METHOD TO DEPOSIT THIN FILM HIGH QUALITY ABSORBER LAYER

Abstract

The present invention proposes a method to form a CdSeTe thin film with a defined amount of selenium and with a high quality. The method comprises the steps of providing a base substrate and of depositing a partial CdSeTe layer on a first portion of the base substrate. The step of depositing a partial CdSeTe layer is performed at least twice, wherein a predetermined time period without deposition of a partial CdSeTe layer on the first portion of the base substrate is provided between two subsequent steps of depositing a partial CdSeTe layer. The temperature of the base substrate and the CdSeTe layer already deposited on the first portion of the base substrate is controlled during the predetermined time period such that re-evaporation of Cd and/or Te from the CdSeTe layer already deposited takes place.

Claims

1. Method for forming a CdSeTe thin film comprising the steps: a) providing a base substrate (50, 50a-50e), and b) depositing a partial CdSeTe layer on a first portion of the base substrate (50, 50a-50e), wherein the first portion is held at a first temperature (T.sub.1) during deposition, characterized in that step b) is performed at least twice, wherein a predetermined time period (t.sub.re1-t.sub.re3) without deposition of a partial CdSeTe layer on the first portion of the base substrate (50, 50a-50e) is provided between two subsequent steps b) and wherein the temperature of the base substrate (50, 50a-50e) and the CdSeTe layer already deposited on the first portion of the base substrate (50, 50a-50e) is controlled during the predetermined time period (t.sub.re1-t.sub.re3) such that re-evaporation of Cd and/or Te from the CdSeTe layer already deposited takes place.

2. Method according to claim 1, characterized in that the first portion of the base substrate (50, 50a-50e) and the CdSeTe layer already deposited are held at a second temperature (T.sub.2) during the predetermined time period (t.sub.re1-t.sub.re3) for at least a part of the predetermined time period (t.sub.re1-t.sub.re3), the second temperature (T.sub.2) lying in the range between 400° C. and 570° C.

3. Method according to claim 2, characterized in that the second temperature (T.sub.2) lies in the range between the first temperature (T.sub.1) and a temperature being 50 K smaller or 50 K higher than the first temperature (T1).

4. Method according to anyone of claim 1, characterized in that step b) is performed more than twice.

5. Method according to claim 4, characterized in that step b) is performed more than ten times.

6. Method according to claim 1, characterized in that the thickness of one partial CdSeTe layer deposited in step b) is smaller than half of the whole thickness of the CdSeTe thin film or smaller than 150 nm.

7. Method according to claim 1, characterized in that the predetermined time period (t.sub.re1-t.sub.re3) between two subsequent steps b) is larger than 2 seconds.

8. Method according to claim 7, characterized in that the predetermined time period (t.sub.re1-t.sub.re3) between two subsequent steps b) is smaller than 10 seconds.

9. Method according to claim 1, characterized in that each partial CdSeTe layer is deposited by evaporation or sublimation or sputtering the respective elements from one source (20a-20c) or a group of sources and in that the base substrate (50a-50e) is moved linearly along a first direction over or below different sources (20a-20c) or different groups of sources during performing the steps b), wherein the different sources (20a-20c) or different groups of sources are arranged separated from each other along the first direction with a predetermined distance (d.sub.ab, d.sub.bc) between them.

10. Method according to claim 1, characterized in that each partial CdSeTe layer is deposited by evaporation or sublimation or sputtering the respective elements from one source (20) or a group of sources and in that the base substrate (50) is moved over or below the source (20) or the group of sources during performing a first step b), moved away from the source (20) or the group of sources during the predetermined time period (t.sub.re1-t.sub.re3) and moved over or below the same source (20) or the same group of sources during performing a second step b) performed subsequently to the first step b).

11. Method according to claim 10, characterized in that the base substrate (50) is moved linearly over or below the source (20) or the group of sources along a first direction during performing the first step b) and along a second direction opposite to the first direction during performing the second step b).

Description

FIGURES

[0019] FIG. 1 schematically shows an embodiment of the method according to the invention.

[0020] FIG. 2 schematically shows an exemplary embodiment of an apparatus suitable for performing a first embodiment of the method according to the invention.

[0021] FIG. 3 schematically shows an exemplary embodiment of an apparatus suitable for performing a second embodiment of the method according to the invention.

[0022] FIG. 4 schematically shows an exemplary embodiment of a course of a temperature of the base substrate over time for performing an embodiment of the method according to the invention.

[0023] FIG. 5 schematically shows the EQE of solar cells comprising different CdSeTe thin films formed according to the invention.

EXEMPLARY EMBODIMENTS

[0024] FIG. 1 schematically shows an embodiment of the method according to the invention. First of all, a base substrate is provided (step S1). The base substrate may comprise different layers and materials along a thickness direction of the base substrate and/or along a direction perpendicular to the thickness direction of the base substrate, i.e. in a lateral direction. The base substrate may be, for instance, a semi-finished CdTe solar cell device, wherein different layers according to a used production configuration are present. That is, if the solar cell device is formed in superstrate configuration, the base substrate usually comprises a glass substrate, a transparent conducting oxide layer (TCO) as a front contact and a layer of cadmium sulfide (CdS) as a window layer on top of each other along the thickness direction of the base substrate. The TCO layer can include a high resistive buffer layer. If the solar cell device is formed in substrate configuration, the base substrate usually comprises a backside substrate, e.g. of glass, on which a backside contact layer is deposited.

[0025] On the top layer of the base substrate, i.e. the window layer or the backside contact layer, a first partial CdSeTe layer is deposited at least in a first portion of the base substrate in step S21. Although not shown in FIG. 1, any preparation steps, e.g. a cleaning step or a heating step, may be performed between steps S1 and S21. The first portion is a lateral portion of the base substrate and extends over at least a part of a surface of the base substrate, the surface extending in a plane perpendicular to the thickness direction of the base substrate. Usually, the whole surface of the base substrate is applied with the deposited CdSeTe layer, except of edge portions where the base substrate is held during deposition. At least the first portion of the base substrate is held at a first temperature during the deposition step.

[0026] After deposition, the base substrate with the deposited partial CdSeTe layer is removed for a predetermined time period from the deposition position (step S31). That is, no deposition on the base substrate is performed during the predetermined time period. However, the temperature of the base substrate, and the first partial CdSeTe layer, is controlled at least in the first portion of the base substrate during the predetermined time period such that re-evaporation of Cd and/or Te from the first partial CdSeTe layer takes place in the first portion. By selecting a specific temperature and a specific length of the predetermined time period, the ratio of selenium within the first partial CdSeTe layer can be adjusted with respect to the ratio of selenium as deposited.

[0027] Subsequent to the predetermined time period without deposition, a second partial CdSeTe layer is deposited on the first CdSeTe layer (step S22). This step is in principal equal to step S21 and is followed by another predetermined time period without deposition.

[0028] As can be seen in FIG. 1, deposition steps and predetermined time periods without deposition are performed alternating, wherein the whole method comprises n deposition steps and n predetermined time periods without deposition. That is, a last deposition step S2n causing the deposition of a n.sup.th partial CdSeTe layer on the previous partial CdSeTe layer, i.e. the (n−1). partial CdSeTe layer, is performed after a directly preceding predetermined time period without deposition and is followed by a last predetermined time period without deposition (step S3n).

[0029] In the result, a CdSeTe layer is formed by depositing n partial CdSeTe layers wherein the ratio of selenium within the CdSeTe layer is controlled and adjusted by re-evaporation of Cd and/or Te from the respective partial CdSeTe layer during predetermined time periods without deposition. The amount n is equal or larger than 2 and may, for instance, lie in the range between 2 and 20 (including the edges). Afterwards, the base substrate is usually further processed to fabricate a solar cell device (step S4).

[0030] Depending on the temperature needed for re-evaporation of Cd and/or Te from the deposited CdSeTe layer, the last predetermined time period without deposition may also be a normal handling period used for transporting the base substrate to a next processing step.

[0031] FIG. 2 schematically shows an exemplary embodiment of an apparatus 100 suitable for performing a first embodiment of the method according to the invention. The first embodiment of the method is characterized by a linear movement of the base substrate along a first direction over or below different sources, each source providing the elements for the deposition of the partial CdSeTe layers. “Linear movement” does not necessarily mean movement along a straight line, but means that the base substrate is moved forward all the time from one source to the next source. That is, the line might be a straight line, a curved line or even a line forming a turn. In the result, “the first direction” is not understood as a concrete direction in the x-y-z-coordinate system, but as a direction connecting one source with the next source in line. In other words: The first embodiment of the method is an in-line method and the apparatus 100 is an in-line apparatus having an inlet 101 for introducing a base substrate at a first side of the apparatus 100 and an outlet 102 for removing a base substrate at a second side of the apparatus 100, wherein the base substrate is moved forward along the first direction from the inlet 101 to the outlet 102 without any backward movement. Often, the second side of the apparatus 100 is opposite to the first side of the apparatus 100, but there might be other configurations.

[0032] The apparatus 100 comprises three sources 20a to 20c, each source providing all of the elements for depositing a CdSeTe layer. The sources 20a to 20c may be sputter targets or evaporation or sublimation crucibles. Each source 20a to 20c may provide the elements cadmium, selenium and tellurium from one single target or crucible or may be a group of different targets or crucibles providing one or two of the mentioned elements or even providing further elements, for instance doping elements. The number of sources 20a to 20c equals the number of deposition steps of partial CdSeTe layers and is not limited to three as shown in FIG. 2. It may reach from 2 to n, for instance to 20. According to the embodiment shown in FIG. 2, all sources 20a to 20c are arranged in a common chamber 10, but they may also be arranged in different chambers. The apparatus 100 further comprises temperature control devices 30, 31a to 31c and 32a to 32c and a transport system 40 suitable for moving base substrates 50a to 50e through the chamber along the first direction connecting the inlet 101 with the outlet 102. The movement of the base substrates 50a to 50e is indicated by the solid arrows. The substrates 50a to 50e may be moved at a steady pace, i.e. constantly, or may be moved with different velocities and may even pause for a while. The transport system 40 may comprise rollers or shafts or belts or any other suitable device for holding and moving the base substrates 50a to 50e through the apparatus 100.

[0033] While the base substrates 50a to 50e are moved through the apparatus 100, they subsequently pass the sources 20a to 20c, wherein each time a partial CdSeTe layer is deposited on the base substrates 50a to 50e. Although a bottom-up deposition is shown in FIG. 2, the deposition of the partial CdSeTe layers may also be performed in a top-down deposition in other embodiments. The sources 20a to 20c are arranged with a distance between adjacent sources, i.e. the source 20b is arranged from the source 20a with a distance d.sub.ab and the source 20c is arranged from the source 20b with a distance d.sub.bc. The distances are measured along the first direction. Due to these distances, each base substrate 50a to 50e moves along the first direction over predetermined time periods without depositing CdSeTe onto the base substrate. In these time periods, in addition to the crystallization process, the re-evaporation of Cd and/or Te from the deposited CdSeTe layer occurs. The distances d.sub.ab and d.sub.bc between the different sources 20a to 20b may be equal or different, depending on the desired amount of re-evaporation of Cd and/or Te during the respective predetermined time periods.

[0034] The temperature control devices 31a to 31c control the temperature of the base substrates 50a to 50e at locations where the sources 20a to 20c are arranged, whereas the temperature control devices 32a to 32c control the temperature of the base substrates 50a to 50e at locations between the sources 20a to 20c and after the last source 20c with respect to the first direction. Thus the temperature control devices 31a to 31c hold the base substrates 50a to 50e at the first temperature during deposition of the partial CdSeTe layers, and the temperature control devices 32a to 32c hold the base substrates 50a to 50e at the second temperature for at least a part of the predetermined time periods without deposition. The first temperature is chosen such that a partial CdSeTe layer with desired characteristics, e.g. grain sizes and density, is formed on the first portion of the base substrate. The second temperature is chosen such that a desired amount of Cd and/or Te re-evaporates from the deposited partial CdSeTe layer. The temperature control device 30 is a device which helps to bring the base substrates 50a to 50e form a starting temperature the base substrates have when entering the apparatus 100 to a temperature near the first temperature. However, the temperature control device 30 may also be omitted, for instance, if the starting temperature is already near the first temperature. Since the first temperature and the second temperature are usually in the range between 300° C. and 700° C., the temperature control devices 30, 31a to 31c and 32a to 32c often comprise a heater. However, they may comprise a cooling device instead or additionally.

[0035] The advantage of the first embodiment of the method according to the invention is that a plurality of base substrates may be processed simultaneously in one apparatus. Furthermore, different amounts of cadmium, selenium and tellurium or of further elements may be provided in different partial CdSeTe layers in a simple manner by providing differing sources or differing deposition conditions or differing second temperatures during the predetermined time periods. The length of the different predetermined time periods may be adjusted primarily by different distances between different adjacent sources, since usually the velocity of the movement of the base substrates is constant throughout the whole apparatus.

[0036] FIG. 3 schematically shows an exemplary embodiment of an apparatus 200 suitable for performing a second embodiment of the method according to the invention. The second embodiment of the method is characterized by moving a base substrate 50 over or below one and the same source 20 at least twice. That is, only one source 20 is arranged in a chamber 10 of the apparatus 200 and the base substrate 50 is moved over or below the source 20 during a first step of depositing a partial CdSeTe layer, moved and may be held away from the source 20 during the predetermined time period and again moved over or below the same source 20 during a second step of depositing a partial CdSeTe layer. In the exemplary embodiment of FIG. 3, the base substrate 50 is moved linearly along a first direction over the source 20 during the first deposition step, for instance away from an opening 103 in the chamber 10, held on one side of the source 20, e.g. the right side, and away from the source 20 during a first predetermined time period, and moved linearly along a second direction over the source 20 during the second deposition step, for instance to the opening 103. That is, the first and the second direction are opposite to each other, as indicated by the solid arrows in FIG. 3. The for- and backward moving of the base substrate 50 within the chamber 10 may be repeated several times, wherein the velocity of movement may differ for different deposition steps. Furthermore, the length of different predetermined time periods without deposition may be adjusted easily, as well as the number of deposition steps. In the shown embodiment of the apparatus 200, only even numbers of deposition steps can be performed, since only one opening 103 is provided in the chamber 10 for entering and removing the base substrate 50. If two openings 103 on opposite sides of the chamber 10 are provided, also an odd number of deposition steps, the number being larger than 2, can be performed.

[0037] The temperature of the base substrate 50 as well as of the CdSeTe layer already deposited on the base substrate 50 is controlled by a temperature control device 31 during deposition steps and by temperature control devices 32a and 32b during the predetermined time periods without deposition.

[0038] In other embodiments of the apparatus suitable for performing the second embodiment of the method according to the invention, the base substrate may be arranged on a rotating holder, wherein the rotation axis of the rotating holder is arranged on a side of the source. Thus, the base substrate is moved over or below the source for a first time period and then is moved over a region without a source for the predetermined time period while the rotating holder rotates.

[0039] In other embodiments of the apparatus suitable for performing the method according to the invention, shutters may be used for interrupting the deposition of the CdSeTe layer and dividing it into a plurality of steps of depositing a partial CdSeTe layer, wherein the predetermined time periods without deposition are the time periods the shutter is placed between the source and the base substrate.

[0040] As can be seen, a person skilled in the art knows may derive a lot of embodiments of the method according to the invention and a lot of apparatuses suitable for performing the method according to the invention.

[0041] With respect to FIG. 4, an exemplary embodiment of a course of a temperature of the base substrate over time is explained. When the method starts at to, the base substrate has a starting temperature T.sub.0, for instance 100° C. Before the first deposition step begins at t.sub.1, the temperature is increased by heaters from T.sub.0 to the first temperature T.sub.1, for instance 450° C. Deposition of a first partial CdSeTe layer takes place between t.sub.1 and t.sub.2, called a first time period of deposition t.sub.dep1. During t.sub.dep1, the base substrate is, for instance, moved over or below a first source, and the temperature of the base substrate is held at the first temperature T.sub.1. When the deposition of the first partial CdSeTe layer is finished at t.sub.2, a first predetermined time period without deposition t.sub.re1 starts, the first predetermined time period t.sub.re1 lasting to t.sub.3. During t.sub.re1, the base substrate is held at the second temperature T.sub.2 for a part of t.sub.re1. The second temperature T.sub.2 is larger than T.sub.1, for instance 500° C., allowing the re-evaporation of Cd and/or Te from the first partial CdSeTe layer. At t.sub.3, the base substrate again has the first temperature T.sub.1 and a second time period of deposition t.sub.dep2 starts, wherein the base substrate is moved over or below a second source, and the temperature of the base substrate is held at the first temperature T.sub.1. At the end of t.sub.dep2, i.e. at t.sub.4, a second partial CdSeTe layer on the base substrate and the CdSeTe layer resulting from the first deposition process and the re-evaporation during the first predetermined time period t.sub.re1 is deposited. A second predetermined time period without deposition t.sub.re2 starts at t.sub.4 and ends at t.sub.5. For at least a part of t.sub.re2, the base substrate is held again at the second temperature T.sub.2 and Cd and/or Te is partially re-evaporated from the second partial CdSeTe layer. A t.sub.5, a third time period of deposition t.sub.dep3 starts, during which the base substrate again is held at the first temperature T.sub.1. When t.sub.dep3 ends at t.sub.6, a third partial CdSeTe layer is deposited on the base substrate and the CdSeTe layer formed before.

[0042] Subsequently, a third predetermined time period without deposition t.sub.re3 follows, wherein the base substrate is held at T.sub.2 for at least a part of t.sub.re3. The third predetermined time period without deposition t.sub.re3, during which re-evaporation of Cd and/or Te from the third partial CdSeTe layer occurs, starts at t.sub.6 and ends at t.sub.7, when the temperature of the base substrate reaches the first temperature T.sub.1. After t.sub.7, the temperature of the base substrate further decreases till it reaches a temperature of the next processing step or a handling step performed with respect to the base substrate and the CdSeTe thin film formed thereon.

[0043] Although a second temperature T.sub.2 higher than the first temperature T.sub.1 is shown in FIG. 4, the second temperature T.sub.2 may be equal to the first temperature T.sub.1 or even lower than the first temperature T.sub.1 as long as partial re-evaporation of Cd and/or Te occurs from the respective deposited CdSeTe layer. Furthermore, the starting temperature T.sub.0 may be lower or higher than the mentioned temperature value and may even be higher than the first temperature T.sub.1. Similarly, the temperature of the base substrate may not decrease as much as it is shown in FIG. 4 or even not decrease at all after the third predetermined time period without deposition t.sub.re3 ends, as long as the temperature of the base substrate does not reach a temperature resulting in an undesired change of the composition or structure of the CdSeTe layer or in re-evaporation of the whole CdSeTe layer. Further, all temperature rises and decreases are shown as linear processes. However, the temperature may increase or fall also non-linearly, wherein the decrease of the temperature of the base substrate may be effected actively, i.e. by cooling of the base substrate, or passively, i.e. only by removing a heat source. Moreover, the temperature of the substrate between two predetermined time periods, i.e. the temperature during a time period of deposition, might not be constant. It might be linearly increasing or decreasing or varying in any other way. In such case the variation in the temperature can be in the range of the T.sub.1±10° C., or more preferable T.sub.1±5° C.

[0044] Further, the individual time periods of deposition t.sub.dep1 to t.sub.dep3 may have the same length or different lengths. The same is true for the individual predetermined time periods without deposition t.sub.re1 to t.sub.re3. Moreover, the second temperature T.sub.2 may differ for the different predetermined time periods without deposition t.sub.re1 to t.sub.re3. Even the first temperature T.sub.1 may differ for the different time periods of deposition t.sub.dep1 to t.sub.dep3.

[0045] FIG. 5 schematically shows the EQE (external quantum efficiency) of fabricated solar cells comprising different CdSeTe thin films. Some of the CdSeTe thin films are formed using the method according to the invention, wherein different numbers of steps for depositing a partial CdSeTe layer and of predetermined time periods without deposition are performed. The graph characterized by only one deposition step is measured for a solar cell fabricated using a conventional method of forming a CdSeTe thin film, i.e. the CdSeTe thin film is deposited as a whole without predetermined time periods of re-evaporation. The measured solar cells are all formed with the same materials and thicknesses for the individual layers of the solar cell. Also the formed CdSeTe thin film has the same thickness and is formed using the same sources, i.e. a CdSeTe crucible for sublimation, for all solar cells.

[0046] As can be seen, as the number of deposition steps and of predetermined time periods without deposition and with re-evaporation of Cd and/or Te increases, the EQE increases in the range of 700 nm to 800 nm. This means that an enhanced charge carrier collection in this range of wavelengths occurs, which is related to a high quality of the formed CdSeTe thin film with reduced recombination of charge carriers, i.e. a longer charge carrier lifetime. Furthermore, the absorption band edge is shifted to higher wavelength as the number of deposition steps increases. This indicates that the band gap is reduced in the formed CdSeTe thin film and more light can be converted into electrical energy by the solar cell.

[0047] The embodiments of the invention described in the foregoing description are examples given by way of illustration and the invention is nowise limited thereto. Any modification, variation and equivalent arrangement as well as combinations of embodiments should be considered as being included within the scope of the invention.

REFERENCE NUMERALS

[0048] 100, 200 Apparatus [0049] 10 Chamber [0050] 101 Inlet [0051] 102 Outlet [0052] 103 Opening [0053] 20, 20a-20c Deposition source [0054] 30, 31, 31a-31c, Temperature control device [0055] 32a-32c [0056] 40 Transportation system [0057] 50, 50a-50e Base substrate [0058] d.sub.ab, d.sub.bc Distance between sources [0059] T.sub.0 Starting temperature [0060] T.sub.1 First temperature [0061] T.sub.2 Second temperature [0062] t.sub.0-t.sub.7 Instant of time [0063] t.sub.dep1-t.sub.dep3 Time period of deposition [0064] t.sub.re1-t.sub.re3 Time period without deposition