Effect of the side-chain size on the optical and electrical properties of confined-PPV derivatives

Superlattices and Microstructures 85 (2015) 469–481

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Effect of the side-chain size on the optical and electrical properties of confined-PPV derivatives Maha Benzarti-Ghédira ⇑, Haikel Hrichi, Nejmeddine Jaballah, Rafik Ben Chaâbane, Mustapha Majdoub, Hafedh Ben Ouada Laboratoire des Interfaces et Matériaux Avancés (LIMA), Faculté des Sciences de Monastir, Avenue de l’Environnement, 5019 Monastir, Tunisia

a r t i c l e

i n f o

Article history: Received 1 May 2015 Accepted 5 May 2015

Keywords: Semi-conducting polymers Poly(p-phenylene vinylene (R) p–p interaction Space-charge-limited current (SCLC) I–V characteristics Impedance spectroscopy (IS)

a b s t r a c t We have investigated the influence of side-chain size on the optical and charge transport behavior of thin layers of new conjugated polymers based on separated PPV-type chromophores (P1, P2 and P3). The polymers are soluble in common organic solvents. The optical properties of these materials were investigated by UV–Vis absorption and PL spectroscopy. In thin solid films, the polymers show side-group dependent optical behavior; the PL spectra of polymers P2 and P3 showed a blue emission, whereas a green emission was observed for the polymer P1. The optical gaps of these thin layers have been estimated to be 2.93, 2.96 and 2.98 eV for P1, P2 and P3, respectively. The optical study showed a stronger p–p interaction in the P1 film. The electrical properties of ITO/PPV derivative/Al diodes base on these PPV derivatives were investigated by the current/tension characteristics and modeled by the current space-charge-limited (SCLC) mechanism; a higher mobility was obtained in the P2 thin layer. The morphology of the polymer films was studied and correlated to the optical and electrical properties. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, studies of electrical and optical properties of conjugated polymer have attracted much attention in view of their application in electronic and optical devices such as chemical sensors [1,2], solar cells [3,4], electroluminescent devices [5,6] and organic thin-film transistors (OTFTs) based on conjugated polymers [7,8] have undergone significant progress due to many advantages such as light weight, low-cost, and good compatibility with solution-processes that are promising for the fabrication of flexible large-area devices [9–11]. It is of essential importance to study the optical properties and understand the charge-carrier transport in these materials in order to design and synthesis appropriate structures [12] and to improve the efficiency of devices. Poly(p-phenylene vinylene) (PPV) has been the subject of significant research because of its electrochemical activity and high electronic conductivity after chemical or electrochemical doping [13]. In general, PPV derivatives composed of two parts, p-conjugated backbone and flexible solubilizing side chains. A first part, the p-Conjugated backbone, defined the optoelectronic properties of the conjugated polymers [14–16]. A second part, namely the side chain play an important role in improving the solubility of the conjugated polymers in common organic solvents and has a critical effect on the intermolecular interactions, also the side chains play a important role in a molecular packing and essential impact the charge transport ⇑ Corresponding author. E-mail address: [email protected] (M. Benzarti-Ghédira). http://dx.doi.org/10.1016/j.spmi.2015.05.006 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.

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properties of thin-film based on conjugated polymers [17]. A good number of researches have been focused on conjugated backbone, whereas, the study of the effect of the side chain has not been fully exploited and have begun to attract attention in recent years [18–20]. The introduction of substituents into the phenylene group can modify the p-electron density in the polymer backbone and leads to a change in the HOMO and LUMO positions. A modification of the electronic properties of PPV can also be achieved by introducing substituents to the vinylene segment. For example, the presence of a strongly attracting electron group, like CN, results in simultaneous lowering of the polymer HOMO and LUMO levels by 0.5 eV in MEH–CN–PPV with respect to the case of MEH–PPV. Al though the band gap position is changed, the bandwidth remains essentially the same [21]. In this paper, we report the use of the PPV based polymer as semiconductors in diode device. In order to investigate the side effect, PPV-based polymers P1–P3 (Fig. 1) with different side chains were intentionally designed. The impacts on optical and electrochemical properties were also investigated. For the analysis of the charge properties of transport in organic semiconductors, the relation between the nature of the side chain and the electronic properties will be discussed. The structures of diode of ITO/PPV derivative/Al will be characterized by current –measurements of tension in order to determine the mechanisms of conduction and these parameters, particularly effective mobility, will be derived for the various structures. A study of the organic diodes by the spectroscopy of impedance (IS) in the frequency band of 100 Hz to 10 MHz will be presented and we deduce the nature of the traps for an electric model is equivalent of circuit for various polarizations.

2. Experimental 2.1. Organic materials Our study focuses on a series of three conjugated polymers (P1–3) based on separated PPV-type chromophores and incorporating different types of solubilizing side-groups (Fig. 1). These materials were synthesized via the Wittig polycondensation according to the procedure described in the reference [22]: The synthetic route to the PPV-type p-conjugated polymers is illustrated in Fig. 1. To a stirred mixture of an equimolar amount of the terephthaldicarboxaldehyde (1 mmol) and the diphosphonium salt (1 mmol) in 10 ml of anhydrous THF, 10 ml of a 0.5 M t-BuOK solution in THF (5 mmol) was added dropwise at room temperature under an argon atmosphere. The reaction mixture was stirred for 24 h at room temperature then 4 h at 60 °C. The polymerization was quenched by addition of 3% aqueous hydrochloric acid then the reaction mixture was poured into water and extracted with chloroform. The organic phase was washed with water, concentrated and then precipitated in methanol. The polymer was then filtered and dried under vacuum for 48 h. The polymer purification was achieved by recrystallization into methanol from chloroform solution. P1: R = ethyl; aspect: yellow powder; 1H NMR (300 MHz, CDCl3, d): 8.10–6.40 (br m, aromatic and vinylic H), 4.10–3.70 (m, OCH2), 1.70–1.50 (m, C(CH3)2), 1.45–1.15 (m, CH3); FTIR (cm1): 3020 (w, aromatic and vinylic CAH stretching), 2973, 2928, 2872 (w, aliphatic CAH stretching), 1684, 1601 (m, C@C stretching), 1246 (s, CAOAC asymmetric stretching), 1043 (m, CAOAC symmetric stretching), 806 (s, aromatic CAH out-of-plane bending). Number-average molar weight (Mn) (determined by 1H NMR analysis): 6050 g mol1. P2: R = hexyl; aspect: yellow powder; 1H NMR (300 MHz, CDCl3, d): 7.80–6.40 (br m, aromatic and vinylic H), 4.15–3.80 (m, OCH2), 2.00–1.20 (br m, A(CH2)4A and C(CH3)2), 0.93 (s, CH3); FTIR (cm1): 3020 (w, aromatic and vinylic CAH stretching), 2955, 2928, 2857 (w, aliphatic CAH stretching), 1687, 1602 (m, C@C stretching), 1246 (s, CAOAC asymmetric

Fig. 1. Synthetic route to the polymers P1–3.

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stretching), 1016 (m, CAOAC symmetric stretching). Number-average molar weight (Mn) (determined by 1H NMR analysis): 11910 g mol1. P3: R = dodecyl; aspect: brown paste; 1H NMR (300 MHz, CDCl3, d): 7.80–6.40 (br m, aromatic and vinylic H), 4.10–3.80 (m, OCH2), 2.00–1.20 (br m, A(CH2)10A and C(CH3)2), 1.00–0.80 (m, CH3); FTIR (cm1): 3020 (w, aromatic and vinylic CAH stretching), 2922, 2852 (w, aliphatic CAH stretching), 1688, 1602 (m, C@C stretching), 1247 (s, CAOAC asymmetric stretching), 1026 (m, CAOAC symmetric stretching), 807 (s, aromatic CAH out-of-plane bending). Number-average molar weight (Mn) (determined by 1H NMR analysis): 17150 g mol1. 2.2. Device elaboration In our study the devices are composed of a single organic layer sandwiched between two electrodes. ITO-coated glass with a sheet resistance of sheet 20 X/cm square was used as an anode in the organic diode fabrication. In this process, the ITO glass was cleaned sequentially in an ultrasonic bath of acetone and isopropanol alcohol; it was then, sonicated in deionized water and finally blown dry with N2 gas. The ITO coated glass substrates are used as the anode due to its superior properties, such as good transparency, high work function, high efficiency, and high conductivity. A solution of PPV derivative was dissolved in a chloroform solvent and then was spin-coated on the cleaned ITO precoated glass substrate at the speed of 1500 rpm for 60 s followed by heating on a hotplate for 30 min. For processing the cathode, samples were put into an evaporator, in which Al metal electrodes (100 nm) were thermally evaporated at 2  106 Torr pressure through a shadow mask to produce simultaneously four diode structures. Fig. 2 shows the typical device structure of the diode investigated in this study. The active areas of the diodes were confined within the overlap of the electrodes which is approximately 3.14 mm2. 2.3. Instrumentation and measurements A Perkin–Elmer UV–Vis Spectrophotometer (Lambda 35) has been used to characterize the optical properties of these thin films A Perkin–Elmer UV–VIS spectrophotometer (Lambda 35) was used to characterize the optical properties of these thin films. Films of polymers were deposited on a glass substrate to prevent the absorption of ITO substrate in the UV region. Photoluminescence spectra have been measured by means of ‘‘JOBIN YVON-SPEX Spectrum One’’ CCD detector, and cooled at liquid nitrogen temperature. All optical measurements have been carried out at ambient conditions in dark. The surface topology (AFM images) of the films was characterized with a Nanoscope IIIa microscope from Digital Instruments Inc. in the tapping mode (25 1C, in air). The current–voltage measurements were determined with an applied bias of 8 to 8 V by using a Keithley 236 measure unit and the impedance measurements were conducted using an impedance analyzer (Hewlett Packard 4192ALF) controlled by a computer acquisition. In general, the excitation potential for dynamic measurements is given by:

V ¼ V 0 þ V mod cosðxtÞ

ð1Þ

where V0 is a DC bias, Vmod is the oscillation level and x/2P is the frequency. In our study, these measurements were released in the following conditions V0: 0–2 V, and Vmod of 50 mV over a frequency range of 5 Hz–13 MHz. All these electrical measurements were performed in dark and at room temperature. 3. Results and discussion 3.1. Optical properties of the thin films The UV–visible absorption of the PPV-type polymers in thin films were recorded at room temperature. The absorption spectra of P1–3 films are shown in Fig. 3; the values of the absorption maxima and edges were listed in Table 1. The UV visible spectra show similar allures with one absorption maxima at 364 nm, which is attributed to the p ? p⁄ transitions in the PPV system [23]. The analysis of the optical absorption spectra reveals the optical energy gap (Eg) between the HUMO and LUMO band due to p ? p⁄ transition in these amorphous organic materials. The absorption coefficient a obeys the Mott and Davis’s model [24,25]:

ahm ¼ Aðhm  Eg Þn

ð2Þ

where A is a constant (also known as disorder parameter that depends on the transition probability and nearly independent of photon energy), Eg is the optical energy gap corresponding to a particular absorbance of photon energy, hm is the energy of the incident photon while h is also known as plank constant and m is angular frequency of incident photon. The parameter n is the power coefficient and its value is determined by the type of possible electronic transitions during absorption processes. Eventually, the Tauc model, in which the n value is of 1/2, can be used as a standard model to determine the optical gap for organic semiconductors [26]. Therefore, by using the Eq. (2), the indirect optical band gap can be evaluated from the linear plot of (ahm)1/2 versus hm graph. The extrapolation of the lines of (ahm)1/2 versus hm for which (ahm)1/2 = 0 give the indirect optical band gap. The

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Fig. 2. Schematic diagram of the ITO/PPV-derivative/Al device.

indirect band gap graph is illustrated inset in Fig. 3. The energy band gaps given from the extrapolation of the graph were 2.93, 2.96 and 2.98 eV for P1, P2 and P3 layers, respectively. There is a shift of energy band gap (Eg) values toward lower energy with increasing of the side-chain length that leads to a shift of optical activation energy (DE) value toward the lower/high energy with increasing of the side-chain length. The results indicate a more important p–p interaction of PPV systems in P1 film, which presents lower Eg [27]. In the case of P2 and P3, this type of interaction is limited by increasing the length of the side chain. The Urbach Energies of P1–3 were determined by taking the inverse of the slope of the linear part of the graph plotted between logarithm of the absorption coefficient (a) and photon energy [28]. The obtained values were 0.0295, 0.0239 and 0.0131 eV respectively, indicating a decrease of the disorder when increasing the side-chain length. Indeed, the Urbach Energy (UE) was considered to be a signature of the presence of different types of structure defects in polymer films [29]. This parameter is generally used to characterize the tail due to localized states in the forbidden gap associated with the amorphous nature of the materials, which are related to the structural disorder [28]. To better understood of the dependence of the disorder and p-stacking with the side-group size in P1–P3, we illustrated the variation of the UE and Eg with the number of carbone in the side aliphatic chain (Fig. 4). We can conclude that in increase of the side-chain size involves a limitation of the p–p interaction and an improvement of the order in the PPV films. The photoluminescent properties of the polymers were investigated in thin film state. The PL spectra of the polymers films present significant differences in the profile and mainly in the spectrum width (Fig. 5). The emission spectra of P2 and P3 lie in the blue region and consist of two maxima (425 nm; 450 nm) and a shoulder (483 nm). The P1 spectrum shows the same maxima (428 nm; 453 nm), but the shoulder is transformed into a discrete band and a new shoulder appears at low energies. Hence, a broader spectrum was obtained and a green emission was observed for the P1 thin layer. The spectrum width decreases upon going to the hexylated polymer P2 and the narrowest spectrum is obtained for polymer P3,

Fig. 3. UV–visible spectra of P1, P2 and P3 thin films. The inset shows the corresponding Tauc plots (ahm)1/2 versus photon energy hm.

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P1

P2

P3

kmax (nm) Eg-op (eV)

364 2.93

364 2.96

364 2.98

Fig. 4. Variation of optical gap energy and Urbach energy with side-chain length for PPV-derivative.

incorporating the dodecyloxy chains. There is, therefore, a clear correlation between the size of the side-chain and emission spectrum width which decreases with increasing the chain length. These results can be explained by the fact that the significant p–p interactions can take place in P1 film and the formation of excimer species is favored. In contrast, in the case of P3 polymer carrying relatively bulky side chains such interchain interactions are considerably limited.

3.2. Frontier orbital energy levels The HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels of the polymers, was estimated with Cyclic voltammetry (CV). The onset oxidation potentials (Eox) of P1, P2 and P3 are 1.06, 1.11 and 1.17 V versus SCE, respectively, while the onset reduction potentials (Ered) are 1.93, 1.96 and 1.98 V, respectively. The HOMO and LUMO energy levels were deduced from the oxidation and reduction onsets under the assumption that the energy level of ferrocene is 4.80 eV below vacuum level [30]. The redox potential of this external standard under the experimental conditions was 0.55 V (E1/2, Fc+/Fc). Accordingly, the energy levels were calculated using the following equation: EHOMO,LUMO = (Eox,red  VFOC + 4.80) eV, where Eox,red is the onset oxidation or reduction potential and VFOC is the ferrocene half-wave potential measured versus SCE. The calculated HOMO and LUMO energy levels of the polymers are depicted in the Fig. 6. In fact, these energy levels is indispensable to determine the energy barriers and select the convenient electrodes in different electronic devices, for optimizing the efficiency of charge-carrier photogeneration in, and charge injection in electronic devices like PLEDs, thin-film transistors, and polymeric solar cells [31–33].

3.3. Static electrical study The examination of the transport mechanism of all these polymers provides further insights in understanding the effect of side chain length. Charge transport was investigated using current–voltage measurements for the diode structures ITO/PPV derivative/Al. Fig. 7 shows that the I–V characteristic exhibited typical diode behavior in both forward and reverse bias. The electrically I–V characteristic shows two different behaviors depending on the voltage. For low voltages, the I–V characteristic is symmetric which can be explained by the localized-state theory with defects inducing localized gap states [34]. At higher voltages, an asymmetric I–V characteristic was observed which is mainly attributed to the difference of injection barriers to electrons and holes due to difference between the work functions of the two electrodes as we can see in Fig. 6. The current values obtained are higher for P2 than for P1 and P3 with a threshold bias voltage of 3.67 V, 2.45 V and 3.54 V, these results are in contrariness with the optical results estimated from their p-stacking behavior. To understand this attitude, the conduction mode governing these materials was studied.

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Fig. 5. Photoluminescence spectra of P1, P2 and P3 thin films.

Log–log plots of the I–V characteristics for the three devices are shown inset in Fig. 7. The current dependence of the applied voltage appears to follow power law behavior (J / Vm), described as follows. At low bias, the current increases linearly with voltage for the three devices, indicating an ohmic conduction (J / V) for this regime (segment m / 1), for which the current density is described by:

J X ¼ qn0 l

V d

ð3Þ

where q is the electronic charge, m is the charge carrier mobility, n0 is the free carrier density, V is the applied voltage and d is the film thickness. For medium bias voltage, the current density increases rapidly with voltage and it is described by J / Vm+1. This is attributed to SCLC conduction mechanism limited by distributed traps since the injected charge carriers will fill the traps of the material [35]. If traps are present in the PPV derivative structure, they are due to defects in the chemical structure of the organic material and/or impurities. In this region, the current increases in a power law (J / Vm; m > 2) and it is not observed in the case of ITO/P1/Al structure. At higher bias, all traps are full and the current density depends quadratically on the voltage (J / V2; segment m = 2). This behavior is characteristic of space-charge-limited current (SCLC), for which the current density in the absence of traps in the organic film can be expressed as [36].

J¼

9 V2 e0 es l 3 8 d

ð4Þ

where er is the permittivity of the organic material and e0 is the permittivity of vacuum. The mobility is one of the most relevant parameters to select the materials for optoelectronic applications. Indeed, the performance of the device depends on the mobility which governs the recombination between the injected holes and

Fig. 6. Representation of energy band diagram of the fabricated diode based on P1, P2 and P3.

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Fig. 7. Current–voltage (I–V) characteristics of ITO/PPV-derivative/Al. The inset shows the log–log I–V curves of ITO/P1/Al, ITO/P2/Al and ITO/P3/Al.

electrons. There are different techniques used to estimate the value of mobility such as time of flight (TOF) method [37], field effect transistor (FET) configuration [38] and SCLC experiment [33]. The charge mobility in the semi-conducting polymers P1–3 has been evaluated from SCLC technique. In fact, the intersection of JO (V) and JSCLC (V) log–log plots (Fig. 7b) defines the bias voltage Vtr as follows [39]: 2

V tr ¼

8 d qp 9 0 e0 er

ð5Þ

The electrical parameters for the ITO/PPV-derivative/Al devices are given in Table 2, and show that the charge mobility in the polymer P2 is significantly greater than the cases of P1 and P3. From the length of side-chain, it can be inferred that the introduction of the hexyl side-chain is the most favorable with the higher carrier mobility in the polymer P2, than the ethylated polymer P1 and the least in polymer P3 incorporating the dodecyl side-chains. However, to understand the disparity in the (p–p-stacking ability) p–p interaction between the conjugated segments in the solid state and the mobility result, leads us to investigate the morphological features of the films. The influence of alkyl chain length on the device performance is complicated. Many factors may together contribute to the different device performance. For example, P1 showed the highest HOMO level in the form of film, which facilitated efficient hole injection from Al electrode. Furthermore, P2 showed the most intense mobility in electrical measurements. Indeed, thin film morphology is the most determining factor in this study. To understand such variations of the device characteristic, we performed further studies on the morphology of the P1, P2 and P3 layers deposited onto ITO has been studied by Atomic Force Microscopy (AFM). It is found that the three films showed different morphologies. As shown in Fig. 8, that the RMS surface roughness values for the three films vary from 6.42 nm for P1 to 1.75 and 2.5 nm for P2 and P3 respectively. Then, the P2 film with (6C) alkyl chains exhibit low RMS values, and a very homogenous film compared with two other films. Provided hence, the best mobility of structure ITO/P2/Al, confirmed in Fig. 9. Where the difference in the surface roughness has significant effect on the device performance [40]. 3.4. Dielectrical study Impedance spectroscopy (IS) is a powerful method to understand the charge transport and the conduction mechanism in different organic electronic and optoelectronic components [41]. The IS was applied to investigate the dielectric behavior of the devices P1, P2 and P3 at different dc forward bias voltages. For all devices, the obtained Cole–Cole plots show only one semicircle which is close to the origin and decreases in size with increasing bias voltage, as illustrated in the Fig. 10 for P1 as example; this behavior is in fact typical for single-layer devices [42]. The minimum Re Z value (at the highest frequency) represents a series resistance Rs attributed to the ohmic contact at the hole injected ITO/polymer interface [43]. The maximum Re Z value (at the lowest frequency) represents the sum of the series resistance and the parallel resistance to the capacitance. This structure can be modeled by an equivalent electrical circuit designed as a parallel resistor Rp and capacitor Cp network with a series resistance Rs (insute in Fig. 10). Rs is bias and frequency independent and should be associated to the ITO/polymer contact; Rs value is relatively small compared to the volume resistance Rp. We illustrate afterward, the frequency-dependent real and imaginary part of the impedance of the device based on the polymer. But before starting this study we will recall that the complex impedance Z(x) under sinusoidal regime can be expressed as [44]:

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Table 2 Ac electrical parameters for the ITO/PPVderivative/Al devices, derived from the impedance spectroscopy study, evaluated using the equivalent circuit at different applied bias voltages. Device bias (v)

0 0.5 1 1.5 2

ITO/P1/Al

ITO/P2/Al

ITO/P3/Al

Rs (O)

Rp (kO)

Cp (nF)

Rs (O)

Rp (kO)

Cp (nF)

Rs (O)

Rp (kO)

Cp (nF)

76378.105 513.840 503.299 425.766 6315.605

8664.602 2799.974 600.984 1142.946 375.598

33.329 26.147 26.830 26.537 33.677

146.822 188.348 107.863 115.688 158.108

1384.090 1368.419 744.662 529.132 329.960

30.036 29.310 30.818 30.557 30.975

117.30 117.03 111.7 115.19 117.09

22.840 10.55 5.783 5.022 3.278

29.932 27.819 27.382 20.733 25.28

Fig. 8. AFM topographic images of P1, P2 and P3 thin films.

ZðxÞ ¼ Re ðZÞ þ jImðZÞ ¼ Z 0 þ jZ Z 0 ¼ Rs þ

Z 00 ¼

Rp 1 þ R2p C 2p x2 R2p C p x

1 þ R2p C 2p x2

00

ð6Þ ð7Þ

ð8Þ

where Z0 and Z00 represent real and imaginary parts of the impedance Z(x), respectively and x is the angular frequency of the ac excitation. In our case, it is important to note that the maximum of Z0 value corresponds to the sum of the device

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477

Fig. 9. Surface roughness and average hole mobilities of PPV-derivative plotted as a function of alkyl chain length.

resistance and the resistance of the ITO substrate whereas the maxima peak of Z00 corresponding to the relaxation frequency f 0 and s0 indicate the relaxation time. s0 values (ms) indicate a dipolar relaxation type. We presented in Fig. 11(a) and (b) the real (Z0 ) and imaginary (Z00 ) parts of the complex impedance in the wide frequency range at different dc voltages. This figure shows that the real part of the impedance decreases as the applied dc bias increases. We also observe that the relaxation frequencies of the hopping charge carriers in the polymers are shifted and the relaxation times are seen to get shorter with the bias increase as seen in Table 2. The decrease of s0 is due to injection of a large number of charge carriers into the device when the bias voltage is increased and consequently the Rp decreases. Experimental data corresponding to the impedance measurement are well fitted for different bias voltages by the equivalent circuit proposed before. As represented in Fig. 12(a) and (b), fitted parameters of the device have been plotted versus bias voltages. We show that the Rp decreases for the ITO/PPV-derivative/Al devices as the dc bias voltage increases from 0 to 2 V as a result of the enhancement in the number of injected charge carriers into the polymer. However, Cp remains rather constant, in despite of the relatively high level of injected charges, which indicates that the devices still effectively acting as simple parallel plate capacitors [45]. In fact, the injected charges can be trapped during hopping movement with increased bias voltage [46]. According to the SCLC theory with an exponential trap distribution, the voltage-dependent current density is given by [47]: 2mþ1

J ¼ KV mþ1 =d

;

ð9Þ

where d is the film thickness and K is a constant. The voltage dependence of Rp can be elucidated as [48]:

Rp / V=J / V m

ð10Þ

To determine the properties of the trap distri x bution in the ITO/PPV-derivative/Al devices, the Rp were plotted versus V on a log–log scale (Fig. 13); a linear variation were observed for all devices (with m = 1.41, m = 0.90 and m = 1.07 for P1, P2 and P3, respectively), which shows that hole conductance in the polymer thin films is consistent with an exponential trap distribution.

Fig. 10. Cole–Cole plots of complex impedance with a variation of bias voltages in device P1. Symbols are experimental results and lines are fits according to the equivalent circuit. The inset shows the electrical equivalent circuit of the ITO/PPV-derivative/Al sandwich structure.

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Fig. 11. The frequency dependent real and imaginary parts of the impedance for (a) ITO/P1/Al (device P1), (b) ITO/P2/Al (device P2) and (c) ITO/P3/Al (device P3) at different bias voltages.

The investigation of variation of conductance versus frequency at different bias voltages for our three devices show that conductance characteristic remains constant at low frequencies and power-law behavior was observed at higher frequencies. Moreover, the conductance increased with the applied bias voltage. In general, the variation of conductance according to frequency in disordered materials obeys the relation [49]:

GðxÞ ¼ Gdc þ Gac ðxÞ; where Gac ðxÞ  xs

ð11Þ

where Gdc is the dc conductance, is the angular frequency of the applied excitation, and s is the critical exponent (0 < s < 1). Fig. 14 shows that P2 is the most conductive device at 0 V. This result is in accordance with the dc electrical study. The frequency dependence of conductivity can be expressed as [50,51]

r ¼ rdc þ rac ðxÞ ¼ rdc þ Axs :

ð12Þ

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479

Fig. 12. Variation of the fitting parameters (a) resistance Rp and (b) capacitance Cp with bias voltages for device P1, P2 and device P3.

Fig. 13. Log–log plot of Rp with applied bias voltages for device P1, device P2 and device P3.

where rdc is the dc electrical conductivity, rac ðxÞ; the ac conductivity, A, the dispersion parameter and s the dimension less critical exponent (0 < s < 1) characterizing hopping conduction [51]. The dc conductivity remains constant at low frequency (x ? 0). However, after a critical frequency (fc), the as conductivity starts to increase with the frequency (Fig. 14). Thus, a hopping transport mechanism can be postulated [51]. The beginning of the conductivity relaxation phenomenon is indicated by the change from frequency-independent to frequency-dependent behavior. For ac conductivity, the charge carrier will hop from site i to site j (during sH ), where it will relax during s0 (related to the ac electrical parameters

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Fig. 14. The superposition conductivity r (x) as a function of frequency for device P1, device P2 and device P3 at 0 volt.

Table 3 Hopping parameters for the [ITO/Polymer/Al] devices, derived from the impedance spectroscopy study at 0 V bias voltages. Device structure

ITO/P1/Al

ITO/P2/Al

ITO/P3/Al

Relaxation frequency (kHz) Relaxation time s0 (ms) rdc (S m1) A (S m1 rad1) s Hopping frequency, fH (kHz) Hopping relaxation time, sH (ms)

3.343 0.299 1.433 0.016 0.979 98.627 0.010

26.819 0.037 50.416 0.047 0.959 1445.545 6.917E4

1.495 0.668 3.824 0.013 1.000 294.153 0.003

described in the previous section). This is typical behavior for a wide variety of materials and is called the Jonscher universal dynamic response (UDR).The frequency corresponding to the beginning of the conductivity dispersion is known as the hopping frequency f H [52,53]. By fitting the plot of Fig. 14 at 0 V and extracting the parameters A and s, we can obtain f H from the universal law

f H ¼ ðrdc =AÞ1=s

ð14Þ

For devices P1, P2 and P3 we deduced the hopping relaxation time at 0 V according to:

sH ¼ ð1=f H Þ

ð15Þ

Table 3 summarizes the hopping conduction parameters dc, A, s, f H and sH at 0 V. s0 and sH are the relaxation time for the charge carrier in a site i and the time to jump from a site i to another j, respectively. The best conductive material will have the lowest sum of relaxation time and the Hopping time s0 + sH , which is the case for the ITO/P2/Al device. This behavior confirms the static electrical results. 4. Conclusion In conclusion, we have synthesized a series of PPV-derivative and systematically investigated the influence of side-chain length on the optical, morphological and electrical properties of the thin polymer film. The results showed that different side chain length appreciably influenced the solid-state properties, such as the UV–visible absorption and photoluminescence, the energy levels and the film morphology, leading to significant differences in the performances of the ITO/PPV derivative/Al devices. The electrical study demonstrates that introduction of medium-size hexyl chains on the PPV backbone enhances its carrier mobility. References [1] A. Rouis, M. Echabaane, N. Sakly, I. Bonnamour, H. Ben Ouada, Characterization of a sensitive and selective copper optode based on b-ketoimine modified calix [4] arene derivative, Mater. Sci. Eng., C 46 (2015) 125–131. [2] M. Bouzitoun, C. Dridi, R. Mlika, R. Ben Chaabane, H. Ben Ouada, N. Jabballah, H. Gam, M. Majdoub, Electrical and sensing properties of partially benzylated b-cyclodextrin: effect of benzyl chain length, Sens. Actuat. B: Chem. 126 (2007) 91–96. [3] C. Brabec, V. Dyakonov, J. Parisi, N.S. Sariciftci, Organic Photovoltaic’s: Concepts and Realization, Springer, 2003.

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