Al structures using optical and capacitance spectroscopy

Synthetic Metals 138 (2003) 33–37

Characterization of ITO/CuPc/AI and ITO/ZnPc/Al structures using optical and capacitance spectroscopy F.T. Reisa,*, D. Mencaragliaa, S. Oould Saada, I. Se´guyb, M. Oukachmihb, P. Jolinatb, P. Destruelb Laboratoire de Ge´nie E´lectrique de Paris (CNRS, UMR 8507)—E´cole Supe´rieure d’E´lectricite´, Universite´s Paris VI et Paris XI, Plateau de Moulon, F-91192 Gif-sur-Yvette Cedex, France b Laboratoire de Ge´nie E´lectrique de Toulouse (CNRS, UMR 5003)—Universite´ Paul Sabatier, 118 Route de Narbonne, 31062 Toulouse Cedex, France

a

Abstract Metal-substituted phthalocyanine (MPc) thin films as zinc- or copper-phthalocyanine are often used as charge injection layers for organic electroluminescent or photovoltaic devices. It is then important to characterize their electronic defect density and band structure near their gap. In this work the monolayer structures were prepared by vacuum sublimation of the organic thin film sandwiched between indium tin oxide (ITO) and aluminum electrodes. Electrically active defects were investigated with space-charge capacitance spectroscopy, as a function of temperature and frequency, in the range 80–330 K and 40 Hz to 10 MHz, respectively. Organic materials are best described on the basis of individual molecular orbital (HOMO and LUMO) energies instead of valence and conduction band. Such energies were derived from cyclic voltammetry and optical absorption spectroscopy measurements. Experimental results were correlated to the electrical J–V characteristics of these MPc based devices to gain more insight on the charge injection processes and their limitations. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Admittance spectroscopy; Metal-substituted phthalocyanine; Optoelectronic properties

1. Introduction Phthalocyanines (Pc) compounds with the formula MPc (M ¼ divalent metals) were traditionally used as dyes and pigments. MPc present a special interest due to its low cost and potentially high photoelectronic properties. They are also used for field-effect transistors [1], non-linear optics, catalysis [2] and molecular sensors [3]. Moreover, MPc are the most promising materials for future molecular optoelectronic devices like organic light emitting diodes (OLEDs) [4] and solar cells [5]. The importance of capacitance techniques for the study of inorganic semiconductors junctions is already well established. It has been used to determine band discontinuities and barriers height on heterostructures, electric active dopant densities, localized bulk or surface states densities, as well as the width of the space-charge region, for example [6]. Nevertheless, not as many detailed studies based on this technique have been reported on organic semiconductors devices. Some research groups have studied the capacitance–voltage * Corresponding author. Tel.: þ33-1698-516-80; fax: þ33-1694-183-18. E-mail address: [email protected] (F.T. Reis).

characteristics of PPV LEDs [7], CuPc (buffer layer)/NPB/ Alq3 devices [8] or PPHT Schottky junctions [9] and reported the obtained values of dopant densities, built-in potential and width of space-charge region. There are even fewer detailed analysis of the capacitance dependency on frequency for a large range of temperatures on organic semiconductor devices in the literature. Scherbel et al. [10] and Dyakonov et al. [11] have studied temperature dependent impedance spectroscopy on PPV LEDs and polymer–fullerene composites, respectively, but they both focused on the acquirement of the activation energies of the localized and ionized acceptor density of states. The most powerful methods to simplify the analysis in the case of a continuum of gap states take advantage of the existence of an abrupt cut-off frequency [12] and a corresponding abrupt cut-off abscissa, to distinguish between states that can follow the ac modulation frequency to those that cannot. The introduction of an abrupt cut-off model is possible when the dynamic response of the sample is controlled by a thermally activated physical parameter, such as the conductivity or the thermal emission rate of carriers towards the delocalized states. In this work we have analyzed our results with two methods based on an abrupt

0379-6779/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0379-6779(02)01284-5

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cut-off model. Firstly, to derive the gap states spectroscopy in a given energy range, we have used the method introduced by Walter et al. [13] to determine the defect distributions in Cu(In, Ga)Se2 based heterojunctions. Secondly, to determine the position of the Fermi level at equilibrium in these structures along with the density of localized states at this level, we have used the basis of a model developed by Cohen and Lang [14] to analyze the dynamic response of Schottky barriers with a continuum of gap states. To our knowledge, this is the first time that both models are applied to investigate the characteristics of the defect density of states in organic semiconductors. Having in mind the limitations of these models, we will present a brief discussion of their results when applied to our devices characterized by a very thin active layer thickness of the order of a few tens of nanometers. The detailed presentation and discussion of the assumptions underlying these models are beyond the scope of the present paper and this first analysis applied to organic semiconductors should be considered as a test of the applicability of the involved concepts.

w ¼ LD log

2. Experimental and theoretical details The organic films were deposited at a pressure of 106 Torr on glass substrates coated with two parallel strips of ITO. The ˚ s1) and deposition rate of the evaporated materials (2–3 A the thickness of each layer (a few tens of nanometers) were controlled by a quartz vibrating monitor placed near the samples. The upper electrodes were obtained by evaporation of aluminum through an appropriate mask. The resulting area S of each individual device is equal to 12 mm2. ˚ The optical absorption spectrum was recorded on 1000 A thick CuPc and ZnPc layers evaporated onto quartz slides, with a Shimadzu 2100 UV-Vis spectrophotometer in a twobeam configuration. The cyclic voltammetry experiments were performed with an EG&G 362 model potentiostat, at room temperature in a conventional three electrodes cell with MPc thin films on ITO as the working electrode. For the temperature dependent admittance spectroscopy measurements, the samples were placed in a liquid nitrogen Oxford cryostat, which enables the coverage of a temperature range between 80 and 480 K, under 105 mbar vacuum. An Agilent 4294A impedance analyzer was used in the parallel RC equivalent circuit mode to measure the zero dc bias complex admittance of the samples, in the frequency range between 40 Hz and 100 MHz. The amplitude of the ac signal was fixed at 50 mV. To evaluate the defect distribution we have used the following formula derived by Walter et al. [13]: NðEo Þ ¼

Vbi dðC=SÞ ewkT dðln oÞ

temperature T, and o the angular frequency of the ac signal. As we do not know precisely w, nor Vbi, because C–V measurements for reverse bias do not exhibit the classical behavior observed in doped crystalline materials, as a first guess Vbi was fixed equal to 0.4 V, of the order of the opencircuit voltage found in the literature for photovoltaic cells based on ZnPc monolayer [15], and an estimate of w was calculated as explained at the end of this section. We will keep in mind in the discussion, that this may introduce some additional uncertainty in the absolute values of the derived defect distribution. The density of states at Fermi level and the position of the bulk equilibrium Fermi level were derived from the capacitance measurements versus temperature and frequency using an expression of the capacitance obtained in the framework of the Cohen and Lang model [14], with the assumption of a slowly varying density of states around Fermi level [16]. In this case, LD being the Debye length associated to the localized states at Fermi level, an order of magnitude of the depletion width w can be obtained as [17]:

(1)

where Vbi is the built-in potential, e the electronic charge, w the space-charge region width, kT the Boltzmann factor at

  eVbi kT

(2)

This procedure to determine LD and w is valid only if the angular dielectric relaxation frequency of the sample is higher than o0, the cut-off angular frequency for which states at the bulk Fermi level begin to follow the ac modulation. The opposite case precludes these determinations but EF0 can still be derived if the bulk conductivity of the layer is thermally activated. Indeed, in this case, the thermal emission rate of states at the bulk Fermi level has to be replaced by the inverse of the dielectric relaxation time, i.e. by the ratio of the bulk conductivity to the dielectric permittivity of the layer [18].

3. Results and discussion 3.1. Optical measurements and J–V characteristics The energy gap was measured from the optical absorption curves, fitting data on the visible energy range. The Tauc energy gap EGopt is related to the absorbance a by the Eq. (3), where hn is the photon energy and A is a proportionality constant: ahn ¼ Aðhn  EGopt Þ2

(3)

The values of EGopt determined by the extrapolation of the linear region to zero absorption [19] are reported in Table 1. The energy levels for HOMO and LUMO of both materials were obtained by cyclic voltammetry with the method described by Bre´ das et al. [20]. For ZnPc HOMO ¼ 5:17 eV, while LUMO ¼ 3:78 eV. For CuPc HOMO ¼ 5:38 eV, and LUMO ¼ 3:77 eV. These values lead us to electrochemical gaps of 1.39 and 1.61 eV for ZnPc and CuPc, respectively,

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Table 1 Electrical and optical parameters derived from the experimental measurements (see text for details) and from the Poole–Frenkel-type fit of the J–V characteristics

ZnPc CuPc

EGopt (eV)

N (Etpeak) (eV1 cm3)

Etpeak (eV)

A (V1/2)

Cd/S (F cm2)

er

d (nm)

b (eV m1/2 V1/2)

1.54 1.52

8.2  1018 1.5  1018

0.319 0.320

13.4 8.2

7.3  108 2.9  108

2.0 2.1

24 64

5.3  105 5.2  105

which are in good agreement with those reported in the literature from photoemission spectroscopy and cyclic voltammetry measurements [21]. The dark J–V characteristics of the investigated devices at room temperature, are presented in Fig. 1. For both ZnPc and CuPc devices, the forward bias was obtained with the positive bias applied to the ITO contact. At low positive bias, V < 2 V for the ZnPc device and V < 1 V for the CuPc device, the current density, J, is slowly varying with the applied voltage. In this range, the bias dependence is characteristic of ohmic and space-charge limited conduction, two conduction mechanisms generally found in MPc thin films in this applied field range [22]. At a higher bias range between 2.3 and 4.3 V for the ZnPc device and 1.8–4 V for the CuPc device, we observed a change in the dominant conduction mechanism. In the inset of Fig. 1 the logarithmic scale of the current density is plotted against V1/2, showing a linear variation in this region. This type of behavior can be generally attributed to either a Poole–Frenkel (PF) or a Schottky effect [22]. Both mechanisms present a voltage variation described by J ¼ J0 expðAV 1=2 Þ

(4)

Fig. 1. Semi-logarithmic plot of the current density vs. bias for the ITO/ ZnPc/Al (open circles) and ITO/CuPc/Al (solid squares) devices. In the inset the semi-logarithmic plot of the current density as a function of V1/2 is presented.

where A ¼ b/kTd1/2, d is the film thickness and b is the field lowering coefficient, which is twice higher in the PF effect, when compared to the Schottky effect. bPF is defined as  bPF ¼

e3 per e0

1=2 (5)

where e is the electron charge, er is the relative dielectric constant and e0 is the vacuum permeability. From the J–V1/2 plot we have obtained the parameter A defined in Eq. (3) and consequently, the ratio bPF/d1/2. From the capacitance measurements at high frequency and low temperature (see Fig. 2 for the ZnPc case) we measure the value of the dielectric capacitance, which is defined as Cd ¼

er e0 S d

(6)

Then, with the derived values of A and of the ratio Cd/S, er and d can be obtained independently. The derived parameters are shown in Table 1. Generally, it is difficult to distinguish between the Poole–Frenkel and the Schottky effect [22]. Nevertheless, our results show that a Poole– Frenkel effect is more probable in the present case. Firstly the value of b ¼ 5:2  105 eV m1/2 V1/2 obtained for

Fig. 2. Capacitance spectra vs. frequency, at different temperatures between 146 and 327 K, for the ITO/ZnPc/Al device.

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CuPc is consistent with the values reported in the literature for Poole–Frenkel conduction in CuPc Schottky junctions [23]. Moreover, the Schottky effect should lead us to values of the film thickness that would be too low, in contradiction with the estimation from the monitoring of the thickness during the layers deposition. At this point we can notice that the optical and electrical parameters obtained up to now in this paper for both ZnPc and CuPc devices, show a great similarity between them. Also, the values found here agree well with those reported in the literature for both materials. 3.2. Capacitance spectroscopy In Fig. 2 we present the capacitance spectra at zero dc bias of the ZnPc device at temperatures ranging between 146 and 327 K, with steps of 10 K and for frequencies ranging between 40 Hz and 5  105 Hz. We can notice from the figure the presence of steps, which are due to the release of trapped charges. There is a shift of these steps to higher frequencies with increasing T, according to the temperature dependence of the detrapping time constants. At frequencies higher than 5  105 Hz the effect of a resonance begins to show, preventing an analysis in this region. Using the model of Walter et al. [13], from the minima position of the capacitance derivative respective to the frequency at each temperature, we derive the Arrhenius plot presented in Fig. 3. Both the activation energy and the thermal emission rate prefactor of a main defect contribution np can be derived. We have obtained Eact ¼ 319 meV, and np ¼ 7:6  109 s1 for the ZnPc device, and Eact ¼ 320 meV, np ¼ 2:6  1010 s1 for the CuPc device. We have neglected here the temperature dependence of np , since it does not affect significantly the results. Also from this model, scaling the energy scale with the frequency using the derived thermal

Fig. 3. Arrhenius plot of the inflexion point angular frequency, derived for the ITO/ZnPc/Al device.

Fig. 4. Density of localized states as a function of energy, for the ITO/ CuPc/Al (solid squares) and ITO/ZnPc/Al device (all other symbols). In the later case, all the spectra corresponding to temperatures between 205 and 295 K are presented. For clarity, only one temperature spectrum has been plotted for the CuPc device. The dashed vertical line corresponds to the position of the bulk Fermi level (see text).

emission prefactor, we have obtained the defect distribution, N(Et), as a function of the defect energy for different temperatures, which is presented in Fig. 4. The density of states at the peak of the defect distribution was found to be 8:2  1018 eV1 cm3 for the ZnPc device and 1:5  1018 eV1 cm3 for the CuPc device. Considering the similarity of the characteristics parameters of these defects distributions derived for both materials, a first interpretation could be that they are the signature of the same kind of defect. A very plausible candidate is an oxygen-related defect, owing to the well known high sensitivity of these MPc to this element, particularly in our devices fitted with an ITO electrode. An important point, however, to consider when performing ac space-charge capacitance spectroscopy, is the energy range that can be investigated by this technique. Indeed, the very general rule is that only states deeper than the energy equal to the Fermi level position in the neutral bulk can be probed. This comes from the fact that only these states can contribute to the steady-state space-charge formation. Then, it is crucial to know the Fermi level position in the bulk to assess the validity of the derived defect distribution. However, it is not an easy task to derive precisely the bulk Fermi level position in such thin films. Extrapolating this value to a sandwich structure configuration from coplanar measurements on such thin films is generally not valid, because interface states at the substrate, with a Debye length of the order of the thickness, may play a dominant role in both configurations. We have then derived an estimation of the ‘‘bulk’’ Fermi level position directly in our sandwich structures from the analysis of the capacitance measurements, using the model of Cohen and Lang [14] as mentioned in Section 2. For both ZnPc and CuPc devices, we have determined a ‘‘bulk’’ Fermi level position equal to 0.28 eV. As

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these materials are known to exhibit a p-type behavior, this value refers to the HOMO level. This identical value for both cases argues for a Fermi level position imposed by the boundary conditions at the interfaces (most probably due to oxygen doping at the MPc/ITO one). Considering the above discussion on the investigated energy range by the ac admittance spectroscopy, we then conclude that the distribution at the left of the vertical line located at 0.28 eV in Fig. 4 could be due to a cut-off imposed by the limit of the technique. One could then also question the reality of a peak in the derived spectroscopy as its energy position is very close to the derived ‘‘bulk’’ Fermi level one, their energy separation being of the order of the Boltzmann factor, kT, which controls the extent of the gap states occupancy at these finite temperatures. The alternative interpretation to the existence of a peak-like distribution is that the valid spectroscopy at the right of the ‘‘peak’’ represents an exponential distribution of traps, as often reported for MPc from space-charge limited current measurements on sandwich structures [22]. Under this hypothesis, the temperature parameter characteristic of these distributions, derived from Fig. 4, are equal to 350 and 510 K for ZnPc and CuPc, respectively. The extrapolated density of states at 0.28 eV on this exponential tail would then be equal to 4:5  1019 eV1 cm3 and 5  1018 eV1 cm3 for ZnPc and CuPc, respectively. The corresponding values at the Fermi level, directly derived from the Debye length (cf. end of Section 2) are respectively equal to 7  1018 and 1  1019 eV1 cm3. We note a poorer agreement for the thinnest sample (ZnPc), which can be explained by the fact that in this case the assumption of a semi-infinite thickness, implicit in both analysis of our capacitance measurements, is not fulfilled [16]. Given also the uncertainty due to the difficulty to derive a precise value of the built-in potential we can consider that the agreement is rather good, but further work will be necessary for a clear-cut interpretation of the results. In particular, it will be helpful to investigate devices exhibiting a larger range of active layer thickness and different preparation conditions.

4. Conclusions In this work we have studied the optoelectronical properties of both ITO/ZnPc/Al and ITO/CuPc/Al junctions. Using optical absorption and cyclic voltammetry techniques, we have obtained the gap energies EGopt ¼ 1:54 eV and EHOMOLUMO ¼ 1:39 eV for the ZnPc device and EGopt ¼ 1:52 eV; EHOMOLUMO ¼ 1:61 eV for the CuPc device. These values agree well with the ones previously reported for these materials. From the dark J–V measurements, for both devices we have observed a Poole–Frenkel type conduction in the bias range around 2–4 V. Associating the derived Poole–Frenkel coefficient with the dielectric capacitance determination, we have proposed a method to

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precisely derive independently the dielectric permittivitty and the thickness of the active layer. Combining two complementary analysis of ac admittance spectroscopy versus temperature and frequency, applied for the first time to our knowledge to organic materials, we have discuss their limitations and shown that these measurements can reveal very useful to gain some insight on the defect distribution. In particular, we have shown that the existence of an exponential band tail near the HOMO side of the bandgap, the characteristic temperatures of which have been derived to be equal to 350 and 510 K for ZnPc and CuPc, respectively, is a plausible interpretation of our data.

Acknowledgements This work was financially supported by the Brazilian agency CAPES and by the French Ministry of Research. We acknowledge also the French contribution of the STIC Department of CNRS. References [1] K. Kudo, M. Iizuka, S. Kuniyoshi, K. Tanaka, Thin Solid Films 393 (2001) 362. [2] T.J. Mafatle, T. Nyokong, J. Electroanal. Chem. 408 (1996) 213. [3] S. Pizzini, G.L. Timo, M. Beghi, N. Butta, C.M. Mari, J. Faltenmaier, Sens. Actuators 17 (1989) 481. [4] D. Ammerman, C. Rompf, W. Kowalsky, Jpn J. Appl. Phys. 34 (1995) 1293. [5] J. Rostalski, D. Meissner, Sol. Energ. Mat. Sol. C 61 (2000) 87. [6] J.R. MacDonald, Impedance Spectroscopy, Wiley, New York, 1987 (Chapter 1). [7] M. Meier, S. Karg, W. Riess, J. Appl. Phys. 82 (1997) 1961. [8] W. Bru¨ tting, H. Riel, T. Beierlein, W. Riess, J. Appl. Phys. 89 (2001) 1704. [9] G.D. Sharma, S.K. Gupta, M.S. Roy, Synth. Met. 95 (1998) 225. [10] J. Scherbel, P.H. Nguyen, G. Paasch, W. Bru¨ tting, M. Schwoerer, J. Appl. Phys. 83 (1998) 5045. [11] V. Dyakonov, D. Godovsky, J. Parisi, C. Brabec, N.S. Sariciftci, J.C. Hummelen, J. De Ceuster, E. Goodvaerts, Synth. Met. 121 (2001) 1529. [12] I.G. Gibb, A.R. Long, Philos. Mag. B 49 (1984) 565. [13] T. Walter, R. Herberholz, C. Mu¨ ller, H.W. Schock, J. Appl. Phys. 80 (1996) 441. [14] J.D. Cohen, D.V. Lang, Phys. Rev. B 25 (1982) 5321. [15] B.A. Gregg, Chem. Phys. Lett. 258 (1996) 376. [16] J.-P. Kleider, D. Mencaraglia, Z. Djebbour, J. Non-Cryst. Solids 114 (1989) 432. [17] P. Viktorovitch, G. Moddel, J. Appl. Phys. 51 (1980) 4847. [18] D. Mencaraglia, A. Amaral, J.-P. Kleider, J. Appl. Phys. 58 (1985) 1292. [19] S. Fujita, T. Nakuzawa, M. Asano, S. Fujita, Jpn. J. Appl. Phys. 34 (2000) 5301. [20] J.L. Bre´ das, R. Silbey, D.S. Boudreux, R.R. Chance, J. Am. Chem. Soc. 105 (1983) 6555. [21] L. Zhu, H. Tang, Y. Harima, Y. Kunugi, K. Yamashita, J. Ohshita, A. Kunai, Thin Solid Films 396 (2001) 213. [22] R.D. Gould, Coord. Chem. Rev. 156 (1996) 237. [23] A.K. Rassan, R.D. Gould, Int. J. Electron. 69 (1990) 11.