azo-calix[4]arene composite layers

Physica B 474 (2015) 70–76

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Physica B journal homepage: www.elsevier.com/locate/physb

Studies of morphological optical and electrical properties of the MEH-PPV/azo-calix[4]arene composite layers A. Rouis a,n, J. Davenas b, I. Bonnamour c, H. Ben Ouada a a

Laboratoire des Interfaces et Matériaux Avancés (LIMA), Faculté des Sciences de Monastir, Avenue de l’environnement, 5000 Monastir, Tunisia Polymer Materials Engineering Laboratory IMP, UMR CNRS 5223, Université Claude Bernard Lyon 1, 15 boulevard Latarjet, 69622 Villeurbanne, France c Institut de Chimie & Biochimie Moléculaires & Supramoléculaires (ICBMS), UMR CNRS 5246, 43 Boulevard du 11 Novembre 1918, Université Claude Bernard Lyon 1, 69100 Villeurbanne, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 4 March 2015 Received in revised form 10 June 2015 Accepted 13 June 2015 Available online 20 June 2015

Thin films of poly[2-methoxy-5-(20-ethylhexyloxy)-1,4 phenylenevinylene] (MEH-PPV), 5,17-bis(4-nitrophenylazo)-26,28-dihydroxy-25,27-di(ethoxycarbonylmethoxy)-calix[4]arene (azo-calix[4]arene) and MEH-PPV doped azo-calix[4]arene, with 30 wt% and 70 wt% doping ratios, were prepared from chloroform solution by spin coating technique on quartz and ITO substrates. Morphological and optical properties of the samples were investigated by scanning electron microscopy (SEM) and UV–visible spectrophotometry techniques, respectively. Further, the charge carrier transport properties and conduction mechanism of the composite MEH-PPV:azo-calix[4]arene thin films based junction were studied by using current–voltage (I–V) characteristics and dielectric spectroscopy technique. I–V characteristic of ITO/MEH-PPV:azo-calix[4]arene/Al devices showed that the space charge limited conduction (SCLC) dominates in the high voltage region. Moreover, frequency dependence of ac conductivity obeys Jonscher's universal power law. Finally, dielectric constant (ε′), dielectric loss (ε″) and loss tangent (tan δ) were investigated as function of amount of azo-calix[4]arene in the MEH-PPV polymer matrix. & 2015 Published by Elsevier B.V.

Keywords: MEH-PPV Azo-calix[4]arene Morphological properties Electrical properties

1. Introduction Calixarenes, a versatile class of macrocyclic compounds possessing soft π-donor cavities composed of benzene rings and hard oxygen cavities constructed on the hydroxyl lower rim, have been extensively investigated in recent years because of their unique characteristics in structure and properties. During the past decades, considerable efforts were made to develop various calixarene derivatives with unique structure and to study their supramolecular characteristics. These materials were often highly selective molecular receptors for various metal ions and organic compounds, making them suitable for separation and analysis applications [1]. In recent years, calix[n]arenes and their derivatives have been extensively studied for their possible application as sensors and electronic devices. Mlika et al. have elaborated and characterized chemical sensors based on evaporated calix[4,6]arenes [2], calix[8] arene [3], calix[9,11]arenes [4] and calix[10,12]arenes [5] thin films for the detection of Na þ , Ca2 þ , Cu2 þ , Ag þ and Fe3 þ cations. Ben Ali et al. [6,7] have developed chemical sensors for copper ion detection based on evaporated thiacalix[4]arene thin films deposited on n

Corresponding author. Fax: þ216 73 332 258. E-mail address: [email protected] (A. Rouis).

http://dx.doi.org/10.1016/j.physb.2015.06.010 0921-4526/& 2015 Published by Elsevier B.V.

different transducers. Dridi et al. [8] have demonstrated the fabrication of mercury ion sensor based on evaporated thin films of thiacalix[4]arene amide. Ebdelli et al. [9] have interested in developing an impedimetric sensor based on azo-calix[4]arene amide derivative thin film spin coated on gold electrode for the detection of Hg2 þ , Ni2 þ and Eu3 þ cations. In previous work [10], we have investigated by impedance spectroscopy (IS), the electrical properties of diode structures ITO/calix[4]arenes derivatives/Al. Conduction mechanism, dielectric behavior and equivalent circuit analysis were discussed. Afterwards Dridi et al. [11,12] have studied nanomorphology, optical and electrical properties of vacuum-deposited azo-calix[4]arene thin films. Recently, Echabaane et al. [13] have studied optical, electrical and sensing properties of β-ketoimine calix[4]arene derivative thin films deposited by spin-coating technique. The ITO/β-ketoimine calix[4]arene/Al diode structure was characterized by current–voltage and impedance analysis to investigate the transport mechanism of electronic conduction and the ac behavior of the organic diode, respectively. Additionally, polymeric materials have been the subject of intense scientific and technological research because of their potential applications. In particular, conducting polymers (CP) have been extensively investigated in the area of electronics and optoelectronics due to their attractive properties [14,15]. Many scientists have been working on finding applications for the

A. Rouis et al. / Physica B 474 (2015) 70–76

conducting polymers, such as thin film transistors [16], polymer light emitting diodes (LEDs) [17], corrosion resistance [18], electromagnetic shielding [19], sensor technology [20], molecular electronics [21], supercapacitors [22], and electrochromic devices [23]. Among various conducting polymers, poly[2-methoxy-5-(20ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), as one of the most promising candidates, has recently received considerable interest, owing to its numerous potential applications in electronic components, such as Schottky diodes and photovoltaic cells. In recent years, the inclusion of cage molecules into conducting polymers has opened new perspectives of applications. In particular, a number of reports have appeared dealing with the immobilization of calixarene molecules into conducting polymers. Bidan et al. [24] have investigated the incorporation of sulfonated calixarenes (calix [4]-p-tetrasulfonate and calix[6]-p-hexasulfonate) into polypyrrole films during their electrodeposition at the surface of a glassy carbon electrode. Kaneto et al. [25] have reported electrochemical recognition and immobilization of uranyl ions by polypyrrole film doped with calix[6]arene-p-hexasulphonate. Mouzavi et al. [26] have studied potentiometric Ag þ sensors based on poly(3,4-ethylenedioxythiophene) and Polypyrrole conducting polymers doped with sulfonated calixarenes. In previously works [27,28], we have reported impedance spectroscopic study of chemical sensor based on a derivative of PPV (MEH-PPV) doped with chromogenic calixarene derivatives, deposited on ITO substrate, for Eu3 þ , Cu2 þ and Hg2 þ cations detection. Echabaane et al. [29] have studied electrical and electrochemical properties of the MEH-PPV and MEH-PPV doped calix[4]arene derivative layers for the detection of Cu2 þ and Na þ ions. In the light of these investigations, we reported the preparation and characterization of MEH-PPV:azo-calix[4]arene composite membranes. Firstly, we investigated the effect of azo-calix[4]arene weight concentrations on the morphology and optical properties of the composite films. Afterwards, we determined the electrical properties of the ITO/MEH-PPV:azo-calix[4]arene/Al devices by current–voltage measurements. The transport mechanism was discussed in the different regimes of electronic conduction. Finally, the impedance analysis was applied to investigate the conductivity, dielectric relaxation and loss tangent for these composites.

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Fig. 1. (a) Azo-calix[4]arene, (b) MEH-PPV, (c) ITO/Azo-calix[4]arene/Al and ITO/ MEH-PPV:azo-calix[4]arene/Al diodes heterostructures.

MEH-PPV matrix. Moreover, the MEH-PPV and azo-calix[4]arene solutions were dissolved in chloroform solvent at different concentrations (5 mg/ml and 10  2 M, respectively). Cleaned ITO and glass substrates were functionalized by MEH-PPV, azo-calix[4] arene and composites MEH-PPV:azo-calix[4]arene solutions by the spin-coating technique, at a controlled speed of 2300 rpm. The optimized volume of the deposited drop of the solutions was 15 ml. Finally, recuperated samples were thoroughly baked at 80 °C for at least 1 h before use. 2.3. Morphology The morphology of MEH-PPV, azo-calix[4]arene derivative and composites MEH-PPV:azo-calix[4]arene modified ITO surfaces were carried out using a Hitachi S800 SEM in the secondary electron mode since enabling a large depth of field. A Pd–Au alloy coating of the sample surface was done before observations to prevent charging artifacts by the electron beam. 2.4. Optical measurements A Perkin-Elmer Lambda 35 UV–visible spectrophotometer was used for UV–visible measurements. Optical measurements were carried out at room temperature. 2.5. Electrical measurements

2. Experimental section 2.1. Synthesis of azo-calix[4]arene molecule 5,17-bis(4-nitrophenylazo)-26,28-dihydroxy-25,27-di(ethoxycarbonylmethoxy)-calix[4]arene (azo-calix[4]arene) was prepared according to the previously reported synthesis route [30]. This compound was characterized by both azo groups on the upper rim and ester functions on the lower rim. The chemical structure of the molecule was shown in Fig. 1a. 2.2. Membranes preparation ITO substrates (ITO-thickness 100 nm, sheet resistance 20 Ω/cm  2) were purchased from Merck Display Technologies and were cut into 1 cm  1 cm square slides. The ITO electrodes were successively cleaned for 20 min in acetone and isopropyl alcohol in an ultrasonic bath and finally dried by a nitrogen gas flow. The same cleaning protocol is applied for glass suprasil substrates. Composites solutions were prepared by incorporating azo-calix[4] arene derivative into poly[2-methoxy-5-(20-ethylhexyloxy)-1,4phenylenevinylene] (MEH-PPV) (Fig. 1b), which is purchased from Aldrich. The two components were mixed in chloroform solvent (1 ml) with 30 wt% and 70 wt% of azo-calix[4]arene relative to the

The current–voltage characteristics were measured by using a Keithley 236 source measure unit. The diode structure studied consisted of indium tin oxide as positive contact and an Aluminum electrode as the negative contact as shown in Fig. 1c. Dynamic measurements of ITO/MEH-PPV:azo-calix[4]arene/Al devices with 30 wt% and 70 wt% of azo-calix[4]arene in MEH-PPV polymer matrix, in the range of 0.1 Hz–1 MHz, were performed with a Solartron impedance/gain-phase analyzer SI 1260. The measurements were carried out at oscillation levels below 50 mV (amplitude of alternative signal) without dc voltage, in order to work close to ohmic regime. All measurements were performed in dark at ambient conditions.

3. Results and discussions 3.1. Morphological characterization (SEM) 3.1.1. Microphotograph of azo-calix[4]arene thin film: solvent effect The influence of the solvent on the morphologies of azo-calix [4]arene thin films was investigated with scanning electron microscopy (SEM). Fig. 2 shows typical images of the azo-calix[4] arene modified ITO electrodes using 2 mm scale, when calixarene molecules were dissolved in chloroform and acetonitrile solvents,

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Fig. 2. SEM images of ITO substrates coated azo-calix[4]arene thin films: (a) in chloroform, (b) in acetonitrile.

respectively. We observed that the difference between the two solvents is much bigger. Fig. 2a reveals that the azo-calix[4]arene thin film (with chloroform solvent) has the sheet-like morphology and homogeneous surface. This observation can be due to the densely packed arrangement of azo-calix[4]arene molecules surface. Concerning the morphological feature of azo-calix[4]arene surface (with acetonitrile solvent), globular-shaped topography structure was obtained with different sizes of globules and aggregates. The rough film might be attributed to the poor dispersion capability of azo-calix[4]arene layer on ITO substrate (Fig. 2b). From the result, we can deduce that azo-calix[4]arene surface, when calixarene molecules are dissolved in chloroform, was more

smooth and covered the totality of the ITO surface . So, the chloroform solvent was chosen for the following studies. 3.1.2. Morphology of MEH-PPV and MEH-PPV:azo-calix[4]arene thin films Fig. 3 shows the topographical images of MEH-PPV and MEHPPV:azo-calix[4]arene thin films deposited on ITO electrodes using 5 mm and 500 nm scales. As it can be observed, the MEH-PPV surface depicted a few apparent pores and small aggregations (Fig. 3a). The effects of azo-calix[4]arene ratio on the composite surfaces morphologies were investigated in Fig. 3b–e. On the one hand, the incorporation of calixarene derivative into MEH-PPV

Fig. 3. SEM pictures of (a) MEH-PPV thin film, (b, c and d, e) MEH-PPV doped azo-calix[4]arene thin films deposited on ITO electrodes, with 30 wt% and 70 wt% doping ratios, respectively, at 5 mm and 500 nm, (f) SEM cross-sectional view of MEH-PPV:azo-calix[4]arene thin film with 30 wt% of azo-calix[4]arene.

A. Rouis et al. / Physica B 474 (2015) 70–76

Absorbance (Normalized)

Azo-calix[4]arene thin film 403 nm

1.0

3.2.1. Optical The UV–visible spectrum of the azo-calix[4]arene thin film was represented in Fig. 4a. It was found an absorption band below 300 nm, due to the aromatic macrocycle and another main absorption band at 403 nm resulting from the azo-calix[4]arene π–πn transition. This absorption, appearing at the lower energy, corresponds to the extended π orbital of the whole chromophore system and is at the origin of many applications of azo-compounds as dyes.

0.0 300

400

500

600

700

Wavelength (nm)

30 wt% azo-calix[4]arene Gaussian fit

0.8

Absorbance

0.6 511 nm

413 nm

0.4

0.2

0.0 300

400

500

600

700

800

900

Wavelength (nm) 0,6 70 wt% azo-calix[4]arene Gaussian fit 403 nm

Absorbance

calix[4]arene thin film at 30 wt% of azo-calix[4]arene doped MEHPPV. The composite film thickness was estimated to be 200 nm. 3.2. Optical studies

0.5

0,4

503 nm

0,2

200

73

300

400

500

600

700

800

900

Wavelength (nm) Fig. 4. (a) Normalized UV–visible spectra of the azo-calix[4]arene thin film. Typical deconvolution procedure applied to the UV–visible spectra corresponded to MEHPPV doped azo-calix[4]arene thin films: (b) 30 wt% and (c) 70 wt% of azo-calix[4] arene doping ratios.

polymer improved the surface smoothness and all pores were covered. On the other hand, there were more and bigger aggregates in the composite surfaces obtained at 30 wt% amount of azo-calix[4]arene doped MEH-PPV polymer, compared to those observed at 70 wt% amount of azo-calix[4]arene in MEH-PPV. Fig. 3f presents the SEM cross-sectional view of MEH-PPV:azo-

3.2.2. Optical absorption of MEH-PPV:azo-calix[4]arene composite films Fig. 4b and c shows the optical absorption spectra of MEH-PPV: azo-calix[4]arene composite films containing 30 wt% and 70 wt% of azo-calix[4]arene in the MEH-PPV polymer. The peak at about 500 nm is attributed to MEH-PPV absorption and the absorption band, at about 400 nm, was due to absorption of azo-calix[4]arene. The absorption spectra of composite films with 30 wt% and 70 wt% doping levels of calixarene molecules were deconvoluted to two peaks using a Gaussian function. Table 1 summarizes the information obtained from the analysis of the deconvoluted spectra shown in Fig. 4. We deduce that the optical absorption edges of the composite films are shifted towards the shorter wavelength region by increasing the MEH-PPV polymer concentration. This blue shift in the optical absorption edge indicates the formation of azo-calix[4]arene particles in the MEH-PPV matrix in the nanometer regime. These results confirm those obtained above by SEM topography study. 3.3. Electrical and impedance spectroscopy studies of ITO/MEH-PPV: azo-calix[4]arene/Al Schottky type diode 3.3.1. DC electrical properties The most common and simple way to investigate the electrical properties of organic semiconductors is the measurement of its I– V characteristics. It has been used to observe the charge transport properties in space charge limited current (SCLC) and Ohmic regime as well as the injection properties of metal-organic interface [31,32]. Fig. 5a shows the forward and reverse bias I–V characteristics of the ITO/MEH-PPV:azo-calix[4]arene/Al devices with 30 wt% and 70 wt% of azo-calix[4]arene in MEH-PPV polymer matrix. Devices based on the composite layers were compared to the ITO/azo-calix[4]arene/Al diode structure. The I–V characteristics show typical Schottky diode behavior for all devices. The electrically symmetric I–V characteristic, for low voltages, can be explained by the localized-state theory with defects providing the localized gap states. The asymmetric I–V characteristic at higher voltages is due to the difference of injection barriers to electron and hole due to different work functions for the ITO anode (4.7 eV) and the Al cathode (4.3 eV). The dynamic resistance (Rd), threshold Table 1 Absorption band wavelength of MEH-PPV:azo-calix[4]arene composite thin films with 30 wt% and 70 wt% of azo-calix[4]arene doping levels. MEH-PPV:azo-calix[4] arene 30 wt% azo-calix[4] arene 70 wt% azo-calix[4] arene

Azo-calix[4]arene π → π⁎ transition

MEH-PPV π → π⁎ transition

413

511

403

503

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1

voltages (region 1) an ohmic conduction mechanism is dominant where the logarithmic slope is in relation to unity. In this region, injection of charge transporters from the electrodes into the semiconductor material is significantly reduced due to the low bias voltage. The ohmic regime with a current density is described by Eq. (1)

0

J=

3

Current (µA)

2

-1 -2

ITO/azo-calix[4]arene/Al ITO/MEH-PPV:azo-calix[4]arene (30 wt%)/Al ITO/MEH-PPV:azo-calix[4]arene (70 wt%)/Al

-3 -4

-3

-2

-1

0

1

2

3

4

Bias voltages (V)

3.6 0

Current (µA)

10

2.94 2.04

10

-1

2.25 3.45 -2

10

-3

1.01

1.36

0.1

1 Bias voltages (V)

Fig. 5. (a) I–V charactristics for ITO/azo-calix[4]arene/Al and ITO/MEH-PPV:azocalix[4]arene/Al devices in forward and reverse bias modes. (b) In log–log representation. Squares correspond to experimental data and the continuous line is the best fit.

Table 2 Electrical parameters of the ITO/azo-calix[4]arene/Al, ITO/MEH-PPV:azo-calix[4] arene (30 wt%)/Al and ITO/MEH-PPV:azo-calix[4]arene (70 wt%)/Al devices from I– V characteristics. Device structures

Rd (MΩ) Vth (V) Rr

ITO/azo-calix[4]arene/Al 1.7029 ITO/MEH-PPV:azo-calix[4]arene (30 wt%)/Al 0.2509 ITO/MEH-PPV:azo-calix[4]arene (70 wt%)/Al 0.3578

9 V2 ε 0 εs μ 8 L3

(2)

The third regime (region 3) for higher bias voltages, characterized by I∝Vn 4 2, is attributed to the trap free space-chargelimited-current regime, obeying the Mott–Gurney law [35,36]

J=

9 V2 ε 0 εs μθ 8 L3

(3)

where εs is the organic material permittivity, θ relating the proportion of trapped charges (nt, pt) to free charges (n, p) then θ = (n/n + nt ) = (p/p + pt ) where n and nt are the densities of free and trapped electrons, respectively, p and pt the densities of free and trapped hole, respectively. The traps present in our case are due to defects in the chemical structure of organic material and impurities [37].

1.22

10

(1)

where V is the applied voltage, L is the thickness of the film, μ is the field-independent mobility, e is the elementary charge and η is the free charge carrier densities. As the voltage increases (region 2), the current varies with voltage as I∝V2 (i.e., it follows the square law), indicating the presence of space charge limited conduction (SCLC) and follows the Eq. (2) [33,34]

J=

ITO/azo-calix[4]arene/Al ITO/MEH-PPV:azo-calix[4]arene (30 wt%)/Al ITO/MEH-PPV:azo-calix[4]arene (70 wt%)/Al

eμηV L

2.30 1.80 2.06

0.46 ( 73 V) 0.94 ( 72 V) 0.68 ( 72.5 V)

bias voltage (Vth) and rectifying ratios (Rr) are extracted and presented in Table 2. It is indicated from the corresponding table that Vth and Rd parameters are increased, when the amount of calix[4] arene derivative molecules incorporated into the polymer matrix increased. This result shows that the ITO/MEH-PPV:azo-calix[4] arene/Al device with 30 wt% of azo-calix[4]arene in MEH-PPV polymer is the more conductive structure. Fig. 5b, represents the forward bias logarithmic plots of the I–V description for the Al/azo-calix[4]arene and Al/MEH-PPV:azo-calix [4]arene contacts. It is represented by three distinct linear regions, corresponding to changed conduction mechanisms. The existence of these limits is explained by the charge transport profile which influences the slopes of the current–voltage characteristics. At low

3.3.2. Impedance spectroscopy analysis 3.3.2.1. Conductivity analysis. The frequency dependence of electrical conductivity can be represented by an equation proposed by Jonscher [38]

σac = σdc + Aωn

(4)

where sdc is the independent frequency component of the conductance due to the excitation of electrons from a localized state to the conduction band, and Aωn is the ac conductivity which consists of all dispersion phenomena. A is the frequency independent constant and n a critical exponent which follows the inequality 0 on o1, characterizing hopping conduction [39]. For illustration, Fig. 6a and b shows the dependence of real and imaginary parts of conductivity (s′, s″) of the ITO/MEH-PPV:azocalix[4]arene/Al devices with 30 wt% and 70 wt% of azo-calix[4] arene relative to the MEH-PPV matrix as a function of frequency at 0 bias voltage. The conductivity pattern shows that it is strongly frequency dependent and obeys Jonscher's power relation, as given above. It clearly indicates that low and high frequency dispersive regions are separated by a change in slope at a particular frequency. The frequency at which a change in the slope occurs is called the hopping frequency. The inset of Fig. 6, represents the frequency dependence on ac conductivity of the studied diode devices on a log–log scale. We note, on the one hand, that the ac conductivity at the lower and medium frequencies of measurement increases with increasing frequency for the two devices. On the other hand, the conductivity of 70 wt% of azo-calix[4]arene sample is far below that of 30 wt% of azo-calix[4]arene structure. 3.3.2.2. Dielectric relaxation analysis. A dielectric response can result from a transport of mobile carriers in a polymer under an applied alternating electric bias. Measurement of the dielectric

A. Rouis et al. / Physica B 474 (2015) 70–76

30 wt% azo-calix[4]arene 70 wt% azo-calix[4]arene

0.025

75

30 wt% azo-calix[4]arene 70 wt% azo-calix[4]arene

40

0.020

30

0.015

20

0.010 0.005

10

0.000 -2

10

-1

10

10

0

1

10

2

10

3

10

4

10

5

10

6

10

7

-2

10

Frequency (Hz)

30 wt% azo-calix[4]arene 70 wt% azo-calix[4]arene

0.025

0

10

-1

10

0

10

10

0.015

8

2

3

4

10

5

6

10

10

7

30 wt% azo-calix[4]arene 70 wt% azo-calix[4]arene

12

0.020

1

10 10 10 10 Frequency (Hz)

6

0.010

4 0.005

2 0.000 -2

10

10

-1

0

10

1

10

2

10

10

3

4

10

5

10

6

10

7

10

Frequency (Hz) Fig. 6. Frequency dependencies of (a) real part (s′) and (b) imaginary part (s″) of capacitance for ITO/MEH-PPV:azo-calix[4]arene/Al devices with 30 wt% and 70 wt% doping ratios. The insets depict s′(f) and s″(f) in log–log scale.

relaxation gives information on the charge transport mechanism. The observed complex permittivity ε* is defined as

ε′ (ω) = ε′(ω)–iε ′′(ω)

(5)

where ε′ is the dielectric constant, ε″ the dielectric loss, and ω the angular frequency. The loss tangent (tan δ) is calculated from the relationship [40,41]

tan δ = ε ′′/ε′

0

10

(6)

The real permittivity ε′ and the dielectric loss factor ε″, at 0 bias voltage, are represented according to the frequency for the ITO/ MEH-PPV:azo-calix[4]arene/Al structures with 30 wt% and 70 wt% of azo-calix[4]arene (Fig. 7). Fig. 7a shows that ε′¼f(f) presents inflexion points related to dipolar relaxation phenomena. The curve of ε″ ¼f(f) (Fig. 7b) exhibits a loss peak characteristics of dipolar relaxations and shows a conduction process in the material at low frequencies. Fig. 8a illustrates the variation of tangent loss with frequency of the composite films. The most significant changes of tan δ are observed in the medium frequency range. The loss spectra characterized by peaks appearing at a characteristic frequency for both samples suggest the presence of relaxing dipoles. The peak in tan δ increases with increasing frequency passes through a maximum value and thereafter decreases. Meanwhile, two obvious dielectric loss peaks in tan δ plot are observed, at around 102 Hz and 103 Hz respectively, for the MEH-PPV:azo-calix

-2

-1

10

0

10

1

2

3

4

10 10 10 10 Frequency (Hz)

10

5

6

10

10

7

Fig. 7. Frequency dependencies of (a) ε′ and (b) ε″ for ITO/MEH-PPV:azo-calix[4] arene/Al devices with 30 wt% and 70 wt% doping ratios. The insets depict ε′(f) and ε ″(f) in log–log scale.

[4]arene thin film at 30 wt% of azo-calix[4]arene doped MEH-PPV. Therefore only one dielectric loss peak is observed at about 3 Hz for the MEH-PPV:azo-calix[4]arene thin film with 70 wt% of azocalix[4]arene. From the Fig. 8a, it depicts that the maxima of tan δ (peak 1) shifts towards lower frequencies side with increase of the doping concentration. As the peak shifts towards lower frequency, the relaxation time is increased. As shown in Fig. 8b, the complex permittivity may be presented on a Cole–Cole diagram by plotting the imaginary part ε″ on the vertical axis and the real part ε′ on the horizontal axis with frequency as the independent parameter. Each relaxation peak corresponds to each semicircle of Cole–Cole plot.

4. Conclusion In summary, different complementary techniques were used in this work to investigate structural, optical and electrical properties of the MEH-PPV, azo-calix[4]arene and composite MEH-PPV:azocalix[4]arene thin films. In the first hand, SEM images provided, in particular, the information regarding the morphology of the MEHPPV:azo-calix[4]arene surfaces with 30 wt% and 70 wt% of doping levels. The degree of agglomeration increased as the amount of MEH-PPV in the composite increased. Besides, optical characteristics of the MEH-PPV:azo-calix[4]arene thin films showed a blue

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References

30 wt% azo-calix[4]arene 70 wt% azo-calix[4]arene

0.4

peak 1

tan δ

0.3

peak 2 peak 1

0.2 0.1 0.0 10

-2

-1

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10 8 6 4 2 0 0

10

20

30

40

Fig. 8. (a) Variation of dielectric loss (tan δ) with frequency and (b) Cole–Cole plots for ITO/MEH-PPV:azo-calix[4]arene/Al devices with 30 wt% and 70 wt% doping ratios.

shift devoting the formation of azo-calix[4]arene particles in the MEH-PPV matrix in the nanometer regime. In the second hand, current–voltage and electric impedance measurements were used to investigate the transport characteristics of ITO/MEH-PPV:azo-calix[4]arene/Al diodes structures with 30 wt% and 70 wt% doping ratios. The dc electrical characterizations showed an ohmic behavior at low voltages and switches to space charge limited current (SCLC) conduction with exponential trap distribution at higher voltage. Therefore, the frequency dependence of ac electrical conductivity (sac), dielectric constant (ε′), dielectric loss (ε″) and loss tangent (tan δ) were investigated as function of amount of azo-calix[4]arene in the MEH-PPV polymer matrix.

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