MORALES MASIS MONICA (NL)
RIJNDERS AUGUSTINUS JOSEPHUS HELENA MARIA (NL)
TEN ELSHOF JOHAN EVERT (NL)
KOSTER GERTJAN (NL)
| US20170271622A1 | 2017-09-21 | |||
| US20150048282A1 | 2015-02-19 |
JAMES K K ET AL: "Structural and optical properties of La-doped BaSnO3thin films grown by PLD", JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS, PERGAMON PRESS, LONDON, GB, vol. 76, 14 August 2014 (2014-08-14), pages 64 - 69, XP029018362, ISSN: 0022-3697, DOI: 10.1016/J.JPCS.2014.07.024
| Claims 1. Method for producing a transparent conductive oxide on a substrate, comprising the steps of: a) providing a substrate; b) applying a buffer layer comprising a material with layered perovskite structure onto the substrate; c) applying doped BaSnO3 onto said layer; wherein the material with layered perovskite structure has a lattice constant of 0.37 to 0.45 nm. 2. Method according to claim 1, wherein the substrate is an amorphous substrate. 3. Method according to claim 1 or 2, wherein the substrate is glass. 4. Method according to any of the previous claims, wherein the material with layered perovskite structure is Ca2Nb3O10. 5. Method according to any of the previous claims, wherein BaSnO3 is doped with La. 6. Method according to claim 5, wherein the La-doped BaSnCL has a composition of Bai-xLaxSn03, and wherein 0 < x < 0.1. 7. Method according to any of the previous claims, wherein step c) is performed using physical vapor deposition. 8. Method according to any of the previous claims, wherein the doped BaSnO3 is applied as a layer with a thickness of 10-1000 nm, preferably 20-500 nm, more preferably 50-200 nm. 9. Transparent conductive oxide on a substrate obtainable by the method according to any of claims 1-8. 10. Transparent conductive oxide on a substrate according to claim 9, wherein the thickness of the applied doped BaSnO3 is adjusted such that the transparent conductive oxide has a transmittance of 70% or higher at a wavelength of 1000 nm. 11. Transparent conductive oxide on a substrate according to claim 9 or 10, wherein the thickness of the applied doped BaSnO3 is 10-1000 nm, preferably 20-500 nm, more preferably 50-200 nm. 12. Transparent conductive oxide on a substrate according to any of claims 9-11, having a resistivity of 102 Ωcm or less. 13. Solar cell comprising a transparent conductive oxide on a substrate according to any of claims 9-12. 14. Use of a transparent conductive oxide on a substrate according to any of claims 9-12 in an optoelectronic device. |
BACKGROUND OF THE INVENTION
The invention is in the field of optoelectronic devices and the like. In particular the present invention is directed to transparent conductive oxides (TCOs), a method for producing TCOs, and the use thereof.
In the field of optoelectronic devices, transparent conductive oxides can be used in many different applications, such as solar cells, light emitting diodes (LEDs), and transparent transistors. Currently available transparent conductive oxides (TCOs) are typically based on zinc oxide, tin oxide and/or indium oxide. The material that is currently used the most is indium tin oxide. However, indium resources are scarce, and prices for indium are high. Another drawback of the currently available TCOs is that although they have a transparency window in the visible part of the electromagnetic spectrum, transmittance in the infrared (JR) is typically low due to fundamental physical limitations, specifically due to free carrier absorption.
Solar cells are becoming increasingly important as a source for renewable energy, and as an alternative for fossil fuels for generating electricity. In order to make efficient use of the entire solar spectrum, multi- junction or tandem solar cells have been developed, which enable electricity generation from photons of several different wavelengths. A significant part of the photons from the sun that reach the earth’s surface have energy levels in the IR region. However, using the currently available TCOs, these photons can not be used for generating electricity, because they are absorbed in the TCO.
Other applications that would benefit from TCOs with a broader transparency window include light emitting diodes (LEDs) and transparent transistors. Therefore, other materials are studied for their applicability as TCO. Promising candidates for this application are doped BaSnO 3 (BSO) materials.
Lanthanum-doped BSO materials are disclosed in US201548282. These materials show a combination of conductivity and a degree of transparency in the visible region of the electromagnetic spectrum.
However, in order to obtain films of these materials with acceptable conductivity, the materials need to be deposited on a substrate having perovskite (ABO3) structure with a specific lattice constant.
In order for practical applicability in devices, however, it is required that films of TCO material can be deposited on various substrates, including amorphous substrates, while maintaining good conductivity and transparency.
An object of the present invention is to overcome one or more problems of the known TCO materials.
Another object of the invention is to enable the deposition of doped BSO materials on arbitrary substrates.
Another object of the invention is to provide a transparent conductive oxide material with improved transparency in the infrared (IR) region, while maintaining transparency in the visible region of the electromagnetic spectrum.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention there is provided a method for producing a transparent conductive oxide on a substrate.
There is also provided a transparent conductive oxide obtainable by the method described herein.
In addition, there is provided a solar cell comprising a transparent conductive oxide as described herein. Furthermore, the use of a transparent conductive oxide according to the invention in an optoelectronic device is provided.
The present invention provides means to use doped BaSnCL as transparent conductive oxide for various applications, by enabling it to be apphed on arbitrary substrates. Furthermore, the transparent conductive oxide has favorable optical properties, such as transparency in the IR region.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a transparent conductive oxide according to the invention.
Figure 2 is a schematic representation of a solar cell according to an embodiment of the invention.
Figure 3 is a graph showing increased crystalhnity of doped BaSnCL when applied on a buffer layer according to the invention.
Figure 4 is a graph showing transmittance in the visible and infrared region of a transparent conductive oxide according to the invention. DETAILED DESCRIPTION OF THE INVENTION
According to the invention there is provided a method for producing a transparent conductive oxide on a substrate, comprising the steps of: a) providing a substrate; b) applying a buffer layer comprising a material with layered perovskite structure onto the substrate; c) applying doped BaSnCL onto said layer; wherein the material with layered perovskite structure has a lattice constant of 0.37 to 0.45 nm. There is also provided a transparent conductive oxide obtainable by the method described herein, specifically a transparent conductive oxide on a substrate.
Figure 1 is a schematic representation of a transparent conductive oxide according to the invention. Substrate (1), buffer layer comprising a material with layered perovskite structure (2) and doped BaSnO 3 (3) are shown.
Surprisingly, according to the present invention, this method results in a transparent conductive oxide with low resistivity, irrespective of the used substrate, while maintaining transparency in the IR region and low resistivity.
A wide range of crystalline and amorphous substrates may be used. Preferably, the substrate is an amorphous substrate, such as glass. This enables application of the TCO in many applications, because there is no need for special and/or expensive substrates.
ABX 3 structure, also known as perovskite structure, refers to lattice structures with general formula ABX 3 , wherein A and B are cations of different size and X is an anion.
The buffer layer comprises a material with layered perovskite structure. Layered perovskite structure refers to a structure that is built up of one or more 2-dimensional sheets having perovskite structure, resulting in a formula A (n-1) B n X 3n+1 , wherein n is the number of octahedral layers in a 2-dimensional sheet. The buffer layer may be applied using solution-based or vacuum-based methods available in the art. Preferably, the material with layered perovskite structure is applied by Langmuir-Blodgett deposition. Other methods include spin-coating, electrophoresis, inkjet printing.
Preferably, the buffer layer promotes strong out-of-plane orientation in the doped BaSnO 3 that is applied onto the buffer layer. This may be achieved by choosing a material with layered perovskite structure that exhibits strong preferential orientation, and/or which has a lattice constant close to the lattice constant of the doped BaSnO 3 . In case of La- doped BaSnO 3 , the lattice constant a c = 4.12 A. Preferably, the material with layered perovskite structure is Ca 2 Nb 3 O 10 , which has a lattice constant a c of 3.86 Å.
It is further preferred that the buffer layer exhibits a high coverage of the surface of the substrate.
The material with layered perovskite structure may be applied as a layer with a thickness of several 2-dimensional sheets, such as 1-3 sheets.
In addition to the material with layered perovskite structure, the buffer layer may also comprise other materials, for example doped BaSnO 3 , undoped BaSnO 3, and/or other oxides.
The BSO may be doped with La, Sb, Gd or H. Preferably, the BSO is doped with La. The doped BaSnCL material can be described with general chemical formula Ba x La(i- x) Sn0 3 , wherein 0 < x < 0.1.
Application of the doped BSO layer may be performed using physical vapor deposition techniques such as Pulsed Laser Deposition or magnetron sputtering, and/or using chemical vapor deposition techniques. Preferably, the doped BSO is applied using physical vapor deposition as these techniques are compatible with current industrial process flows and do not require chemical precursors.
Generally, it is preferable that the resistivity of the TCO material is as low as possible. Preferably, the TCO has a resistivity of 1x 10 -2 Ωcm or less, more preferably 1x10 -3 Ωcm or less, even more preferably 5x10 -4 Ωcm or less, such as 1x10 -4 Ωcm or less. Without wishing to be bound by theory, it is believed that the resistivity of the TCOs made with the method as described herein are low, because of the high degree of crystallinity of the doped BaSnO 3 thin films.
The doped BaSnO 3 may be applied as a layer with a thickness of 10-1000 nm, preferably 20-500 nm, such as 50-200 nm. Such a layer may be referred to as a thin film of doped BaSnO 3 .
Preferably, the transparent conductive oxide is transparent for both visible light and infrared light, i.e., light with a wavelength of 800-1200 nm. For quantification of the transparency in the IR region, the transmittance at a wavelength of e.g. 1000 nm can be used. For application of the TCO in optoelectronic devices which require transparency in the IR region, it is preferred that a large fraction of IR radiation is transmitted by the TCO material. Preferably, the TCO has a transmittance of 70% or higher at a wavelength of 1000 nm, more preferably of 80% or higher, such as 90% or higher. Because the transmittance is dependent on layer thickness, the thickness of the doped BaSnO 3 layer may be adjusted in order to obtain the desired transmittance at a wavelength of 1000. In the case of solar cells, good transparency in the IR region means that IR light can be used for generation of electricity.
According to another aspect of the invention, there is provided a solar cell comprising a transparent conductive oxide as described herein, specifically a solar cell comprising a transparent conductive oxide on a substrate.
There is also provided the use of a transparent conductive oxide as described herein in an optoelectronic device, specifically the use of a transparent conductive oxide on a substrate as described herein. Examples of optoelectronic device include, but are not limited to solar cells, light emitting diodes (LEDs), displays, and transparent transistors.
Figure 2 is a schematic representation of a solar cell according to an embodiment of the present invention, comprising substrate (1), doped BaSnCL (3), electron selective layer (13), absorber (14), hole transport layer (15), and rear electrode (16). Between the substrate (1), and the doped BaSnCL (3), there is buffer layer comprising a material with layered perovskite structure (2).
The TCO according to the invention may be used as front electrode of a solar cell, as rear electrode, or both.
With reference to figure 2, the TCO according to the invention allows infrared light to reach the absorber, where, depending on the band gap of the absorber, infrared light can be converted into electric energy. If the rear electrode is also transparent for visible and/or infrared light, additional absorber materials can be provided below the rear electrode. In this way, a larger fraction of the solar spectrum, including infrared light, can be used for the generation of electricity.
In the case of solar cells, specifically for perovskite solar cells, the doped BSO material may act as electrode, as electron selective layer (ESL), or both. The ESL plays an important role in solar cells, with regard to surface passivation, and electron extraction in perovskite solar cells, leading to more efficient electricity generation. Generally, it is believed that for the electron selective layer its conduction band minimum (CBM) and valence band maximum (VBM) of the electron selective layer have to be lower than those of the perovskite absorber. ESLs based on T 1 O 2 are commonly applied in perovskite solar cells. Without wishing to be bound by theory, the inventors believe that the band offsets, i.e., the (positions of CBM and VMB, respectively,) of this material make it suitable as electron selective layer. Analogously, the doped BSO may also act as hole selective layer.
Optimization of the properties of the transparent conductive oxide on a substrate, such as thickness of the doped BaSnO 3 and amount of dopant introduced in the material, may be required, depending on the desired functionality. For example, in electron selective layers, there typically is a tradeoff between better carrier selectivity, e.g. achievable by a thicker ESL and better carrier extraction, e.g. achievable by a thinner ESL. Similar tradeoffs may apply to other applications as well.
EXAMPLES
Example 1. Two samples of La-doped BaSnO 3 on a fused silica substrate were prepared. In one sample, doped BaSnCL was applied directly on the substrate, whereas in the other sample the doped BaSnCL was applied onto a buffer layer according to the invention. An X-ray diffraction pattern of the two samples is shown in figure 3. The increase in intensity of the peak at 44° 2Q and the appearance of a smaller peak around 21° 2θ indicate a higher degree of crystallinity for the doped BaSnO 3 applied onto a buffer layer according to the invention (indicated as ‘w/seed layer’ in figure 3). It is believed that the increased crystallinity forms the basis of the enhanced functional properties of the material, e.g. low resistivity. In figure 4, transmittance in the visible and infrared region is shown for the substrate alone, and for the two samples. It can be seen that the transmittance of the two samples is very similar, indicating that the transparency of the doped BaSnCL is not negatively effected by the buffer layer according to the invention.