WO2013171286A1

WO2013171286A1 - Solar cells having a nanostructured antireflection layer

Info

Publication number: WO2013171286A1
Application number: PCT/EP2013/060096
Authority: WO
Inventors: Haiyan Ou
Original assignee: Danmarks Tekniske Universitet
Priority date: 2012-05-15
Filing date: 2013-05-15
Publication date: 2013-11-21

Abstract

An solar cell having a surface in a first material is provided, the optical device having a non-periodic nanostructure formed in the surface, the nanostructure comprising a plurality of cone -haped structures wherein the cones are distributed non-periodically on the surface and have a random height distribution, at least a part of the cone-shaped structures having a height of at least 100 nm. The first material may be SiC or GaN. A method of manufacturing a non-periodic nanostructured surface on a solar cell, is furthermore provided, the method comprising the steps of providing a surface comprising SiC or GaN, forming a thin film of a masking material on at least a part of the substrate, treating the thin film to form nano islands of the thin film material, etching the SiC or GaN in a substantially anisotropic etch and concurrently etching at least a part of the thin film masking material to form a non-periodical nano structure, the nano structure comprising a plurality of cone-shaped surface structures, whereby the structures have a random height distribution, at least a part of the structures having a height of at least 100 nm.

Description

SOLAR CELLS HAVING A NANOSTRUCTURED AN IREFLEC ION LAYER

FIELD OF INVENTION The present invention relates to antireflective surfaces, especially to antireflective surfaces having non-periodic nano structures formed therein and to a method of fabricating such structures. The invention furthermore relates to solar cells having a non-periodic nano structure provided as a window for the solar cell. In one or more embodiments, the non-periodic nano structure is formed in silicon carbide or gallium nitride.

BACKGROUND

Renewable sources of energy are gaining increasing interest as the world faces global warming, environmental issues, and possibly a shortage of conventional energy sources. These renewable sources of energy includes wave energy, wind energy, geothermal heating, solar energy, etc. Solar energy is one of the renewable energy sources which have received increased interest, which has resulted in a significant development of the solar cells.

However, the commercial solar cell market, or photovoltaic market, is still dominated by first generation solar cells based on silicon wafer technology. The production volume for these first generation solar cells is growing rapidly, with the technological emphasis upon streamlining manufacturing to reduce costs while, at the same time, improving the energy conversion efficiency of the product. The costs may be reduced by reducing the thickness of the starting silicon wafer without losing performance, to save on material use or new materials may be used to reduce the production costs. The efficiency of the commercially available solar cells is well below 40%, typically around

10% to 20%. Thus, there is a huge potential in increasing the efficiency of the solar cells. Thin film solar cells also use silicon to a large degree, and thin film silicon is deposited on substrates, such as ceramic substrates.

A simple solar cell is a pn junction having a contact to both the p-type layer and the n-type layer. The principles in obtaining electrical energy from the solar energy includes a light absorption process which causes a transition in a material (the absorber) from a ground state to an excited state upon absorption of electromagnetic light, the excited state is converted into (at least) a free negative- and a free positive-charge carrier pair. The resulting free negative-charge carriers are moved in one direction to a cathode contact and the resulting free positive-charge carriers are moved in another direction to an anode contact by a discriminating transport mechanism and an electrical current is generated when the anode contact and cathode contact form part of an electrical circuit. The efficiency of the solar cell depends on a number of factors, and may be increased by increasing the amount of sunlight absorbed by the solar cell, by increasing the short circuit current density and the open circuit voltage for the solar cell component. Especially, the absorption of larger amount of sunlight by the solar cell can potentially increase the efficiency of the solar cells significantly.

Periodic photonic crystals have been demonstrated as an effective way to enhance the antireflection, for example by Ou, Y et al. Optics Express, vol. 20, No. 7, 7575-7579, "Broadband and omnidirectional light harvesting enhancement of fluorescent SiC". However, these structures are made using expensive and time-consuming electron beam lithography, which brings huge extra costs and restrains the scalability.

By e.g. Song et al, in Applied physics letters 97, D931 10-1 -3, "Disordered antireflective nanostructures on GaN based light-emitting diodes using Ag nanoparticles for improved light extraction efficiency", it has been suggested to provide a disordered subwavelength structure in an ITO coating. The method includes the deposition of a silicon dioxide layer on the ITO contact layer as a durable etch mask and a buffer layer to form Ag nanoparticles. A Ag thin film layer is deposited on top of the S1O2 layer, the layer is annealed to form separated nanoparticles by self-assembled agglomeration. Hereafter, the S1O2 is patterned using the Ag nanoscale mask whereafter, the ITO is processed in a further etching process to create subwavelength structures in the ITO.

It is a disadvantage of having the nanostructure in a coating layer in that there is always a certain loss in the transition between two materials, furthermore, the method as suggested is quite complex and requires two masking steps and two etching steps to create a subwavelength

nanostructure in the ITO layer. It has furthermore been suggested by Dylewicz in Appl. Phys B (2012) 107: 393-399, "Nanostructured graded-index antireflection layer formation on GaN for enhancing light extraction from light-emitting diodes" to provide a random surface roughening with submicron spatial structures below 100nm.

However, it is a disadvantage of the surface roughening that the

nanostructures are too small to achieve a guiding of the light, and thereby too small to efficiently increase the transmittance.

SUMMARY OF INVENTION It is an object of the present invention to provide an optical device having an increased antireflection.

According to the above and other objects, a device with at least one surface having a non-periodical nano structure is provided. The device may be an optical device, such as an optical device comprising a window, such as a solar cell comprising an input area, etc., and the optical device may have at least one surface in a material such as silicon carbide or gallium nitride. The at least one surface may have the non-periodical nano structure formed in the silicon carbide or gallium nitride material. The nano structure may comprise a plurality of cone shaped structures wherein the cone shaped structures are distributed non-periodically on the surface. The cone shaped structures, which may be referred to hereinafter as cones, may be nano- sized cone shape structures, such as subwavelength cone shaped

structures. The material may be single-crystalline, poly-crystalline, or amorphous, and the non-periodical nano structures may be formed in the respective material.

According to an aspect of the present invention, a solar cell is provided, the solar cell having at least one input area configured to receive incoming electromagnetic radiation, at least a part of an input area surface being provided in a SiC or GaN material. At least a pn junction or a pin junction is formed between a p-type layer and an n-type layer configured to receive the incoming electromagnetic radiation, wherein at least a part of the input area surface has a subwavelength nanostructure formed in the SiC or GaN material, the subwavelength nanostructure comprising a plurality of cone shaped structures, wherein the cone shaped structures are distributed non- periodically in the at least one surface. The cone shaped structures may - have a random height distribution. At least a part of the cone shaped structures may have a height of at least 100 nm. It is an advantage of distributing the cone shaped structures non-periodically on the surface in that the reflectivity of the surface is significantly reduced. An optical device as used herein may be any device, including any surface, configured for receiving or transmitting light of any wavelength. In another aspect of the present invention, a method of manufacturing at least one non-periodical nano structured surface on a device, such as on an optical device, is provided. The method comprising the steps of providing a surface in a silicon carbide or gallium nitride material . The surface may be provided as the surface of the silicon carbide or gallium nitride material, such as a coating material, provided on the device. A thin film of a masking material may be formed on at least a part of the surface, and the thin film may be treated to form nano islands of the thin film material. The silicon carbide or gallium nitride material may be etched in a mostly anisotropic etch which concurrently may etch at least a part of the thin film material to form a non-periodical nanostructure, the nanostructure comprising a plurality of cone shaped surface structures. After the formation of the nano structures, the thin film material is removed, for example using a wet etch.

In a further aspect of the present invention, a method of providing an antireflective structure in a surface of a solar cell device is provided. The surface comprises a silicon carbide or gallium nitride material, and the method comprises providing a surface of the solar cell device in a silicon carbide or gallium nitride material, providing a thin film material on at least a part of the surface and treating the thin film to form self-assembled nano islands of the thin film material, the nano islands being configured to mask the surface during at least a part of the etching. The silicon carbide or gallium nitride material is etched at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, etching at least a part of the nano islands at a second etch rate, the second etch rate being lower than first etch rate. Thereby a subwavelength nanostructure may be formed in the silicon carbide or gallium nitride material, the subwavelength nanostructure comprising a plurality of cone shaped structures, wherein the cone shaped are distributed non-periodically in the at least one surface. The cone shaped structures may structures have a random height distribution and at least a part of the cone shaped structures may have a height of at least 100 nm. In a final step the remaining thin film material, i.e. the remaining nano islands, may be removed. In another aspect of the present invention, a solar cell is provided. The solar cell may have at least one surface in a silicon carbide or gallium nitride material. The at least one surface may have a nano structure formed in the silicon carbide or gallium nitride material, and the nano structure may comprise a cone shaped surface structure wherein the cones are distributed non-periodically on the surface.

In a still further aspect of the present invention, a method of providing an antireflective coating on a solar cell surface may be provided. The method may comprise coating at least a part of a solar cell surface with a silicon carbide or gallium nitride material and forming a thin film of a masking material on at least a part of the coating material. The thin film may be treated to form nano islands of the thin film material, etching the coating material in a mostly anisotropic etch and concurrently etching at least a part of the thin film material to form a non-periodically nanostructure, the nanostructure comprising a plurality of cone shaped surface structures. The cone shape structures may have a random height distribution and at least some of the cone shaped structures may have a height of at least 100nm. The remaining thin film may be removed, for example by using a wet etch.

The silicon carbide or gallium nitride material may be a poly-crystalline or an amorphous material deposited on the solar cell input area. Alternatively, or additionally, the solar cell may comprise silicon carbide or gallium nitride as a part of the solar cell, such as a part of an active or integral part of the solar cell, an active part forming for example part of the pn junction or the pin junction.

In a solar cell, the at least one surface of the input area may be provided in the silicon carbide or gallium nitride material and may be at least one surface of a substrate and/or an active element of the solar cell provided in the silicon carbide or a gallium nitride material. The silicon carbide or galliunn nitride material and/or the silicon carbide or gallium nitride coating material may be a poly crystalline material, an amorphous material or a single crystalline material. It is an advantage of the poly crystalline and amorphous materials that the processing, such as the growing or deposition of the poly crystalline and amorphous materials are faster, and thus less expensive, than the growing of a single crystalline material. It is a further advantage that the processing temperature may be reduced, for example, a micro crystalline SiC may be deposited at about 200°C, whereas a single crystalline SiC layer is typically grown at more than 1000 °C.

It is an advantage of being able to provide the nanostructure directly in the silicon carbide or gallium nitride material, that is in the surface of the silicon carbide or gallium nitride material, whether provided as a coating material, or as an integral part of the solar cell device that there is no difference in thermal expansion coefficients, and furthermore, that there is no difference in refractive index between the silicon carbide or gallium nitride material and the cone shaped nano structure, i.e. the substrate and the cone shaped nano structure has a same refractive index. Thereby, a gradient refractive index may be achieved and the light will not, or substantially not, experience any interface. For example, if the refractive index of the bottom of the

nanostructures is the substrate refractive index, the effective refractive index of the nano structures will gradiently change from the substrate refractive index to the surrounding refractive index as the nanocones become narrow from bottom to top.

It is a further advantage that no materials foreign to the standard processing of the poly crystalline material need to be introduced neither in the optical device nor in the process for manufacturing the optical device. The surface being provided with the nano structure may have a very low reflectance of light in the visible wavelength range, such as an average surface reflectance below 10 %, such as below 5 %, such as an average surface reflectance in the visible wavelength range below 2 %, such as below 1 .7 %, such as below 1 .5%, such as below 1 .0%, such as below 0.9 %. For some materials, due to the low reflectance, the surface may appear black.

In one or more embodiments, the silicon carbide or gallium nitride material, may be a compound material, such as SiC, GaN, InGaN material. The gallium nitride material may be any gallium nitride based material, and the gallium nitride material may for example comprise GaN, InGaN, etc.

The material, such as the single crystalline material or compound material, may have a wide bandgap and strong bonding energy.

The compound material may be composed of at least two compounds, and the compound material, such as the first material or the coating material, may typically be characterised by a strong bond between the different compounds and the compound materials, thus, typically have a high bonding energy and typically a high chemical resistance as any chemical process will need to have an activation energy being higher than the bonding energy between the compounds.

These materials therefore require either high temperature or a physical reaction to etch, and the silicon carbide or gallium nitride material may be characterized by etching anisotropically in a reactive ion etching process, such as characterized by etching anisotropically in a reactive ion etching process using a fluoride based gas, such as SF₆. Therefore, these materials are often used in micro machining because deep structures may be etched with a minimum of undercutting. The thin film material may be any material having the required masking capabilities, and the thin film may be made of any material comprising silver, gold, platinum, aluminum or palladium, or any combination thereof. The thin film of masking material may have an etching rate which is much lower than the etching rate of the silicon carbide or gallium nitride material, such as an etching rate being 2, 5 or 10 times lower than the etching rate for the silicon carbide or gallium nitride material. The ratio between the first etching rate and the second etching rate may be above 1 , such as 5, above 5, such as above 10, such as 10, such as 100, etc.

The masking material is preferably capable of forming nano islands upon thin film treatment, thus the thin film may be treated to form nano islands by either a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the masking material to thereby fabricate discontinuous nano islands with half sphere-like shapes or dome shapes. The average size and density of the nano islands may be controlled by adjusting the processing parameters as well as thin film layer thickness. The nano islands may have an average particle size of 1 nm to 800 nm, such as between 10 nm and 380 nm and/or wherein an average interval between the nano islands is between 1 nm and 800 nm, such as between 10 nm and 380 nm. It is, however, an advantage of the present invention that the size of the nano islands does not need to be rigorously controlled as the nano structures, i.e. the plurality of cone shaped structures, are preferably randomly distributed over the surface, and have a random height distribution, thus the plurality of cone shaped structures do not need to be identical in neither height nor width. Thereby, any intermediate steps in the method of fabricating the nano structured surfaces, i.e.

planarizing, etc. may be eliminated. In one or more embodiments, the cones may have a base width of less than 1000 nm, such as less than 800 nm, such as less than 500 nm, such as less than 400 nm, such as less than 300 nm, such as less than 200 nm, such as less than 100 nm. The base width may be between 20 nm and 1000 nm, such as between 50 and 800 nm, such as between 100 nm and 500 nm, such as between 100 nm and 300 nm.

Each of the plurality of cone shaped structures may have a height of at least 100 nm, such as at least 200 nm,, such as at least 300 nm, such as at least 400 nm, such as at least 500 nm, such as at least 800 nm, such as at least 1000 nm, such as at least 1500 nm, such as at least 2000 nm. The cone shaped structures may have a height between 100 nm and 2000 nm, such as between 1000 nm and 2000 nm, between 500 and 1500 nm, between 500 nm and 1000 nm, between 100 nm and 800 nm, such as between 100 nm and 500 nm, such as between 200 nm and 400 nm.

Any combination of cone heights and cone widths is achievable and the cone heights and cone widths may be tailored to achieve for example specific reflectance for a specific wavelength or a specific wavelength range. In some embodiments, the cones may have a base width of less than 400 nm and a height of at least 400 nm. In some embodiments the cones may have a height distribution between 100 nm and 350 nm. The cones may have different base widths and different heights within the at least one surface. In some embodiments, the height of the plurality of cone shaped structures may vary randomly, such as pseudo randomly, between 100 nm and 1 100 nm to thereby reduce the reflectivity of the at least one surface for

electromagnetic radiation in a wavelength range between 350 nm and 1 100 nm, such as between 370 and 770 nm. The aspect ratio of the cone shaped structures may be between 2 and 15, such as between 3 and 10, between 7 and 13, etc., the aspect ratio being a height/width ratio. The height distribution of the plurality of cone shaped structures may be selected for optimum performance in a wavelength range having a center wavelength. The height of the plurality of cone shaped structures may vary randomly between 1/3 of the center wavelength and at least half the center wavelength, such as between 100 nm and at least half the center

wavelength, such as between 100 nm and up to the center wavelength.

It is envisaged that the cone shaped structure parameters (base width, pitch, and height) may be engineered for desired optical performance. For example for LED applications, the parameters are mainly optimized for visible light range, and for example for solar cell applications, the sun light includes infrared light, so that the parameters are optimized for the visible range and the infrared range. Thereby, for example, when the incoming wavelength is known to include longer wavelengths, such as include infra red wavelengths, the cone shaped structure parameters may generally be shifted to larger values as well.

The cone shaped structures may be distributed non-periodically on the at least one surface. That the plurality of cone shaped structures are distributed non-periodically infers that the distribution of the cones are not periodical and that the distance between any two cones is not necessarily the same as the distance between any two other cones, the non-periodical distribution may be a random, a non-periodic or a pseudo-periodic distribution. Furthermore, the individual cones need not to be identical, the height of the cones may vary, for example, the height distribution of the plurality of cone shaped structures may be random, such as pseudo-random, and likewise the width of the cones may vary on a same surface, so that both the height and the width of cones may vary from 100 nm to 2000 nm, from 500 nm to 1500 nm, from 100 nm to 800 nm, such as from 100 nm to 500 nm. Thus, the size distribution for the cones may extend over more than 1000 nm and a mean value may be given with regards to the cone width and height with a possible height and/or width variation of 1000 nm, such as 900 nm, such as of 500 nm, such as 300 nm, etc.

For a plurality of cone shaped structures having a random height variation, such as a pseudo random variation, at least a first part of the plurality of cone shaped structures may have a height in a first height interval and at least a second part of the plurality of cone shaped structures may have a height in a second height interval, different from the first height interval. The plurality of cone shaped structures may have a random distribution in among the first, second, and possibly further height intervals.

The random height distribution ensures optimized or improved transmission and/or reflection properties for the nano structured surface over a wavelength range, Typically, the height is measured from a selected base plane, such a base plane comprising a lowest etch point, such as a base plane proving a base plane for a plurality of cone shaped structures. Typically, also the width of a cone shaped structure is measured along a selected base plane. The plurality of cone shaped structures may have a random height distribution. Thus, the height of individual cone shaped structures forming a nano structure may vary randomly. For example, if a mean height of the plurality of cone shaped structures is 240 nm, the standard deviation of the height of the plurality of cones may be 80, if a mean height of the plurality of cone shaped structures is 500 nm, the standard deviation may be 300 nm. Thus, the standard deviation may range from 30% to 60 % of a mean height distribution.

In some embodiments, the height of the plurality of cone shaped structures may vary randomly between 300 nm and 1200 nm to thereby allow transmission of diffused light in a wavelength range, such as between 450 nm and 1 100 nm, such as between 390 and 700 nm.

The cones may be distributed with an average of 1 .0E8 -2.0E1 1 cones pr cm².

The density of the self-assembled nano islands may be between 1 and 2000 nanoparticles/ μηη2, such as from 100 and 200 nanoparticles/μηη². The nano island particle area coverage may be between 20% and 35 %, such as between 25 % and 35 %. Typically, the density of the plurality of cone shaped nano structures may correspond to the density of the self-assembled nano islands, and thus the density of the plurality of cone shaped structures may be between 1 and 2000 cone shaped structures/ μηη2, such as from 100 and 200 cone shaped structures/μηη².

In one or more embodiments of the present invention, the material of the input surface is silicon carbide or gallium nitride the nano structure comprising a plurality of cone shaped structures may also be silicon carbide or gallium nitride, respectively, so that the cone shaped structures are fabricated in silicon carbide or gallium nitride, respectively.

Both silicon carbide and gallium nitride have been suggested as basis materials for a solar cells and a silicon carbide substrate or gallium nitride substrate may thus form the basis for the solar cell, however, silicon carbide or gallium nitride may also be provided on top of any solar cell, to provide a window into the solar cell. However, even though silicon carbide or gallium nitride may improve the electrical characteristics of the solar cell, silicon carbide as well as gallium nitride has a high refractive index. Thus, much light may be reflected from the solar cell surface, i.e. the interface between in this example silicon carbide and surroundings, i.e. air, or gallium nitride and the surroundings, in this case air, without even reaching the active photo voltaic device.

The surroundings may encompass any material surrounding the optical device and/or the nano structured surface, such as air, such as ambient air, such as a protected environment, such as a liquid, such as water, etc. The light may thus be emitted from within a device towards the surroundings, such as through an output surface. The output surface may be a

nanostructured silicon carbide or gallium nitride surface. Silicon carbide is an inert material having a high chemical resistance, and a good temperature stability. It is therefore a material being well suited for a harsh environment such as the solar cell experiences on-site, including temperature fluctuations, heavy rain, humidity, snow and ice, and dirt. The same applies mutatis mutandis for gallium nitride.

Silicon carbide is normally a transparent material, and is as such used as material for optical devices allowing for transmission of the light. Silicon carbide having a periodic nano-structure manufactured in the surface layer is also a transparent material, however, providing a non-periodic nano structure in a silicon carbide substrate renders the silicon carbide to appear black. The surface having a non-periodical nano structure has a very low reflectance of light in the visible wavelength range and therefore the transmittance will be increased. Typically, the surface reflection of silicon carbide is about 20% for light in the visible wavelength range, however, by applying a non-periodic nano structure on the silicon carbide surface, the reflectance of the surface material may be reduced by at least a factor 2, such as at least a factor 5, such as at least a factor 15, such as for example from 20.5% to about 10 %, to about 4%, to about 1 .62% and even lower, depending on the cone distribution, the cone width and the cone height. Similar results may be found for gallium nitride.

In one or more embodiments of the present invention a poly crystalline material or the amorphous silicon carbide or gallium nitride material is used in the manufacturing of the solar cells, i.e. providing at least a part of the active photovoltaic device.

A solar cell typically has an input area configured to receive incoming electromagnetic radiation, and typically, this input area corresponds to the active area for the solar cell or the photovoltaic device. Often, an

antireflective surface coating may cover at least a part of the input area to decrease the reflectivity of the surface and to thereby ensure that as much incoming electromagnetic radiation as possible reaches the active area of the solar cell. The active area of a solar cell typically comprises at least a pn junction formed between a p-type layer and an n-type layer and the active area may be configured to receive the incoming electromagnetic radiation. The materials and the structures of the solar cells are getting increasingly advanced to improve the efficiency of the solar cells. The materials forming the active area include the traditional wafer based crystalline silicon solar cells, but also thin films including thin films of micro crystalline silicon, amorphous silicon, CdTe, etc. deposited typically on lower cost substrates, such as ceramic substrates are used. The wafer based crystalline silicon solar cells has so far been found to have a better conversion efficiency and long-term stability than the thin film based solar cells. It is envisaged that the present invention is applicable to wafer based silicon solar cells as well as thin film based solar cells. To extract the electrical energy generated by the solar cell, an anode and a cathode may be connected to the p-type layer and the n-type layer, respectively, independently of the detailed structure of the solar cell.

The material deposited on the surface may be an antireflective coating, and the anti reflective coating may comprise a plurality of nano-sized cone shaped structures wherein the cones are distributed non-periodically on the surface.

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like reference numerals refer to like elements throughout. Like elements will, thus, not be described in detail with respect to the description of each figure.

BRIEF DESCRIPTION OF THE DRAWING

Fig. 1 a shows schematic illustrations of a non-periodic antireflective sub- wavelength structures fabrication process,

Fig. 1 c shows a SEM picture of the metal nano islands formed,

Figs. 1 b, 1d and 1 e show SEM pictures of the nano structures formed, Fig. 1f shows schematically the nano structures as seen in the SEM picture in Fig. 1 d.

Figs. 2a and b shows the water droplet contact angle on a bare silicon carbide substrate,

Figs. 3a and b shows the water droplet contact angle on a silicon carbide substrate having non-periodic nanostructure, Fig. 4 shows the surface reflectance of the bare silicon carbide and a silicon carbide substrate having a non-periodic nanostructure,

Fig. 5 shows schematically a test sample for absorption measurements, Fig. 6 shows absorbance by the test cells over a wavelength range,

Fig. 7 shows schematically a solar cell,

Fig. 8 shows schematically a process for fabricating a non-periodic

nanostructure,

Fig. 9 shows SEM pictures of samples with Au thickness of 7 nm, 9 nm, 1 1 nm, 13 nm and 15nm, respectively,

Fig. 10 shows the relation between Au thickness and particle diameter, NP density and area coverage, respectively,

Fig. 1 1 shows SEM pictures of the antireflective structures formed with Au film thickness of 7 nm, 9 nm, 1 1 nm, 13 nm and 15 nm, respectively,

Fig. 12 shows measured transmittance and reflectance as a function of wavelength, and calculated absorbance as a function of wavelength,

Fig. 13 shows the measured reflectance, absorbance and transmittance as a function of Au thickness.

DETAILED DESCRIPTION OF THE DRAWING

In the present invention an optical device is fabricated, and in Fig. 1 , the device is shown. The device has a substrate 2, such as a solar cell with a coating, having a surface 3. The substrate may be a silicon carbide substrate or a gallium nitride substrate, and the coating may comprise a silicon carbide or a gallium nitride material. In Fig. 1 a, the process of fabricating a

nanostructure comprising a plurality of cone shaped structures on the surface wherein the cones are distributed non-periodically on the surface is briefly illustrated. Firstly, a thin film layer of a metal, typically Au, is deposited on the surface 3 and the thin film is treated to form nano islands by either a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the masking material to thereby fabricate discontinuous nano-islands 4 with half spherelike shape or dome like shapes. (Note, however, that periodic structures are schematically drawn in steps (ii) and (iii) to simplify the illustrations). The average size and density of the nano-islands may be controlled by adjusting the processing parameters as well thin film layer thickness. In step (iii) reactive-ion etching (RIE) is applied with SF₆ and O2 gases mixture, and the non-periodic cone-shaped nano structures are formed on the surface 3 using the thin film nano-islands as a mask layer. The residual thin film is removed to get an optical device 1 having a substrate 2, such as a solar cell with a coating having non-periodical cone shaped nano structures.

In the present description of the drawing, the material in which the non- periodic nano structure is formed is a poly crystalline material, however, it is envisaged that for applications in which the single crystalline properties are not exploited, a non single crystalline material, such as a polycrystalline substrate or an amorphous substrate may be used.

In a specific example, non-periodic cone-shaped anti reflective nano structure are formed on a SiC coating using self-assembled etch mask.

An exemplary sample uses a silicon carbide substrate, for illustrative purposes, and the intermediate thin film nano-islands have been

characterized by scanning electron microscope, SEM. In Fig. 1 c, the nano-islands 4 are seen in the SEM picture. The nano islands 4 are seen as bright spots against the dark substrate 2.

Figs. 1 b, 1 d and 1 e, show a same sample from different angles. In Fig. 1 b, the non-periodic nano structures 5 are shown from the top. In Fig. 1 d, the non-periodic nano structures 5 are shown from the side, and in Fig. 1 e, the non-periodic nano structures 5 are shown from an elevated angle. It is seen that the nano-structures are cone shaped; the top of the cone may be slightly rounded. It is further seen that the height and width of the structures differ so that not two cones may be identical. Fig. 1f shows schematically the non- periodic nano structures as seen in Fig. 1 d with cones 5 on substrate 2.

The non-periodic or pseudo-periodic nano structure has a mean pitch of approximately 1 15 to 230 nm, that is the mean distance between successive cone shaped nano structures, and the structure height varies from 400 to 850 nm.

It is envisaged that although silicon carbide and to some extent gallium nitride are used to describe the effects and devices herein, also other materials may be used preferably materials having a high chemical resistance, such as for example Sapphire.

The nano structured surface has been characterized, and Figs 2a and 2b show the water contact angle measurements realized by using a drop shape analyzer (Kruss DSA 100S). The bare substrate 2 with a droplet 6 is shown schematically in Fig. 2a, and as DSA picture in Fig. 2b. In this case the substrate is fluorescent silicon carbide, and it is seen to be hydrophilic with a contact angle of 49°. After providing the fluorescent silicon carbide substrate 2 with the non-periodic nano structure 7, it is seen schematically in Fig. 3a, and in the form of a DSA picture in Fig. 3b, that the surface turns

hydrophobic with a contact angle of 98°. The nano structure 7 is not visible in the DSA picture. It is an advantage of being able to provide a hydrophobic surface especially for solar cell applications as this reduces the collection of dirt on the surface and is an advantage in humid environments.

The antireflection properties of the nano structure surface is shown in Fig. 4, wherein a bare silicon carbide surface is compared with the nano structured silicon carbide surface. The antireflection properties may depend on structure height and typically at least 100 nm high structures are required to achieve fairly good antireflection performance and in the present case the average height of the non-periodic nano-structures is controlled to be larger than 400 nm. This may be obtained when using a reactive ion etch, RIE, for etching the nano structures in the silicon carbide or gallium nitride substrate. The RIE conditions for a silicon carbide substrate may for example be: process pressure of 30 mT, RF power of 100 W, gases flow rates of 24 seem SF6 and 6 seem O2, and process time of 15 minutes. The surface reflectance obtained is illustrated in Fig. 4, where the reflection has been measured by using a calibrated goniometer system (Instrument Systems GON360) at near-normal incidence of 6° over a wavelength range of 390-785 nm which covers the entire visible spectral range (typically from 390 to 750 nm). The reflectance spectra are shown in Fig. 4, the bare silicon carbide substrate, i.e. the bare SiC, having a reflectance curve 8 showing a reflectance of about 20 % and the substrate surface having a non-periodic nano structure, i.e. the ARS SiC, has a reflectance curve 9 showing a reflectance of between 0.1 to a few percent. It may be seen that the average surface reflectance may be significantly suppressed from 20.5 % to 1 .62 % by a factor of 1 1 .6 after introducing the non-periodic nano structure. It is seen that the reflectance at the luminescence peak (576 nm) is lower than 2 % and the minimum value of 0.05 % is obtained at 405 nm. Although the reflectance starts to increase at longer wavelengths, the reflectance through the whole measured spectral range is below 4 %. It may be seen that the silicon carbide surface turns from shiny light green colour (transparent) to dark green-black colour (black, transparent) after introducing the non-periodic nano structure on the surface.

Fig. 5 shows a test sample 13 for measuring absorption in a SiC substrate 2 having a non-periodical nano structured surface 7. Incident light is directed at the surface 7 and the reflected light 1 1 and the transmitted light 12 is measured. The transmitted light is measured using a goniometer and the reflected light is measured using a microscope system. The absorption, A, is calculated as one minus transmittance (T) minus reflection (R), i.e. A = 1 -T- R. The measured spectral range is from 420-900 nm;

Fig. 6 shows the absorption for different samples. A bare SiC substrate is measured as the reference, and the curve for this reference sample is shown in curve 30, having a peak at about 400 nm, with a maximum absorption of about 0.75 and another peak at about 630 nm with a maximum absorption of about 0.5.

The test substrate, i.e. samples of single crystalline SiC having a thickness of about 330 μιτι were prepared according to the present invention: 10 nm Au film was deposited on bare 6H-SiC substrates; the substrates were annealed at 650°C for 10 minutes to form the self-assembled nano-islands; the samples were etched with three different etch time (10, 20, 30 minutes) by using reactive-ion etching (RIE) with the same other conditions; the residual nano island material, in the present case the residual or remaining Au, was removed where after the absorption measurements were performed.

The samples being etched for 10 minutes has a largest cone height of 1 .07 μιτι, the samples being etched for 20 minutes has a largest cone height of 1 .69 μιτι and the samples being etched for 30 minutes has a largest cone height of 1 .73 μιτι.

The curves 31 , 32, 33 shows the absorption for the samples being etched for 10 min, 20 min and 30 min, respectively. It is seen that for the sample being etched for 30 min, the absorption is constantly high in the entire wavelength range, about 0.95 - 0.98. Thus a significant increase compared to the bare SiC substrates. In Fig. 7, a solar cell 35 is shown schematically. The layer 38 may be any n- type doped material, and may be a substrate material or a layer deposited on a substrate. The thickness of the intrinsic layer, or depletion layer, 37 may be tailored to optimize the quantum efficiency and frequency response of the solar cell. The top layer 36 is in the present example a p-type doped material, such as silicon carbide or gallium nitride. The pin junction is formed by the depletion layer 37, and the interface 40 towards the p-type layer 36 and the interface 34 between the depletion layer 37 and the n-type layer 38. The surface of the top layer 36, i.e. the silicon carbide or gallium nitride, is manufactured to have an antireflective coating 29, such as a non-periodic nano structure 29. The electromagnetic radiation 39 is incident on the surface of the layer 36 and the entire surface functions as input area for the solar cell.

It is envisaged that also a refracting gallium nitride surface, such as a refracting gallium nitride surface, may be provided with a nanostructure as described above.

In Fig. 8, a process or a method for fabricating the non-period nanostructure is provided. The coating 42 is fabricated of a poly crystalline material, but it may also be an amorphous material, a single crystalline material or a poly crystalline material, such as silicon carbide or gallium nitride material. A thin film, such as a metal thin film, such as a Au thin film, 41 is provided on top of the coating 42, e.g. by e-beam evaporation, step (a). The thin film 41 may be between 1 and 50 nm, such as between 5 nm and 20 nm, preferably such as between 7 and 15 nm thick, such as between 9 and 15 nm thick, such as about 10 nm thick. The thin film may be an Au film and in step (b), the thin film is treated so as to form self-assembled nano islands 43 on the surface of the coating 42. In the present example, the thin film is treated with rapid thermal processing at 350°C for 5 minutes in an N₂ ambient. Hereby, the thin film layer turns into discontinuous self-assembled nano islands with half sphere like or dome like shapes. The size and shape of the nano islands may be controlled by adjusting the annealing conditions as well as the layer thickness of the thin film 41 . In steps (c), (d) and (e) reactive-ion etching (RIE) 44 is applied with SF₆ and O2 gases in a mixture of 4:1 . It is seen that the RIE 44 etches trenches 47 in the silicon carbide coating 42, and furthermore that while the nano islands 43 are used as a mask, the nano islands 43 are being gradually etched and over etching of at least some of the nano islands may occur so that at least some of the nano islands are etched away during the process. As the silicon carbide coating is chemically resistant to the SF₆ and O2 gases, substantially no undercutting of the thin film nano islands 43 occurs and the etching is thus anisotropic. The total etching time may wary depending on the thickness of the thin film 41 , the predetermined height of cone structures to be reached etc, and may be between 5 and 30 minutes, such as 15 minutes. After the etching non- periodic cone-shaped nano structures on the surface of the SiC coating are formed. In step (f), the residual nano islands, such as the residual Au islands, are removed by using an iodine based solution of KI:l₂:H₂O-100 g:25 g:500 ml. Hereafter, the coating 42 has a surface 48 having a nano structure 46 formed in the single crystalline material. It is seen that the nanostructure comprises a plurality of cone shaped structures 49 wherein the cones are distributed non-periodically on the surface.

Thus, the nano islands are configured to mask the silicon carbide substrate during at least a part of the etching. It is seen that the silicon carbide substrate is etched at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and concurrently, at least a part of the nano islands are etched at a second etch rate, the second etch rate being lower than first etch rate.

A thin film of Au has been deposited on silicon carbide wafers, the thickness of the film ranging from 3 to 21 nm (see table 1 ). The silicon carbide wafers are double-side polished 6H-SiC samples and the thin film has been deposited by using e-beam deposition (Alcatel) with a deposition rate of 1 A s. The samples were treated using thermal annealing to form self- assembled nano islands of the thin film material. A first annealing process included thermally annealing the samples for 3 minutes at 650 degree Celsius, and for samples with a Au thin film thickness from 3 to 1 1 nm, this annealing step was sufficient to form self-assembled Au nano-islands. A second annealing process included thermally annealing the samples for 33 minutes at 650 degree Celsius, was needed to form Au nano- islands on samples with a Au thin film thickness from 13 to 21 .

The self-assembled Au nano-islands have been observed by SEM and related calculations of particle density, particle area coverage, mean effective diameter and spread in diameter have been performed for the samples with different Au thin film thickness, see table 1 .

Nano islands comprising Au nanoparticles (i.e. Au particles size range is between 1 nm and 100 nm) were formed when the Au thin film thickness was below 13 nm and nano islands comprising Au nano clusters (i.e. Au cluster size range is between sub-micrometer and 10 μιτι) were formed when the Au thickness was above 13 nm.

Table 1 Fig. 9 shows SEM pictures of samples with Au thin film thicknesses of 7 nm, 9 nm, 1 1 nm, 13 nm and 15 nm, respectively. It is seen that by increasing the Au thin film thickness the nanostructure particle density decreases from approximately 0,1 to 2 particles pr. μιτι².

Fig. 10a shows the relation between particle diameter and Au thin film thickness. According to the figure, it is seen that when increasing the Au thickness the particle diameter increases almost proportionally. Fig 10b shows the relation between particle density and Au thickness. Fig 10c shows the relation between area coverage and Au thickness.

Table 2(a) includes calculation of particle area coverage, mean effective diameter, estimated mask thickness and RIE etching time as a function of Au film thickness. The samples are conducted by RIE with different etching time and the estimated mask thickness is calculated by dividing Au thickness x 100 % with particle area coverage. Table 2(b) includes measured average cone height for samples with different Au thickness. When the Au thickness is between 7 nm and 15 nm the measured average cone height is varying between 156 and 1040 nm.

Fig. 1 1 shows SEM pictures of the antireflective structures formed with Au film thickness of 7 nm, 9 nm, 1 1 nm, 13 nm and 15 nm, respectively. For an Au film thickness of 7 nm, 9 nm, 1 1 nm, 13 nm and 15 nm it is seen in Fig. 1 1 , that the cone shaped structures are distributed non-periodically and that the height of the cones are varying, respectively. According to table 2(b), the cone height is varying between 100 and 1 100 nm when the Au thickness is between 7 nm and 15 nm.

Fig. 12(a) and (b), shows measured surface diffuse reflectance and transmittance, respectively. The absorbance as a function of wavelength has been calculated by; Absorbance(A) = 1 - Transmittance(A) - Reflectance(A).

Figure 12(c) shows the calculated absorbance as a function of wavelength.

* Estimated mask thickness = Au thiekness x 100% / panicle area coverage

a

b

Table 2

Fig. 13 shows an average measured reflectance and transmittance as a function of Au thickness, and an average calculated absorbance as a function of Au thickness, the measurements being averaged over the wavelength range in question, i.e. from 370 nm to 770 nm. When the Au thickness is between 7 and 15 nm the absorbance is below 0.60 reflectance is below 0.125 and the transmission is above 0.15.

Claims

1 . A solar cell having at least one input area configured to receive incoming electromagnetic radiation, at least a part of an input area surface being provided in a silicon carbide or gallium nitride material, and at least a pn junction or a pin junction formed between a p-type layer and an n-type layer configured to receive the incoming electromagnetic radiation, wherein at least a part of the input area surface has a subwavelength nanostructure formed in the silicon carbide or gallium nitride material, the subwavelength

nanostructure comprising a plurality of cone shaped structures, wherein the cone shaped structures have a random height distribution and are distributed non-periodically in the at least one surface, at least a part of the cone shaped structures having a height of at least 100 nm.

2. An optical device according to claim 1 , wherein the silicon carbide or gallium nitride material is a poly-crystalline, or an amorphous material deposited on the solar cell input area.

3. A solar cell according to claim 1 , wherein the silicon carbide material comprises poly crystalline silicon carbide, amorphous silicon carbide, or single crystalline silicon carbide.

4. A solar cell according to claim 1 , wherein the at least one surface of the input area in the silicon carbide or gallium nitride material is at least one surface of a substrate and/or an active element of the solar cell provided in the silicon carbide or a gallium nitride material.

5. A solar cell according to any of the previous claims, wherein the height of the plurality of cone shaped structures varies randomly between 100 nm and 1 100 nm to thereby reduce the reflectivity of the at least one surface for electromagnetic radiation in a wavelength range between 350 nm and 1 100 nm, such as between 370 and 770 nm.

6. A solar cell according to any of the previous claims, wherein each cone has a base width of less than 400 nm.

7. A solar cell according to any of the previous claims, wherein the aspect ratio of the cone shaped structures is between 3 and 15.

8. A solar cell according to any of the previous claims, wherein the

nanostructured surface has a mean reflection of less than 10 %, such as less than 2% for incoming electromagnetic radiation in the visible light range.

9. A solar cell according to any of the previous items wherein the cones are distributed with an average of 1 .0E8 -1 .0E12 cones pr cm².

10. A method of providing an antireflective structure in a surface of a solar cell device, the surface comprising a silicon carbide or a gallium nitride material, the method comprising

providing a surface of the solar cell device in silicon carbide or gallium nitride, providing a thin film material on at least a part of the surface,

treating the thin film to form self-assembled nano islands of the thin film material, the nano islands being configured to mask the surface during at least a part of the etching,

etching the silicon carbide or gallium nitride material at a first etch rate in a substantially anisotropic etch using the nano islands as mask, and

concurrently, etching at least a part of the nano islands at a second etch rate, the second etch rate being lower than first etch rate,

to thereby form a subwavelength nanostructure in the silicon carbide or the gallium nitride material, the subwavelength nanostructure comprising a plurality of cone shaped structures, wherein the cone shaped structures have a random height distribution and are distributed non-periodically in the at least one surface, at least a part of the cone shaped structures having a height of at least 100 nm, and

removing the remaining thin film material.

1 1 . A method according to claim 10, wherein the silicon carbide or gallium nitride material is a poly-crystalline material or an amorphous material deposited on a solar cell.

12. A method according to any of claims 10-1 1 , wherein the step of treating the thin film to form self-assembled nano islands comprising using a heating reaction, a chemical reaction, a photoreaction or any combination of these reactions to cause aggregation, nucleation or decomposition of the thin film material.

13. A method according to any of claims 10-12, wherein the thin film material is a material comprising silver, gold, platinum, aluminum or palladium.

14. A method according to any of claims 10-13, wherein the nano islands have an average particle size of 10 nm to 380 nm and/or wherein an average interval between the nano islands is between 10 nm and 380 nm.

15. A method according to any of claims 10-14 wherein the density of the self-assembled nano islands is between 1 and 2000 nanoparticles/μηη², such as from 100 and 200 nanoparticles/μηη², and/or wherein the particle area coverage is between 20% and 35%.

16. A method according to any of claims 10-15, wherein the etching is a Reactive ion etch process, the reactive ion etch process reacting with the material in a physical etching process and a chemical etching process and wherein at least one gas for use in the RIE comprises a fluoride component, such as SF₆.

17. A method according to claim 16, wherein the silicon carbide or the gallium nitride material is primarily etched by the physical reaction between the silicon carbide and the gallium nitride material and reactive ions in the reactive ion etch process.

18. A method according to any of claims 10-17, wherein the etching rate for the silicon carbide or gallium nitride material is higher, such as at least 10 times higher, than the etching rate of the thin film material.

19. A method according to any of claims 10-18, wherein the shape of the nano islands corresponds substantially to a dome.