(CoPc) thin films

Eur. Phys. J. Appl. Phys. 37, 1–9 (2007) DOI: 10.1051/epjap:2006135

THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS

Structural and electrical properties of thermally evaporated cobalt phthalocyanine (CoPc) thin films H.S. Soliman1 , A.M.A. El-Barry2,a , N.M. Khosifan1 , and M.M. El Nahass2 1 2

Physics department, faculty of girls, Jedda, Saudi Arabia Kingdom Physics Department, Faculty of Education, Ain Shams University, Cairo, Egypt Received: 31 May 2006 / Received in final form: 22 August 2006 / Accepted: 28 August 2006 c EDP Sciences Published online: 15 November 2006 – Abstract. Thin films of CoPc of various thickness have been deposited onto glass substrates using thermal evaporation technique at room temperature. The dark electrical resistivity measurements were carried out at different temperature range (298–423 K). An estimation of mean free path (0 ) of charge carriers in CoPc thin films and bulk resistivity, ρB was attempted. Measurements of thermoelectric power confirm that CoPc thin films behave as p-type semiconductors. The ac conductivity (σac ) has been investigated in the frequency range (102 –106 Hz) and temperature range (298–407 K). σac is found to be proportional to ω s where s ≈ 0.879 which is frequency and temperature independence. The ac conductivity interpreted by the correlated barrier hopping (CBH) model with centers of intimate valence alternation pairs type with a maximum barrier height, WM ≈ 1.594 eV. PACS. 72.20.-i Conductivity phenomena in semiconductors and insulators – 77.55.+f Dielectric thin films – 73.61.-r Electrical properties of specific thin films

1 Introduction The phthalocyanines (Pc’s) have been become one of the most studied of all organic functional materials [1] and have recently attracted considerable interest due to their high thermal and chemical stability. Phthalocyanine exists in several crystalline polymorphs, including the α-, βand γ-structures [2]. Metal – substituted and metal – free phthalocyanine such as MnPc, CuPc, NiPc, FePc, CoPc, etc. have been studied and have been widely used as gas sensors, optical logic displays, solar energy conversion [3], colour filters and organic laser materials [4–8]. Most of the phthalocyanines are p-type due to an absorbed Oxygen which acts as an acceptor level in band gap [9]. The oxygen impurities, which are unavoidably introduced in the preparation of organic semiconductors [10] appear to play the dual role of both acceptors and traps levels [11]. However, these impurities, as well as voids and defects which give rise to extrinsic conductivity [12] can be removed by heat treatment. The Dc electrical properties of phthalocyanine have received the greatest attention [13,14], with most work to data focusing on metal-substituted phthalocyanine, such as Nickel phthalocyanine, NiPc [15] and Molybdenum phthalocyanine, MoPc [14]. However, CoPc has received considerable less attention [16]. The ac conductivity, σac , in various phthalocyanine films has been a

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shown to obey the law σac = Aω s , where ω is the angular frequency and s is an index less than unity. This behaviour was ascribed to the in homogeneity with the solid caused by the absence of long range crystalline order [17]. Carrier transport via a hopping mechanism was identified with this type of dielectric response [18], and Jonscher [19] has proposed that such dependence represents a universal law, applicable very wide materials irrespective of their chemical and physical structure and the type of dominant charge carrier. In the present work, the dc-conductivity, thermoelectric power and ac-conductivity measurements have been performed, on thermally evaporated CoPc thin films in the frequency range (102 –106 Hz) and over a temperature range (∼298–423 K), to determine some reasonable electrical parameters.

2 Experimental details The CoPc powder is obtained from Kodak company, UK. Thin films of CoPc have been prepared by thermal evaporation technique with thickness rang (∼62–∼518 nm). The deposition temperature was kept at room temperature. Used a high vacuum coating unit (Edwars type E 306A, England). Thin films were deposited from a quartz crucible source heated by a tungsten coil in a vacuum

Article published by EDP Sciences and available at http://www.edpsciences.org/epjap or http://dx.doi.org/10.1051/epjap:2006135

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The European Physical Journal Applied Physics

of 10−4 Pa. The deposition rate and the film thickness were controlled using a quartz crystal thickness monitor (Model FTM4, Edward co., England). X-ray diffractometer (Philips X’ pert), using Cu radiation operating at 40 kV and 30 mA, was used to investigate the structure. The electrical resistance, R of planner structure CoPc thin films of different thickness were measured in a temperature range (∼298–423 K), according to two point probe method and using an electrometer with high input impedance (Keithley 617). The ohmic contacts were made by evaporating gold electrodes. The electrical resistivity, ρ was determined by measuring the film resistance, R, where ρ=

RA L

(1a)

A is the active area of the film and L is the distance between the two Au-electrodes. In order to study the effect of annealing, CoPc films were heated, in vacuum, at 623 K for two hours. The thermoelectric power was measured using the differential technique with copper electrodes, based on the following equation [20]: d∆E ∗ = S12 = S2 (T12 ) − S1 (T12 ) dT

(1b)

where S12 is the relative thermoelectric power between the materials 1 and 2 at temperatures T1 and T2 , S1 (T12 ) and S2 (T12 ) are the thermoelectric power between the film and Cu electrode and ∆E ∗ is the electromotive force. The temperature T1 and T2 of the two ends were increased by using two different high power resistance R1 and R2 as a heat source and heat sink across the thin film under test. The film has a dimension ∼ 3.5 × 0.5 cm2 . The electromotive force, ∆E, associated with the temperature gradient along the film was measured using an electrometer (keithly 617). The temperatures, T1 and T2 , were measured using two cromel–alumel thermocouples. For ac measurements, CoPc film with ∼518 nm thickness were sandwiched between two evaporated Au electrodes. The ac conductivity and some dielectric properties were measured over 100 Hz to 1 MHz frequency range and 298 to 423 K temperature range. A programmable automatic RLC bridge was used to measure the impedance Z, the capacitance C, similar to that described in [21].

3 Results and discussion

Fig. 1. (a) XRD pattern of CoPc in the powder form at room temperature. (b) XRD pattern of as deposited CoPc film (∼518 nm) at room temperature. (c) XRD pattern of an annealed CoPc film (∼518 nm) at 623 K/2 h under vacuum.

˚ and β = 120.7◦ . X-ray a = 20.5 ˚ A, b = 4.78 ˚ A, c = 14.8 A diffraction patterns of as deposited CoPc thin film, with 518 nm thickness, as a representative example, is shown in Figure 1b. It can be seen that X-ray diffraction of CoPc film shows similar characteristics of that of CuPc, CoPc and H2 Pc films [2,22–24]. As observed from the figure, there is only a single peak and a prefer orientation around 2θ ≈ 6.88◦ . A similar observation was obtained in the structure study of α-phase of CoPc thin films deposited onto glass substrates held at room temperature [2]. Figure 1c shows that the X-ray diffraction pattern of the same CoPc thin film (with 518 nm), annealed at 623 K for two hours under vacuum. The figure shows that the degree of crystallinity increases with heat treating for the CoPc film. Also, there is only one significant peak, its intensity increased with the annealing process, while there is no change, (around 2θ ≈ 6.88◦ ), in the preferred orientation [25]. The mean crystallite size (L) is estimated by using the Scherrer’s expression [26]:

3.1 Structure investigation X-ray diffraction pattern derived from CoPc in the powder form is shown in Figure 1a. The powder of CoPc identified as a mixture of α-form and β-form. Lattice spacing dhkl were calculated using Bragg,s equation together with Miller indices as compared with the corresponding values given in Card No. 44-1994 for α-CoPc and ICCD Card No. 14-0948 for β-CoPc. Bragg,s angle, θ, lattice spacing, d, and a valuable Miller indices, hkl, are listed in Table 1. The analysis of the obtained data indicated that CoPc has a monoclinic system. The calculated lattice parameters are

L=

KS λ β cos(θ)

(2)

where λ the X-ray wavelength of CuKα(0.15418 nm), β is the width of the strong peak at half maximum intensity for the thin film, θ is the corresponding Bragg’s angle and KS is the Scherrer’s constant. The strain (ζ) was calculated from the relation [26]: β=

λ − ζ tan(θ). L cos(θ)

(3)

H.S. Soliman et al.: Structural and electrical properties of thermally evaporated cobalt...

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Table 1. The experimental Bragg, angles, the relative intensity and the available Miller indices in comparison with ICCD card No. 44-1994 for α-Co-Pc and 14-0948 for β-CoPc. ICCD card No. 44-1994 for α-CoPc 2Θ I/I0 6.8 1 7.3 0.72 10.00 0.17 15.6 0.26 16.2 0.24 24.1 0.29 24.8 0.14 25.3 0.2 26.8 0.46 27.8 0.58 -

ICCD card No. 14-0948 for β-CoPc 2Θ hkl I/I0 1 7.072 100 0.95 9.26 102 1 10.607 002 0.16 11.582 0.06 12.591 202 0.3 14.102 200 0.06 15.464 102 0.06 18.254 104 0.45 18.68 0.4 21.102 202 0.2 21.622 111 0.1 22.337 212 0.02 0.08 23.162 210 23.92 112 0.4 26.368 311 0.3 28.24 106 0.16 28.517 212 0.06 29.679 115 0.04 0.2 30.724 502 0.06 31.615 411 0.06 32.318 415 33.307 312 0.06

Also, the dislocation density (δ), which is defined as the length of dislocation lines per unit volume, was evaluated from the relation; δ = 1/L2 . Table 2 lists a comparison of the mean crystallite size (L), dislocation density (δ) and strain (ζ) for CoPc thin film (518 nm), before and after annealing, at 623 K for two hours under vacuum. The crystallite size increased from 13.99 to 97.92 nm by annealing compared with 24 nm in [27]. Moreover, other authors [28,29] have reported the values of the crystallite size in the order of 10 to 150 nm for various phthalocyanines. Similar observations for mean crystallite size were reported [30] for an α-CuPc films. While the dislocation density (δ) and the strain (ζ) decreased by annealing process indicates the formation of high quality films with annealing process. 3.2 Dark electrical resistivity measurements Figures 2a and 2b show the dependence of the planner electrical resistivity, ρ of CoPc films (as-deposited and annealed respectively) for film thickness, d, at different

Experimental results 2Θ 6.88 9.38 10.625 12.69 14.175 15.526 18.125 18.75 21.675 24.167 26.875 28.7 30.729 31.875 32.188 33.33

I/I0 1 1 0.092 0.13 0.05 0.025 0.184 0.18 0.02 0.025 0.08 0.05 0.065 0.026 0.039 0.021

Co-Pc form α-Co-Pc β-CoPc. β-CoPc. β-CoPc. β-CoPc. α-Co-Pc β-CoPc. β-CoPc. β-CoPc. α-Co-Pc α-Co-Pc β-CoPc. β-CoPc. β-CoPc. β-CoPc. β-CoPc.

temperatures. As illustrated the resistivity decreases with increasing film thickness in accordance with the results for inorganic [31] semiconductor films and organic films [32]. The behaviour indicates that the electrical resistivity exhibits size effect phenomena in CoPc films. According to Tellier’s [33] model for effective mean free path, which takes to account surface scattering in addition to bulk scattering, ρf , the resistivity, of a film of thickness, d, can be represented by the following relation [34,35]: ρf = ρB

1 + 3 0 (1 − p) 8d

(4)

where ρB is the bulk resistivity, 0 is the bulk electron mean free path and p is the specularity parameter, which gives the fraction of electrons incident on the surface that are specularity scattered. This equation is, in fact, valid only at certain limiting conductions, i.e. large film thickness and/or small mean free path. However, in our case, if we assume a complete diffuse scattering (p = 0) so that

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Table 2. The values the lattice parameters and the mean crystallite size (L), dislocation density (δ) and strain (ζ) for Co-Pc thin film (518 nm). Physical parameter State ˚) a (A b (˚ A) c (˚ A) β◦ L (nm)

Present work powder As deposited CoPc films ˚ a = 19.5 A – b = 4.78 ˚ A, – c = 14.8 ˚ A – β = 120.7◦ – – 13.99

(δ) × 10−4 (nm−2 ) (ζ) × 10−3

– –

51.09 136.6

Reference An annealed CoPc films at 623 K/2 h – – – – 97.92 1.04 52.24

˚ [27] a = 19.3 A b = 4.77 ˚ A [27] c = 14.54 ˚ A [27] β = 120.82◦ [27] 24 nm [27] (10–150) nm [28, 29] 1.28 nm−2 [27] 23.23 [27]

Fig. 2. (a) The dependence of the dark electrical resistivity of CoPc, ρ, on film thickness, d, for as-deposited films CoPc; the inset figures: ρ vs. 1/d. (b) The dependence of the dark electrical resistivity of CoPc, ρ, on film thickness, d, for annealed thin films of CoPc at 623 K/2 h, the inset figures: ρ vs. 1/d.

equation (4) can be rewritten as follows: 3ρB 0 ρf = ρB + . 8d

(5)

It is seen from these equations that ρf is a function of film thickness. Hence, a plot of ρf vs. 1/d should be a straight line at different temperatures as shown in the inset of Figure 2. Similar linear dependences of electrical conductivity were explained by Das and Bhat for H2 Pc [36,37]. The intercept and the slope of the inset of Figure 2 gives the ability to determine the bulk resistivity, ρB and the mean free path, 0 . Figures 3a, 3b show the variation of the mean free, 0 path vs. T for both as-deposited and annealed CoPc films respectively. For as deposited CoPc films the mean free path increases with temperature until a cer-

tain temperature ≥ 350 K, the mean free path starts to decrease with increasing of the temperature, which may be due to increasing of scattering for the charge carriers. On the other hand the mean free path for the annealed films increases with temperature to reach a value nearly equal to that observed for as deposited CoPc films. On the other hand, Figure 3b illustrates the effect of temperature on the bulk resistivity for both as deposited and annealed films. As compared the values of mean free path and bulk resistivity before and after annealing are listed in Table 3. Then, the electrical resistivity studies of planner CoPc thin films were performed to obtain the thermal activation energy. It was carried out in the temperature range (298–423 K) for films with different thickness ranging from 62 to 403 nm. The temperature dependence of the


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Table 3. Values of the bulk resistivity, ρf = ρB , and the mean free path, 0 , for as-deposited and annealed CoPc thin films, at different temperatures. As deposited CoPc T (K) ρB (Ω m) 298 4031 303 1938 313 1434 323 1072 333 548 343 506 353 355 363 322 373 320

films L0 (µm) 2.368 2.76 3.72 6.59 13.07 10 7 4.31 4.12

Annealed CoPc films ρB (Ω m) 0 (µm) 33796 0.41 28081 0.439 16612 0.464 11493 0.51 7613 0.416 5977 0.53 4206.9 0.82 1243 1.5 500.4 2.5

Fig. 3. (a) Variation of the mean free path, l0, vs. T for asdeposited and annealed CoPc films. (b) Variation of the bulk resistivity ρB vs. T for as-deposited and annealed CoPc films.

resistivity can be expressed by Arrhenius equation [38]: ∆E ρf = ρ0 exp (6a) kB T where ∆E is the thermal activation energy, ρ0 is the pre-exponentional resistivity and kB is the Boltzmann’s constant. A plot of log (ρf ) vs. (1000/T ) yields straight lines whose slope can be used to determine the thermal activation energy of the films. Figure 4a shows the dependence of the dark electrical resistivity, of as-deposited CoPc films of different thickness, on the temperature. As seen from the figure, there are two distinct linear parts, which correspond to two activation energies ∆E1 and ∆E2 , where equation (6a) can be represented as, ∆E1 ∆E2 ρf = ρ01 exp + ρ02 exp . (6b) kB T kB T The activation energies ∆E1 and ∆E2 were obtained at T < 350 K and T > 350 K respectively. ∆E1 corresponds to extrinsic conduction, and ∆E2 is corresponding to intrinsic conduction, and hence the activation energy is interpreted as a change from extrinsic to intrinsic conduction [39]. The value of the thermal activation energies,

Fig. 4. (a) Variation of the dark electrical resistivity, p, against 1000/T for as-deposited CoPc thin films. (b) Variation of the dark electrical resistivity, ρ, against 1000/T for as-deposited CoPc thin films, after annealing at 623 K/2 h.

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∆E1 is nearly equals to 0.15 ± 0.025 eV and ∆E2 is nearly equals to 0.74 ± 0.02 eV, which is in agreement with other workers [40–42] where the value of ∆E2 ≈ 1/2 the value of the onset optical gap which illustrated in [27]. The temperature dependence of the resistivity for annealed films shown in Figure 4b. The values of activation energies ∆E1 and ∆E2 decreased after annealing to 0.152 ± 0.006 eV, and 0.57 ± 0.04 eV respectively. 3.3 Thermoelectric power measurements Seebeck measurements are a powerful tool to characterize organic semiconductors [43–45] because they provide information about the energetic difference between the relevant transport state and the Fermi level in bulk material, regardless of the details of the transport mechanisms [46]. For transport in the valence band or, alternatively, hopping of holes at one energy level Eµ , S is given by [46]; −kB Ef − Eµ S= + Aµ (7) e kB T where S is Seebeck coefficient, Ef is the energy of Fermi level, Eµ is the energy of the transport state and Aµ is a constant [46]. The Seebeck coefficient directly reveals (i) the conduction type (n or p transport) by its sign and (ii) the energy difference between the Fermi level and the transport state labeled as Eµ . The variation of Seebeck coefficient, S, for CoPc thin films with temperature is shown in Figure 5. It can be seen from the figure that the value of S is positive over the entire temperature range as observed for other metal–phthalocyanine [39,47,48]. The positive value of Seebeck coefficient indicates p-type conduction of CoPc, i.e., (the conduction is due to holes moving in the valence states of the matrix molecules and not by a hopping of electrons between acceptor states). S continuously decreases with temperature, indicating a negative shift of the Fermi-level, Ef towards the transport level Eµ , The general behavior of conductivity and Fermi level thus seemingly follows the situation in inorganic semiconductors [49]. By combining Seebeck and conductivity results, we can determine the carrier density and the mobility, respectively. The only assumption in this analysis is that the effective density of states Nµ at the transport level Eµ is comparable to the density of molecules, provided each molecule contributes one state [50], where the calculated values of the effective density of states Nµ , in the present work ≈ (1.058 ± 2.67) × 1024 /m3 . The hole density can then be directly calculated from the Seebeck coefficient, neglecting Aµ and taking into account that the Maxwell Boltzmann approximation is justified since (Ef − Eµ ) (kB T ) [39,49,50]: −(Ef − Eµ ) −eS p = Nµ exp ≈ Nµ exp kB T kB 3/ 2 2Πm∗ kB T ∆E =2 exp (8) h2 kB T

Fig. 5. Variation of Seebeck coefficient, S, of CoPc thin films vs. temperature; the inset figure: variation of Seebeck coefficient, S, of CoPc thin films vs. 1000/T .

where mh * is the effective hole mass ≈ 0.1me [25] and h is Plank, s constant. ∆E is the thermal activation energy, which was obtained from temperature dependence of resistivity. In the intrinsic region, the Seebeck coefficient can be given as [39]; S=

−kB e

c−1 c+1

∆E +2 kB T

(9)

where c is the mobility ratio i.e., µe /µh . The application of the above equations lead to calculate the concentration of charge carrier, p. In addition, the value of c can be determined from the slope of the graph of S against 1/T in the intrinsic region [see inset of Figure 5, and then by substituting p and c in equations (8, 9) we obtain the holes mobility. The mean values of p, c and µh are 8.358×1023 m−3 , 0.972 and 4.42 × 10−11 m2 v−1 s−1 respectively for as deposited CoPc films. A positive value of Seebeck coefficient in the intrinsic region, can arise for a semiconductor in which the mobility of the positive charge carriers is greater than that of the negative carriers. The sharp decreasing in S, with temperature may be attributed to the large mobility of the generated electrons associated with intrinsic conduction [51].


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3.4 Temperature and frequency dependence of ac conductivity and some related dielectric parameters of CoPc thin film The total conductivity, σtot (ω) of sandwiched CoPc film, with ∼ 518 nm thickness, was measured as a function of both temperature and frequency in the temperature range (298–407 K) and frequency range (102 –106 Hz). Hence, the total conductivity is a sum of the two components: dc conductivity (σdc ), which is independent of frequency; and frequency dependent conductivity σac (ω), i.e σtot (ω) = σdc + σac (ω).

(10)

In the present work, the values of σdc were obtained by extrapolating σtot (ω) to ω = 0 and then the values of the ac conductivity, obtained by subtracting the dc conductivity from the measured total conductivity. The behaviour of log(σac ) vs. 1000/T , at constants frequencies, is represented in Figure 6a. This figure depicts the temperature independent and the frequency dependence of σac (ω) in the investigated temperature and frequency ranges. This means that σac (ω) is not thermally activated in this range of temperature (298–407). A common feature to the ac conductivity that, it changes with frequency according to the following formula [52]: σac (ω) = σtot − σdc = Aω s

(11)

where A is a constant independent on frequency and also weakly dependent on temperature, ω is the angular frequency and the exponent s denotes the frequency dependence of σac (ω). Figure 6b represents the frequency dependence of σac (ω) at different temperatures. The frequency exponent s was computed from the slope of the straight ac lines where s = ∂σ ∂ω . It was found that the average value of the frequency exponent, s, is temperature independent in the investigated range of temperatures, and the average value of s ≈ 0.879. Regarding the experimental results given above for the temperature dependence of σac (ω) and the value of the frequency exponent, s, one can explain the charge transfer process via CBH model with centers of intimate valence alternation pairs (IVAPs) type. These states were consider due to a group of defects made of D+ and D− centers due to their mutual coulomb interaction. Since CBH model considered a simultaneous hopping of two electrons from one (negatively charged) center D− to another (positively charged) center D+ over the barrier height, moreover, being correlate with the inter site separation via the coulomb interaction between centers are associated into pairs IVAP [53,54]. When all charged, spin paired defect centers are totally associated into close pairs; the distribution of the resulting (neutral) IVAP pairs of centers may however be random, all charged centers are associated into close pairs IVAP [55]. For approximation [56], the ac conductivity may be represented by the following relation; 2 6 8e Π2 ωs 2 σac ≈ (NIV AP ) ε (12a) (1−s) 24 εβm τ 0

Fig. 6. (a) Temperature dependence of the ac conductivity of as deposited CoPc film (∼518 nm) at fixed frequencies. (b) Frequency dependence of the ac conductivity for as deposited CoPc film (∼518 nm) at fixed temperatures.

where NIVAP is the spatial concentration of IVAP centers, τ0 is of the order of a typical inverse phonon frequency, ε is the effective dielectric constant, βm is the optical band gap and s is given by the following relation [57]; 6kB T s=1− (12b) βm by substituting the average determined value of s ∼ 0.879 in equation (12b) at a certain temperature, T = 350 K, the calculating value of the optical band gap, βm was found to be βm = 1.49 eV in agreement with 1.5 eV [27]. The spatial concentration of IVAP centers N (Ef ) through CoPc films, could be extracted for ε = 3.56 [27]. NIVAP was found to slightly decreased from 2.54 × 1018 to 2.448 × 1018 /eV.cm−3 as the frequency increased from 102 to 106 Hz. Since the maximum barrier height, WM , at infinite separation can be related to the optical gap, βm and the dc electrical activation energy, where WM ≈ 2(βm − ∆E). By substituting, βm = 1.497 and ∆E2 = 0.744, WM was found to be 1.594 eV for CoPc films with 518 nm

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Fig. 7. Frequency dependence of the capacitance for as deposited CoPc film (∼518 nm) at fixed temperatures, the inset figure; frequency dependence of the real part of the dielectric constant, ε1 , for as deposited CoPc film (∼518 nm) at fixed temperatures.

thickness. Also, the real part of the complex dielectric constant, ε1 , for CoPc film could be determined from the measured capacitance, C as ε1 = Cd/(A ε0 ) [58], where d is the thickness of the tested CoPc film which is equals to 518 nm, A is the area ≈ 4 × 106 m2 and ε0 is the electric permittivity of free space. Capacitance measurements were taken over the same temperature and frequency ranges. The capacitance C as a function of frequency at different temperatures is shown in Figure 7. It is clear that the capacitance for CoPc film decreases from 1.1 × 10−10 to 5.79 × 10−11 F as the frequency increases from 100 Hz to 1 MHz, while it increases slightly from 5.79 × 10−11 to 6.13 × 10−11 F as the temperature increases from 298 to 407 K [59,60]. The real part of dielectric constant, ε1 , [see the inset of Fig. 7], decreases from 1.622 to 1.76 as the frequency increases from 100 Hz to 1 MHz, while it increases slightly from 0.85 to 0.898 as the temperature increases from 298 to 407 K. This behavior can be attributed to the effect of charge redistribution by mean carrier hopping on defects [21,60–62].

4 Conclusions Throughout the present work we can conclude that:

1- The X-ray diffraction pattern (XRD) of Cobalt phthalocyanine (CoPc) in the powder form showed that the crystal structure is a mixture of α- and β-forms with monoclinic unit cell. The lattice parameters: a = 20.5 ˚ A, b = 4.78 ˚ A, c = 14.8 ˚ A and β = 120.70◦. XRD pattern of CoPc thin film (518 nm) have a single peak with a prefer orientation around 2θ ≈ 6.88. The mean crystallite size of Co-Pc increases with annealing temperature at 623 K for two hours under vacuum. The dislocation density and the strain decease with the annealing temperature. 2- The dark electrical resistivity, ρf , measured for CoPc thin films, with different thickness (62–407 nm) decreased with increasing the film thickness. Two activation energies can be determined below and above 350 K, ∆E1 and ∆E2 , which are equal to 0.1495 ± 0.0025 and 0.744 ± 0.018 eV respectively as a change from extrinsic to intrinsic conduction. The values of activation energies decreased by annealing process for CoPc thin films at 623 K/2 h under vacuum. 3- Seebeck coefficient measurements showed that CoPc thin films behave as p-type semiconductors, where the mobility of the positive charge carrier is higher than that of the negative charge carriers. 4- The ac conductivity was found to be temperature independent and proportional to ω s where s ∼ 0.879. The conductivity has been interpreted in the basis of CBH model with centers of intimate valence alteration pairs, IVAPs. The maximum barrier height, WM ≈ 1.594 eV.

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