Morphology and AC Electrical Properties of Chloroindium Phthalocyanine Thin Film
Mohammad Esmail Azim Araghi, Samaneh Mahmoudi*
Faculty of Physics, Kharazmi University, 49 Mofateh Avenue, Tehran, Iran
*Corresponding Author: Samaneh Mahmoudi, E-mail: asamanehmahmoudi@gmail.com, tel:00989107355129
Abstract: The AC electrical properties of Chloroindium phthalocyanine (ClInPc) are investigated to study Conduction mechanism in this material. For this purpose, sandwich devices of ClInPc with aluminum electrodes (Al/ClInPc/Al) are deposited on pre-cleaned glass substrate, using electron beam gun evaporation technique. Capacitance and Dissipation factor are measured over the frequency range of 102-105 Hz and temperature range of 307-383 K. Capacitance increased with increasing temperature and decreased with increasing frequency. Dissipation factor increased by increasing temperature and decreased with increasing frequency. The AC electrical properties are in good agreement with Goswami and Goswami model. The frequency dependence of AC conductivity is analyzed by the equation σ(ω) = AωS which is typical for charge transport by hopping or tunneling mechanism. In this present research, Conduction mechanism is specified with correlated barrier hopping (CBH) and overlapping large-polaron tunneling (OLPT) models. furthermore, the activation energies of the device are achieved as a function of frequency. Morphology of the samples was investigated by scanning electron microscope (SEM) images, X-ray diffraction (XRD) and optical absorption.
Keywords: Chloroindium Phthalocyanine, AC conductivity, Conduction mechanism, Optical band gap.
1 Introduction
In the last few decades, researchers are too busy with organic semiconductors like phthalocyanines. The behavior electrical and optical of phthalocyanines is receiving great attention because of the potential use in a wide range of large area electronic, and photonic devices, such as organic light-emitting diodes (OLEDs) [1,2], gas sensors [3–5], organic thin film transistors (OTFTs) [6,7], and solar cells [8,9]. Phthalocyanines (Pc’s) have planar structures with high thermally and chemically stability. [10]. Phthalocyanines are aromatic porphyrins synthetic 18 π-electron, including four isoindole units connected together via nitrogen atoms [11]. Phthalocyanines classified with respect to the atom that is in the benzene ring. If two hydrogen atoms are in the ring Benzene is called metal-free phthalocyanine. If the atom is of a particular metal in between Benzene rings are indicated called metal phthalocyanine compounds, and if the atoms Hydrogen is replaced by a metal atom and a halogen atom is called halogenated Phthalocyanine. Electrical behavior phthalocyanine is usually investigated by Planar or sandwich devices. Planar devices containing interdigital electrodes are mainly applied as gas sensors of different gases through DC electrical properties, while sandwich devices can also be applied for AC electrical properties [12]. AC conductivity in thin film structures of Phthalocyanine has a frequency dependence of the form σ = AωS, where ω is the angular frequency and S is an index generally less than unity [13]. Correlated barrier hopping model (CBH), quantum-mechanical tunneling model (QMT), overlapping large-polaron tunneling model (OLPT) and Non-small polaron tunneling model (NSPT) can be used to describe principal mechanisms of conduction.
Saleh and Gould et al. obtained the activation energy, capacitance and dielectric loss as the function of temperature and frequency on ZnPc, CuPc, FePc, and H2Pc films [14–17]. Azim-Araghi et al. [12,18,19] studied the electrical properties of PbPc, CuPc , and ClAlPc thin films. Their results demonstrate that the hopping model is dominant in describing the conduction mechanism in films. The AC properties in phthalocyanine thin films have been studied extremely less than the DC properties [20,21]; this is especially correct of halogenated phthalocyanines. ChloroIndium phthalocyanine (ClInPc) is a halogenated phthalocyanine which only a few types of research have been performed on electrical properties of this material. In the present research, we studied morphology, optical and AC electrical properties of ClInPc sandwich devices with aluminum electrodes.
2 Experimental details
2.1 Materials preparation
ClInPc material was synthesized by mixed of 50 g phthalonitrile, 15 g InCl3, and 200 ml quinoline for 2 h. The outcoming mixture was then filtered in a glass filter. The segregated solid washed successively with toluene, carbon tetrachloride and acetone then dried under vacuum at 110 ⁰C. synthesized ClInPc have considerable amounts of impurities, so purification required. The sublimation has been performed for purification.
2.2 Device preparation and characterization
Glass slides with dimensions of 1 × 1 × 0.2 cm3 were used as substrates for devices. substrates were cleaned entirely using acetone and deionized water to remove any contamination. Thin film sandwich devices of ClInPc were provided by electron beam gun (EBG) ‘‘HIND HIVAC-PC-3K’’ model under a vacuum of 10-5 mbar. Aluminum electrodes of thickness 100 ± 5 nm were deposited Both the bottom and top contacts to the phthalocyanine film. the layer of ClInPc with the thickness of 750 ± 5 nm was deposited between the Al layers. The deposition rate was 0.6 nm per second pending evaporation for all layers. Figs.1 and 2 show the cross-sectional FESEM image and Schematic image of a sandwich device respectively. Capacitance and loss factor measurements were used to determine the electrical properties of AC in dark conditions using MT4080A LCR in the temperature range and frequency of 307-383 K and 102-105 Hz, respectively. ClInPc was deposited on glass in the same proceeding of devices fabrication, and the surface morphology of ClInPc thin films was studied. The crystalline structure of thin films was determined by X-ray diffraction (XRD) and surface morphology of ClInPc thin films was characterized by scanning electron microscopy (SEM). UV–Visible spectra are recorded in the wavelength range 300–800 nm using Lambada25.
Fig. 1 Cross-sectional FESEM image of a device on the glass substrate.
Fig. 2 Schematic image of a sandwich device.
3 Results and discussion
3.1 Structural characteristics
Fig. 3a shows the SEM image of ClInPc thin films deposited on the glass substrate. Nanoparticles have the uniform distribution of the almost spherical-shaped particles. histogram the distribution of ClInPc nanoparticles diameter demonstrate the average size of nanoparticles 23.15 nm (fig.3b).
Fig.3 SEM images of ClInPc thin films (a), The distribution of nanoparticles diameter (b).
The X-ray diffraction pattern of ClInPc thin films is shown in Fig. 4. The pattern shows that there is the peak at 2θ = 24.4⁰. The grain size L is measured using the Scherrer equation (1) [22].
L=(k_s λ)/(β cosθ) (1)
where λ is the X-ray wavelength (1.540 Å), β the full width at half maximum (FWHM) of the most intense peak in radians, θ is Bragg's angle and ks is the Scherrer constant which has a value of 0.9 [23]. The crystallite size was measured to be about 4.8 ± 0.03 nm.
Fig. 4 X-ray diffraction pattern of ClInPc.
3.2 Optical measurement
Fig. 5 shows the spectrum of absorption against wavelength for the films. The optical absorption spectra were measured for the ClInPc thin film deposited onto the glass substrate using UV–Vis spectrophotometer. The ClInPc has two peaks in the visible region at 589 and 731 nm called Q-band and single peak 358 nm called B-band, both of the transitions are from (π) to (π*) transitions [24].
Fig. 5 Optical absorbance spectra of ClInPc thin film at room temperature.
The absorption coefficient α is measured according to the equation (2).
α = 2.303 A/t (2)
where A is the absorbance of the thin film and t is its thickness. For the direct allowed transition, the absorption coefficient α is related to the photon energy (hν) by the following equation (3).
α= α_0 〖(hν-E_g)〗^n (3)
where Eg is the optical band gap, α0 is a constant and n determines the type of transition [25] (n = 0.5 for the direct transition [26]). By plotting (αhν) 2 versus hν and extrapolating to zero absorption, the band gap Eg is obtained. The Plot of (αhν)2 versus hν for ClInPc film is shown in Fig. 5. According to Fig. 6, the optical band gap energy of ClInPc is 2.9 ± 0.01 eV.
Fig. 6 (αhν)2 as a function of photon energy (hν) for ClInPc thin film at room temperature.
3.3 AC electrical measurements
3.3.1 Capacitance and dissipation factor
Capacitance and dissipation factor of samples were measured in the frequency range and temperature range of 102–105 Hz and 307-383 K respectively. The frequency dependence of capacitance for the Al/ ClInPc /Al structure is shown in Fig. 7. The capacitance decreases with increasing frequency for all temperatures and attains to a constant at frequencies more than 104 Hz. This type of behavior of capacitance with frequency could more exactly be explained in terms of Goswami and Goswami model [27]. In this proposed model each capacitor system is supposed to contain a capacitance element, C, which is not impressed by frequency and temperature and a discrete resistance element, R, due to the dielectric film in parallel with Cs. Both C and R are in series with a resistance, r, due to lead lengths, etc. In the above model, it is supposed that whereas R will be impressed via temperature due to the equation (4).
R=R_0 exp(∆E/(K_B T)) (4)
where R0 is constant, ΔE an activation energy and KB is the Boltzmann constant T is the utter temperature. The lead resistance, r, will be a constant. Such a model beseems to be more appropriate for the intention of analysis of this research work. According to the model of Goswami and Goswami, the capacitance Cs is given by equation (5).
C_S=(1+ω^2 C^2 R^2)/(ω^2 R^2 C)=C+1/(ω^2 R^2 C) (5)
where ω is the angular frequency. According to the equation (5), capacitance Cs should decrease with increasing frequency and achieving to a constant C at frequencies more than 104 Hz. For high frequencies greater than 104 Hz, the delocalized charges of the ClInPc will not be able response easily to the frequency fluctuations with no contribution to the induced capacitance. Fig. 8 shows the capacitance as a function of temperature for frequencies of 102–105 Hz over the temperature range of 307–383 K. capacitance will increase with increasing temperature. The effect of increasing capacitance with increasing temperature is depended increasing the number of carriers with increasing temperature [28]. These effects were clearly observed in Figs. 7 and 8. Similar observations have been made by various authors [29,30].
Fig. 7 Variation of capacitance with frequency at different temperatures.
Fig. 8 Variation of capacitance with the temperature at different frequencies.
Fig. 9 and Fig. 10 show the variation of the dissipation factor with frequency and temperature. The dissipation factor (tanδ) is the power dissipated in the sample. It is given in terms of the ratio of the imaginary part (ε′′) to the real part (ε′) of the complex dielectric permittivity as:
tanδ = ε′′/ ε′ (6)
ε′ = C d/(ε_0 A) (7)
where C is the capacitance of the capacitor containing the dielectric material, ω is the angular frequency and ε_0= 8.85×10-12 F/m is the vacuum permittivity [31]. Dissipation factor is found to decreases with the increasing in frequency and increases with the increasing temperature. This behavior may be attributed to the dipole relaxation phenomenon in films. The frequency dependence of dissipation factor or loss tangent can be represented by the equation (8).
tanδ= 1/ωRC+r/(ωR^2 C)+ωrC (8)
The equation (8) clearly clarifies the behavior of the dissipation factor with frequency. The dissipation factor tanδ is expected to decrease with increasing frequency where the ω-1 term is dominant.
Dissipation factor is found to decreases with the increasing in frequency and increases with the increasing temperature. This behavior may be ascribed to the dipole relaxation phenomenon in films. The frequency dependence of dissipation factor or loss tangent can be expressed by the equation (9).
tanδ= 1/ωRC+r/(ωR^2 C)+ωrC (9)
The equation (8) clearly illustrates the behavior of the dissipation factor with frequency. The dissipation factor tanδ is anticipated to decrease with increasing frequency where the ω-1 term is predominant.
Fig. 9 Variation of loss factor with frequency at different temperatures.
Fig. 10 Variation of loss factor with temperature at different frequencies.
3.3.2 AC conductivity
The AC conductivity, capacitance, and dissipation factor can be described by the following equation (10).
σ_AC=ωC tanδ ( d)/A (10)
Where ω is the angular frequency, d is the thickness of the thin film, and A is the active area of the sample. The typical dependence of AC electrical conductivity with frequency at different temperatures is shown in Fig. 11. Generally, the conductivity increases slowly with frequency at low frequencies and very low temperatures, while the increase is stronger at higher frequencies and higher temperatures. An increase in AC conductivity can be relevant with the increase of possibility of charge carriers tunneling, which is appertained to the thermal fluctuation of sites [32].
Fig. 11 Dependence of conductivity with frequency at different temperatures.
moreover, the AC conductivity follows to the Jonscher’s power law given by equation (11).
σ=Aω^S (11)
where A is a constant, ω is the angular frequency and S is an index generally less than unity. The mechanism of conductivity is analyzed of the exponent S and temperature relation by diverse models based on the hopping or tunneling of electrons or atoms between equilibrium sites [33]. Fig. 12 shows the function of S against temperature over two different frequency ranges. The values of the exponent S were concluded from the slopes of logarithmic conductivity versus frequency for different frequency ranges. At the low frequency region (100 Hz-1 KHz), a decrease in the S value was observed, which can be affiliated with the correlated barrier hopping (CBH) model. This model clarifies charge carrier hops between sites over the potential barrier separating them. At higher frequency region (1 KHz-100 KHz), the overlapping large-polaron tunneling model (OLPT) can be applied to describe the S behavior. In this model, the frequency exponent, S, depends on both frequency and temperature and it drops to a minimum value with rising temperature and then increases to approach the value of the quantum-mechanical tunneling model (QMT). In (QMT) model, the exponent S is nearly equal to 0.8 and increases slightly with increasing temperature or it is temperature independent. so a tunneling mechanism is responsible for conductivity at higher frequency region.
Fig. 12 Variation of S with versus the temperature.
The AC conductivity as a function of temperature can be related by the following equation (12).
σ_AC= σ_0 exp((-∆E_a)/(K_B T)) (12)
where σ0 is a constant, ΔEa is the activation energy for electrical conductivity, KB is the Boltzmann constant, T is the temperature. Fig. 13 shows the Ln σAC versus 1000/T of ClInPc thin films with the temperature at different frequencies. Activation energy is measured from the slope of the plots for two different temperature regions and the obtained results are given in Table І. E1 is the activation energy in the higher temperature region and E2 is in the lower temperature region. The activation energies decrease with increasing frequency. The increase of the applied frequency enhances the electronic jumps between the localized states; accordingly, the activation energy decreases with increasing frequency.
Fig. 13 Variation of AC conductivity with the temperature at different frequencies.
Table 1 Activation energies determined at different frequencies for ClInPc thin films.
Frequencies (Hz) activation energy (eV)
(E1) (E2)
100 0.98 0.50
120 0.94 0.48
1000 0.88 0.44
10000 0.87 0.42
100000 0.73 0.37
4 Conclusion
sandwich devices of Chloroindium phthalocyanine thin films with aluminum electrodes (Al/ClInPc/Al) were fabricated by electron beam gun evaporation at room temperature. The AC electrical properties of nanostructured thin films were examined at the temperature range of 307–383 K and the frequency range of 102–105 Hz. SEM images show ClInPc nanoparticles with an average size of 23.15 nm. XRD results shown that there is the peak at 2θ = 24.4⁰ and the grain size of ClInPc thin films is 4.8 ± 0.03 nm. Optical band gap energy of ClInPc thin films was obtained as 2.9 ± 0.01 eV from optical absorption spectra. Capacitance has been observed to decrease with increasing frequency and also to increase with increasing temperatures. Dissipation factor decreased with increasing frequency and enhanced with increasing temperature. The frequency and temperature dependence of the capacitance and dissipation factor is in conformity with the model of Goswami and Goswami. AC conductivity in ClInPc follows the Jonscher’s power law σ(ω) = AωS, the variation of the exponent S as a function of temperature verified that the conduction mechanism assigned to the correlated barrier hopping (CBH) model at low frequency range of (100 Hz-1 KHz) and overlapping large-polaron tunneling model at higher frequency range of (1 KHz-100 KHz). The temperature dependence of the AC conductivity showed activation energies, the activation energies are determined and it is found to decrease with increasing frequency.
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