RU2806180C1 - METHOD FOR MANUFACTURING FLEXIBLE SOLAR CELLS WITH CdTe ABSORBING LAYER ON POLYMER FILM - Google Patents

Abstract

FIELD: solar cells.

SUBSTANCE: technology for manufacturing flexible thin-film solar cells with a heterojunction in the Mo/MoO_x//CdTe//CdS//i-ZnO//ITO structure. At the first stage, a polyimide film with a tensile strength of 170-250 MPa is used under standard conditions for a film thickness of 38-50 mcm. The film is pre-annealed in vacuum at temperature T≈ +250°C in a vacuum oven for at least 1 hour. Then a molybdenum coating is applied to it on both sides to reduce its deformation. In this case, a layer of MoO_x is applied to the working surface of Mo using the magnetron sputtering method, by replacing the working gas in the magnetron chamber from argon to oxygen. At the second stage, an absorbing layer - cadmium telluride - is applied using the PVD method. In this case, the size of the evaporator is less than the distance between the substrate and the evaporator and can be considered a point size. At the third stage, photoconductivity of the absorbing layer is activated, in which a NaF layer is applied before applying the activator layer (CdCb). At the fourth stage, a CdS buffer layer is applied. ZnO was then deposited onto the samples. After this, annealing was carried out at T=450°C for at least 120 minutes in an inert atmosphere. A layer of ITO is then applied.

EFFECT: manufacture of flexible thin-film solar cells with a CdTe absorbing layer; reduced deformation and increase the thermal stability of the solar cell.

1 cl, 5 dwg

Description

The invention relates to a technology for creating flexible thin-film solar cells with a CdTe/CdS heterojunction. More specifically, the invention relates to a technology for creating flexible thin-film solar cells of a frontal “substrate” configuration with a CdTe absorbing layer on a flexible polyimide film.

The assembly of thin-film solar cells with a front-facing “substrate” configuration is a sequential application of layers on a base (Fig. 1. Sequence of layers - substrate - back contact layer - absorbing layer - buffer layer - optical window layer - layer of the upper transparent electrode - contact grid). In this case, sunlight passes through the upper transparent electrode and the optical window. Since in this case there are no requirements for the transparency of the base, an opaque flexible lightweight polymer film can be used as a substrate.

Assembling a solar battery on such a flexible basis can significantly reduce the specific weight of the device, as well as facilitate its installation. This is especially important for the so-called. building photovoltaics (BIPV), which involves the integration of solar panels with residential buildings or industrial facilities.

Currently, the following methods are known for creating flexible solar cells with a CdTe absorbing layer. However, they have a number of disadvantages.

In the patent [A process for large-scale production of CdTe/CdS thin film solar cells [text]: patent WO 2003032406 A2 / N. Romeo, A. Bosio, A. Romeo; applicant Solar Systems & Equipments SRL; 10/04/2002] describes a large-scale process for creating flexible solar cells with a rear “superstrate” configuration on a transparent substrate. This method imposes a requirement on the transparency of the base, since light with this (rear) configuration of the device first passes through the base, which is why not every film can act as it. In addition, the polymer film also requires sufficient heat resistance, since during the production of the device using the proposed technology, at the stage of activation of the photoconductivity of the absorbing layer (cadmium telluride), the layered structure with the activator (CdCl ₂ :) deposited on it must be annealed at temperatures of +380°C... +420°C in a vacuum chamber and at an inert gas pressure of 300-1000 mbar.

In the patent [Method for producing thin-film solar cells [text]: patent. WO 2015028520 A1 / K. Velappan, B. Siepchen, et al.; applicant China Triumpf International Engineering Co., Ltd., Ctf Solar Gmbh; 08/27/2014] describes a method for creating thin-film solar cells with a rear “superstrate” and front “substrate” configuration. It describes the application of a method of sputtering metal halides (activators) between layers of CdS (buffer) and CdTe (absorbent). It is also proposed to use ZnCl ₂ as an activator of photoconductivity. This method can be used to create solar cells with a frontal “substrate” configuration, and therefore avoids the requirement for substrate transparency. However, at one of the stages of creating a solar cell in this patent, temperatures up to +550 ° C are used, which makes it difficult to use lightweight, but not sufficiently heat-resistant polyimide film substrates.

In the patent [CdTe thin-film solar cell with an npp<+>structure [text]: patent. CN 203103315U / Zhang Chuanjun et. al.; applicant Shanghai Solar Battery Research and Development Center; 10/17/2012] describes a thin-film solar cell consisting of a substrate, a lower contact layer of metal Mo, a heavily doped p ^<+> -CdTe layer, a weakly doped p-type CdTe layer, an n-CdS layer and a transparent conducting oxide film of a top transparent layer electrode. This method consists of using the so-called. Back Surface Field (BSF), which is obtained by applying a highly alloyed absorbent layer to the back contact. The use of such layers unnecessarily complicates the solar cell design and also requires precise temperature control.

The closest to the proposed development is the patent [Method of manufacturing base layers of flexible photovoltaic converters based on CdTe in a quasi-closed volume [text]: patent. RU2675403 C1 / Kryukov Yu.A., Fursaev D.V. and etc.; patent holder State University "Dubna"; 11/14/2017], which describes the method of deposition of the main layers of a solar cell in a quasi-closed volume. In this case, the distance from the evaporation zone (evaporator) to the condensation zone (substrate) is commensurate with the diameter of the reactor. This solution has the disadvantage that with such a small distance between the evaporator and the substrate, the temperature of the evaporation zone affects the temperature of the substrate on which condensation occurs. So, for example, when spraying the absorbing layer of a solar battery - cadmium telluride using this method at an evaporator temperature of 425-460°C, the substrate temperature is 319-353°C. However, the high temperature of the substrate at this stage can affect its mechanical properties, therefore, in the case of a lightweight substrate, it is necessary to use only heat-resistant films with a low coefficient of thermal expansion. The authors also note that polyimide films with a thickness of 7-20 microns, thermally stable up to a temperature of 450°C, can be used as flexible substrates. However, films of such a small thickness are prone to undesirable significant deformation during the creation of the device. In addition, this invention uses an expensive and hard-to-find variety of polyimide, UPILEX-S.

The objective of the present invention is to create flexible solar cells of a front design with a CdTe/CdS heterojunction on other types of polyimide film.

A distinctive feature of our invention is that when applying a CdTe layer, the distance between the evaporation zone and the condensation zone significantly exceeds their dimensions, thus the evaporator is point-like in relation to the substrate. This avoids additional unwanted heating of the substrate by the evaporator. Another feature is the use of polyimide film as a substrate, stable in the temperature range T=300-450°C, the tensile strength of which is 170-250 MPa under standard conditions with a thickness of 38-50 microns. In this case, the film is pre-annealed in vacuum at a temperature of T=+250°C in a vacuum oven for at least 1 hour. Another feature is that the molybdenum layer (bottom contact) is applied on both sides of the substrate, which makes it possible to reduce its deformation when applying subsequent layers of the solar cell. In this case, a layer of _MoOx is applied to the working surface of Mo using the magnetron sputtering method by replacing the working gas in the magnetron chamber from argon to oxygen. In addition, before applying the CdCl ₃ layer, 10 nm NaF is applied, after which annealing is carried out in an inert atmosphere at T = 430°C for 30 minutes, after which the samples are washed and dried, then a layer of CdS and ZnO is applied using thermal vacuum evaporation using magnetron spraying. After this, the solar cell is annealed at T=450°C for at least 120 minutes in an inert atmosphere, then an upper transparent contact made of a transparent conductive oxide and a metal contact grid are applied.

Thus, the proposed invention makes it possible to create flexible solar cells with a front-facing CdTe/CdS heterojunction on polyimide films other than UPILEX-S.

The claimed invention is illustrated, but not limited in any way, by the following example.

Example 1. Creation of a flexible solar cell with efficiency = 7.6%

As a substrate, a polyimide film of the “Kargop” type was used, with a thickness of 50 microns, the tensile strength of which was not lower than 176 MPa (load F _{max =} 80.4 N) (Fig. 2). This film was pre-annealed in vacuum at T=250°C. Then 1.5 μm Mo was applied to it using a previously developed magnetron sputtering technique [Method of metallization of polyimide film [text]: Pat. RU2673294 C2 / Gapanovich M.V., Tikhonina N.A., Novikov G.F.; patent holder Gapanovich M.V.; 02/07/2017] on both sides. In the case of applying this layer on only one side, noticeable deformation was observed when applying the CdTe layer (Fig. 3. 1 - molybdenum layer on one side of the substrate, 2 - molybdenum layer applied on both sides of the substrate). In this case, a layer of MoO _x is applied to the working surface of Mo using the magnetron sputtering method, by replacing the working gas in the magnetron chamber from argon to oxygen. Then, 5 µm CdTe was deposited onto the substrate, heated by a layer of halogen lamps to T=350°C. The spraying scheme is shown in Fig. 4 (1 - crucible with sprayed substance, 2 - substrate, 3 - heater, 4 - thermocouple, R=15 cm). In this case, the size of the evaporator was 4 cm, and the distance between the substrate and the evaporator was 15 cm. In Fig. Figure 5 shows a polyimide film, half of which, after applying molybdenum, before applying MoO _x , was covered with aluminum foil, after which cadmium telluride was applied over a mask. As can be seen from FIG. 5 (1 - Mo/MoO _x , 2 - Mo/MoO _x // CdTe, 3 - Mo), the presence of a MoO _x layer improved the adhesion of CdTe.

Then 10 nm NaF and 1 μm CdCb were applied, after which they were annealed in a nitrogen stream at T = 430°C for 30 min. After washing with water and drying, 200 nm CdS was applied to the resulting samples by thermal evaporation, similar to that shown in Fig. 4. In this case, the substrate temperature was T=200°C. Then 50 nm ZnO was applied to the samples. After this, the samples were annealed in the time range from 30 to 120 minutes in an inert atmosphere at T=450°C. After this, an ITO layer was deposited using magnetron sputtering and a contact grid made of metallic indium. In Table. Figure 1 shows the current-voltage characteristics of the resulting solar cells, measured under AM 1.5 lighting conditions (solar spectrum emulator with Keithley 2401 meter). As can be seen from FIG. 6, the efficiency of the solar cell is maximum at an annealing time of t=120 min, while when applying a NaF layer during the annealing process it increases significantly and amounts to 7.6%.

Claims (1)

A method for manufacturing flexible solar cells with a frontal configuration, including the following layers: substrate - polyimide film, lower contact - molybdenum layer, absorbing layer - cadmium telluride, buffer layer - cadmium sulfide, optical window made of zinc oxide, upper transparent contact made of transparent conductive oxide with metal mesh; characterized in that polyimide films are taken with a tensile strength of 170-250 MPa with a thickness in the range from 38 to 50 microns; the polyimide film is additionally coated with a layer of molybdenum on the back side, while a layer of MoO _x is additionally applied to one of the molybdenum layers using magnetron sputtering by replacing the working gas in the magnetron chamber from argon to oxygen; the fact that the linear dimensions of the evaporator are smaller than the distance between the evaporator and the substrate; the fact that the absorbing layer of cadmium telluride is coated sequentially with layers of NaF, CdCl ₂ , then annealed in an inert atmosphere at T=430°C for 30 minutes, after which it is washed and dried; in that after applying the buffer layer of cadmium sulfide and the optical window of zinc oxide, annealing is carried out in an inert atmosphere at T = 450°C for at least 120 minutes.