Patent Application
20110017283
This application claims the priority, under 35 U.S.C. §119, of European application EP 09 009 994.6, filed Jul. 24, 2009; the prior application is here-with incorporated by reference in its entirety.
The present invention relates to a method for depositing a layer of an indium chalcogenide onto a substrate and an apparatus for performing the method.
Indium chalcogenide layers, sometimes also referred to as indium chalcogenide films, may be used in several applications, particularly in the field of semiconductor technology. For the purpose of the present invention indium chalcogenides are indium compounds containing a chalcogen, i.e. elements of group 16 of the periodic table of elements like oxygen, sulphur, selenium etc. For example, indium sulphide, indium selenide or indium oxide and mixtures thereof are such indium chalcogenides. For the purposes of the present invention, the term chalcogenide is also defined as including an oxide material which has been at least partly converted to hydroxide.
In particular, indium chalcogenides have been investigated in recent years in connection with solar cells of the second generation, also referred to as compound thin film solar cells. Such solar cells usually contain a substrate for structurally supporting the solar cell, e.g. a glass substrate, a back contact forming metal layer arranged upon the glass substrate, an absorber layer formed on the metal layer for example by copper indium selenide (CIS), copper indium gallium selenide (CIGS) or copper indium gallium sulphur selenide (CIGSSe), and a buffer layer deposited onto the absorber layer for which cadmium sulphide is widely used. Further on, the buffer layer is covered by a transparent conducting material like zinc oxide. The different layers can be formed by evaporation under vacuum conditions. However, this technique is not well-suited for low cost production at an industrial scale.
This problem has been partly addressed by the invention described in international patent disclosure WO 2009/033674. It provides for a thermal conversion of metallic precursor layers into a semiconducting absorber layer like CIGS at approximately atmospheric pressure. In this way absorber layers can be produced in an industrial scale at low complexity. It even allows an inline production of absorber layers.
However, thin film solar cells also require a buffer layer. As the deposition of cadmium sulphide as a buffer layer is problematic with respect to environmental risks and the prohibition of the usage of cadmium in certain countries, alternative materials have been investigated. Indium chalcogenides like indium sulphide have proven as promising alternatives to cadmium sulphide.
Indium chalcogenide layers may be applied by chemical bath deposition. This method, however, requires a quite long processing time and produces contaminated wastewater which requires costly special treatment. Indium chalcogenide deposition by atomic layer deposition (ALD) or ion layer gas reaction (ILGAR) also requires long processing times. Physical vapour deposition (PVD) and sputtering are performed under vacuum and are consequently quite elaborate and costly. This is also the case for metal organic chemical vapour deposition (MOCVD).
It is accordingly an object of the invention to provide a method and an apparatus for deposition of a layer of an indium chalcogenide onto a substrate which overcome the above-mentioned disadvantages of the prior art methods and devices of this general type, which provides a low-complexity method to deposit an indium chalcogenide layer onto a substrate. Moreover, it is a further object of the invention to provide a solar cell that can be produced at an industrial scale at favourable cost.
The method according to the invention is based on the concept of providing an indium source in a reaction zone, providing a gaseous source of a chalcogen in the reaction zone, and heating the substrate. Thereby in the reaction zone, at a pressure of approximately atmospheric ambient pressure indium originating from the indium source and chalcogen originating from the source of a chalcogen are converted to an indium chalcogenide being deposited onto a surface of a substrate.
For the purpose of the present invention, reaction zone refers to an area, in which conditions like temperature etc. are such that indium originating from an indium source located in the reaction zone and chalcogen originating from a source of chalcogen located in the reaction zone can be converted into indium chalcogenide and such that the indium chalcogenide can be deposited onto the surface of the substrate. In most cases the reaction zone will therefore be formed in the near neighborhood of the substrate and may include the surface of the substrate itself. Furthermore, substrate refers in principle to anything an indium chalcogenide layer can be deposited on. It might for example be a glass substrate with or without any layers deposited or formed before the deposition of the indium chalcogenide.
For the purpose of the present invention approximately atmospheric ambient pressure means that the conversion of indium originating from the indium source and chalcogen originating from the source of a chalcogen to an indium chalcogenide takes place under a pressure having nearly the same pressure value as the atmospheric ambient pressure. The values might slightly differ due to gas flows within or out of the reactor or other effects. But in each case the reactor is not in any way evacuated and the pressure value at the substrate is not more than 50 mbar lower than the atmospheric ambient pressure. However, it might be more than 50 mbar higher.
Heating might be performed in the reaction zone or before the substrates are fed into the reaction zone. In the latter case the substrates have to be heated such that their temperature at the time when they reach the reaction zone is still high enough to enable the described conversion in the reaction zone.
The method according to the invention reduces complexity of indium chalcogenide deposition, as no vacuum is needed and as processing time is short as compared to chemical bath deposition. Furthermore, the method can be applied in inline production lines.
It shall be mentioned that instead of a gaseous source of a chalcogen other sources such as a liquid or solid source of a chalcogen can be provided within the reaction zone such that the liquid or solid source of a chalcogen is converted directly into an indium chalcogenide without being previously transformed into a gaseous phase. Compared to providing, in accordance with the method according to the invention, a gaseous source of a chalcogen within the reaction zone, this is, however, less advantageous.
In a preferred embodiment of the method according to the invention the indium chalcogenide to be deposited is selected from a group consisting of indium sulphide, indium selenide and indium oxide and the gaseous source of a chalcogen correspondingly provides a gaseous source of a chalcogen selected from a group consisting of sulphur, tellurium, selenium and oxygen. These materials have proven themselves. In an especially preferred embodiment, indium sulphide is deposited and the gaseous source of a chalcogen correspondingly provides a gaseous source of sulphur. In each embodiment the gaseous source can be one or more compounds providing the respective chalcogen.
In an advantageous embodiment the substrate is heated to a temperature between 130° C. and 380° C., preferably to a temperature between 230° C. and 330° C.
Preferably, the substrate is moved during the depositing of the indium chalcogenide, as it has turned out that in this way the homogeneity of the deposited indium chalcogenide layer can be improved.
In an advantageous embodiment several indium chalcogenide layers are deposited sequentially. The resulting several sub-layers form together the final layer having the desired thickness. In doing so, properties of the deposited indium chalcogenide layer can be improved. Furthermore, it allows a high throughput in manufacturing of thin film solar cells, for example, particularly in connection with inline production lines.
Using the method according to the invention indium chalcogenide layers can be deposited at significant higher growth rates as it is possible using other techniques like vacuum MOCVD. In a preferred embodiment therefore the indium chalcogenide layer is deposited at a growth rate between 1 and 60 nm/s, especially preferred at a growth rate greater than 5 nm/s.
In a preferred embodiment of the invention the substrate is preheated before entering the reaction zone or even before entering an apparatus for carrying out the method according to the invention. The latter is especially advantageous for an inline production, for example. Preheating might be performed such that on entering the reaction zone the temperature of the substrates is still high enough to enable the described conversion of indium and a chalcogen into an indium chalcogenide, so that no further heating of the substrates needs to be done. Or preheating might be performed such that an additional heating within the reaction zone is necessary. In an embodiment of the invention according to the latter case, such additional heating is performed in order to enable the desired conversion of indium originating from the indium source and the chalcogen originating from the source of a chalcogen to an indium chalcogenide.
In one embodiment of the method according to the invention the indium source is provided in the reaction zone by depositing an indium containing compound, preferably indium oxide, onto the surface of the substrate and arranging the substrate at least partly in the reaction zone. The indium containing compound might be deposited onto the substrate by sputtering or chemical vapor deposition. In particular, the indium containing compound might be itself an indium chalcogenide which has been previously deposited onto the substrate by the method according to the invention. In case of using indium oxide as an indium source, another indium chalcogenide other than indium oxide will be formed by applying the method according to the invention. For example, a gaseous source of sulphur, preferably hydrogen sulphide, might be fed into the reaction zone and in this way be provided therein. In this case indium originating for the indium oxide and sulphur originating from the gaseous source of sulphur would be converted into indium sulphide being deposited onto the surface of the substrate.
In an alternative embodiment of the method according to the invention a gaseous mixture containing a carrier gas, the indium source and the chalcogen source is provided in the reaction zone. Preferably an inert gas is used as carrier gas; especially preferred is the usage of nitrogen or argon gas.
The gaseous mixture is preferably delivered to the substrate to be coated in a laminar flow. In this way the homogeneity of the deposited indium chalcogenide layers can be improved.
Until reaching the reaction zone, the gaseous mixture is advantageously kept at a temperature chosen such that gas phase reaction of the gaseous mixture is sufficiently controlled. Therefore the gaseous mixture is kept at a temperature such that in the gaseous mixture the conversion of indium and of a chalcogen into an indium chalcogenide does not, or at least only to a non-relevant extent, take place until the gaseous mixture reaches the reaction zone. Advantageously, the gaseous mixture is kept at a temperature between 30° C. and 240° C., preferably at a temperature between 40° C. and 210° C., and even more preferably at a temperature between 50° C. and 200° C., until it reaches the reaction zone.
In a preferred embodiment an indium containing a precursor gas is premixed with a chalcogen containing a precursor gas and a carrier gas and the resulting mixture is delivered as gaseous mixture into the reaction zone. Advantageously, a gaseous component already contained in the indium and/or the chalcogen containing precursor gas is used as a carrier gas, so that no further carrier gas has to be added in order to obtain the desired gaseous mixture. For example an inert gas like nitrogen or argon contained in one of the precursor gases can be used as carrier gas.
Advantageously, at least one of the precursor gases contains a stabilizing adduct group. For the purpose of the present invention, a stabilizing adduct group is an adduct group that stabilizes the gaseous mixture such that the conversion of the precursor gases into indium chalcogenide is suppressed until the gaseous mixture reaches the reaction zone. The stabilizing adduct group is preferably based on an element selected from the group consisting of sulphur, oxygen and electron donating nitrogen. Electron donating nitrogen based adduct groups are e.g. amine containing compounds. Moreover, chelating agents can be used as stabilizing adduct groups of the indium containing precursor gas, for example tetramethylethylendiamine. Another example is the usage of 1,2-etanedithiol in or as a chalcogen containing precursor gas, as it simultaneously stabilizes certain indium containing precursor gases like precursor gases containing trimethylindium.
Instead of premixing precursor gases, it is also possible to form the gaseous mixture by mixing an indium containing precursor gas with a chalcogen containing precursor gas and a carrier gas close to or in the reaction zone. Again, a gaseous component contained in the indium and/or the chalcogen containing a precursor gas is preferably used as carrier gas.
According to a preferred embodiment of the invention, in both cases, precursor gases premixed or not, at least one precursor gas is provided via at least one distribution head. Such distribution heads allow a controlled gas flow and are per se known from methods and apparatus of chemical vapour deposition. Preferably, temperature controlled distribution heads are used, which make it possible to improve uniformity of the deposited layer. As already discussed above, the substrate is advantageously moved during the deposition of the indium chalcogenide. Preferably, the substrate is moved relatively to the at least one distribution head or vice versa.
In a preferred embodiment of the method according to the invention, an at atmospheric ambient pressure volatile chalcogen compound, preferably a sulphur compound and especially preferably a hydrogen sulphide or an organosulphur compound, is used as the gaseous chalcogen source. Organosulphur compounds are for example alkyl sulphides. Also an ethanethiol, particularly 1,2-ethanedithiol, may be used as a sulphur source. Particularly, sulphur sources being solid at room temperature might be advantageous against the background of maintenance and handling aspects. Obviously, different gaseous chalcogen sources can be mixed.
In both cases, precursor gases premixed or not, the usage of an indium containing precursor gas containing trimethylindium and nitrogen carrier gas in combination with a chalcogenide containing a precursor gas containing hydrogen sulphide diluted into argon carrier gas has proven itself.
In one embodiment of the method according to the invention the indium chalcogenide layer, preferably an indium sulphide layer, is deposited as a buffer layer of a solar cell.
In a preferred embodiment a metal layer, preferably a molybdenum layer, and subsequently an absorber layer, preferably an absorber layer containing a compound represented by the formula Cu(In,Ga)(S,Se)2—or more precisely Cu(In1-aGaa)(Sb,Se1-b)2, where 0≦a,b≦1—, are deposited onto the substrate before the indium chalcogenide is deposited onto the surface of the substrate. This, for example, allows a fabrication of CIS or CIGS solar cells at industrial scale and favorable cost. In connection with this embodiment the deposition of indium sulphide as indium chalcogenide is especially preferred.
The solar cell according to the invention is based on the concept of forming a buffer layer by chemical vapor deposition of an indium chalcogenide at approximately atmospheric ambient pressure. In this way thin film solar cells of reduced complexity can be provided.
In a preferred embodiment of the solar cell according to the invention indium sulphide is used as indium chalcogenide.
The apparatus according to the invention contains a reactor, into which substrates can be introduced, a gas inlet slot for feeding gases into the reactor, a transport device for moving the substrates through the reactor, and at least one premixing device for premixing an indium containing a precursor gas with a chalcogen containing the precursor gas. A gaseous mixture produced in the premixing device can be fed from the premixing device into the reactor via the gas inlet slot. The transport device can be realized by known transport systems, e.g. push or pull rod systems or a conveyor belt.
In an advantageous embodiment of the apparatus several gas inlet slots are provided, which are arranged in a distance from each other in a transport direction of the transport device. Preferably, the gas inlet slots are arranged such that with each gas inlet slot a further sublayer can be deposited. In this way several sublayers can comfortably be deposited on a substrate passing the several gas inlet slots.
Each gas inlet slot might be connected to the same premixing device or to a separate premixing device.
Advantageously, a laminar flow of the gaseous mixture from the gas inlet slots to the substrates can be realized.
In a preferred embodiment, the apparatus contains at least one temperature control device for controlling the temperature of the gaseous mixture in at least one premixing device or at least one gas inlet slot. The at least one control unit might be realized as an open-loop control device or as a closed-loop device, the latter one being connected to heating or cooling devices for the gaseous mixture or materials fed into the at least one premixing device.
In a preferred embodiment at least one coater head is provided as a gas inlet slot. As mentioned above, such coater heads are known from chemical vapor deposition apparatus. Preferably, the at least one coater head is a temperature controlled coater head.
In yet another preferred embodiment the apparatus contains a heating device for heating the substrates. In this way, substrates can be preheated outside of the apparatus and additionally heated within the apparatus to a temperature required in the reaction zone. Also it is possible to heat the substrates to the required temperature within the apparatus without any previous heating of the substrates. If instead the apparatus does not contain any heating device, the substrates have to be heated before entering the apparatus in such that their temperature on entering the reaction zone is still high enough to enable the conversion of indium originating from the indium source and the chalcogen originating from the source of a chalcogen to an indium chalcogenide.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and an apparatus for deposition of a layer of an indium chalcogenide onto a substrate, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
The invention will be explained in more detail below with reference to figures. Elements exercising essentially similar effects are, as far as this appears appropriate, marked with equal reference signs.
Referring now to the figures of the drawing in detail and first, particularly, to
In the embodiments illustrated in
Further on, the substrate 1 carrying the indium sulphide sub-layer 18a is transported into a second reaction zone 18b. All transport of the substrates 1 is preferably done continuously. This means that the substrates 1 are continuously moved relatively to the reactor 70.
Via gas inlets 75c and 75d again a mixture 77b of trimethylindium and the carrier gas nitrogen and a mixture 79b of hydrogen sulphide and argon gas as the carrier gas are fed into the second reaction zone 18b. Therefore, by analogy with the indium sulphide sublayer 17a in the first reaction zone 18a, a further indium sulphide sublayer 17b is deposited onto the previously deposited indium sulphide sublayer 17a. Both indium sulphide sublayers 17a, 17b form together the final indium sulphide layer. Such a sequential deposition of indium sulphide sublayers makes it possible to produce indium sulphide layers of enhanced homogeneity. Furthermore, a thickness of the indium sulphide layer can be quite easily controlled. In principle, any number of indium sulphide sublayers can be deposited by elongating the reactor 70 and providing additional gas inlets.
Under atmospheric pressure trimethylindium is solid at room temperature. In order to provide the indium containing a precursor gas, a trimethylindium source is therefore preferably heated to a temperature between 25° C. and 75° C., so that the material partly sublimes. Further on, nitrogen is preferably provided as the carrier gas for trimethylindium vapor. Flowing above the trimethylindium source it carries away trimethylindium vapor and delivers it into the premixing devices. In this way growth rates of the indium sulphide layer can be improved.
In the embodiment of
By providing one premixing device 82 for several, in this embodiment two, distribution heads 91a, 91b or gas inlets in general, complexity of the apparatus can be reduced. Apparently, the number of premixing devices and the number of gas inlets or distribution heads each premixing device supplies with the gaseous mixture can be adapted to individual needs.
Distribution heads, and particularly temperature controlled distribution heads, can obviously also be provided instead of gas inlets in the embodiments of
All the apparatus depicted in
In each of the embodiments of the apparatus depicted in
The solar cell 49 can be manufactured using one of the methods and apparatus illustrated in
| Number | Date | Country | Kind |
|---|---|---|---|
| 09 009 994.6 | Jul 2009 | EP | regional |
