METHOD FOR PRODUCING A DOUBLE GRADED CDSETE THIN FILM STRUCTURE

Abstract

The present invention proposes a method to form a double-graded CdSeTe thin film. The method comprises providing a base substrate, forming a first CdSe.sub.wTe.sub.1-w layer having a first amount w1 of selenium in it, forming a second CdSe.sub.wTe.sub.1-w layer having a second amount w2 of selenium in it and forming a third CdSe.sub.wTe.sub.1-w layer having a third amount w3 of selenium in it. The second amount w2 lies in the range between 0.25 and 0.4, whereas each of the amounts w1 and w3 lies in the range extending from 0 to 1. According to the present invention, the energy gap in the first and the third CdSe.sub.wTe.sub.1-w layers is equal to or higher than 1.45 eV and the energy gap in the second CdSe.sub.wTe.sub.1-w layer lies in the range between 1.38 eV and 1.45 eV and is smaller than the energy gap in the first and the third CdSe.sub.wTe.sub.1-w layers.

Claims

1. Method for forming a double-graded CdSeTe thin film comprising the steps: a) providing a base substrate, b) forming a first CdSe.sub.wTe.sub.1-w layer having a first thickness d1 and a first amount w1 of selenium in it on the base substrate, c) forming a second CdSe.sub.wTe.sub.1-w layer having a second thickness d2 and a second amount w2 of selenium in it on the first CdSe.sub.wTe.sub.1-w layer, wherein the second amount w2 lies in the range between 0.25 and 0.4, and d) forming a third CdSe.sub.wTe.sub.1-w layer having a third thickness d3 and a third amount w3 of selenium in it on the second CdSe.sub.wTe.sub.1-w layer, wherein a maximum of the energy gap in the first CdSe.sub.wTe.sub.1-w layer and a maximum of the energy gap in the third CdSe.sub.wTe.sub.1-w layer are equal to or higher than 1.45 eV and the energy gap in the second CdSe.sub.wTe.sub.1-w layer lies in the range between 1.38 eV and 1.45 eV and is smaller than the maximum of the energy gap in the first CdSe.sub.wTe.sub.1-w layer and smaller than the maximum of the energy gap in the third CdSe.sub.wTe.sub.1-w layer.

2. Method according to claim 1, characterized in that at least the second CdSe.sub.wTe.sub.1-w layer is formed using co-deposition of cadmium, selenium and tellurium and annealing the deposited layer under an atmosphere containing gaseous selenium.

3. Method according to claim 2, characterized in that: the first CdSe.sub.wTe.sub.1-w layer is formed using co-deposition of cadmium, selenium and tellurium and annealing the deposited layer at a first temperature and under a first atmosphere containing a first amount c1 of gaseous selenium in it for a first time period, the second CdSe.sub.wTe.sub.1-w layer is formed using co-deposition of cadmium, selenium and tellurium and annealing the deposited layer at a second temperature and under a second atmosphere containing a second amount c2 of gaseous selenium in it for a second time period, and the third CdSe.sub.wTe.sub.1-w layer is formed using co-deposition of cadmium, selenium and tellurium and annealing the deposited layer at a third temperature and under a third atmosphere containing a third amount c3 of gaseous selenium in it for a third time period, wherein the first amount w1 and the third amount w3 are larger than zero and smaller than 1 and wherein all amounts c1 to c3 are higher than zero.

4. Method according to claim 1, characterized in that at least the first or the third CdSe.sub.wTe.sub.1-w layer is formed using deposition of a layer of CdSe and/or a layer of CdTe and annealing the deposited layers.

5. Method according to claim 4, characterized in that: the first CdSe.sub.wTe.sub.1-w layer is formed using consecutive deposition of a first layer of CdSe with a first thickness d11 and a second layer of CdTe with a second thickness d22 and annealing the deposited layers at a first temperature and under a first atmosphere for a first time period, the second CdSe.sub.wTe.sub.1-w layer is formed using consecutive deposition of a second layer of CdSe with a third thickness d21 and a second layer of CdTe with a fourth thickness d22 and annealing the deposited layers at a second temperature and under a second atmosphere for a second time period, and the third CdSe.sub.wTe.sub.1-w layer is formed using consecutive deposition of a third layer of CdSe with a fifth thickness d31 and a third layer of CdTe with a sixth thickness d32 and annealing the deposited layers at a third temperature and under a third atmosphere for a third time period, wherein the ratio of the respective thickness of the CdSe layer and the respective thickness of the CdTe layer is different for each of the first, the second and the third CdSe.sub.wTe.sub.1-w layer.

6. Method according to claim 1, characterized in that the base substrate comprises a front contact layer and a window layer, wherein the window layer forms a surface of the base substrate, the first CdSe.sub.wTe.sub.1-w layer is formed on the window layer, a back contact layer is formed on the third CdSe.sub.wTe.sub.1-w layer, and the third amount w3 is in relation with the first amount w1 and the second amount w2 so that the maximum of the energy gap in the third CdSe.sub.wTe.sub.1-w layer is larger than the maximum of the energy gap in the first CdSe.sub.wTe.sub.1-w layer and the maximum of the energy gap in the first CdSe.sub.wTe.sub.1-w layer is larger than the energy gap in the second CdSe.sub.wTe.sub.1-w layer.

7. Method according to claim 6, wherein the first thickness d1 is in the range of 1 nm to 100 nm and smaller than the third thickness d3 lying in the range of 10 nm to 1500 nm and the third thickness d3 is smaller than the second thickness d2 lying in the range of 50 nm to 2000 nm.

8. Method according to claim 1, characterized in that the base substrate comprises a back contact layer forming a surface of the base substrate, the first CdSe.sub.wTe.sub.1-w layer is formed on the back contact layer, a layer stack comprising a window layer and a front contact layer is formed on the third CdSe.sub.wTe.sub.1-w layer, wherein the window layer is formed adjacent to the third CdSe.sub.wTe.sub.1-w layer, and the first amount w1 is. in relation with the third amount w3 and the second amount w2 so that the maximum of the energy gap in the first CdSe.sub.wTe.sub.1-w layer is larger than the maximum of the energy gap in the third CdSe.sub.wTe.sub.1-w layer and the maximum of the energy gap in the third CdSe.sub.wTe.sub.1-w layer is larger than the energy gap in the second CdSe.sub.wTe.sub.1-w layer.

9. Method according to claim 8, wherein the first thickness d1 is in the range of 10 nm to 1500 nm and larger than the third thickness d3 lying in the range of 1 nm to 100 nm and the first thickness d1 is smaller than the second thickness d2 lying in the range of 50 nm to 2000 nm.

10. Method according to claim 1, characterized in that a dopant is inserted into at least one of the first, the second or the third CdSe.sub.wTe.sub.1-w layer having an amount of selenium higher than 0.3.

11. Method according to claim 10, characterized in that the dopant is selected from the group of Zn, Mg and Mn and combinations thereof.

12. Method according to claim 10, characterized in that the dopant is inserted using co-deposition of the dopant for at least a part of the time of co-deposition of cadmium, selenium and tellurium during forming at least the second CdSe.sub.wTe.sub.1-w layer using co-deposition of cadmium, selenium and tellurium.

13. Method according to claim 10, characterized in that the dopant is inserted using forming one or more layers of a composition of tellurium with the dopant within or adjacent to the layer stack comprising the first, the second and the third CdSe.sub.wTe.sub.1-w layers.

14. Method according to claim 1, characterized in that a barrier layer is formed at least between the first and the second CdSe.sub.wTe.sub.1-w layers or between the second and the third CdSe.sub.wTe.sub.1-w layers, the barrier layer reducing the diffusion of selenium.

15. Method according to claim 14, characterised in that the barrier layer is a thin film comprising one of ZnO, MnO or MgO or combinations thereof and has a thickness in the range from 1 nm to 50 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] FIG. 1 schematically shows the layer structure of a solar cell produced by an embodiment of the method according to the invention, wherein the solar cell is formed in a superstrate configuration.

[0038] FIG. 2 schematically shows an exemplary distribution of the selenium content within the photoactive layer.

[0039] FIG. 3 schematically shows the dependence of the energy gap E.sub.g of a CdSe.sub.wTe.sub.1-w layer on the amount w of selenium.

[0040] FIG. 4 schematically shows the different phases of a CdSe.sub.wTe.sub.1-w layer in dependence on the amount of selenium within the CdSe.sub.wTe.sub.1-w layer and on the temperature of the CdSe.sub.wTe.sub.1-w layer.

[0041] FIG. 5 schematically shows the method for forming a double-graded CdSeTe thin film according to the invention.

[0042] FIGS. 6A and 6B schematically show methods for forming a solar cell comprising the double-graded CdSeTe thin film in a superstrate configuration or a substrate configuration, respectively.

[0043] FIG. 7 schematically shows a first embodiment of the inventive method for forming a double-graded CdSeTe thin film, wherein each CdSe.sub.wTe.sub.1-w layer is formed using a co-deposition of cadmium, selenium and tellurium and a consecutive annealing under a selenium containing atmosphere.

[0044] FIG. 8 schematically shows a second embodiment of the inventive method for forming a double-graded CdSeTe thin film, wherein each CdSe.sub.wTe.sub.1-w layer is formed using consecutive deposition of a CdSe layer and a CdTe layer and an annealing step.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0045] FIG. 1 schematically shows a solar cell 100 formed in a superstrate configuration by an embodiment of the method according to the invention. The layer structure of the solar cell 100 is shown, wherein the solar cell 100 comprises a substrate 10, a front contact layer 11, a window layer 12, a photoactive layer 13 and a back contact layer 14. The substrate 10 may be a transparent substrate, e.g. made of glass or transparent polymer. The front contact layer 11 may be a transparent conductive oxide, e.g. indium tin oxide (ITO) or aluminum doped zinc oxide (AZO), fluorine doped-tin oxide, or a layer stack of such a layer and a buffer layer, e.g. of cadmium stannate, intrinsic tin oxide, or intrinsic zinc oxide. The window layer 12 is a layer of cadmium sulfide (CdS) having an n-conductivity. However, the solar cell may also be formed without the window layer 12 or with a window layer formed of other materials. The photoactive layer 13 is a CdSeTe thin film comprising three different CdSe.sub.wTe.sub.1-w layers: a first CdSe.sub.wTe.sub.1-w layer 131 having a first thickness d1 and a first amount w1 of selenium in it, a second CdSe.sub.wTe.sub.1-w layer 132 having a second thickness d2 and a second amount w2 of selenium in it and a third CdSe.sub.wTe.sub.1-w layer 133 having a third thickness d3 and a third amount w3 of selenium in it. The back contact layer 14 is also a layer stack comprising a buffer layer 141 of, but not limited to, Te, MgTe, ZnTe, CdZnTe, As.sub.2Te.sub.3, Sb.sub.2Te.sub.3, Bi.sub.2Te.sub.3, Cu.sub.xTe.sub.1-x, MoO.sub.y, MoN, and a metal layer 142, e.g. made of, but not limited to, Mo, W, Ta, Hf, Al, Cr, Ni, Ti, Au, Ag as well as multi-layer combinations.

[0046] The photoactive layer 13 has a p-conductivity. Therefore, a p-n junction 15 is formed between the window layer 12 and the photoactive layer 13. The energy gap E.sub.g varies over the thickness of the photoactive layer 13, wherein the first CdSe.sub.wTe.sub.1-w layer 131 has a first energy gap E.sub.g1 equal to or larger than 1.45 eV, the second CdSe.sub.wTe.sub.1-w layer 132 has a second energy gap E.sub.g2 lying in the range between 1.38 eV and 1.45 eV, and the third CdSe.sub.wTe.sub.1-w layer 133 has a third energy gap E.sub.g3 equal to or larger than 1.45 eV. The energy gap E.sub.g strongly depends on the amount w of selenium within the respective CdSe.sub.wTe.sub.1-w layer, as schematically shown in FIG. 3. The figure shows different measured energy gap values and two fitting curves. The measured values referenced as PL1 (RT) are measured by photoluminescence spectroscopy for CdSe.sub.wTe.sub.1-w layers which are deposited on Si substrates by molecular beam epitaxy (MBE) technique, the measured values referenced as CL (RT) are measured by chatodoluminescence for the same CdSe.sub.wTe.sub.1-w layers, and the simulated values referenced as TB (RT) are calculated by means of the tight-binding method for similar CdSe.sub.wTe.sub.1-w layers. The fitting curves refer to the respective measured values as their respective reference name indicates. [Reference: J. Phys.: Condens. Matter 21 (2009) 075802]

[0047] FIG. 2 schematically shows an exemplary distribution of the selenium content w within the photoactive layer, i.e. the CdSeTe thin film, over the thickness of the CdSeTe thin film, the thickness extending along the x-axis. Where x=0, the p-n junction 15 between the window layer 12 and the photoactive layer 13 from FIG. 1 lies. The first CdSe.sub.wTe.sub.1-w layer 131 extends from x=0 to x=x.sub.1, the second CdSeTe layer 132 extends from x=x.sub.1 to x=x.sub.2, and the third CdSe.sub.wTe.sub.1-w layer 133 extends from x=x.sub.2 to x=x.sub.3. Within the first CdSe.sub.wTe.sub.1-w layer 131, the amount of selenium lies over 25% and falls from a maximum lying in the range between 60% and 90% at the p-n junction to a minimum of 25% at the interface to the second CdSe.sub.wTe.sub.1-w layer 132. Within the second CdSe.sub.wTe.sub.1-w layer 132, the amount of selenium is about 25% and essentially constant resulting in a thick layer being most effective in photoelectric conversion of sunlight. Within the third CdSe.sub.wTe.sub.1-w layer 133, the amount of selenium again raises from a minimum of 25% at the interface to the second CdSe.sub.wTe.sub.1-w layer 132 to a maximum of 90% at the interface to the back contact layer 14. The first CdSe.sub.wTe.sub.1-w layer 131 has a smaller thickness d1 than the thicknesses d2 and d3, and the second CdSe.sub.wTe.sub.1-w layer 132 has the largest thickness of all three CdSe.sub.wTe.sub.1-w layers 131 to 133. Although linear courses of the selenium amount are shown for the first and the third CdSe.sub.wTe.sub.1-w layers 131 and 133, there might also be other, nonlinear or even discontinuous courses. Furthermore, the selenium amount within the first and the third CdSe.sub.wTe.sub.1-w layers 131 and 133 may also be constant over the whole thickness of the first or the third CdSe.sub.wTe.sub.1-w layer 131 or 133, respectively, or may vary over the respective thickness, but without reaching the amount of selenium within the second CdSe.sub.wTe.sub.1-w layer 132. The last possibilities can be reached by a barrier layer formed between the first CdSe.sub.wTe.sub.1-w layer 131 and the second CdSe.sub.wTe.sub.1-w layer 132 or the second CdSe.sub.wTe.sub.1-w layer 132 and the third CdSe.sub.wTe.sub.1-w layer 133, respectively, wherein the barrier layer prevents the migration of selenium from the first or the third CdSe.sub.wTe.sub.1-w layers 131 or 133 into the second CdSe.sub.wTe.sub.1-w layer 132.

[0048] Moreover, the mentioned energy gaps in the first and the third CdSe.sub.wTe.sub.1-w layers 131 and 133 may also be obtained by amounts of selenium smaller than 10%, as can be seen from FIG. 3.

[0049] FIG. 4 schematically shows the relation between the amount of selenium within a CdSe.sub.wTe.sub.1-w layer, a temperature a CdSe.sub.wTe.sub.1-w layer is exposed to during formation and the phase of the CdSe.sub.wTe.sub.1-w layer. As can be seen, for amounts of selenium smaller than 60% and temperatures smaller than 600° C., the zinc-blende phase is obtained, whereas for higher amounts of selenium or even for smaller amounts of selenium, but higher temperatures, the wurtzite phase is obtained. Therefore, high amounts of selenium and/or high temperatures during formation of the CdSe.sub.wTe.sub.1-w layer are critical for obtaining photoactive layers having high efficiency and good electrical properties. [Reference: J. D. Poplawsky et al., Nature Communications, July 2016]

[0050] FIG. 5 schematically shows the process steps of the method for forming a double-graded CdSeTe thin film according to the invention. First, a base substrate is provided (step S1). Then, a first CdSe.sub.wTe.sub.1-w layer is formed on the base substrate, wherein the first CdSe.sub.wTe.sub.1-w layer has a first amount w1 of selenium in it with 0≤w1≤1 and a first energy gap E.sub.g1 being larger than or equal to 1.45 eV (step S2). Further, a second CdSe.sub.wTe.sub.1-w layer is formed on the first CdSe.sub.wTe.sub.1-w layer, wherein the second CdSe.sub.wTe.sub.1-w layer has a second amount w2 of selenium in it with 0≤w2≤1 and a second energy gap E.sub.g2 with 1.38 eV≤E.sub.g2≤1.45 eV, E.sub.g2 being smaller than the first energy gap E.sub.g1 (step S3). Finally, a third CdSe.sub.wTe.sub.1-w layer is formed on the second CdSe.sub.wTe.sub.1-w layer, wherein the third CdSe.sub.wTe.sub.1-w layer has a third amount w3 of selenium in it with 0≤w3≤1 and a third energy gap E.sub.g3 being larger than or equal to 1.45 eV and larger than the second energy gap E.sub.g2 (step S4). Each of the steps S1 to S4 may comprise a plurality of substeps, wherein some of the substeps of the steps S2 to S4 may be performed subsequent to other substeps of the steps S2 to S4 or may be performed simultaneously with each other. For instance, temperature treatment steps may be performed simultaneously. Therefore the steps S2 and S3 may be completed not until step S4 has been completed.

[0051] With respect to FIGS. 6A and 6B, a method for forming a solar cell comprising the double-grades CdSeTe thin film is described, wherein FIG. 6A schematically shows a superstrate configuration and FIG. 6B schematically shows a substrate configuration.

[0052] In the superstrate configuration of FIG. 6A, a first step S11 being an exemplary embodiment of step S1 of FIG. 5 comprises the substeps of providing a transparent substrate, for instance made of glass, (step S111), forming a front contact layer on the transparent substrate (step S112) and forming a window layer on the front contact layer (step S113). In the result, the base substrate onto which the first CdSe.sub.wTe.sub.1-w layer is formed is provided. The front contact layer may comprise a transparent oxide or any other electrode material suitable for the whole process of forming a solar cell and suitable for operating a solar cell. The step S112 may comprises depositing a continuous layer of the front contact material, structuring that layer and forming a transparent material in between the structures of the front contact material. Further, the step S112 may comprises depositing and/or structuring different front contact material layers thereby forming, for instance, thin lines of metal electrodes and a continuous layer of a transparent conductive oxide. The window layer may comprise a CdS layer or any other suitable layer forming a p-n junction with the CdSeTe thin film.

[0053] After forming the double-graded CdSeTe thin film by process steps S2 to S4 as described with respect to FIG. 5, a back contact layer is formed on the CdSeTe thin film (step S51). Also a Cl-based treatment step can be included in the process sequence of steps S2 to S4. The back contact layer may comprise a metal layer or different layers of different materials as known from the state of the art. Further, the step S51 may comprise a further substep for activation of the CdSeTe thin film or cleaning the surface of the CdSeTe thin film. By having an adapted CdSe.sub.wTe.sub.1-w material at the back contact, the interface to the metal-based back contact can be optimized and this can avoid the use of complicated structure stacks or specific buffer materials in between for ensuring a good ohmic back contact.

[0054] In the substrate configuration of FIG. 6B, a first step S12 being another exemplary embodiment of step S1 of FIG. 5 comprises providing or forming a back contact layer. That is, the back contact layer may be provided in form of a metal foil or any other electrical conductive substrate suitable as a back electrode of the solar cell or may be formed on any suitable substrate which may also be an electrical isolating material by, for instance, depositing a back contact layer or a plurality of layers forming a back contact layer. In the result, the base substrate onto which the first CdSe.sub.wTe.sub.1-w layer is formed is provided.

[0055] After forming the double-graded CdSeTe thin film by process steps S2 to S4 as described with respect to FIG. 5 (but in reverse order with respect to the selenium contents and the resulting energy gaps), a window layer is formed on the CdSeTe thin film (step S521) and a front contact layer is formed on the window layer (step S522). The window layer and the front contact layer may be formed similar to the way described above with respect to FIG. 6A, but in reverse order.

[0056] In both embodiments of the method for forming a solar cell, one or all of the front contact layer or the window layer or the back contact layer may comprise a buffer layer or barrier layers or passivation layer or any other layer improving the efficiency of the solar cell.

[0057] FIG. 7 schematically shows a first embodiment of the inventive method for forming a double-graded CdSeTe thin film. In this embodiment, each CdSe.sub.wTe.sub.1-w layer is formed using a co-deposition of cadmium, selenium and tellurium and a consecutive annealing under a selenium-containing atmosphere.

[0058] Step S21 being an embodiment of step S2 of FIG. 5 comprises a first substep S211 of co-depositing cadmium, selenium and tellurium, wherein the elements, in particular selenium and tellurium, have a first relation to each other. For instance, the amount w1 of selenium integrated in the deposited CdSe.sub.wTe.sub.1-w layer lies in the range between 0.5 and 0.9. The deposited CdSe.sub.wTe.sub.1-w layer has a thickness in the range of 1 nm to 100 nm. Subsequent, in a second substep S212, the deposited CdSe.sub.wTe.sub.1-w layer is annealed under an atmosphere containing a first amount c1 of gaseous selenium, wherein c1 is chosen such that out-diffusion of selenium during annealing can be prevented. The annealing is performed under a first temperature T1 for a first time period t1, for instance for 20 min to 30 min.

[0059] In step S61, a first barrier layer, made for instance of ZnO with a thickness of 1 nm to 5 nm, is formed on the first CdSe.sub.wTe.sub.1-w layer, for instance by sputtering.

[0060] After step S61, a second CdSe.sub.wTe.sub.1-w layer is formed on the first barrier layer in step S31 comprising a first substep S311 of co-deposition of Cd, Se and Te and a second substep S312. The elements Cd, Se and Te have a second relation to each other in substep S311, such that the amount w2 of selenium integrated in the deposited CdSe.sub.wTe.sub.1-w layer lies in the range between 0.25 and 0.4. The deposited CdSe.sub.wTe.sub.1-w layer has a thickness in the range of 500 nm to 2000 nm. The atmosphere used in substep S312 contains a second amount c2 of gaseous selenium, wherein c2 is chosen such that out-diffusion of selenium during annealing can be prevented, as described above. The annealing is performed under a second temperature T2 for a second time period t2, for instance for 20 min to 30 min.

[0061] Subsequent to step S31, a second barrier layer, made for instance of ZnO with a thickness of 1 nm to 5 nm, is formed on the second CdSe.sub.wTe.sub.1-w layer, for instance by sputtering, in step S62.

[0062] After step S62, a third CdSe.sub.wTe.sub.1-w layer is formed on the second barrier layer in step S41 comprising a first substep S411 of co-deposition of Cd, Se and Te and a second substep S412. The elements Cd, Se and Te have a third relation to each other in substep S411, such that the amount w3 of selenium integrated in the deposited CdSe.sub.wTe.sub.1-w layer lies in the range between 0.5 and 0.9. The deposited CdSe.sub.wTe.sub.1-w layer has a thickness in the range of 100 nm to 1500 nm. The atmosphere used in substep S412 contains a third amount c3 of gaseous selenium, wherein c3 is chosen such that out-diffusion of selenium during annealing can be prevented, as described above. The annealing is performed under a third temperature T3 for a third time period t3, for instance for 20 min to 30 min.

[0063] The co-deposition of Cd, Se and Te may be performed by sputtering, evaporation or sublimation, in particular closed space sublimation (CSS) as known from the state of the art. The substrate, onto which cadmium, selenium and tellurium are deposited, has preferably a substrate temperature in the range between 300° C. and 550° C., for instance 500° C., during co-deposition and/or annealing. The substrate temperature should not exceed 700° C. in any of these substeps for glass substrates. The time periods of the annealing substeps depend on the thickness of the respective deposited CdSe.sub.wTe.sub.1-w layer. Due to co-deposition, the desired amount w of the respective CdSe.sub.wTe.sub.1-w layer formed in the whole forming step may be controlled and adjusted in a good manner already during the deposition of the respective CdSe.sub.wTe.sub.1-w layer. Furthermore, the concentration of gaseous selenium within the annealing atmosphere ensures control of the desired amount w of the respective CdSe.sub.wTe.sub.1-w layer. Co-deposition and annealing at temperatures in the given ranges result in forming the zinc-blende phase of the respective CdSe.sub.wTe.sub.1-w layer.

[0064] The first and the second barrier layer prevent the cross-diffusion of selenium between the second CdSe.sub.wTe.sub.1-w layer on one side and the first or the third CdSe.sub.wTe.sub.1-w layer on the other side. However, the barrier layers may also be omitted resulting in smoother transition of the selenium content and the energy gaps between the different CdSe.sub.wTe.sub.1-w layers. The barrier layers may be formed by directly depositing the compound material specified, or by co-deposition of the contained elements using sputtering, evaporation or sublimation or by chemical vapour deposition or may be formed by deposition of an elemental dopant layer, for instance a zinc layer, and a subsequent oxidation.

[0065] Additionally, a dopant, for instance Zn, may be inserted into the first and/or the third CdSe.sub.wTe.sub.1-w layer by co-deposition during the respective substeps of co-deposition of Cd, Se and Te. The dopant may be inserted into the deposited CdSe.sub.wTe.sub.1-w layer with an amount in the range from 0.001 to 0.2, for instance with an amount of 0.01.

[0066] FIG. 8 schematically shows a second embodiment of the inventive method for forming a double-graded CdSeTe thin film. In this embodiment, each CdSe.sub.wTe.sub.1-w layer is formed using consecutive deposition of a CdSe layer and a CdTe layer and a consecutive annealing step.

[0067] Step S22 being an embodiment of step S2 of FIG. 5 comprises a first substep S221 of depositing a first CdSe layer having a first thickness d11 onto the base substrate. The first thickness d11 lies in the range of 1 nm to 100 nm. Subsequent, in a second substep S222, a first dopant containing layer, for instance of Zn.sub.yTe.sub.1-y with an amount y1 of Zn in the range between 0.001 and 0.1, is deposited on the first CdSe layer with a thickness dm1 in the range between 1 nm to 10 nm, for instance with a thickness dm1 of 2 nm. In a third substep S223, a first CdTe layer having a second thickness d12 is deposited onto the first dopant containing layer. The second thickness d12 lies in the range of 1 nm to 100 nm, wherein the ratio of the first thickness d11 to the second thickness d12 and the thickness dm1 of the first dopant containing layer determines the amount w1 of selenium incorporated in the CdSe.sub.wTe.sub.1-w layer resulting from the whole step S22. After substep S223, an annealing step (substep S224) is performed at a first temperature T1 under a first atmosphere for a first time period t1, for instance for 20 min to 30 min. In the result, the first CdSe.sub.wTe.sub.1-w layer, which is a doped CdSe.sub.wTe.sub.1-w layer, is formed.

[0068] The second CdSe.sub.wTe.sub.1-w layer is formed by a similar sequence of substeps in step S32, wherein however no dopant containing layer is deposited. That is, in a first substep S321, a second CdSe layer having a third thickness d21 is deposited onto the first CdSe.sub.wTe.sub.1-w layer. The third thickness d21 lies in the range of 50 nm to 2000 nm. Subsequent, in a second substep S322, a second CdTe layer having a fourth thickness d22 is deposited onto the second CdSe layer. The fourth thickness d22 lies in the range of 50 nm to 2000 nm, wherein the ratio of the third thickness d21 to the fourth thickness d22 determines the amount w2 of selenium incorporated in the CdSe.sub.wTe.sub.1-w layer resulting from the whole step S32. After substep S322, an annealing step (substep S323) is performed at a second temperature T2 under a second atmosphere for a second time period t2, for instance for 20 min to 30 min. In the result, the second CdSe.sub.wTe.sub.1-w layer is formed.

[0069] In step S42, a third CdSe.sub.wTe.sub.1-w layer is formed. Similar to step S22, step S42 comprises a first substep S421 of depositing a third CdSe layer having a fifth thickness d31 onto the second CdSe.sub.wTe.sub.1-w layer. The fifth thickness d31 lies in the range of 10 nm to 1500 nm. Subsequent, in a second substep S422, a second dopant containing layer, for instance of Zn.sub.yTe.sub.1-y with an amount y2 of Zn in the range between 0.001 and 0.1, is deposited on the third CdSe layer with a thickness dm2 in the range between 1 nm to 10 nm, for instance with a thickness dm2 of 5 nm. In a third substep S423, a third CdTe layer having a sixth thickness d32 is deposited onto the second dopant containing layer. The sixth thickness d32 lies in the range of 10 nm to 1500 nm, wherein the ratio of the fifth thickness d31 to the sixth thickness d32 and the thickness dm2 of the second dopant containing layer determines the amount w3 of selenium incorporated in the CdSe.sub.wTe.sub.1-w layer resulting from the whole step S42. After substep S423, an annealing step (substep S424) is performed at a third temperature T3 under a third atmosphere for a third time period t3, for instance for 20 min to 30 min.

[0070] Although the consecutive deposition of one CdSe layer and one CdTe layer is described above, the formation of a CdSe.sub.wTe.sub.1-w layer may also comprise a plurality of consecutive steps of depositing a layer stack comprising a CdSe layer and a CdTe layer and, if applicable, a dopant containing layer. In particular for forming a thick CdSe.sub.wTe.sub.1-w layer, such a multistack process may be advantageous for achieving a constant selenium amount throughout the whole formed CdSe.sub.wTe.sub.1-w layer and/or for reducing the annealing time.

[0071] The deposition of the respective CdSe layers and CdTe layers and the dopant containing layers may be performed by sputtering, evaporation or sublimation, in particular closed space sublimation (CSS) as known from the state of the art. The substrate, onto which these layers are deposited, has preferably a substrate temperature in the range between 300° C. and 700° C., for instance 500° C., during deposition of the layers. During annealing, the substrate temperature lies in the range between 300° C. and 700° C., for instance 400° C. The temperature should not exceed 700° C. in any of the substeps. The time periods of the annealing substeps depend on the thicknesses of the respective deposited CdSe layer and CdTe layer. The atmosphere during annealing may contain selenium or/and chlorine or any other suitable gases. The concentration of selenium or chlorine, if present, lies in the range between 0.1% and 100%. Due to annealing, the respective CdSe layer and CdTe layer intermixe with each other thereby forming the respective CdSe.sub.wTe.sub.1-w layer. Furthermore, the dopant dispersing throughout the resulting CdSe.sub.wTe.sub.1-w layer ensures forming the zinc-blende phase of the respective CdSe.sub.wTe.sub.1-w layer.

[0072] Additionally, a barrier layer may be formed between the second CdSe.sub.wTe.sub.1-w layer on one side and the first or the third CdSe.sub.wTe.sub.1-w layer on the other side as described with respect to FIG. 7.

[0073] 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

[0074] 100 Solar cell [0075] 10 Substrate [0076] 11 Front contact layer [0077] 12 Window layer [0078] 13 Photoactive layer [0079] 131 First CdSe.sub.wTe.sub.1-w layer [0080] 132 Second CdSe.sub.wTe.sub.1-w layer [0081] 133 Third CdSe.sub.wTe.sub.1-w layer [0082] 14 Back contact layer [0083] 141 Buffer layer [0084] 142 Metal layer [0085] 15 p-n junction [0086] c1-c3 Amount of gaseous selenium in an annealing atmosphere [0087] d1-d3 Thickness of a CdSe.sub.wTe.sub.1-w layer [0088] d11, d21, d31 Thickness of a CdSe layer [0089] d21, d22, d23 Thickness of a CdTe layer [0090] E.sub.g1-E.sub.g3 Energy gap of a CdSe.sub.wTe.sub.1-w layer [0091] T1-T3 Annealing temperature [0092] t1-t3 Time period of annealing [0093] w, w1-w3 Amount of selenium in a CdSe.sub.wTe.sub.1-w layer