WO2014095347A2

WO2014095347A2 - Formation of a doped transparent conductive oxide layer by solution deposition

Info

Publication number: WO2014095347A2
Application number: PCT/EP2013/075380
Authority: WO
Inventors: Harald HAGENDORFER; Yaroslav E. ROMANYUK; Ayodhya N. TIWARI
Original assignee: Empa Eidgenössische Materialprüfungs- Und Forschungsanstalt
Priority date: 2012-12-18
Filing date: 2013-12-03
Publication date: 2014-06-26
Also published as: WO2014095347A9; EP2935650A2; WO2014095347A3; WO2014095347A4

Abstract

A non-vacuum, low temperature (<200°C) method for formation of a doped transparent conductive oxide layer (5) by solution deposition, using a precursor solution comprising ammonia with pH ≥ 10 and metal tetraamine complexes, usable for forming doped transparent conductive oxide layers (5) on a substrate providing a suitable nucleation surface (2) should be optimized so that thin films of doped transparent conductive oxide layers (5) could be reached in a fast and reliable way. This will be reached, by a controllable continuous variable adding of a metallic doping element to an initial dopant-free precursor solution (1) from a dopant source (3) for adjusting the actual dopant concentration precisely from zero to a desired dopant concentration, turning the dopant-free precursor solution (1) into a precursor solution with dopant(4), while the growing process of the doped transparent conductive oxide layer (5) is taking place for a deposition time, in order to change the actual dopant concentration or to keep it constant during the deposition process from outside of an deposition container (6).

Description

Formation of a doped transparent conductive oxide layer by solution deposition

TECHNICAL FIELD

The present invention describes a method for formation of a doped transparent conductive oxide layer by solution deposition, using a precursor solution comprising ammonia and metal tetraamine complexes, usable for forming doped transparent conductive oxide layers on a substrate which can serve as a suitable nucleation surface, the use of the method for producing any optoelectronic, photovoltaic, or other devices and an apparatus for formation of a doped transparent conductive oxide layer by solution deposition on a substrate providing a suitable nucleation surface, comprising a deposition container, capable of holding a dopant free precursor solution based on ammonia and metal tetraamine complexes.

STATE OF THE ART

Transparent conductive oxides (TCO) are a class of doped semiconductor materials where the property of low resistivity (p = 10^"2 to 10^"5 Qcm) is paired with a wide band gap ( > 3 eV) resulting in transparency of light in the visible and near infrared region (wavelengths between λ = 0.4-1.5 μπτι). They are widely used in optoelectronics, photovoltaics, window and display technologies.

Popular TCO materials are currently based on Indium oxide (ln₂0₃), Tin oxide (Sn0₂), and Zinc oxide (ZnO) . Doping of ln₂0₃ with Sn(IV) (Tin doped Indium oxide, Sn :ln₂0₃, or ITO) results in, until now, lowest reported resistivity (p ≤ 10^~4 Ωαη) at excellent visible transparency (T > 90%) . Thus ITO is currently used as a high performance TCO. However, because of the abundance of materials compared to ITO, increased attention is paid on doped ZnO as a more cost efficient alternative material .

Doping of ZnO with a doping element, for example Aluminium, leads to an increase in carrier concentration, and hence, decreases resistivity of ZnO. Aluminium doped ZnO (AI : ZnO, AZO) is one of the most notable representatives with resistivity low as 1 -2 x 10^"4 Ωατι for vacuum deposited layers.

Thin film deposition techniques for TCOs can generally be divided into vacuum and non-vacuum techniques. Non-vacuum deposition techniques have a higher potential for low cost manufacturing because of a lower degree of sophistication as well as higher deposition rates and a higher material utilization. Up to now employed non-vacuum techniques for thin film formation of TCOs utilize a liquid solution or a reactive gas with suitable precursors. Well known solution based techniques are electro-deposition, sol-gel deposition, spray assisted deposition, knife blading, ink-jet printing, chemical vapour deposition (CVD), and chemical bath deposition (CBD) . Solution based deposition methods are of increased interest for metal doped ZnO or other transparent conducting oxide thin films because of the opportunity to use aqueous-, non-toxic solutions, low deposition temperatures (< 100°C), fast growth rates, and easy upscaling.

A very close prior art for a solution deposition method for doped ZnO thin film growth on a substrate, is disclosed in Miyake, et al . , Physica Status Solidi (a), 209, (2012), 945. The deposition was performed with a crystal growth reactor where a precursor solution can be stirred and heated, and with dimensions suitable to immerse an appropriate substrate for deposition of the doped ZnO transparent conductive thin film. A pre-deposited layer of ZnO functioning as a nucleation surface was used to promote the growth of a high quality crystalline thin film; in that case Al uminium doped ZnO. Doping was performed with an Aluminium nitrate salt added to the initial precursor solution before thin film deposition.

The growth of the Aluminium doped ZnO film was performed in a continuous aqueous solution process. A ZnO-saturated ammonia solution with a pH of 10.7 at room temperature, comprising an amount of sodium citrate, distilled water and an amount of Aluminium nitrate was used . The amount of ZnO is added in form of a ZnO powder, leading to the formation of a zinc tetraamine complex, and the saturated precursor solution was agitated for more than a day. After inserting the substrate with the nucleation layer the film growth of ZnO was performed for 5h in the crystal growth reactor after increasing the precursor solution temperature in the crystal growth reactor to 90°C. The concentration of Zn ions was kept constant, while the crystal growth was performed for several hours. The dopant concentration, due to the added Aluminium nitrate to the initial precursor solution, decreases with the crystal growth or fil m growth process, resulting i n a doping of the transparent conductive oxide. After an annealing step at 300°C in air the resistivity of the Aluminium doped ZnO films was decreased by more than two orders of magnitude compared to as deposited layers, as low as 3xl0^"2 Ωαη .

As known from WO2010/121060, US2010/263586 and US2011/101414, ZnO layers using aqueous precursor solutions zinc ammine complexes were formed. The doping of resulting ZnO layers can be achieved by addition of dopant additives to the solution prior the doping process or deposition process. The dopants used should be highly soluble and are solved completely i n the precursor solution . Al nitrate is completely solved in the precursor solution . Also AI 2O3 can be mixed up with ZnO powder before the deposition process. Also prefabricated Al doped ZnO can be used building part of the precursor solution. Additives of the precursor solution in different forms, for example metal citrate salts, citric acid, other salts or acids which produce stable anions that interact with the surfaces of the ZnO, surfactants, polymers, and/or biomolecules are known. All these additives are added to the precursor solution prior the thin film deposition process, leading to a fixed consumable dopant concentration . The dopant concentration decreases with the crystal growth or film growth process. To assure, that the doping is sufficient, a maximal amount of additives is solved i n the precursor solution before starting the deposition process.

The doping of ZnO is necessary to obtain low resistivity and is the most critical step in formation of a doped ZnO transparent conductive thin film with solution deposition. Doping with metal salts added to the precursor solution, as described i n Miyake et al . , slow down the deposition rate and alters the morphology, resulting in an unfavourable, non-compact polycrystalline layer. Appropriate film thickness is only achieved at very long deposition times and films with not-satisfactory resistivity ( > 10^"2 Qcm) are obtained due to a high density of structural defects. Appropriate doping procedures to control the metallic dopant concentration i n the growing thin film are necessary to obtain desired optoelectronic properties (p < 10^"2 Qcm and T% > 80% at wavelengths between λ = 0.4- 1.5 pm) .

For ensuring low resistivity (p ≤ 10^"2 Ωατι) and good transparency (T% > 80% at wavelengths between λ = 0.4- 1.5 μιτι) an annealing step is necessary. It is common for non-vacuum deposition techniques to perform an annealing step at higher temperatures (300 - 600°C) after thin film formation, in order to recrystallize the deposited layer and achieve a resistivity < 10^"2 Qcm.

Non-vacuum deposition techniques for TCOs applicable for temperature sensitive substrates (e.g. photoelectric converters of the type Cu(In,Ga)_x(S,Se)_y (CIGS), organic photovoltaic devices (OPV), or polymer substrates such as polyethylene terephthalate) have to fulfil the criteria to be a low temperature (< 200°C) process. Thus non-vacuum solution based deposition techniques for TCOs so far show limited applicability for temperature sensitive substrates, although being more cost effective with the same or faster deposition rates compared to common vacuum techniques.

DESCRIPTION OF THE INVENTION

The object of the present invention is to provide a non-vacuum, solution based method for deposition of a metal-doped ZnO (AZO) transparent conductive thin film with solution deposition at low process temperatures from aqueous solutions via an in-situ controllable doping step while the thin film deposition takes place.

The method leads to a fast and reliable formation of a doped transparent conductive oxide layer with a favourable compact polycrystalline form, with electrical resistivity < 10^"2 Ωατι and optical transparency > 80% at wavelengths between λ = 0.4- 1.5 μιτι.

Another object of the invention is to provide a simplified method where the used temperature in any method step does not exceed 200°C. With this method temperature sensitive materials can be used as substrates.

The process comprises the following steps:

Preparation of a dopant free precursor solution, so that the deposited ZnO thin film can be doped simultaneously with the metallic dopant in with controllable doping levels of 0 to 5% and higher can be reached.

Deposition of the Aluminium doped ZnO thin film on a substrate which can serve as a suitable nucleation layer or which was coated with a nucleation layer beforehand, followed by a washing and a drying step to remove solution residues, in order to obtain low resistivity (< 10^"1 Ωαη) and high transparency ( > 80% at wavelengths between λ = 0.4-1.5 μιη) at enhanced temperatures, usually not exceeding 200°C. Using in particular a photon assisted annealing step (e.g. Infrared or UV light) to improve the above given electro-optical properties to p ~5xl0^"3 Ωαη and T > 90% at wavelengths between wavelengths of λ = 0.4-1.5 μιτι without exceeding the temperature of 200°C.

With respect to the current state of the art, this invention allows to obtain a doped transparent conductive ZnO thin film (p≤lxl0 ² Ωαη, T > 90% at wavelengths between λ = 0.4- 1.5 μιτι) by a non- vacuum process without exceeding temperatures higher than 200°C at any process step.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic chart of the disclosed method for formation of a deposited layer of a doped transparent conductive oxide on a substrate.

Figure 2 shows a schematic setup to implement the described method for formation of a doped transparent conductive oxide layer by solution deposition. Figure 3 shows the time dependence of the Zn (dark grey diamonds) and Al (light grey triangles) concentrations in the deposition container during the growth.

Figure 4 shows a transmission (T%) and reflectance (R%) spectra of an undoped and an Al doped ZnO thin film, prepared with the claimed process, compared to a transmission spectrum of a glass substrate. Figure 5a shows a secondary electron microscopy image of a cross section of an Al doped ZnO thin film layer with a thickness of 2600 nm prepared according Example 1 in two different magnifications, while

Figure 5b shows a X-ray diffraction (XRD) pattern of the Al- doped ZnO thin film of figure 5a.

Figure 5c shows an energy dispersive X-ray spectrum of the Al- doped ZnO thin film of figure 5a (2.5 at% Al in respect to Zn) prepared with the claimed process.

Description

Figure 1 shows a flow-chart of the disclosed and claimed method for formation of a doped transparent conductive oxide layer by solution deposition.

A solution comprising ammonia with pH > 10 and metal tetraamine complexes is prepared as a dopant-free precursor in a preparation container. In a first step I the dopant-free precursor solution and a substrate with a suitable nucleation surface is introduced in a deposition container heated to a temperature of at least 60 °C. A doping source is added in a controllable and continuous way while deposition takes place to achieve a gradually increasing amount of metallic dopant in the precursor solution, usually by immersing a group IIIA element of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry in its metallic state. As depicted in figure 1, the order of the sub-steps of the first step I are exchangeable. In a second step II the substrate will be removed from the precursor solution with added dopant, followed by a subsequent washing of the substrate with deposited doped transparent conductive oxide layer to remove solution residues before the substrate is dried in air, vacuum, inert, or reactive atmosphere at elevated temperature usually not exceeding 200°C.

The final third step III comprises an annealing of the dried doped transparent conductive oxide layer employing elevated temperatures usually not exceeding 200°C, or treatment with electromagnetic radiation such as UV or IR light in an air, vacuum, inert, or reactive atmosphere. As disclosed in figure 1 and the following experimental results, experiments have been carried out using Zinc oxide (ZnO) as a Zn source to form a Zn tetraamine complex, and Aluminium metal as a dopant source to deposit a transparent conductive oxide layer.

As metallic dopant source not only Aluminium but also other group IIIA elements of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry can be used, such as B, Ga, and In .

In the following figures the method will be described i n detail, by forming an Al uminium doped ZnO thin film as one possible doped transparent conductive oxide layer 5. In a preparation container 7 a dopant-free precursor solution 1 , comprising ammonia and metal tetraamine complexes is prepared at room temperature.

The dopant-free precursor solution 1 is a true, aqueous basic solution of a soluble Zinc salt, an aqueous basic suspension of an insoluble Zn salt, a dispersion of ZnO (nano)-particles/powder. The pH of the solution is adjusted to have a value of 10 and above. Usually N H₄OH is used to adjust the pH .

NH₄OH also acts as a complexant for the Zn ion . The solubility of Zn in the dopant-free precursor solution 1 is provided by the formation of the zinc tetraamine complex [Zn(NH₃)4] ²⁺ (see Eq. 1 ) .

Eq. 1 Zn²⁺ + 4 NH4OH→ [Zn(NH₃)₄]²⁺ + 4 H₂O

The Zinc concentration in the aqueous basic solution, arising from the soluble Zi nc salt or insoluble Zi nc salt/ZnO (nano)particles/powder is between 1 and 1000 mmol/L and typically 50 mmol/L. The Zn concentration i n the aqueous basic suspension, arising from the insoluble Zinc salt/ZnO (nano)particles/powder, is dependent on the pH of the solution and the solubility product of the compound.

To control the morphology of the later deposited Al uminium doped ZnO transparent conductive thin film 5 water soluble carboxylic acids (e.g . Citric acid), salts of carboxylic acids (e.g . Ammonium Citrate), esters of carboxylic acids (e. g. Triethyl Citrate), or alcohols (e.g. Ethanol) can be added to the dopant-free precursor solution 1. The morphology of the ZnO thin film 5 can be altered by changing their concentration so that the thin film surface features an increased roughness with reduced reflectivity and increased light trapping . The concentration of the compounds can be up to 10 mmol/L and is typically about 1 mmol/L. To control the stability of the solution pH a stabilizing compound, typically an ammonium salt, has to be added to the dopant-free precursor solution 1. The concentration of the salt can be up to 100 mmol/L and typically is 50 mmol/L. The dopant-free precursor solution 1 is supplied through pipes 9 into a deposition container 6 in which the deposition process takes place.

A substrate 2 which can serve as a suitable nucieation surface for growing a doped ZnO transparent conductive thin film 5, is introduced i n the deposition container 6 and immersed into the dopant-free precursor solution 1. A suitable nucieation surface can be any surface which allows to grow a crystalline doped ZnO thin film, and can be composed either of ZnO based material (e.g . thin film of ZnO deposited by gas-phase or solution method, ZnO nanoparticles) or any other suitable material (e.g. CdS, Mn0₂, CIGS, etc. ) - The substrate 2 can be flexible or rigid, e.g . glass, ceramic, polymer or metal foil, etc.

In the next sub-step of step I the dopant-free precursor solution 1 is heated in the deposition container 6 to temperatures of at least 60 °C and the solution is kept at this elevated temperature. This is done by heating an oil bath 8 in which the deposition container 6 is placed. A doping source 3, comprising a doping element, typically in pure metallic form or as an alloy, is placed to be introduced into the dopant-free precursor solution 1 in a controlled way. According to figure 2 the doping source 3 is a solid body of a metal or a metal comprising alloy, which can be immersed into the deposition container 6 and extracted from the container 6. The doping element is capable to enhance the electrical conductivity of the growing ZnO layer and is added/removed in a controllable way to achieve a gradually increasing/decreasing amount of dopant i n the precursor solution .

Metallic Aluminium (or B, Ga, In) or alloys of these elements can be added to the heated precursor solution with dopant 4. The dopant source 3 depicted in figure 2 is an Aluminium metal foil, comprising metallic Aluminium as doping element. With such a dopant source 3 a controllable way to achieve doping of the growing ZnO layer is possible.

Also other forms of solid bodies comprising a dopi ng element of the above mentioned list could be used, for example rods of pure metal or pressed metal powder, metal alloys, or pressed metal oxide powders. The dopant supply can be controlled by the immersed surface, type, or added quantity of the dopant source 3. The possibility of immersion and removal of the dopant source 3 is indicated by the double arrow in figure 2. If the dopant source 3 is removed from the precursor solution than the supply of doping element is stopped and gradually decreasing.

The doping element can also be added by a dopant source 3 comprising a liquid with doping element by controlled infilling into the deposition container 6 while the deposition process is running . In each case the dopant source allows a supply of the doping element from outside of the deposition container 6 and not from the initial precursor solution from the preparation container.

The doping element is typically added in its metallic form or in form of an alloy. The doping element can be dissolved at elevated pH values and temperatures because of the formation of a soluble complex, e.g. a hydroxide complex Eq.2) .

Eq. 2 2M (s) + 20H^" + 6H₂0→ 2[M (OH)₄]^" + 3H₂† (g)

M = hydroxide forming group IIIA metal as dopant

The pH of the solution has to be high enough to dissolve appropriate amounts of the doping element in form of a hydroxide complex. The temperature of the precursor solution with dopant 4 is increased to 60 °C and higher. Because the [Zn(NH₃)4] ²⁺ complex hydrolyses to the zinc hydroxide (Zn(OH)₂) species at elevated temperatures, the solubility of Zn is decreased when the reaction solution is heated (see Eq .3) . The Zn(OH)₂ species is known to dissociate i n water and ZnO at temperatures higher than 60°C (Eq .4) . The net reaction (Eq .5) results in the formation of ZnO directly from the [Zn(NH₃)₄]²⁺ complex at elevated temperatures. Thus the increase of the temperature leads to a deposition of ZnO on the surface of the substrate.

+

Eq. 3 [Zn(NH₃)₄] + 2 H₂0→ Zn(OH)₂j + 4 NH₃ + 2 H

Eq. 4 Zn(OH)₂→ ZnO + H₂O

+

Eq. 5 [Zn(NH₃)₄] + H₂O→ ZnO + 4 NH₃ + 2 H

The doping element, according to figure 2 in its metallic form, which is dissolved as complex i n the aqueous basic precursor solution/suspension with dopant 4 at minimum 60°C, undergoes co- precipitation and is incorporated during the ZnO thin film formation . Deposition of a metal doped ZnO thin film 5 takes place.

The concentration of the dopant in the precursor solution with dopant 4 is dependent on the temperature, pH of the solution, the doping element itself, as well as the surface/quantity dopant immersed/added.

The growth speed as well as the morphology of the metal doped ZnO thin film is dependent on the concentration of the doping element itself. Gradually increase of the dopant concentration results in a low doping level in the beginning of the growth and an increasing doping level in the course of the deposition. This favours a self-controlled and fast growth of a dense and doped ZnO layer.

According to figure 2 a setup of preparation container 7 and deposition container 6 is used, where through the pipes 9 freshly prepared dopant free precursor solution 1 can be refilled in the deposition container 6 and used precursor solution with dopant 4 can be removed. Figure 3 demonstrates the course of the Zn (dark grey diamonds) and Al (light grey triangles) concentration in the heated precursor solution with dopant 4 during deposition of the Al doped ZnO transparent conducting thin film 5. The low level of Al in the beginning favours a fast ZnO growth whereas increasing Al concentrations in the course of the deposition are necessary for successful doping resulting in low resistivity of the transparent conductive thin film 5. The amount of doping in the ZnO transparent conductive thin film 5 can be controlled by changing the immersed surface/added quantity or the immersion time of the metal/alloy. This offers the possibility to change the doping levels and doping regions in the ZnO transparent conductive thin film 5 and obtaining a doping gradient. The doping element in its metallic form can be reused as long it is not completely dissolved in the precursor solution.

The immersion of the substrate 2 can be performed either before preparation of the dopant-free precursor solution 1, before or after adding the doping element in its metallic or alloy form, or any time during or after the precursor solution is heated. Addition of the dopant can be performed at any stage before or during deposition. Also the heating of the precursor solution in the deposition container 6 can be carried out after addition of the doping element or before addition of the doping element.

Achievable Transmission (T%) and Reflectance (R%) spectra of an Al doped ZnO thin film, prepared with the claimed method, compared to a transmission spectrum of a glass substrate and an undoped ZnO thin film are shown in figure 4. In the second step II the substrate 2 with deposited doped conductive oxide layer 5 is removed from the deposition container.

The washing step by rinsing the surface with a liquid (e.g . deionised water), is performed after the deposition of a ZnO transparent conductive thin film 5 to clean the surface from undesired ZnO precipitates and solution residues. The washing/drying step is necessary to remove solution residues after the deposition of a ZnO transparent conductive thin film 5.

The drying step after deposition can be performed in in air, vacuum, inert, or reactive atmosphere. Drying is usually performed at 200°C in a nitrogen atmosphere for 60 minutes.

The third step III, the annealing step improves the optoelectronic properties of the deposited metal doped ZnO thin film 5. Annealing can be done either using elevated temperature i n air, vacuum, inert, or reactive atmosphere, or employing an electromagnetic radiation source such as UV light (λ = 200 - 400 nm) or IR-light (λ > 900 nm) in ai r, vacuum, inert, or reactive atmosphere. Typically UV-light is employed for annealing, not increasing the temperature of the substrate 2 with deposited doped conductive oxide layer 5 above 200°C. The annealing step enhances the optoelectronic properties such that T% > 90% (at wavelengths of λ = 0.4- 1.5 m) and p≤ 1 x 10^"2 Qcm.

While the deposition process is running the doping element can be added in a controlled way as described above. If the doping element concentration i n the doped transparent conductive oxide layer should be changed, the precursor solution with dopant 4 in the deposition container 6 can be exchanged or diluted with more dopant free precursor solution 1 , or the immersed surface/added amount of the dopant source in the deposition container 6 is changed .

In the following example the method is explained in more detail . EXAMPLE 1 (Al doped ZnO based TCO with metal aluminum)

As precursor solution, 10 g ZnO powder is immersed in a 1 mol/L NH₄OH in DI (distilled) water solution. To the dispersion 1 mM Ammonium citrate and 50 mM Ammonium nitrate are added . The final solution is stirred overnight. The Zn concentration determined by Inductively Coupled Plasma Mass Spectroscopy (ICPMS) i n the solution is 38 ± 2 mmol/L.

A glass substrate, containing a nucleation layer of non-conductive, intrinsic ZnO with a thickness of 50 nm is immersed in the precursor solution .

A metal Al foil is immersed into the precursor solution. The solution is heated to 90°C in 5 minutes and is kept at 90°C for 30 minutes.

The Al concentration in the solution is determined to be between 0.5 mmol/L and 4 mmol/L depending on the surface of Al-foil immersed into the solution . The dependence of the Al concentration in the precursor solution versus the immersed Al surface is linear. The dependence of the Al concentration in the grown film on the Al concentration in the precursor solution is also linear, with a slope of 0.5.

After a deposition time of 30 minutes a thin film layer with a thickness of about 2500 nm is formed . The substrate is removed from the deposition bath and washed with DI water. Immediately afterwards the substrate with the grown film is dried at 200°C in a nitrogen atmosphere for 60 minutes.

Afterwards annealing is performed under UV light for additional 10 minutes.

The optoelectronic properties are : T% < 90% between wavelengths of λ = 0.4- 1.5 pm ; p = 7.8 x 10^"3 Qcm. The experimental results of Example 1 are depicted in figures 5a) to 5 c) .

The aim of the invention is a non-vacuum, low temperature process to fabricate doped transparent conductive oxide thin films 5 with a solution deposition method on a flexible or rigid substrate 3 that serves as a suitable nucleation surface. Such a substrate 3 is immersed in the deposition container 6. The doping is performed in- situ from the dopant source 2 gradually increasing/decreasing the dopant concentration during deposition of the doped transparent conductive oxide thin film 5 with controlled doping levels from 0 to 5 at% .

Due to the fact, that temperatures lower than 200°C can be used in all of the method steps, this method is predestined for use of temperature sensitive substrates. These include flexible substrates ( e.g . Polyethylene, Polypropylene, etc. ) that are used i n roll-to-roll processing or rigid substrates that are used in batch-to-batch or inline processing . Therewith a new low temperature method applicable for forming a doped transparent conductive metal oxide layer in batch-to-batch, in-line, and roll-to-roll setups is found . Due to the fact that a liquid precursor solution is used, a variety of methods for solution deposition of doped transparent conductive oxide layers can be employed such as chemical bath deposition (CBD), spray assisted deposition, knife blading, spin coating, dip coating, ink-jet printing, etc

Reference numerals

1 dopant free precursor solution

2 substrate

3 dopant source

4 precursor solution with dopant

5 doped transparent conductive oxide layer/ deposited layer

6 deposition container

7 preparation container

8 oil bath

9 pipe

I first step

II second step

III third step

Claims

PATENT CLAIMS

Method for formation of a doped transparent conductive oxide layer (5) by solution deposition, using a precursor solution comprising ammonia and metal tetraamine complexes, usable for forming transparent conductive oxide layers (5) on a substrate (2) which can serve as a suitable nucieation surface, characterized in,

a controllable adding to/removing of an doping element to an initial dopant-free precursor solution (1) from a dopant source (3) from outside of a deposition container (6)

for adjusting the actual dopant concentration precisely between zero and a desired dopant concentration, turning the dopant-free precursor solution (1) into a precursor solution with dopant (4) and back,

while the growing process of the transparent conductive oxide layer (5) is taking place for a deposition time, in order to change the actual dopant concentration or to keep it constant during the deposition process.

Method according to claim 1, wherein

first step comprising

a supply of a dopant-free precursor solution (1) in the deposition container (6), insertion of the substrate (2) in the deposition container (6), the controllable adding/removing of the doping element and heating the dopant-free precursor solution (1) respectively the precursor solution with dopant (4) to temperatures of at least 60 °C and keeping this elevated temperature leading to enhanced crystal growth, in order to start growth of the doped conductive oxide layer (5) for a deposition time of several minutes in the precursor solution with dopant (4), before

a second step comprising

removing the substrate (2) from the precursor solution with added dopant (4), subsequent washing the substrate (2) with a deposited doped transparent conductive oxide layer (5) to remove solution residues, and drying the substrate (2) in air, vacuum, inert, or reactive atmosphere at elevated

temperatures of usually 200°C is carried out followed by third step comprising

an annealing procedure or treatment with electromagnetic radiation of the dried substrate (2) in air, vacuum, inert, or reactive atmosphere to enhance the conductivity of the doped transparent conductive oxide layer (5), usually not exceeding 200°C.

Method according to claim 1 or 2, wherein

the suitable substrate (2) can be introduced in the empty deposition container (6) or in the deposition container (6) filled with dopant-free precursor solution (1) or in the precursor solution with dopant (4) after starting the addition of the dopant or after start of increasing the temperature of the dopant free precursor solution (1) or precursor solution with dopant (4).

Method according to one of the preceding claims, wherein the dopant source (3) is a solid body, comprising the doping element in elemental form or the solid body is a mixture comprising the doping element, which can be immersed into the dopant-free precursor solution (1) or took out of the precursor solution with dopant (4) in a controlled way while the deposition time, to achieve a precise regulation of the actual dopant concentration.

Method according to one of the preceding claims, wherein the doping element is capable of forming metal complexes soluble in the ammonia comprising dopant-free precursor solution (1) respectively precursor solution with dopant (4).

Method according to claim 4, wherein the dopant source (3) is a metal foil, metal rod, pressed metal powder, pressed metal oxide, or metal plate comprising the doping element, where the dopant supply is controlled by the immersed surface of the dopant source (3).

Method according to one of the preceding claims, wherein the variability of the dopant concentration in the initial dopant-free precursor solution (1) is adjustable from zero to a concentration up to 100 mmol/l, offering the possibility to change the doping levels and doping regions in the doped conductive oxide layer (5).

Method according to one of the preceding claims, wherein the material of the substrate (2) is chosen to provide a suitable nucleation surface which allows to grow a transparent conductive thin film (5), and can be composed either of ZnO based material (e.g. thin film of ZnO deposited by gas-phase or solution method, ZnO nanoparticles) or another material such as CdS, Mn0₂ or CIGS.

9. Method according to one of the preceding claims, wherein the material of the substrate (2) can be flexible or rigid, comprising glass, ceramic, polymer or metal foil.

10. Method according to one of the claims 2 to 9, wherein in the second step (II) the deposited doped transparent conductive oxide layer (2) is dried at elevated temperatures, in particular lower than 200°C, in a nitrogen atmosphere for several minutes, in particular sixty minutes.

11. Method according to one of the claims 2 to 10, wherein in the third step (III) the doped transparent conductive thin film (5) is annealed by exposure to electromagnetic radiation such as UV or IR-radiation in air, vacuum, inert, or reactive atmosphere .

12. Method according to one of the preceding claims, wherein the doping element comprises at least of one element of group IIIA of the periodic table of chemical elements as defined by the International Union of Pure and Applied

Chemistry.

13. Method according to one of the preceding claims, wherein the dopant-free precursor solution (1) is a true, aqueous solution of a soluble Zinc salt or an aqueous suspension of an insoluble Zinc salt in NH₄OH.

14. Method according to claim 13, wherein the Zinc

concentration arising from the addition of soluble Zinc salts or insoluble Zinc salts/(nano)particles can be up to 1000 mmol/l and in particular 50 mmol/l.

15. Method according to one of the claims 13 to 14, wherein the doping element is Aluminium or an Aluminium comprising alloy or compound.

16. Method according to one of the preceding claims, wherein water soluble carboxylic acids, salts of carboxylic acids, esters of carboxylic acids or an alcohol are added to the dopant-free precursor solution (1), with concentration of the compounds up to 10 mmol/L.

17. Method according to one of the preceding claims, wherein a water soluble pH stabilizing compound, typically an

ammonium salt, is added to the dopant-free precursor solution, with concentrations up to 100 mmol/l.

18. Use of the method according to one of the preceding claims, wherein a transparent conductive oxide is deposited, that can be used in any optoelectronic, photovoltaic, or other devices or structures where optical transparency and/or electrical conductivity are required, in an in-line, batch to batch or roll to roll setup.

19. Use of the method according to one of the preceding claims, wherein a transparent conductive oxide is deposited, with a setup that can be configured to allow any deposition technique that utilize a liquid precursor solution which is applied on a flexible or rigid substrate on a roll-to-roll, in-line, or batch to batch manner.

20. Apparatus for formation of a doped transparent conductive oxide layer (5) by solution deposition on a substrate providing a suitable nucleation surface (2), comprising a deposition container (6), capable of holding a dopant free precursor solution (1) based on ammonia and metal tetraamine complexes, wherein the apparatus comprises a dopant source (3) with an doping element, which is capable for a controllable adding of the doping element to the initially dopant free precursor solution (1), while the deposition of the transparent conductive oxide layer (5) takes place.

21. Apparatus according to claim 20, wherein the dopant source (3) is a foil, rod, plate, powder or any other form comprising the doping element, where the dopant supply is controlled by the immersed surface or added quantity of the dopant source (3) into the precursor solution (1, 4).