing the illumination geometry, the amplitudes as well as the phases of the SPV response bands at the wavelength where Me-PTCDI strongly absorbs significantly changed, while those of the SPV response bands at the wavelength where Me-PTCDI weakly absorbs hardly changed. It was explained based on the different absorption lengths of Me-PTCDI at different wavelengths. The experiment results suggest that the photo-induced charge carriers generated at the surface and the interface region are separated by the action of the built-in electric field and move in the opposite directions. From the dependence of the work function on the film thickness, it is inferred that the interface and surface are all depleted with the interface space charge region extending to be about 200 nm. In addition, it is found that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface. © 2005 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Surface; Electronic structure; Thin films
1. Introduction Perylene and its derivatives, as organic n-type semiconductors with high optical and thermal stability, have attracted much attention due to their potential applications in various electronic and optoelectronic devices, such as the organic solar cells [1–4], organic light-emitting diodes (OLEDs) [5], and organic fieldeffect transistors [6]. The understanding of the transport of the photo-induced charge carriers and the related electronic structure in organic semiconductors is crucially important for the optimization of the devices’ performance. As a result, many studies have focused on the electronic structure and the transport of the photo-induced charge carriers of perylene (or its derivatives) films evaporated on the metals or inorganic semiconductors substrates. The organic films have been studied using the ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy
∗
Corresponding author. Tel.: +86 431 8499172; fax: +86 431 8499158. E-mail address: [email protected] (D. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.12.034
(XPS) [7–11]. Although they can give the information on the organic/metal or inorganic semiconductor interface electronic structure and electrostatic information, such as interface dipole, work function, electronic energy level, and chemical reaction at interfaces, they do not provide insight into the transport and excitation processes of the photo-induced carriers. The measurement of photocurrent allows one to investigate the photo-induced carrier excitation and transport processes in organic films [12,13]. Nevertheless, ohmic contact between the electrode and the tested sample, required for that technique may substantially change the surface/interface electronic structures [14]. Therefore, an informative and nondestructive characterization technique for these organic devices is desirable. Surface photovoltage (SPV) spectroscopy technique may fulfill the demands for the organic devices. SPV technique is a wellestablished contactless method for the characterization of semiconductors, which relies on the analysis of the photo-induced changes in surface voltage [15]. The SPV measurements can give information concerning not only the photo-induced carrier excitation and transport processes, but also on the electronic structure of the surface/interface as well as the bulk. As a result,
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SPV technique has been used as an efficient tool for characterizing the optoelectronic and electronic properties of inorganic semiconductors [15,16]. Recently, this technique has also been used to investigate the electronic structure and photo-induced charge transport processes of organic semiconductors [17–19]. In this paper, we have used the SPV technique to study the organic film of N,N-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) evaporated on ITO substrate. The transport of photo-generated carriers and the electronic structure in the organic film have been studied. 2. Experiment Me-PTCDI was kindly provided by Prof. Mang Wang from Zhejiang University, and it was synthesized according to the reported procedure [20] and purified by temperature gradient sublimation. The molecular structure of Me-PTCDI was depicted in the inset of Fig. 2. Me-PTCDI films were vacuum evaporated at a pressure about 5 × 10−4 Pa and a temperature about 200 ◦ C onto ITO substrates, ˚ s−1 . Samples of three thicknesses were faband the deposition rate was 1–2 A ricated: 456 nm (thick), 135 nm (medium), 50 nm (thin), and the different film thickness was controlled by the different deposition time, and monitored independently by quartz crystal oscillators. The uncertainties of the film thickness measurements are about ±10%. UV–vis spectra were taken on a Varian CARY 100 Bio spectrophotometer. The transient SPV measurements were carried out in house system (Fig. 1a). Spectral SPV measurements were carried out in a standard parallel plate capacitor arrangement that was like a sandwich (Fig. 1b). It consists of the Me-PTCDI layer on ITO, a 10 m thick mica spacer and a platinum wire gauze electrode (with the transparency of about 40%, and the diameter of about 5 mm). The platinum wire gauze worked as the transparent electrode. A 500 W Xe lamp with a double grating monochromator was used to excite the spectral depen-
Fig. 2. The UV–vis absorption spectrum of Me-PTCDI film with the thickness of 50 nm. The inset is the molecular structure of Me-PTCDI.
dent SPV in the wavelength range of 400–800 nm. The light was chopped by a chopper (SR 540, Stanford Research Systems Inc.). The spectral SPV signals and the phase of the SPV were recorded by lock-in amplifier (SR 830, Stanford Research Systems Inc.). During the measurement, the gauze platinum electrode was connected to the detector and the ITO electrode was grounded. Illumination was performed through the gauze platinum electrode (top illumination) or through the ITO electrode (bottom illumination), which is depicted in Fig. 1b. The work function measurements were carried out on a commercial Kelvin Probe (KP) technique (KP Technology Ltd., England). The Kelvin Probe is a vibrating capacitor device that is used to measure the work function difference between the sample and vibrating tip (gold tip with the work function of 5.1 eV is used in this paper). The details of the measurement mechanisms and the setup of the KP technique have been described in Ref. [15]. All the measurements were performed at ambient temperature and atmospheric pressure under atmospheric ambient and at ambient.
3. Results and discussion
Fig. 1. The setup diagrams of the surface photovoltage technique (a), and the marked part in (a) is the sandwiched sample and the details are depicted in (b). The illumination geometries used in this paper are also depicted in (b).
Fig. 2 shows the UV–vis absorption spectrum of the thin sample. In this spectrum three bands could be distinguished, at 460, 480 and 570 nm, the former two of which are strongly overlapping. This is consistent with the previous reports [21,22], and these absorption bands are attributed to –* transition. The abrupt increase of the light absorption at the wavelength of 600 nm indicates that the band gap of Me-PTCDI is about 2.0 eV, which is consistent with a previous report [21]. The broad tail at the wavelength longer than 600 nm indicates that defect states exist in band gap of this Me-PTCDI film. Fig. 3a shows the SPV spectra for thick Me-PTCDI film measured under top and bottom illumination. It is interesting to observe the quite different SPV responses when the sample is illuminated from the top and bottom side. The profile of the SPV spectrum resembles the absorption spectrum (Fig. 2), when the sample is under bottom illumination. However, when the sample is under top illumination, the SPV spectrum differs markedly from the absorption spectrum. The signal is much weaker showing peaks at 610, 475 and 410 nm. We note that the change of the illumination from the top to the bottom side has no effects on the UV–vis absorption.
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Fig. 3. (a) The SPV spectra of thick Me-PTCDI film under top and bottom illumination and (b) the phases of the SPV signals in (a).
It is well known that the formation of the SPV depends not only on the light absorption in the sample, but also on the photoinduced carrier transport which is determined by the electronic structure in the absorption region [15]. Therefore, the results indicate that the transports of the excess carriers excited by the light strongly absorbed by Me-PTCDI under top and bottom illumination may be charged by the different electronic structures, but the transports of the excess carriers excited by the light weakly absorbed by Me-PTCDI under top and bottom illumination may be charged by the same electronic structures. This may be further proved by the phase measurement of the SPV signals under top and bottom illumination (Fig. 3b). It is clearly observed that at the wavelength between 570 and 425 nm where Me-PTCDI strongly absorbs, the difference between the phase of SPV under top and bottom illumination is more than 160◦ which is close to 180◦ . This suggests that the excess carriers excited by the light strongly absorbed by Me-PTCDI under top and bottom illumination move in the opposite directions [19]. However, at the wavelength longer than 600 nm, almost no difference is observed between the phase of the SPV under top and bottom illumination. This indicates that the separation of the excess carriers excited by the light at this wavelength range move in the same directions [19]. These may be explained as follows. For a uniform bulk sample, far thicker than the absorption length (α−1 ), the top and bottom illumination are expected to excite the excess carriers at the free surface/air interface (surface) and Me-PTCDI/ITO interface (interface) region, respectively. However, for the thin film sample, thinner than α−1 , the excess carriers may be excited uniformly in the film. As a result, the top and bottom illumination have no effect on the generation site of the excess carriers. Hence the change of the illumination geometry should have no effect on the SPV. According to the Beer’s law, the absorption length of the light of 470 nm strongly absorbed by Me-PTCDI can be deduced to be about 145 nm, which is thinner than the film thickness. Thus, at the wavelength strongly absorbed by Me-PTCDI, the top and bottom illumi-
Fig. 4. The SPV spectra of the medium (a) and thin (c) Me-PTCDI films under top and bottom illumination, and the phase of the SPV signals for medium (b) and thin (d) film.
nation may excite the excess carriers only at the surface and interface region, respectively, and the different SPV responses under the top and bottom illumination demonstrate the different transport of the excess carriers at the surface and interface region. However, the absorption length of the light of 608 nm light weakly absorbed by Me-PTCDI is about 405 nm, which is almost equal to the film thickness, and the absorption length will increase with increasing the wavelength. That is, the light at the band edge of Me-PTCDI is absorbed uniformly in the film. According to the above discussion, the change of the illumination geometry has no effects on the SPV, which is in good agreement with our experiment results. We also measured the SPV spectra as well as the phase of the SPV signals for medium and thin film under top and bottom illumination (Fig. 4a–d). It is clearly observed that with decreasing the film thickness, the profiles of the SPV spectra under top and bottom illumination resemble better and better with each other
Q. Zhang et al. / Materials Chemistry and Physics 100 (2006) 230–235
appear more similar (Fig. 4a and c), and the difference between the phase of the SPV signal under top and bottom illumination becomes more and more unconspicuous also becomes smaller (Fig. 4b and d). This is because the light absorption in the film is uniform, owing to the films are thinner than α−1 even at the wavelength strongly absorbed by Me-PTCDI. This is consistent with the above discussion. The above results indicate that the photo-induced carriers can be separated at both the surface and the interface region, and the higher magnitude of the SPV under bottom illumination than that of the SPV under top illumination suggests that the separation efficiency of the photo-induced carriers at interface region is higher than that of the photo-induced carriers at surface region [15]. It is well known that for the conventional inorganic semiconductors, the SPV may result from the separation of the photo-generated carriers assisted by the built-in electric field, owing to the band bending existing at the surface and/or at the interface [15]. However, for the organic semiconductors, whether or not the band bending can be formed is still in debate. Usually, it is considered that it is the interface dipole, rather than band bending, that exists at the interface of the organic film/metal or organic semiconductor substrate [8,13]. If this is the case for our samples, because the interface dipole is formed within several atomic thicknesses, the majority of the photoinduced carriers are generated and separated in the region which is electrically neutral in equilibrium. According to the SPV formation mechanism [15], the excess carrier should be separated by the diffusion of the excess electrons and holes with different diffusion coefficients, and the SPV should be the Dember photovoltage. The sole driving force for separation of the excess carrier is the charge carrier concentration gradient caused by the nonuniform absorption of light in the sample. If it is assumed that the diffusion coefficient of the electron is larger than that of the hole in Me-PTCDI, top and bottom illumination would cause the electron to be more close to the ITO electrode and the platinum wire gauze electrode, respectively. This also causes the difference between the phases of SPV signals measured under top and bottom illumination to be 180◦ , which is consistent with the above experiment result. However, using the theory of the Dember photovoltage, it cannot be reasonably explained why the change of the illumination geometry can cause so significant a change of the magnitude of the SPV response at the wavelength strongly absorbed by Me-PTCDI. Instead, if it is assumed that the excess carriers were separated by the built-in electric fields existing at the surface and the interface, the results would be explained reasonably. The fact that the excess carriers at the surface and the interface region are separated in the opposite directions indicates that the builtin electric fields existing at the surface and the interface have the opposite directions. The higher separation efficiency of the excess carriers at the interface than that of the excess carriers at the surface suggests that the strength of the electric field existing at the interface is stronger than that of the electric field existing at the surface. In order to further confirm whether or not there is an electric field in the Me-PTCDI/ITO film, the work functions of these three Me-PTCDI/ITO films as well as the bare ITO were
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Fig. 5. Work functions measured by Kelvin Probe method (KPM) for bare ITO and Me-PTCDI/ITO films with different thickness. The dotted line is presented to guide the eyes.
measured by Kelvin Probe (KP) method (Fig. 5). It is clearly observed that with increasing the film thickness, the work function decreases and tends to saturate at thicker film thickness. This is an indication of the existence of the built-in electric field at the interface [23]. Further, from the KP measurements, it can be inferred that the work function of the ITO is larger than that of Me-PTCDI. According to the Schottky model, when the MePTCDI is brought in contact with ITO, an n-type depletion region will be formed in Me-PTCDI layer, i.e., an upward band bending exists at the interface. That is, a built-in electric field is formed at the interface directed from Me-PTCDI to ITO. It can be inferred from the thickness dependence of the measured work function that the built-in electric field extends over a thickness of about 200 nm. This is in agreement with the recent report on the width of the interface space charge region of C60 layers evaporated on metal substrates (100 nm) [24]. According to the above discussion, it can also be concluded that the built-in electric field formed at the surface directs from the Me-PTCDI to the surface. That is, the surface band bending is also upward, i.e., a depletion region exists at the surface. It is clearly shown that the formation of the interface depletion region causes the work function to decrease with increasing the film thickness. On the contrary, the formation of the surface depletion region is expected to cause the work function to increase with increasing the film thickness. However, such surface depletion region is not evident from the results of the work function measurements. Nevertheless, the fact that the work function of bare ITO is larger than the work function of the Me-PTCDI means that the interface band bending is larger than the surface band bending. This is consistent with the above discussion that the strength of the electric field existing at the interface is stronger than that of the electric field existing at the surface. Therefore, when the excess carriers are excited at the interface (the surface), the excess electrons will move to the bulk of the film, and the excess holes will move to the interface (the surface). From the phase of the SPV signal under bottom illumination
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of 456 nm at the wavelength strongly and weakly absorbed by Me-PTCDI. This is attributed to the different absorption lengths at these wavelength regions. Taking into account the change of the amplitude (about 15 times) and the phase (about 180◦ ) of the SPV at the wavelength strongly absorbed by Me-PTCDI, we conclude that the photo-induced charge carriers are not separated by Dember potential, but by the action of the built-in electric field. This is further confirmed by the dependence of the work function on the film thickness. Based on these results, we infer that the surface and the interface of Me-PTCDI/ITO film are all depleted with the interface barrier height being larger than surface barrier height, and the width of the interface space charge region is about 200 nm. In addition, it is also inferred that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface. Acknowledgements Fig. 6. The energy diagram of thick Me-PTCDI film, where VS and VS is the barrier height at the interface and surface, respectively.
(Fig. 3b), it can be clearly observed that hardly any phase difference is observed between the SPV resulting from the band–band transition and the defect state transition. This indicates that this defect state transition is due to the photo-excitation of electrons from an occupied interface acceptor state to the conduction band of Me-PTCDI. Subsequently, the holes are trapped in the defect state, while the electrons are swept to the bulk of the film by the action of the built-in electric field. In fact, we have proved that the defect state also exists at the surface, using the transient SPV measurements. This will be discussed elsewhere [25]. Almost no observation of the SPV resulting from the transition of defect state at the surface is attributed to the lower separation efficiency of the excess carriers at the surface as compared to that at the interface. According to the UV–vis absorption and the SPV spectra measurements, it can be inferred that the transition energy locates the gap state at 0.1 eV above the valence band. Therefore, above discussion about the band structure of Me-PTCDI/ITO film is depicted in Fig. 6. The present results, together with the previous reports [17,18,23,24,26,27], demonstrate the existence of the band bending of Me-PTCDI films. It should be noted that whether or not the band bending in organic semiconductors can be understood as the band bending in conventional inorganic semiconductors is still in debate. It has been previously proposed that the formation of the band bending in -conjugated organic semiconductor was similar to that in inorganic semiconductors [27]. 4. Conclusion We have characterized the electronic structure and photoinduced charge carriers transport of Me-PTCDI film evaporated on ITO substrate, using SPV technique. It is found that the change of the illumination geometry has different influence on the SPV spectrum of the Me-PTCDI/ITO film with the thickness
The authors would like to thank Prof. Mang Wang of Zhejiang University for kindly supplying us the sample, and thank Dr. Xiaoke Hou for helping us to evaporate the film. This work is supported by National Natural Science Foundation of China (Nos. 20273027 and 20473033), the Research Fund for the Doctoral Program of Higher Education (RFDP) of China (No. 020183008) and the Science and Technology Developing Funding of Jilin Province (No. 20040503). References [1] C.W. Tang, Appl. Phys. Lett. 48 (1986) 183. [2] A. Yakimov, S.R. Forrest, Appl. Phys. Lett. 80 (2002) 1667. [3] A.J. Breeze, A. Salomn, D.S. Ginley, B.A. Gregg, H. Tillmann, H.H. H¨orhold, Appl. Phys. Lett. 81 (2002) 3085. [4] L. Schmidt-Mende, A. Fechtenk¨otter, K. M¨ullen, E. Moons, R.H. Friend, J.D. MacKenzie, Science 293 (2001) 1119. [5] P. Ranke, L. Bleyl, J. Simmerer, D. Haarer, A. Bacher, H.W. Schmidt, Appl. Phys. Lett. 71 (1997) 1332. [6] J.H. Sch¨on, C. Kloc, B. Batlogg, Appl. Phys. Lett. 77 (2000) 3776. [7] R. Schlaf, C.D. Merrittt, L.A. Crisafulli, Z.H. Kafafi, J. Appl. Phys. 86 (1999) 5678. [8] I.G. Hill, A.J. M¨akinen, Z.H. Kafafi, Appl. Phys. Lett. 77 (2000) 1825. [9] T.U. Kampen, G. Gavrila, H. M´endez, D.R.T. Zahn, A.R. Vearey-Roberts, D.A. Evans, J. Wells, I. McGovern, W. Braun, J. Phys. Condens. Matter 15 (2003) S2679. [10] H. Peisert, M. Knupfer, T. Schwieger, J. Fink, Appl. Phys. Lett. 80 (2002) 2916. [11] S. Piclzzi, A. Pecchia, M. Gheorghe, A. Di Carlo, P. Lugli, B. Delley, M. Elstner, Phys. Rev. B 68 (2003) 195309. [12] J.Y. Kim, I.J. Chung, Y.C. Kim, J.-W. Yu, Chem. Phys. Lett. 398 (2004) 367. [13] B.A. Gregg, Appl. Phys. Lett. 67 (1995) 1271. [14] L. Kronic, M. Leibovitch, E. Fefer, L. Burstein, Y. Shapira, J. Electron. Mater. 24 (1995) 379. [15] L. Kronic, Y. Shapira, Surf. Sci. Rep. 37 (1999) 1. [16] Q. Zhang, D. Wang, X. Wei, T. Xie, Z. Li, Y. Lin, M. Yang, Thin Solid Films 491 (2005) 242. [17] J. Yang, K.C. Gordon, A.J. McQuillan, Y. Zidon, Y. Shapira, Phys. Rev. B 71 (2005) 155209. [18] J. Yang, I. Shalish, Y. Shapira, Phys. Rev. B 64 (2001) 035325. [19] Q. Zhang, D. Wang, J. Xu, J. Cao, J. Sun, M. Wang, Mater. Chem. Phys. 82 (2003) 525.
Q. Zhang et al. / Materials Chemistry and Physics 100 (2006) 230–235 [20] H.E. Katz, J. Johnson, A.J. Lovinger, W. Li, J. Am. Chem. Soc. 122 (2000) 7787. [21] M. Hiramoto, K. Ihara, H. Fukusumi, M. Yokoyam, J. Appl. Phys. 78 (1995) 7153. [22] E. Lifshitz, A. Kaplan, E. Ehrenfreund, D. Meissner, J. Phys. Chem. B 102 (1998) 967. [23] E. Moons, A. Goossens, T. Savenije, J. Phys. Chem. B 101 (1997) 8492.
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[24] H. Ishii, N. Hayashi, E. Ito, Y. Washizu, K. Sugi, Y. Kimura, M. Niwano, Y. Ouchi, K. Seki, Phys. Stat. Sol. (a) 201 (2004) 1075. [25] Q. Zhang, D. Wang, X. Wei, Q. Zhao, Y. Lin, M. Yang, J. Chem. Phys. B, submitted for publication. [26] R. Schlaf, P.G. Schroeder, M.W. Nelson, B.A. Parkinson, P.A. Lee, K.W. Nebesny, N.R. Armstrong, J. Appl. Phys. 86 (1999) 1499. [27] J. Kanicki, in: T.A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986, p. 543.
The electronic and optoelectronic properties study of N,N-dimethylperylene-3,4,9,10-dicarboximide/ITO film using surface photovoltage technique Qinglin Zhang, Dejun Wang ∗ , Xiao Wei, Qidong Zhao, Yanhong Lin, Min Yang College of Chemistry, Jilin University, Changchun 130023, PR China Received 13 July 2005; received in revised form 24 November 2005; accepted 19 December 2005
Abstract The electronic structure and photo-induced charge carriers transport have been studied in N,N-dimethylperylene-3,4,9,10-dicarboximide (MePTCDI) films evaporated on ITO substrate using surface photovoltage (SPV) technique. For the Me-PTCDI film with the thickness of 456 nm, with changing the illumination geometry, the amplitudes as well as the phases of the SPV response bands at the wavelength where Me-PTCDI strongly absorbs significantly changed, while those of the SPV response bands at the wavelength where Me-PTCDI weakly absorbs hardly changed. It was explained based on the different absorption lengths of Me-PTCDI at different wavelengths. The experiment results suggest that the photo-induced charge carriers generated at the surface and the interface region are separated by the action of the built-in electric field and move in the opposite directions. From the dependence of the work function on the film thickness, it is inferred that the interface and surface are all depleted with the interface space charge region extending to be about 200 nm. In addition, it is found that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface. © 2005 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Surface; Electronic structure; Thin films
1. Introduction Perylene and its derivatives, as organic n-type semiconductors with high optical and thermal stability, have attracted much attention due to their potential applications in various electronic and optoelectronic devices, such as the organic solar cells [1–4], organic light-emitting diodes (OLEDs) [5], and organic fieldeffect transistors [6]. The understanding of the transport of the photo-induced charge carriers and the related electronic structure in organic semiconductors is crucially important for the optimization of the devices’ performance. As a result, many studies have focused on the electronic structure and the transport of the photo-induced charge carriers of perylene (or its derivatives) films evaporated on the metals or inorganic semiconductors substrates. The organic films have been studied using the ultraviolet photoemission spectroscopy (UPS) and X-ray photoelectron spectroscopy
∗
Corresponding author. Tel.: +86 431 8499172; fax: +86 431 8499158. E-mail address: [email protected] (D. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.12.034
(XPS) [7–11]. Although they can give the information on the organic/metal or inorganic semiconductor interface electronic structure and electrostatic information, such as interface dipole, work function, electronic energy level, and chemical reaction at interfaces, they do not provide insight into the transport and excitation processes of the photo-induced carriers. The measurement of photocurrent allows one to investigate the photo-induced carrier excitation and transport processes in organic films [12,13]. Nevertheless, ohmic contact between the electrode and the tested sample, required for that technique may substantially change the surface/interface electronic structures [14]. Therefore, an informative and nondestructive characterization technique for these organic devices is desirable. Surface photovoltage (SPV) spectroscopy technique may fulfill the demands for the organic devices. SPV technique is a wellestablished contactless method for the characterization of semiconductors, which relies on the analysis of the photo-induced changes in surface voltage [15]. The SPV measurements can give information concerning not only the photo-induced carrier excitation and transport processes, but also on the electronic structure of the surface/interface as well as the bulk. As a result,
Q. Zhang et al. / Materials Chemistry and Physics 100 (2006) 230–235
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SPV technique has been used as an efficient tool for characterizing the optoelectronic and electronic properties of inorganic semiconductors [15,16]. Recently, this technique has also been used to investigate the electronic structure and photo-induced charge transport processes of organic semiconductors [17–19]. In this paper, we have used the SPV technique to study the organic film of N,N-dimethylperylene-3,4,9,10-dicarboximide (Me-PTCDI) evaporated on ITO substrate. The transport of photo-generated carriers and the electronic structure in the organic film have been studied. 2. Experiment Me-PTCDI was kindly provided by Prof. Mang Wang from Zhejiang University, and it was synthesized according to the reported procedure [20] and purified by temperature gradient sublimation. The molecular structure of Me-PTCDI was depicted in the inset of Fig. 2. Me-PTCDI films were vacuum evaporated at a pressure about 5 × 10−4 Pa and a temperature about 200 ◦ C onto ITO substrates, ˚ s−1 . Samples of three thicknesses were faband the deposition rate was 1–2 A ricated: 456 nm (thick), 135 nm (medium), 50 nm (thin), and the different film thickness was controlled by the different deposition time, and monitored independently by quartz crystal oscillators. The uncertainties of the film thickness measurements are about ±10%. UV–vis spectra were taken on a Varian CARY 100 Bio spectrophotometer. The transient SPV measurements were carried out in house system (Fig. 1a). Spectral SPV measurements were carried out in a standard parallel plate capacitor arrangement that was like a sandwich (Fig. 1b). It consists of the Me-PTCDI layer on ITO, a 10 m thick mica spacer and a platinum wire gauze electrode (with the transparency of about 40%, and the diameter of about 5 mm). The platinum wire gauze worked as the transparent electrode. A 500 W Xe lamp with a double grating monochromator was used to excite the spectral depen-
Fig. 2. The UV–vis absorption spectrum of Me-PTCDI film with the thickness of 50 nm. The inset is the molecular structure of Me-PTCDI.
dent SPV in the wavelength range of 400–800 nm. The light was chopped by a chopper (SR 540, Stanford Research Systems Inc.). The spectral SPV signals and the phase of the SPV were recorded by lock-in amplifier (SR 830, Stanford Research Systems Inc.). During the measurement, the gauze platinum electrode was connected to the detector and the ITO electrode was grounded. Illumination was performed through the gauze platinum electrode (top illumination) or through the ITO electrode (bottom illumination), which is depicted in Fig. 1b. The work function measurements were carried out on a commercial Kelvin Probe (KP) technique (KP Technology Ltd., England). The Kelvin Probe is a vibrating capacitor device that is used to measure the work function difference between the sample and vibrating tip (gold tip with the work function of 5.1 eV is used in this paper). The details of the measurement mechanisms and the setup of the KP technique have been described in Ref. [15]. All the measurements were performed at ambient temperature and atmospheric pressure under atmospheric ambient and at ambient.
3. Results and discussion
Fig. 1. The setup diagrams of the surface photovoltage technique (a), and the marked part in (a) is the sandwiched sample and the details are depicted in (b). The illumination geometries used in this paper are also depicted in (b).
Fig. 2 shows the UV–vis absorption spectrum of the thin sample. In this spectrum three bands could be distinguished, at 460, 480 and 570 nm, the former two of which are strongly overlapping. This is consistent with the previous reports [21,22], and these absorption bands are attributed to –* transition. The abrupt increase of the light absorption at the wavelength of 600 nm indicates that the band gap of Me-PTCDI is about 2.0 eV, which is consistent with a previous report [21]. The broad tail at the wavelength longer than 600 nm indicates that defect states exist in band gap of this Me-PTCDI film. Fig. 3a shows the SPV spectra for thick Me-PTCDI film measured under top and bottom illumination. It is interesting to observe the quite different SPV responses when the sample is illuminated from the top and bottom side. The profile of the SPV spectrum resembles the absorption spectrum (Fig. 2), when the sample is under bottom illumination. However, when the sample is under top illumination, the SPV spectrum differs markedly from the absorption spectrum. The signal is much weaker showing peaks at 610, 475 and 410 nm. We note that the change of the illumination from the top to the bottom side has no effects on the UV–vis absorption.
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Fig. 3. (a) The SPV spectra of thick Me-PTCDI film under top and bottom illumination and (b) the phases of the SPV signals in (a).
It is well known that the formation of the SPV depends not only on the light absorption in the sample, but also on the photoinduced carrier transport which is determined by the electronic structure in the absorption region [15]. Therefore, the results indicate that the transports of the excess carriers excited by the light strongly absorbed by Me-PTCDI under top and bottom illumination may be charged by the different electronic structures, but the transports of the excess carriers excited by the light weakly absorbed by Me-PTCDI under top and bottom illumination may be charged by the same electronic structures. This may be further proved by the phase measurement of the SPV signals under top and bottom illumination (Fig. 3b). It is clearly observed that at the wavelength between 570 and 425 nm where Me-PTCDI strongly absorbs, the difference between the phase of SPV under top and bottom illumination is more than 160◦ which is close to 180◦ . This suggests that the excess carriers excited by the light strongly absorbed by Me-PTCDI under top and bottom illumination move in the opposite directions [19]. However, at the wavelength longer than 600 nm, almost no difference is observed between the phase of the SPV under top and bottom illumination. This indicates that the separation of the excess carriers excited by the light at this wavelength range move in the same directions [19]. These may be explained as follows. For a uniform bulk sample, far thicker than the absorption length (α−1 ), the top and bottom illumination are expected to excite the excess carriers at the free surface/air interface (surface) and Me-PTCDI/ITO interface (interface) region, respectively. However, for the thin film sample, thinner than α−1 , the excess carriers may be excited uniformly in the film. As a result, the top and bottom illumination have no effect on the generation site of the excess carriers. Hence the change of the illumination geometry should have no effect on the SPV. According to the Beer’s law, the absorption length of the light of 470 nm strongly absorbed by Me-PTCDI can be deduced to be about 145 nm, which is thinner than the film thickness. Thus, at the wavelength strongly absorbed by Me-PTCDI, the top and bottom illumi-
Fig. 4. The SPV spectra of the medium (a) and thin (c) Me-PTCDI films under top and bottom illumination, and the phase of the SPV signals for medium (b) and thin (d) film.
nation may excite the excess carriers only at the surface and interface region, respectively, and the different SPV responses under the top and bottom illumination demonstrate the different transport of the excess carriers at the surface and interface region. However, the absorption length of the light of 608 nm light weakly absorbed by Me-PTCDI is about 405 nm, which is almost equal to the film thickness, and the absorption length will increase with increasing the wavelength. That is, the light at the band edge of Me-PTCDI is absorbed uniformly in the film. According to the above discussion, the change of the illumination geometry has no effects on the SPV, which is in good agreement with our experiment results. We also measured the SPV spectra as well as the phase of the SPV signals for medium and thin film under top and bottom illumination (Fig. 4a–d). It is clearly observed that with decreasing the film thickness, the profiles of the SPV spectra under top and bottom illumination resemble better and better with each other
Q. Zhang et al. / Materials Chemistry and Physics 100 (2006) 230–235
appear more similar (Fig. 4a and c), and the difference between the phase of the SPV signal under top and bottom illumination becomes more and more unconspicuous also becomes smaller (Fig. 4b and d). This is because the light absorption in the film is uniform, owing to the films are thinner than α−1 even at the wavelength strongly absorbed by Me-PTCDI. This is consistent with the above discussion. The above results indicate that the photo-induced carriers can be separated at both the surface and the interface region, and the higher magnitude of the SPV under bottom illumination than that of the SPV under top illumination suggests that the separation efficiency of the photo-induced carriers at interface region is higher than that of the photo-induced carriers at surface region [15]. It is well known that for the conventional inorganic semiconductors, the SPV may result from the separation of the photo-generated carriers assisted by the built-in electric field, owing to the band bending existing at the surface and/or at the interface [15]. However, for the organic semiconductors, whether or not the band bending can be formed is still in debate. Usually, it is considered that it is the interface dipole, rather than band bending, that exists at the interface of the organic film/metal or organic semiconductor substrate [8,13]. If this is the case for our samples, because the interface dipole is formed within several atomic thicknesses, the majority of the photoinduced carriers are generated and separated in the region which is electrically neutral in equilibrium. According to the SPV formation mechanism [15], the excess carrier should be separated by the diffusion of the excess electrons and holes with different diffusion coefficients, and the SPV should be the Dember photovoltage. The sole driving force for separation of the excess carrier is the charge carrier concentration gradient caused by the nonuniform absorption of light in the sample. If it is assumed that the diffusion coefficient of the electron is larger than that of the hole in Me-PTCDI, top and bottom illumination would cause the electron to be more close to the ITO electrode and the platinum wire gauze electrode, respectively. This also causes the difference between the phases of SPV signals measured under top and bottom illumination to be 180◦ , which is consistent with the above experiment result. However, using the theory of the Dember photovoltage, it cannot be reasonably explained why the change of the illumination geometry can cause so significant a change of the magnitude of the SPV response at the wavelength strongly absorbed by Me-PTCDI. Instead, if it is assumed that the excess carriers were separated by the built-in electric fields existing at the surface and the interface, the results would be explained reasonably. The fact that the excess carriers at the surface and the interface region are separated in the opposite directions indicates that the builtin electric fields existing at the surface and the interface have the opposite directions. The higher separation efficiency of the excess carriers at the interface than that of the excess carriers at the surface suggests that the strength of the electric field existing at the interface is stronger than that of the electric field existing at the surface. In order to further confirm whether or not there is an electric field in the Me-PTCDI/ITO film, the work functions of these three Me-PTCDI/ITO films as well as the bare ITO were
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Fig. 5. Work functions measured by Kelvin Probe method (KPM) for bare ITO and Me-PTCDI/ITO films with different thickness. The dotted line is presented to guide the eyes.
measured by Kelvin Probe (KP) method (Fig. 5). It is clearly observed that with increasing the film thickness, the work function decreases and tends to saturate at thicker film thickness. This is an indication of the existence of the built-in electric field at the interface [23]. Further, from the KP measurements, it can be inferred that the work function of the ITO is larger than that of Me-PTCDI. According to the Schottky model, when the MePTCDI is brought in contact with ITO, an n-type depletion region will be formed in Me-PTCDI layer, i.e., an upward band bending exists at the interface. That is, a built-in electric field is formed at the interface directed from Me-PTCDI to ITO. It can be inferred from the thickness dependence of the measured work function that the built-in electric field extends over a thickness of about 200 nm. This is in agreement with the recent report on the width of the interface space charge region of C60 layers evaporated on metal substrates (100 nm) [24]. According to the above discussion, it can also be concluded that the built-in electric field formed at the surface directs from the Me-PTCDI to the surface. That is, the surface band bending is also upward, i.e., a depletion region exists at the surface. It is clearly shown that the formation of the interface depletion region causes the work function to decrease with increasing the film thickness. On the contrary, the formation of the surface depletion region is expected to cause the work function to increase with increasing the film thickness. However, such surface depletion region is not evident from the results of the work function measurements. Nevertheless, the fact that the work function of bare ITO is larger than the work function of the Me-PTCDI means that the interface band bending is larger than the surface band bending. This is consistent with the above discussion that the strength of the electric field existing at the interface is stronger than that of the electric field existing at the surface. Therefore, when the excess carriers are excited at the interface (the surface), the excess electrons will move to the bulk of the film, and the excess holes will move to the interface (the surface). From the phase of the SPV signal under bottom illumination
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of 456 nm at the wavelength strongly and weakly absorbed by Me-PTCDI. This is attributed to the different absorption lengths at these wavelength regions. Taking into account the change of the amplitude (about 15 times) and the phase (about 180◦ ) of the SPV at the wavelength strongly absorbed by Me-PTCDI, we conclude that the photo-induced charge carriers are not separated by Dember potential, but by the action of the built-in electric field. This is further confirmed by the dependence of the work function on the film thickness. Based on these results, we infer that the surface and the interface of Me-PTCDI/ITO film are all depleted with the interface barrier height being larger than surface barrier height, and the width of the interface space charge region is about 200 nm. In addition, it is also inferred that an acceptor state at 0.1 eV above the valence band exists at the surface as well as at the interface. Acknowledgements Fig. 6. The energy diagram of thick Me-PTCDI film, where VS and VS is the barrier height at the interface and surface, respectively.
(Fig. 3b), it can be clearly observed that hardly any phase difference is observed between the SPV resulting from the band–band transition and the defect state transition. This indicates that this defect state transition is due to the photo-excitation of electrons from an occupied interface acceptor state to the conduction band of Me-PTCDI. Subsequently, the holes are trapped in the defect state, while the electrons are swept to the bulk of the film by the action of the built-in electric field. In fact, we have proved that the defect state also exists at the surface, using the transient SPV measurements. This will be discussed elsewhere [25]. Almost no observation of the SPV resulting from the transition of defect state at the surface is attributed to the lower separation efficiency of the excess carriers at the surface as compared to that at the interface. According to the UV–vis absorption and the SPV spectra measurements, it can be inferred that the transition energy locates the gap state at 0.1 eV above the valence band. Therefore, above discussion about the band structure of Me-PTCDI/ITO film is depicted in Fig. 6. The present results, together with the previous reports [17,18,23,24,26,27], demonstrate the existence of the band bending of Me-PTCDI films. It should be noted that whether or not the band bending in organic semiconductors can be understood as the band bending in conventional inorganic semiconductors is still in debate. It has been previously proposed that the formation of the band bending in -conjugated organic semiconductor was similar to that in inorganic semiconductors [27]. 4. Conclusion We have characterized the electronic structure and photoinduced charge carriers transport of Me-PTCDI film evaporated on ITO substrate, using SPV technique. It is found that the change of the illumination geometry has different influence on the SPV spectrum of the Me-PTCDI/ITO film with the thickness
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