CA1171505A

CA1171505A - Conductive elements for photovoltaic cells

Info

Publication number: CA1171505A
Application number: CA000379284A
Authority: CA
Inventors: Katherine V. Clem
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 1980-07-23
Filing date: 1981-06-08
Publication date: 1984-07-24
Also published as: GB2080275B; GB2080275A; DE3128806A1; FR2487584B1; JPS5758374A; FR2487584A1

Abstract

CONDUCTIVE ELEMENTS FOR PHOTOVOLTAIC CELLS
Abstract of the Disclosure A conductive element such as an electrode useful in photovoltaic cells comprises a soda-lime glass support having thereon a layer containing polycrystalline SnO2 and a fluorine dopant, said conductive electrode being substan-tially haze-free and having a transmittance of radiation between 400 and 800 nm greater than 70% and an electrical resistance less than 30 ohm per square. The conductive element is particularly useful in thin film cadmium sulfide/-cadmium telluride photovoltaic cells.

Description

`` ~31 ~

CONDUCTIVE ELE ENTS_FOR PHOTOVOLTAIC CELLS
BACKGROUND OF THE INVENTIOM
This invention relates to SnO2 conductive elements which are particularly useful in thin film photovoltaic cell~.
Effective conversion from expensive petroleum-based ener~y sources to solar energy sources, such as photovoltaic cells, has been delayed by two factors - the cost of mass-producing such and the low conversion efficiency achieved by such cells. Any improvement in either factor is 10 capable of movin~ industry towards the use of more solar cells, and an improvement in both has been a lon~_sou~ht ~oal.
The use of p-n cadmium telluride - cadmium sulfide photovoltaic cells having thin layers as descr;bed in 15 U.S. Patent No. 4,207,119 has considerably improved the efficiency of the cells. A limitin~ factor to further improvements in the efficiency of such cells is the window electrode through which the solar cell is illuminated.
Conventionally, the window electrode comprises a glass 20 support with a transparent and conductive coating as for example In203 such as is available under the trade-mark Nesatron0 from PPG Industries. These materials and others such as Cd2SnO4 and CdSnO3 yield films of low resistivity and hi~h transmittance, but such materials 25 are not readily avai]able and the processes required for their preparation make them extremely expensive for use in photovoltaic cells.
Glass electrodes containing more available and less expensive coatings have been sought. R. G. Livesey, 30 E. Lyford and H. Moore, J. of Physics E: J. of Scientific Instruments, 1, 947 (1968) describe a transparent conductive tin oxide film on glass prepared by flowing oxy~en through a flask of heated SoCl2-2H20 onto glass substra~es.
These tin oxide films had 85% transmittance but electrical 35 resistivities of 100-500 ohm per square. Films having lower resistance were acknowledged by the authors to be undesirable due to haze.

~:17~S~35 James Kane, H. P~ Schwizer and Werner Kern in Volume 123, No. 2 of J. Electrochem. Soc Solid-State Science and Technolo~y, pa~es 270-276 (February, 1976) describe the use of a soda-lime glass support for a tin oxide film wherein the soda-lime ~lass surface is necessarily treated to remove sodium from the soda-lime glass a~ the surface to prevent haze from forming.
U.S. Patent No. 3,880,633 describes a tin oxide film on ~lass prepared by sprayin~ a solution of SnCl2 in 10 methanol with small amounts of ammonia bifluoride. This method discloses an acid pretreatment of the glass support prior to the application of the SnO2 layer, formin~ a silica film over the support, to not only lessen the result-ing ha7e in the SnO2 layer and glass support but also to 15 be instrumental in obtainin~ a satîsfactory layer resistance and high transmittance. This method achieves 78% transmit-tance and e]ectrical resistance as low as 10 ohm/square, but the resulting tin oxide films are still hazy. The appearance of haze on the electrode film causes light scattering (a loss 20 of tratlsmittance).
Thus, the prior art is replete with references to the desirability of using soda-lime glass as the support for conductive elements as this material is extremely inexpen-sive, but has not found an acceptable way of usin~ the 25 material without involvin~ the expense of first pretreatin~
the support to remove sodium so that the support is no lon~er soda-lime glass per se or to add a layer of silica on the soda-lime glass. A substantially haze-free element formed from a support of soda-lime glass containin~ a layer 30 of SnO2 directly on the soda-lime glass support is deemed to be highly useful in this art.
SUMMARY OF THE INVENTION
In accordance with the present invention there is formed a conductive elemen~ useful in photovoltaic cells 35 which has a transmittance at 400-800 nm greater than 70%;
an electrical resistance 30 ohm/square or less; is substan-tially haze-free; and reguires no pretreated support or extra layer on the support.

~7lS~5 A conductive element in accordance with the present invention comprises a soda-lime glass support having thereon a layer containing polycrystalline SnO2 and a fluorine dopant, said conductive element being substantially haze-free and having a total transmittance of radiation between 400 and 800 nm greater than 70~/O and an electrical resistance less than 30 ohm/square.
Total transmittance is the percent transmittance measured by an integratin~ sphere while specular transmit-lO tance is the percent transmittance measured with a smallangle detector. The specular transmittance is, of course, always less than total transmittance.
In accordance with another aspect of this invention a method of preparing a polycrystalline SnO2 conductive 15 element comprises heatinR a soda_lime glass support at a temperature of at least about 450C in the presence of a source or sources of SnCl2 and the fluorine dopant, said heating step being carried out in an oxy~en-containing atmosphere and wherein the source or sources of SnCl2 and 20 dopant are heated to a temperature of less than about 480C.
In accordance with yet another aspect of the present invention a photovoltaic cell comprises crystalline layers of p-type cadmium telluride and n-type cadmium sulfide in operative low impedance contact with a conductive element as 25 described above.
BRIEF DESCRIPTION OF THE DRAWINGS
Fi~. 1 is a view of a source of SnCl2 and dopant;
Fig. 2 is a view of a support to be coated; and Fi~. 3 is a view of apparatus useful in performing the 30 method of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
- The conductive element of the present invention com-prises a soda-lime glass support having thereon a tin oxide layer containing a fluorine dopant.
By "soda-lime glass" support is meant the support material is strictly soda 1ime ~lass and is not pretreated to remove sodium from the surface of the support to result in ~L~ 715~5 a surface layer of something other than soda-lime &lass or to add a protective layer over the soda~lime glass. The support used in this invention requires no additional expensive treatment in order to achieve good transmittance, low resistance, and the substantial absence of ha~e. The suppo~t is preferably soda-lime ~lass with 90~/O transmit-tance. Although the thickness of the glass suppor~ is not critical, thicknesses of from about 0.5 to about 5 mm are preferred.
It is noted that the method of the present invention is useful with any inorganic, hiRh temperature-resistant, nonconductive materials as the support, such as silica, quartz, borosilicate and other ~lasses, alumina and ceramics. These supports, however, result in relatively 15 expensive electrodes which have limited use in photovoltaic cells.
The layer on the support comprises tin oxide and a fluorine dopant. The fluorine dopant is basically any fluorine-containin~ material such as SnClF, SnF2, 20 H2SiF6 and NH4FHF. The only requirements for the fluorine dopant are that it is volatile at any processin~
temperature used in heating it and the SnCl2 to form SnO2 on the support.
By a soda_lime glass support "havin~ thereon a layer 25 containin~ polycrystalline SnO2 and a fluorine dopant" is meant the layer is formed directly on the soda-lime glass support and not on an intervening layer of any kind or on a support that is treated so that the surface of the support is no lon~er soda-lime glass.
In photovoltaic cells, it is desirable for the conduc-tive element to have low electrical resistance so that the Joule loss is reduced and the cell efficiency is therefore increased. Thus the SnO2 layer has a resistance of less than 30 ohm/square and preferably less than 20 ohm/square.
35 It is also desirable that the layer possess ~ood li~ht trans-mit~ance. Thus, at 400-800 nm the transmittance of radia-tion is advantageously greater than 70V/o and preferably greater than 80%.

.

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The conductive elements of the present invention are substantially haze-free. This is important in the use of the conductive elements in photovoltaic cells as haze leads to undesirable interactions between the conductive element and other layers in such cells such as CdTe and CdS.
An example of such interaction is given in U.S. Patent No.
3,880,633. Further, if the conductive element is hazy, light-scattering occurs, which results in loss of transmit-tance. By substantially haze-free it is meant that a visual examination of the material gives the impression of a relatively clear material. That is, there is no detection of fog by the naked eye.
Conductive elements, as described herein, include electrodes such as are used with photovoltaic cells, display electrodes, and electrodes for electrophotographic plates;
optical filters; and antistatic elements. A particularly preferred conductive element is an electrode such as a window electrode for photovoltaic cells.
The layer containing tin oxide and fluorine dopant is preferably thin. Preferred conductive element layers are from 1000 to about lO,OOOA. The layer potentially comprises the fluorine dopant in any concentration but the preferred layer contains from about 0.001 to about 5 wt/% of the fluorine dopant.
The layer containing the doped SnO2 is poly-crystalline and the crystallites preferably are oriented relative to the glass support such that the (200) and (110) crystallographic planes are oriented parallel to the plane of the glass surface. Although the crystallites are of any size it is preferred that the SnO2 crystals be less than one micrometer and more preferably less than 0.5 micrometer.
The polycrystalline SnO2 conductive element is prepared by heating a support of soda-lime glass to a temp-erature of at least about 450C in an environment containing a source of SNCl2 and, either in the same source or a separate source, a fluorine dopant. The source of '~' ~ ~ 7 ~

SnCl2 and dopant is heated to a tempera~ure of less than 480C but high enou~h to volatili~e the SnCI2 and dopant toward ~he support. Preferably the temperature to which the source is heated is from 200C to 400C if the method used is close-spaced evaporation. If the method of deposition is by transport mode (the source vapors are transported over long distances, such as in Example 7~, ~hen temperatures up to 480C are useful. This operation is carried out in an atmosphere where the oxygen content is at least about 15%.
A particularly preferred method of forming poly-crystalline SnO2 conductive elements is illustrated in the drawings.
In Fi~. 1 a source of SnCl2 and fluorine dopant A
is contained in holder B. In Fi8. 2 a soda_lime glass 15 support D is attached to a holder E.
The method of coating the glass support is preferably chemical vapor deposition in a close-space confi~uration.
Using this method, vapors are evaporated from a source to a support positioned from the source a distance no ~reater than 20 the SqlJare root of the smaller of the surface areas of the source and of the support.
The chemical vapor deposition is illustrated in Fig. 3 where a glass enclosure & contains an inlet for oxygen or oxygen-enriched air H. The flow of gas is admitted into 25 the enclosure to provide a suitable oxygen-rich atmosphere for the reaction of A and deposition of the oxide onto the support D. The gas flow rate is adjustable so that the desired gas pressure is maintained in the enclosure.
Holder E is heated by lamp F and holder B is heated by 30 lamp C. The holders are formed from graphite or other heat tolerant substances and are heated by the lamps or by other techniques such as resistance heating and induction heating.
After the heating process, the excess SnCl2 and dopant vapors are preferably removed from the presence of the 35 support (generally by the flow of oxygen before heatin~ of the support is terminated)O

131~7~S(~

The atmosphere for the vapor-phase depositin~ can be either pure oxyRen, oxygen artifically admixed with other gasses, or air. As will be readily apparent, the actual amount of the oxygen present durinR deposition will depend upon the specific form of vapor-phase depositin~ that is selec~ed. For example, chemical vapor deposition in a close-space configuration, a highly preferred form of the process of the invention, is generally carried out at atmos-pheric pressure. The o~her forms of vapor-phase depositing 10 mentioned above have known or standard tolerance levels of ~as, and the amount of oxygen pressure or partial pressure is selected to comply with such tolerance levels.
The vapor-phase depositin~ is done either as a batch process, e.g., in a process chamber containin~ a single 15 source and a single support, or as a continuous process in which a support is moved through appropriate zones of treat-ment.
The evaporation is ~enerally run at atmospheric pressure or sli~htly above atmospheric pressure. The spacing 20 between source and support preferably is between about 2 and about 10 mm althou~h distances of between one aDd 100 mm are useful. The temperature of the support îs variable dependin~
on which material is bein~ evaporated. Preferably, the source material is deposited for a time of between about 0.1 25 second to about 10 minutes Onto a support held at a tempera-ture of between 450C and about 630C. The source tempera-ture is maintained in each instance between about 200~C and about 400C.
A photovoltaic cell is formed simply by using the 30 formed electrode as a window electrode. A preferred cell is similar to those described in U.S. Patent No. 4,207,119 except that the window electrode is as described above.
Thus, the preferred cell comprises ~irst and second contiguous polycrystalline layers containing respectively 35 p-type cadmium telluride and n-type cadmium sulfide and the electrode described above in operative, low-imped-ance-contact with at least part of said layers. The con-struction and use of photovoltaic cells is disclosed in detail in U.S. Patent No. 4,207,119 which is herein incorpo-rated by reference.
The followin~ examples further illustrate the inven-tion.
Example 1 Seven samples of transparent and electrically conduc-tive tin oxide were prepared usin~ the close_space evapo-rator described herein. The source was anhydrous SnCl2 doped with one mole percent fluorine added as SnClF. The 10 support was soda_lime glass wi~h 90% transmittance. The spacing between the source and suppor~ was 5 mm. The process was carried out at atmospheric pressure with an oxy~en flow of 1220 cc/min. The support was heated to 550C; and, immediately thereafter, the source was heated to 325C. The 15 deposition time was 1 min, 15 second, starting at the time the source reached 325C. The avera~e resistance of these seveo samples was 12 ohm/sguare and the ~otal transmittance was 80V/o of visible light between 400-800 nm. The thickness o~ the films varied between 0.37 and 0.43 ~m. The films 20 were haze-freeO
Example 2 Ei~ht samples of transparent and electrically conduc-tive tin oxide were prepared using the same close-space evapnrator and essentially the same conditions described in 25 Example 1 except that the dopant was 0.9 mole % fluorine and was added as SnF2 to the anhydrous SnCl2. The average of the results of this experiment was a resistance o 14 ohm/square and 79% total transmittance between 400 and 800 nm. The thickness of the films varied between 0~36 and 0.52 30 ~m and the films were haze free.
Example 3 - This is a comparative example. One sample of trans-parent and electrically conductive tin oxide was prepared using the close-space evaporator and essentially the same 35 conditions described in Example 1 except DO dopants were added to ~he anhydrous SnCl2 source. The results of this experiment were a resistance of 63 ohm/square and 80 percent ~7~S~ S

_9_ total transmittance between 400 and 800 nm. The thickness of this film was 0.57 ~m. The importance of the fluorine dopin~ is demonstrated with this example.
Example 4 One sample of transparent and electrically conductive tin oxide was prepared using the close-space evaporator and essentially the same conditions described in Example 1, except the oxy~en flow rate was 4000 cc/min. The results of this experiment were 9 ohm/square resistance and 80V/~ total 10 transmittance to visible light between 400-800 nm. The thickness of this sample was 0.39 ~m. A ~olar cell of 9.5%
efficiency was made on this glass, as described in U.S.
Patent No. 4,207,119, example 1, with the exception that the window electrode of this example was substituted for the 15 Nesatron~ window used in U.S. Patent No. 4,207,119.
Example 5 Six samples of transparent, electrically-conducting, tin oxide-coated ~lass were prepared as described in Example 1, except that the source temperature was 315C and 20 the support temperature was chan~ed from sample to sample.
The electrical resistance and avera~e transmittance to visible and near IR li~ht (400-800 nm) of the resultin~
coatin~s was:

25 Support Avera~e % Specular Temperature Resistance Transmittance (C) (ohm/square? Visible Li~ht 400 2.5 x 10- 4 83%
~30 360 79V/~
450 96 78%
500 18 77%
550 10 76%
600 5.5 68%

It is seen that support temperatures above 450C are desirable.

Example 6 Three samples of transparent, electricallg_conduct-ing, tin oxide-coated ~lass were prepared in a close-space evaporator of similar design to that employed in Example 1, and utilizin~ the same experimental conditions excep~ as noted below:
Air was used instead of oxy~en, the flow-rate was 410 cc/min; the source temperature was 320C, deposition time was 1 min; and the spacin~ between source and support was 2.5 mm.
The avera~e electrical resistance of the three result-ing samples was 15 ohm/sguare, and the avera~e specular transmittance to visible and near IR light (400-800 nm) was 74%.
Example 7 An apparatus was constructed in which a stream of gas flowed, in a confined channel, over a heated source-holder containing a mixture of SnCl2 (99 mole %) and SnClF (1 mole %). After passin~ over the source, the stream of ~as carried the sncl2lsnclF vapors over a distance of about 20 five inches, and was then deflected to impinge On a heated soda-lime glass support, thus depositin~ on the glass a layer of doped tin oxide.
One sample of such tin oxide-coated ~lass was pre-pared in the above-described apparatus, while holdin~ the 25 source at 475C, the support at 550C, utilizin~ an oxy~en flow of 3000 cc/min, and carryin~ out the deposition for 30 seconds. The resultin~ coatin~ had an electrical resistance of 18 ohm/square, a thickness of 0.26 ~m, and 75% avera~e specular transmittance to visible and near IR li~ht (400-800 nm).
The invention has been described in detail with par-ticular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A conductive element comprising a soda-lime glass support having thereon a layer containing poly-crystalline SnO2 and a fluorine dopant, said conduc-tive element being substantially haze-free and having a total transmittance to radiation between 400 and 800 nm greater than 70% and an electrical resistance less than 30 ohm/square.

2. The conductive element of Claim 1 wherein the fluorine dopant is selected from the group consisting of SnF2 and SnFCl.

3. The conductive element of Claim 1 wherein the thickness of the layer is from 1000A to 10,000A.

4. The conductive element of Claim 1 wherein the polycrystalline SnO2 has (200) and (110) planes that are oriented parallel to the glass surface.

5. A method of preparing a polycrystalline SnO2 conductive element comprising heating a soda-lime glass support at a temperature of at least about 450°C in the presence of a source or sources of SnCl2 and fluorine dopant, said heating step being carried out in an oxygen atmosphere wherein the oxygen content is at least 15%, and said source or sources of SnCl2 and dopant are heated at a temperature sufficient to evaporate the SnCl2 and dopant onto the support, but at a temperature less than 480°C.

6. The method of Claim 5 wherein the source of SnCl2 and dopant is heated to a temperature between about 200°C and 400°C.

7. The method of Claim 5 wherein said source of SnCl2 is also the source of fluorine dopant.

8. The method of Claim 5 wherein the heating step is carried out in a glass enclosure.

9. The method of Claim 5 wherein excess SnCl2 and dopant vapors are removed from the presence of the support before the heating is terminated.

10. The method of Claim 5 wherein the dopant is selected from the group consisting of SnF2 and SnFCl.

11. The method of Claim 6 wherein the source or sources of SnCl2 and fluorine dopant are spaced apart from said glass support at a distance of from about 2 to about 10 mm.

12. The method of Claim 5 wherein the tempera-ture to which the support is heated is from 450°C to 630°C.

13. A photovoltaic cell comprising contiguous crystalline layers and the conductive element of Claim 1 in operative, low-impedance contact with at least part of said layers.

14. A photovoltaic cell comprising first and second contiguous crystalline layers containing, respec-tively, p-type cadmium telluride and n-type cadmium sulfide and the conductive element of Claim 1 in opera-tive, low-impedance contact with at least part of the layers of said cell.

15. The photovoltaic cell of Claim 13 wherein the fluorine dopant is selected from the group consisting of SnF2 and SnFCl.