Method for manufacturing CdTe-based thin film solar cell with graded refractive index distribution in CdTe (cadmium telluride)-based absorption layer and CdTe-based thin film solar cell with graded refractive index distribution

Abstract

A method of manufacturing a CdTe-based thin film solar cell device having a graded refractive index profile within a CdTe (cadmium telluride)-based absorber layer, comprising the steps of: a) providing a transparent substrate comprising a front electrode, b) forming a doped CdTe-based absorber layer on the substrate, c) performing an activation treatment after step b). The doped CdTe-based absorber layer in step b) is a doped CdTe-based absorber layer stack comprising a first layer and a second layer. The first layer is a first doping element-containing layer including vanadium as a first doping element, which is formed by depositing a first doping element-enriched layer followed by depositing a CdSe (cadmium selenide) layer or a CdSeTe (cadmium selenium telluride) layer, or by depositing a CdSe layer or a CdSeTe layer each doped with a first doping element. The second layer is formed by depositing a CdTe layer. The invention also relates to a CdTe-based thin-film solar cell device with gradient refractive index distribution.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a CdTe (cadmium telluride) based thin film solar cell having a gradient refractive index distribution in a CdTe based absorption layer and a CdTe based thin film solar cell having a gradient refractive index distribution. Background Art

[0002] In the prior art, CdTe-based thin film solar cell devices are produced using a top substrate configuration, and the process sequence is as follows: a first electrode layer is deposited as a front contact on a generally transparent substrate (such as a glass substrate); on the first electrode layer, a CdTe-based absorption layer is deposited; the CdTe-based absorption layer is then activated by an activator (e.g., CdCl2) and a temperature treatment step; finally, a second electrode layer (e.g., a metal layer or a metal compound layer) is applied as a back contact to collect charge carriers. Substrate-configured CdTe-based thin film solar cell devices are also known in the prior art. It is also well known that other layers, such as a buffer layer, are included before the deposition of the CdTe-based absorption layer and the back contact.

[0003] The difference between the refractive index of the glass substrate including the front electrode and the CdTe-based absorption layer results in a degradation in the ability of light to couple into the CdTe-based absorption layer, thereby limiting the photovoltaic efficiency of the thin-film solar cell device. In particular, in the case where no CdS layer is formed between the front electrode and the CdTe absorption layer, the difference between the front glass with a refractive index of 1.5 to 1.7 and the CdTe layer with a refractive index of 2.95 is very large, resulting in a large amount of incident light being reflected at the interface with the CdTe layer. In some devices, a CdSeTe (cadmium telluride selenide) absorption layer with a high Se content is formed at the interface of the CdTe layer, such as disclosed in US2014 / 0360565A1. However, even pure CdSe (cadmium selenide) has a refractive index of 2.54, which still results in a large difference in refractive index.

[0004] Furthermore, doping (particularly p-type doping, or increasing the p-type doping level of the CdTe-based absorber layer) is extremely important for improving the efficiency of solar cells. P-type doping with copper is well known, but also has disadvantages, such as lack of long-term stability.

[0005] WO 2017 / 081477 A1 discloses a method for preparing a Cu-doped CdTe-based thin film solar cell, wherein a continuous organic layer is coated between the absorber layer and the back contact. Copper can be incorporated into the CdTe absorber layer by providing copper to the CdTe absorber layer by thermal evaporation or by treating the surface of the organic layer with copper.

[0006] EP 2 337 088 A2 discloses a method for p-doping cadmium telluride (CdTe), wherein the CdTe has an interface region and at least a portion of the interface region is subjected to a heat treatment. The heat treatment is performed in the presence of a first material and a second material, wherein the first material comprises a p-type dopant, such as Bi, P, As, Sb, Au, Ag or Cu, and the second material comprises a halogen, such as cadmium chloride, hydrochloric acid or chlorine.

[0007] However, some of the doping elements used can create sub-bandgap defects that reduce the efficiency of solar devices. Summary of the invention

[0008] Therefore, an object of the present invention is to provide a method for preparing a CdTe-based thin film solar cell device with improved efficiency and a corresponding CdTe-based thin film solar cell device.

[0009] This object is solved by a method for preparing a CdTe-based thin film solar cell device having a graded refractive index profile in a CdTe-based absorption layer and a CdTe-based thin film solar cell device having a graded refractive index profile according to the independent claims. Preferred embodiments are given in the dependent claims.

[0010] According to the present invention, a method for preparing a CdTe-based thin-film solar cell device having a gradient refractive index distribution in a CdTe-based absorption layer comprises at least the following steps: a) providing a transparent substrate comprising a front electrode, b) forming a doped CdTe-based absorption layer on the substrate, and c) performing an activation treatment after step b). According to the present invention, the doped CdTe-based absorption layer in step b) is formed as a doped CdTe-based absorption layer stack comprising a first layer and a second layer. The first layer is formed as a first doping element-containing layer comprising vanadium as a first doping element. The first layer is formed by depositing a first doping element-enriched layer and then depositing a CdSe layer or a CdSeTe layer or by depositing a CdSe layer or a CdSeTe layer each doped with the first doping element. The second layer is formed by depositing a CdTe layer.

[0011] Advantageously, the method is capable of manufacturing a CdTe-based thin film solar cell device having a gradient of a first doping element within a CdTe-based absorbing layer, with a high concentration of the first doping element at a first interface of the CdTe-based absorbing layer facing the substrate. The gradient may have any known nonlinear shape along the thickness direction of the CdTe-based absorbing layer. More advantageously, the method is capable of manufacturing a CdTe-based thin film solar cell device having a gradient refractive index distribution within the CdTe-based absorbing layer, and having a lowest refractive index at a first interface of the CdTe-based absorbing layer. The refractive index at the first interface of the CdTe-based absorbing layer is in the range of 2.0 to 2.5, while the refractive index increases with increasing thickness of the CdTe-based absorbing layer, and reaches a value of between 2.5 and 3 at a second interface of the CdTe-based absorbing layer facing the back contact. The refractive index is indirectly related to the concentration of the first doping element. In an embodiment, the concentration of the first doping element within the entire CdTe-based absorbing layer is in the range of 0.001 to 1 wt.-%, wherein the concentration of vanadium at the first interface of the CdTe-based absorbing layer is in the range of 10 15 Up to 10 20 cm -3 In addition, the gradient refractive index forms a smooth transition of the refractive index from the substrate to the CdTe-based absorption layer at the first interface of the absorption layer. The activation treatment in step c) advantageously promotes the formation of the gradient refractive index distribution and the doping of the CdTe-based absorption layer. In addition, this helps to couple light from the front electrode into the CdTe-based absorption layer.

[0012] According to the present invention, substrate refers to any transparent substrate on which a CdTe-based absorption layer is formed in step b) and includes a front electrode. The substrate may include a transparent base substrate, such as a glass substrate, a transparent front electrode and other layers, such as a buffer layer, a window layer or any other layer. In an embodiment, the front electrode is a front electrode layer or a front electrode layer stack. In other embodiments, the front electrode is an n-type front electrode. The front electrode may be made of a transparent conductive oxide or may include a transparent conductive oxide. The buffer layer refers to a layer or a layer stack, for example, for band alignment.

[0013] The doped CdTe-based absorbing layer can be deposited by various methods known in the art, such as physical vapor deposition (PVD), electrochemical deposition, etc. In an embodiment, the doped CdTe-based absorbing layer is deposited by close space sublimation (CSS). In other embodiments, the doped CdTe-based absorbing layer is a p-type doped CdTe-based absorbing layer or at least includes a p-type doped region.

[0014] The CdSeTe layer refers to a layer having a composition of CdSe x Te 1-xwherein x varies between 0 (zero) and 1, preferably between 0 and 0.4.

[0015] The first doping element is vanadium. Advantageously, vanadium as a doping element achieves a high doping density, almost two orders of magnitude higher than copper doping, resulting in a higher open circuit voltage of the thin film solar cell device. Furthermore, advantageously, due to the amphoteric characteristics of vanadium depending on the oxidation state, p-type doping as well as n-type doping of the CdTe-based absorption layer can be achieved. Vanadium as the first doping element advantageously reduces the characteristics associated with the sub-band gap, such as the recombination of defects and the resulting quantum efficiency loss, and thus reduces the overall efficiency of the thin film solar cell device, which is known from other doping elements (such as Group 15 elements). Furthermore, advantageously, vanadium as the first doping element affects the refractive index of the CdTe-based absorption layer.

[0016] In some embodiments, additional O2 partial pressure is required, which is beneficial, either by providing an oxygen source via an additionally deposited oxygen-containing layer or by depositing the first layer of the doped CdTe-based absorber in a vacuum or inert atmosphere.

[0017] In an embodiment, the first layer of the doped CdTe-based absorber layer stack is formed by depositing a CdSe layer or a CdSeTe layer, each doped with vanadium, under a vacuum or inert atmosphere with an additional O2 partial pressure of 10 - 3 In other embodiments, the first layer of the doped CdTe-based absorber layer stack is formed by depositing a vanadium-rich layer and then depositing a CdSe layer or a CdSeTe layer in a vacuum or inert atmosphere with an additional O2 partial pressure of 10 -3 Pa to 1 Pa. Advantageously, the doping type in the CdTe based absorber layer can be varied. All deposition processes can be achieved by any known method, such as by evaporation or sputtering using corresponding sources such as CdSe and CdSeTe materials premixed with vanadium, respectively.

[0018] Vacuum means pressure below 10 -4 Pa to 100 Pa. Inert atmosphere refers to any dry atmosphere with a relative oxygen content of less than 0.05%.

[0019] In other embodiments, the second layer of the CdTe based absorber layer stack is formed as a layer stack comprising at least a CdTe layer.In other embodiments, the layer stack may comprise additional layers, for example layers comprising like a second doping element.

[0020] The activation treatment of the present invention can induce recrystallization, reduce lattice defects and improve pn junction or its formation. In addition, this step also improves the mixing of different compounds and / or elements, resulting in the formation of mixed or doped compounds. Activation treatments are known in the prior art, however, the parameters of the activation treatment of the present invention may be different from the parameters of the prior art. Such parameters are, for example, temperature, time or duration, the type or amount of chemical activators, or the composition and pressure of the surrounding atmosphere. In an embodiment, the activation treatment includes a step of applying an activator and a step of heat treatment. The activator can be, for example, CdCl2 or a composition including chlorine or chloride ions or any other reagent including halogen ions. The activator can be applied to the CdTe-based absorption layer in the form of a solid, liquid or gaseous material using techniques known to those skilled in the art. In addition, the heat treatment is carried out in a temperature range of 350°C to 500°C, preferably in a temperature range of 400°C to 450°C, for a duration of 5 to 30 minutes.

[0021] In an embodiment, the doped CdTe-based absorption layer is formed to have a total thickness in the range of 2 μm to 5 μm, preferably 3 μm.

[0022] In other embodiments, the first layer of the CdTe-based absorption layer is formed with a thickness in the range of 0.5 μm to 3.5 μm. In an embodiment, the first doping element-enriched layer is formed with a thickness in the range of 5 nm to 500 nm. In other embodiments, a CdSe layer or a CdSeTe layer with a thickness in the range of 5 nm to 500 nm is deposited, respectively, wherein the thickness of the first doping element-enriched layer and the undoped CdSe layer or CdSeTe layer corresponds to each other according to the vanadium concentration to be achieved in the first layer of the CdTe-based absorption layer. The deposition thickness of the CdSe and CdSeTe layers, each doped with the first doping element, is in the range of 5 nm to 500 nm, respectively. In an embodiment, the second layer of the CdTe-based absorption layer is formed with a thickness in the range of 2 μm to 4 μm.

[0023] In an embodiment, the first doping element rich layer is at least one of the group consisting of VTe2, VSe2, V, VO2, NH4VO2, VCl2, VCl4.

[0024] Such a first doping element enriched layer may be formed by any known method, such as physical vapor deposition or chemical vapor deposition methods, and methods including applying chemical reagents and then annealing under a specific atmosphere. In an embodiment, the VCl4 or VCl2 layer may be formed by the reaction 2VCl3→VCl4+VCl2.

[0025] In an embodiment, the substrate provided in step a) further comprises an oxygen-containing layer.

[0026] Advantageously, this layer can be used as an oxygen source in subsequent process steps.

[0027] In other embodiments, the oxygen-containing layer is a layer or layer stack having a high sheet resistance, a band gap greater than 3.4 eV, a valence band edge lower than the valence band edge of the CdTe-based absorber layer, and a conduction band edge comparable to the conduction band edge of the CdTe-based absorber layer. In embodiments, the oxygen-containing layer has a thickness in the range of 1 nm to 50 nm. In other embodiments, the oxygen-containing layer has an average transmittance of at least 80% for incident electromagnetic radiation having a wavelength in the range of about 250 nm to about 1050 nm.

[0028] In an embodiment, the oxygen-containing layer is an oxide buffer layer.

[0029] Advantageously, the oxide buffer layer as an oxygen source can affect the amphoteric characteristics of the first doping element vanadium and thus affect the doping type that can be achieved in the CdTe-based absorber layer. In other embodiments, the oxide buffer layer can be (but not limited to) ZnO, SnO2 or Mg-doped ZnO.

[0030] In an embodiment, the second layer of the CdTe based absorber layer stack is doped with the first and / or the second doping element.

[0031] Advantageously, the second doping element forms a gradient of the second doping element in the CdTe-based absorbing layer through the promotion of the activation treatment in step c), and has a high concentration of the second doping element at the second interface of the CdTe-based absorbing layer opposite to the first interface. In addition, advantageously, the second doping element forms a diffusion back pressure in the CdTe-based absorbing layer, and restricts the diffusion of the first doping element toward the second interface of the CdTe-based absorbing layer during the activation treatment by occupying a lattice position that is also suitable for the first doping element.

[0032] Obviously, the second doping element is different from the first doping element. In an embodiment, the first doping element and the second doping element are elements of the same type or doping elements of different types. The types of doping elements refer to p-type and n-type doping elements, respectively. For example, it may be advantageous if the first doping element is an n-type doping element and the second doping element is a p-type doping element, or vice versa, or if both the first doping material and the second doping material are p-type doping materials or both are n-type doping materials. In addition, the type of doping element may vary depending on the oxidation state of the doping material.

[0033] In an embodiment, the second doping element is selected from Group 11 and Group 15 elements of the periodic table. Advantageously, this enables the manufacture of copper-free CdTe-based thin film solar cell devices. In other embodiments, the second doping element is selected from N, P, As, Ag, Cu and Sb. In an embodiment, the concentration of the second doping element in the CdTe-based absorption layer is between 10 15 Up to 1020 cm -3 within the range.

[0034] In an embodiment, the doped second layer of the CdTe-based absorber layer stack is formed by any known method, such as co-deposition of CdTe and a second doping element, depositing a layer containing the second doping element first and then depositing CdTe, or vice versa, or between two depositions of CdTe. In addition, an ex-situ doping method for the second layer of the CdTe-based absorber layer is also possible and can be performed before or after step c), such as disclosed in US202180735 A1.

[0035] In an embodiment, the method further comprises a step d) of forming a back contact.

[0036] In other embodiments, the back contact may be formed as a back contact layer stack.The formation of the back contact may be performed by any known method.

[0037] In other embodiments, the method further comprises other steps before forming the back contact, such as a known Cd etching step, to form a Te-rich surface layer at the second interface of the CdTe-based absorbing layer. The second interface of the CdTe-based absorbing layer faces the back contact.

[0038] In an embodiment, the back contact is formed by forming a back contact layer stack, the back contact layer stack comprising a first back contact layer and a second back contact layer, wherein the first back contact layer is a Te-rich layer and the second back contact layer is a metal layer or a high resistance layer.

[0039] Advantageously, such a Te-rich layer acts as a buffer layer for the metal layer or metal nitride layer and forms a barrier layer. In addition, advantageously, by forming such a back contact layer stack, the known Cd etching step of forming a Te-rich surface layer at the second interface of the CdTe-based absorber layer can be omitted. In an embodiment, the Te-rich layer is selected from ZnTe, SbTe, Te.

[0040] In other embodiments, the Te-rich layer may be formed by a known etching step, such as a NP etching step.

[0041] In other embodiments, the Te-rich layer can be doped with group 11 or group 15 elements, preferably doped with Ag, Cu or N, P, As, Sb. It is advantageous if the second layer of the CdTe-based absorption layer stack is undoped, i.e., does not include the first doping element and / or the second doping element. In this case, the group 11 or group 15 doping element of the Te-rich layer acts as a second doping element, forms a gradient of the second doping element within the CdTe-based absorption layer during the additional heat treatment, and has a high concentration of the second doping element at the second interface of the CdTe-based absorption layer. The additional heat treatment is performed after the back contact is formed by annealing in air at a temperature in the range of 200°C to 280°C for 5 minutes to 60 minutes. In other embodiments, the Te-rich layer includes a group 11 or group 15 element in the range of 0.01 wt.-% to 3 wt.-%.

[0042] In other embodiments, the metal layer is a highly conductive metal layer with a sheet resistance of 5Ω·sq -1 The highly conductive metal layer is, for example but not limited to, Mo, Al, Cr or Au.

[0043] The high resistance layer has a sheet resistance of at least 100Ω·sq -1 In an embodiment, the high resistance layer is a metal nitride layer, such as but not limited to MoN, AlN.

[0044] In an embodiment, the thickness of the Te-rich layer is in the range of 50 nm to 500 nm. In addition, the metal layer or the metal nitride layer may be formed to a thickness in the range of 10 nm to 1000 nm and in the range of 10 nm to 100 nm, respectively.

[0045] In an embodiment, the activation treatment is performed under an inert atmosphere or under vacuum.

[0046] In an embodiment, the activation treatment is carried out in an inert atmosphere with a partial pressure of H2 or in a vacuum. Advantageously, H2 can remove substances bound to the first doping element, such as removing oxygen from vanadium. Vacuum refers to a temperature of 10 -4 Inert atmosphere refers to any dry atmosphere with a total oxygen content of less than 0.05%. In other embodiments, the H2 partial pressure is 10 -3 In the range of Pa to 1Pa.

[0047] The present invention also relates to a CdTe-based thin-film solar cell device with a gradient refractive index distribution. The CdTe-based thin-film solar cell device comprises at least a transparent substrate including a front electrode, a back contact, and a doped CdTe-based absorption layer, wherein the doped CdTe-based absorption layer is located between the front electrode and the back contact and comprises vanadium as a first doping element. According to the present invention, the doped CdTe absorption layer comprises a gradient refractive index along the thickness direction of the CdTe-based absorption layer, which has a lowest refractive index at a first interface of the CdTe-based absorption layer facing the substrate, and has a highest refractive index at a second interface of the CdTe-based absorption layer facing the back contact.

[0048] Advantageously, such CdTe-based thin film solar cell devices provide a graded refractive index having a minimum refractive index at a first interface of the CdTe-based absorption layer facing the substrate, and when the thin film solar cell device is used, the first interface faces sunlight, thereby promoting light coupling from the front electrode into the CdTe-based absorption layer. In addition, such CdTe-based thin film solar cell devices improve photovoltaic efficiency.

[0049] Vanadium advantageously enables high doping density, almost two orders of magnitude higher than copper doping, resulting in a higher open circuit voltage of the thin film solar cell device. Furthermore, vanadium as the first doping element advantageously reduces sub-bandgap-related features, such as the recombination of defects and the resulting quantum efficiency loss, and thus the overall efficiency of the thin film solar cell device, which is known from other doping elements such as Group 15 elements.

[0050] In an embodiment, the substrate comprises a transparent base substrate, preferably a glass substrate. In other embodiments, the front electrode is a front electrode layer or a front electrode layer stack. In an embodiment, the front electrode is an n-type front electrode. The front electrode may be made of or may include a transparent conductive oxide.

[0051] In an embodiment, the substrate further comprises an oxygen-containing layer, the oxygen-containing layer being a layer or a stack of layers having a high sheet resistance, a band gap higher than 3.4 eV, a valence band edge lower than the valence band edge of the CdTe-based absorbing layer, and a conduction band edge comparable to the CdTe-based absorbing layer. In other embodiments, the oxygen-containing layer is an oxidized buffer layer. In other embodiments, the oxidized buffer layer may be (but is not limited to) ZnO, SnO2, or Mg-doped ZnO. In an embodiment, the thickness of the oxygen-containing buffer layer is in the range of 1 nm to 50 nm.

[0052] In an embodiment, the thickness of the doped CdTe-based absorber layer is in the range of 2 μm to 5 μm, preferably 3 μm.

[0053] In an embodiment, the back contact is a back contact layer stack including a first back contact layer and a second back contact layer, wherein the first back contact layer is a Te-rich layer and the second back contact layer is a metal layer or a high resistance layer. In an embodiment, the Te-rich layer is selected from ZnTe, SbTe, Te. In other embodiments, the metal layer is a highly conductive metal layer with a sheet resistance of 5Ω·sq -1 or lower. The highly conductive metal layer is, for example, but not limited to, Mo, Al, Cr or Au. The high resistance layer is a layer with a sheet resistance of at least 100Ω·sq -1 In an embodiment, the high resistance layer is a metal nitride layer, such as but not limited to MoN, AlN.

[0054] In an embodiment, the thickness of the Te-rich layer is in the range of 5 nm to 500 nm. In addition, the thickness of the metal layer and the high resistance layer are in the range of 10 nm to 1000 nm and in the range of 1 nm to 100 nm, respectively.

[0055] In other embodiments, the Te-rich layer may be doped with group 11 or group 15 elements, preferably Ag, Cu or N, P, As, Sb. In an embodiment, the Te-rich layer comprises group 11 or group 15 elements in the range of 0.01 wt.-% to 3 wt.-%.

[0056] The CdTe-based thin film solar cell device in the present invention is preferably formed by the above method.

[0057] In an embodiment, the doped CdTe-based absorber layer includes a first doping element (ie, vanadium) gradient along a thickness direction of the CdTe-based absorber layer, and has a highest concentration of the first doping element at a first interface of the CdTe-based absorber layer.

[0058] Advantageously, the concentration of the first doping element is indirectly related to the refractive index. This means that a high concentration of the first doping element leads to a low refractive index and vice versa.

[0059] In other embodiments, the doped CdTe-based absorbing layer can be a p-type doped CdTe-based absorbing layer, an n-type doped CdTe-based absorbing layer, or a CdTe-based absorbing layer with varying doping types within the CdTe-based absorbing layer. In other embodiments, the concentration of the first doping element within the CdTe-based absorbing layer is in the range of 0.001 wt.-% to 1 wt.-%. In other embodiments, the concentration of the first doping element at the first interface of the CdTe-based absorbing layer is up to 1 wt.-%.

[0060] In an embodiment, the doped CdTe-based absorbing layer includes a second doping element having a gradient along a thickness direction of the CdTe-based absorbing layer, and has a highest concentration of the second doping element at a second interface of the CdTe-based absorbing layer.

[0061] Advantageously, during the activation process or during operation of the CdTe-based thin film solar cell device, the gradient of the second doping element limits the diffusion of the first doping element to the second interface of the CdTe-based absorber layer by occupying lattice sites also suitable for the first doping element. The first doping element and the second doping element are different doping elements.

[0062] In an embodiment, the first doping element and the second doping element are doping elements of the same type or different types. In other embodiments, the first doping element may be an n-type doping element and the second doping element may be a p-type doping element, or vice versa. In addition, the type of doping element may depend on the oxidation state of the element in the CdTe-based thin film solar cell.

[0063] In other embodiments, the concentration of the second doping element within the CdTe based absorber layer is in the range of 0.001 wt.-% to 1 wt.-%. In other embodiments, the concentration of the second doping element at the second interface of the CdTe based absorber layer is up to 1 wt.-%.

[0064] In an embodiment, the second doping element is a Group 11 or Group 15 element.

[0065] Advantageously, Group 11 and Group 15 elements enable the fabrication of copper-free CdTe-based thin film solar cell devices. In other embodiments, the second doping element is selected from N, P, As, Ag, Cu and Sb.

[0066] Combining the features of the above embodiments and claims is beneficial to the implementation of the present invention. However, the embodiments of the present invention described in the above description are examples given as illustrations, and the present invention is by no means limited thereto. Any modifications, variations and equivalent configurations are deemed to be included within the scope of the present invention.

[0067] Although specific embodiments have been shown and described herein, it will be appreciated by those skilled in the art that various alternative and / or equivalent implementations may be used to replace the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptation or variation of the specific embodiments discussed herein. Therefore, the present invention is intended to be limited only by the claims and their equivalents. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] The accompanying drawings are included to provide a further understanding of embodiments of the invention and are incorporated into and constitute a part of this specification. The accompanying drawings illustrate embodiments of the invention and together with the description are used to explain the principles. Other embodiments of the invention and many of the expected advantages will become more readily understood as they can be better understood by reference to the following detailed description. The elements of the accompanying drawings are not necessarily to scale with respect to each other. The same reference numerals represent corresponding similar parts.

[0069] Figure 1A The invention shows a method for preparing a CdTe-based thin-film solar cell device having a graded refractive index distribution in a CdTe-based absorption layer in one embodiment of the invention.

[0070] Figure 1B The invention shows a method for preparing a CdTe-based thin-film solar cell device having a graded refractive index distribution in a CdTe-based absorption layer in another embodiment of the invention.

[0071] Figure 2 A CdTe-based thin film solar cell device with a gradient refractive index distribution in an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0072] In accordance with Figure 1A In an embodiment of the present invention, a method for preparing a CdTe-based thin-film solar cell device having a gradient refractive index distribution in a CdTe-based absorption layer comprises at least: step S1, providing a transparent substrate, the transparent substrate comprising a transparent base substrate (e.g., a glass substrate), a front electrode (e.g., an n-type transparent conductive oxide, such as fluorine-doped tin oxide) and an oxide buffer layer (e.g., magnesium-doped zinc oxide (MZO)); step S2, forming a doped CdTe-based absorption layer, the doped CdTe-based absorption layer being a CdTe-based absorption layer stack comprising a first layer and a second layer, the first layer of the CdTe-based absorption layer stack being formed as a layer containing a first doping element, wherein the first doping element is vanadium; in an embodiment, in step S2.1, a layer containing a first doping element is formed by depositing a CdSe layer or a CdSeTe layer doped with vanadium, for example, by depositing from a vanadium-doped CdSe or CdSeTe source or by co-depositing vanadium with CdSe or CdSeTe, to form a layer containing a first doping element The thickness of the layer is 300nm; in addition, in step S2.2, a second layer of the CdTe-based absorption layer stack is formed by depositing a CdTe layer with a thickness of 3.2μm in a vacuum of 1Pa or under an inert atmosphere. In an embodiment, the second layer of the CdTe-based absorption layer stack can be deposited as a doped layer, such as a p-type doped CdTe layer doped with As, and the method of depositing the doped CdTe layer is well known; step S3, after forming the CdTe-based absorption layer stack, an activation treatment is performed, including applying an activator (such as CdCl2), and heat treatment at 400°C to 450°C in a vacuum of 10Pa with a H2 partial pressure of 5×10Pa for 15 minutes; finally, step S4, forming a back contact, the back contact is a back contact layer stack including a first back contact layer and a second back contact layer, the first back contact layer is formed by depositing highly p-type doped ZnTe with a thickness of 30nm, and the second back contact layer is deposited as a metal layer, such as Mo with a thickness of 300nm.

[0073] Figure 1B Examples include Figure 1A In step S2, the doped CdTe-based absorption layer is formed into a CdTe-based absorption layer stack including a first layer and a second layer. The first layer of the CdTe-based absorption layer stack is formed into a layer containing a first doping element, wherein the first doping element is vanadium. Figure 1B , a layer containing the first doping element is formed by depositing a first doping element-enriched layer in step S2.10, for example, a VSe2 layer with a thickness of 50 nm is deposited in a vacuum of 1 Pa, and then a CdSe layer or CdSeTe layer with a thickness of 250 nm is deposited also in a vacuum in step S2.11.

[0074] Figure 2 Schematically shows the Figure 1A or Figure 1B An embodiment of a CdTe-based thin-film solar cell device 1 having a gradient refractive index distribution prepared by a method, which is not true to scale. Figure 2 The upper CdTe-based thin film solar cell device 1 comprises at least: a transparent substrate 10, which comprises a glass substrate, a fluorine-doped tin oxide front electrode and an oxide buffer layer doped with magnesium zinc oxide (MZO); a doped CdTe-based absorption layer 11; and a back contact 12, which comprises a first back contact layer containing ZnTe and a second back contact layer containing Mo. Figure 2 As shown in the middle part, the doped CdTe absorption layer 11 includes a graded refractive index n along the thickness x direction of the CdTe-based absorption layer 11, with a minimum refractive index n of 2.0 to 2.5 at a first interface of the CdTe-based absorption layer 11 facing the substrate, and a maximum refractive index n of 2.5 to 3 at a second interface of the CdTe-based absorption layer 11 facing the back contact. Figure 2 As shown in the lower part, the CdTe-based absorption layer 11 also includes a gradient of the first doping element (ie, vanadium) along the thickness x direction of the CdTe-based absorption layer 11. At the first interface of the CdTe-based absorption layer 11, the highest concentration of vanadium is C V In addition, the doped CdTe-based absorber layer 11 comprises a gradient of the second doping element (herein arsenic) along the thickness x direction of the CdTe-based absorber layer 11, wherein also in Figure 2 At the second interface of the CdTe-based absorption layer 11 shown in the lower part, the highest arsenic concentration C As Up to 1 wt.-%. The CdTe based absorber layer 11 comprises up to 1 wt.-% vanadium and up to 0.5 wt.-% arsenic.

Claims

1. A method for preparing a CdTe-based thin film solar cell device having a gradient refractive index distribution in a CdTe-based absorption layer, comprising at least the following steps: a) providing a transparent substrate including a front electrode, b) forming a doped CdTe-based absorption layer on the substrate, c) performing an activation treatment after step b), It is characterized in that the doped CdTe-based absorption layer in step b) is a doped CdTe-based absorption layer stack including a first layer and a second layer, wherein The first layer is formed into a first doping element-containing layer including vanadium as a first doping element by the following steps: Depositing a first doping element-enriched layer and subsequently depositing a CdSe layer or a CdSeTe layer; or depositing a CdSe layer or a CdSeTe layer each doped with the first doping element; And, the second layer is formed by depositing a CdTe layer.

2. The method according to claim 1, characterized in that The first doping element-rich layer is at least one of the group consisting of VTe2, VSe2, VTe2, VSe2, V, VO2, NH4VO2, VCl2, and VCl4.

3. The method according to claim 1 or 2, characterized in that: The substrate provided in step a) further comprises an oxygen-containing layer.

4. The method according to claim 3, characterized in that The oxygen-containing layer is an oxidation buffer layer.

5. The method according to any one of claims 1 to 4, characterized in that The second layer of the CdTe based absorber layer stack is doped with a second doping element.

6. The method according to any one of claims 1 to 5, characterized in that The method further comprises a step d) of forming a back contact after step c).

7. The method according to claim 6, characterized in that The back contact is a back contact layer stack including a first back contact layer and a second back contact layer, wherein the first back contact layer is a Te-rich layer, and the second back contact layer is a metal layer or a high resistance layer.

8. The method according to any one of claims 1 to 7, characterized in that The activation treatment is carried out under an inert atmosphere or in vacuum.

9. A CdTe-based thin film solar cell device with a gradient refractive index distribution, comprising at least a transparent substrate, the transparent substrate comprising a front electrode; Back contact; and a doped CdTe-based absorbing layer, the doped CdTe-based absorbing layer comprising vanadium as a first doping element and being located between the front electrode and the back contact, in, The doped CdTe absorption layer comprises a graded refractive index along the thickness direction of the CdTe-based absorption layer, having a lowest refractive index at a first interface of the CdTe-based absorption layer toward the substrate, and having a highest refractive index at a second interface of the CdTe-based absorption layer toward the back contact.

10. The CdTe-based thin film solar cell device according to claim 9, characterized in that: The doped CdTe-based absorbing layer comprises a gradient of the first doping element along a thickness direction of the CdTe-based absorbing layer, with a highest concentration of the first doping element at the first interface of the CdTe-based absorbing layer.

11. The CdTe-based thin film solar cell device according to claim 9 or 10, characterized in that: The doped CdTe-based absorption layer includes a second doping element, the second doping element has a gradient along the thickness direction of the CdTe-based absorption layer, and has a highest concentration of the second doping element at the second interface of the CdTe-based absorption layer.

12. The CdTe-based thin film solar cell device according to any one of claims 9 or 11, characterized in that: The second doping element is a Group 11 or Group 15 element.

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