Title: Method and apparatus for applying an active substance to a substrate
The invention relates to a method and an apparatus for depositing a layer of an active substance on a substrate, wherein the active substance is chosen from the group of substances consisting of an alkali metal, an alkali earth metal, a lanthanide, or an alloy or intermetallic mixture of one or more of the said alkali metal, alkali earth metal or lanthanide elements. Applying such layers is for instance carried out in the manufacture of OLEDs and the layers particularly serve to increase the life of the OLED and to reduce the work function of the cathode so that an efficient electron injection at lower voltages is obtained. More information about this is for instance described in American patent US-A-6,255,774 and the Journal of Applied Physics, Volume 88, Number 6, of September 15, 2000, pp. 3618- 3623 whose contents are incorporated herein by reference. However, the problem of at least a number of the substances mentioned is that they are very reactive in that they spontaneously react with the atmosphere. Therefore applying such a layer is a problem in practice. The invention contemplates providing a solution to this problem. For this purpose, the invention provides a method for depositing a layer of an active substance on a substrate, wherein the active substance is chosen from the group of substances consisting of an alkali metal, an alkali earth metal, a lanthanide, or an alloy or intermetallic mixture of one or more of the said alkali metal, alkali earth metal or lanthanide elements, which method is, according to the invention, characterized in that the active substance is bound in a compound which is stable at room temperature, wherein the compound is incorporated in a wire, which wire forms a connection between two electrodes, wherein, for releasing the active
substance, a current is fed through the wire, so that it heats up to a temperature at which the compound decomposes and the active substance evaporates and then precipitates on the substrate. Because the active substance is incorporated in a compound which is stable at room temperature, the active substance is easy to handle. In this context, the term room temperature is to be understood in a relatively broad sense, in the sense that it is to be understood to mean temperatures at which the handling, transporting, mounting and other manipulating of the wire normally take place. This should be understood to involve temperatures of less than approx. 50 degrees Celsius, more particularly temperatures in the range of 10-30 degrees Celsius. Placing the active substance in a process chamber, the transport and similar actions are therefore no problem anymore. Because the active substance of the compound is incorporated in a wire, the compound can be heated and cool down very quickly by feeding a strong electric current through at least a part of the respective wire during a certain desired interval. As a result of the temperature increase, the compound will decompose and the active substance will evaporate. The active substance released in a vapor state will then precipitate on the substrate on which the layer with the desired active substance is to be deposited. Due to the fact that the compound containing active substance is provided in the shape of a wire, the time interval during which the active substance is released can be controlled very accurately. In this manner, the thickness of the layer which is deposited on the substrate can thus be controlled very accurately. Due to the quick heating which can be realized with the wire, a relatively large amount of active substance can be released in a relatively short time. This quick evaporation has the advantage that less stringent requirements need to be set with regard to the background pressure; for instance, a background pressure of 106 mbar instead of 10 9 mbar can be used.
The invention further provides a method for manufacturing an OLED by using the above-described method. According to a further elaboration of this method, the active substance which is deposited on the substrate is a hole injection layer which is part of an OLED, wherein the method further comprises applying an anode, a light -emitting organic material and a cathode, wherein the light-emitting organic material is located between the anode and the cathode. Up to date, in the manufacture of OLEDs, applying the hole injection layer, which layer serves to reduce the working current of the OLED, has been a problem because the active substances suitable for this purpose very easily react with oxygen and water. Such reaction need to be minimized prior to and during the application and for this purpose the OLED manufacturing method with the associated advantages as described hereinabove is now provided according to the present invention. Preferably, the active substance which is deposited on the substrate is applied immediately before or immediately after application of a cathode-forming layer. Thus, optimal use is made of the working current-reducing properties of the hole injection layer. According to a further elaboration of the invention, it is particularly favorable if the active substance is barium and if the compound is a barium aluminum compound. Particularly in the manufacture of OLEDs, it has been found that barium has excellent work function-reducing properties and besides has a favorable influence on the life of the OLED. According to a further elaboration of the invention, optionally the time interval during which the wire is excited by current can be made to depend on the desired layer thickness of the active substance. For instance, after a first substrate, the layer thickness obtained can be measured and then it can be determined whether the time interval of excitement of the wire with electric current for heating thereof which is used for that substrate has yielded the desired result. If the layer thickness is found to be
too small, the time interval can be increased and/or the current intensity can be increased. If the layer thickness is found to be too large, the time interval can be shortened and/or the current intensity can be reduced. However, according to a further elaboration of the invention, it is also possible that the layer thickness of the active substance is measured during the application process, while the measurement data are used for determining the moment when the feeding of the current through the wire is stopped. With such a course of action, no substrate at all is lost in experimentation and the desired result is obtained immediately. According to a further elaboration of the method according to the invention, the wire can be provided by filling a casing from a carrier material, such as for instance steel, with the above-mentioned compound, while, in longitudinal direction, the casing is provided with at least one opening extending in longitudinal direction, through which the active substance can leave the casing during evaporation of this active substance. Because at least one opening is oriented in a particular direction, the active substance will particularly be released in that direction defined by the opening. The advantage thereof is that the substrate can be evaporated in any orientation. By providing the wire with multiple openings oriented in different directions, active substance can be released in different directions. As indicated hereinabove, the method can be used for manufacturing an OLED. However, according to a further elaboration of the invention, it is also possible to use the method for manufacturing an organic solar cell and other organic electronics. The invention further provides an apparatus for carrying out the method according to the invention, wherein the apparatus comprises a process chamber arranged for providing a conditioned environment therein, wherein, in the process chamber, two electrodes are provided between which a wire extends, which wire comprises a compound which is stable at room temperature of an active substance and at least one stabilizing substance,
wherein the active substance is chosen from the group of substances consisting of an alkali metal, an alkali earth metal, a lanthanide, or an alloy or intermetallic mixture of one or more of the said alkali metal, alkali earth metal or lanthanide elements, wherein the electrodes are connectable with a power source. With such an apparatus, the above-described method can be carried out and the associated advantages can be realized in a simple manner. Further elaborations are described subclaims and will hereinafter be explained in more detail on the basis of an exemplary embodiment, with reference to the drawing, in which: Fig. 1 shows a schematic cross-sectional view of an exemplary embodiment of an apparatus according to the invention; and Fig. 2 shows a perspective view of a part of the wire. Fig. 1 schematically shows a process chamber 1 in which a substrate 2 is present which is supported by a substrate carrier 3. The substrate is for instance a substrate for manufacturing an OLED. Further, in the process chamber, a wire 4 is present which extends between two electrodes 5, 6. The electrodes 5, 6 are connected to a power source 7 which is in connection with a control 8. Further, in the process chamber 1, a feed reel 9 from which the wire 4 is fed and a take-up reel 10 on which the wire 4 is taken up are accommodated. Fig. 2 shows the wire 4 in a perspective schematic view. The wire 4 is provided with a casing 11 from steel which is bent in a cu -shaped configuration and is provided with an opening 12 extending in longitudinal direction. In the casing, the compound 13 containing active substance is present. As already indicated hereinabove, the wire may also be provided with openings oriented in different directions, so that it is possible to evaporate in multiple directions. In the present exemplary embodiment, the compound 13 is formed by BaAl4; the active substance is barium. Other active substances are also possible; options to be considered are substances
chosen from the group consisting of an alkali metal (Li, Na, K, Rb or Cs), an alkali earth metal (Mg, Ca, Sr or Ba), a lanthanide (Sm, Eu, Tb or Yb), or an alloy or intermetallic mixture of one or more of the said alkali metal, alkali earth metal or lanthanide elements (such as for instance Mg:Al, Mg:In, Mg:Ag, Al:Li, BaAl4 or CaAl2). When the wire 4 is heated as a result of a large amount of electric current being fed through it, the compound 13 will decompose and the active substance from the compound 13 will evaporate. As soon as no current is fed through the wire 4 anymore, as a result of the small heat content of the wire 4, the temperature of the wire 4 will fall quickly and the evaporation of the active substance will stop. With the aid of the control 8, the current intensity fed through the wire 4 can be controlled. In addition, the time interval during which current is fed through the wire 4 can be controlled. Since, after a certain lapse of time, the active substance from the compound 13 from the part of the wire 4 which extends between the electrodes 5, 6 has evaporated completely, a new piece of wire 4 needs to be stretched between the two electrodes 5, 6. For this purpose, in the present exemplary embodiment, the apparatus is provided with the feed reel 9 and the take-up reel 10. The take-up reel 10 is provided with a drive which is controllable with the aid of the control 8. By switching on the drive, a part of the wire 4 can be taken up on take-up reel 10, and further a new part of wire will automatically be fed from feed reel 9. Optionally, the drive of the take-up reel 10 can be switched on continuously, so that each time an new wire part containing compound is provided between the electrodes 5, 6. Fig. 1 further shows a measuring provision 14 which is connected to the control 8 for measuring the thickness of the layer of the active substance which is applied to the substrate 2. With the aid of the data obtained by the measuring device 14, the control 8 can control the current intensity of the source 7 and/or the time interval during which the current is fed through the wire 4. It will be clear that the measuring device can also be
arranged outside the process chamber 1 and for instance carries out measurements of the substrate 2 through an inspection window. It will be clear that the invention is not limited to the exemplary embodiment described but that various modifications are possible within the framework of the invention as defined by the claims.