DE102004020879A1 - Process and device for reductive deposition of a powder layer of Cd- or Zn sulfide on Chalco-pyrities solar cells by a roll-to-roll process with shortened time in the deposition bath - Google Patents

Abstract

Process and device for reductive deposition of a powder layer of Cd- or Zn sulfide on Chalco-pyrities (sic) solar cells, with wet spraying chemical treatment to produce the power layer using at least two stable aqueous solvent mixtures, one of which contains a reduction agent and a S-containing component, while the other mixture a Cd or Zn salt, and a substance dependent on the pH value.

Description

Under takes the known technologies for the production of thin-film solar cells the so-called CIS cell (copper indium / gallium selenium / sulfur) a special position insofar as such cells are potentially low-degradation, high efficiency, low cost and environmentally friendly can be produced. There are both processes for applying the so-called precursor layers (Copper, indium / gallium, selenium / sulfur) in a high vacuum by sputtering or vapor deposition as well as non-vacuum method has become known e.g. the electrochemical deposition of said elements or their alloys (copper selenide, indium selenide, gallium selenide) or the compound semiconductor (copper indium selenide = ternary deposition, Copper indium gallium selenide = quaternary deposition). Produced on a laboratory scale were also CIS cells whose precursor layer by mechanical Application of a dispersion of nanoparticles of said substances on the so-called substrate (substrate plus Fitting, barrier and contact layer) is produced.

In usually goes through the precursor subsequently an annealing step, wherein the individual Elements or alloys to the actual, microcrystalline absorber connect. In practically all known methods, following the tempering step, a so-called "buffer layer" is applied, before the CIS solar cell is removed by the deposition of highly transparent, electrically conductive window layers (TCO) is completed.

For the production The mentioned buffer layer has become worldwide (if at all solar modules of CIS technology are already available on the market) established a method called CdS-CBD. It deals this is the deposition of cadmium sulfide from a chemical Bath (CBD = chemical bath deposition), also known as "immersion doping", as it The application of cadmium sulfide is less an extremely thin layer acts (about 40 nm "thickness"), rather than an incorporation of Cd atoms in the CIS lattice, in context with the formation of a for the function of the photoelectric solar cell essential p / n junction. The exact procedures were previously only partially explored by basic science, because under certain conditions, functional CIS cells (usually with a worse Efficiency) can be produced without the said CdS buffer layer could.

The "classic" CdS-CBD process is that a cadmium salt (CdJ2 or CdSO4) and thiourea at room temperature in an aqueous, ammoniacal solution be introduced. At the same time CIS cells or CIS modules with finished absorber layer completely immersed in the bath; the bath including solution mixture and CIS cells / modules be together within about 6 min. heated to 60 ° C, after another approx. 4 min. The CIS samples are removed from the solution. The remaining one bath liquid must be disposed of, the containers careful be cleaned of the everywhere deposited / adherent CdS.

Although the CIS cells produced in this way have the highest efficiencies, the described CdS-CBD process obviously has two very significant disadvantages:
Firstly, the process is not environmentally friendly, not so much because of the traces of cadmium that is ultimately present as an alloy or compound in the modules, but rather because of the considerable volumes of wastewater produced by the CIS coatings Heavy metal compound are loaded.

Of the second disadvantage are the serious production engineering difficulties, caused by the fact that not only the residence time of the cells or Module in the deposition bath with 10-15 min is relatively long, but by sufficient amounts of bath liquid again and again set and must be discarded after use, including the vessel cleaning. A continuous production flow is not possible, just a "batch process", whereby the batch size determines becomes by the bath size and the (possible high) stack density of the samples in the bath.- Further complications like the unwanted Deposition of CdS on the cell back and unwanted incorporation flocculation / bad contaminants in the buffer layer may seem marginal mentioned be.

It has now, especially those involved in CIS cell development University institutes, in the past years at efforts not missing, replacement processes that improve both environmental and environmental performance Should reduce manufacturing issues. However, one is in the Rule of a CIS production on glass, i. a continuous roll-to-roll manufacturing process, as he sought in the deposition of CIS on metal or plastic film is not assumed.

Probably the best-known alternative to the "classic CdS-CBD" is the ILGAR method of the Hahn-Meitner-Institut Berlin, where a dipping process (alternately into different but stable bath fluids) results in a CdS or ZnS layer of only a few Atomic layers thickness formed, so that for 40 nm total layer thickness about 40 Wech sel-dipping operations are required. ILGAR achieves good cell efficiencies and a solution to the problem of "environmental compatibility" with ZnS, but the process is anything but suitable for rapid, easy and continuous "on-the-fly" cell production.

The The present invention now overcomes both deficiencies: the use of an environmentally harmful cadmium compound can be omitted In addition, fall in the manufacturing process far smaller amounts of liquid at. Above all, a continuous run of the module or CIS cell production through the "buffer layer" manufacturing step even for roll-to-roll ribbon cell production (under atmospheric pressure conditions) guaranteed for the first time.

The Invention idea was developed in analogy to the seeding of plastic parts with silver for the subsequent electrochemical silver coating. This is on the one hand a water-dissolved one Silver salt, on the other hand, a reducing agent plus ammonia in aqueous solution sprayed. The result is a roughly 0.05 μm thick silver layer on the plastic.

For the environment- and production-friendly wet chemical deposition of ZnS as Buffer layer is now, derived therefrom proposed, initially 2 systems with stable solutions to use. The reactive mixture, from which CdS or (in the case the environmental friendliness) ZnS is excreted, only arises in that both liquids in finely sprayed Mix the shape on the pobe.

The inventive device thus consists of at least 2 containers ( 4.1 and 4.2 ), each with pump ( 3.1 and 3.2 ) and spray nozzle ( 2.1 and 2.2 ), wherein in one container a sulfur-donating substance (as mentioned below) and a reducing agent (examples also below), in the other container a zinc or cadmium salt (examples below) and a pH-determining agent, for example ammonia, each in aqueous Solution, is included. The solutions are maintained by heating at a temperature of 25 to 45 ° C, depending on the desired reaction rate.

The spray nozzles ( 2.1 and 2.2 ) of the two systems are adjusted so that the spray cones on the passing CIS cell band ( 1 ), so that only at this point a mixing of the liquids in exactly the amount required for the buffer layer deposition on the CIS solar cell takes place. The stoichiometric equilibrium of the reactants is adjusted on the one hand by the concentration of the solutions, and on the other hand by the flow rate determined by the pumps.

• • 2-Mercaptobenzothiazol
• • 3-Mercaptobenzimidazol
• • 3-Mercapto-1,2-propandiol
• • Thioacetamid
• • Polysulfide
• • Thiophen-derivate
• • Di-iso-butylsulfid
• • Di-tetra-butylsulfid
• • Dichlorobenzolthiol
• • Diäthyldithiocarbamat
• • Thiocyanat
• • Thioglykolsäure
• • Thiocarbamid

As sulfur-donating substances are preferably in question:

• • 2-mercaptobenzothiazole
• • 3-mercaptobenzimidazole
• • 3-mercapto-1,2-propanediol
• • thioacetamide
• • polysulfides
• • thiophene derivatives
• • Diisobutyl sulfide
• • Di-tetra-butyl sulfide
• • dichlorobenzenethiol
• Diethyl dithiocarbamate
• • thiocyanate
• • thioglycolic acid
• • thiocarbamide

• • Zink-Acetat
• • Zink-Acetylacetonat
• • Zink-Carbamat (basisch)
• • Zink-p-Toluolsulfonat

As zinc or cadmium-containing salts are preferably own

• • zinc acetate
• • zinc acetylacetonate
• Zinc carbamate (basic)
• Zinc p-toluenesulfonate

Such as the corresponding salts of cadmium.

• • Hydrazin-
• • Boran-
• • Borhydrid-
• • Alkoholat- oder
• • Phosphit- Verbindungen

Suitable reducing agents with simultaneous catalyst function are suitable

• • hydrazine
• • Boran
• • borohydride
• • Alcoholate or
• • phosphite compounds

Around the surface tension to reduce the use of known, non-iogenic wetting agents use.

PH value determining additives are known Ammonia, potash, amines and organic or inorganic acids.

Execution example:

A light yellow highly transparent CdS buffer layer of 0.04-0.05 μm thickness was prepared in less than 1 min. Spraying time and in a few minutes post-reaction time generated by misting on the tempered CIS absorber from 2 separate systems (as shown) via moving special nozzles:
Solution mixture 1, temperature about 40 ° C:
Sulfur-containing component: 1.5 mol / l
Reducing agent: 0.8-8 mol / l
Wetting agent: 0,001-0,01 Vol%
Solvent mixture 2, temperature about 40 ° C:
Cd-containing component: 1-5 mol / l
PH-determining substance: as needed

The After treatment of the samples was carried out as usual by rinsing with deionized water and hot air drying.

The Follow-up stories to complete the CIS solar cell, highly transparent and electrically highly conductive, so-called "window layers", are below in usual Applied to the cell. Then, a strip conductor grid ("grid") vapor-deposited, sprayed or be printed and the hereby complete CIS-cell by means of a varnish layer be covered and completed.

The The above method for producing the buffer layer makes it possible for the first time, to perform the entire process on a continuous tape, i. a roll-to-roll process to realize instead of a batch process.

1

CIS cells (Band), moved

2.1

Spray nozzle, system 1

3.1

Pump, System 1

4.1

Liquid container, system 1

2.2

Spray nozzle, system 2

3.2

Pump, System 2

4.2

Liquid container, system 2

Claims (4)

Method and device for the reductive deposition of a buffer layer of cadmium or zinc sulfide in chalcopyrite solar cells, characterized in that the wet-chemical treatment and production of a buffer layer by a spray process, wherein at least two stable aqueous solution mixtures are used, one of which containing the reducing agent and sulfur-containing component, the other a cadmium or zinc salt and a pH-determining substance. A method according to claim 1, characterized in that a or both aqueous Mixtures warmed up be, before or during they are fed to the spray nozzles. Method according to claim 1, characterized in that the aqueous Mixture of the reducing agent and a sulfur-containing component Wetting agent added becomes. A method according to claim 1, characterized in that the Substrate on which the CIS absorber layer is deposited, before or during the spray treatment heated becomes.