WO2019083451A1 - A method of reducing surface reflectance in a photovoltaic device - Google Patents

A method of reducing surface reflectance in a photovoltaic device

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Publication number
WO2019083451A1
WO2019083451A1 PCT/SG2018/050530 SG2018050530W WO2019083451A1 WO 2019083451 A1 WO2019083451 A1 WO 2019083451A1 SG 2018050530 W SG2018050530 W SG 2018050530W WO 2019083451 A1 WO2019083451 A1 WO 2019083451A1
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WO
WIPO (PCT)
Prior art keywords
heating
coating
hydrolysing
precursor mixture
metal
Prior art date
Application number
PCT/SG2018/050530
Other languages
French (fr)
Inventor
Zhi Ming KAM
Bingrui Joel Li
Fen Lin
Thomas GASCOU
Original Assignee
National University Of Singapore
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by National University Of Singapore filed Critical National University Of Singapore
Publication of WO2019083451A1 publication Critical patent/WO2019083451A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02167—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells
    • H01L31/02168—Coatings for devices characterised by at least one potential jump barrier or surface barrier for solar cells the coatings being antireflective or having enhancing optical properties for the solar cells
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
    • G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
    • G02B1/11—Anti-reflection coatings
    • G02B1/118—Anti-reflection coatings having sub-optical wavelength surface structures designed to provide an enhanced transmittance, e.g. moth-eye structures
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the present invention relates to a method of reducing surface reflectance in a photovoltaic (PV) device.
  • PV photovoltaic
  • PV devices The power conversion efficiency of PV devices is typically described by the devices' parametric properties such as open-circuit voltage (V oc ), short-circuit current (J sc ), resistances, where direct improvements in any of these parameters will result in a corresponding increase in the power conversion efficiency.
  • V oc open-circuit voltage
  • J sc short-circuit current
  • PV devices A significant influence on the J sc of PV devices is their surface reflectance, where a low reflectance is desirable to maximize light absorption by the devices.
  • reduction in surface reflectance in PV devices has typically been achieved by two distinct physical mechanisms: texturing the surface of the light-absorbing substrate to induce light-trapping, and growth of anti-reflective coatings composed of dielectric materials via vacuum-based tools like plasma-enhanced chemical vapour deposition (PECVD) on the PV device surface to induce optical wavelength-specific destructive interference based on the dielectric materials' refractive indices and thicknesses.
  • PECVD plasma-enhanced chemical vapour deposition
  • the present invention seeks to address these problems, and/or to provide an improved method for improving the power conversion efficiency of PV devices.
  • the present invention relates to an improved method for reducing the surface reflectance in a PV device.
  • the invention relates to a facile and low cost method to improve the efficiency of PV devices by further reducing the reflectance of the PV devices by introducing an additional light-trapping layer in a way that is complementary to existing reflectance reduction mechanisms.
  • the present invention provides a method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising: depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition;
  • the liquid-based precursor mixture may comprise any suitable metal and/or metalloid containing compounds.
  • the metal and/or metalloid containing compounds may comprise, but is not limited to, zinc (Zn), titanium (Ti), aluminium (Al), silicon (Si), or a combination thereof.
  • the precursor mixture may comprise a suitable amount of the metal and/or metalloid containing compound.
  • the precursor mixture may have a concentration of 0.005-1.0 M, with respect to the metal or metalloid content.
  • the depositing may be by any suitable method. According to a particular aspect, the depositing may be by liquid deposition.
  • the liquid deposition may comprise, but is not limited to, spin coating, spray coating, ink jet printing, slot-die coating, dip coating, roller coating, blade coating, or a combination thereof.
  • the deposited layer may have a suitable thickness.
  • the deposited layer may have a thickness of 30-1000 nm.
  • the heating may be carried out under suitable conditions.
  • the heating may comprise heating at a temperature of ⁇ 400°C.
  • the heating may be in normal or inert atmosphere.
  • the inert atmosphere may be in a nitrogen or argon atmosphere.
  • the heating may be for a suitable time, such as ⁇ 20 minutes.
  • the hydrolysing may be by any suitable method.
  • the hydrolysing may be by, but not limited to: immersion in water, exposure to steam, water curtain, water spray, or a combination thereof.
  • the hydrolysing may form nano-sized structures on the surface of the PV device onto which the deposited layer is formed.
  • the nano-sized structures may have an average size of 1-800 nm.
  • Figure 1 shows a top down scanning electron microscope (SEM) image of a light- trapping layer according to one embodiment of the present invention deposited on a PV device substrate;
  • Figure 2(a) shows the cross-sectional SEM image of a light-trapping layer according to one embodiment of the present invention deposited on a PV device substrate and Figure 2(b) shows the cross-sectional SEM image of another embodiment of the present invention deposited on a PV device substrate;
  • Figure 3(a) shows the reflectance spectra of a PV device before and after coating with a light-trapping layer according to one embodiment of the present invention
  • Figure 3(b) shows the reflectance spectra of a PV device before and after coating with a light- trapping layer according to another embodiment of the present invention
  • Figure 4 shows the external quantum efficiency of Cell 1 as shown in Tables 2 and 3;
  • Figure 5 shows the internal quantum efficiency of Cell 1 as shown in Tables 2 and 3. Detailed Description
  • the method of the present invention enables a lower reflectance to be achieved using a low-cost and facile method in which an additional light-trapping layer is provided on top of existing anti-reflective coatings.
  • the method does not utilise high temperature or high vacuum conditions which are typically required for material deposition in solar cell manufacturing, thus keeping costs low.
  • the materials involved in the method are also simple, non-hazardous and easily available, thus avoiding safety and material scarcity issues.
  • the light-trapping layer formed by the method of the present invention reduces the surface reflectance and thereby immediately results in the improvement of device J sc and therefore, power conversion efficiency of the PV device.
  • the method may be applied on pre-metallization or completed PV devices.
  • the advantage of the method of the present invention is that it only affects the optical transmission of irradiance onto the semiconducting material and the method is insensitive to the PV device in any other way.
  • the method generally involves depositing the additional light-trapping layer on a surface of a PV device.
  • the surface may comprise an existing anti-reflective coating.
  • the additional light-trapping layer may be deposited on the surface as a pre-cursor layer by liquid deposition, followed by a low temperature curing process.
  • the precursor layer may then undergo further hydrolysis to yield a highly surface conformal layer of a material with surface morphology with light-trapping properties.
  • the present invention provides a method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising: depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition;
  • the metal and/or metalloid containing compounds comprised in the liquid-based precursor mixture may be any suitable metal and/or metalloid containing compound.
  • the metal and/or metalloid comprised in the metal and/or metalloid containing compound may be ionic or covalent in nature.
  • the metal or metalloid comprised in the metal and/or metalloid containing compound may be, but not limited to, zinc (Zn), titanium (Ti), aluminium (Al), silicon (Si), or a combination thereof.
  • the metal and/or metalloid containing compound may comprise Zn or Al.
  • the precursor mixture may comprise a suitable amount of the metal and/or metalloid containing compounds.
  • the precursor mixture may have a concentration of 0.005-1.0 M, with respect to the metal or metalloid content.
  • the concentration may be 0.1-0.3 M.
  • liquid-based precursor mixture may be in the form of a sol or solution.
  • the precursor mixture may be formed by any suitable method.
  • the precursor mixture may be formed by dispersion or dissolution of a metal and/or metalloid source in a solvent.
  • the precursor mixture may further comprise a suitable amount of additives for stabilising the precursor mixture.
  • the additives may be, but not limited to, ammonia, potassium hydroxide, sodium hydroxide, acetyl acetone, ethanolamine, oxalic acid, citric acid, hydrochloric acid, nitric acid, sulphuric acid, or a combination thereof.
  • the precursor mixture may comprise additives in an amount up to 20 v/v % or thrice the stoichiometric quantity with respect to the metal and/or metalloid content in the precursor mixture.
  • the precursor mixture may be aged before it is either further diluted, or directly used in liquid deposition. The extent of dilution may be dependent on the method of deposition and condition of the PV device onto which the precursor mixture is to be deposited.
  • the metal and/or metalloid containing compound may comprise the metal and/or metalloid comprised in the precursor mixture.
  • the metal and/or metalloid containing compound may comprise, but is not limited to, anhydrous zinc oxide, zinc nitrate, titanium tetraisopropoxide, aluminium tri-isopropoxide, aluminium tri-sec- butoxide, aluminium nitrate anhydrate, tetraethyl orthosilicate, or a combination thereof.
  • the solvent used in the preparation of the precursor mixture may be any suitable solvent for the purposes of the present invention.
  • the solvent may be, but is not limited to, methanol, ethanol, isopropanol, deionized water, or a combination thereof.
  • the depositing may be by any suitable method.
  • the depositing may be by liquid deposition.
  • the liquid deposition may comprise, but is not limited to, spin coating, spray coating, ink jet printing, slot-die coating, dip coating, roller coating, blade coating, or a combination thereof.
  • the liquid deposition may comprise spin-coating, spray-coating or a combination thereof. Since the depositing is by liquid deposition, this step avoids low pressure, high temperature conditions which are typical in gaseous phase deposition of a precursor mixture. Accordingly, the depositing of the precursor mixture may be carried out more easily and more cost effectively.
  • the depositing of the precursor mixture may be on a suitable surface of the PV device.
  • the depositing may be on the front and/or rear surface of the PV device.
  • the depositing may achieve a macroscopically uniform distribution of metal or metalloid content on the surface onto which the precursor mixture is being deposited.
  • depositing of the precursor mixture by spin-coating and spray-coating may result in good surface conformity and reflectance improvements.
  • the deposited layer may have a suitable thickness.
  • the deposited layer may have a thickness of 30-1000 nm.
  • the thickness of the deposited layer may be 50-900 nm, 80-800 nm, 100-700 nm, 200-600 nm, 300-500 nm, 350-400 nm. Even more in particular, the thickness may be 50-300 nm.
  • the heating may be any suitable heating.
  • heating may be defined as comprising any suitable heat treatment of the deposited layer.
  • the heating may comprise curing, annealing, or a combination thereof.
  • the heating may comprise annealing the deposited layer.
  • the heating may be by any suitable means.
  • the heating may be, but is not limited to, radiative or conductive heating.
  • Radiative heating may be by, but not limited to, using an infrared lamp and conductive heating may be by, but not limited to, an oven or furnace or a heated surface.
  • the heating may be carried out under suitable conditions.
  • the heating may comprise heating at a temperature of ⁇ 400°C.
  • the heating may be at a temperature of about 100-400°C, 120-380°C, 140-360°C, 150-350°C, 175-325°C, 200-300°C, 220-280°C, 240-260°C, 245-250°C.
  • the heating may be at a temperature of ⁇ 275°C, preferably 140-275°C.
  • the heating may be for a suitable period of time.
  • the time for heating may be dependent on the heating means used for the heating.
  • the heating may be for ⁇ 20 minutes.
  • the heating may be for 2 seconds - 20 minutes, 30 seconds - 15 minutes, 1-12 minutes, 2-10 minutes, 5-8 minutes. Even more in particular, the heating may be for 1-2 minutes.
  • the heating may be in an inert atmosphere or in normal atmosphere.
  • heating in an inert atmosphere may comprise heating in a nitrogen or argon atmosphere.
  • the hydrolysing may be by any suitable method.
  • the hydrolysing may be by, but not limited to: immersion in water, exposure to steam, water curtain, water spray, or a combination thereof.
  • the hydrolysing may be under suitable conditions.
  • the hydrolysing may be for a suitable period of time.
  • the hydrolysing may be for ⁇ 20 minutes.
  • the hydrolysing may be for 2 seconds - 20 minutes, 30 seconds - 15 minutes, 1-12 minutes, 2-10 minutes, 5-8 minutes. Even more in particular, the hydrolysing may be for 2-5 minutes.
  • the hydrolysing may be carried out at a suitable temperature.
  • the hydrolysing may comprise hydrolysing at a temperature of ⁇ 100°C.
  • the hydrolysing may be at a temperature of about 50-100°C, 60-90°C, 70-80°C.
  • the hydrolysing may comprise direct immersion of the PV device comprising the deposited layer in deionised water at a temperature of 60-100°C.
  • the hydrolysing may comprise exposing the PV device comprising the deposited layer to steam at a temperature of ⁇ 80°C.
  • the hydrolysing may form a surface-conformal coating comprising nano-sized structures on the surface of the PV device.
  • the resultant surface-conformal coating may be chemically composed of metal or metalloid oxides, which may contain hydroxide stoichiometry.
  • the surface-conformal coating may comprise a random distribution of the nano-sized structures.
  • the nano-sized structures may comprise high aspect ratio surface features.
  • the nano- sized structures may comprise nanospheres, nanohairs, nanowires, nanoplatelets, nanoridges, or a combination thereof.
  • the nano-sized structures may have an average size of 1-800 nm.
  • the average size may refer to at least one of the height of nano- sized structures or the width of nano-sized structures.
  • the nano-sized structure may have an average size of diameter of 30-800 nm.
  • the nano-sized structure may have an average size of 50-750 nm, 75-700 nm, 90-600 nm, 100-500 nm, 200-400 nm, 250-300 nm.
  • the average size of the nano-sized structure may be 50-300 nm.
  • the surface geometry of the nano-sized structures formed may be modified through the temperature and duration of the heating and hydrolysing. For example, time and temperature at which hydrolysing takes place may affect the extent of reaction for each metal/metalloid base layer. For example, below a certain temperature, the reaction may be either too slow or non-existent such that properties of the deposited layer may not change even after a period of time.
  • nano-sized structures results in additional fine surface roughening due to the irregular distribution of the nano-sized structures. This enhances surface light-trapping on the surface of the PV device on which the light-trapping layer is formed where the optical effect is more pronounced for substrates of PV devices which do not already have nano-sized structures like reactive-ion etched (RIE) or metal- assisted chemical etched (MACE) silicon surfaces.
  • RIE reactive-ion etched
  • MACE metal- assisted chemical etched
  • Figure 1 shows an example of a light- trapping layer which is formed by the method of the present invention in which the hydrolysing comprises hot water washing.
  • the PV device comprises an isotropically- etched multi-crystalline silicon substrate.
  • the PV device may comprise any suitable PV device, including but not limited to multi-crystalline and mono-crystalline substrates.
  • the method of the present invention does not involve high temperatures or high vacuum conditions, the PV device onto which the additional light-trapping layer may be provided is not damaged in the course of the method. There is also minimal accumulation in thermal budget.
  • a further advantage of the method of the present invention is that not only does the method result in the reduction of surface reflectance and therefore improvement in power conversion efficiency by way of the light-trapping layer, but the addition of the light-trapping layer may also increase the apparent thickness of the underlying anti- reflective coating. Accordingly, the thickness of the anti-reflective coating may be reduced, thereby resulting in reduced material usage and processing time for deposition of the anti-reflective coating.
  • Two different light-trapping layers were formed on a surface of two PV devices. One was a spin-coated oxidised aluminium and the other was a spray-coated oxidised zinc layer. The light-trapping layers were formed on isotropically-etched multi-crystalline silicon substrates.
  • the spin-coated oxidised aluminium layer was formed as follows.
  • a solution of aluminium nitrate in ethanol was prepared by mixing 33.3 mmol of aluminium nitrate with 100 ml of ethanol and 6.67 ml of acetylacetone to form a precursor mixture.
  • the precursor mixture was then left to stir for at least 24 hours at room temperature.
  • the precursor mixture was then deposited on an isotropically-etched multi-crystalline silicon substrate at room temperature and normal atmosphere via spin coating at 3000 rpm.
  • the silicon substrate comprising the deposited layer was then placed in an oven at 200°C for 10 minutes for heating, following which it was removed to cool down to room temperature again.
  • the resultant light-trapping layer was hydrolysed via submersion into an agitated water bath at a temperature of about 80°C for 5 minutes before it was removed for blow drying.
  • the spray-coated oxidized zinc layer was formed as follows. A solution of zinc nitrate in ethanol was prepared by mixing 60 mmol of zinc nitrate with 600 ml of ethanol to form a precursor mixture. The precursor mixture was then left to stir for at least 24 hours at room temperature.
  • the precursor mixture was then deposited on an isotropically- etched multi-crystalline silicon substrate via spray coating by scanning the surface of the substrate with the precursor mixture with a spray rate of 3.5 ml/min from a single nozzle where the total amount of precursor mixture sprayed across the substrate surface was 5.5 ml, and the surface of the substrate was maintained at about 100°C.
  • the solar cell was then placed in an oven at 280°C for 1 minute for heating, following which it was removed to cool down to room temperature again.
  • the resultant light- trapping layer was hydrolysed via submersion into an agitated water bath about 80°C for 5 minutes before it was removed for blow drying.
  • Figures 2(a) and 2(b) show the cross-sectional SEM images of the coated substrates with the light-trapping layer formed by the spin coated oxidised aluminium and spray coated oxidised zinc, respectively. From Figures 2(a) and 2(b), it can be seen that the deposited light-trapping layer easily achieves conformal coating quality that mimics the initial substrate surface topography. This demonstrates a ubiquity in the method of the present invention. Accordingly, the light-trapping layer may be applied onto surface textures found on most PV devices.
  • the surface texture may be typically an ordered distribution of square pyramids, whereas in multi-crystalline silicon solar cells, the surface texture may be a random distribution of surface features with high surface roughness. In both cases, surface feature scales are typically characterized as being in the order of microns.
  • the method of forming the light-trapping layer according to the present invention deposits and achieves a surface-conformal coating that can be described as being a random distribution of fine nano-sized structures between 1-500 nm in size.
  • Figures 3(a) and 3(b) show the reflectance spectra of the solar cells before and after coating with the light-trapping layer formed by the spin coated oxidised aluminium and spray coated oxidised zinc, respectively.
  • the reflectance reduction induced by the metal oxide coating described is a result of two optical phenomena - a shift in the minima of the pristine surface's reflectance spectrum towards longer wavelengths due to higher apparent thickness of the existing anti-reflective coating, and a wideband reduction in reflectance due to enhanced light-trapping on the device surface due to the additional deposited light-trapping layer.
  • Table 1 Changes in WAR 30 o-iooo of different crystalline Si substrates before and after introduction of a spin-coated oxidised aluminium light-trapping layer
  • Table 2 Electrical performance of three example PV devices under 1 sun irradiance prior to the introduction of spin-coated oxidised aluminium light- trapping layer
  • Table 3 Electrical performance of three example PV devices under 1 sun irradiance following introduction of spin-coated oxidised aluminium light- trapping layer

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Abstract

A method of reducing surface reflectance in a photovoltaic device There is provided a method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising: depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition; heating the deposited layer; and hydrolysing the layer.

Description

A method of reducing surface reflectance in a photovoltaic device Technical Field
The present invention relates to a method of reducing surface reflectance in a photovoltaic (PV) device.
Background
The power conversion efficiency of PV devices is typically described by the devices' parametric properties such as open-circuit voltage (Voc), short-circuit current (Jsc), resistances, where direct improvements in any of these parameters will result in a corresponding increase in the power conversion efficiency.
A significant influence on the Jsc of PV devices is their surface reflectance, where a low reflectance is desirable to maximize light absorption by the devices. Currently, reduction in surface reflectance in PV devices has typically been achieved by two distinct physical mechanisms: texturing the surface of the light-absorbing substrate to induce light-trapping, and growth of anti-reflective coatings composed of dielectric materials via vacuum-based tools like plasma-enhanced chemical vapour deposition (PECVD) on the PV device surface to induce optical wavelength-specific destructive interference based on the dielectric materials' refractive indices and thicknesses. These methods typically require high vacuum or high temperature, which adds to the cost of the method.
There is therefore a need for an improved method for improving the power conversion efficiency of PV devices.
Summary of the invention
The present invention seeks to address these problems, and/or to provide an improved method for improving the power conversion efficiency of PV devices. In particular, the present invention relates to an improved method for reducing the surface reflectance in a PV device.
In general terms, the invention relates to a facile and low cost method to improve the efficiency of PV devices by further reducing the reflectance of the PV devices by introducing an additional light-trapping layer in a way that is complementary to existing reflectance reduction mechanisms. According to a first aspect, the present invention provides a method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising: depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition;
heating the deposited layer; and
hydrolysing the deposited layer.
According to a particular aspect, the liquid-based precursor mixture may comprise any suitable metal and/or metalloid containing compounds. In particular, the metal and/or metalloid containing compounds may comprise, but is not limited to, zinc (Zn), titanium (Ti), aluminium (Al), silicon (Si), or a combination thereof.
The precursor mixture may comprise a suitable amount of the metal and/or metalloid containing compound. For example, the precursor mixture may have a concentration of 0.005-1.0 M, with respect to the metal or metalloid content.
The depositing may be by any suitable method. According to a particular aspect, the depositing may be by liquid deposition. The liquid deposition may comprise, but is not limited to, spin coating, spray coating, ink jet printing, slot-die coating, dip coating, roller coating, blade coating, or a combination thereof.
The deposited layer may have a suitable thickness. For example, the deposited layer may have a thickness of 30-1000 nm.
The heating may be carried out under suitable conditions. For example, the heating may comprise heating at a temperature of ≤ 400°C. The heating may be in normal or inert atmosphere. The inert atmosphere may be in a nitrogen or argon atmosphere. According to a particular aspect, the heating may be for a suitable time, such as≤ 20 minutes.
The hydrolysing may be by any suitable method. For example, the hydrolysing may be by, but not limited to: immersion in water, exposure to steam, water curtain, water spray, or a combination thereof. According to a particular aspect, the hydrolysing may form nano-sized structures on the surface of the PV device onto which the deposited layer is formed. For example, the nano-sized structures may have an average size of 1-800 nm.
Brief Description of the Drawings
In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:
Figure 1 shows a top down scanning electron microscope (SEM) image of a light- trapping layer according to one embodiment of the present invention deposited on a PV device substrate;
Figure 2(a) shows the cross-sectional SEM image of a light-trapping layer according to one embodiment of the present invention deposited on a PV device substrate and Figure 2(b) shows the cross-sectional SEM image of another embodiment of the present invention deposited on a PV device substrate;
Figure 3(a) shows the reflectance spectra of a PV device before and after coating with a light-trapping layer according to one embodiment of the present invention and Figure 3(b) shows the reflectance spectra of a PV device before and after coating with a light- trapping layer according to another embodiment of the present invention;
Figure 4 shows the external quantum efficiency of Cell 1 as shown in Tables 2 and 3; and
Figure 5 shows the internal quantum efficiency of Cell 1 as shown in Tables 2 and 3. Detailed Description
As explained above, there is a need for an improved method of increasing the power conversion efficiency of PV devices while at the same time being a cost-effective method. One way of achieving this is by reducing the surface reflectance so as to maximise light absorption by the PV device.
The method of the present invention enables a lower reflectance to be achieved using a low-cost and facile method in which an additional light-trapping layer is provided on top of existing anti-reflective coatings. In particular, the method does not utilise high temperature or high vacuum conditions which are typically required for material deposition in solar cell manufacturing, thus keeping costs low. There is also no need for a seed layer to be formed to ensure surface conformity. The materials involved in the method are also simple, non-hazardous and easily available, thus avoiding safety and material scarcity issues.
The light-trapping layer formed by the method of the present invention reduces the surface reflectance and thereby immediately results in the improvement of device Jsc and therefore, power conversion efficiency of the PV device.
The method may be applied on pre-metallization or completed PV devices. The advantage of the method of the present invention is that it only affects the optical transmission of irradiance onto the semiconducting material and the method is insensitive to the PV device in any other way.
The method generally involves depositing the additional light-trapping layer on a surface of a PV device. The surface may comprise an existing anti-reflective coating. The additional light-trapping layer may be deposited on the surface as a pre-cursor layer by liquid deposition, followed by a low temperature curing process. The precursor layer may then undergo further hydrolysis to yield a highly surface conformal layer of a material with surface morphology with light-trapping properties.
According to a first aspect, the present invention provides a method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising: depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition;
heating the deposited layer; and
hydrolysing the deposited layer.
According to a particular aspect, the metal and/or metalloid containing compounds comprised in the liquid-based precursor mixture may be any suitable metal and/or metalloid containing compound. The metal and/or metalloid comprised in the metal and/or metalloid containing compound may be ionic or covalent in nature. For example, the metal or metalloid comprised in the metal and/or metalloid containing compound may be, but not limited to, zinc (Zn), titanium (Ti), aluminium (Al), silicon (Si), or a combination thereof. In particular, the metal and/or metalloid containing compound may comprise Zn or Al.
The precursor mixture may comprise a suitable amount of the metal and/or metalloid containing compounds. For example, the precursor mixture may have a concentration of 0.005-1.0 M, with respect to the metal or metalloid content. In particular, the concentration may be 0.1-0.3 M.
For the purposes of the present invention, liquid-based precursor mixture may be in the form of a sol or solution.
The precursor mixture may be formed by any suitable method. According to a particular aspect, the precursor mixture may be formed by dispersion or dissolution of a metal and/or metalloid source in a solvent. The precursor mixture may further comprise a suitable amount of additives for stabilising the precursor mixture. For example, the additives may be, but not limited to, ammonia, potassium hydroxide, sodium hydroxide, acetyl acetone, ethanolamine, oxalic acid, citric acid, hydrochloric acid, nitric acid, sulphuric acid, or a combination thereof. In particular, the precursor mixture may comprise additives in an amount up to 20 v/v % or thrice the stoichiometric quantity with respect to the metal and/or metalloid content in the precursor mixture. The precursor mixture may be aged before it is either further diluted, or directly used in liquid deposition. The extent of dilution may be dependent on the method of deposition and condition of the PV device onto which the precursor mixture is to be deposited.
The metal and/or metalloid containing compound may comprise the metal and/or metalloid comprised in the precursor mixture. In particular, the metal and/or metalloid containing compound may comprise, but is not limited to, anhydrous zinc oxide, zinc nitrate, titanium tetraisopropoxide, aluminium tri-isopropoxide, aluminium tri-sec- butoxide, aluminium nitrate anhydrate, tetraethyl orthosilicate, or a combination thereof.
The solvent used in the preparation of the precursor mixture may be any suitable solvent for the purposes of the present invention. For example, the solvent may be, but is not limited to, methanol, ethanol, isopropanol, deionized water, or a combination thereof. The depositing may be by any suitable method. According to a particular aspect, the depositing may be by liquid deposition. The liquid deposition may comprise, but is not limited to, spin coating, spray coating, ink jet printing, slot-die coating, dip coating, roller coating, blade coating, or a combination thereof. In particular, the liquid deposition may comprise spin-coating, spray-coating or a combination thereof. Since the depositing is by liquid deposition, this step avoids low pressure, high temperature conditions which are typical in gaseous phase deposition of a precursor mixture. Accordingly, the depositing of the precursor mixture may be carried out more easily and more cost effectively.
The depositing of the precursor mixture may be on a suitable surface of the PV device. For example, the depositing may be on the front and/or rear surface of the PV device. The depositing may achieve a macroscopically uniform distribution of metal or metalloid content on the surface onto which the precursor mixture is being deposited. In particular, depositing of the precursor mixture by spin-coating and spray-coating may result in good surface conformity and reflectance improvements.
The deposited layer may have a suitable thickness. For example, the deposited layer may have a thickness of 30-1000 nm. In particular, the thickness of the deposited layer may be 50-900 nm, 80-800 nm, 100-700 nm, 200-600 nm, 300-500 nm, 350-400 nm. Even more in particular, the thickness may be 50-300 nm.
The heating may be any suitable heating. For the purposes of the present invention, heating may be defined as comprising any suitable heat treatment of the deposited layer. For example, the heating may comprise curing, annealing, or a combination thereof. According to a particular aspect, the heating may comprise annealing the deposited layer.
The heating may be by any suitable means. For example, the heating may be, but is not limited to, radiative or conductive heating. Radiative heating may be by, but not limited to, using an infrared lamp and conductive heating may be by, but not limited to, an oven or furnace or a heated surface.
The heating may be carried out under suitable conditions. For example, the heating may comprise heating at a temperature of≤ 400°C. In particular, the heating may be at a temperature of about 100-400°C, 120-380°C, 140-360°C, 150-350°C, 175-325°C, 200-300°C, 220-280°C, 240-260°C, 245-250°C. Even more in particular, the heating may be at a temperature of≤ 275°C, preferably 140-275°C.
The heating may be for a suitable period of time. The time for heating may be dependent on the heating means used for the heating. For example, the heating may be for≤ 20 minutes. In particular, the heating may be for 2 seconds - 20 minutes, 30 seconds - 15 minutes, 1-12 minutes, 2-10 minutes, 5-8 minutes. Even more in particular, the heating may be for 1-2 minutes.
The heating may be in an inert atmosphere or in normal atmosphere. For example, heating in an inert atmosphere may comprise heating in a nitrogen or argon atmosphere.
The hydrolysing may be by any suitable method. For example, the hydrolysing may be by, but not limited to: immersion in water, exposure to steam, water curtain, water spray, or a combination thereof. The hydrolysing may be under suitable conditions.
According to a particular aspect, the hydrolysing may be for a suitable period of time. For example, the hydrolysing may be for≤ 20 minutes. In particular, the hydrolysing may be for 2 seconds - 20 minutes, 30 seconds - 15 minutes, 1-12 minutes, 2-10 minutes, 5-8 minutes. Even more in particular, the hydrolysing may be for 2-5 minutes.
The hydrolysing may be carried out at a suitable temperature. For example, the hydrolysing may comprise hydrolysing at a temperature of ≤ 100°C. In particular, the hydrolysing may be at a temperature of about 50-100°C, 60-90°C, 70-80°C. According to a particular aspect, the hydrolysing may comprise direct immersion of the PV device comprising the deposited layer in deionised water at a temperature of 60-100°C. According to a particular aspect, the hydrolysing may comprise exposing the PV device comprising the deposited layer to steam at a temperature of≥ 80°C.
According to a particular aspect, the hydrolysing may form a surface-conformal coating comprising nano-sized structures on the surface of the PV device. The resultant surface-conformal coating may be chemically composed of metal or metalloid oxides, which may contain hydroxide stoichiometry. In particular, the surface-conformal coating may comprise a random distribution of the nano-sized structures. The nano-sized structures may comprise high aspect ratio surface features. For example, the nano- sized structures may comprise nanospheres, nanohairs, nanowires, nanoplatelets, nanoridges, or a combination thereof.
The nano-sized structures may have an average size of 1-800 nm. For the purposes of the present invention, the average size may refer to at least one of the height of nano- sized structures or the width of nano-sized structures.
According to a particular aspect, the nano-sized structure may have an average size of diameter of 30-800 nm. For example, the nano-sized structure may have an average size of 50-750 nm, 75-700 nm, 90-600 nm, 100-500 nm, 200-400 nm, 250-300 nm. In particular, the average size of the nano-sized structure may be 50-300 nm.
The surface geometry of the nano-sized structures formed may be modified through the temperature and duration of the heating and hydrolysing. For example, time and temperature at which hydrolysing takes place may affect the extent of reaction for each metal/metalloid base layer. For example, below a certain temperature, the reaction may be either too slow or non-existent such that properties of the deposited layer may not change even after a period of time.
The formation of the nano-sized structures results in additional fine surface roughening due to the irregular distribution of the nano-sized structures. This enhances surface light-trapping on the surface of the PV device on which the light-trapping layer is formed where the optical effect is more pronounced for substrates of PV devices which do not already have nano-sized structures like reactive-ion etched (RIE) or metal- assisted chemical etched (MACE) silicon surfaces.
An example of the additional light-trapping layer formed by the method of the present invention is shown in Figure 1. In particular, Figure 1 shows an example of a light- trapping layer which is formed by the method of the present invention in which the hydrolysing comprises hot water washing. The PV device comprises an isotropically- etched multi-crystalline silicon substrate. However, it would be clear to a person skilled in the art that the PV device may comprise any suitable PV device, including but not limited to multi-crystalline and mono-crystalline substrates.
As can be seen from the above, since the method of the present invention does not involve high temperatures or high vacuum conditions, the PV device onto which the additional light-trapping layer may be provided is not damaged in the course of the method. There is also minimal accumulation in thermal budget.
A further advantage of the method of the present invention is that not only does the method result in the reduction of surface reflectance and therefore improvement in power conversion efficiency by way of the light-trapping layer, but the addition of the light-trapping layer may also increase the apparent thickness of the underlying anti- reflective coating. Accordingly, the thickness of the anti-reflective coating may be reduced, thereby resulting in reduced material usage and processing time for deposition of the anti-reflective coating.
Having now generally described the invention, the same will be more readily understood through reference to the following embodiment which is provided by way of illustration, and is not intended to be limiting.
Example
Two different light-trapping layers were formed on a surface of two PV devices. One was a spin-coated oxidised aluminium and the other was a spray-coated oxidised zinc layer. The light-trapping layers were formed on isotropically-etched multi-crystalline silicon substrates.
Method
The spin-coated oxidised aluminium layer was formed as follows. A solution of aluminium nitrate in ethanol was prepared by mixing 33.3 mmol of aluminium nitrate with 100 ml of ethanol and 6.67 ml of acetylacetone to form a precursor mixture. The precursor mixture was then left to stir for at least 24 hours at room temperature. The precursor mixture was then deposited on an isotropically-etched multi-crystalline silicon substrate at room temperature and normal atmosphere via spin coating at 3000 rpm. The silicon substrate comprising the deposited layer was then placed in an oven at 200°C for 10 minutes for heating, following which it was removed to cool down to room temperature again. The resultant light-trapping layer was hydrolysed via submersion into an agitated water bath at a temperature of about 80°C for 5 minutes before it was removed for blow drying. The spray-coated oxidized zinc layer was formed as follows. A solution of zinc nitrate in ethanol was prepared by mixing 60 mmol of zinc nitrate with 600 ml of ethanol to form a precursor mixture. The precursor mixture was then left to stir for at least 24 hours at room temperature. The precursor mixture was then deposited on an isotropically- etched multi-crystalline silicon substrate via spray coating by scanning the surface of the substrate with the precursor mixture with a spray rate of 3.5 ml/min from a single nozzle where the total amount of precursor mixture sprayed across the substrate surface was 5.5 ml, and the surface of the substrate was maintained at about 100°C. The solar cell was then placed in an oven at 280°C for 1 minute for heating, following which it was removed to cool down to room temperature again. The resultant light- trapping layer was hydrolysed via submersion into an agitated water bath about 80°C for 5 minutes before it was removed for blow drying.
Results and discussion
Figures 2(a) and 2(b) show the cross-sectional SEM images of the coated substrates with the light-trapping layer formed by the spin coated oxidised aluminium and spray coated oxidised zinc, respectively. From Figures 2(a) and 2(b), it can be seen that the deposited light-trapping layer easily achieves conformal coating quality that mimics the initial substrate surface topography. This demonstrates a ubiquity in the method of the present invention. Accordingly, the light-trapping layer may be applied onto surface textures found on most PV devices.
For mono-crystalline silicon solar cells, the surface texture may be typically an ordered distribution of square pyramids, whereas in multi-crystalline silicon solar cells, the surface texture may be a random distribution of surface features with high surface roughness. In both cases, surface feature scales are typically characterized as being in the order of microns. As seen from Figure 2, the method of forming the light-trapping layer according to the present invention deposits and achieves a surface-conformal coating that can be described as being a random distribution of fine nano-sized structures between 1-500 nm in size.
(i) Reflectance modification
Figures 3(a) and 3(b) show the reflectance spectra of the solar cells before and after coating with the light-trapping layer formed by the spin coated oxidised aluminium and spray coated oxidised zinc, respectively. The reflectance reduction induced by the metal oxide coating described is a result of two optical phenomena - a shift in the minima of the pristine surface's reflectance spectrum towards longer wavelengths due to higher apparent thickness of the existing anti-reflective coating, and a wideband reduction in reflectance due to enhanced light-trapping on the device surface due to the additional deposited light-trapping layer.
Due to the interaction between the two phenomena, the magnitude of improvements in reflectance and Jsc are dependent on the original PV device surface's reflectance profile. Aside for the possibility for achieving lower reflectance than permissible by the pristine cell, most importantly, addition of the metal oxide light-trapping layer allows for reduced material usage and process time for deposition of the underlying anti-reflective coating.
As observed from Figures 3(a) and 3(b), some optical wavelength bands experience an increase in reflectance, as opposed to a spectral wide reduction. Nevertheless, the weighted-average-reflectance between 300-1000 nm (WAR30o-iooo) still experiences a significant absolute reduction between 1 -2 % as can be seen in Table 1. Table 1 provides the changes in WAR30o-iooo based on light-trapping layer shown in Figure 2(a) and whose reflectance spectra is shown in Figure 3(a).
Figure imgf000013_0001
Table 1 : Changes in WAR30o-iooo of different crystalline Si substrates before and after introduction of a spin-coated oxidised aluminium light-trapping layer
(ii) Device efficiency improvement
Sample photovoltaic devices were electrically characterized under a calibrated AM1.5G spectrum (1 sun) prior to the introduction of the light-trapping layers as formed in this example, and again characterized after its introduction. The results obtained are shown in Tables 2 and 3.
Figure imgf000014_0001
Table 2: Electrical performance of three example PV devices under 1 sun irradiance prior to the introduction of spin-coated oxidised aluminium light- trapping layer
Figure imgf000014_0002
Table 3: Electrical performance of three example PV devices under 1 sun irradiance following introduction of spin-coated oxidised aluminium light- trapping layer
As can be seen from Tables 2 and 3, an absolute efficiency improvement of 0.2-0.4% was observed.
In these examples, absolute efficiency gains as high as 0.4% were observed, primarily due to a proportional improvement in Jsc. The coherence between the addition of a layer and improved Jsc values shows that the light-trapping layer indeed has light- trapping properties.
External quantum efficiencies (EQE) for an example photovoltaic device were characterized via a spectral-response tool. The results are shown in Figure 4. As expected from the observed reflectance spectral changes above, EQE improvements were observed between 600-1000 nm, with losses between 400-600 nm. However, there was still an observed net gain in device Jsc coherent with expected Jsc calculated from the EQE spectra integrated with AM1.5G photon flux. Internal quantum efficiencies (IQE) for this device were also derived before and after introduction of the spin coated oxidised aluminium light-trapping layer; the similarity between the two spectra indicates that no electrical function was affected aside from the improved light absorption as seen in Figure 5.
Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Claims

Claims
1. A method of reducing surface reflectance in a photovoltaic (PV) device, the method comprising:
depositing a liquid-based precursor mixture comprising metal and/or metalloid containing compounds on a surface of a PV device to form a deposited layer, wherein the depositing is by liquid deposition;
heating the deposited layer; and
hydrolysing the deposited layer.
2. The method according to claim 1 , wherein the metal and/or metalloid containing compounds comprise metal and/or metalloid selected from the group consisting of: zinc (Zn), titanium (Ti), aluminium (Al), silicon (Si), and a combination thereof.
3. The method according to claim 1 or 2, wherein the precursor mixture has a concentration of 0.005-1.0 M.
4. The method according to any preceding claim, wherein the liquid deposition comprises: spin coating, spray coating, ink jet printing, slot-die coating, dip coating, roller coating, blade coating or a combination thereof.
5. The method according to any preceding claim, wherein the deposited layer has a thickness of 30-1000 nm.
6. The method according to any preceding claim, wherein the heating is at a temperature of≤ 400°C.
7. The method according to any preceding claim, wherein the heating is in a normal or an inert atmosphere.
8. The method according to claim 7, wherein the inert atmosphere comprises a nitrogen or argon atmosphere.
9. The method according to any preceding claim, wherein the heating is for≤ 20 minutes.
10. The method according to any preceding claim, wherein the hydrolysing is by: immersion in water, exposure to steam, water curtain, water spray or a combination thereof.
5
1 1. The method according to any preceding claim, wherein the hydrolysing forms nano-sized structures on the surface of the PV device.
12. The method according to claim 11 , wherein the nano-sized structures have an i o average size of 1-800 nm.
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US20090253227A1 (en) * 2008-04-08 2009-10-08 Defries Anthony Engineered or structured coatings for light manipulation in solar cells and other materials
CN102222728A (en) * 2011-05-24 2011-10-19 中国科学院上海技术物理研究所 Preparation method for zinc oxide nanoarray antireflection layer on surface of silicone-based solar battery
US20140004709A1 (en) * 2009-07-03 2014-01-02 National Tsing Hua University Antireflection structures with an exceptional low refractive index and devices containing the same
US20150276989A1 (en) * 2014-03-25 2015-10-01 Seung-Yeol Han Low temperature-curable antireflective coatings having tunable properties including optical, hydrophobicity and abrasion resistance

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4190321A (en) * 1977-02-18 1980-02-26 Minnesota Mining And Manufacturing Company Microstructured transmission and reflectance modifying coating
US20090253227A1 (en) * 2008-04-08 2009-10-08 Defries Anthony Engineered or structured coatings for light manipulation in solar cells and other materials
US20140004709A1 (en) * 2009-07-03 2014-01-02 National Tsing Hua University Antireflection structures with an exceptional low refractive index and devices containing the same
CN102222728A (en) * 2011-05-24 2011-10-19 中国科学院上海技术物理研究所 Preparation method for zinc oxide nanoarray antireflection layer on surface of silicone-based solar battery
US20150276989A1 (en) * 2014-03-25 2015-10-01 Seung-Yeol Han Low temperature-curable antireflective coatings having tunable properties including optical, hydrophobicity and abrasion resistance

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