Synthetic Metals 88 (1997) 197–199
Junction with tunable current–voltage characteristics: n-doped Si/poly(vinyl chloride)–poly(dithienopyrrole) composite N. Camaioni a,U, G. Casalbore-Miceli a, A. Geri a, F. Capuano b, F. Croce c, F. Ronci c a
Istituto di Fotochimica e Radiazioni d’Alta Energia del CNR, via P. Gobetti 101, 40129 Bologna, Italy b ENIRICERCHE, via Rimarini 32, 00016 Monterotondo Scalo (Rome), Italy c Dipartimento di Chimica, Universita` ‘La Sapienza’, piazza le A. Moro 5, 00185 Rome, Italy Received 28 October 1996; revised 6 February 1997; accepted 7 February 1997
Abstract A device was realized by interfacing the rectifying junction silicon/poly(vinyl chloride)–poly(dithienopyrrole) composite with a second lithium electrode through a polymer gel. The purpose was to tune the characteristics of the junction by changing the potential delivered to the polymer. Current–voltage plots of the junction were recorded at different voltages applied between the polymer and the lithium electrode; preliminary results indicate a variation of the current–voltage characteristics by changing the polymer potential; in particular, an increase of the reverse saturation current and a decrease of the series resistance by increasing the oxidation level of the polymer were observed. Keywords: Junctions; Poly(vinyl chloride)–poly(dithienopyrrole) composites; Electrical properties; Doping
1. Introduction An advantage of the utilization of conducting polymers in the field of electronic technology lies in the variety of potentially available materials and in the possibility of changing the redox characteristics and doping level as a function of preparation details. In previous papers the formation of rectifying junctions, by interfacing inorganic semiconductors to conducting polymers, was put in evidence [1,2]. In particular, the characteristics of rectifying junctions between n-doped silicon and the conducting polymer composite poly(vinyl chloride)– poly(dithienopyrrole) (p(PVC–DP)) were described [3]. The properties of Schottky junctions, obtained by interfacing n-doped silicon to conducting dithienothiophene–dithienopyrrole copolymers with different redox potentials, were the subject of a previous paper [4], where it was reported that no meaningful differences can be observed in the current– voltage (I–V) characteristics of junctions made by interfacing n-doped silicon to copolymers with redox potentials varying in the order of 0.3 V. In the present paper some preliminary measurements are reported, inherent to a device constructed by interfacing the previously characterized Si–p(PVC–DP) junction [3] with U
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a solid-state electrochemical cell equivalent to an electrochromic device (Al–p(PVC–DP)–gel electrolyte–Li) (Fig. 1). In this manner, we tried to obtain a rectifying junction based on a redox polymer which can vary the optical spectrum as a function of the electrochemical cell potential: this property is of utmost importance if photovoltaic applications of the junction are taken into consideration. This device could also be considered a rectifying junction with variable I–V characteristics; in effect, by changing the voltage of the electrochemical cell, one can modify the nature and density of carriers inside the conducting polymer composite.
2. Experimental results and discussion Junctions Si–p(PVC–DP) were realized in the following way: n-doped silicon plates (rs0.5 V cm) were coated by PVC films (about 0.7–0.8 mm thick) by spinning a 50 g/l PVC solution in tetrahydrofuran. The silicon plates were previously deoxidized by rinsing them in a diluted aqueous HF solution. Then, the PVC-coated silicon plates were used as working electrodes in the DP polymerization by cyclic voltammetry, as reported in a previous paper [3]. The chemical and electrochemical properties of the p(PVC–DP) composite are reported in [5]. As the charge density is 1020–1021 charges per cm3 in wholly oxidized p(PVC–DP) and 1016 charges
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Article: 4972
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N. Camaioni et al. / Synthetic Metals 88 (1997) 197–199
the possibility to use this polymer for the construction of electrochromic solid-state windows. In Fig. 3 the I–V characteristics of the RJ are reported for three different polymer electrode potentials, in sequence 1.6, 2.3, 3.8 V; the response of RJ depends on the ECC potential. In order to explain the results, the expression can be considered which gives the current flowing across a Schottky junction by thermionic emission: qV y1 nkT
≥ ž / ¥
IsIS exp
Fig. 1. Scheme of the device.
per cm3 in n-doped silicon, the interface between Si and polymer was previously considered a Schottky junction [3]. The complete device, shown in Fig. 1, was assembled inside a Braun Labmaster 130 glove box, in argon atmosphere with O2 and H2O concentrations less than 1 ppm. A membrane of gel electrolyte (GE), based on poly(methyl methacrylate), LiClO4, ethylene carbonate and propylene carbonate, was spread on the conducting polymer film; the characteristics and the properties of this gel electrolyte are reported in Ref. [6]. Finally, a Li plate was interfaced to the GE membrane. The I–V plots of the junction were recorded by a Solartron 1286 electrochemical interface. The potential, referred to Li–Liq, was supplied to the electrochemical cell by an AMEL model 549 potentiostat. All the outputs of the instruments were floating. The complete device can be seen as composed of two different interfaced circuits: Al/n-Si/p(PVC–DP)/Al, the rectifying junction (RJ) and Al/p(PVC–DP)/GE/Li, the electrochemical cell (ECC). Before testing the device, the ECC was repeatedly cycled between 1.5 and 4.5 V versus Li/Liq in order to activate the conducting polymer (Fig. 2). The electrochemical oxidation–reduction process can be reproducibly reversed for many cycles; this result, considering the electrochromic properties of p(PVC–DP) evidenced in a previous paper [5], suggests
Fig. 2. Subsequent cyclic voltammetries of the ECC, assembled in the device of Fig. 1, with the rectifying junction (RJ) non-connected.
(1)
where q is the charge of the electron, V the applied voltage, n the ideality factor of the junction, k the Boltzmann constant and T the absolute temperature. IS, the reverse saturation current, is given by qfB ISsAUT2 exp y kT
ž /
(2)
where AU is the Richardson constant and fB the height of the contact barrier. In the plots of Fig. 3, the forward current increases exponentially only for low applied voltages (V-0.8 V), being limited for higher values by Rs. Therefore, a modification for a series resistance (Rs) must be introduced into Eq. (1): q(VyIRs) y1 nkT
≥ ž
IsIS exp
/ ¥
(3)
Many components can contribute to Rs, such as the resistance of electrical contacts, silicon plate, polymer film, etc.
Fig. 3. I–V plot of the RJ assembled in the device of Fig. 1 as a function of the ECC voltage, in sequence: - - -, 1.6 V; ———, 2.3 V; —— ——, 3.8 V.
Journal: SYNMET (Synthetic Metals)
Article: 4972
N. Camaioni et al. / Synthetic Metals 88 (1997) 197–199
Fig. 4. Incremental resistance as a function of forward voltage, inherent to the I–V characteristics of the RJ reported in Fig. 3.
The voltage drop across the series resistance decreases by increasing the ECC voltage from 1.6 to 3.8 V. A gradual lowering of Rs, due to the influence of ECC potential on the doping level and, therefore, to the resistivity of the polymer, may be the reason for this behaviour; the trend of the incremental resistance (rsdV/dI) of RJ, shown in Fig. 4 as a function of forward voltage, strengthens this hypothesis. The values of r tend, for high voltages, to quasi-constant values, i.e. to Rs values, which decrease by increasing ECC potential. The voltage variation of the polymer electrode can also cause the change, other than of the polymer doping level, in the nature of cation states with a consequent variation of the Fermi level inside the polymer, and then of the contact barrier height fB. For this reason, the pre-exponential factor IS in Eq. (1) can vary. In fact, it was found that, by increasing ECC potential, an increase of IS occurs from 1.92=10y5 to 2.92=10y5 A (the reverse saturation current values were obtained by extrapolating the experimental current to Vs0). Changes in the rectification properties of polymeric Schottky junctions, as a function of the doping level of the organic material, have already been observed [7]. Unfortunately, the device tends to damage after the first cycles; the reason can be found in the imperfect construction
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of some parts of the device, as metal contacts and interfaces between the components of the structure. Another reason for the short lifetime of the device may be the heating that takes place during the working process, which can damage the thin polymer films (less than 1 mm) or induce the diffusion of metal from the contacts into the polymer. Recently, some signs of reversibility have been recorded, but further work should be done in order to reach this goal; consequently, the meaning of the relation existing between the potential of the Al/p(PVC–DP)/GE/Li electrochemical cell and the I–V characteristics of the rectifying junction, Al/ n-Si/p(PVC–DP)/Al, will be more deeply investigated in a next paper. However, these preliminary results indicate the possibility of constructing rectifying junctions, based on conducting polymers, that are able to change their working characteristics as a function of the voltage of the conducting polymer electrode; this possibility could open up interesting opportunities of investigation. In fact, the nature of the cation species present in the doped polymers could be studied as a function of the oxidation level by evaluating the energy of the Fermi level of the ‘organic metal’; furthermore, a photoreceptor device with a tunable spectral region of maximum response could be constructed.
References [1] H. Nguyen Cong, C. Sene and P. Chartier, Sol. Energy Mater. Sol. Cells, 40 (1996) 261. [2] L. Torsi, L. Sabbatini and P.G. Zambonin, Adv. Mater. 7 (1995) 417. [3] M. Campos, N. Camaioni, G. Casalbore-Miceli, A. Geri, G. Giro and Q. Zini, Synth. Met., 75 (1995) 61. [4] M. Campos, G. Casalbore-Miceli and N. Camaioni, J. Phys. D: Appl. Phys., 28 (1995) 2123. [5] G. Beggiato, G. Casalbore-Miceli, V. Fattori, A. Geri, A. Berlin and G. Zotti, Synth. Met., 55–57 (1993) 3495. [6] G.B. Appetecchi, F. Croce and B. Scrosati, Electrochim.. Acta, 40 (1995) 991. [7] S. Glenis, G. Tourillon and F. Garnier, Thin Solid Films, 139 (1986) 221.
Journal: SYNMET (Synthetic Metals)
Article: 4972
