GB2080275A - Conductive element, method of preparing the conductive element and photovoltaic cell comprising the conductive element - Google Patents

Abstract

A conductive element comprises a sodalime glass support having a coating of polycrystalline stannic oxide (SnO2) and a fluorine dopant and has a transmittance greater than 70% to radiation of a wavelength between 400 and 800 nm and an electrical resistance of less than 30 ohm/square. It is useful in thin film cadmium sulphide/cadmium telluride photovoltaic cells. The conductive element is made by heating the sodalime glass support to 450 DEG C in a 15% by volume oxygen atmosphere in the presence of stannous chloride (SnCl2) and a fluorine dopant, which is heated to a temperature less than 480 DEG C that is sufficient to evaporate the SnCl2 and the fluorine dopant onto the support.

Description

SPECIFICATION Conductive element, method of preparing the conductive element and photovoltaic cell comprising the conductive element This invention relates to a conductive element, a method of preparing the conductive element and a photovoltaic cell comprising the conductive element.

Effective conversion from expensive petroleumbased energy sources to solar energy sources, such as photovoltaic cells, has been delayed by two factors, namely, the cost of mass-producing the cells and the low conversion efficiency achieved by such cells. Any improvement in either factor is capable of moving industry towards the use of more solar cells, and an improvement in both has been a long-sought goal.

The use of p-n cadmium telluride - cadmium sulphide photovoltaic cells having thin layers as described in European Patent Application 0006025 has considerably improved the efficiency of the cells.

A limiting 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 support with a transparent and conductive coating as for example indium oxide (In203) such as is available under the trademark Nesatron from PPG Industries. These materials and others such as cadmium ortho stannate (Cd2SnO4) and cadmium stannate (CdSnO3) yield films of low resistivity and high transmittance, but such materials are not readily available 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, 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 oxygen through a flask of heated stannous chloride dihydrate (SnCl2 ~ 2H20) onto glass substrates. These tin oxide films had 85% transmittance but electrical resistivities of 100 to 500 ohm per square. Films having low resistance were acknowledged by the authors to be undesirable due to haze.

James Kane, H. P. Schwizer and Werner Kern, 2,J.

Electrochem. Soc: Solid-State Science and Technology, Volume 123, No. pages 270-276 (February 1976) describe the use of a soda-lime glass support for a tin oxide film wherein the soda-lime glass surface is necessarily treated to remove sodium from the soda-lime glass at the surface to prevent haze from forming.

U.S. Patent 3,880,633 describes a tin oxide film on glass prepared by spraying a solution of stannous chloride in in methanol with small amounts of ammonia bifluoride. This method discloses an acid pretreatmentofthe glass support prior to the application of the stannic oxide (SnO2) layer, thus forming a silica film over the support, to not only lessen the resulting haze in the SnO2 layer and glass support but also to be instrumental in obtaining a satisfactory layer resistance and high transmittance. This method achieves 78% transmittance and electrical 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 of transmittance).

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 inexpensive, but has not found an acceptable way of using the material without first pretreating the support to remove sodium. The pretreated support is no longer soda-lime glassper se. It can be regarded as a soda-lime glass with an added layer of silica.

The problem the invention intends to solve is to provide a conductive element which is particularly useful in thin film photovoltaic cells and which can be prepared at relatively low cost.

According to the present invention there is provided a conductive element comprising a sodalime glass support having thereon a layer containing polycrystalline SnO2 and a fluorine dopant and having a total transmittance to radiation of wavelengths between 400 and 800 nm greater than 70% and an electrical resistance less than 30 ohm/square area.

The invention also provides a method of preparing a conductive element comprising heating a sodalime glass support to a temperature of at least 450 C in the presence of a source of SnC12 and fluorine dopant, the heating step being carried out in an oxygen atmosphere wherein the oxygen content is at least 15% by volume, and wherein the source of SnCl2 and dopant is heated to a temperature sufficient to evaporate the SnCI2 and fluorine dopant onto the support, but at a temperature less than 480 C.

The invention further provides a photovoltaic cell comprising contiguous crystalline layers and including a conductive element as defined above in operative, low-impedance contact with at least part of one of said layers.

Total transmittance is the percent transmittance measured by an integrating sphere while specular transmittance is the percent transmittance measured with a small angle detector. The speculartransmittance is, of course, always less than total transmittance.

The conductive element of the present invention comprises 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-lime glass and is not pretreated to remove sodium from the surface of the support to result in a surface layer of something other than soda-lime glass orto 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 haze. The support is preferably soda-lime glass with 90% transmittance.

Although the thickness of the glass support is not critical, thicknesses of from 0.5 to 5 mm are preferred.

It is noted that the method of the present invention is useful with any inorganic, high temperature resistant, nonconductive materials as the support, such as silica, quartz, borosilicate and other glasses, alumina and ceramics. These supports, however, result in relatively 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 fluorinecontaining material such as stannous chloro fluoride (SnCIF), stannous fluoride (SnF2), hydrofluosilic acid (H2SiF6) and acid acid ammonium fluoride (NH4F.HF). The only requirements for the fluorine dopant are that it is volatile at any processing temperature used in heating it and the SnCl2 to form SnO2 on the support.

By a soda-lime glass support "having thereon a layer containing polycrystalline Snow and a fluorine dopant" is meant the layer is formed directly on the sodalime 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 longer soda-lime glass.

In photovoltaic cells, it is desirable for the conductive 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 area and preferably less than 20 ohm/square area. It is also desirable that the layer possess good light transmittance. Thus, at 400400 nm the transmittance of radiation is advantageously greater than 70% and preferably greater than 80%.

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 cadmium telluride (CdTe) and cadmium sulphide (CdS). An example of such interaction is given in U.S. Patent 3,880,633. Further, if the conductive element is hazy, light-scattering occurs, which results in loss of transmittance. By substantially hazefree 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 anti-static 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 100 to 1,000 nm. The layer potentially comprises the fluorine dopant in any concentration but the preferred layer contains from 0.001 to 5% by weight of the fluorine dopant The layer containing the doped SnO2 is polycrystalline 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 temperature of at least 4500C in an environment containing a source of SnCI2 and, either in the same source or a separate source, a fluorine dopant. The source of SnCl2 and dopant is heated to a temperature of less than 480"C but high enough to volatilize the SnCI2 and dopanttoward the support. Preferably the temperature to which the source is heated is from 2000C to 4000C 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), then temperatures up to 4800C are useful.This operation is carried out in an atmosphere where the oxygen content Is at least 15% by volume.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. isa view of a source of SnCl2 and fluorine dopant; Fig. 2 is a view of a support to be coated; and Fig. 3 is a view of apparatus useful in performing the method of this invention.

In Fig. 1 a source of SnCl2 and fluorine dopant A is contained in a holder B. In Fig. 2 a soda-lime glass support D is attached to a holder E.

The method of coating the glass support is preferably chemical vapor deposition in a closespace configuration. Using this method, vapors are evaporated from a source to a support positioned from the source a distance no greater than the square 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 G contains an inlet for oxygen or oxygenenriched air H. The flow of gas is admitted into the enclosure to provide a suitable oxygen-rich atmosphere for the reaction of A and deposition of the tin 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 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 SnCI2 and dopant vapors are preferably removed from the support (generally by the flow of oxygen before heating of the support is terminated).

The atmosphere for the vapor-phase depositing can be either pure oxygen, oxygen artificially admixed with other gases, or air. As will be readily apparent, the actual amount of the oxygen present during deposition will depend upon the specific forrh of vapor-phase depositing that is selected. For example, chemical vapor deposition in a close-spa configuration, a highly preferred form of the process of the invention, is generally carried out at atmospheric pressure. The other forms of vapor-phase depositing mentioned above have known or standard tolerance levels of gas, and the amount of oxygen pressure or partiai pressure is selected to comply with such tolerance levels.

The vapor-phase depositing is done either as a batch process, e.g., in a process chamber containing a single source and a single support, or as a continuous process in which a support is moved through appropriate zones of treatment.

The evaporation is generally carried out at atmospheric pressure or slightly above atmospheric pressure. The spacing between source and support pref erablyis between 2 and 10 mm although distances of between one and 100 mm are useful. The temperature of the support is variable depending on which material is being evaporated. Preferably, the source material is deposited for a time of between 0.1 second to 10 minutes onto a support held at a temperature of between 450 C and about 630"C. The source temperature is maintained in each instance between 200t and 400 C.

A photovoltaic cell is formed simply by using the formed electrode as a window electrode. A preferred cell is similartothose described in U.S. Patent 4,207,119 except that the window electrode is as described above. Thus, the preferred cell comprises first and second contiguous polycrystalline layers containing respectively p-type cadmium telluride and n-type cadmium sulfide and the electrode described above in operative, low-impedancecontact with at least part of said layers. The construction and use of photovoltaic cells is disclosed in detail in U.S. Patent 4,207,119.

The following examples further illustrate the invention.

Example I Seven samples of transparent and electrically conductive tin oxide were prepared using the closespace evaporator described herein. The source was anhydrous SnCl2 doped with one mole percent fluorine added as SnCIF. The support was soda-lime glass with 90% transmittance. The spacing between the source and support was 5 mm. The process was carried out at atmospheric pressure with an oxygen flow of 1220 cm3/min. The support was heated to 550 C; and, immediatelythereafler, the source was heated to 325 C. The deposition time was 1 min, 15 seconds, starting at the time that the source temperature reached 325 C. The average resistance of these seven samples was 12 ohmlsquare area and the total transmittance was 80% of visible light between 400 and 800 nm.The thickness of the films varied between 0.37 and 0.43,am. The films were haze-free.

Example 2 Eight samples of transparent and electrically conductive tin oxide were prepared using the same close-spaced evaporator and essentially the same conditions described in Example 1 except that the dopant was 0.9 mole % fluorine and was added as SnF2to the anhydrous SnCl2. The average of the results of this experiment was a resistance of 14 ohm/square area and 79% total transmittance of visible light between 400 and 800 nm. The thickness of the films varied between 0.36 and 0.52,am and the films were haze-free.

Example 3~Comparative Example 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 no do pants were added to the anhydrous SnCI2 source. The results of this experiment were a resistance of 63 ohm/square area and 80 percent total transmittance of visible light between 400 and 800 nm. The thickness of this film was 0.57 corm. The importance of the fluorine doping 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 oxygen flow rate was 4000 cm5lmin. The results of this experiment were 9 ohm/square area resistance and 80% total transmittance to visible light between 400 and 800 nm. The thickness of this sample was 0.39,am. A solar cell of 9.5% efficiency was made on this glass, as described in U.S. Patent 4,207,119, example 1, with the exception that the window electrode of this example was substituted for the Nesatron (trade mark) window used in U.S. Patent 4,207,119.

Example 5 Six samples of transparent, electricallyconducting, tin oxidecoated glass were prepared as described in Example 1, except that the source temperature was 315 C and the support temperature was changed from sample to sample.

The electrical resistance and average transmittance to visible light and near infra red radiation (400 to 800 nm) of the resulting coatings was: Average % Support Resistance Specular Temperature {ohmisquare Transmittance { C) area) Visible Light 400 2.5 x 10+4 83% 430 360 79% 450 96 78% 500 18 77% 550 10 76% 600 5.5 68% It is seen that support temperatures above 4500C are desirable.

Example 6 Three samples of transparent, electricallyconducting, tin oxide-coated glass were prepared in a close-space evaporator of similar design to that employed in Example 1, and utilizing the same experimental conditions except as noted below: Air was used instead of oxygen, the flow-rate was 410 cma/min; the source temperature was 320"C, deposition time was 1 min; and the spacing between source and support was 2.5 mm.

The average electrical resistance of the three resulting samples was 15 ohm/square area, and the average specular transmittance to visible light and near infra red light (400 to 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 SnCIF (1 mole %). After passing over the source, the stream of gas carried the SnCl,'SnClF vapors over a distance of about five inches, and was then deflected to impinge on a heated soda-lime glass support, thus depositing on the glass a layer of doped tin oxide.

One sample of such tin oxidecoated glass was prepared in the above-described apparatus, while holding the source at4750C, the support at 550 C, utilizing an oxygen flow of 3000 cm3/min, and carrying out the deposition for 30 seconds. The resulting coating had an electrical resistance of 18 ohm/square, a thickness of 0.26 ELm, and 75% average specular transmittance to visible and near IR light (400 to 800 nm).

Claims (17)

1. A conductive element comprising a sodalime glass support having therein a layer containing polycrystalline stannic oxide and a fluorine dopant, and having a total transmittance to radiation of wavelengths between 400 and 800 nm greaterthan 70% and an electrical resistance less than 30 ohmlsquare area.

2. A conductive element according to claim 1, wherein the fluorine dopant is selected from the group consisting of stannous fluoride and stannous fluoro chloride.

3. A conductive element according to claim 1 or 2, wherein the thickness of the layer is from 100 nm to 1,000 nm.

4. A conductive element according to claim 1, 2 or 3, wherein the polycrystalline stannic oxide has (200) and (110) planesthat are oriented parallel to the surface of the glass support.

5. A method of preparing a conductive element comprising heating a sodalime glass support to a temperature of at least 450 C in the presence of a source of stannous chloride and fluorine dopant, the heating step being carried out in an oxygen atmosphere wherein the oxygen content is at least 15% by volume, and wherein the source of stannous chloride and fluorine dopant is heated to a temperature sufficientto evaporate the stannous chloride and fluorine dopant onto the support, but at a temp erature less than 480 C.

6. A method according to claim 5 wherein the source of stannous chloride and fluorine dopant is heated to a temperature between 200 C and 400 C.

7. A method according to claim 5 or 6 wherein the source of stannous chloride is also the source of fluorine dopant.

8. A method according to claim 5, 6 or 7 wherein the heating step is carried out in a glass enclosure.

9. A method according to any one of claims 5 to 8 wherein excess stannous chloride and fluorine dopant vapors are removed from the presence of the support before the heating is terminated.

10. A method according to any one of claims 5 to 9 wherein the fluorine dopant is selected from the group consisting of stannous fluoride and stannous fluoro chloride.

11. A method according to any one of claims 6 to 10 wherein the source of stannous chloride and fluorine dopant is spaced from the glass support at a distance of 2 to 10 mm.

12. A method according to any one of claims 5 to 11 wherein the temperature to which the support is heated is from 4500C to 630 C.

13. A photovoltaic cell comprising contiguous crystalline layers and including the conductive element of any one of claims 1 to 4 in operative, lowimpedance contact with at least part of one of said layers.

14. A photovoltaic cell according to claim 13 wherein the contiguous crystalline layers comprise p-type cadmium telluride and n-type cadmium sulphide.

15. A conductive element substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

16. A method of preparing a photoconductive w element substantially as hereinbefore described with reference to the accompanying drawings.

17. A photovoltaic cell including a conductive element substantially as hereinbefore described with reference to the accompanying drawings.