Electrosynthesis of organic–inorganic compounds (p–n heterojunction)

Materials Science in Semiconductor Processing 13 (2010) 141–146

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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Electrosynthesis of organic–inorganic compounds (p–n heterojunction) F. Habelhames a,, B. Nessark a, M. Girtan b a b

´lectrochimie et mate´riaux, De´partement de Ge´nie des proce´de´s, Faculte´ des Sciences de l’inge´nieur, Universite´ Ferhat Abbas, 19000 Se´tif, Alge´rie Laboratoire d’e Laboratoire de Photonique Universite´ d’Angers, 2, Bd. Lavoisier, 49045 Angers, France

a r t i c l e in fo

abstract

Available online 3 July 2010

An electrochemical deposition procedure by cyclic voltammetry, in an electrolyte solution was adopted for the preparation of thin films of polypyrrole–gallium arsenide composite materials. The properties of the composite layers were studied by cyclic voltammetry, electrochemical impedance spectroscopy and photoelectrochemical measurements. The p- and n-type semiconductor behaviour of the polypyrrole (PPy) and gallium arsenide (GaAs) were studied by photocurrent measurements. It was found that the composite material (PPy–GaAs) had a (p–n) heterojunction behaviour. & 2010 Elsevier Ltd. All rights reserved.

Keywords: organic–inorganic compounds Semiconductors Electrochemical properties p–n Heterojunction

1. Introduction A hybrid solar cell consists of a combination of both organic and inorganic semiconductor materials. It combines the unique properties of inorganic semiconductors with the film-forming properties of the organic-conjugated polymers [1]. Organic materials usually are inexpensive, easily processable, and their functionality can be tailored by molecular design and chemical or electrochemical synthesis. On the other hand, inorganic semiconductors can also be manufactured as processable nanoparticulate colloids. By varying the size of the nanoparticles, their band gap can be tuned and their absorption/emission spectra can be modified [2]. An effective strategy for hybrid solar cell fabrication is to use blends of nanocrystals with organic conjugated polymers as bulk heterojunction [1–6]. Excitons created upon photoexcitation are separated very efficiently into free-charge carriers at interfaces between organic semiconductors and inorganic semiconductor nanoparticles in

 Corresponding author. Tel.: + 213 36 92 51 21 fax: + 213 36 92 51 33.

E-mail address: [email protected] (F. Habelhames). 1369-8001/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2010.06.001

a hybrid composite thin film. Hybrid solar cell concept has been demonstrated for conjugated polymer blends containing CdSe [3,6,7], CuInS2 [1,4], CdS [5] or PbS [8,9] nanocrystals. The usage of these inorganic semiconductor nanoparticles embedded into organic-conjugated polymer blends is interesting for two reasons at least: first the inorganic semiconductor nanoparticles can have high absorption coefficients and higher photoconductivity compared to many organic semiconductor materials, second the n- or p-type character of the nanocrystals can be varied by synthetic routes [4]. Polypyrrole (PPy) is an especially promising conducting polymer for different applications, due to its good conductivity, environmental stability and ease of synthesis. Its use as a new material has opened up entirely new field for polymeric materials. Therefore, several approaches to prepare composite materials consisting of nanoparticles and polypyrrole have been reported [10–12]. The principal advantage of the electrochemical polymerisation method is related to the better conducting properties and long-term stability [13,14]. In the present study, PPy–GaAs composite is electrochemically prepared using cyclic voltammetry on platinum electrode. The microstructures PPy–GaAs

F. Habelhames et al. / Materials Science in Semiconductor Processing 13 (2010) 141–146

composite was analysed by Scanning Electron Microscopy (SEM) coupled with EDX analyser. The characterizations of samples were carried out by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and photoelectrochemical analyses were done by photocurrent measurements.

2. Experimental Pyrrole (Aldrich product) was used after distillation under reduced pressure. The electrolyte used was the lithium perchlorate (LiClO4, Fluka product) dissolved in the acetonitrile (CH3CN, Aldrich). The gallium arsenide (GaAs) particles having 20–40 nm of diameter (Sigma-Aldrich product) were used as inorganic semiconductors. The cyclic voltammetry study and the electrochemical syntheses were carried out in a one-compartment cell containing the solution of electrolysis, using three electrodes. The working electrode was a platinum disc (Ø= 2 mm), the reference electrode was a saturated calomel electrode (SCE) and the auxiliary electrode was a platinum plate. The system solvent/supporting electrolyte used CH3CN/LiClO410 1 mol l 1. The elctropolymerization of Py was carried out from the solution containing Pyrrole (10 2 mol l 1) and LiClO4 (10 1 mol l 1) in CH3CN with gallium arsenide powder (GaAs) at a concentration of 5  10 2 mol l 1 and dispersed by a weak agitation. Prior to polymerization, the solution was deoxygenated by passing nitrogen gas. Electrochemical impedance spectroscopy (EIS) measurements were performed, using an alternative current voltage of 10 mV, in the frequency range from 100 mHz to 100 kHz. The cell and the electrodes used were identical to those used in cyclic voltammetry. The study was related to films of polypyrrole doped or not by GaAs particles. In this case, the working electrode was the platinum disc (Ø= 2 mm) modified by this film, obtained electrochemically by cycling on a potential domain ranging between 0.3 and 1.3 V/SCE, at v =100 mV/s. The analysis of film was carried out in a system solvent/supporting electrolyte, in the absence of the monomer. The SEM micrographs of PPy and composite material (PPy–GaAs) deposited on platinum (used as a working electrode), were taken out by using a Sirion-200, FEI, USA scanning electron microscope. The elemental compositions of the composite film surface were determined by using an energy-dispersive X-ray detector coupled to this microscope (EDX; Serion-200 132-10). The photoelectrochemical analysis was recorded with a 500 W halogen lamp as a polychromatic light source, and the illumination intensity is 100 mW cm 2. The composite layers for photocurrent measurements were prepared on platinum electrodes with an illuminated area of 1 cm2 in the electrolyte solution. All the electrochemical and photoelectrochemical measurements were carried out using a standard potentiostat PGZ-301 Voltalab, coupled with a microcomputer equipped with a software voltamaster 4 program.

3. Results and discussion 3.1. Electrochemical synthesis of film Fig. 1 shows a successive cyclic voltammogram corresponding to CH3CN/(LiClO4 10 1 mol l 1) solution containing the pyrrole (Py) 10 2 mol l 1 and gallium arsenide powder (GaAs) 5  10 2 mol l 1 (dispersed by a weak agitation) and recorded in the potential range of 0.3 and 1.3 V/ECS, with a scan rate of 100 mV/s. During the positive potential scan, the voltammograms show an anodic wave, which is observed at the potential 0.5 V/ECS and during negative potential scan a cathodic wave at 0.1 V/ECS corresponding to the oxidation and reduction of the polypyrrole, respectively. During the successive recording of the cyclic voltammogram, the anodic peak potential move weakly to the more positive values and the cathodic peak potential to the more negative value. The displacement of the potential peaks is accompanied by an increase in the current intensity of the oxidation and reduction peaks. Also, the difference between anodic and cathodic peak DEp (DEp =EPa EPc) increases during cycling. This corresponds in the same time to the accumulation of the deposit of polypyrrole on the electrode surface. 3.2. Morphology of films and EDX analysis Fig. 2 shows SEM micrographs of the PPy and PPy– GaAs composite films electrochemically prepared. In Fig. 2a, the typical granular raspberry morphology of pure PPy is presented [15,16]. Scanning electron microscopy analyses showed gallium arsenide distribution at the PPy surface (Fig. 2b). The white dots represent the gallium arsenide pigment. This pigment may be found

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100 50 0 -50 -100 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

E (V vs SCE) Fig. 1. Cyclic voltammogram corresponding to the polymerisation of Py in the presence of GaAs on Pt electrode, between 0.3 and 1.3 V, with a scan rate of 100 mV/s.

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in agglomerated form and is distributed in a homogeneous way compared to the distribution of other inorganic materials in the PPy matrix [16–18]. However, the surface morphology change of PPy–GaAs is ascribable to the existence of GaAs nanoparticles in the electropolymerization process. The incorporation of gallium arsenide in the polymer is confirmed by the EDX (energy-dispersive X-ray spectroscopy) analysis, which shows the presence of the intense rays of gallium (Ga) located at 1.1 keV and of arsenide (As) at 1.3 keV (Fig. 3). Also, the EDX spectrum of the electrochemically prepared composite material film shows a signal of carbon (C) at 0.25 keV and nitrogen (N) at 0.4 keV characteristic of the PPy polymer. The signals of Cl at 2.62 and 2.82 keV and oxygen (O) at

143

0.53 keV indicate that the PPy film is doped by the perchlorate (ClO–4) ions [16]. This anion results from the LiClO4, which was used as a supporting electrolyte. Thus, the GaAs particles can be incorporated in the polymer during the electropolymerization of the pyrrole, which led to the pigmentation of the polymer film, and the obtaining of the composite material PPy–GaAs on the electrode. 3.2. Cyclic voltammetry Fig. 4 shows cyclic voltammogram (CV) corresponding to the electrochemical analysis composite material in electrolyte solution, carried out in the potential range between 0.3 and 1.3 V, with scan rate of 50 mV/s. The

Fig. 2. SEM micrographs of PPy (a) and PPy–GaAs (b) films electrochemically prepared.

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N O

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Fig. 3. EDX spectrum of PPy–GaAs, deposited on platinum electrode.

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cyclic voltammogram shows the anodic and cathodic peaks at 0.6 and 0.1 V vs. ECS, respectively, corresponding to oxidation and reduction of the polypyrrole film [16]. It was found that the composite material has stable electrochemical properties because the GaAs semiconductor showed no electrochemical activity. 3.3. Electrochemical impedance spectroscopy (EIS) The impedance responses (Nyquist diagrams) of PPy and composite film (PPy–GaAs) are shown in Fig. 5. They were analysed in (CH3CN/LiClO4 10 1 mol l 1) solution without monomer and semiconductor. The nyquist diagrams were

plotted in a frequency range between 100 mHz and 100 kHz, with an alternative current voltage of 10 mV. As can be seen, EIS spectra have a single semicircle in the high frequency region and a straight line in the low-frequency region [19,20], which are characteristic of a process of charge transfer and diffusion, respectively. The semicircle results from the parallel combination of resistance and capacitance and the linear region is due to the diffusion process. From PPy–GaAs composite film, it is observed that the semicircle which is more capacitative and resistant than the PPy film, suggests the incorporation of GaAs semiconductor in polymer.

3.4. Photo-electrochemistry

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100 50 0 -50 -100 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

After the preparation of PPy film, the electrode was further rinsed with acetonitrile and transferred into photoelectrochemical cell. Typical photocurrent–potential curves for experiments run in CH3CN/LiClO4 electrolyte without the monomer are given in Fig. 6a. It can be seen that the polymer presents a cathodic photocurrent, which is typical for polypyrrole and polybithiophene polymers [16,21–23], and we observe a cathodic photocurrent under negative potential confirming that under these conditions the polymer behaves as a p-type semiconductor. The presence of the space–charge region suggests that these polymers may produce photocurrents under illumination. Fig. 6b shows the photocurrent–potential curves of GaAs electrode in CH3CN/LiClO4 electrolyte. The gallium arsenide presents an anodic photocurrent under positive potential confirming the n-type inorganic semiconductor [24,25].

E (V vs.SCE) Fig. 4. CV corresponding to PPy–GaAs film in CH3CN/LiClO4 (10 1 mol l 1) solution, between 0.3 and 1.3 V vs.SCE, scan rate v =50 mV/s.

-Im Z (K ohm.cm-2)

0.6

3.5. p–n heterojunction The p–n heterojunction behaviour was determined by photoelectrochemical method (Fig. 7). The inorganic semiconductor (GaAs) presents a photocurrent response in the positive-field potential (n-type) and the organic polymer conductor (PPy) presents a photocurrent response in the negative-field potential (p-type), which gives information on the possibility of photovoltaic cells’ application.

PPy PPy - GaAs

0.5 0.4

4. Conclusion

0.3 0.2 0.1 0.0 0.8

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Re Z (K ohm.cm ) Fig. 5. Impedance diagrams of polypyrrole and composite film (PPy-GaAs).

Composite material PPy–GaAs has been successfully prepared by cyclic voltammetry method. The films were synthesized on platinum electrode. The effect of gallium arsenide content was analysed by cyclic voltammetry and impedance spectroscopy. We observed that the incorporation of an amount of GaAs particles modified the morphology properties of the PPy film. Polymer photocurrent increased with inorganic semiconductor GaAs concentration and the p–n heterojunction behaviour was observed. This indicated the importance of composite films and their possible application for photovoltaic cells.

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Photocurrent density (µA.cm-2)

Photocurrent density (mA.cm-2)

F. Habelhames et al. / Materials Science in Semiconductor Processing 13 (2010) 141–146

-10

Fig. 6. Photocurrent–potential dependence of GaAs (a) and PPy (b) electrodes in electrolyte solution.

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-20 -30 -40 -50 Fig. 7. Photocurrent–potential dependence of (p-PPy)–(n-GaAs) composite materials measured at a light intensity of 100 mW cm

References [1] Arici E, Sariciftci NS, Meissner D. Hybrid solar cells based on nanoparticles of CuInS2 in organic matrices. Adv Funct Mater 2003; 13:165–71. [2] Weller HA. Colloidal semiconductor Q-particles: chemistry in the transition region between solid state and molecules. Chem Int Ed Engl 1993;32:41–53. [3] Hunyh W, Dittmer J, Alivisatos AP. Hybrid nanorod-polymer solar cells. Science 2002;295:2425–7.

2

.

¨ [4] Arici E, Meissner D, Schaffler F, Sariciftci NS. Core/shell nanomaterials in photovoltaics. Int J Photoenergy 2003;5:199–208. [5] Greenham NC, Peng X, Alivisatos AP. Charge separation and transport in conjugated-polymer/semiconductor–nanocrystal composites studied by photoluminescence quenching and photoconductivity. Phys Rev B 1996;54:17628–37. [6] Huynh W, Peng X, Alivisatos AP. CdSe nanocrystal rods/poly (3-hexylthiophene) composite photovoltaic devices. Adv Mater 1999;11:923–7. [7] Alivisatos AP. Semiconductor clusters, nanocrystals and quantum dots. Science 1996;271:933–7.

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F. Habelhames et al. / Materials Science in Semiconductor Processing 13 (2010) 141–146

[8] McDonald SA, Konstantatos G, Zhang S, Cyr PW, Klem EJD, Levina L, et al. Solution-processed PbS quantum dot infrared photodetectors and photovoltaics. Nat Mater 2005;4:138. ¨ [9] Gunes S, Fritz K, Neugebauer H, Sariciftci NS, Kumar S, Scholes G. Hybrid solar cells using PbS nanoparticles. Sol Energy Mater Sol Cells 2007;91:420–3. [10] Miyauchi S, Abiko H, Sorimashi Y, Tsubata I. Preparation of barium titanate–polypyrrole compositions and their electrical properties. J Appl Polym Sci 1989;37:289–93. [11] Yoneyama H, Kishimoto A, Kuwabata S. Charge–discharge properties of polypyrrole films containing manganese dioxide particles. J Chem Soc Chem Commun 1991;15:986–7. [12] Bhattaeharya A, Ganguly KM, De A, Sarkar S. A new conducting nanocomposite PPy–zirconium (IV) oxide. Mater Res Bull 1996;31: 527–530. [13] Avlyanov JK, Kuhn HH, Josefowicz JY, MacDiarmid AG. In-situ deposited thin films of polypyrrole: conformational changes induced by variation of dopant and substrate surface. Synth Met 1997;84:153–4. [14] Balci N, Bayramli E, Toppare L. Conducting polymer composites: polypyrrole and poly (vinyl chloride–vinyl acetate) copolymer. J Appl Polym Sci 1997;64:667–71. [15] Skotheim TA. Handbook of conducting polymers. New York: Marcel Dekker; 1986. [16] Habelhames F, Nessark B, Bouhafs D, Cheriet A, Derbal H. Synthesis and characterisation of polypyrrole–indium phosphide composite film. Ionics 2010;16:177–84.

[17] Lenz DM, Delamar M, Ferreira CA. Improvement of the anticorrosion properties of polypyrrole by zinc phosphate pigment incorporation. Prog Org Coat 2007;58:64–9. [18] Chen W, Li CM, Chen P, Sun CQ. Electrosynthesis and characterization of polypyrrole/Au nanocomposite. Electrochim Acta 2007;52: 2845–9. [19] Wang J, Xu Y, Chen X, Sun X. Capacitance properties of single wall carbon nanotube/polypyrrole composite films. Compos Sci Technol 2007;67:2981–5. [20] Ximin H, Gaoquan S. Electrochemical actuator based on monolithic polypyrrole–TiO2 nanoparticle composite film. Sens Actuators B 2006;115:488–93. [21] Kim H, Chang W. Preparation and photoelectrochemical behaviour of polypyrrole with platinum nanoparticles. Synth Met 1999;101: 150–1. [22] HabElhames F, Nessark B, Boumaza N, Bahloul A, Bouhafs D, Cheriet A. Electrochemical and photoelectrochemical study of gallium arsenide–polybithiophene composite. Synth Met 2009; 159:1349–52. [23] Zhao C, Wang H, Jiang Z. Photocurrent and photothermal current of polypyrrole (PPy) film. Appl Surf Sci 2003;207:6–12. [24] Laflere WH, Cardon F, Gomes WP. On the differential capacitance of the n- and p-type gallium arsenide electrode. Surf Sci 1974;44: 541–52. [25] Yao H, Yau S-L, Itaya K. Atomic resolution images of GaAs(1 1 1) A surfaces in sulfuric acid solution. Surf Sci 1995;335: 166–70.