Solar Cells, 23 (1988) 233 - 244 233

THE SEMICONDUCTOR SEPTUM PHOTOELECTROCHEMICAL CELL

KE XIAO and H. TI TIEN*

Membrane Biophysics Laboratory (Giltner Hall), Department of Physiology, Michigan State University, East Lansing, MI 48824 (U.S.A.)

(Received September 23, 1987; accepted in revised form November 23, 1987 )

Summary

CdSe semiconductor septum photoelectrochemical cells have been constructed and their photoelectrochemical characteristics studied using cyclic voltammetry. Photovoltages and photocurrent densities as high as 1.8 V and 7 mA cm -2 respectively were obtained. The changes in polarity of open-circuit voltage and short-circuit current of the cell upon illumina- tion suggest that a rechargeable photoelectrochemical cell may be developed using dual electrolyte semiconductor contacts.

1. Introduct ion

Photoelectrochemical (PEC) cells of various designs have been con- structed and tested, the aim being to convert solar energy into suitable forms for more efficient use [1 - 6]. To date no commercially viable PEC cells have been developed; however, the PEC cells based on CdSe and CdS appear to be promising [2, 7, 8]. Further, the construction of PEC cells based on ultrathin lipid membranes has been at tempted by several groups of investigators [9 - 11]. The central idea of a membrane-based PEC cell is a pigmented lipid bilayer separating two aqueous solutions containing redox agents. To a very large extent, the essential aspect of membrane-based PEC cells is modeled after natural green plant photosynthesis, where under illumination one side of the membrane solution interface is reducing while the other side of the membrane solution interface is oxidizing. In this scheme, a transmembrane movement of electrons is assumed [12, 13]. Experimental evidence for these photoinit iated transmembrane redox reactions has been provided by a number of laboratories [ 14 - 16]. Closely related to the membrane-based PEC cells are lipid vesicles, micelles and organized assemblies [ 1 7 - 2 2 ] , which have provided additional support for light-induced electronic processes in ultrathin lipid membranes in aqueous media.

*Author to whom correspondence should be addressed.

0379-6787/88/$3.50 © Elsevier Sequoia/Printed in The Netherlands

234

In all previous work, with one notable exception, the photoactive compounds in the membrane phase were either natural products or organic pigments whose long-term stability is limited in comparison with inorganic semiconductors. The exception mentioned involved the use of CdSe or CdS in place of the membrane [23]. In the case reported, a PEC cell was made with a CdSe pellet separating two aqueous solutions, which had an open- circuit voltage Voe of 1.3 V, a short-circuit current density Isc of 6 mA cm -2 and a cell efficiency of 6% [24]. In this paper we report a more de- tailed characterization and further improvement of semiconductor septum PEC cells.

2. Materials and methods

The CdSe pellets (0.1 cm thick, I crn 2 area) were made with CdSe powder (99.999% Alpha Products, Aldrich Chemical Co. Inc.) under a pressure of 2 × 106 kg crn 2. They were then sandwiched between two glass slides and sintered in air at 540 °C for 4 h. The cooled CdSe pellets were etched in HCI:HNO3 (3:1 v/v) aqueous solution for i0- 15 rain, rinsed in distilled water, then dipped into a mixture of 1 M Na2S and I M NaOH for I h and rinsed again. All chemicals were analytical grade and were used without further purification.

2.1. The cell The measuring cell had two compartments each containing different

electrolytes which were separated by a CdSe pellet, called a semiconductor

)

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CdSe Pellet

] 2 . . . . . --~i POTENTIOSTAT/GALVANOSTAT

(EG&G MODEL 173 WITH 276 INTERFACE)

I I G IB PC INTERFACE I

I COMPUTER

(IBM XT)

I PRINTER/PLOTTER

Fig. 1. Block diagram of the set-up used for investigating semiconductor septum photo- electrochemical cells. The numerals I and 2 refer to chambers I and 2 in the text, M stands for metal electrode.

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236

septum electrode [24]. The solution in chamber 1 adjacent to the light was always 1 M Na2S and 1 M NaOH, while chamber 2 contained different redox couples (Fig. 1). To complete the cell arrangement, a reference electrode (RE) and an auxiliary electrode (AE) together with a platinum electrode were used. The working electrode (WE) was either the standard calomel electrode, silver, copper, lead, palladium or platinum and was placed in chamber 2. The RE, AE and SCE were all saturated calomel electrodes.

2.2. M e a s u r e m e n t s The open-circuit trans-septum photovoltage Vop was measured with an

HP-3460B digital voltmeter. The cyclic voltammograms were obtained with a computer-controlled electrochemical system (IBM-PC XT computer , GPIB- PC interface, PAR Model 173 Potentiostat , Model 267 interface and Model 175 programmer, Epson FX-86 printer and an X-Y recorder). The light source was a 650 W halogen-tungsten lamp, the light intensity of which was measured at the place of the septum electrode with a Solar Meter (model 776).

3. Results and discussion

The outstanding feature of the CdSe SS-PEC (semiconductor septum photoelectrochemical) cell is that it has an open-circuit photovoltage Vop approaching that of the band gap (approximately 1.7 eV), in contrast to a typical CdSe PEC cell which usually displays a Vo, less than 0.9 V. The CdSe SS-PEC reported here has an average Vop of 1.75 V, 1.86 V being the highest recorded. The highest short-circuit photocurrent density observed w a s 7 m A c m 2.

The idea of trans-septum cyclic voltammograms is taken from the trans-membrane cyclic voltammograms used in black lipid membrane re- search [ 12, 25]. The difference between trans-septum cyclic voltammograms and conventional cyclic voltammograms is that they involve a series of complex reactions occurring at three interfaces: two semiconductor- l iquid junctions (redox 1-semiconductor and semiconductor- redox 2) and one metal-solut ion interface (redox 2-metal) . One of the three interfaces should be a controlling interface which limits the overall reaction rate of the SS-PEC system. In our experiments, we are more interested in the semiconductor- liquid junction than the metal-solut ion interface. The different redox couples and corresponding metal electrode which i's reversible to the metallic ions presented in chamber 2 were used. Figure 2 shows a voltammogram for an SS-PEC cell. In this case, a copper electrode in chamber 2 was used. The dark current is much smaller than the photocurrents, and the curves of photocurrents change very little when different sweep rates are used {Fig. 2(a)). This means that a photo-anodic and/or a photo-cathodic reaction is occurring at the interfaces 1 and 2. Figure 2(b) shows voltammograms under different light intensities I, from which d i /d I are calculated (Fig. 2(c)). The

237

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239

onset potential at the higher light intensity is about --1.2 V (SCE). It should be considered to be the flat band potential U~b [26]. The U~b here is equiv- alent to the inner potential of a CdSe pellet because a septum electrode has two semiconductor- l iquid junctions. Here, we assume that the semiconduc- tor septum has a continuity of energy levels. However, it seems likely that there could be junctions in the CdSe semiconductor septum electrode; this is a very interesting subject to be researched in the future. The SS-PEC cell of 1 M Na2S , 1 M NaOH//CdSe/ /0.5 M AgNO a has similar voltammo- grams to the Cu(NO3): system (Fig. 3). However, the value of di/dI (0.857 mA mW -1) is less than that of Cu(NO3)2. It should be pointed out that the metal copper or silver was deposited onto the surface of the CdSe septum during the measurement of the cyclic voltammograms. The effects of metal deposit ion on the SS-PEC cell are still unclear. Figures 4 and 5 show voltam- mograms of the SS-PEC cells where 0.1 M PbC12 and 0.1 M PdCI: were used in chamber 2, with a lead and a palladium electrode respectively. The photo- current curves are similar to those obtained for photoresistance. The dif- ference between the PbCI2 and PdCl: photoresistance is that the former is non-linear and the latter is linear. As a group I I - IV semiconductor, CdSe reacts easily with many metal ions, which changes it to a small band gap semiconductor (for example, the band gap Eg of PbSe is 0.26 eV) [26]. Another interesting phenomenon of the SS-PEC cell is its capacitance. Figure 6 shows voltammograms of the SS-PEC where the 0.1 M FeSO4, 0.5 M H:SO4 solution and a platinum electrode were placed in chamber 2. When the concentrat ion of FeSO4 was changed from 0.1 M to 0.01 M, the capacitance-voltage curves as a function of light intensity are altered. The effect of the concentrat ion of the metal ion in chamber 2 is very dramatic. There is also an oxidation peak in the voltammograms, bu t no reduction

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Voltage vs. SCE (volts)

Fig. 4. A compos i te vo l t ammogram at a light intensi ty of 50 mW cm -2 at d i f ferent sweep rates for the system 1 M Na2S, 1 M NaOHl/CdSe//O.1 M PbC12/Pb at 200, 100, 50, 20 and 1 0 m V s -1.

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Fig. 5. A compos i t e v o l t a m m o g r a m at a l ight in tens i ty of 50 mW cm -2 at d i f f e ren t sweep ra tes for the sys tem 1 M Na2S, 1 M N a O H / / C d S e / / 0 . 1 M PdCl /Pd at 2 0 0 , 1 0 0 , 50, 20 and l O m V s -I"

peaks can be discerned. One conclusion is that the reaction in chamber 2 is irreversible.

An ideal SS-PEC cell should be a combined system of photoenergy con- version and storage. By replacing the FeSO4-H:SO4 solution with the K4Fe- (CN)6-H2SO 4 solution in chamber 2, voltammograms of different forms are obtained (Fig. 7). Figure 7(a) shows the voltammograms as a funct ion of sweep rate v. The photocur ren t was dependent on the sweep rate. When the concentra t ion of ferrocyanide is reduced to 10 -3 M, one oxidat ion peak and one small reduct ion peak were observed under illumination. Our observa- tions are consistent with those reported recently by Van Ryswyk [27], who reported an insoluble Kx [CdFe(CN)6] layer on the CdSe surface. Moreover, the capacitance of the cell is increased as the concentrat ion of ferrocyanide is reduced.

Finally, we made a durat ion test of our "bes t " SS-PEC cell over a period of 10 days, a port ion of the results are shown in Fig. 8. A fairly steady increase in trans-septum potential difference was observed, which peaked after about 40 h. At the end of the test period, the pellet was exam- ined. The surface exposed to AgNO3 was found to be reddish in color whereas the CdSe side facing the Na2S-NaOH solution was visually un- changed.

In concluding, a semiconductor septum PEC cell has been character- ized. In terms of photovoltage we have obtained the highest (1.86 V) value ever reported, whereas pho tocur ren t densities are of the order of 7 mA cm -2. The SS-PEC system mimics the natural photosynthe t ic membranes of the chloroplast. The conventional cyclic vol tammetry has been found to be a suit- able method for determining the parameters of the SS-PEC system with the two semiconduc tor - redox interfaces. Some metal salts such as AgNO3,

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Fig. 7. (a) Voltammograms at a light intensity of 50 mW cm 2 at different sweep rates for the system 1 M Na2S, 1 M NaOH//CdSe//0.1 M K4Fe(CN)6 , 0.5 M H2SO4/Pt: curve 1, 200 mV s 1; curve 2, 100 mV s 1; curve 3, 50 mV s-l; curve 4, 20 mV s-l; curve 5, 10 mV s 1; curve 6, 5 mV s -1. (b) Voltammograms at a light intensity of 50 mW cm -2 at dif- ferent sweep rates for the system 1 M Na2S, I M NaOHHCdSe//1 × 10 .3 M K4Fe(CN)6, 0.5 M H2SO4/Pt.

Cu(NO3)2 , PdC12 and PbC12 have b e e n i n v e s t i g a t e d . A l t h o u g h t h e SS-PEC s y s t e m d e s c r i b e d here has m a n y i n t e r e s t i n g and n o v e l f ea tures , th ere are m a n y a s p e c t s t h a t n e e d a d d i t i o n a l i n v e s t i g a t i o n . N e v e r t h e l e s s , SS-PEC s y s t e m o f f e r s a u n i q u e o p p o r t u n i t y for fur th er d e v e l o p m e n t in t h e c o n s t r u c - t i o n o f e n e r g y c o n v e r s i o n and s torage dev i ce s .

243

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~ o.~ E ~-" 0 .7 -

0.6- 0 . 5 -

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TIME (hour=)

Fig. 8. Duration test of the CdSe SS-PEC system. Pt/1 M Na2S, 1 M NaOH//CdSe// 0.5 M AgNO3/Ag. The SS-PEC cell was illuminated with a halogen-tungsten lamp at an intensity of 75 mW cm -2.

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