Cold pressed cadmium selenide photoanodes for electrochemical solar cells

Solar Energy Materials 9 (1983) 69-75 North-Holland

69

COLD PRESSED CADMIUM SELENIDE PHOTOANODES TROCHEMICAL SOLAR CELLS

FOR ELEC-

A. M A C K I N T O S H , S. W E S S E L , F. E1 G U I B A L Y and K. C O L B O W * Department of Physics, Simon Fraser University, Burnaby, BC, Canada V5A 1S6

Received 28 September 1982; in revised form 8 February 1983 Cold pressing of cadmium selenide powder was investigated as a technique for producing photoanodes for electrochemical solar cells. Physical properties such as density, resistivity and surface morphology were determined and related to solar cell performance via wavelength response, quantum efficiency and white light current-voltage characteristics. The spectral response indicated a bandgap of 1.7 eV. Pellets pressed at higher pressure showed an improved quantum efficiency. Pressures above 69 MPa produced fractures in the pellets. Conversion efficiencies under white light (tungsten halide lamp) at 100 mWcm -2 were on the order of 1.5%.

1. Introduction Polycrystalline semiconductor liquid junction solar cells offer the possibility of giving energy conversion efficiency close to that of cells employing single crystal electrodes [1], but at a much lower cost. C a d m i u m selenide is one of the materials that is the subject of m u c h research because of its ideal direct b a n d gap (1.7 eV) relative to the solar spectrum. Several preparation techniques to obtain discs or sheets of polycrystalline materials have been tried, such as cold pressing [2], pressure sintering [3,4], spray techniques [4], painting from a slurry [5] and electrodeposition [6-9]. In the present paper we are reporting the o p t i m u m conditions for cold pressing of CdSe powder, an uncomplicated yet promising technique after appropriate heat treatment.

2. Experimental CdSe power (99.99%-5 F m mesh) from C o m i n c o Ltd., Trail, BC (Canada), was cold pressed in a c a r b o n steel press at r o o m temperature at different pressures and for varying periods of time. Emission spectrographic analysis of the high purity p o w d e r showed the following impurities (ppm): A1

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The resulting discs of 10 mm diameter and 0.5-0.8 m m thickness were annealed at 600°C for 6 h followed by cooling to room temperature under vacuum. The density of the discs were measured before and after annealing. 600°C was found suitable to reduce the resistivity of the discs without evaporating excessive amounts of material. One half of each disc was used as photoanode to determine its solar cell parameters while the other half was used for resistivity measurement. Ohmic contacts were made by rubbing a thin layer of l n G a (25%:75% by weight) onto a slightly abraded sample surface. The sample was attached to a clean Cu disk using silver dag and the assembly was mounted on a plexiglass disk using Devcon 5 min epoxy. The photoanodes were etched, prior to testing, for 1 min in 1 : 3 H202 : CH3CO2H. Light entered the test cell through a quartz bottom window, allowing = 3 m m of electrolyte between the window and the CdSe electrode which was surrounded by a Pt counter electrode. In all cases the sulfide/polysulfide redox electrolyte was unstirred and consisted of 1 M N a O H , 2 M N a z S (saturated), 1 M S and 0.01 M Se. N 2 gas continuously flushed the system since the electrolyte is sensitive to oxygen. The current-voltage ( I - V ) characteristics of the cell were determined in the dark and under 100 m W c m 2 white light illumination from a 100 W tungsten halide lamp by manually adjusting the bias voltage and recording the stable value of the current. The spectral response of the cell was obtained under short-circuit conditions using a 100 W tungsten halide lamp chopped at 20 Hz, followed by a Jarrel-Ash grating monochromator with 8 nm resolution, and a Princeton Applied Research lock-in amplifier (model HR-8). The quantum efficiency (7) was measured using the lock-in amplifier and a Spectra-Physics model 125 H e - N e laser source (14 m W c m - 2 ) . In obtaining the ~/- V curves care was taken to adjust the bias voltage in the presence of illumination to compensate for the voltage drop across the series resistance of the cell.

3. Results and discussion Fig. 1 illustrates the effects of pressure, its duration and subsequent heat treatment on the density of the CdSe discs. It is observed that duration of pressure has only a small effect on the density of the pressed sample. Larger pressures lead to densities closer to the single crystal density of 5.81 gcm-3. However. for pressures larger than 69 MPa, fractures occurred either on pressure release or during the following heat treatment. Annealing the pellets at 600°C for 6 h under vacuum increases the density a further 13 to 22%, with the larger percent increase occurring for the samples pressed at lower pressure. The surface morphology of the annealed pressed pellets was examined with a scanning electron microscope. Fig. 2a shows the amorphous nature of a sample pressed at 11.5 MPa and the presence of crystallites which disappear after etching the sample prior to testing its solar cell characteristics. For a sample pressed at 23 M P a (fig. 2b), the surface is less amorphous and less crystallites are present. The crystallites disappear completely for samples pressed at 46 MPa (fig. 2c), and the surface has a more polycrystalline structure as evident in fig. 2d on a 5 times

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expanded scale. Fig. 2e shows a sample pressed at 69 MPa, which is similar in appearance as the 46 MPa sample. Typical l- 1 characteristics under white light illumination are illustrated in fig. 3 (I) for samples pressed for 5 rain and in fig. 3 (II) for samples pressed for 2 h at different pressures. Both figures show that Dower pressures give rise to a higher short-circuit current (I,,,,) while the open-circuit potential (1,~,~) is lower compared to samples pressed at higher pressure. For comparison the dark currents are sho,an in fig. 3(right). Comparison with fig. 3(left) shows that the "superposition principle" [ 11 ] is reasonably well obeyed in these samples, that is the photoresponse of the cell deteriorates when the opposing dark current becomes large. The results from fig. 3 are summarized in fig. 4, which relates 1~. and F[,~, to the density of the annealed pellets, The observed dependence of IJi,~, 1~. and the I - I," characteristics on the pellet density could be ascribed to several factors which include series and shunt resistances of each cell, crystallinity of the samples and variations in the intrinsic

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A. Mackintosh et aL / Cold pressed cadmium selenide photoanodes

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quantum efficiency with voltage and wavelength of the incident light. Fig. 5 illustrates the effect of pressure and its duration on the 7/- V curves. A general trend is a deteriorating efficiency around 0.30-0.35 V, also lower pressure leads to generally lower quantum efficiency. The highest density pellet (69 MPa, 2 h pressure time) shows the best efficiency at low forward bias, but a faster decrease in efficiency with increasing forward bias, most likely due to a decrease in shunt resistance resulting from microcracks. All curves in fig. 5 exhibit the familiar "S-shape", indicating a loss mechanism related to recombination of carriers in the quasi neutral region, the depletion region or the surface of the semiconductor [10-13]. The spectral response of the photoanodes in fig. 6 shows a well-defined absorption edge of CdSe at 1.7 eV for the two samples pressed at higher pressures indicating the increased crystallinity of the pellets, while the two samples that were pressed at lower pressure show a more gradual transition of the absorption edge because of the more amorphous nature of their structure and due probably to the increased density of defects in their bandgap. It is apparent that while the quantum efficiency of the photoanodes is quite high, the external power conversion efficiency of the cells is low, similar to earlier observations for CdS and TiO 2 [14]. On comparing the general features of the voltage dependence, one notices the presence of S-shape curves for the quantum efficiency (fig. 5) as previously discussed, yet these features are not seen in the 1 - V characteristics (fig. 3). This also indicates that the dominant mechanism effecting the

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A. Mackintosh et al. / Cold pressed cadmium selenide photoanodes

75

shape of the I - V curves is the large bucking (dark) currents that flow opposite to the photocurrent.

Acknowledgements This research was supported by the BC Science Council and the Natural Science and Engineering Research Council.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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