The invention is directed to a method for preparing a layer of In4Sn30i2, to a layer of In4Sn30i2 obtainable by said method, and to an electronic device comprising said layer.

The n-type doped oxides of indium, tin, and zinc form the basis of compounds referred to as transparent conductive oxides (TCOs). These materials offer a combination of high electrical conductivity with optical transparency over the visible wavelengths, which is highly desirable in many electronic and optoelectronic applications (mostly in the form as a transparent electrode). To date, the industry standard transparent conductive oxides is tin- doped indium oxide, also referred to as ITO. This material is a diluted solid solution of tin(IV) oxide (SnCh) in indium(III)oxide (Ιη2θ3) and has proven its potential in widespread applications such as electroluminescent devices, flat panel displays, and photovoltaic cells. ITO has a low resistivity and high transmittance, but it has the drawback of being rather expensive.

Indium, the primary metal of ITO, is rare and there is an increasing technological demand. In order to meet the increasing demand, the indium production has grown from 70 000 to 500 000 kg per year over the last 20 years to meet demand. This increasing technological demand for indium has lead to escalating costs.

There is a need for new materials that can be used as transparent conductive oxides and which are less expensive than the present industry standard tin-doped indium oxide.

Object of the present invention is to at least partly fulfil this need in the art. Further object of the invention is to provide a projected capacitive touchscreen with a low cost and highly efficient transparent conductive oxide.

In4Sn30i2 has been suggested as a promising candidate to replace conventional ITO, although the conductivity of a layer of this material when obtained with a low pressure technique is lower than ceramic ITO. Nevertheless, the known preparation of highly conductive In4Sn30i2 layers is rather elaborated, involving e.g. DC magnetron sputtering (Minami et al., Thin Solid Films 1997, 308-309, 13-18).

The invention is based on the insight that a layer of In4Sn30i2 having comparable conductivity and transparency over the visible spectrum to ITO can be prepared in a relatively simple manner from a solution. The layer of In4Sn30i2 has a significantly lower materials cost than a layer of ITO, because the net amount of required indium is much less.

Accordingly, in a first aspect the present invention is directed to a method for preparing a layer of In4Sn30i2, comprising

mixing a solution of In and Sn salts in an atomic ratio of In and Sn of about 4:3 with a complexing agent for In; and

applying the mixed solution onto a surface of a substrate, said surface having a temperature in the range of 250-400 °C, preferably 300-350 °C.

In accordance with the method of the invention a solution of In and Sn salts is mixed with a complexing agent for indium. The In and Sn salts can suitably be In and/or Sn halides, such as In and/or Sn chlorides. In order to achieve the right stoichiometry, the In and Sn salts should be provided in such a ratio that the In and Sn are present in an atomic ratio of about 4:3. The term "atomic ratio of about 4:3" in the context of this invention is meant to refer to an atomic ratio in the range of 4.4:2.6 to 3.6:3.4, preferably in the range of 4.2:2.8 to 3.8:3.2.

The solution of In and Sn can comprise a suitable solvent, such as isopropylalcohol and/or water. Preferably, the solvent comprises a mixture of isopropylalcohol and water. More preferably, a mixture of isopropylalcohol and water in a ratio of 70:30-99:1, such as in a ratio of 80:20-95:5, is applied.

The complexing agent for In can be a chelating ligand capable of binding In and can for instance comprise one or more from the group consisting of acetyl acetonate, ethyl-acetoacetate, ethylenediaminetetraacetic acid and triethanolamine. Preferably, the chelating ligand comprises acetyl acetonate.

In a further step of the method of the invention the mixed solution is applied onto a surface that has a temperature in the range of 250-400 °C, preferably in the range of 300-350 °C. Preferably, the mixed solution is applied onto to the surface of a substrate by spraying. The mixed solution is suitably sprayed in a pulsed mode. Spraying the solution in pulsed mode has the advantage that the surface has time to recover from the cooling effect during the spray process. In a preferred embodiment, the mixed solution is applied onto the surface of a substrate under standard atmospheric pressure. Although the mixed solution is applied onto a heated surface, the mixed solution itself can advantageously be at room temperature.

The substrate can suitably comprise one or more from the group consisting of glass, silicon, ceramic materials, and transparent transitional metal oxides. Furthermore, organic polymers have been developed which can be heated up to temperatures of, for instance, 300 °C. Organic substrates based on such polymers can also be used as suitable substrates.

The In4Sn30i2 is preferably doped with one or more charge carrier dopants. Any element which is stable as a 3+ valence can suitably be used as charge carrier dopant. Preferably, the In4Sn30i2 is doped with at least aluminium as charge carrier dopant. Such doping improves the electrical conductivity of the layer. Although the inventors presently believe that the charge carrier is present in the In4Sn30i2 as a dopant, it could be that the introduction of the charge carrier leads to the formation of a different bulk phase.

The concentration of Al in the prepared layer of In4Sn30i2 is preferably 0.1-5 mol% relative to the total amount of metal, preferably 1-4 mol%, more preferably 1.5-3 mol%, such as about 2 mol%.

The method of the invention results in a layer of In4Sn30i2 with unique properties, such as a unique layer morphology. Therefore, in a further aspect the invention is directed to a layer of In4Sn30i2 obtainable by the method of the invention. It will be clear that the layer of In4Sn30i2 does not necessarily consist of In4Sn30i2 only, but that the layer can further comprise impurities such as the above mentioned charge carrier dopants.

The layer of In4Sn30i2 according to the invention suitably has a thickness of 50-1000 nm, preferably 100-500 nm, more preferably 150-300 nm. These layer thicknesses are suitable for most applications in electronic devices, such as transparent electrodes.

The unique properties of a In4Sn30i2 layer prepared by the method of the invention include, for instance, that the layer is homogeneous. In addition, the layer can have a grain size of 150-200 nm for a thickness larger than 400 nm. Moreover, the oxygen content of a layer obtainable by the method of the invention is slightly higher than a similar layer prepared by conventional vacuum techniques, such as magnetron sputtering or pulsed laser deposition. This is a result of the fact that the method of the invention can be performed under atmospheric conditions.

It was found that the method of the invention allows the preparation of a In4Sn30i2 layer that has a light transmittance in the visible spectrum (350-700 nm) in the range of 80-95 %, preferably in the range of 85-95 %, as measured on a 350 nm thick layer. Light transmittance can be measured using a UV-VIS spectrophotometer.

Furthermore, In4Sn30i2 layers prepared by the method of the invention preferably have a sheet resistance of less than 7 Ω/sq, preferably less than 5 Ω/sq, as measured on a 350 nm thick film. Sheet resistance can be measured, for instance, with a 4 points multimeter using the van der Pauw method which is well-known to the person skilled in the art.

The excellent electrical conductive properties of the layers of the invention enable the use of these layers in a wide variety of applications, such as in electronic devices. Accordingly, in a further aspect the invention is directed to an electronic device comprising a layer of In4Sn30i2 according to the invention.

In a highly advantageous embodiment, the layer of In4Sn30i2 according to the invention is applied in a projected capacitive touchscreen. Traditional touchscreen sensing technologies, such as capacitive, resistive and surface acoustic wave are surface-active which renders them susceptible to surface variance effects during the lifetime of the touchscreen. Capacitive sensor arrays are often subject to drift, which means that regular recalibration is required and consequently, maintenance is more expensive. Resistive touch technology is more prone to damage caused by sharp pointing devices such as a pen ,the corner of a credit card, or other sharp objects. A projected capacitive touchscreen eliminates all of these problems by providing a robust, reliable and calibration-free, non-surface active glass fronted touch solution.

Projected capacitive touchscreens are based on the principle of embedding an array of micro-fine sensing wires within a multi-layer laminated screen behind a protective front surface, ensuring that the sensing medium is well-protected from accidental and malicious damage. This technology is extremely robust and reliable, offers one of the industry's highest light transmission characteristics and can be operated with a gloved or ungloved hand. While projected capacitive touchscreens are conventionally prepared using standard sputtered tin- doped indium oxide, the use of a layer of

In4Sn30i2 of the invention allows sensitivity even through protective glass or plastic.

Therefore, in yet a further aspect the invention is directed to a projected capacitive touchscreen comprising one or more layers of In4Sn30i2 according to the invention. Typically, one or more of the micro-fine sensing transparent conductive oxide pads in the embedded array can be In4Sn30i2 layers according to the invention. Preferably, all of the micro-fine sensing transparent conductive oxide pads in the embedded array are In4Sn30i2 layers according to the invention. During preparation of the projected capacitive touchscreen, the one or more In4Sn30i2 layers may be deposited on the protective front surface (such as on a glass substrate with a thickness of 4 mm or more), but it is also possible that the one or more In4Sn30i2 layers are laminated.

When used in a projected capacitive touchscreen, it is preferred to include one or more charge carrier dopants in the In4Sn30i2 layer, more preferably aluminium.

The Example will now be illustrated by the following non-limiting

Example.

Example

A mix solution of In and Sn chlorine salts in fix ratio of 4/3 was solved in a mix of isopropylalcohol/water 90/10, acetyl acetonate as surfactant and complexing agent for In, and aluminiumtrichloride. The concentration of Al was kept constant at 2 mol%, relative to the total amount of metal. The mixed solution was sprayed over a heated glass surface (325 °C) in a pulsed mode. A film with a thickness of 350 nm was obtained which was highly electrically conductive (less than 5 Ω/sq). The 350 nm thick film had a average light transmission in the spectrum of 400-700 nm of 86 % as measured on a UV-VIS spectrophotometer.

The obtained layer was further examined using X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results are shown in the Figures (Figure 1: photograph of the deposited layer; Figure 2: UV-VIS spectrum; Figure 3: XRD spectrum; Figure 4: SEM micrograph; Figure 5: AFM microgarph).