Novel photoelectrochromic cells containing a polyaniline layer and a dye-sensitized nanocrystalline TiO2 photovoltaic cell

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S I'fllTIHI[[TIIC IIliiI|IRLK Synthetic Metals 94 (1998) 273-277

Novel photoelectrochromic cells containing a polyaniline layer and a dye-sensitized nanocrystalline TiO2 photovoltaic cell Yongxiang Li a,b J~irgen Hagen a,b,* Dietrich Haarer a,b ~Lehrstuhl fiir Experimentalphysik IV, Universimt Bayreuth, D-95440 Bayreuth, Germany hBayreuther lnstitutfiir Makromolekiilforschung (BIMF), Universitdt Bayreuth, D-95440 Bayreuth, Germany Received 22 December 1997; accepted 6 January 1998

Abstract

We present a novel photoelectrochromic (PEC) cell containing a polyaniline layer and a dye-sensitized nanocrystalline TiO2 layer. Electrochromic thin films of polyaniline layers have been prepared by electrochemical deposition and by spin-coating a dispersion of a polyaniline lacquer. The influences of the preparing parameters on the micromorphologies of the polyaniline layers were investigated. Under illumination of about one sunlight intensity, this self-powered PEC cell can modulate its transmission, averaged over the whole visible spectral region. The structure-property relations of the photoelectrochromic cells are discussed. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: Photoelectrochromic cells; Photovoltaic cells; Polyaniline; Electrochromism; Nanocrystalline TiO2

1. Introduction

Systems, which change color electrochemically after being subjected to illumination, are referred to as photoelectrochromic (PEC) devices [ l ] . PEC devices can be operated in two different modes. In the first, photo-activated mode, the potential required to cause the electrochromic reaction is continuously applied to the system, but can act only through a photo-activated switch or trigger. A separate photoconducting layer or another photocell can serve as a switch. Alternatively, the electrochromic electrode surface itself can be a photoconductor or can be sandwiched together with a photoconducting layer. The second mode is a photo-driven device, in which illumination of one part of the cell can produce the photovoltaic potential required to drive the electrochromic layer. WO3 is one of the best-known electrochromic inorganic oxide materials and changes its color in response to an electrically induced change in the oxidation state. A low-cost, high-efficiency photovoltaic cell based on dye-sensitized colloidal TiOz films was developed by Gr~itzel's group and has been extensively studied [ 2 - 5 ] . Recently, Bechinger et al. [6] reported the principle o f a PEC cell with a dye-sensitized nanocrystalline TiOz electrode and a WO3 electrochromic thin film on the counter electrode. This * Corresponding author. Tel.: +49 921 55 3218; fax: +49 921 55 3250; e-mail: j uergen.hagen @ep4.phy.uni-bayreuth.de 0379-0779/98/$19.0(1 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 3 7 9 - 6 7 7 9 ( 9 8 )000 13-7

type of self-powered smart window, which uses the energy of the incident sunlight to modulate its own transmissivity, can in principle be easily installed in buildings without requiring additional electrical connections. It has several distinct advantages over conventional photovoltaic cells or other semiconductor electrodes for use as large-area windows or display applications. Conducting polymers are also being considered as attractive materials for optical devices and display applications due to the electrochromic phenomena associated with the redox processes of the polymers involved. Among them, polyaniline (PANI) has been predicted as a promising electronic material due to its unique conduction mechanism and due to its high environmental stability. In this respect, it is a new electrochromic material with interesting multiple coloring properties. By electrochemical oxidation or reduction, the color of a PAN1 film can be reversibly switched between yellow and green at relatively low voltages ( - 0.2 to 1.0 V). It turns colored when it is oxidized and bleaches when it is reduced, which is the opposite electrochromic behavior as compared to WO3. In this paper we describe a novel PEC cell containing a PANI layer and a dye-sensitized nanocrystalline TiO2 photovoltaic cell, which is quite similar to the configuration of Bechinger's PEC cell. The advantages of the present cell are the fast color/bleach transition and, when it is short-circuited

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Y.Li et al. / Synthetic Metals 94 (1998) 273-277

under illumination by white light, it changes to its transparent (or decolored) state, yet it is colored again when it is switched back to an open circuit. It can be used as a photo-switch, as a shutter and as a smart window or for contrast enhancement.

2. Experimental 2.1. Preparations of P A N l films The anodic deposition of a PANI film is carried out from a 1 M HC1 medium containing aniline in 0.1 M concentration onto an indium-tin oxide (ITO) electrode (sheet resistivity 17.1 fl/l-]) by the application of an alternating voltage ( - 0 . 1 to 1.0 Vp_p value, 0.1 Hz). A platinum plate is used as counter electrode. The electrodeposition time is typically 10 min. The film is rinsed in ethanol to remove the very rough surface layer. The advantages of using electrochemically deposited polymer layers are the ease of preparation and the uniformity of the prepared films. Another method of preparing PANI films is spin-coating or dip-coating of a dispersion of ORMECON Lacquer 900187/34. This lacquer is a dispersion of the intrinsically conductive PANI nanoparticles in a solvent containing a binder (polyamide). The mean particle size is approximately 10 rim. It can be used to produce virtually translucent and electrically conductive coatings. The solid content is 10%. To make a thin film by the spin-coating process, the dispersion is diluted (1:4) with a solvent mixture of isopropyl alcohol and toluene (1:1). A 1 txm thick film is spin-coated for 20 s at a speed of 2000 rpm. Good coating results can also be realized by dip-coating the substrate into the dispersion. The wet films are heated in an oven at a temperature of 180°C for 2 h before preparing the PEC cells, in order to obtain dry and compact films. The color of the films is blue or dark blue. 2.2. PEC cell configuration Fig. 1 shows a schematic PEC cell configuration. It is a multilayer structure of TiO2/dye molecule/electrolyte/ PANI between two ITO substrate electrodes. A glass coated with nanocrystalline TiO2 particles (P25 Degussa) is heated to a temperature of 450°(2 for 30 min and then, while still warm (80°C), immersed in a 30 IxM ethanolic solution of cis-di ( thiocyanato )-bis-( 2,2'-bipyridyl-4,4'-dicarboxylate ) ruthenium(II). It is kept in the dye solution for 1 day. The light-absorbing dye layer can be adjusted by either reducing the thickness of the TiO2 layer or by decreasing the concentration of the adsorbed dye molecules. In most cases the TiO2 layers in the PEC cells are about 1 to 3 p~mthick. The counter electrode is an electrodeposited PANI layer on an ITO glass which has a thickness of about 140 rim. The two electrodes are then sealed by a UV hardening epoxy. The PEC cells are fabricated with a typical size of 2 cm 2. The electrolyte is

Fig. 1. SchematicPEC cell setup. The arrows indicate the direction of the incident light. typically 0.5 M LiI and 0.04 M 12 in a mixture of propylene carbonate and acetonitrile (4:1 by volume). 2.3. Measurements The absorption spectra of the various states of the PANI layers are recorded with a PE lambda 9 UV-Vis-NIR spectrophotometer. A JSM-840A scanning electron microscope is employed to analyze the morphology of the PANI films. The thickness of the PANI films is measured with a surface profiler (Dektak 3030ST VEEO Instruments). Finally, we use a 75 W xenon short arc lamp and a power meter with wavelength calibration (Newport Corp. 835) to detect the transmitted light intensity after the transmittance change of the PEC cells (colored/decolored). A digital oscilloscope (PM 3323/41, Philips) is used to store the data.

3. Results and discussion 3.1. PANl films Aniline molecules can be polymerized electrochemically. The mechanism of the electropolymerization proceeds via a radical cation [7]. Aniline molecules can be protonated in a strongly acidic solution, and PANI is well formed in a solution at lower pH value (e.g. p H - 2 ) . The scheme in Fig. 2 depicts the chain initiation process. PANI layers change their color from pale green to dark blue depending on the redox reaction at different potentials. PANI has two oxidation stages in the electrochemical process, namely, leucoemeraldine salt (LS) and emeraldine salt (ES). The most stable PANI operates over a restricted poten-

7C. 2 x--k_g-..2

~-20" ,. -x=/ Fig. 2. Initiationprocessof aniline polymerization.

E L i et al. / Synthetic Metals 94 (1998) 273-277

1.6

Spin-coated PANI Electrodeposited

]

1.4 1.2

g

o~

1.0 0.8


~JJS ~f

0.6 0.4 0.2

0.0

400

450

500

550 600 650 Wavelength (nm)

700

750

800

Fig. 3. Absorption spectra of the polyaniline layers coated on ITO glass substrates.

275

ately conductive PANI layer. The thicknesses of the films are 140 nm in (b) and 1 ixm in (c), respectively, as determined with the surface profiler. Fig. 5 shows scanning electron micrographs of a spincoated PANI film from a PANI suspension lacquer. The film contains many aggregated particles and many pores with a size of about 1 Ixm. The thickness of the film is 1 Ixm. As compared to the electrodeposited PANI films, the quality of this film is worse. The only advantage of this film is its simpler preparation. Although the conditions chosen for the preparation of the PANI films may not be fully optimized, the observed photoelectrochromic properties of our PEC cells constructed from the electrochemically deposited PANI films seem to be satisfactory with respect to their long-term stability and rapid response.

3.2. PEC cell properties tial range to allow only the yellow and green states. Such an electrochromic cell can have a cycling life in excess of 10 6 cycles [8]. Since an alternating potential is applied to the PANI film during electrodeposition, we can see that the film changes its color between pale green and blue in each cycle of the electrochemical deposition. The UV-Vis absorption spectra reveal that the spin-coated and electrodeposited PAN1 films have the emeraldine salt (green color) structure as shown in Fig. 3. Fig. 4 shows scanning electron micrographs of PANI films which were electrodeposited on ITO glasses. Fig. 4(a) shows that there are two different structures in the PANI layer. One is the primary PANI layer on the ITO glass as shown in (b), which is an even, compact microspheroid surface morphology layer with a uniform thickness. The grain size is about 50 rim. On top of this layer there is another layer, which has an open microstructure (as shown in (c) ). This top layer can be easily rinsed off in ethanol, and a PANI colloidal suspension is formed. This reveals that the loose top layer consists of PANI nanoparticles. The reason of the formation of the two layers is assumed to be due to different nucleus formation and growth speeds of PANI layers on different substrate surfaces. The lower layer is polymerized on a highly conductive ITO substrate, and the top layer is polymerized on a moder-

The large surface area TiO2 semiconductor electrodes with a nanoporous morphology are sensitized with a Ru(bpy) complex. A photocurrent is generated when photons are absorbed by the dye molecules at the TiOz/electrolyte interface and electrons are injected into the conduction band of the TiO2 nanocrystalline particles. At open circuit, the dyesensitization process produces a photovoltage of 0.4-0.6 V, but does not result in decoloration of the cell. When the electrodes are short-circuited, electrons move from the TiO2 layer through the external circuit to the PANI film, and the PANI film is reduced and bleached. The following redox reaction can be proposed:

+2e g 3 - a O - q

Iredu=,o.l__ k~.~(/ x-

~

x~._]n ~ (oxidation) -2e"

(emeraldine salt, colored)

rt

(leuco-emeraldine, decolored)

where X - is an anion ( C I - and/or I - ). Fig. 6 shows the change of light intensity transmitted through a PEC cell when it is short-circuited and open-circuited for time intervals of each 1 rain at an incident light intensity of 80.4 m W / c m 2, illustrating the decolored and

Fig. 4. Scanning electron micrographs of the PANI films grown on an ITO glass substrate: (a) × 20 000, area is 4.4 X 4.4 p~m2; (b) X 50 000, area is 1.75 X 1.75 ~m2; (c) × 15 000, area is 5.8 × 5.8 ixm2.

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Y. Li et a l . / Synthetic Metals 94 (1998) 273-277

Fig. 5. Scanning electron micrographs of the PANI films spin-coated on an ITO glass substrate: (a) × 5000, area is 17.5 × 17.5 ixm2; (b) × 40 000, area is 2.2 × 2.2 ixm2. Table 1 PEC cell characteristics at different illuminating light intensities 5

0.5

(decolored)(colored) 0.4

Tshort Topen

o~

~c~

0.3

~ t ~ r

II

c,rcuit [

"1-

t

short open short open circuit cwcuit circuit circuit,

=F

rr

Filter

wG345NG9

WG345 (NG4) WG345

Light intensity ( m W / c m 2)

64

80.4 25 1.4

V,,~ (mV)

J~ ( m A / c m 2)

2 2004

416 516

0.37 0.78

AT (%)

24

5. I 5.8

"

0.2 ~-

0.1 0.0

0

i

i

i

50

1O0

150

i

i

200 250 Time(Sec.)

i

i

300

350

400

Fig. 6. Change of the light intensity transmitted through the PEC cell.

colored states, respectively. The short-circuit current density J~c and the open-circuit voltage Voc are 0.37 m A / c m 2 and 416 mV, respectively. The light intensity transmitted through the PEC cell changes by 5.1%. It should be noted that this value represents an average value over the whole visible spectral region. The transmission change at certain fixed wavelengths must be significantly larger, but since the PANI films can switch to various colors, we chose to detect the whole spectrum. The photoelectrochromic process is highly reversible and a lifetime of more than 106 cycles can be expected. The reduction (decoloring) occurs in about 2-3 s. The oxidation (coloring) occurs in two steps. The first step takes 4 s, and then a slower process of about 30 s follows. This means that anions from the electrolyte move into the PANI film during the oxidative coloration process and are then eliminated from the film during the reductive decolorization process, as was also reported for other conductive polymers [9], The reason for the different time scales is that the reduction process is driven by an internal electrical field, whereas the oxidation is a diffusion process. Also, in the case of WO3, coloring and bleaching (charge injection and extraction) are not completely symmetric phenomena. Here on illumination, transmission decreased to its saturation level after 100 s. Recovery

to the bleached state after blocking the light was somewhat slower (4 rain) [6], Table 1 shows the PEC cell properties illuminated at different light intensities. A WG345 filter (Schott Glaswerke, Germany) was used to block UV light (Ac= 345_+6 nm), and neutral density filters NG4 and NG9 were used to reduce the light intensity in the visible region. It is obvious that a higher light intensity generates higher Vo~and J~c values, and therefore a stronger color change can be achieved.

4. Conclusions We have investigated a novel photoelectrochromic cell consisting of a PANI layer combined with a dye-sensitized nanocrystalline TiO2 layer. The structure of the cell was ITO/ TiOJdye/electrolyte/PANI/ITO. The homogeneity and compactness of electrochemically deposited PANI films were better than that of the films spin-coated from PANI suspension (Ormecon Lacquer). Our PEC cells are colored when they are open-circuited and decolored when short-circuited under visible light illumination. The coloring and decoloring times are on the order of a few seconds, which is much faster than in the case of WO3-type PEC cells. Under white light illumination with an intensity of 80.4 m W / c m 2, values for Voc and J~c of 416 mV and 0.37 m A / c m 2 were obtained, and the light intensity transmitted through the PEC cell (color/ decolor states) changed by 5.1% averaged over the whole visible spectral region. The driving voltages of our PEC cells generated internally are lower than that used in conventional

E L i et al. ~Synthetic Metals 94 (1998) 273-277

EC devices. To develop this novel PEC cell for practical applications as smart windows or as photo-switches, infraredabsorbing dyes must be used as sensitizers. Such a device would then be transparent to visible light when it is shortcircuited, but can be colored when it is open-circuited. The most important advantages of the new PEC cells are that they are inexpensive, robust, fast and can be easily scaled up to large-area devices,

Acknowledgements We thank the Bundesministerium ftir Bildung, Wissenschaft, Forschung und Technologie for financial support. Furthermore, we thank the Degussa AG for supplying the P25 material, C. Drummer and W. Reichstein (BIMF, Bayreuth) for taking the scanning electron micrographs and W. Joy for

27 "7

Ru(bpy) dye synthesis. Y. Li thanks the 'Alfried Krupp yon

Bohlen und Halbach Stiftung' for support. References [11 P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism Fundamentals and Applications, VCH, Weinheim, 1995, p. 192. [21 B. O'Regan, M. Gr~.tzel, Nature 353 ~1991 ) 737. [3] M. Gr~itzel, Comments Inorg, Chem. 12 ( 1991 ) 93. [4] S. St~dergren, A. Hagfeldt, J. Olsson, S.E. Lindquist, J. Phys. Chem. 98 (1994) 5552. [5] J. Hagen, W. Schaffrath, P. Otschik, R. Fink, A. Bacher, H.-W. Schmidt, D. Haarer, Synth. Met. 89 ( 1997 ) 215-220. [6J C. Bechinger, S. Ferrere, A. Zaban, J. Sprague, B.A. Gregg, Nature 383 (1996) 608-610. [7] Y. Wei, G.-W. Jang, C.-C. Chan, K.F. Hsueh, R. Hariharan, S.A. PateL C.K. Whitercar, J. Phys. Chem. 94 (1990) 7716. [8] W. Takashima, M. Kaneko, K. Kaneto. A.G. MacDiarmid, Synth. Met. 69-71 (1995) 2265-2266. [ 91 P.K. Shen, H.T. Huang, A.C. Tseung, J. Electrochem. Soc. 139 (1992) 1840-1844.