WO2018119679A1

WO2018119679A1 - Method for activation of cdte layer of cdte thin-film solar cells with calcium hypochlorite

Info

Publication number: WO2018119679A1
Application number: PCT/CN2016/112405
Authority: WO
Inventors: Drost CHRISTIAN; Frauenstein SVEN; Harr MICHAEL; Peng SHOU
Original assignee: China Triumph International Engineering Co Ltd; CTF Solar GmbH
Current assignee: China Triumph International Engineering Co Ltd; CTF Solar GmbH
Priority date: 2016-12-27
Filing date: 2016-12-27
Publication date: 2018-07-05
Anticipated expiration: 2019-06-27
Also published as: CN108604617A; CN108604617B

Abstract

A method for activating the CdTe laye r(4) of semi-finished thin-film CdTe solar cell is described, in which the CdCl ₂ used in prior art is at least partially replaced by a less hazardous, cheaper and easy to handle substance, wherein the overall method of producing a CdTe thin-film solar cell is maintained. This is achieved by applying calcium hypochlorite onto the CdTe layer (4) and the subsequent heat treatment of the semi-finished thin-film CdTe solar cells.

Description

[Title established by the ISA under Rule 37.2] METHOD FOR ACTIVATION OF CDTE LAYER OF CDTE THIN-FILM SOLAR CELLS WITH CALCIUM HYPOCHLORITE

The subject of the present application is a method for activation of the CdTe layer of CdTe thin-film solar cells without the use of CdCl₂.

Producing CdTe thin-film solar cells in superstrate configuration according to the state of the art involves applying a transparent front contact layer (TCO, i.e. transparent conductive oxide) onto a substrate (preferably glass) . The front contact layer may also be a layer stack consisting of several different layers. Onto this, a layer of pure or modified CdS (cadmium sulfide) is deposited. Modified CdS is henceforth understood as CdS with dopings, variations in crystal shape or in grain sizes, or as a mixture of CdS with other substances. On top of the CdS layer, a layer of CdTe (cadmium telluride) is applied. Then, a back contact layer, or a back layer sequence, is deposited onto the CdTe layer.

An issue arising from this is the difficulty of applying a metallic contact layer onto the CdTe, since this would create a rectifying Schottky contact. What is desired, however, is the creation of an ohmic contact. Therefore, in the state of the art, an interface layer sequence is used. Its object is to realise the approximation of energy levels of the individual layer materials so that an ohmic contact is created, with a metallic layer being the preferred top back contact layer.

After applying the CdTe, usually an activation of CdTe by means of CdCl₂ (cadmium chloride) and heatingfollowsin the production processaccording to the state of the art. To this end, a CdCl₂ layer is applied to the CdTe layer using methods according to the state of the art (preferably wet-chemical or CVD or PVD methods) . Afterwards, the CdCl₂ is left to react with the CdS/CdTe layer stack at a raised temperature (usually about 380℃ to 430℃) . Reaction time is about 15 minutes to 45 minutes. Here, the CdCl₂ acts as fluxing agent and supports a recrystallisation of the CdTe layer.

After activation, any surplus CdCl₂ is rinsed off from the CdTe layer surface using deionized water. A problem arising from the activation process according to the state of the art is that CdCl₂ is an easily water-soluble, toxic chemical and highly hazardous for the environment. It is proven to have carcinogenic and mutagenic properties and to be toxic for reproduction. These properties require special care in handling when processing the CdCl₂ in the production process. Furthermore, the generated contaminated waste water requires costly cleaning or waste disposal.

Therefore, other substances were investigated, which could replace the CdCl₂ or at least reduce the amount of the CdCl₂ necessary for achieving activation of the CdTe layer and good electrical characteristics of produced CdTe thin-film solar cells.

In US 9,287,439 B1, calcium chloride, zinc chloride hydrate, adducts of zinc chloride, in particular tmeda^. ZnCl₂ (tmeda: tetramethylethylenediamine) , and tetrachlorozincates, particularly (NH₄) ₂ZnCl₄, and further double salts containing zinc and chloride ions were investigated. Most of these compounds turned out to be not suitable for replacing CdCl₂. Only calcium tetrachlorozincate has proven suitable.

Thus the object consists in finding alternative compounds suited for at least partially replacing the CdCl₂ used in the process, beingless hazardous for the environment, in particular less carcinogenic and mutagenic, easy to handle and a cost-effective substance. Furthermore, the overall method of producing a CdTe thin-film solar cell should be maintained.

According to the invention, the object is achieved using the method according to claim 1. Advantageous embodiments are disclosed in the corresponding dependent sub-claims.

The inventors have conducted research investigating various, less toxic and non-carcinogenic, respectively non-mutagenic, chemical compounds, which also react as fluxing agents in the activation process. In order to secure a stable availability at low costs, inventors considered different hypochlorite derivatives which are used as bleaching, disinfection and water treatment agents, for instance hypochlorites of sodium, calcium or other alkaline or earth alkaline metals. In a series of trials, some of them were investigated. In those parameter ranges largely corresponding to the parameters of the existing activation process, however, most of the investigated compounds proved to be unsuitable. For these compounds no results regarding efficiency, electric characteristics, and reproducibility were achieved which would be comparable to the results obtained by a solar cell produced by means of a CdCl₂ activation process. Surprisingly, however, calcium hypochlorite (Ca (OCl) ₂) has proven suitable, with the process steps and parameters to be used (temperature, duration of activation) being similar to those of CdCl₂ activation.

Therefore, according to the invention, it is intended to use calcium hypochlorite, e.g. a calcium hypochlorite solution, instead of or in combination with CdCl₂ for activation, and to keep or even improve the tried and tested process parameters. Thus, hazards and environmental impact caused by CdCl₂ can be reduced without the need of developingcompletely new processes. Furthermore, negative effects of the process on other layers of the CdTe solar cell, e.g. the CdS layer or the front contact layer, as well as costs may be reduced. Calcium hypochlorite is manufactured on industrial scale and therefore very cheap.

The calcium hypochlorite is applied to the CdTe layer preferably as a solution. If the calcium hypochlorite is used in combination with CdCl₂, the calcium hypochlorite solutioncan easily be mixed with a solution of CdCl₂. Preferably calcium hypochlorite is added to CdCl₂ in small amounts, i.e. in the range of 1％to 10％. Suitable methods for applying a solution are known from the state of the art. Such methods are, e.g. :

- roller coating with an aqueous salt solution,

- spraying an aqueousor aqueous salt solution,

- aerosol coating,

- dipping bath.

Preferably, the calcium hypochlorite solution is produced directly before applying the solution to the CdTe layer. Thus, degradation of the components can be reduced, and a stable concentration of the components having a good impact on the CdTe layer, in particular the chlorine and oxygen, can be achieved. “Directly before” means less than 6 hours in advance to the application onto the surface of the CdTe layer.

Producing the calcium hydrochlorite solutionis achieved, for instance, by dissolving calcium hypochorite powder in deionized water. Since the calcium hypochlorite is often sold as a powder which comprises also other components like fillers or additives, it may be necessary to filtrate the produced calcium hydrochlorite solution before applying it to the surface of the CdTe layer. However, all compounds comprising calcium hypochlorite are understood by the term “calcium hypochlorite” as used in the application

The calcium hypochlorite is preferably present in an aqueous5％to 20％solution, particularly preferably in a 15％solution.

Preferably the calcium hypochlorite is applied to an unheated semi-finished CdTesolar cell, i.e. a semi-finished CdTe solar cell at room temperature (20℃ to 25℃) . According to a further preferred embodiment, the semi-finished CdTe solar cell is heated up to a higher temperature, e.g. 60℃, before or during applying the calcium hypochlorite.

Applying the layer of calcium hypochlorite onto the semi-finished CdTe solar cell is followed by the heat treatment required for activating the CdTe layer, at temperatures which are preferably between 250℃ and 450℃, particularly preferably between 300℃ and 430℃.

Treatment time ranges preferably from 15 minutes to 45 minutes, particularly preferably from 20 minutes to 30 minutes.

In a special embodiment, the semi-finished CdTe solar cell is held for some time, in particular in the range of 2 to 10 minutes, e.g. for 5 minutes, at a temperaturenot higher than 170℃after applying the calcium hypochlorite onto the semi-finished CdTe solar cell and before performing the heat treatment step. This gives the calcium hypochlorite or its dissociated components the possibility to migrate into the CdTe layer. Furthermore, the heat treatment step as described above may be made shorter than without this intermediate temperature step.

Calcium hypochlorite decomposes at temperatures above 175℃, wherein compounds containing oxygen and compounds containing chlorine are set free. Chlorine helps recrystallization as known from the process using cadmium chlorine. Oxygen containing compounds result in integration of oxygen into the CdTe layer which has positive effects on electrical characteristics of the produced CdTe solar cell.

Due to the activation, the CdTe layer recrystallizes. However, if high temperatures are used during heat treatment, thismay result in mixing of the CdS layerwith the CdTe layer. A very high degree of mixing of these layers involving the inclusion of sulphur into the CdTe crystals, the so-called over-activation of the CdTe layer, would have an unfavourable effect on the properties of the CdTe solar cell. Surprisingly, a combination of calcium hypochlorite and cadmium chloride results in better recrystallization of the CdTe layer. Thus, temperature used during heat treatment and/or the time of the heat treatment may be reduced without a negative effect on the recrystallization. Advantageously, lower temperatures and/or shorter times, i.e. a lower thermal budget which is the product of temperature and time of a process, result in lower mixing of the CdS layer and the CdTe layer and in lower degradation of other layers, e.g. of the front contact layer. Preferably, the ratio of calcium hypochlorite to cadmium chloride within the aqueous solution applied to the surface of theCdTe layer is in the range of 1: 100 to 1: 10.

After heat treatment, preferably a rinsing step using deionized water and a drying process, both corresponding to the process steps in CdCl₂ activation, are carried out.

A further step to eliminate any residual calcium hypochlorite and/or cadmium chloride not yet removed by the previous rinsing is carried out. To this end, a treatment with an acid, e.g. HCl or H₃PO₄, with a complexing agent, e.g. EDA (ethylenediamine) , or a combination of some of these compounds is performed, wherein all of the mentioned compounds are used in an aqueous solution in the range of 0.5％to 5％. The semi-finished CdTe solar cell may be dipped, by way of example, for 1 minute into such an aqueous solution held at room temperature (20℃ to 25℃) . This treatment is followed by another rinsing step using deionized water, and a drying step. Alternatively or even additionally, a step of mechanical polishing the surface of the CdTe layer may be performed in order to remove any residuals. This step is also followed by a rinsing and drying step.

Instead of the previously mentioned treatment step, a standard etching step may be performed, for example with the well-known nitric-phosphoric (NP) acid etching solution or with the well-known bromine-methanol (BM) etching solution, to treat the activated cadmium telluride layer.

After the following rinsing using deionized water, and after drying the semi-finished CdTe solar cell, a back contact layer, for instance a molybdenum layer, is applied using methods from prior art, preferably by sputtering. However, it is also possible to use other materials known from prior art for the back contact of the CdTe solar cell, for instance Sb₂Te₃, gold (Au) and nickel containing further admixtures, and layer sequences made from various contact layers.

Figures

The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate an embodiment of the present invention and together with the description serve to explain the principles. Other embodiments of the invention are possible and lie within the scope of the invention. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.

Figures 1 to 5 schematically show the sequence of process steps, including the activation step according to the invention using calcium hypochlorite.

Fig. 1 shows the prepared solar cell with the substrate (1) , onto which the transparent front contact (21) , the CdS layer (3) , and the CdTe layer (4) on top of it have already been applied.

Fig. 2 schematically shows the application of an aqueous calcium hypochlorite solution (5) with a concentration of 15％. After application, the semi-finished CdTe solar cell is held at a temperature of 170℃ for 5 min, followed by a temperature of 400℃ for 20 min.

As schematically shown in Fig. 3, an activated CdTe layer (41) remainsafter the activation process.

Fig. 4 schematically shows the cleaning step after the activation process. Here, a cleaning solution (6) containing EDAis used to remove residuals of the calcium hypochlorite.

Fig. 5 schematically shows the layer sequence of the finished solar cell after applying aMo layer (22) of the back contact onto the CdTe layer (41) .

Reference numbers

1 Substrate (glass)

21 Front contact (transparent, TCO)

22 Mo back contact

3 CdS layer (pure or modified CdS (cadmium sulfide) )

4 CdTe layer

41 activated CdTe layer

5 Aqueous solution containing calcium hypochlorite

6 Cleaning solution

Claims

Method for activating the CdTe layer (4) of semi-finished thin-film CdTe solar cells, characterized in that calcium hypochlorite is applied to the CdTe layer (4) and the semi-finished thin-film CdTe solar cell subsequently undergoes a heat treatment.
Method according to claim 1, characterized in that the calcium hypochlorite is present in an aqueous solution (6) .
Method according to claim 1, characterized in that the calcium hypochlorite is present in a methanolic solution.
Method according to claim 3, characterized in that the calcium hypochlorite is enriched with water.
Method according to any of the claims 2 to 4, characterized in that the calcium hypochlorite is present in a solution of 5％ to 20％, preferably in a solution of 15％.
Method according to any of the claims 2 to 5, characterized in that the aqueous solution (6) further comprises cadmium chloride.
Method according to claim 6, characterized in that the ratio of calcium hypochlorite to cadmium chloride within the aqueous solution is in the range of 1: 100 to 1: 10.
Method according to any of the preceding claims, characterized in that the calcium hypochlorite is applied onto the CdTe layer (4) by means of roller coating, or by spraying a solution containing the calcium hypochlorite, or by means of aerosol coating or by means of dipping the semi-finished thin-film CdTe solar cell into a solution containing the calcium hypochlorite.
Method according to any of the preceding claims, characterized in that the heat treatment is carried out at a temperature in the range between 250℃ and 450℃, and preferably in the range between 300℃ and 430℃.
Method according to any of the preceding claims, characterized in that the heat treatment is carried out for a duration in the range between 15 minutes and 45 minutes, and preferably in the range between 20 minutes and 30 minutes.
Method according to claim 9 or 10, characterized in that the heat treatment comprises at least two steps, wherein, in a first step, the semi-finished CdTe solar cell is held at a temperature of lower than or equal to 170℃ for a first time period in the range between 2 minutes and 10 minutes, and wherein, in a second step performed after the first step, thesemi-finished CdTe solar cell is held at a temperature in the range between 250℃ and 450℃ for a second time period in the range between 5 minutes and 45 minutes.
Method according to any of the preceding claims, further comprising a cleaning step performed subsequently to the heat treatment, wherein the cleaning step comprises a treatment of a surface of the CdTe layer (4) with an acid, with a complexing agent, or a combination of some of these compounds.
Method according to claim 12, characterized in that the acid is HCl or H₃PO₄or in that the complexing agent is ethylenediamine.
Method according to any of the preceding claims, further comprising a step of mechanical polishing a surface of the CdTe layer (4) , wherein the step of mechanical polishing is performed subsequently to the heat treatment.