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n-Si heterojunction diode
Synthetic Metals 214 (2016) 92–99
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Structural and optical characteristics of PEDOT/n-Si heterojunction diode A. Asherya , G. Saidb , W.A. Arafac , A.E.H. Gaballahd, A.A.M. Farage,f,* a
Solid State Physics Department, Physics Division, National Research Center, Dokki, Cairo 12311, Egypt Physics Department, Faculty of Science, Fayoum University, Fayoum, Egypt Chemistry Department, Faculty of Science, Fayom University, Fayoum, Egypt d Research Scholar, Physics Department, Faculty of Science, Fayoum University, Egypt e Department of Physics, Faculty of Science and Arts, Aljouf University, Aljouf, Saudi Arabia f Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Roxy 11757 Cairo, Egypt b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 November 2015 Received in revised form 5 January 2016 Accepted 13 January 2016 Available online 10 February 2016
In this work, Poly(3,4-ethylenedioxythiophene), Tetra Methacrylate (PEDOTTM) film was deposited by spin coating on glass substrates. Surface topography and crystalline properties of PEDOTTM were carried out by scanning electron microscopy and transmission electron microscopy, respectively. Fourier transform infrared spectroscopy was used for investigating vibrational properties.Optical characteristics were investigated using spectrophotometric measurements in the wavelength range 200–2500 nm. Absorption characteristic in the UV region shows a well-defined absorption band. The UV–vis absorption spectrum was resolved by molecular orbital and band theories and the optical transition of the films is allowed and direct. Optical dispersion parameters namely oscillator energy,dispersion energy, lattice dielectric constant and high frequency dielectric constant were determined. Temperature dependent current density–voltage (J–V) characteristics was studied to clarify the dominant charge transport in the temperature range 398–373 K. Moreover, temperature dependent of barrier height, ideality factor and series resistance were also considered. The presented results offer a new perspective in optoelectronic applications. ã 2016 Published by Elsevier B.V.
Keywords: Thin film Optical properties Energy gap Heterojunction
1. Introduction Organic semiconductors thin films for electronic devices have been attracting great considerable attention due their particular characteristics such as low cost, low weight, good mechanical flexibility, easy processing from solution, large-scale fabrication, and the ability to modify their structure to obtain a desired electrical and optical characteristics [1,2]. Moreover, these materials possess favorable electrical, optical and optoelectronic characteristics applicable for designing new systems of solar energy conversion, laser electrophotography, photoelectrochemical cells, photosensitization, electrocatalysis and electrophotography [3–5] Conjugated polymers like Poly(3,4-ethylenedioxythiophene) (PEDOT) and their derivatives have been known to have defined electronic properties among the conjugated polymers [6]. A large
* Corresponding author at: Ain Shams University, Thin Film Laboratory, Physics Department, Faculty of Education, Roxy, 11757, Egypt. Fax: +20 22581243. E-mail addresses: [email protected], [email protected] (A.A.M. Farag). http://dx.doi.org/10.1016/j.synthmet.2016.01.008 0379-6779/ ã 2016 Published by Elsevier B.V.
number of Schottky barrier diodes have been fabricated and characterized using organic conductive polymers together with metals and substantial research studies have been made on the preparation and characterization of conducting polymer/inorganic semiconductor heterojunction [6,7]. Poly(3,4-ethylenedioxythiophene), Tetra Methacrylate (PEDOTTM) has attracted considerable attention due to its excellent environmental stability, low oxidation potential and high electrical conductivity as well as its good electrochromic characteristics [8]. Most of literature review confirms that a detailed correlation between the morphology of PEDOT film and the preparation route has not yet been established [2–8]. Therefore, this work presents some of the main experimental results of the morphological and optical characteristics of the spin coated PEDOTTM film. The using of spin coating technique is based on the fact that this method is more easy to manufacture, low coast, as well as characterized by more reliable seed coatings with better particle adhesion to the substrate. In addition, optical band gap is also estimated based on the application of the energy band theory. Furthermore, the current–voltage characteristics of PEDOTTM/n-Si heterojunction
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are also studied for check the availability of this type of heterojunction for device application.
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resistance with each material and have a lower barrier which can be vanishes as compared to the barrier originated from PEDOTTM/ n-Si interface.
2. Experimental details 2.2. Characterization tools 2.1. Materials and thin film preparation Poly(3,4-ethylenedioxythiophene),Tetra Methacrylate (PEDOTTM) was purchased from Sigma–Aldrich Chem. Co. and used as received without further purification (0.5 wt.% (dispersion in propylene carbonate), contains p-toluenesulfonate as dopant). The schematic diagram of the molecular structure of PEDOTTM is shown in Fig. 1. Thin films of PEDOTTM were prepared by spin coating on suitable substrates such as corning glass for optical measurements and n-Si substrates for heterojunction measurements. The glass substrates were carefully cleaned by a detergent, and then in methanol and acetone each for 10 min by using an ultrasonic cleaner. At last, the glass substrates were rinsed with deionized water and dried with nitrogen gun. n-Type Si (111) single crystals substrates was etched by HF/NH4F/H2O (2:7:1) for 1.5 min to remove the surface layer of silicon dioxide, SiO2 and then washed for 2 min in pure alcohol and distilled water and finally dried with nitrogen. Solution of PEDOTTM was prepared using pure ethanol as a solvent and adequately stirred. The film of this material was prepared by spin coating technique at a spinning speed of 800 rpm for 60 s at room temperature (300 K). After the spin coating, the film was dried at 373 K for 10 min to evaporate the solvent and to remove the organic residuals. This coating/drying procedure was repeated for five times to obtain a suitable thickness for measurements. The thickness of the films was determined with Mettler Toledo MX5 microbalance. This thickness was also determined by the cross sectional imaging of the deposited film and the substrate edge using scanning electron microscopy (SEM) of type JEOL JEM1230. The obtained heterojunctions were coated from the back side (n-Si side) by thin films of Au-ohmic contact using the high vacuum sputtering unit (Turbo Sputtering RF & DC Power Supplies Deposition System Model Hummer 8.1) was used for evaporating ohmic contact of high purity of gold onto the top of the PEDOTTM film and the bottom of the n-Si substrate. These contacts has a low
In this work, the organo-functionalized surface chemistry of filler nanoparticles was characterized by a 4100 Jasco-Japan FTIR spectrophotometer. The spectrum was collected in the range from 4000 to 400 cm1. The morphology of the prepared TiO2 samples were investigated by scanning electron microscopy (SEM) type JEOL-JAX-840A, with accelerating voltage 30 kV. Transmission electron microscopy (TEM) study includes transmission electron micrographs study type JEOL JEM-1230, with maximum resolving power 0.2 nm, energy 40–120 kV on steps, maximum magnification power 600,000 and computerized with Ultra Scan 1000 2 k 2 k CCD. The UV–vis spectra of the spin coated films (absorbing) on corning glass substrates (nonabsorbing) were recorded from 200 nm to 2500 nm wavelength using SHIMADZU UV-3600 UV–vis-NIR spectrophotometer at room temperature (300 K). The current–voltage (I–V) measurements were performed by the use of a high impedance Keithley 617 programmable constant current source electrometer. 3. Results and discussion 3.1. Structural and molecular characteristics of PEDOTTM SEM images as indicated in Fig. 2(a) and (b) clearly show the surface morphology of PEDOTTM film deposited on glass substrate. This figure clarifies that the surface is composed of particles uniformly distributed over all the scanned area as well as numerous nanoparticles joined together to form an aggregate. This structure turned to be randomly oriented nanoparticles with average grain size 30 nm which agree with the obtained data of TEM images. The thickness of the film can be extracted from the cross sectional image of SEM at edge between the film and the substrate (film–substrate interface). A highly adhesion of the film to the substrate was observed. The vertical boundary of the film at the broken edge is shown in the inset of Fig. 2(b) and the average calculated thickness is about 300 nm. Fig. 3 presents the TEM images of PEDOTTM particles dispersed in propylene carbonate. The specimen was taken from a droplet of PEDOTTM suspension, was treated by ultrasonic dispersion.It is observed that nearly spherical nanoparticles are dispersed in propylene carbonate with diameter ranging from 25 to 35 nm. In order to detect the vibration characteristics (stretch, contract and bend etc.) of chemical functional groups in PEDOTTM, IR spectroscopy is employed in the wavenumber range 400– 4000 cm1 as shown in Fig. 4. The absorption bands at 3428 can be assigned the O H stretching vibration mode [9]. Absorption signal at 2989 is associated to the stretching C H ring vibration [10]. The strong absorption band at 1795 cm1 is due to the presence of C¼O stretch vibrations [11]. The absorption bands at 1478 and 1400 cm1 can be attributed to the stretching modes of C¼C and C C modes in the thiophene ring [12]. Absorption bands at 1183, 1036, 772 and 710 cm1 can be assigned to the ethylenedioxy group [13–15]. Moreover, the bands occurring at 772 and 710 cm1 correspond to the interaction in the CS of the thiophene ring modes [11–15]. 3.2. Optical characteristics
Fig. 1. Schematic diagram of PEDOTTM.
The transmittance, T (l) and reflectance, R (l) spectra of PEDOTTM thin films were measured at normal incidence at room temperature in the spectral range of 200–2500 nm with
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Fig. 2. (a,b) SEM micrograph of PEDOTTM thin film with different magnifications. The inset of Fig.2(b) shows the cross section image.
Fig. 3. (a,b) TEM micrograph of PEDOTTM nanoparticles.
r
where a is the absorption coefficient, M is molecular weight (55,000) and r is the sample density (1 g/cm3 at 20 C) [22,23]. Spectral distribution dependence of emolar for PEDOTTM thin film is shown in Fig. 6. Well Gaussian fit for the molar extinction coefficient data is shown in Fig. 7. The main important absorption parameters are calculated and listed in Table 1, in comparison with
2989
1478
710 772
80
3562
100
1400
60 40 1036 1183
20 0
1795
Transmittance %
uncertainty of 1% as given by the manufacturer. Fig. 5 shows the optical transmittance spectra of the used PEDOTTM film with two different thicknesses of 220 and 300 nm. It is clear that the value of optical transmission increases with increasing the wavelength. The higher film thickness of 300 nm has a lower transparency as compared to the lower one (220 nm), especially in the higher wavelength range l > 600 nm The optical reflectance (R) for the PEDOTTM thin film is found to decrease with increasing the wavelength in the lower wavelength region l < 900 nm. While value of R is slightly increases with increasing wavelength, in the region l > 900 nm. This behavior is similar to those published elsewhere by Medina et al. [16]. As can be seen in Fig. 6(a), the absorption spectra exhibit an excitonic absorption band peaked around 284 nm typical for the film thickness 220 nm. Upon increasing the film thickness to 300 nm the excitonic absorption is slightly blue-shifted by 2.5 nm. This behavior is similar to those observed by Li and Zhang [17]. In the present work, one can deduce different important absorption parameters such as molar extinction, em coefficient, oscillator strength, f (a dimensionless parameters of the electromagnetic radiation that indicates the absorption probability of the electromagnetic radiation for transition between energy levels of a molecule) and electric dipole strength EDS using the following expressions [18–21] Z M em ¼ 2300 a; f ¼ 4:38 109 em dy ð1Þ
500
1000
1500
2000
2500
3000
3500
-1
Wavenumber, (cm ) Fig. 4. FTIR spectrum of PEDOTTM nanoparticles.
4000
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elsewhere [27–31]. In the present work, the higher calculated energy gap of PEDOTTM can be attributed to some reasons such as the polymerization process, the deposition technique, the doping process, and others [24]. Groenendaal et al. [25] reported that the energy gap of PEDOTTM can change from 1.4 to 2.5 eV depending on the doping ratio. This highly change gives contribution of PEDOTTM for various applications such as solar cells, and smart windows as well as photographic films. 3.3. Determination of the optical constants
Fig. 5. Transmittance and reflectance spectra of PEDOTTM films.
other organic polymers thin films. It is observed that the obtained results are in good agreement with the previously reported results of other polymers [22,23]. The valence band and conduction band are the solid state analogue of the highest occupied molecular orbitals, HOMO and the lowest unoccupied molecular orbitals, LUMO of molecules. In many organic materials, it is usual to analyze the optical absorption at the fundamental edge in terms of band-to-band transitions theory [24]. The fundamental absorption edge of PEDOTTM film is formed by a direct allowed transition. Optical gap of PEDOTTM film can be obtained from the analysis of the spectral dependence of the absorption near the fundamental absorption edge which is found to be direct allowed transition. Accordingly, the absorption coefficient can easily be described by the following expression [23] ð2Þ ðahyÞ2 ¼ A hy Eg ; where hy is the photon energy, A is a factor depends on the transition probability and Eg is the optical band gap. Fig. 6(b) shows the plot of ðahyÞ2 vs. hy near the absorption edge for PEDOTTM films. This plot gives a best linear fit over a wide range of photon energy, near the band gap edge. Eg can easily be estimated from the linear extrapolation to the photon energy axis, where ðahyÞ2 = 0 (and listed in Table 2), in comparison with those published
The refractive index n and excitation coefficient k (al/4p) of the films were determined using the well known equations stated in [23,26] and plotted as a function of wavelength (not shown here) (Fig. 8). The n values decrease with the increase of wavelength. The refractive index of the films in the visible range was found about less than 1.8 in this study. So, the decrease in refractive index may be attributed to the increase in film thickness, where the film thickness has an important effect on the refractive index [26]. Moreover, the k values increase with the increasing of wavelength. 3.4. Dispersion characteristics of PEDOTTM films Dispersion characteristics supply a demonstration of the change of the index of refraction (n) at higher wavelength region, where the extinction index (k) is considered to be negligible. A better description for this dispersion was introduced by Wemple and DiDomenico using the following well known expression [27]: 1 Eo 1 ¼ ðhyÞ2 ðn2 1Þ Ed Eo Ed
ð3Þ
where Ed is the dispersion energy and Eo is the single oscillator energy. Graphical representation of ðn2 1Þ1 vs. ðhyÞ2 is plotted and shown in Fig. 9(b) for the two thicknesses and by the fitting of the data to a straight line, Eo and Ed can be determined from the intercept and the slope. The calculated dispersion values of Eo and Ed as well as the corresponding high-frequency dielectric constant (e1 = n21 ) for the films are also listed in Table 3 in comparison with those published dispersion data of related organic films [23,28]. As observed, a good agreement is observed which gives an indication
Fig. 6. (a) Absorption spectrum of PEDOTTM films and (b) plots of (ahn)2 vs. photon energy of PEDOTTM films.
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It is seen from Fig. 9(a) and (b) that the values of e1 are higher than that of e2 values in the same photon energy range and both characteristics are nearly different. The variation of the dielectric constant with photon energy indicates the probability of the presence of some interactions between photons and electrons in the films that produced in this energy range. These interactions can cause formation of peaks in dielectric spectra that depends on the type of the material. 3.5. Current density–voltage characteristics of PEDOTTM/p-Si heterojunction
Fig. 7. Plot of emolar vs. wavenumber for PEDOTTM films.
for the suitability of the prepared films in the field of optoelectronic devices. Real part of the dielectric constant e1has a relation with wavelength (l) according to the following [28]: e2 N e1 ¼ n2 ¼ eL l2 ; ð4Þ 4p2 c2 e0 m where eL is known as lattice dielectric constant e0 is the free space permittivity, c is velocity of light and (N/m*) is the ratio of free carrier concentration to its effective mass. Spectral distribution of e1 = n2, vs. l2 is shown in Fig. 9(a). e1 decreases with increasing l2 and the representation can be linearly fit in the chosen range. Both values of eL and N/m* for the two thicknesses can easily be obtained from the extrapolation of the straight line section ofthe curve to intersect with the ordinate axis and the slope, respectively. Value of eL is listed in Table 3 in comparison with those published before for some related organic films. In addition, the disagreement between eL and e1 gives support for the availability for the presence of free carrier contribution [23,28]. The dielectric properties are concerned with the storage and dissipation of electric and magnetic energy in materials [29]. The e and its real, e1 and imaginary,e2 , complex dielectric constant, ~ parts are described by the following relations [29]: ~ eðhyÞ ¼ e1 ðhyÞ ie2 ðhyÞ;
ð5Þ
where e1 ¼ ðn k Þ and e2 ¼ 2nk. 2
2
Temperature dependence of semi-logarithmic current density– voltage (J–V) characteristics of PEDOTTM/p-Si heterojunction is shown in Fig. 10. A nearly non-symmetric shape under forward and reverse bias confirms the diode—like characteristics with weak rectification property. As observed, the forward and reverse currents for all the studied temperatures are exponentially increase with increasing bias. Moreover, the reverse currents have not saturation nature, indicating a weak characteristic for the heterojunction. In addition, a little voltage drop across a forwardbiased with the current is also observed. Furthermore, a noticeable temperature effect is also recorded which gives a confirmation for the validity of the device as a temperature sensor. The overall dark current in the heterojunction is basically estimated to be the totality of several components; one of them is normally predominant in a definite voltage range. Generally, at the higher temperature range the thermionic emission mechanism is predominant as compared to other ones [30]. Current density– voltage characteristics are demonstrated in the narrow biasing voltages (0.1 V V 0.6 V) using thermionic emission mechanism as follows [31]: qðV IRs Þ 1 ð6Þ J ¼ J0 exp hkT where Rs is the series resistance, V is the applied voltage, h is the ideality factor, k is the Boltzmann’s constant, and J0 is the saturation current density given by the following expression [31]: qFb ð7Þ ; J0 ¼ A T 2 exp kT where A* is the effective Richardson constant and Fb is the effective barrier height. For emphasizing that the thermionic
Table 1 Absorption parameters of PEDOTTM films in comparison with other organic films. Compound
em
f
EDS
Reference
PEDOTTM (Thickness 220 nm) PEDOTTM (Thickness 300 nm) Rhodamine B Poly(o-toluidine) (POT)
5.23 107 1.17 108 3 104 1.67 107
1108.14 5650.2 0.214 0.278
2335.27 9534.24 0.99 0.15
Present work Present work [22] [23]
Table 2 Values of energy gaps of PEDOTTM films in comparison with other organic films. Compound TM
(thickness 220 nm) PEDOT PEDOTTM (thickness 300 nm) PEDOT Poly(o-toluidine) (POT) Polypyrrol Polypyrrole-chitosan (PPy-CHI) depending on the chitosan content Undoped ploypyrrol film
Eg (eV)
References
1.93 1.80 1.4 1.2 1.60 1.30 1.72
Present work Present work [27] [28] [29] [30] [31]
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Fig. 8. (a) Photon energy dependence of real parts of the dielectric constant and (b) Photon energy dependence of imaginary parts of the dielectric constants of the PEDOTTM films.
Fig. 9. (a) Variation of n2 vs. l2 and (b) plot of (n2 1)1 vs. photon energy (hn)2 for PEDOTTM films.
conduction is the dominant conduction mechanism at the desired narrow voltage range, the dependence of ln (J0/T2) vs. 1/T is checked as shown in Fig. 10(b). The linear fit of this dependence gives support for the availability of this mechanism. From the slope of this straight line, one can obtain the value of the effective barrier height which is found to be 0.12 eV. Moreover, A* can also be obtained from the intercept which is found to be 8.39 A/cm2 K2 which is lower than those obtained by Ashery et al. [32] for p-Si. This disagreement in A* can be attributed to the presence of inhomogeneous barrier height and potential fluctuation at the interface that consist of low and high barrier areas [33]. Values of Fb and h were determined from the forward I–V characteristics at different temperatures (Fig. 11(a)) using Eqs. (1) & (2), respectively and plotted as function of temperature as shown in Fig. 11(b). As observed, values of h show non-ideal diode characteristics (deviates from unity). Moreover, values of h are found to be high in the lower temperature region and gradually decrease with increasing temperature. High values of the ideality factor can be attributed to various factors such as inhomogeneities
of organic film thickness, non-uniformity of the interfacial charges and the effect of series resistance. Moreover, another probability of the presence of insulating oxide layer at the interface of PEDOT/ n-Si which may be originated from any adsorbed water vapor onto the Si surface before the deposition process. This deviation of h from unity indicates that the device is not ideal and then there is no ideal thermionic emission process can be taken place [31,33]. On the contrary, values of Fb were found to increase with increasing temperature. Such temperature dependence can be due to the
Table 3 Dispersion parameters of PEDOTTM films in comparison with other organic films. Compound
Eo (eV)
Ed (eV)
e1
eL
References
PEDOTTM (thickness 220 nm) PEDOTTM (thickness 300 nm) Polypyrrol Polyaniline
6.39 5.37 3.71 4.56
9.4 11.2 4.61 11.03
2.47 3.08 2.24 3.43
3.32 3.29 2.34 3.85
Present work Present work [23] [28]
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Fig. 10. (a) Plot of J–V characteristics of PEDOTTM/n-Si heterojunction at different films temperatures and (b) plot J0/T2 vs. 1000/T.
Fig. 11. (a) Plot of forward J–V characteristics and (b) temperature dependence of effective barrier height and ideality factor.
Fig. 12. (a) RJ–V characteristics and (b) temperature dependence of series resistance, Rs.
A. Ashery et al. / Synthetic Metals 214 (2016) 92–99
presence of non-uniformity of the interface charge or spatial inhomogeneities of barrier height [34]. Series resistance has effectiveness on the electrical parameters of junctions and then several methods were used to determine its value. In the present work, Rs is determined from the dependence of junction resistance, RJ, on bias (i.e. RJ vs. V) as shown in Fig. 12(a). As observed, values of RJ is found to decrease with increasing bias but higher bias, RJ reaches a nearly saturation which corresponds to Rs. Fig. 12(b) shows the temperature dependence of Rs. Decreasing of the series resistance with increasing temperature can be attributed to some factors reliable for the decrease of h. Decreasing in Rs with increasing temperature means improving for the junction performance as the temperature increase [35]. 4. Conclusions Morphology characteristics of PEDOTTM indicate that the surface is composed of aggregation and a randomly oriented nanoparticles with average grain size 30 nm. Results of FT-IR indicates that the used spin coating is considered as a suitable technique for obtaining undissociated PEDOTTM films and confirmed that the molecular structure has stable structure. Absorption spectra exhibit an excitonic absorption band peaked around 284 nm. Optical dispersion was interpreted by single oscillator model. A direct allowed transition was characterized for the film. Current density–voltage characteristics of the PEDOTTM/p-Si heterojunction showed temperature dependence in the temperature range 298–373 K. Thermionic emission was suggested as a dominant charge transport mechanism with regarding of inhomogeneity of barrier height due to the mismatch over the interface. All the prepared junction parameters showed temperature dependence which open a new view-point in electronic device applications. References [1] Z. Li, F. Qin, T. Liu, R. Ge, W. Meng, J. Tong, S. Xiong, Y. Zhou, Org. Electron. 21 (2015) 144–148. [2] F. Zabihi, Y. Xie, S. Gao, M. Eslamian, Appl. Surf. Sci. 338 (2015) 163–177. [3] Q. Wang, M. Ahmadian-Yazdi, M. Eslamian, Synth. Met. 209 (2015) 521–527.
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[4] M.M. Makhlouf, H.M. Zeyada, Synth. Met. 211 (2016) 1–13. [5] C. Constantinescu, A. Diallo, A. D’Aleo, F. Fages, P. Rotaru, C. VidelotAckermann, P. Delaporte, A. Alloncle, Synth. Met. 209 (2015) 29–33. [6] F. Zabihi, Y. Xie, S. Gao, M. Eslamian, Appl. Surf. Sci. 338 (2015) 163–177. [7] Ö. Yagci, S.S. Yesilkaya, S.A. Yüksel, F. Ongül, N.M. Varal, M. Kus, S. Günes, O. Icell, Synth. Met. 212 (2016) 12–18. [8] C. Tozlu, A. Mutlu, Synth. Met. 211 (2016) 99–106. [9] B.J. Saikia, G. Parthasarathy, J. Mod. Phys. 1 (2010) 206–210. [10] K. Motobayashi, K. Minami, N. Nishi, T. Sakka, M. Osawa, J. Phys. Chem. Lett. 4 (2013) 3110–3114. [11] P. Domlin, C. Kvarnstom, A. Ivaska, J. Electroanal. Chem. 570 (2004) 113–122. [12] S. Radhakrishnan, C. Sumathi, V. Dharuman, J. Wilson, Anal. Methods 5 (2013) 684–689. [13] A. Balamurugan, K.C. Ho, S.-M. Chen, Synth. Met. 159 (2009) 2544–2549. [14] S.V. Selvaganesh, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, Nanoscale Res. Lett. 2 (2007) 546–549. [15] S.S. Vinod, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, Nanoscale Res. Lett. 2 (2007) 546–549. [16] B.M. Medina, D. Wasserberg, S.C.J. Meskers, E.M. Osteritz, P. Bäuerle, J. Gierschner, J. Phys. Chem. 112 (2008) 13282–13286. [17] J. Li, J.Z. Zhang, Coord. Chem. Rev. 253 (2009) 3015–3041. [18] W. Demtröder, Laser Spectroscopy: Basic Concepts and Instrumentation, Springer, New York, 2007. [19] A.A.M. Farag, Opt. Laser Technol. 39 (2007) 728–732. [20] B. Friedel, Thomas J.K. Brenner, Christopher R.. McNeill, U. Steiner, N.C. Greenham, Org. Electron. 12 (2011) 1736–1745. [21] http://nano.tau.ac.il/mncf/images/MSDS/MSDS_for_PEDOTPSS_AI4083.pdf. [22] A.A.M. Farag, I.S. Yahia, Opt. Commun. 283 (2010) 4310–4317. [23] A.A.M. Farag, A. Ashery, M.A. Shenashen, Physica B 407 (2012) 2404–2411. [24] A. Elmansouri, A. Outzourhit, A. Oueriagli, A. Lachkar, N. Hadik, M.E. Achour, A. Abouelaoualim, A. Malaoui, K. Berrada, E.L. Ameziane, Synth. Met. 160 (2010) 1487. [25] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494. [26] F. Yakuphanoglu, M. Sekerci, A. Balaban, Opt. Mater. 27 (2005) 1369–1374. [27] J.N. Hodgson, Optical Absorption and Dispersion in Solids, Chapman and Hall Ltd., London, 1970. [28] A. Ashery, A.A.M. Farag, M. Shenashen, Synth. Met. 162 (2012) 1357–1363. [29] A.A.M. Farag, I.S. Yahia, Synth. Met. 161 (2011) 32–39. [30] A.A.M. Farag, M. Fadel, I.S. Yahia, Curr. Appl. Phys. 12 (2012) 1436–1444. [31] M.C. Arenas, N. Mendoza, H. Cortina, M.E. Nicho, H. Hu, Sol. Energy Mater. Sol. Cells 94 (2010) 29–39. [32] A. Ashery, A.A.M. Farag, M.A. Salem, Microelectro. Eng. 85 (2008) 2309–2315. [33] H. Uslu, A. Bengi, S.S. Cetin, U. Aydemir, S. Altındal, S.T. Aghaliyeva, S. Ozcelik, J. Alloys Compd. 507 (2010) 190–195. [34] A.A.M. Farag, B. Gunduz, F. Yakuphanoglu, W.A. Farooq, Synth. Met. 160 (2010) 2559–2563. [35] A.A.M. Farag, I.S. Yahia, T. Wojtowicz, G. Karczewski, J. Phys. D: Appl. Phys. 43 (2010) 215102–21509.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Structural and optical characteristics of PEDOT/n-Si heterojunction diode A. Asherya , G. Saidb , W.A. Arafac , A.E.H. Gaballahd, A.A.M. Farage,f,* a
Solid State Physics Department, Physics Division, National Research Center, Dokki, Cairo 12311, Egypt Physics Department, Faculty of Science, Fayoum University, Fayoum, Egypt Chemistry Department, Faculty of Science, Fayom University, Fayoum, Egypt d Research Scholar, Physics Department, Faculty of Science, Fayoum University, Egypt e Department of Physics, Faculty of Science and Arts, Aljouf University, Aljouf, Saudi Arabia f Thin Film Laboratory, Physics Department, Faculty of Education, Ain Shams University, Roxy 11757 Cairo, Egypt b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 November 2015 Received in revised form 5 January 2016 Accepted 13 January 2016 Available online 10 February 2016
In this work, Poly(3,4-ethylenedioxythiophene), Tetra Methacrylate (PEDOTTM) film was deposited by spin coating on glass substrates. Surface topography and crystalline properties of PEDOTTM were carried out by scanning electron microscopy and transmission electron microscopy, respectively. Fourier transform infrared spectroscopy was used for investigating vibrational properties.Optical characteristics were investigated using spectrophotometric measurements in the wavelength range 200–2500 nm. Absorption characteristic in the UV region shows a well-defined absorption band. The UV–vis absorption spectrum was resolved by molecular orbital and band theories and the optical transition of the films is allowed and direct. Optical dispersion parameters namely oscillator energy,dispersion energy, lattice dielectric constant and high frequency dielectric constant were determined. Temperature dependent current density–voltage (J–V) characteristics was studied to clarify the dominant charge transport in the temperature range 398–373 K. Moreover, temperature dependent of barrier height, ideality factor and series resistance were also considered. The presented results offer a new perspective in optoelectronic applications. ã 2016 Published by Elsevier B.V.
Keywords: Thin film Optical properties Energy gap Heterojunction
1. Introduction Organic semiconductors thin films for electronic devices have been attracting great considerable attention due their particular characteristics such as low cost, low weight, good mechanical flexibility, easy processing from solution, large-scale fabrication, and the ability to modify their structure to obtain a desired electrical and optical characteristics [1,2]. Moreover, these materials possess favorable electrical, optical and optoelectronic characteristics applicable for designing new systems of solar energy conversion, laser electrophotography, photoelectrochemical cells, photosensitization, electrocatalysis and electrophotography [3–5] Conjugated polymers like Poly(3,4-ethylenedioxythiophene) (PEDOT) and their derivatives have been known to have defined electronic properties among the conjugated polymers [6]. A large
* Corresponding author at: Ain Shams University, Thin Film Laboratory, Physics Department, Faculty of Education, Roxy, 11757, Egypt. Fax: +20 22581243. E-mail addresses: [email protected], [email protected] (A.A.M. Farag). http://dx.doi.org/10.1016/j.synthmet.2016.01.008 0379-6779/ ã 2016 Published by Elsevier B.V.
number of Schottky barrier diodes have been fabricated and characterized using organic conductive polymers together with metals and substantial research studies have been made on the preparation and characterization of conducting polymer/inorganic semiconductor heterojunction [6,7]. Poly(3,4-ethylenedioxythiophene), Tetra Methacrylate (PEDOTTM) has attracted considerable attention due to its excellent environmental stability, low oxidation potential and high electrical conductivity as well as its good electrochromic characteristics [8]. Most of literature review confirms that a detailed correlation between the morphology of PEDOT film and the preparation route has not yet been established [2–8]. Therefore, this work presents some of the main experimental results of the morphological and optical characteristics of the spin coated PEDOTTM film. The using of spin coating technique is based on the fact that this method is more easy to manufacture, low coast, as well as characterized by more reliable seed coatings with better particle adhesion to the substrate. In addition, optical band gap is also estimated based on the application of the energy band theory. Furthermore, the current–voltage characteristics of PEDOTTM/n-Si heterojunction
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are also studied for check the availability of this type of heterojunction for device application.
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resistance with each material and have a lower barrier which can be vanishes as compared to the barrier originated from PEDOTTM/ n-Si interface.
2. Experimental details 2.2. Characterization tools 2.1. Materials and thin film preparation Poly(3,4-ethylenedioxythiophene),Tetra Methacrylate (PEDOTTM) was purchased from Sigma–Aldrich Chem. Co. and used as received without further purification (0.5 wt.% (dispersion in propylene carbonate), contains p-toluenesulfonate as dopant). The schematic diagram of the molecular structure of PEDOTTM is shown in Fig. 1. Thin films of PEDOTTM were prepared by spin coating on suitable substrates such as corning glass for optical measurements and n-Si substrates for heterojunction measurements. The glass substrates were carefully cleaned by a detergent, and then in methanol and acetone each for 10 min by using an ultrasonic cleaner. At last, the glass substrates were rinsed with deionized water and dried with nitrogen gun. n-Type Si (111) single crystals substrates was etched by HF/NH4F/H2O (2:7:1) for 1.5 min to remove the surface layer of silicon dioxide, SiO2 and then washed for 2 min in pure alcohol and distilled water and finally dried with nitrogen. Solution of PEDOTTM was prepared using pure ethanol as a solvent and adequately stirred. The film of this material was prepared by spin coating technique at a spinning speed of 800 rpm for 60 s at room temperature (300 K). After the spin coating, the film was dried at 373 K for 10 min to evaporate the solvent and to remove the organic residuals. This coating/drying procedure was repeated for five times to obtain a suitable thickness for measurements. The thickness of the films was determined with Mettler Toledo MX5 microbalance. This thickness was also determined by the cross sectional imaging of the deposited film and the substrate edge using scanning electron microscopy (SEM) of type JEOL JEM1230. The obtained heterojunctions were coated from the back side (n-Si side) by thin films of Au-ohmic contact using the high vacuum sputtering unit (Turbo Sputtering RF & DC Power Supplies Deposition System Model Hummer 8.1) was used for evaporating ohmic contact of high purity of gold onto the top of the PEDOTTM film and the bottom of the n-Si substrate. These contacts has a low
In this work, the organo-functionalized surface chemistry of filler nanoparticles was characterized by a 4100 Jasco-Japan FTIR spectrophotometer. The spectrum was collected in the range from 4000 to 400 cm1. The morphology of the prepared TiO2 samples were investigated by scanning electron microscopy (SEM) type JEOL-JAX-840A, with accelerating voltage 30 kV. Transmission electron microscopy (TEM) study includes transmission electron micrographs study type JEOL JEM-1230, with maximum resolving power 0.2 nm, energy 40–120 kV on steps, maximum magnification power 600,000 and computerized with Ultra Scan 1000 2 k 2 k CCD. The UV–vis spectra of the spin coated films (absorbing) on corning glass substrates (nonabsorbing) were recorded from 200 nm to 2500 nm wavelength using SHIMADZU UV-3600 UV–vis-NIR spectrophotometer at room temperature (300 K). The current–voltage (I–V) measurements were performed by the use of a high impedance Keithley 617 programmable constant current source electrometer. 3. Results and discussion 3.1. Structural and molecular characteristics of PEDOTTM SEM images as indicated in Fig. 2(a) and (b) clearly show the surface morphology of PEDOTTM film deposited on glass substrate. This figure clarifies that the surface is composed of particles uniformly distributed over all the scanned area as well as numerous nanoparticles joined together to form an aggregate. This structure turned to be randomly oriented nanoparticles with average grain size 30 nm which agree with the obtained data of TEM images. The thickness of the film can be extracted from the cross sectional image of SEM at edge between the film and the substrate (film–substrate interface). A highly adhesion of the film to the substrate was observed. The vertical boundary of the film at the broken edge is shown in the inset of Fig. 2(b) and the average calculated thickness is about 300 nm. Fig. 3 presents the TEM images of PEDOTTM particles dispersed in propylene carbonate. The specimen was taken from a droplet of PEDOTTM suspension, was treated by ultrasonic dispersion.It is observed that nearly spherical nanoparticles are dispersed in propylene carbonate with diameter ranging from 25 to 35 nm. In order to detect the vibration characteristics (stretch, contract and bend etc.) of chemical functional groups in PEDOTTM, IR spectroscopy is employed in the wavenumber range 400– 4000 cm1 as shown in Fig. 4. The absorption bands at 3428 can be assigned the O H stretching vibration mode [9]. Absorption signal at 2989 is associated to the stretching C H ring vibration [10]. The strong absorption band at 1795 cm1 is due to the presence of C¼O stretch vibrations [11]. The absorption bands at 1478 and 1400 cm1 can be attributed to the stretching modes of C¼C and C C modes in the thiophene ring [12]. Absorption bands at 1183, 1036, 772 and 710 cm1 can be assigned to the ethylenedioxy group [13–15]. Moreover, the bands occurring at 772 and 710 cm1 correspond to the interaction in the CS of the thiophene ring modes [11–15]. 3.2. Optical characteristics
Fig. 1. Schematic diagram of PEDOTTM.
The transmittance, T (l) and reflectance, R (l) spectra of PEDOTTM thin films were measured at normal incidence at room temperature in the spectral range of 200–2500 nm with
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Fig. 2. (a,b) SEM micrograph of PEDOTTM thin film with different magnifications. The inset of Fig.2(b) shows the cross section image.
Fig. 3. (a,b) TEM micrograph of PEDOTTM nanoparticles.
r
where a is the absorption coefficient, M is molecular weight (55,000) and r is the sample density (1 g/cm3 at 20 C) [22,23]. Spectral distribution dependence of emolar for PEDOTTM thin film is shown in Fig. 6. Well Gaussian fit for the molar extinction coefficient data is shown in Fig. 7. The main important absorption parameters are calculated and listed in Table 1, in comparison with
2989
1478
710 772
80
3562
100
1400
60 40 1036 1183
20 0
1795
Transmittance %
uncertainty of 1% as given by the manufacturer. Fig. 5 shows the optical transmittance spectra of the used PEDOTTM film with two different thicknesses of 220 and 300 nm. It is clear that the value of optical transmission increases with increasing the wavelength. The higher film thickness of 300 nm has a lower transparency as compared to the lower one (220 nm), especially in the higher wavelength range l > 600 nm The optical reflectance (R) for the PEDOTTM thin film is found to decrease with increasing the wavelength in the lower wavelength region l < 900 nm. While value of R is slightly increases with increasing wavelength, in the region l > 900 nm. This behavior is similar to those published elsewhere by Medina et al. [16]. As can be seen in Fig. 6(a), the absorption spectra exhibit an excitonic absorption band peaked around 284 nm typical for the film thickness 220 nm. Upon increasing the film thickness to 300 nm the excitonic absorption is slightly blue-shifted by 2.5 nm. This behavior is similar to those observed by Li and Zhang [17]. In the present work, one can deduce different important absorption parameters such as molar extinction, em coefficient, oscillator strength, f (a dimensionless parameters of the electromagnetic radiation that indicates the absorption probability of the electromagnetic radiation for transition between energy levels of a molecule) and electric dipole strength EDS using the following expressions [18–21] Z M em ¼ 2300 a; f ¼ 4:38 109 em dy ð1Þ
500
1000
1500
2000
2500
3000
3500
-1
Wavenumber, (cm ) Fig. 4. FTIR spectrum of PEDOTTM nanoparticles.
4000
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elsewhere [27–31]. In the present work, the higher calculated energy gap of PEDOTTM can be attributed to some reasons such as the polymerization process, the deposition technique, the doping process, and others [24]. Groenendaal et al. [25] reported that the energy gap of PEDOTTM can change from 1.4 to 2.5 eV depending on the doping ratio. This highly change gives contribution of PEDOTTM for various applications such as solar cells, and smart windows as well as photographic films. 3.3. Determination of the optical constants
Fig. 5. Transmittance and reflectance spectra of PEDOTTM films.
other organic polymers thin films. It is observed that the obtained results are in good agreement with the previously reported results of other polymers [22,23]. The valence band and conduction band are the solid state analogue of the highest occupied molecular orbitals, HOMO and the lowest unoccupied molecular orbitals, LUMO of molecules. In many organic materials, it is usual to analyze the optical absorption at the fundamental edge in terms of band-to-band transitions theory [24]. The fundamental absorption edge of PEDOTTM film is formed by a direct allowed transition. Optical gap of PEDOTTM film can be obtained from the analysis of the spectral dependence of the absorption near the fundamental absorption edge which is found to be direct allowed transition. Accordingly, the absorption coefficient can easily be described by the following expression [23] ð2Þ ðahyÞ2 ¼ A hy Eg ; where hy is the photon energy, A is a factor depends on the transition probability and Eg is the optical band gap. Fig. 6(b) shows the plot of ðahyÞ2 vs. hy near the absorption edge for PEDOTTM films. This plot gives a best linear fit over a wide range of photon energy, near the band gap edge. Eg can easily be estimated from the linear extrapolation to the photon energy axis, where ðahyÞ2 = 0 (and listed in Table 2), in comparison with those published
The refractive index n and excitation coefficient k (al/4p) of the films were determined using the well known equations stated in [23,26] and plotted as a function of wavelength (not shown here) (Fig. 8). The n values decrease with the increase of wavelength. The refractive index of the films in the visible range was found about less than 1.8 in this study. So, the decrease in refractive index may be attributed to the increase in film thickness, where the film thickness has an important effect on the refractive index [26]. Moreover, the k values increase with the increasing of wavelength. 3.4. Dispersion characteristics of PEDOTTM films Dispersion characteristics supply a demonstration of the change of the index of refraction (n) at higher wavelength region, where the extinction index (k) is considered to be negligible. A better description for this dispersion was introduced by Wemple and DiDomenico using the following well known expression [27]: 1 Eo 1 ¼ ðhyÞ2 ðn2 1Þ Ed Eo Ed
ð3Þ
where Ed is the dispersion energy and Eo is the single oscillator energy. Graphical representation of ðn2 1Þ1 vs. ðhyÞ2 is plotted and shown in Fig. 9(b) for the two thicknesses and by the fitting of the data to a straight line, Eo and Ed can be determined from the intercept and the slope. The calculated dispersion values of Eo and Ed as well as the corresponding high-frequency dielectric constant (e1 = n21 ) for the films are also listed in Table 3 in comparison with those published dispersion data of related organic films [23,28]. As observed, a good agreement is observed which gives an indication
Fig. 6. (a) Absorption spectrum of PEDOTTM films and (b) plots of (ahn)2 vs. photon energy of PEDOTTM films.
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It is seen from Fig. 9(a) and (b) that the values of e1 are higher than that of e2 values in the same photon energy range and both characteristics are nearly different. The variation of the dielectric constant with photon energy indicates the probability of the presence of some interactions between photons and electrons in the films that produced in this energy range. These interactions can cause formation of peaks in dielectric spectra that depends on the type of the material. 3.5. Current density–voltage characteristics of PEDOTTM/p-Si heterojunction
Fig. 7. Plot of emolar vs. wavenumber for PEDOTTM films.
for the suitability of the prepared films in the field of optoelectronic devices. Real part of the dielectric constant e1has a relation with wavelength (l) according to the following [28]: e2 N e1 ¼ n2 ¼ eL l2 ; ð4Þ 4p2 c2 e0 m where eL is known as lattice dielectric constant e0 is the free space permittivity, c is velocity of light and (N/m*) is the ratio of free carrier concentration to its effective mass. Spectral distribution of e1 = n2, vs. l2 is shown in Fig. 9(a). e1 decreases with increasing l2 and the representation can be linearly fit in the chosen range. Both values of eL and N/m* for the two thicknesses can easily be obtained from the extrapolation of the straight line section ofthe curve to intersect with the ordinate axis and the slope, respectively. Value of eL is listed in Table 3 in comparison with those published before for some related organic films. In addition, the disagreement between eL and e1 gives support for the availability for the presence of free carrier contribution [23,28]. The dielectric properties are concerned with the storage and dissipation of electric and magnetic energy in materials [29]. The e and its real, e1 and imaginary,e2 , complex dielectric constant, ~ parts are described by the following relations [29]: ~ eðhyÞ ¼ e1 ðhyÞ ie2 ðhyÞ;
ð5Þ
where e1 ¼ ðn k Þ and e2 ¼ 2nk. 2
2
Temperature dependence of semi-logarithmic current density– voltage (J–V) characteristics of PEDOTTM/p-Si heterojunction is shown in Fig. 10. A nearly non-symmetric shape under forward and reverse bias confirms the diode—like characteristics with weak rectification property. As observed, the forward and reverse currents for all the studied temperatures are exponentially increase with increasing bias. Moreover, the reverse currents have not saturation nature, indicating a weak characteristic for the heterojunction. In addition, a little voltage drop across a forwardbiased with the current is also observed. Furthermore, a noticeable temperature effect is also recorded which gives a confirmation for the validity of the device as a temperature sensor. The overall dark current in the heterojunction is basically estimated to be the totality of several components; one of them is normally predominant in a definite voltage range. Generally, at the higher temperature range the thermionic emission mechanism is predominant as compared to other ones [30]. Current density– voltage characteristics are demonstrated in the narrow biasing voltages (0.1 V V 0.6 V) using thermionic emission mechanism as follows [31]: qðV IRs Þ 1 ð6Þ J ¼ J0 exp hkT where Rs is the series resistance, V is the applied voltage, h is the ideality factor, k is the Boltzmann’s constant, and J0 is the saturation current density given by the following expression [31]: qFb ð7Þ ; J0 ¼ A T 2 exp kT where A* is the effective Richardson constant and Fb is the effective barrier height. For emphasizing that the thermionic
Table 1 Absorption parameters of PEDOTTM films in comparison with other organic films. Compound
em
f
EDS
Reference
PEDOTTM (Thickness 220 nm) PEDOTTM (Thickness 300 nm) Rhodamine B Poly(o-toluidine) (POT)
5.23 107 1.17 108 3 104 1.67 107
1108.14 5650.2 0.214 0.278
2335.27 9534.24 0.99 0.15
Present work Present work [22] [23]
Table 2 Values of energy gaps of PEDOTTM films in comparison with other organic films. Compound TM
(thickness 220 nm) PEDOT PEDOTTM (thickness 300 nm) PEDOT Poly(o-toluidine) (POT) Polypyrrol Polypyrrole-chitosan (PPy-CHI) depending on the chitosan content Undoped ploypyrrol film
Eg (eV)
References
1.93 1.80 1.4 1.2 1.60 1.30 1.72
Present work Present work [27] [28] [29] [30] [31]
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Fig. 8. (a) Photon energy dependence of real parts of the dielectric constant and (b) Photon energy dependence of imaginary parts of the dielectric constants of the PEDOTTM films.
Fig. 9. (a) Variation of n2 vs. l2 and (b) plot of (n2 1)1 vs. photon energy (hn)2 for PEDOTTM films.
conduction is the dominant conduction mechanism at the desired narrow voltage range, the dependence of ln (J0/T2) vs. 1/T is checked as shown in Fig. 10(b). The linear fit of this dependence gives support for the availability of this mechanism. From the slope of this straight line, one can obtain the value of the effective barrier height which is found to be 0.12 eV. Moreover, A* can also be obtained from the intercept which is found to be 8.39 A/cm2 K2 which is lower than those obtained by Ashery et al. [32] for p-Si. This disagreement in A* can be attributed to the presence of inhomogeneous barrier height and potential fluctuation at the interface that consist of low and high barrier areas [33]. Values of Fb and h were determined from the forward I–V characteristics at different temperatures (Fig. 11(a)) using Eqs. (1) & (2), respectively and plotted as function of temperature as shown in Fig. 11(b). As observed, values of h show non-ideal diode characteristics (deviates from unity). Moreover, values of h are found to be high in the lower temperature region and gradually decrease with increasing temperature. High values of the ideality factor can be attributed to various factors such as inhomogeneities
of organic film thickness, non-uniformity of the interfacial charges and the effect of series resistance. Moreover, another probability of the presence of insulating oxide layer at the interface of PEDOT/ n-Si which may be originated from any adsorbed water vapor onto the Si surface before the deposition process. This deviation of h from unity indicates that the device is not ideal and then there is no ideal thermionic emission process can be taken place [31,33]. On the contrary, values of Fb were found to increase with increasing temperature. Such temperature dependence can be due to the
Table 3 Dispersion parameters of PEDOTTM films in comparison with other organic films. Compound
Eo (eV)
Ed (eV)
e1
eL
References
PEDOTTM (thickness 220 nm) PEDOTTM (thickness 300 nm) Polypyrrol Polyaniline
6.39 5.37 3.71 4.56
9.4 11.2 4.61 11.03
2.47 3.08 2.24 3.43
3.32 3.29 2.34 3.85
Present work Present work [23] [28]
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Fig. 10. (a) Plot of J–V characteristics of PEDOTTM/n-Si heterojunction at different films temperatures and (b) plot J0/T2 vs. 1000/T.
Fig. 11. (a) Plot of forward J–V characteristics and (b) temperature dependence of effective barrier height and ideality factor.
Fig. 12. (a) RJ–V characteristics and (b) temperature dependence of series resistance, Rs.
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presence of non-uniformity of the interface charge or spatial inhomogeneities of barrier height [34]. Series resistance has effectiveness on the electrical parameters of junctions and then several methods were used to determine its value. In the present work, Rs is determined from the dependence of junction resistance, RJ, on bias (i.e. RJ vs. V) as shown in Fig. 12(a). As observed, values of RJ is found to decrease with increasing bias but higher bias, RJ reaches a nearly saturation which corresponds to Rs. Fig. 12(b) shows the temperature dependence of Rs. Decreasing of the series resistance with increasing temperature can be attributed to some factors reliable for the decrease of h. Decreasing in Rs with increasing temperature means improving for the junction performance as the temperature increase [35]. 4. Conclusions Morphology characteristics of PEDOTTM indicate that the surface is composed of aggregation and a randomly oriented nanoparticles with average grain size 30 nm. Results of FT-IR indicates that the used spin coating is considered as a suitable technique for obtaining undissociated PEDOTTM films and confirmed that the molecular structure has stable structure. Absorption spectra exhibit an excitonic absorption band peaked around 284 nm. Optical dispersion was interpreted by single oscillator model. A direct allowed transition was characterized for the film. Current density–voltage characteristics of the PEDOTTM/p-Si heterojunction showed temperature dependence in the temperature range 298–373 K. Thermionic emission was suggested as a dominant charge transport mechanism with regarding of inhomogeneity of barrier height due to the mismatch over the interface. All the prepared junction parameters showed temperature dependence which open a new view-point in electronic device applications. References [1] Z. Li, F. Qin, T. Liu, R. Ge, W. Meng, J. Tong, S. Xiong, Y. Zhou, Org. Electron. 21 (2015) 144–148. [2] F. Zabihi, Y. Xie, S. Gao, M. Eslamian, Appl. Surf. Sci. 338 (2015) 163–177. [3] Q. Wang, M. Ahmadian-Yazdi, M. Eslamian, Synth. Met. 209 (2015) 521–527.
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[4] M.M. Makhlouf, H.M. Zeyada, Synth. Met. 211 (2016) 1–13. [5] C. Constantinescu, A. Diallo, A. D’Aleo, F. Fages, P. Rotaru, C. VidelotAckermann, P. Delaporte, A. Alloncle, Synth. Met. 209 (2015) 29–33. [6] F. Zabihi, Y. Xie, S. Gao, M. Eslamian, Appl. Surf. Sci. 338 (2015) 163–177. [7] Ö. Yagci, S.S. Yesilkaya, S.A. Yüksel, F. Ongül, N.M. Varal, M. Kus, S. Günes, O. Icell, Synth. Met. 212 (2016) 12–18. [8] C. Tozlu, A. Mutlu, Synth. Met. 211 (2016) 99–106. [9] B.J. Saikia, G. Parthasarathy, J. Mod. Phys. 1 (2010) 206–210. [10] K. Motobayashi, K. Minami, N. Nishi, T. Sakka, M. Osawa, J. Phys. Chem. Lett. 4 (2013) 3110–3114. [11] P. Domlin, C. Kvarnstom, A. Ivaska, J. Electroanal. Chem. 570 (2004) 113–122. [12] S. Radhakrishnan, C. Sumathi, V. Dharuman, J. Wilson, Anal. Methods 5 (2013) 684–689. [13] A. Balamurugan, K.C. Ho, S.-M. Chen, Synth. Met. 159 (2009) 2544–2549. [14] S.V. Selvaganesh, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, Nanoscale Res. Lett. 2 (2007) 546–549. [15] S.S. Vinod, J. Mathiyarasu, K.L.N. Phani, V. Yegnaraman, Nanoscale Res. Lett. 2 (2007) 546–549. [16] B.M. Medina, D. Wasserberg, S.C.J. Meskers, E.M. Osteritz, P. Bäuerle, J. Gierschner, J. Phys. Chem. 112 (2008) 13282–13286. [17] J. Li, J.Z. Zhang, Coord. Chem. Rev. 253 (2009) 3015–3041. [18] W. Demtröder, Laser Spectroscopy: Basic Concepts and Instrumentation, Springer, New York, 2007. [19] A.A.M. Farag, Opt. Laser Technol. 39 (2007) 728–732. [20] B. Friedel, Thomas J.K. Brenner, Christopher R.. McNeill, U. Steiner, N.C. Greenham, Org. Electron. 12 (2011) 1736–1745. [21] http://nano.tau.ac.il/mncf/images/MSDS/MSDS_for_PEDOTPSS_AI4083.pdf. [22] A.A.M. Farag, I.S. Yahia, Opt. Commun. 283 (2010) 4310–4317. [23] A.A.M. Farag, A. Ashery, M.A. Shenashen, Physica B 407 (2012) 2404–2411. [24] A. Elmansouri, A. Outzourhit, A. Oueriagli, A. Lachkar, N. Hadik, M.E. Achour, A. Abouelaoualim, A. Malaoui, K. Berrada, E.L. Ameziane, Synth. Met. 160 (2010) 1487. [25] L.B. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater. 12 (2000) 481–494. [26] F. Yakuphanoglu, M. Sekerci, A. Balaban, Opt. Mater. 27 (2005) 1369–1374. [27] J.N. Hodgson, Optical Absorption and Dispersion in Solids, Chapman and Hall Ltd., London, 1970. [28] A. Ashery, A.A.M. Farag, M. Shenashen, Synth. Met. 162 (2012) 1357–1363. [29] A.A.M. Farag, I.S. Yahia, Synth. Met. 161 (2011) 32–39. [30] A.A.M. Farag, M. Fadel, I.S. Yahia, Curr. Appl. Phys. 12 (2012) 1436–1444. [31] M.C. Arenas, N. Mendoza, H. Cortina, M.E. Nicho, H. Hu, Sol. Energy Mater. Sol. Cells 94 (2010) 29–39. [32] A. Ashery, A.A.M. Farag, M.A. Salem, Microelectro. Eng. 85 (2008) 2309–2315. [33] H. Uslu, A. Bengi, S.S. Cetin, U. Aydemir, S. Altındal, S.T. Aghaliyeva, S. Ozcelik, J. Alloys Compd. 507 (2010) 190–195. [34] A.A.M. Farag, B. Gunduz, F. Yakuphanoglu, W.A. Farooq, Synth. Met. 160 (2010) 2559–2563. [35] A.A.M. Farag, I.S. Yahia, T. Wojtowicz, G. Karczewski, J. Phys. D: Appl. Phys. 43 (2010) 215102–21509.