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Investigations on a SeCdO photovoltaic cell
Solar Energy Materials 5 (1981) 391-401 North-Holland Publishing Company
391
I N V E S T I G A T I O N S ON A S e - C d O P H O T O V O L T A I C C E L L C. H. C H A M P N E S S , S. F U K U D A * and S. JATAR Electrical Enoineeriny Department, McGill University, 3480 University Street, Montreal, P.Q., Canada H3A 2A7
Received 17 June 1981 A photovoltaic Se-CdO thin film cell has been fabricated by reactive sputtering of CdO on a crystallized selenium film using a cadmium target in the presence of argon plus residual air. Critical control of pressure is needed to obtain the appropriate excess of cadmium in the CdO to realize low enough resistivity coupled with sufficient transparency. The spectral response lies between 350 and 700 nm and is controlled on the long wavelength side by the selenium and on the short wavelength side by the CdO. The CdO dominates the series resistance, so that the resistivity of the selenium is unimportant. A recent improvementin the fabrication processhas resulted in an increase of cell conversion efficiency to about 1.7°~,, which is well above values observed on commercial selenium photovoltaic cells. Some preliminary experiments have been made to sputter CdO reactively on glass from gas pre-mixtures of oxygen and argon to obtain easier pressure control.
I. Introduction At the present time silicon and gallium arsenide appear to be the most promising materials for photovoltaic solar cells. However, there are many other possible materials that have not been as extensively studied for use in the more distant future. Many of these other candidate materials could be used in thin film form leading to reduction in material costs. One such material is selenium, which has been used for many years in photovoltaic cells employed principally in photometers [1]. This application arises not only from the low cost of making the devices but also because the photoresponse is quite close to the spectral sensitivity of the human eye. The most detailed description of the fabrication of a selenium photovoltaic cell was given by Preston [2] some three decades ago. He first crystallized a layer o f selenium on a roughened steel plate and then deposited a transparent C d O layer on top of this by reactive dc sputtering from a metallic cadmium target. He obtained the best results by utilizing residual air left in the system after only moderate pumping. This method was later used by Altmejd and Champness [3] to make cell structures where a monocrystalline selenium film was employed for the first time by epitaxial deposition on a single crystal tellurium substrate. The benefits of the orientation of the film and its small thickness were not realized however, because of the high series resistance of the C d O counter electrode. Very recently Shaw and Ghosh [-4] described work on a selenium cell where CdSe and C d O layers were deposited on selen*Now at Department of Electronics, Kyoto University. Japan. 0165-1633/81/0000~000/$02.75 t~) 1981 North-Holland
392
(" f t ('hampne~s et al.
4 Se ('dO photovolt.ic ce/I
ium by rfsputtering in air at 10- 2 Torr from a cadmium target. Using a gold currentcollecting grid they reported efficiencies about Y',,, which are much higher than previously measured values for commercial selenium cells. These workers estimated that an efficiency of over 10,,;, (AM1) should eventually be possible with this type of cell. The investigation on Se CdO cells reported in this paper arises out of the initial earlier studies of Altmejd and Champness [3] and consists of an investigation aimed specifically at determining the counter electrode deposition conditions. In this stage of the work, monocrystalline selenium films were not used in the structures to shorten the time of the fabrication processes. The first part of the paper describes the making of the cell structures essentially by the Preston method using residual air. This is followed by results obtained with changes in the fabrication processes. The second part describes some preliminary work using premixed argon-oxygen gases to sputter reactively conducting CdO on glass in experiments to get better pressure control.
2. Cells
2.1. Cell Jabrication procedure The Se CdO photovoltaic cell structures studied in this investigation were fabricated basically in the following way (see table 1). One of the flat ends of a machined cylindrical aluminum stud, 2 cm in diameter, was roughened slightly by rubbing on fine emery paper. This surface was then coated with a thin film of bismuth using vacuum deposition. Next a film of selenium was evaporated on top of the bismuth, with the substrate maintained at a temperature of 130° C to convert the selenium from the amorphous to the polycrystalline form. After allowing the temperature to decrease slowly to room temperature to minimize peeling off of the film, the structure was remounted in another vacuum system so that the selenium surface was about 1.2 cm above a flat target of metallic cadmium (dia. 5 cm). A film of CdO was then reactively dc-sputtered on the selenium through a metal mask containing four quadrant-shaped openings. The first discharge was carried out for 3 rain at a pressure near 100 m T o r r with argon flowing into the bell-jar via a needle valve. At the end of the three minutes. air was allowed into the chamber to bring the pressure back to atmospheric level for a period of three to five minutes. Then the pressure was again reduced to about 100 r e t o r t and a second 3 min sputtering discharge made. This procedure of letting in air and sputtering under lower pressure was repeated usually 4 to 7 times, as indicated in column 7 of table 1. The resulting CdO film was thus formed by reaction of the cadmium atoms with the residual low pressure oxygen left in the vacuum chamber. With this done, two parallel gold stripes were sputtered on to each quadrant using a gold target and a suitable mask. For this, three sputtering bursts of 2 rain with 2 rain intervals were used. Finally with the stud mounted in a plastic holder, fine wires were soldered between the gold stripes and terminal posts using Wood's metal alloy. The thickness of the CdO layer was estimated from the interference colours which are listed in column 9 of table 1. Thus, for instance, the blue magenta colour for unit
60
60
60
60
60
60
60
0
60
K25
K26
K27
K28
K29
K30
K31
K34
S1
10
10
10
10
10
7 8
10
7
7
depos, time (min)
8 10
7
10
10
10
7 10
7 10
7-10
1
1
initial pressure (mTorr)
100
100
110
110
110
105 110
105 110
105-110
100
100
spurt, press, (mTorr)
18 20
18
12-15
12 15
12 15
12 15
12 15
12-15
25
25
needle valve setting (arb)
23
21
18
17
15
19.5
19.5
19.5
14
14
total time (min)
7 x 3 min lx2
7 × 3 min
5×3min 1×2 6 × 3 min
5x3min
ditto
6 x 3 min 1 ×1.5 ditto
4 × 3 min 1×2 ditto
no. of bursts
C d O sputtering ~
yellow red dark b.m. green
b.m. and green dark b.m. blueand yellow b.m.
blue magenta blue magenta green
colour of film
Au
Au
Cd
Au
Au
Au
Au
Au
Au
Au
Sputt. metal contact
Se film left overnight in air at room temperature
Se film heated 150°C in air for 24 h Se film heated 150°C in pure 0 2 for 24 h Se film heated 150°C in pure argon for 24 h Cd contacts used instead of Au u n d o p e d Se used
Remarks
"AI subst, evap. with Bi layer prior to Se evap.: subst, temp. during Se evap. 130°C. ~CdO sputtering: 20 mA dc; voltage 750 to 900 V; s u b s t r a t e - t a r g e t dist. 1.2-1.3 cm" Ar gas flow with line pressure of 5 psi (34 x 103 Pa).
60
K24
Device no.
CI dopant (ppm)
Se evaporation"
Table 1 Fabrication of Se C d O cells
~a
,2
e5
e~
e~
394
C. H. Champness et al.
4 Se CdO photoroltai( cc/l
K24 arises from second order interference and corresponds to a thickness of about 2100
2.2. Effect of fabrication changes on cell output In making the cells it was evident that the deposition of the counter electrode is the most critical process affecting photovoltaic performance. In the reactive sputtering, the resistance of the CdO film was greatly affected by the initial pressure after pumpdown (column 4 of table l) and the pressure during discharge (column 5). The pressure was controlled by the extent of opening the high vacuum valve to the diffusion pump: too low a pressure during sputtering (i.e. below 100 mTorr) resulted in deposition of metallic cadmium; too high a pressure yielded high resistivity CdO. To obtain sufficiently low resistivity and high enough transparency required careful control of this pressure. Frequently, nonreproducible results would occur from one run to another, possibly due to superficial oxidation of the cadmium target. As mentioned above, gold stripes were used as the electrodes on the CdO. Resistances measured between the stripes are listed in column 3 of table 2, which gives also the results of other measurements on the cells. The short-circuit current density (j~c) and open-circuit voltage (Vc~)figures in columns 4, 5 and 6 were taken under a 150 W reflector spot lamp positioned to give approximately AM1 illumination as detected by a silicon Solarex reference cell. While such lamp illumination has insufficient energy at the blue end of the spectrum (where Se is relatively more sensitive than Si), it was adequate for the relative comparison of cells. Thus comparing cell number K31 with K24 for example, it is evident from thejsc values in table 2 that the use of cadmium instead of gold for the metal contacts did not lead to any significant change in performance. Comparing cell K34, where undoped selenium was used, with the other cells in table 2, it is clear that in these devices the chlorine dopant in'the selenium plays no role in determining the photovoltaic performance. This is quite different from selenium rectifiers, where the series resistance is greatly affected by the presence of halogens in the selenium [5]. It clearly shows that the series resistance in the present devices arises from the CdO layer and not the selenium. The most interesting new feature in the present work is shown in cells K28, K29 and K30. In the first of these, the selenium film (together with its supporting aluminum mount) was maintained, at 150°C in air for 24 h prior to CdO deposition. As a result, the open circuit voltage was found to be 0.68 V, which is some 50% higher than the average value of 0.45 V obtained previously. This same value was again found in cell K29, where the selenium film was maintained in pure oxygen at 150°C for 24 h. However, in cell K30, where a 24 h 150°C treatment in pure argon was employed, the Vo~was the normal value of 0.45 V. It is thus evident that the presence of oxygen is necessary to obtain the increase of Vow. Among the 10 cells reported in table 2, the jsc values (2 stripes) on the better cells were about 2.5 mA/cm 2 under lamp illumination, with the highest value of 2.9 mA/cm 2 observed on device S1, which was the best cell fabricated. Under solar illumination of 90 mW/cm 2, this cell gave a jsc of 5.5 mA/cm 2 and a Vo~ of 0.65 V.
SI
Se left overnight in air at room temperature
Cd contacts used instead of Au undoped Se used
Se film heated 150° C in air 24 h Se film heated 150°C in pure Oz 24 h Se film heated 150~C in pure argon 24 h
Special fabrication features
1.0
2.8
2.5
2.6
5.2
1.0 7.8 1.9
3.3
Dark resist. between stripes (k~)
2.9
2.2 12.1) 2.0
2.2 0.4 2.2 1.2 2.2 (1.5) ~ 2.2 (1.3) 2.0
l stripe
2.9
2.5 (3.0) 2.2
(2.1)
(2.0)
2.4
2.6
2 stripes
j~¢(mA/cm 2)
0.6
0.59 (0.52) 0.4
0.4 0.4 0.44 0.56 0.68 (0.46) 0.68 (0.62) 0.45
Vo~ (V)
Measurements under lamp ~
5.5 (2 stripes)
0.65
0.68
(0.53)
(1.9)(1 stripe) 1.65 (1 stripe)
0.48
3.1 (1 stripe)
j~(mA/cm 2)
Vo~ (V)
90
36
60
80
solar b ilium. (mW/cm 2)
Measurements under sunlight
a 150 W reflector spot lamp adjusted in position for AM 1 using Solarex Si. ref. cell. ~Solar ilium, est. using Solarex Si ref. cell. CValues in brackets those remeasured after 1 to 2 months.
K34
K31
K30
K29
K24 K25 K26 K27 K28
Cell no.
Table 2 Measurements on Se CdO cells
open-circuit voltage not increased output unaffected by Cd contacts output essentially unaffected by doping of Se best cell fabricated, max. eff. 1.7°,~,
Increased opencircuit voltage ditto
Remarks
I
g~
e~
.:z
396
('. t1. Champne.ss ct ~tl. . 4 Sc ( ' d O p h o t m , o l t a i c cell
It had a conversion efficiency (AM 1 ) of about 1.7"., which is some 40". higher than on cells made earlier in the program. 2.3. Current-voltage measurements Dark characteristics Dark current-voltage characteristics for cells K24 and S1 are shown in fig. 1, indicating both to be good rectifiers. The rapid rise of current with voltage in the forward direction for cell S 1 is indicative of its comparatively low series resistance. This fact is brought out more strikingly above about 0.6 V in fig. 2 showing a plot of current density against forward voltage on semi-logarithmic scales; at l V the series area-resistances are about 150 and 670 f~ cm 2 for cells S1 and K24, respectively. 0.3 K24 Dark Current-Voltage
Characteristics 0-2
011
Vottage , V (volt) -7 I
-6 _._.--t---
K2~
-5 I ~
-4
-3
-2
-1
I
I
I
J,
0
1
S1
Fig. I. Dark dc current voltage characteristics of two laboratory-fabricated Bi Se C d O Au photovoltaic cells at room temperature.
Characteristics under illumination Fig. 3 shows current density versus voltage plots for cells S1, K24 and a commercial selenium cell (of unknown origin) under actual solar illumination. It is noted that cell S1 has higher jsc, Vo~and fill factor values than the other two cells. 2.4. Spectral response
The variation of responsivity (ampere/watt) with wavelength between 500 and 800 nm is given in fig. 4 for samples K24 and S1. With decreasing wavelength the responsivity rises from about 700 nm to a maximum between 500 and 600 nm with little difference between the two samples in this wavelength range, where the photogeneration arises from the selenium. The fall-off beyond the maximum on the short wavelength side is due to absorption in the CdO, where the difference in responsitivity
C. H. Champness et al. / A Se CdO photot,oltaic cell
397
10-2 5-I n:2
~0-3
o...o-K-24
n=3
10-4 E ~ 10-5 Dark Forward Characteristic ~ 10-6
10-7
1o-8 10 0
0.2
9
~ 0.4 06 0.8 10 Forward Voltage, (volt)
1.2
1.4
Fig. 2. Plot on log-linear scales of current density against forward voltage for two laboratory-fabricated Bi S e - C d O - A u photovoltaic cells in darkness at room temperature.
81
I
I
I
I
I
I
0.5
06
I E E
_
+ o +
o
0.I
0.2
0.3
0.4
0.7
Voltage, V (volt)
Fig. 3. Plot of current density against voltage under actual solar illumination of 94 mW/cm 2 (according to calibrated Solarex silicon cell) for cells Sl and K24 and a commercial selenium cell of unknown origin. Readings taken l to 2 pro, 31 May 1981 in Montreal.
398
(2 H. Champne~s ~'t al.
j
l
,
0.14
i
4 Sc ('dO photovoltaic eel~
,
1 ' ' ' ' 1
oeOee~ 4 o
.
012 o
.~" "~
•
0 O
010
O O
0.08 O
e
>.
T. 0.06
O
c
0
0.04 re-
002 0
I , 500
,
,
I
,
,
,
,
~
£
~
9
600 700 Wavelength, ~ , (nm}
a
800
Fig. 4. Variation of photoresponsivity with wavelength between 500 and 800 nm for two photovoltaic cells K24 and S1.
between the two cells could well be accounted for by a difference in the transparency of the counter electrode films arising from a difference in thickness or of cadmium-tooxygen ratio.
3. Experiments using premixed gases during CdO sputtering
3.1. Experimental procedure All the photovoltaic cells described above were fabricated by sputtering CdO in argon plus residual air. However, while this method has given good cells, it is rather difficult to control. Therefore, some preliminary experiments were made to deposit CdO films on glass using premixtures of argon and oxygen as the gas present during the dc sputtering. In these investigations, three gas mixtures were tried (a) pure argon, as before, with residual air, (b) 1% oxygen with 99 % argon and (c) 4.8 % oxygen with 95.2 ~, argon. The gases were admitted into the vacuum chamber through the needle valve, with an external line pressure of 5 psi (34 × 103 Pa). As in the case of the fabrication of the cells, the target was metallic cadmium and the separation between target and glass substrate was 1.3 cm. The glass substrate was a pre-cleaned microscope slide. For pure argon, the vacuum chamber was pumped down initially only to 5 to 10 mTorr, while for the gas mixtures (b) and (c), it was pumped down to a lower pressure of 10 -2 to 10 - 3 mTorr. With the baffle plate firstly closed, the sputtering gas was admitted into the chamber via the needle valve. By partially opening the baffle valve and manipulating the needle valve the pressure was adjusted to the desired value. Sputtering was then started with the discharge current at 20 mA and maintained at
C. H. Champness et al. / A Se-CdO photovohaic cell
399
this value for three minutes. After this period, in the case of the pure argon plus residual air, the needle and baffle valves were closed and air was let into the system; after three to five minutes at atmospheric pressure, pump down was again carried out and a second three minute sputtering discharge was made; this cycle was then repeated eight more times for a total sputtering time of 30 min. In the case of the premixed Ar/O2 gases, the sputtering was simply halted after the first three minutes and the vacuum maintained while the target was allowed to cool for another three minutes. The sputtering was then resumed for a further three minutes and the cycles continued until a total sputtering time of 30 min had been employed. 3.2. Films obtained
The CdO was deposited selectively through a mask so that the thickness of the film could be measured from the displacement of interference fringes using a microscope and monochromatic light from a mercury lamp. The resistivity of each film was determined using a four point probe. The average deposition rate was taken as the thickness divided by the total sputtering time. The variation of film resistivity with sputtering gas pressure is shown in fig. 5a, where it is noted that the resistivity decreases markedly with decreasing pressure, at least for the pure Ar plus residual air and the 4.8% 02 cases; for pressures less than 20C C
E . ~ 15(
Residual air
I
4.8*/. 0 2 ~
I
I
,
1'/, 0 2
o.)
g ~
~u
5O
a
O.
[
[
I
I
CdO
(o)
E o31 Glass ~. O. :>
~
0.1
rY
~
100
-4 125 150 !75 Chamber Pressure (mitbtorr)
1% 02
200
Fig. 5. Plots against vacuum chamber pressure of (a) electrical resiStivity and (b) deposition rate for CdO films reactively dc sputtered on glass in the presence of the gases: pure argon plus residual air. 1% oxygen with 99°/,, argon and 4.8°/,,, oxygen with 95 •-~o argon.
400
C. tt. Champne,ss e t al. , 4 Se CdO photovoltaic cell
90 mTorr (residual air) and 175 mTorr (l o~, O2) metallic cadmium was in fact deposited. The decrease of resistance is due to an increase of excess cadmium over oxygen with decrease in pressure. This may be explained by reduced recombination of oxygen and cadmium to form CdO as the total partial pressure of oxygen is reduced. For a fixed pressure, the resistivity falls from the 4.8'!,~i 0 2 mixture to the one with 1"i, 02. arising again from an increase in the excess cadmium. The CdO films in the 1'~i, O2 case were much less transparent than those for the 4.8";, O2 case, indicating that the excess cadmium also decreases the transparency. Fig. 5b shows that the deposition rate increases with decrease of pressure. Hence the films containing more cadmium are deposited faster than the ones which are more stoichiometric. This may be due to the extra time taken to form more of the oxide at the higher pressures where there is more oxygen. Another possible cause is that for a constant sputtering current, the voltage was higher at lower pressure; this would increase the energy of the gas ions hitting the target, thus leading to an increase in sputtering rate. It may also be noted in fig. 5b that at the higher pressure of 200 mTorr, the deposition rates were much the same for the three gas mixtures. The variation of deposition rate and resistivity with pressure for the residual air case is somewhat different from the results with the other two gas mixtures. The lower deposition rate at low pressure for residual air may be due to the presence of more than 4.8"~i, O2 at the beginning of the sputtering burst and less than this amount at the end of it; in this situation the average measured deposition rate would be dominated by the slower rate at the start; at high pressure, the total partial pressure of oxygen would be greater and the change in oxygen content with time would have less effect. This would also explain why the resistivity with residual air is higher than the 4.8°i, mixture at high pressure and lower at lower pressure.
4. Discussion and conclusions
The measured conversion efficiencies in the first stage of the present work on S e - C d O cells, were found to be just over 1'~,, which is better than observed on available commercial selenium cells measured so far in this laboratory. In the second stage, the effect of heat treatment of the selenium film in oxygen prior to CdO film deposition has raised the open-circuit voltage and fill factor, resulting in a cell giving about 1.7°/,i efficiency, without an optimized grid or antireflecting coating. The nature of this effect is not yet clear and it will be necessary in future work to see if SeOz, for instance, is created between the Se and the CdO. In any case, even with the Voc improvement, the device is still dominated by the high series resistance associated with the counter electrode, so that the resistivity of the selenium does not play a role in limiting the short-circuit current. Hence, until lower resistance counter electrodes can be made, the use of thin, doped monocrystalline selenium films appears to be premature. The selenium is of course important optically, since most of the photogenerated carriers are created in this material, as evidenced by the position of the rise on the long wavelength side of the spectral response maximum. In the reactive sputtering process with residual air, it appears that the first atomic
c. H. Champness et al. / A Se~CdO photovoltaic cell
401
layers deposited are of high resistivity due to a relatively large oxygen content in the gas initially. As the sputtering proceeds and more oxygen is removed by chemical reaction and pumping, the amount in the surrounding gas decreases, resulting in layers richer in excess cadmium and hence of lower resistivity. While this appears to be advantageous, the main problem with the use of residual air is the difficulty of good control. Efforts to get over this were made in the experiments using premixtures of oxygen and argon. Easier pressure control was certainly found but gas compositions of 1 and 4.8~, oxygen in argon at a fixed pressure did not give films of low enough resistivity coupled with high transparency. Thus optimized gas compositions and procedures have yet to be determined.
Acknowledgements The authors wish to acknowledge the support of this work by the Natural Sciences and Engineering Research Council Canada under their strategic grant program.
References [1] [2] [3] [4] [5]
G. Blet, Photopiles au S616nium(Dunod, Paris, 1959). J. S. Preston, Proc. Roy. Soc. London 202A (1950)449. M. Altmejd and C. H. Champness, J. Electron. Mater. 7 (1978)363. R. F. Shaw and A. K. Ghosh, Solar Cells 1 (1979/80)431. M. 1. E1-Azaband C. H. Champness, IEEE Trans. Elect. DevicesED-27 (1980}255.
391
I N V E S T I G A T I O N S ON A S e - C d O P H O T O V O L T A I C C E L L C. H. C H A M P N E S S , S. F U K U D A * and S. JATAR Electrical Enoineeriny Department, McGill University, 3480 University Street, Montreal, P.Q., Canada H3A 2A7
Received 17 June 1981 A photovoltaic Se-CdO thin film cell has been fabricated by reactive sputtering of CdO on a crystallized selenium film using a cadmium target in the presence of argon plus residual air. Critical control of pressure is needed to obtain the appropriate excess of cadmium in the CdO to realize low enough resistivity coupled with sufficient transparency. The spectral response lies between 350 and 700 nm and is controlled on the long wavelength side by the selenium and on the short wavelength side by the CdO. The CdO dominates the series resistance, so that the resistivity of the selenium is unimportant. A recent improvementin the fabrication processhas resulted in an increase of cell conversion efficiency to about 1.7°~,, which is well above values observed on commercial selenium photovoltaic cells. Some preliminary experiments have been made to sputter CdO reactively on glass from gas pre-mixtures of oxygen and argon to obtain easier pressure control.
I. Introduction At the present time silicon and gallium arsenide appear to be the most promising materials for photovoltaic solar cells. However, there are many other possible materials that have not been as extensively studied for use in the more distant future. Many of these other candidate materials could be used in thin film form leading to reduction in material costs. One such material is selenium, which has been used for many years in photovoltaic cells employed principally in photometers [1]. This application arises not only from the low cost of making the devices but also because the photoresponse is quite close to the spectral sensitivity of the human eye. The most detailed description of the fabrication of a selenium photovoltaic cell was given by Preston [2] some three decades ago. He first crystallized a layer o f selenium on a roughened steel plate and then deposited a transparent C d O layer on top of this by reactive dc sputtering from a metallic cadmium target. He obtained the best results by utilizing residual air left in the system after only moderate pumping. This method was later used by Altmejd and Champness [3] to make cell structures where a monocrystalline selenium film was employed for the first time by epitaxial deposition on a single crystal tellurium substrate. The benefits of the orientation of the film and its small thickness were not realized however, because of the high series resistance of the C d O counter electrode. Very recently Shaw and Ghosh [-4] described work on a selenium cell where CdSe and C d O layers were deposited on selen*Now at Department of Electronics, Kyoto University. Japan. 0165-1633/81/0000~000/$02.75 t~) 1981 North-Holland
392
(" f t ('hampne~s et al.
4 Se ('dO photovolt.ic ce/I
ium by rfsputtering in air at 10- 2 Torr from a cadmium target. Using a gold currentcollecting grid they reported efficiencies about Y',,, which are much higher than previously measured values for commercial selenium cells. These workers estimated that an efficiency of over 10,,;, (AM1) should eventually be possible with this type of cell. The investigation on Se CdO cells reported in this paper arises out of the initial earlier studies of Altmejd and Champness [3] and consists of an investigation aimed specifically at determining the counter electrode deposition conditions. In this stage of the work, monocrystalline selenium films were not used in the structures to shorten the time of the fabrication processes. The first part of the paper describes the making of the cell structures essentially by the Preston method using residual air. This is followed by results obtained with changes in the fabrication processes. The second part describes some preliminary work using premixed argon-oxygen gases to sputter reactively conducting CdO on glass in experiments to get better pressure control.
2. Cells
2.1. Cell Jabrication procedure The Se CdO photovoltaic cell structures studied in this investigation were fabricated basically in the following way (see table 1). One of the flat ends of a machined cylindrical aluminum stud, 2 cm in diameter, was roughened slightly by rubbing on fine emery paper. This surface was then coated with a thin film of bismuth using vacuum deposition. Next a film of selenium was evaporated on top of the bismuth, with the substrate maintained at a temperature of 130° C to convert the selenium from the amorphous to the polycrystalline form. After allowing the temperature to decrease slowly to room temperature to minimize peeling off of the film, the structure was remounted in another vacuum system so that the selenium surface was about 1.2 cm above a flat target of metallic cadmium (dia. 5 cm). A film of CdO was then reactively dc-sputtered on the selenium through a metal mask containing four quadrant-shaped openings. The first discharge was carried out for 3 rain at a pressure near 100 m T o r r with argon flowing into the bell-jar via a needle valve. At the end of the three minutes. air was allowed into the chamber to bring the pressure back to atmospheric level for a period of three to five minutes. Then the pressure was again reduced to about 100 r e t o r t and a second 3 min sputtering discharge made. This procedure of letting in air and sputtering under lower pressure was repeated usually 4 to 7 times, as indicated in column 7 of table 1. The resulting CdO film was thus formed by reaction of the cadmium atoms with the residual low pressure oxygen left in the vacuum chamber. With this done, two parallel gold stripes were sputtered on to each quadrant using a gold target and a suitable mask. For this, three sputtering bursts of 2 rain with 2 rain intervals were used. Finally with the stud mounted in a plastic holder, fine wires were soldered between the gold stripes and terminal posts using Wood's metal alloy. The thickness of the CdO layer was estimated from the interference colours which are listed in column 9 of table 1. Thus, for instance, the blue magenta colour for unit
60
60
60
60
60
60
60
0
60
K25
K26
K27
K28
K29
K30
K31
K34
S1
10
10
10
10
10
7 8
10
7
7
depos, time (min)
8 10
7
10
10
10
7 10
7 10
7-10
1
1
initial pressure (mTorr)
100
100
110
110
110
105 110
105 110
105-110
100
100
spurt, press, (mTorr)
18 20
18
12-15
12 15
12 15
12 15
12 15
12-15
25
25
needle valve setting (arb)
23
21
18
17
15
19.5
19.5
19.5
14
14
total time (min)
7 x 3 min lx2
7 × 3 min
5×3min 1×2 6 × 3 min
5x3min
ditto
6 x 3 min 1 ×1.5 ditto
4 × 3 min 1×2 ditto
no. of bursts
C d O sputtering ~
yellow red dark b.m. green
b.m. and green dark b.m. blueand yellow b.m.
blue magenta blue magenta green
colour of film
Au
Au
Cd
Au
Au
Au
Au
Au
Au
Au
Sputt. metal contact
Se film left overnight in air at room temperature
Se film heated 150°C in air for 24 h Se film heated 150°C in pure 0 2 for 24 h Se film heated 150°C in pure argon for 24 h Cd contacts used instead of Au u n d o p e d Se used
Remarks
"AI subst, evap. with Bi layer prior to Se evap.: subst, temp. during Se evap. 130°C. ~CdO sputtering: 20 mA dc; voltage 750 to 900 V; s u b s t r a t e - t a r g e t dist. 1.2-1.3 cm" Ar gas flow with line pressure of 5 psi (34 x 103 Pa).
60
K24
Device no.
CI dopant (ppm)
Se evaporation"
Table 1 Fabrication of Se C d O cells
~a
,2
e5
e~
e~
394
C. H. Champness et al.
4 Se CdO photoroltai( cc/l
K24 arises from second order interference and corresponds to a thickness of about 2100
2.2. Effect of fabrication changes on cell output In making the cells it was evident that the deposition of the counter electrode is the most critical process affecting photovoltaic performance. In the reactive sputtering, the resistance of the CdO film was greatly affected by the initial pressure after pumpdown (column 4 of table l) and the pressure during discharge (column 5). The pressure was controlled by the extent of opening the high vacuum valve to the diffusion pump: too low a pressure during sputtering (i.e. below 100 mTorr) resulted in deposition of metallic cadmium; too high a pressure yielded high resistivity CdO. To obtain sufficiently low resistivity and high enough transparency required careful control of this pressure. Frequently, nonreproducible results would occur from one run to another, possibly due to superficial oxidation of the cadmium target. As mentioned above, gold stripes were used as the electrodes on the CdO. Resistances measured between the stripes are listed in column 3 of table 2, which gives also the results of other measurements on the cells. The short-circuit current density (j~c) and open-circuit voltage (Vc~)figures in columns 4, 5 and 6 were taken under a 150 W reflector spot lamp positioned to give approximately AM1 illumination as detected by a silicon Solarex reference cell. While such lamp illumination has insufficient energy at the blue end of the spectrum (where Se is relatively more sensitive than Si), it was adequate for the relative comparison of cells. Thus comparing cell number K31 with K24 for example, it is evident from thejsc values in table 2 that the use of cadmium instead of gold for the metal contacts did not lead to any significant change in performance. Comparing cell K34, where undoped selenium was used, with the other cells in table 2, it is clear that in these devices the chlorine dopant in'the selenium plays no role in determining the photovoltaic performance. This is quite different from selenium rectifiers, where the series resistance is greatly affected by the presence of halogens in the selenium [5]. It clearly shows that the series resistance in the present devices arises from the CdO layer and not the selenium. The most interesting new feature in the present work is shown in cells K28, K29 and K30. In the first of these, the selenium film (together with its supporting aluminum mount) was maintained, at 150°C in air for 24 h prior to CdO deposition. As a result, the open circuit voltage was found to be 0.68 V, which is some 50% higher than the average value of 0.45 V obtained previously. This same value was again found in cell K29, where the selenium film was maintained in pure oxygen at 150°C for 24 h. However, in cell K30, where a 24 h 150°C treatment in pure argon was employed, the Vo~was the normal value of 0.45 V. It is thus evident that the presence of oxygen is necessary to obtain the increase of Vow. Among the 10 cells reported in table 2, the jsc values (2 stripes) on the better cells were about 2.5 mA/cm 2 under lamp illumination, with the highest value of 2.9 mA/cm 2 observed on device S1, which was the best cell fabricated. Under solar illumination of 90 mW/cm 2, this cell gave a jsc of 5.5 mA/cm 2 and a Vo~ of 0.65 V.
SI
Se left overnight in air at room temperature
Cd contacts used instead of Au undoped Se used
Se film heated 150° C in air 24 h Se film heated 150°C in pure Oz 24 h Se film heated 150~C in pure argon 24 h
Special fabrication features
1.0
2.8
2.5
2.6
5.2
1.0 7.8 1.9
3.3
Dark resist. between stripes (k~)
2.9
2.2 12.1) 2.0
2.2 0.4 2.2 1.2 2.2 (1.5) ~ 2.2 (1.3) 2.0
l stripe
2.9
2.5 (3.0) 2.2
(2.1)
(2.0)
2.4
2.6
2 stripes
j~¢(mA/cm 2)
0.6
0.59 (0.52) 0.4
0.4 0.4 0.44 0.56 0.68 (0.46) 0.68 (0.62) 0.45
Vo~ (V)
Measurements under lamp ~
5.5 (2 stripes)
0.65
0.68
(0.53)
(1.9)(1 stripe) 1.65 (1 stripe)
0.48
3.1 (1 stripe)
j~(mA/cm 2)
Vo~ (V)
90
36
60
80
solar b ilium. (mW/cm 2)
Measurements under sunlight
a 150 W reflector spot lamp adjusted in position for AM 1 using Solarex Si. ref. cell. ~Solar ilium, est. using Solarex Si ref. cell. CValues in brackets those remeasured after 1 to 2 months.
K34
K31
K30
K29
K24 K25 K26 K27 K28
Cell no.
Table 2 Measurements on Se CdO cells
open-circuit voltage not increased output unaffected by Cd contacts output essentially unaffected by doping of Se best cell fabricated, max. eff. 1.7°,~,
Increased opencircuit voltage ditto
Remarks
I
g~
e~
.:z
396
('. t1. Champne.ss ct ~tl. . 4 Sc ( ' d O p h o t m , o l t a i c cell
It had a conversion efficiency (AM 1 ) of about 1.7"., which is some 40". higher than on cells made earlier in the program. 2.3. Current-voltage measurements Dark characteristics Dark current-voltage characteristics for cells K24 and S1 are shown in fig. 1, indicating both to be good rectifiers. The rapid rise of current with voltage in the forward direction for cell S 1 is indicative of its comparatively low series resistance. This fact is brought out more strikingly above about 0.6 V in fig. 2 showing a plot of current density against forward voltage on semi-logarithmic scales; at l V the series area-resistances are about 150 and 670 f~ cm 2 for cells S1 and K24, respectively. 0.3 K24 Dark Current-Voltage
Characteristics 0-2
011
Vottage , V (volt) -7 I
-6 _._.--t---
K2~
-5 I ~
-4
-3
-2
-1
I
I
I
J,
0
1
S1
Fig. I. Dark dc current voltage characteristics of two laboratory-fabricated Bi Se C d O Au photovoltaic cells at room temperature.
Characteristics under illumination Fig. 3 shows current density versus voltage plots for cells S1, K24 and a commercial selenium cell (of unknown origin) under actual solar illumination. It is noted that cell S1 has higher jsc, Vo~and fill factor values than the other two cells. 2.4. Spectral response
The variation of responsivity (ampere/watt) with wavelength between 500 and 800 nm is given in fig. 4 for samples K24 and S1. With decreasing wavelength the responsivity rises from about 700 nm to a maximum between 500 and 600 nm with little difference between the two samples in this wavelength range, where the photogeneration arises from the selenium. The fall-off beyond the maximum on the short wavelength side is due to absorption in the CdO, where the difference in responsitivity
C. H. Champness et al. / A Se CdO photot,oltaic cell
397
10-2 5-I n:2
~0-3
o...o-K-24
n=3
10-4 E ~ 10-5 Dark Forward Characteristic ~ 10-6
10-7
1o-8 10 0
0.2
9
~ 0.4 06 0.8 10 Forward Voltage, (volt)
1.2
1.4
Fig. 2. Plot on log-linear scales of current density against forward voltage for two laboratory-fabricated Bi S e - C d O - A u photovoltaic cells in darkness at room temperature.
81
I
I
I
I
I
I
0.5
06
I E E
_
+ o +
o
0.I
0.2
0.3
0.4
0.7
Voltage, V (volt)
Fig. 3. Plot of current density against voltage under actual solar illumination of 94 mW/cm 2 (according to calibrated Solarex silicon cell) for cells Sl and K24 and a commercial selenium cell of unknown origin. Readings taken l to 2 pro, 31 May 1981 in Montreal.
398
(2 H. Champne~s ~'t al.
j
l
,
0.14
i
4 Sc ('dO photovoltaic eel~
,
1 ' ' ' ' 1
oeOee~ 4 o
.
012 o
.~" "~
•
0 O
010
O O
0.08 O
e
>.
T. 0.06
O
c
0
0.04 re-
002 0
I , 500
,
,
I
,
,
,
,
~
£
~
9
600 700 Wavelength, ~ , (nm}
a
800
Fig. 4. Variation of photoresponsivity with wavelength between 500 and 800 nm for two photovoltaic cells K24 and S1.
between the two cells could well be accounted for by a difference in the transparency of the counter electrode films arising from a difference in thickness or of cadmium-tooxygen ratio.
3. Experiments using premixed gases during CdO sputtering
3.1. Experimental procedure All the photovoltaic cells described above were fabricated by sputtering CdO in argon plus residual air. However, while this method has given good cells, it is rather difficult to control. Therefore, some preliminary experiments were made to deposit CdO films on glass using premixtures of argon and oxygen as the gas present during the dc sputtering. In these investigations, three gas mixtures were tried (a) pure argon, as before, with residual air, (b) 1% oxygen with 99 % argon and (c) 4.8 % oxygen with 95.2 ~, argon. The gases were admitted into the vacuum chamber through the needle valve, with an external line pressure of 5 psi (34 × 103 Pa). As in the case of the fabrication of the cells, the target was metallic cadmium and the separation between target and glass substrate was 1.3 cm. The glass substrate was a pre-cleaned microscope slide. For pure argon, the vacuum chamber was pumped down initially only to 5 to 10 mTorr, while for the gas mixtures (b) and (c), it was pumped down to a lower pressure of 10 -2 to 10 - 3 mTorr. With the baffle plate firstly closed, the sputtering gas was admitted into the chamber via the needle valve. By partially opening the baffle valve and manipulating the needle valve the pressure was adjusted to the desired value. Sputtering was then started with the discharge current at 20 mA and maintained at
C. H. Champness et al. / A Se-CdO photovohaic cell
399
this value for three minutes. After this period, in the case of the pure argon plus residual air, the needle and baffle valves were closed and air was let into the system; after three to five minutes at atmospheric pressure, pump down was again carried out and a second three minute sputtering discharge was made; this cycle was then repeated eight more times for a total sputtering time of 30 min. In the case of the premixed Ar/O2 gases, the sputtering was simply halted after the first three minutes and the vacuum maintained while the target was allowed to cool for another three minutes. The sputtering was then resumed for a further three minutes and the cycles continued until a total sputtering time of 30 min had been employed. 3.2. Films obtained
The CdO was deposited selectively through a mask so that the thickness of the film could be measured from the displacement of interference fringes using a microscope and monochromatic light from a mercury lamp. The resistivity of each film was determined using a four point probe. The average deposition rate was taken as the thickness divided by the total sputtering time. The variation of film resistivity with sputtering gas pressure is shown in fig. 5a, where it is noted that the resistivity decreases markedly with decreasing pressure, at least for the pure Ar plus residual air and the 4.8% 02 cases; for pressures less than 20C C
E . ~ 15(
Residual air
I
4.8*/. 0 2 ~
I
I
,
1'/, 0 2
o.)
g ~
~u
5O
a
O.
[
[
I
I
CdO
(o)
E o31 Glass ~. O. :>
~
0.1
rY
~
100
-4 125 150 !75 Chamber Pressure (mitbtorr)
1% 02
200
Fig. 5. Plots against vacuum chamber pressure of (a) electrical resiStivity and (b) deposition rate for CdO films reactively dc sputtered on glass in the presence of the gases: pure argon plus residual air. 1% oxygen with 99°/,, argon and 4.8°/,,, oxygen with 95 •-~o argon.
400
C. tt. Champne,ss e t al. , 4 Se CdO photovoltaic cell
90 mTorr (residual air) and 175 mTorr (l o~, O2) metallic cadmium was in fact deposited. The decrease of resistance is due to an increase of excess cadmium over oxygen with decrease in pressure. This may be explained by reduced recombination of oxygen and cadmium to form CdO as the total partial pressure of oxygen is reduced. For a fixed pressure, the resistivity falls from the 4.8'!,~i 0 2 mixture to the one with 1"i, 02. arising again from an increase in the excess cadmium. The CdO films in the 1'~i, O2 case were much less transparent than those for the 4.8";, O2 case, indicating that the excess cadmium also decreases the transparency. Fig. 5b shows that the deposition rate increases with decrease of pressure. Hence the films containing more cadmium are deposited faster than the ones which are more stoichiometric. This may be due to the extra time taken to form more of the oxide at the higher pressures where there is more oxygen. Another possible cause is that for a constant sputtering current, the voltage was higher at lower pressure; this would increase the energy of the gas ions hitting the target, thus leading to an increase in sputtering rate. It may also be noted in fig. 5b that at the higher pressure of 200 mTorr, the deposition rates were much the same for the three gas mixtures. The variation of deposition rate and resistivity with pressure for the residual air case is somewhat different from the results with the other two gas mixtures. The lower deposition rate at low pressure for residual air may be due to the presence of more than 4.8"~i, O2 at the beginning of the sputtering burst and less than this amount at the end of it; in this situation the average measured deposition rate would be dominated by the slower rate at the start; at high pressure, the total partial pressure of oxygen would be greater and the change in oxygen content with time would have less effect. This would also explain why the resistivity with residual air is higher than the 4.8°i, mixture at high pressure and lower at lower pressure.
4. Discussion and conclusions
The measured conversion efficiencies in the first stage of the present work on S e - C d O cells, were found to be just over 1'~,, which is better than observed on available commercial selenium cells measured so far in this laboratory. In the second stage, the effect of heat treatment of the selenium film in oxygen prior to CdO film deposition has raised the open-circuit voltage and fill factor, resulting in a cell giving about 1.7°/,i efficiency, without an optimized grid or antireflecting coating. The nature of this effect is not yet clear and it will be necessary in future work to see if SeOz, for instance, is created between the Se and the CdO. In any case, even with the Voc improvement, the device is still dominated by the high series resistance associated with the counter electrode, so that the resistivity of the selenium does not play a role in limiting the short-circuit current. Hence, until lower resistance counter electrodes can be made, the use of thin, doped monocrystalline selenium films appears to be premature. The selenium is of course important optically, since most of the photogenerated carriers are created in this material, as evidenced by the position of the rise on the long wavelength side of the spectral response maximum. In the reactive sputtering process with residual air, it appears that the first atomic
c. H. Champness et al. / A Se~CdO photovoltaic cell
401
layers deposited are of high resistivity due to a relatively large oxygen content in the gas initially. As the sputtering proceeds and more oxygen is removed by chemical reaction and pumping, the amount in the surrounding gas decreases, resulting in layers richer in excess cadmium and hence of lower resistivity. While this appears to be advantageous, the main problem with the use of residual air is the difficulty of good control. Efforts to get over this were made in the experiments using premixtures of oxygen and argon. Easier pressure control was certainly found but gas compositions of 1 and 4.8~, oxygen in argon at a fixed pressure did not give films of low enough resistivity coupled with high transparency. Thus optimized gas compositions and procedures have yet to be determined.
Acknowledgements The authors wish to acknowledge the support of this work by the Natural Sciences and Engineering Research Council Canada under their strategic grant program.
References [1] [2] [3] [4] [5]
G. Blet, Photopiles au S616nium(Dunod, Paris, 1959). J. S. Preston, Proc. Roy. Soc. London 202A (1950)449. M. Altmejd and C. H. Champness, J. Electron. Mater. 7 (1978)363. R. F. Shaw and A. K. Ghosh, Solar Cells 1 (1979/80)431. M. 1. E1-Azaband C. H. Champness, IEEE Trans. Elect. DevicesED-27 (1980}255.