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Optimization of CdO layer in a SeCdO photovoltaic cell
ELSEVIER
Solar Energy Materials and Solar Cells 37 (1995) 75-92
Optimization of CdO layer in a Se-CdO photovoltaic cell C.H. Champness *, C.H. Chan Electrical Engineering Department, McGill University, Montreal, PQ, H3A 2,47, Canada
Received 13 July 1994; revised 25 October 1994
Abstract
An experimental study was made on a Se-CdO photovoltaic cell of conventionali structure to optimize the conditions during the dc reactive sputtering of the CdO layer. Thei parameters varied were: sputtering current, pressure, gas flow rate, deposition time andf ratio of oxygen to argon in the sputtering gas. These parameters were found to be ver~ critical in determining photovoltaic cell performance at higher light levels. The electrica! resistance and optical transparency in the CdO, are determined by its stoichiometry and this in turn was found to be controlled by the film deposition rate, through sputtering curren~ and pressure and by oxygen content in the sputtering gas. A laboratory-fabricated S e - C d O photovoltaic cell with an optimized CdO window layer (but without an optimized collecting grid or antireflecting coating) was found to yield a conversion efficiency of about 2.5% under 100 m W / c m 2 of solar irradiance and about 5% under fluorescent room light o~ irradiance 0.13 m W / c m 2. For a standard commercial selenium photometry cell the corre, sponding efficiency values were 0.3% and 3%, respectively.
I. Introduction
A photovoltaic cell based on a S e - C d O structure, involving crystallized trigonal selenium, has b e e n in use for about half a century for p h o t o m e t r i c applications [1]i This is due to the fact that the device can be m a d e to have a spectral response close to the wavelength sensitivity of the h u m a n eye and because the cell is relatively simple to make, involving only low temperature, low cost fabrication processes. However, it is unsuitable as a solar cell, since it yields a conversion efficiency well below 1% [2] u n d e r solar illumination. This is because it iS insensitive to the red and infrared regions of the solar spectrum, arising from the
* Corresponding author. 0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-0248(94)00199-5
76
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
fact that the energy gap of trigonal selenium, of 1.85 eV [3], is too large to convert these lower energy photons into photocurrent. Nevertheless, under room lighting, which is designed for the wavelength range of the human eye, the selenium cell is more efficient and this offers the opportunity for wider photometric use, as well as for exploitation as a power source for small consumer electronic products, such as pocket calculators, normally used indoors. In such applications, the use of selenium-based photovoltaic cells to replace the present semiconductor materials could well be cost-effective. Demonstration units using Se-CdO ceils have already been fabricated, including one in this laboratory [4] based on present-day technology. With the energy gap of 1.85 eV for selenium, it should be possible theoretically in an ideal pn junction to reach a short circuit current density Jsc of some 15 m A / c m 2, under a 100 m W / c m 2 of solar illumination, which is much higher than the 2 to 3 m A / c m 2 actually observed in commercial photometry selenium photovoltaic cells. While selenium cannot be made n-type to make a pn junction, it can be used in a heterojunction with an n-type window material of a different semiconductor, such as CdO [5], [6], [7], CdSe [8], [9], [10], indium tin oxide [11], [12] or TiO 2 [13]. The commercial Se-CdO photometry cell was developed principally to yield a linear easily-measurable output voltage for light level determinations rather than maximum output power and therefore series electrical resistance was not minimized; consequently Jsc values were low. It is thus evident that higher jsc values should be possible. It has therefore been the objective of a program in this laboratory to find ways of optimizing the fabrication processes of the conventional S e - C d O cell to improve its photovoltaic performance. The present paper is a report on progress towards that goal. The S e - C d O cell is fabricated basically in this laboratory as a substrate-metalS e - C d O - m e t a l sandwich structure, where the back contact metal, usually bismuth, is evaporated onto the substrate, the selenium is deposited by evaporation and the CdO is put down by rf sputtering; evaporation cannot be used for CdO, since it dissociates on heating under vacuum. The cell is thus of conventional form, rather the "inverted" structure investigated elsewhere [9-13]. All steps in the depositions need to be optimized and this is particularly the case for the reactive sputtering of the CdO, which involves the parameters: sputtering current, pressure, gas composition, gas flow rate and deposition time. In this paper, the results of optimizing these parameters are reported. It should be pointed out that optimization of the selenium layer deposition is not treated in this paper. However, one of the most important factors for this layer is to keep the substrate temperature sufficiently high during the selenium deposition, without excessive re-evaporation [14]. This was done in the fabrication of the research cells in the present work. 2. Cell fabrication
Details of the fabrication of the Se-CdO cells studied in this work are given in Ref. [15]. However, a brief description may be helpful. The research device (Fig. 1)
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
77
In or Au CdO
stud
Fig. 1. Schematicdiagram of the structure of the Se-CdO photovoltaiccell studied. consists of an aluminum stud, with one fiat end lapped and etched, on which a t h i n layer of bismuth, greater than about a micron in thickness, is deposited by t h e r m a l evaporation. Next a layer of selenium, typically 20 microns in thickness, is evaporated on top of the bismuth from a selenium source containing selenium d o p e d with nominally 104 ppm of C1. During the evaporation, the substrate is maintained at a temperature of 140°C to crystallize the selenium. Following this, a thin film o f CdO is deposited on the selenium in four circular mask-defined areas, using D C reactive sputtering from a pure cadmium target, with a gas of mixed argon and oxygen flowing through the deposition system. Two thin stripes of metal (indium o r gold) are then deposited on each circular CdO area and finally thin copper wires are soldered to these stripes with low melting point Wood's metal solder (50% bismuth, 25% lead, 12.5% tin and 12.5% cadmium). Electrical contacts are then! made between the aluminum base and terminals connected by the fine wires to the metal stripes. In a fabrication process used in earlier work [6,16], the DC sputtering was carried out in periodic bursts with a small amount of residual air intentionally left in the vacuum system to provide the oxygen needed to form the CdO. This method was taken over from a process employed some time ago by Preston [5]. While this residual air method is satisfactory, it is not as convenient as the mixed gas method [17] employed for the work in this paper, where usually the mixture consisted of 1% oxygen and 99% argon, with the substrate-target separation fixed at 8 cm.
3. M e a s u r e m e n t s m a d e on cells
The evaluation of the effect of changes in the fabrication processes of the cells was carried out by measuring current density-voltage characteristics in darkness and under strong white light of intensity in excess of 100 m W / c m 2 but kept constant throughout the tests. Some later tests were done at a lower intensity n e ~ 100 m W / c m z under actual sunlight and under room light. Thickness t of the CdO film on each cell was determined from its interference colour and order numbe~, while the dark resistance was obtained from the resistance Rst measured between
78
CH. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
the two metal stripes on each CdO area. The product Rst t c a n be taken as a measure of the dark electrical resistivity of the CdO film, if the shunting effect of the underlying, but higher resistance, selenium can be neglected. Other measurements carried out on some of the cells were: spectral response over the wavelength range 400 to 800 nm, junction capacitance and reverse photocurrent to determine minority diffusion lengths [18,19]. 4. Optimization of the CdO sputtering conditions
The parameters which were varied during the reactive sputtering of the CdO were: sputtering current, pressure, gas flow rate, deposition time and oxygen/argon ratio in the flowing gas. Since it was not possible to know the optimum values of the parameters in advance, certain practical initial values were chosen; these were: 20 mA, 75 millitorr, 24 m l / m i n , 30 min and 1% oxygen, 99% argon, respectively, unless otherwise stated. The optimization was done by keeping all but one of these parameters constant and varying the remaining one over a range of values to obtain the best photovoltaic performance.
4.1. Sputtering current Fig. 2 shows a plot of illuminated short circuit current density Jsc and open circuit voltage Voc as a function of sputtering current, with the other sputtering
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Fig. 2. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Vo~ against sputtering current used in the dc reactive sputtering of the CdO layer in the S e - C d O cells. The sputtering pressure, gas flow rate, deposition time and gas composition are respectively 75 millitorr, 24 m l / m i n , 30 min and 1% 0 2, 99% Ar.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 400.
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Sputtering Current (mA) Fig. 3. Plot of CdO film thickness t and Rst t product against sputtering current used in fabricating the S e - C d O cells. The other parameters in the dc reactive sputtering of the CdO are those indicated for Fig. 2.
parameters kept constant. It is noted that J~c shows a maximum near 27 mA, while Voc shows a broad maximum. It was observed that, at a current of 35 mA, a completely black and opaque cadmium film was deposited, corresponding to a zero value of J,c. Fig. 3 shows a plot of the CdO film thickness t, obtained fro m interference colour and order, against sputtering current, along with the product Rst t. It is seen that, with increasing sputtering current, the thickness increases, while Rstt , which is essentially proportional to the CdO resistivity, decreases.
4.2. Sputtering pressure A plot of illuminated Jsc and Voc against sputtering pressure is shown in Fig. 4, where it is noted that Jsc shows a maximum near 60 millitorr and Vo~ a broader maximum. The fast fall-off of Jsc with decreasing pressure below 60 millitorr is doe to increasing optical opacity of the CdO, eventually resulting in the onset of completely opaque cadmium below about 50 millitorr. Fig. 5 is a plot of CdO thickness t and Rstt against sputtering pressure, showing thickness to decrease and resistivity to increase with increase of gas pressure. From plots of dark curr~nt density against voltage (not shown), the current I d at a forward voltage of Vd =i 1, V was measured on each cell. The quantity Vd/(ldt), which can be taken as a measure of the resistivity of the material of the diode, is plotted against sputterihg pressure in Fig. 6, along with the product Rstt. It is noted that the two quantities
80
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 15
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vary in approximately the same way, on this semilogarithmic plot, confirming that they both represent a measure of the same parameter, electrical resistivity of the CdO, which increases with sputtering pressure.
4.3. Gas flow rate In the CdO sputtering, the flow rate of the mixed gas of 1% oxygen and 99% argon was varied from 10 to 102 m l / m i n , keeping the other sputtering parameters constant. Fig. 7 shows illuminated Jsc and Voc plotted against gas flow rate, where js~ exhibits a sharp peak near 10 m l / m i n , with a fast fall-off with decreasing flow rate on the low flow rate side of the maximum. For truly zero flow rate, J~c should be zero but the non-zero j~¢ value plotted at no gas flow is due some residual oxygen still within the sputtering system. By contrast to Js~, Voc shows no maximum but a slight decline with increasing gas flow rate. Thickness and R s t values, measured on the CdO film, were found to exhibit little change with gas flow rate (not shown). Fig. 8 shows the spectral response of the samples, indicating a systematic increase in photoresponse with decreasing gas flow rate. The maximum
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
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C.H. Charnpness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 0.6
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Fig. 7. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against flow rate of gas mixture of 1% 0 2 and 99% Ar used in the dc reactive sputtering of the CdO layer in the Se-CdO cells. The other parameters are 20 mA sputtering current, 75 millitorr pressure and 30 min deposition time.
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Fig. 8. Plot of photoresponsivity against wavelength for Se-CdO cells fabricated with different gas flow rates during the sputtering of the CdO layer. The other sputtering parameters are those indicated for Fig. 7.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 12
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Fig. 9. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against sputtering time of the CdO layer in the Se-CdO cells. An average curve for maximum power per unit area Pmaxis also shown. The other parameters are 20 mA sputtering current, 75 millitorr pressure, 32 ml/min gas flow rate and a gas composition of 1% 0 2 and 99% Ar. photoresponse is seen to lie approximately between wavelengths of 500 and 600 nm for all the gas flow rates. 4.4. @uttering time
The deposition time of CdO, in the continuous sputtering process, was varied from 10 to 240 minutes, with the other sputtering parameters fixed at the above-mentioned values, except for the gas flow rate, which was maintained at 32 m l / m i n . Fig. 9 shows a plot of Jsc and Vo~ against sputtering time, indicating a maximum for Jsc near 60 minutes and a broad maximum for Voc with only a modest decrease with increase of sputtering time. In the same figure is a curve for maximum power per unit area Pro=, obtained from the individual illuminated j - V plots (not shown), indicating a broad maximum. Since the CdO thickness was found to increase linearly with sputtering time (not shown), the plots of j~, Voc and Pmax against sputtering time can be regarded as representing the variations of these quantities with CdO thickness. Spectral response for the cells is shown in Fig. 10. At a wavelength of about 660 nm, the responsivity is seen to be fairly insensitive to sputtering time, except for the cells sputtered at the longer times 0f 180 and 240 minutes. However, for the shorter wavelengths, the response dependent on deposition time. This is particularly the case at 400 nm, where in
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C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 0.4
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Fig. 11, photoresponse is plotted against sputtering time, on semilogarithmic scales, indicating a monotonic decrease of response with deposition time and hence with CdO thickness.
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Fig. 11. Photoresponsivity at a wavelength of 400 nm plotted against deposition time of the CdO layer in Se-CdO cells on semilogarithmic scales. The other sputtering conditions are those indicated for Fig. 9.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 6
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0.2 0.4 Voltage, V (volt) Fig. 12. Illuminated current density - voltage characteristics of four Se-CdO cells fabricated with different concentrations of oxygen in the gas stream (balance argon) during the sputtering of the CdO layer. The other parameters are 20 mA sputtering current, 50 millitorr pressure, 24 ml/min gas flow rate and 30 rain. deposition time.
4.5. Oxygen / argon ratio To see the effect of changing the oxygen content in the sputtering gas, four cells were made with the gas compositions of 1, 5, 18 (air) and 50% oxygen, with the balance argon. The other sputtering conditions of the CdO depositions were kept constant at the previously mentioned values, except that the sputtering pressure was maintained at 50 millitorr. Illuminated j - V characteristics for these cells are shown in Fig. 12, indicating a decrease of photovoltaic output with increase Of oxygen in the gas stream. This is more clearly shown in Fig. 13, indicating a sharp decrease of j~ and a slower decrease of Vo~ with increased oxygen. Fig. 14 shows ia plot of CdO thickness t and Rstt against oxygen content, where it is clear that tl~e thickness decreases and the electrical resistivity increases with increase of oxygen.
4.6. Target conditions The condition of the metal cadmium sputtering target is difficult to quantify but the state of its surface was found to have an important effect on the deposited CdO. For uniformity, it was found necessary to have prior depositions to get the target surface to a " m a t u r e " condition before sputtering CdO on to the seleniu~ itself for a cell. In the case where the target was an evaporated film of pu~re . /
86
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 6
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Fig. 13. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against oxygen content in the gas stream during the sputtering of the CdO layer in the S e - C d O cells. The other parameters are those indicated for Fig. 12.
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C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 I0
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Fig. 15. Comparison of illuminated current density-voltage characteristics of two S e - C d O cells, one fabricated with a C d O layer sputtered from a smooth aluminum target surface and the other from a rough aluminum target surface.
cadmium on aluminum, this pre-sputtering consisted of deposition on glass substrates for four periods of 30 minutes. The importance of the smoothness of the target is brought out in the illuminated j - V plots in Fig. 15. The sputtering conditions for the two ceils here were the same as the above mentioned parameters values, except that the gas flow rate was 36 ml/min. However, in one case the evaporated target surface was rough and in the other case smooth. Fig. 15 shows that the latter yielded a cell with much greater photovoltaic output than the former, even though the CdO thicknesses of 0.24 ~m were found to be the same for both devices.
5.Sputterideposi ng tirate on It was found, with fixed sputtering current, pressure, gas flow rate and oxygen content, that the thickness of the CdO increases linearly with sputtering time. Hence, knowing the thickness and sputtering time, usually 30 minutes, the deposition rate can be determined when the other parameters are varied. Thus, Fig. 16 is
88
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
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Fig. 16. Resistance Rst, measured between the pair of metal stripes on each CdO area, plotted against C d O film deposition rate for all the sputtering p a r a m e t e r variations, except sputtering time. The
broken line represents the average trend of the points for sputtering current and pressure variations only.
a plot of Rst, the resistance between the two metal stripes on each CdO area, against deposition rate for all the parameters varied, except sputtering time. It is clear that, despite the scatter in the points, there is a definite trend, at least for the variation from sputtering current and pressure. This is that Rst decreases with increase of deposition rate. Such a dependence can be understood if there is a certain "residence time" needed for the oxygen and cadmium to recombine to form the CdO. If the deposition rate is high, at low sputtering pressure or high sputtering current, there is insufficient time to form stoichiometric CdO and hence the deposited material is cadmium-rich and therefore highly conducting - - that is of low Rst. In fact, for a high-enough deposition rate, only metallic cadmium is deposited. If, on the other hand, the deposition rate is low, because of high sputtering pressure or low sputtering current, there is now enough time for the oxygen and cadmium to react and the deposited material will be more stoichiometric and hence of high electrical resistivity - - that is of high Rst. For the variation with oxygen content in the gas stream, the points in Fig. 16 also appear to follow somewhat the trend just described but the oxygen concentration itself seems to be at work also, making the deposition rate dependence less important. However, in
C.H. Champness, C.H. Chan /Solar Energy Materials and Solar Cells 37 (1995) 75-92
89
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Irradianceestimate ~ ~ - - ~ ' ~ _ 110 mW/cm2 ~ ~ _ _ . ~
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Fig. 17. Plot of current density S e - C d O cell (CH108) and a commercial selenium photometry cell u n d e r full sunlight at noon in Montreal on March 18th 1994, when the global solar irradiance was estimated to be about 110 m W / c m 2.
respect of gas flow rate variation, the points plotted do not show any discernible correlation of Rst with deposition rate.
6. Optimized cell performance A cell was fabricated under essentially optimized CdO sputtering conditions of: 20 mA sputtering current, 75 millitorr pressure, 24 ml/min gas flow rate, 30 min deposition time, with a gas mixture of 1% oxygen and 99% argon. Furthermore, the base contact to the selenium consisted of a film of platinum sputtered on t o the evaporated bismuth layer, as indicated in the inset to Fig. 17. In this figure, t h e illuminated j - V characteristic of this cell is shown, along with that of a standard commercial selenium photometry cell, both measured under actual sunlight at noon in Montreal on March 18th, 1994, when the global solar irradiance was i estimated to be about 110 m W / c m 2. The maximum power points on the curvesl
90
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 40
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Voltage, V (volt) Fig. 18. Plot of current density - voltage variation for an optimized laboratory-fabricated S e - C d O cell (CH108) and a commercial selenium photometry cell under fluorescent room light of global irradiance estimated to be about 0.13 m W / c m 2,
yield conversion efficiencies of 2.5% and 0.3% for the laboratory-made S e - C d O and photometry cell respectively, clearly showing the superiority of the former device, at least under high illumination. Fig. 18 shows the characteristics of the same two cells under approximately 0.13 m W / c m 2 of fluorescent room light, yielding maximum efficiencies of 4.8% and 3.3% respectively. Thus, while under lower intensity room light, the improvement of the laboratory-made S e - C d O cell over the photometry cell is not as great as under the sun, both cells show higher performance under the narrower wavelength spectrum of artificial room light.
7. Discussion
It is clear that the illuminated short circuit current density in the S e - C d O cell is very sensitive to the reactive sputtering conditions of the CdO window layer. This layer must have an appropriate balance of electrical resistance and optical trans-
C.H. Champness, C.H. Chan /Solar Energy Materials and Solar Cells 37 (1995) 75-92
91
mittance, which are factors controlled by the various sputtering parameters. If t h e CdO film is too thin, its electrical resistance is high, not only due to its narrowness, but also to its being somewhat discontinuous across the rough selenium surface. The result is reduced Jsc in the cell. If it is too thick, less light reaches the active selenium layer and Jsc is again reduced. Dependence on oxide thickness is j particularly evident at short wavelengths from the photoresponse results in Fig. 10. Thus, the results in Fig. 11 represent the exponential decrease of short circuit current with increase in the thickness of the CdO at 400 nm. If the CdO deposition rate of 3.2 X 10 -3 microns/min, determined during these measurements, is applied to the results in Fig. 11, an optical absorption coefficient of 4 x 10 4 cm -~ is obtained at this wavelength. In respect of the CdO stoichiometry, this is controlled by deposition rate through sputtering current and pressure and by oxygen content in the gas but, in contrast, very little by gas flow rate. If the first three quantities are high, low and low respectively, the resulting CdO is cadmium-rich, yielding material which isi highly conducting but optically opaque. On the other hand if the three quantities are low, high and high respectively, the resulting CdO is near-stoichiometric,j yielding material which is highly resistive but optically transparent. Both extremesj lead to low Jsc, so that compromise values of the sputtering parameters are needed] to maximize cell performance. The action of gas flow change, however, is somewhat different. Measurements have shown that the main decrease of Jsc with increase of flow rate is apparently due to a reduction in diffusion length, and hence of lifetime, of the minority electrons in the selenium as a result of increased oxygen presence [19]. Thus, the action of oxygen is both good and bad; it is necessary to form the CdO from the reaction with the cadmium but its presence may be deleterious, if too much oxygen enters the selenium. The tests on the best optimized laboratory-prepared Se-CdO cell so far, gave a 100 m W / c m 2 maximum efficiency of about 2.5%, which is about an order of magnitude greater than that for the commercial selenium photometry cell. Under fluorescent room light of intensity 0.13 m W / c m 2 the respective efficiencies for the laboratory-prepared device and a commercial cell were 4.8% and 3%, which are higher, indicating the potential of this type of cell for indoor applications. Replaces ment of the CdO window layer by indium tin oxide [11], [12], CdSe [9], [10] or TiO~ [13] has been reported to give increases in conversion efficiency in inverted cellsi but, even with CdO in the conventional cell structure, higher efficiencies are still possible with further fabrication changes, such as lowering back contact resistance to the selenium and improving the integrity of the active junction of the celll. Efforts in this direction will therefore be made in future work.
Acknowledgements The authors wish to thank Nam-Hyong Kim for assistance in some of the measurements and to acknowledge partial financial support of this work by the Natural Science and Engineering Research Council of Canada.
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C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
References [1] G. Blet, Photopiles au S61~.nium (Dunod, Paris 1959). [2] E. Billig and K.W. Plessner Phil. Mag. 40 (1949) 518. [3] J. Stuke, in: R.A. Zingaro and W.C. Cooper (Eds.), Selenium (Van Nostrand, New York, 1974) pp. 174-297. [4] C.H. Champness, Z.A. Shukri and Y.F. Go, Proc. 4th Int. Photovoltaic Science and Engineering Conf., 1989, pp. 347-352.. [5] J.S. Preston, Proc. Roy. Soc. Lond. 202A (1950) 449. [6] C.H. Champness, S. Fukuda and S. Jatar, Sol. Energy Mater. 5 (1981) 391-401. [7] C.H. Champness, K. Ghoneim and J.K. Chen, Can. J. Phys. 63 (1985) 767. [8] R.F. Shaw and A.K. Ghosh, Solar Cells 1 (1979/1980) 431. [9] H. Ito, M. Oka, T. Ogino, A. Takeda and Y. Mizushima, Proc. 3rd Photovoltaic Science and Engineering Conf., Jpn. J. Appl. Phys. 21 Suppl. 21-22 (1982) 177. [10] H. Ito, M. Oka, T. Ogino, A. Takeda and Y. Mizushima, Jpn. J. Appl. Phys. 23 (1984) 719. [11] A. Kunioka and T. Nakada, Proc. 3rd Photovoltaic Science and Engineering Conf., Jpn. J. Appl. Phys. 21 Suppl. 21-22 (1982) 73. [12] T. Nakada and A. Kunioka, Jpn. J. Appl. Phys. 23 (1984) L587. [13] T. Nakada and A. Kunioka, Jpn. J. Appl. Phys. 24 (1985) L536. [14] C.H. Champness, Z.A. Shukri and C.H. Chan, Can. J. Phys. 69 (1991) 538-542. [15] C.H. Chan, M. Eng. Thesis, McGill University, 1993. [16] C.H. Champness and K. Ghoneim, Solar Cells 13 (1984) 121-131. [17] C.H. Champness, J.K. Chen and K. Ghoneim, Proc. 17th IEEE Photovoltaic Specialists Conf., 1984, pp. 933-938. [18] C.H. Champness and C.H. Chan, 6th Int. Photovoltaic Science and Engineering Conf., 1992, pp. 827-832. [19] C.H. Champness and C.H. Chan, Sol. Energy Mater. Sol. cells 30 (1993) 65-75.
Solar Energy Materials and Solar Cells 37 (1995) 75-92
Optimization of CdO layer in a Se-CdO photovoltaic cell C.H. Champness *, C.H. Chan Electrical Engineering Department, McGill University, Montreal, PQ, H3A 2,47, Canada
Received 13 July 1994; revised 25 October 1994
Abstract
An experimental study was made on a Se-CdO photovoltaic cell of conventionali structure to optimize the conditions during the dc reactive sputtering of the CdO layer. Thei parameters varied were: sputtering current, pressure, gas flow rate, deposition time andf ratio of oxygen to argon in the sputtering gas. These parameters were found to be ver~ critical in determining photovoltaic cell performance at higher light levels. The electrica! resistance and optical transparency in the CdO, are determined by its stoichiometry and this in turn was found to be controlled by the film deposition rate, through sputtering curren~ and pressure and by oxygen content in the sputtering gas. A laboratory-fabricated S e - C d O photovoltaic cell with an optimized CdO window layer (but without an optimized collecting grid or antireflecting coating) was found to yield a conversion efficiency of about 2.5% under 100 m W / c m 2 of solar irradiance and about 5% under fluorescent room light o~ irradiance 0.13 m W / c m 2. For a standard commercial selenium photometry cell the corre, sponding efficiency values were 0.3% and 3%, respectively.
I. Introduction
A photovoltaic cell based on a S e - C d O structure, involving crystallized trigonal selenium, has b e e n in use for about half a century for p h o t o m e t r i c applications [1]i This is due to the fact that the device can be m a d e to have a spectral response close to the wavelength sensitivity of the h u m a n eye and because the cell is relatively simple to make, involving only low temperature, low cost fabrication processes. However, it is unsuitable as a solar cell, since it yields a conversion efficiency well below 1% [2] u n d e r solar illumination. This is because it iS insensitive to the red and infrared regions of the solar spectrum, arising from the
* Corresponding author. 0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927-0248(94)00199-5
76
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
fact that the energy gap of trigonal selenium, of 1.85 eV [3], is too large to convert these lower energy photons into photocurrent. Nevertheless, under room lighting, which is designed for the wavelength range of the human eye, the selenium cell is more efficient and this offers the opportunity for wider photometric use, as well as for exploitation as a power source for small consumer electronic products, such as pocket calculators, normally used indoors. In such applications, the use of selenium-based photovoltaic cells to replace the present semiconductor materials could well be cost-effective. Demonstration units using Se-CdO ceils have already been fabricated, including one in this laboratory [4] based on present-day technology. With the energy gap of 1.85 eV for selenium, it should be possible theoretically in an ideal pn junction to reach a short circuit current density Jsc of some 15 m A / c m 2, under a 100 m W / c m 2 of solar illumination, which is much higher than the 2 to 3 m A / c m 2 actually observed in commercial photometry selenium photovoltaic cells. While selenium cannot be made n-type to make a pn junction, it can be used in a heterojunction with an n-type window material of a different semiconductor, such as CdO [5], [6], [7], CdSe [8], [9], [10], indium tin oxide [11], [12] or TiO 2 [13]. The commercial Se-CdO photometry cell was developed principally to yield a linear easily-measurable output voltage for light level determinations rather than maximum output power and therefore series electrical resistance was not minimized; consequently Jsc values were low. It is thus evident that higher jsc values should be possible. It has therefore been the objective of a program in this laboratory to find ways of optimizing the fabrication processes of the conventional S e - C d O cell to improve its photovoltaic performance. The present paper is a report on progress towards that goal. The S e - C d O cell is fabricated basically in this laboratory as a substrate-metalS e - C d O - m e t a l sandwich structure, where the back contact metal, usually bismuth, is evaporated onto the substrate, the selenium is deposited by evaporation and the CdO is put down by rf sputtering; evaporation cannot be used for CdO, since it dissociates on heating under vacuum. The cell is thus of conventional form, rather the "inverted" structure investigated elsewhere [9-13]. All steps in the depositions need to be optimized and this is particularly the case for the reactive sputtering of the CdO, which involves the parameters: sputtering current, pressure, gas composition, gas flow rate and deposition time. In this paper, the results of optimizing these parameters are reported. It should be pointed out that optimization of the selenium layer deposition is not treated in this paper. However, one of the most important factors for this layer is to keep the substrate temperature sufficiently high during the selenium deposition, without excessive re-evaporation [14]. This was done in the fabrication of the research cells in the present work. 2. Cell fabrication
Details of the fabrication of the Se-CdO cells studied in this work are given in Ref. [15]. However, a brief description may be helpful. The research device (Fig. 1)
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
77
In or Au CdO
stud
Fig. 1. Schematicdiagram of the structure of the Se-CdO photovoltaiccell studied. consists of an aluminum stud, with one fiat end lapped and etched, on which a t h i n layer of bismuth, greater than about a micron in thickness, is deposited by t h e r m a l evaporation. Next a layer of selenium, typically 20 microns in thickness, is evaporated on top of the bismuth from a selenium source containing selenium d o p e d with nominally 104 ppm of C1. During the evaporation, the substrate is maintained at a temperature of 140°C to crystallize the selenium. Following this, a thin film o f CdO is deposited on the selenium in four circular mask-defined areas, using D C reactive sputtering from a pure cadmium target, with a gas of mixed argon and oxygen flowing through the deposition system. Two thin stripes of metal (indium o r gold) are then deposited on each circular CdO area and finally thin copper wires are soldered to these stripes with low melting point Wood's metal solder (50% bismuth, 25% lead, 12.5% tin and 12.5% cadmium). Electrical contacts are then! made between the aluminum base and terminals connected by the fine wires to the metal stripes. In a fabrication process used in earlier work [6,16], the DC sputtering was carried out in periodic bursts with a small amount of residual air intentionally left in the vacuum system to provide the oxygen needed to form the CdO. This method was taken over from a process employed some time ago by Preston [5]. While this residual air method is satisfactory, it is not as convenient as the mixed gas method [17] employed for the work in this paper, where usually the mixture consisted of 1% oxygen and 99% argon, with the substrate-target separation fixed at 8 cm.
3. M e a s u r e m e n t s m a d e on cells
The evaluation of the effect of changes in the fabrication processes of the cells was carried out by measuring current density-voltage characteristics in darkness and under strong white light of intensity in excess of 100 m W / c m 2 but kept constant throughout the tests. Some later tests were done at a lower intensity n e ~ 100 m W / c m z under actual sunlight and under room light. Thickness t of the CdO film on each cell was determined from its interference colour and order numbe~, while the dark resistance was obtained from the resistance Rst measured between
78
CH. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
the two metal stripes on each CdO area. The product Rst t c a n be taken as a measure of the dark electrical resistivity of the CdO film, if the shunting effect of the underlying, but higher resistance, selenium can be neglected. Other measurements carried out on some of the cells were: spectral response over the wavelength range 400 to 800 nm, junction capacitance and reverse photocurrent to determine minority diffusion lengths [18,19]. 4. Optimization of the CdO sputtering conditions
The parameters which were varied during the reactive sputtering of the CdO were: sputtering current, pressure, gas flow rate, deposition time and oxygen/argon ratio in the flowing gas. Since it was not possible to know the optimum values of the parameters in advance, certain practical initial values were chosen; these were: 20 mA, 75 millitorr, 24 m l / m i n , 30 min and 1% oxygen, 99% argon, respectively, unless otherwise stated. The optimization was done by keeping all but one of these parameters constant and varying the remaining one over a range of values to obtain the best photovoltaic performance.
4.1. Sputtering current Fig. 2 shows a plot of illuminated short circuit current density Jsc and open circuit voltage Voc as a function of sputtering current, with the other sputtering
eq
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Fig. 2. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Vo~ against sputtering current used in the dc reactive sputtering of the CdO layer in the S e - C d O cells. The sputtering pressure, gas flow rate, deposition time and gas composition are respectively 75 millitorr, 24 m l / m i n , 30 min and 1% 0 2, 99% Ar.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 400.
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parameters kept constant. It is noted that J~c shows a maximum near 27 mA, while Voc shows a broad maximum. It was observed that, at a current of 35 mA, a completely black and opaque cadmium film was deposited, corresponding to a zero value of J,c. Fig. 3 shows a plot of the CdO film thickness t, obtained fro m interference colour and order, against sputtering current, along with the product Rst t. It is seen that, with increasing sputtering current, the thickness increases, while Rstt , which is essentially proportional to the CdO resistivity, decreases.
4.2. Sputtering pressure A plot of illuminated Jsc and Voc against sputtering pressure is shown in Fig. 4, where it is noted that Jsc shows a maximum near 60 millitorr and Vo~ a broader maximum. The fast fall-off of Jsc with decreasing pressure below 60 millitorr is doe to increasing optical opacity of the CdO, eventually resulting in the onset of completely opaque cadmium below about 50 millitorr. Fig. 5 is a plot of CdO thickness t and Rstt against sputtering pressure, showing thickness to decrease and resistivity to increase with increase of gas pressure. From plots of dark curr~nt density against voltage (not shown), the current I d at a forward voltage of Vd =i 1, V was measured on each cell. The quantity Vd/(ldt), which can be taken as a measure of the resistivity of the material of the diode, is plotted against sputterihg pressure in Fig. 6, along with the product Rstt. It is noted that the two quantities
80
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 15
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vary in approximately the same way, on this semilogarithmic plot, confirming that they both represent a measure of the same parameter, electrical resistivity of the CdO, which increases with sputtering pressure.
4.3. Gas flow rate In the CdO sputtering, the flow rate of the mixed gas of 1% oxygen and 99% argon was varied from 10 to 102 m l / m i n , keeping the other sputtering parameters constant. Fig. 7 shows illuminated Jsc and Voc plotted against gas flow rate, where js~ exhibits a sharp peak near 10 m l / m i n , with a fast fall-off with decreasing flow rate on the low flow rate side of the maximum. For truly zero flow rate, J~c should be zero but the non-zero j~¢ value plotted at no gas flow is due some residual oxygen still within the sputtering system. By contrast to Js~, Voc shows no maximum but a slight decline with increasing gas flow rate. Thickness and R s t values, measured on the CdO film, were found to exhibit little change with gas flow rate (not shown). Fig. 8 shows the spectral response of the samples, indicating a systematic increase in photoresponse with decreasing gas flow rate. The maximum
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
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82
C.H. Charnpness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 0.6
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Fig. 7. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against flow rate of gas mixture of 1% 0 2 and 99% Ar used in the dc reactive sputtering of the CdO layer in the Se-CdO cells. The other parameters are 20 mA sputtering current, 75 millitorr pressure and 30 min deposition time.
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700
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Fig. 8. Plot of photoresponsivity against wavelength for Se-CdO cells fabricated with different gas flow rates during the sputtering of the CdO layer. The other sputtering parameters are those indicated for Fig. 7.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 12
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Fig. 9. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against sputtering time of the CdO layer in the Se-CdO cells. An average curve for maximum power per unit area Pmaxis also shown. The other parameters are 20 mA sputtering current, 75 millitorr pressure, 32 ml/min gas flow rate and a gas composition of 1% 0 2 and 99% Ar. photoresponse is seen to lie approximately between wavelengths of 500 and 600 nm for all the gas flow rates. 4.4. @uttering time
The deposition time of CdO, in the continuous sputtering process, was varied from 10 to 240 minutes, with the other sputtering parameters fixed at the above-mentioned values, except for the gas flow rate, which was maintained at 32 m l / m i n . Fig. 9 shows a plot of Jsc and Vo~ against sputtering time, indicating a maximum for Jsc near 60 minutes and a broad maximum for Voc with only a modest decrease with increase of sputtering time. In the same figure is a curve for maximum power per unit area Pro=, obtained from the individual illuminated j - V plots (not shown), indicating a broad maximum. Since the CdO thickness was found to increase linearly with sputtering time (not shown), the plots of j~, Voc and Pmax against sputtering time can be regarded as representing the variations of these quantities with CdO thickness. Spectral response for the cells is shown in Fig. 10. At a wavelength of about 660 nm, the responsivity is seen to be fairly insensitive to sputtering time, except for the cells sputtered at the longer times 0f 180 and 240 minutes. However, for the shorter wavelengths, the response dependent on deposition time. This is particularly the case at 400 nm, where in
,s
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C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 0.4
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Fig. 11, photoresponse is plotted against sputtering time, on semilogarithmic scales, indicating a monotonic decrease of response with deposition time and hence with CdO thickness.
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Fig. 11. Photoresponsivity at a wavelength of 400 nm plotted against deposition time of the CdO layer in Se-CdO cells on semilogarithmic scales. The other sputtering conditions are those indicated for Fig. 9.
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 6
i
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Percent of oxygen (%) 5
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~3 ~
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0.2 0.4 Voltage, V (volt) Fig. 12. Illuminated current density - voltage characteristics of four Se-CdO cells fabricated with different concentrations of oxygen in the gas stream (balance argon) during the sputtering of the CdO layer. The other parameters are 20 mA sputtering current, 50 millitorr pressure, 24 ml/min gas flow rate and 30 rain. deposition time.
4.5. Oxygen / argon ratio To see the effect of changing the oxygen content in the sputtering gas, four cells were made with the gas compositions of 1, 5, 18 (air) and 50% oxygen, with the balance argon. The other sputtering conditions of the CdO depositions were kept constant at the previously mentioned values, except that the sputtering pressure was maintained at 50 millitorr. Illuminated j - V characteristics for these cells are shown in Fig. 12, indicating a decrease of photovoltaic output with increase Of oxygen in the gas stream. This is more clearly shown in Fig. 13, indicating a sharp decrease of j~ and a slower decrease of Vo~ with increased oxygen. Fig. 14 shows ia plot of CdO thickness t and Rstt against oxygen content, where it is clear that tl~e thickness decreases and the electrical resistivity increases with increase of oxygen.
4.6. Target conditions The condition of the metal cadmium sputtering target is difficult to quantify but the state of its surface was found to have an important effect on the deposited CdO. For uniformity, it was found necessary to have prior depositions to get the target surface to a " m a t u r e " condition before sputtering CdO on to the seleniu~ itself for a cell. In the case where the target was an evaporated film of pu~re . /
86
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 6
,
,
,
0.6
0.5
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Fig. 13. Plot of illuminated short-circuit current density Jsc and open-circuit voltage Voc against oxygen content in the gas stream during the sputtering of the CdO layer in the S e - C d O cells. The other parameters are those indicated for Fig. 12.
1000
,
,
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C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 I0
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Smooth target s u r f a c e Rough target surface - - - - -
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Fig. 15. Comparison of illuminated current density-voltage characteristics of two S e - C d O cells, one fabricated with a C d O layer sputtered from a smooth aluminum target surface and the other from a rough aluminum target surface.
cadmium on aluminum, this pre-sputtering consisted of deposition on glass substrates for four periods of 30 minutes. The importance of the smoothness of the target is brought out in the illuminated j - V plots in Fig. 15. The sputtering conditions for the two ceils here were the same as the above mentioned parameters values, except that the gas flow rate was 36 ml/min. However, in one case the evaporated target surface was rough and in the other case smooth. Fig. 15 shows that the latter yielded a cell with much greater photovoltaic output than the former, even though the CdO thicknesses of 0.24 ~m were found to be the same for both devices.
5.Sputterideposi ng tirate on It was found, with fixed sputtering current, pressure, gas flow rate and oxygen content, that the thickness of the CdO increases linearly with sputtering time. Hence, knowing the thickness and sputtering time, usually 30 minutes, the deposition rate can be determined when the other parameters are varied. Thus, Fig. 16 is
88
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92
10,000 \
[] gas flow rate
\
5,000
Ot
2,000
•
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\ \
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Fig. 16. Resistance Rst, measured between the pair of metal stripes on each CdO area, plotted against C d O film deposition rate for all the sputtering p a r a m e t e r variations, except sputtering time. The
broken line represents the average trend of the points for sputtering current and pressure variations only.
a plot of Rst, the resistance between the two metal stripes on each CdO area, against deposition rate for all the parameters varied, except sputtering time. It is clear that, despite the scatter in the points, there is a definite trend, at least for the variation from sputtering current and pressure. This is that Rst decreases with increase of deposition rate. Such a dependence can be understood if there is a certain "residence time" needed for the oxygen and cadmium to recombine to form the CdO. If the deposition rate is high, at low sputtering pressure or high sputtering current, there is insufficient time to form stoichiometric CdO and hence the deposited material is cadmium-rich and therefore highly conducting - - that is of low Rst. In fact, for a high-enough deposition rate, only metallic cadmium is deposited. If, on the other hand, the deposition rate is low, because of high sputtering pressure or low sputtering current, there is now enough time for the oxygen and cadmium to react and the deposited material will be more stoichiometric and hence of high electrical resistivity - - that is of high Rst. For the variation with oxygen content in the gas stream, the points in Fig. 16 also appear to follow somewhat the trend just described but the oxygen concentration itself seems to be at work also, making the deposition rate dependence less important. However, in
C.H. Champness, C.H. Chan /Solar Energy Materials and Solar Cells 37 (1995) 75-92
89
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Fig. 17. Plot of current density S e - C d O cell (CH108) and a commercial selenium photometry cell u n d e r full sunlight at noon in Montreal on March 18th 1994, when the global solar irradiance was estimated to be about 110 m W / c m 2.
respect of gas flow rate variation, the points plotted do not show any discernible correlation of Rst with deposition rate.
6. Optimized cell performance A cell was fabricated under essentially optimized CdO sputtering conditions of: 20 mA sputtering current, 75 millitorr pressure, 24 ml/min gas flow rate, 30 min deposition time, with a gas mixture of 1% oxygen and 99% argon. Furthermore, the base contact to the selenium consisted of a film of platinum sputtered on t o the evaporated bismuth layer, as indicated in the inset to Fig. 17. In this figure, t h e illuminated j - V characteristic of this cell is shown, along with that of a standard commercial selenium photometry cell, both measured under actual sunlight at noon in Montreal on March 18th, 1994, when the global solar irradiance was i estimated to be about 110 m W / c m 2. The maximum power points on the curvesl
90
C.H. Champness, C.H. Chan / Solar Energy Materials and Solar Cells 37 (1995) 75-92 40
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Voltage, V (volt) Fig. 18. Plot of current density - voltage variation for an optimized laboratory-fabricated S e - C d O cell (CH108) and a commercial selenium photometry cell under fluorescent room light of global irradiance estimated to be about 0.13 m W / c m 2,
yield conversion efficiencies of 2.5% and 0.3% for the laboratory-made S e - C d O and photometry cell respectively, clearly showing the superiority of the former device, at least under high illumination. Fig. 18 shows the characteristics of the same two cells under approximately 0.13 m W / c m 2 of fluorescent room light, yielding maximum efficiencies of 4.8% and 3.3% respectively. Thus, while under lower intensity room light, the improvement of the laboratory-made S e - C d O cell over the photometry cell is not as great as under the sun, both cells show higher performance under the narrower wavelength spectrum of artificial room light.
7. Discussion
It is clear that the illuminated short circuit current density in the S e - C d O cell is very sensitive to the reactive sputtering conditions of the CdO window layer. This layer must have an appropriate balance of electrical resistance and optical trans-
C.H. Champness, C.H. Chan /Solar Energy Materials and Solar Cells 37 (1995) 75-92
91
mittance, which are factors controlled by the various sputtering parameters. If t h e CdO film is too thin, its electrical resistance is high, not only due to its narrowness, but also to its being somewhat discontinuous across the rough selenium surface. The result is reduced Jsc in the cell. If it is too thick, less light reaches the active selenium layer and Jsc is again reduced. Dependence on oxide thickness is j particularly evident at short wavelengths from the photoresponse results in Fig. 10. Thus, the results in Fig. 11 represent the exponential decrease of short circuit current with increase in the thickness of the CdO at 400 nm. If the CdO deposition rate of 3.2 X 10 -3 microns/min, determined during these measurements, is applied to the results in Fig. 11, an optical absorption coefficient of 4 x 10 4 cm -~ is obtained at this wavelength. In respect of the CdO stoichiometry, this is controlled by deposition rate through sputtering current and pressure and by oxygen content in the gas but, in contrast, very little by gas flow rate. If the first three quantities are high, low and low respectively, the resulting CdO is cadmium-rich, yielding material which isi highly conducting but optically opaque. On the other hand if the three quantities are low, high and high respectively, the resulting CdO is near-stoichiometric,j yielding material which is highly resistive but optically transparent. Both extremesj lead to low Jsc, so that compromise values of the sputtering parameters are needed] to maximize cell performance. The action of gas flow change, however, is somewhat different. Measurements have shown that the main decrease of Jsc with increase of flow rate is apparently due to a reduction in diffusion length, and hence of lifetime, of the minority electrons in the selenium as a result of increased oxygen presence [19]. Thus, the action of oxygen is both good and bad; it is necessary to form the CdO from the reaction with the cadmium but its presence may be deleterious, if too much oxygen enters the selenium. The tests on the best optimized laboratory-prepared Se-CdO cell so far, gave a 100 m W / c m 2 maximum efficiency of about 2.5%, which is about an order of magnitude greater than that for the commercial selenium photometry cell. Under fluorescent room light of intensity 0.13 m W / c m 2 the respective efficiencies for the laboratory-prepared device and a commercial cell were 4.8% and 3%, which are higher, indicating the potential of this type of cell for indoor applications. Replaces ment of the CdO window layer by indium tin oxide [11], [12], CdSe [9], [10] or TiO~ [13] has been reported to give increases in conversion efficiency in inverted cellsi but, even with CdO in the conventional cell structure, higher efficiencies are still possible with further fabrication changes, such as lowering back contact resistance to the selenium and improving the integrity of the active junction of the celll. Efforts in this direction will therefore be made in future work.
Acknowledgements The authors wish to thank Nam-Hyong Kim for assistance in some of the measurements and to acknowledge partial financial support of this work by the Natural Science and Engineering Research Council of Canada.
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References [1] G. Blet, Photopiles au S61~.nium (Dunod, Paris 1959). [2] E. Billig and K.W. Plessner Phil. Mag. 40 (1949) 518. [3] J. Stuke, in: R.A. Zingaro and W.C. Cooper (Eds.), Selenium (Van Nostrand, New York, 1974) pp. 174-297. [4] C.H. Champness, Z.A. Shukri and Y.F. Go, Proc. 4th Int. Photovoltaic Science and Engineering Conf., 1989, pp. 347-352.. [5] J.S. Preston, Proc. Roy. Soc. Lond. 202A (1950) 449. [6] C.H. Champness, S. Fukuda and S. Jatar, Sol. Energy Mater. 5 (1981) 391-401. [7] C.H. Champness, K. Ghoneim and J.K. Chen, Can. J. Phys. 63 (1985) 767. [8] R.F. Shaw and A.K. Ghosh, Solar Cells 1 (1979/1980) 431. [9] H. Ito, M. Oka, T. Ogino, A. Takeda and Y. Mizushima, Proc. 3rd Photovoltaic Science and Engineering Conf., Jpn. J. Appl. Phys. 21 Suppl. 21-22 (1982) 177. [10] H. Ito, M. Oka, T. Ogino, A. Takeda and Y. Mizushima, Jpn. J. Appl. Phys. 23 (1984) 719. [11] A. Kunioka and T. Nakada, Proc. 3rd Photovoltaic Science and Engineering Conf., Jpn. J. Appl. Phys. 21 Suppl. 21-22 (1982) 73. [12] T. Nakada and A. Kunioka, Jpn. J. Appl. Phys. 23 (1984) L587. [13] T. Nakada and A. Kunioka, Jpn. J. Appl. Phys. 24 (1985) L536. [14] C.H. Champness, Z.A. Shukri and C.H. Chan, Can. J. Phys. 69 (1991) 538-542. [15] C.H. Chan, M. Eng. Thesis, McGill University, 1993. [16] C.H. Champness and K. Ghoneim, Solar Cells 13 (1984) 121-131. [17] C.H. Champness, J.K. Chen and K. Ghoneim, Proc. 17th IEEE Photovoltaic Specialists Conf., 1984, pp. 933-938. [18] C.H. Champness and C.H. Chan, 6th Int. Photovoltaic Science and Engineering Conf., 1992, pp. 827-832. [19] C.H. Champness and C.H. Chan, Sol. Energy Mater. Sol. cells 30 (1993) 65-75.