WO2013082246A1

WO2013082246A1 - Controlling alkali in cigs thin films via glass and application of voltage

Info

Publication number: WO2013082246A1
Application number: PCT/US2012/066993
Authority: WO
Inventors: Richard Michael FIACCO; Kenneth Edward HRDINA; Melissann ASHTON-PATTON
Original assignee: Corning Incorporated
Priority date: 2011-11-30
Filing date: 2012-11-29
Publication date: 2013-06-06
Also published as: TW201329009A

Abstract

A method of moving alkali metal ions in a glass substrate to form a glass substrate having an intrinsic alkali metal barrier layer or an enhanced alkali metal layer by applying voltage to at least one of the surfaces of the substrate such that at least a portion of the alkali metal ions, alkaline earth metal ions, or the combination thereof in the at least one surface move into the thickness of the glass substrate.

Description

CONTROLLING ALKALI IN CIGS THIN FILMS VIA GLASS AND

APPLICATION OF VOLTAGE

[0001] This application claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Serial No.

61/565109 filed on November 30, 2011 the content of which is relied upon and incorporated herein by reference in its entirety .

Field

[0002] This disclosure relates generally to glass substrates with diffusion barrier layers or enhanced alkali layers and more particularly to glass substrates with alkali diffusion barrier layers or enhanced alkali layers which may be useful in photovoltaic applications, for example, thin film

photovoltaic (PV) devices and methods of making the same.

Background

[0001] One issue facing the PV industry today in the

development of thin film solar panel technologies is

controlling alkali delivery from a substrate material into a deposited semiconductor thin film. The two PV thin film technologies in development today are Cadmium Telluride (CdTe) and Cadmium Indium Gallium Selenide (CIGS) . In the case of CdTe, the thin film stacks are deposited onto the glass superstrate through which sunlight must pass. It has been established that incorporation of alkali ions into the

transparent conducting oxide (TCO) and/or CdTe thin films can degrade the device's efficiency in converting sunlight into electricity. The most common barrier layers currently

employed in the CdTe thin film stack are sputter-deposited silica (Si02) or alumina (AI₂O3) . These barrier layers can suppress the egress of alkali ions from the glass superstrate, but not completely block it.

[0002] In the case of CIGS, thin films are deposited onto a glass substrate that is not directly exposed to sunlight.

Incorporation of sodium, potassium and possibly other alkali metals into the CIGS semiconductor thin film during deposition can alter its microstructure in a manner that ultimately enhances the device's efficiency in converting sunlight into electricity .

[0003] The most commonly employed substrate material for both the CdTe and CIGS technologies is soda-lime-silica (SLG) float glass. This glass material is primarily used because it is readily available, low cost and contains a high concentration of sodium. This last attribute is desirable in some CIGS processes (e.g., rapid thermal processing and co-evaporation) because a high concentration of sodium can result in the rapid delivery of sodium during thin film deposition and can yield high cell efficiencies. Dopant levels of about ≥ lxlO¹⁹ sodium atoms per cm³ within the CIGS thin film typically results in higher cell efficiency. Typical CIGS films are ≤ 2.5 microns (μιη or urn) thick. Therefore, the required sodium

concentration for every square centimeter of PV cells can be estimated at 95 ng/cm² or less for thinner CIGS films. 1 gram of table salt (NaCl) has about the same amount of sodium in about 1 in² of a 3 mm thick plate of soda-lime-silica float glass (SLG) . This sodium is enough to dope about 41,000 m² of CIGS cells to a level of lxlO¹⁹ atoms/cm³.

[0004] As SLG glass ages in ambient conditions of temperature and humidity, its surface chemistry changes in a manner that results in a spatially non-uniform release of sodium during CIGS thin film deposition. [0005] As a result, some solutions that incorporate an external dopant layer that delivers all of the required sodium into the CIGS film in a controllable manner are being investigated. In these product designs, it is desirable to fully prevent the egress of alkali from the underlying glass substrate. In order to accomplish this, barrier layers are being developed that prevent diffusion of alkali from the glass substrate into the external alkali dopant layer and CIGS film. The barrier layer must also prevent or minimize the diffusion of alkali ions from the external dopant layer back into itself such that all of the alkali ions are available for diffusion into the CIGS film. The most common barrier layers currently employed in the CIGS thin film stack are sputter-deposited silica or alumina. These barrier layers can suppress the egress of alkali ions from the glass substrate, but not completely block it. Additionally, these barrier layers have been found ineffective in preventing the back diffusion of alkali from the external dopant layer into themselves.

[0006] In most CdTe and CIGS PV module product designs, two pieces of glass are employed as both the substrate and

superstrate materials. It is desirable for at least one of the glass pieces to be SLG float glass because of its low cost. In such product configurations, the CTE of both glass pieces should be similar in order to prevent mechanical distortion during the lamination process.

[0007] A further desirable attribute of the glass is stability at high temperatures. The performance of CdTe and CIGS cells can be enhanced at higher processing temperatures. Attributes that best describe this temperature resistance in glass are strain and anneal points. It is difficult to design a single glass composition that can effectively meet the all of the thermal expansion, temperature resistance and variable alkali delivery requirements simultaneously.

SUMMARY

[0003] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the

description or recognized by practicing the invention as described in the written description and claims hereof.

[0004] Typical PV CIGS processing (those without voltage) result in a sodium concentration that is limited in the CIGS and molybdenum (Mo) layers due to either inhibited diffusion of sodium or a concentration of sodium that is limited by the concentration present in the glass. The present disclosure is an improvement over the past PV cells in that both the rate of sodium diffusion is accelerated as well as the quantity of sodium in the cells can be increased to levels exceeding that in the glass. The applied voltage increases the rate of diffusing sodium species by application of the voltage.

Secondly, the application of the voltage serves to create an enrichment of sodium where the voltage is applied and thus may increase the concentration of sodium in the Mo and CIGS layers .

[0005] Application of a negative voltage to the glass surface will cause mobile, positively-charged alkali and alkaline- earth ions to migrate/diffuse toward the glass surface and create a surface layer enriched in these species. Conversely, application of a positive voltage to the glass surface will cause mobile, positively-charged alkali and alkaline-earth ions to migrate/diffuse away from the glass surface and create a surface layer depleted in these species. Applied voltage can be used to create either a surface layer enriched in alkali or a surface layer depleted in alkali. Additionally, the technique can be tailored to control the extent of alkali enrichment or depletion (e.g., partial enrichment or

depletion) as desired to control the amount and rate of alkali release from the glass surface into subsequently deposited thin films. The thickness of the enriched or depleted surface layers can range from a few nanometers to about 10 microns, for example, from 3 nanometers to 10 microns, for example, from 10 nanometers to 1 micron.

[0006] One embodiment is a method of moving alkali ions in a glass substrate to form a glass substrate having an intrinsic alkali barrier layer, the method comprises:

providing a glass substrate comprising alkali metal ions, alkaline earth metal ions, or a combination thereof and having at least two opposing surfaces and a thickness between the surfaces; and

applying voltage to both surfaces of the substrate such that at least a portion of the alkali metal ions, alkaline earth metal ions, or the combination thereof in at least one surface move into the thickness to form the glass substrate having the intrinsic alkali barrier layer.

[0007] Another embodiment is a method of moving alkali ions in a glass substrate to form a glass substrate having an enhanced alkali layer, the method comprises:

applying voltage to both surfaces of the substrate such that at least a portion of the alkali metal ions, alkaline earth metal ions, or the combination thereof near at least one surface move away from a negative voltage and toward a positive voltage on an opposing surface to form the glass substrate having the enhanced alkali layer.

[0008] Another embodiment is an article comprising a glass substrate having a first region depleted in alkali metal ions, alkaline earth metal ions, or a combination thereof; and a second region having alkali metal ions, alkaline earth metal ions, or a combination thereof in a substantially uniform concentration in physical contact with the first region, wherein the glass substrate does not have a third region having an enhanced concentration of alkali metal ions,

alkaline earth metal ions, or a combination thereof.

[0009] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and

character of the invention as it is claimed.

[0010] The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment ( s ) of the invention and together with the description serve to explain the principles and operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention can be understood from the following detailed description either alone or together with the

accompanying drawing figures.

[0012] Figure 1 is a graph showing the net result of diffusion of sodium out of glass substrates when placed in a potassium containing salt bath. [0013] Figure 2 is a schematic of methods according to some embodiments .

[0014] Figure 3 is a graph showing predicted amounts of sodium in ng delivered from the glass as a function of time and temperature if an ion exchange process was operating.

[0015] Figure 4A, Figure 4B, and Figure 4C are schematics of exemplary steps of the method, according to some embodiments.

[0016] Figure 5 is a graph of current as function of time for a SLG type glass.

[0017] Figure 6 is a graph of a sodium profile taken on the non-tin side of SLG glass by secondary ion mass spectrometry (SIMS) .

[0018] Figure 7 is a graph of sodium profiles in CIGS and Mo films by SIMS for Example 1.

[0019] Figure 8 is a graph of current behavior with a 50 volt applied field for Examples 1 and 2.

[0020] Figure 9 is a graph of a sodium profile for Example 2 by SIMS .

[0021] Figure 10 is a graph of potassium profiles in CIGS and Mo films by SIMS for Example 2.

[0022] Figure 11 is a graph of a sodium profile for Example 3 by SIMS.

[0023] Figure 12 is a graph of a sodium profile for Example 4 by SIMS.

[0024] Figure 13 is a graph of a sodium profile for Example 5 by SIMS.

[0025] Figure 14 is an illustration of features of a

photovoltaic device according to one embodiment.

[0026] Figure 15 is an illustration of features of a glass substrate according to some embodiments. DETAILED DESCRIPTION

[0027] Reference will now be made in detail to various

embodiments of the invention.

[0028] As used herein, the term "substrate" can be used to describe either a substrate or a superstrate depending on the configuration of the photovoltaic cell. For example, the substrate is a superstrate, if when assembled into a

photovoltaic cell, it is on the light incident side of a photovoltaic cell. The superstrate can provide protection for the photovoltaic materials from impact and environmental degradation while allowing transmission of the appropriate wavelengths of the solar spectrum. Further, multiple

photovoltaic cells can be arranged into a photovoltaic module. Photovoltaic device can describe either a cell, a module, or both .

[0029] As used herein, the term "adjacent" can be defined as being in close proximity. Adjacent structures may or may not be in physical contact with each other. Adjacent structures can have other layers and/or structures disposed between them.

[0030] One embodiment is a method of moving alkali ions in a glass substrate to form a glass substrate having an intrinsic alkali barrier layer, the method comprises:

applying voltage to both surfaces of the substrate such that at least a portion of the alkali metal ions,

alkaline earth metal ions, or the combination thereof in at least one surface move into the thickness to form the glass substrate having the intrinsic alkali barrier layer. [0031] Another embodiment is a method of moving alkali ions in a glass substrate to form a glass substrate having an enhanced alkali layer, the method comprises:

alkaline earth metal ions, or the combination thereof near at least one surface move away from a negative voltage and toward a positive voltage on an opposing surface to form the glass substrate having the enhanced alkali layer.

[0032] The glass substrate, made according to the disclosed methods can have a first region depleted in alkali metal ions, alkaline earth metal ions, or a combination thereof; and a second region having alkali metal ions, alkaline earth metal ions, or a combination thereof in a substantially uniform concentration in physical contact with the first region, wherein the glass substrate does not have a third region having an enhanced concentration of alkali metal ions,

alkaline earth metal ions, or a combination thereof.

[0033] Another embodiment is an article comprising a glass substrate having a first region depleted in alkali metal ions, alkaline earth metal ions, or a combination thereof; and a second region having alkali metal ions, alkaline earth metal ions, or a combination thereof in a substantially uniform concentration in physical contact with the first region, wherein the glass substrate does not have a third region having an enhanced concentration of alkali metal ions,

alkaline earth metal ions, or a combination thereof. [0034] Voltage, both positive and negative can be used to control the amount of alkali migration seen in the glass substrate and then the amount of alkali entering into, for example, CIGS PV cells from the glass substrate. In some embodiments, voltages at levels in the range of from + 10 kv and -100 kv, for example, -0.5 volts to -100 kv, for example, -0.5 volts to -10 kv, for example, from -0.5 volts to -1 kv, or, for example, -5 volts to -100 kv, for example, from -5 volts to -10 kv, for example, -5 volts to -1 kv are applied to the glass substrate with or without additional layers.

[0035] In some embodiments, voltages are applied at levels of 1 kv or less, for example, 750 volts or less, for example, 700 volts or less, for example, 650 volts or less, for example, 600 volts or less, for example, 550 volts or less, for

example, 500 volts or less, for example, 450 volts or less, for example, 400 volts or less, for example, 350 volts or less, for example 300 volts or less, for example, 250 volts or less to the glass substrate with or without additional layers.

[0036] In some embodiments, voltages are applied at levels of from -5 volts to 1 kv, for example, from -5 volts to 750 volts, for example, from -5 volts to 700 volts, for example, from -5 volts to 650 volts, for example, from -5 volts to 600 volts, for example, from -5 volts to 550 volts, for example, from -5 volts to 500 volts, for example, from -5 volts to 450 volts, for example, from -5 volts to 400 volts, for example, from -5 volts to 350 volts, for example from -5 volts to 300 volts, for example, from -5 volts to 250 volts to the glass substrate with or without additional layers.

[0037] In some embodiments, voltages are applied at levels of from 1 volt to 1000 volts, for example, 1 volt to 950 volts, for example, 1 volt to 900 volts, for example, 1 volt to 900 volts, for example, 1 volt to 850 volts, for example, 1 volt to 800 volts, for example, 1 volt to 750 volts, for example, 1 volt to 700 volts, for example, 1 volt to 650 volts, for example, 1 volt to 600 volts, for example, 1 volt to 550 volts, for example, 1 volt to 500 volts, for example, 2 volts to 500 volts, for example, 3 volts to 500 volts, for example, 4 volts to 500 volts, for example, 5 volts to 500 volts.

[0038] The various voltages can be applied for different lengths of time. Negative voltages will help deliver alkali while positive voltages can shut off or slow down the rate or amount of alkali migration. The time for which the voltages are applied can vary between 1 nanosecond and multiple days. The total current applied in a certain temperature range may be used to quantify the amount of ion migration from the glass .

[0039] Voltage can be applied in a number of different ways including: applying a voltage on the Mo film during CIGS deposition; applying a voltage on the CIGS or Mo film after CIGS deposition but during the high temperature heat

treatment; applying a voltage to the Mo film and then heating the glass/Mo film to temperatures exceeding 50°C; applying a voltage to the multilayered film in order to change the alkali content in the cell at temperatures between 20°C and 400°C;

applying a voltage after CIGS is applied but before the final capping layers such as CdS and or ZnO are deposited; or applying a voltage and thermal lower temperature treatment after all multilayer coatings have been applied.

[0040] The method can comprise applying a voltage to bare glass that is heated prior to application of Mo film to create an alkali enriched surface. As an example, gold, silver,

graphite, platinum, palladium, copper, aluminum or other metallic alloys known to those skilled in the art coated electrodes could be pressed to glass surfaces while heating. [0041] Alkali-silicate glasses, like the glasses being used in this disclosure, are known to be ionic conductors. At room temperature glasses are insulators, however at elevated temperatures the monovalent cations become mobile in the glass. When a glass is at elevated temperatures (T > 75°C) , contains high concentrations of mobile ions, and/or is in the presence of an electric field, alkali ions will move through the glass network towards one surface of the glass and away from the other. For example, glasses with large amounts of sodium, like commercial soda lime silicates, will naturally form a sodium rich layer on the surface of the glass under at temperatures > 50°C, but < 100°C. The presence of the electric field will increase the driving force behind the alkali migration above that of temperature alone, and allows the direction of the migration to be determined, for example, the positive alkali ions will move towards the negatively biased surface and away from the positively biased surface.

[0042] The weak electrolyte theory expresses the ionic conductivity as a product of the concentration of mobile ions and mobility of the conducting ion [Eq 1] :

σ = n * zeu

Equation 1

Where O is the ionic conductivity, n* is the number of mobile ions per unit volume, z is the charge of the mobile ion, e is the electronic charge, andtiis the mobility of the ion. One point in this disclosure is to maintain the conductivity of the alkali ions, but decreasing Π*, the number of mobile ions per unit volume in the bulk of the glass. Controversy exists over exactly why sodium increases cell efficiency, but it indeed does. Perhaps there are multiple effects. This disclosure identifies processes that can utilize a wide variety of glasses. As an example, a glass with less alkali will likely allow CIGS manufacturing at higher temperatures. This may improve efficiency by creating a better performing crystal structure or a change in the grain size or amount of sodium in the grains or the grain boundaries.

[0043] To maintain the conductivity of the ions at the surface of the glass, we are proposing using an electric field to attract the alkali from the bulk of the glass to the surface of where the CIGS layer will be laid down on the glass. The positive alkali ions in the glass will be attracted to the anode, the rate of ionic mobility can be manipulated, through temperature, time, and strength of the voltage placed across the glass, thus allowing a tailored alkali profile at the surface. Tailoring the alkali profile in the glass, allows the concentration of mobile ions to be high at the surface, maintaining the conductivity similar to high alkali glass, but keeps the alkali low in the bulk of the glass, therefore gaining the technical advantage of a high strain point glass.

[0044] Figure 1 is a graph showing the net result of diffusion of sodium out of glass substrates when placed in a potassium containing salt bath. In this example, glass substrates containing sodium are placed in potassium salt baths and heated to 600°C for a fixed length of time, for example, forty minutes. The net result is diffusion of sodium out of the glass substrates, Line 12. The salt bath diffusion data shown in Figure 1 is a predicted sodium profile in the glass that was modeled using the same thermal profile used to apply the CIGS coatings onto the same glass substrate. However, Figure 1 shows that the actual sodium profile, Line 10, of the glass surface below the CIGS film has virtually no detectable sodium loss compared to that which would be expected for a glass surface ion exchanged in a salt bath. This demonstrates that the standard CIGS processing, i.e., one which does not involve application of a voltage or ion exchangeable species, would result in very little sodium migration.

[0045] Figure 2 is a schematic of methods according to some embodiments. A depiction of voltage is being applied to the Mo film during a heat treatment in order to control the amount of sodium getting into the CIGS film. Voltage V can be applied to molybdenum (Mo) film 16 and at least one surface of the glass substrate 14 during a heat treatment. The amount of sodium getting into the CIGS film may be controlled by

applying voltage to the Mo film.

[0046] Figure 3 is a graph showing predicted amounts of sodium in ng delivered from the glass to the multilayers, for

example, CIGS layers as a function of time and temperature. Lines 24, 22, 20, and 18 show temperature curves for 225°C, 250°C, 350°C, and 500°C, respectively. This prediction is extrapolating the sodium diffusion data at 500°C and 600°C down to lower temperatures. It is assumed that the application of the voltage can result in diffusion rates similar to that in the salt bath. In reality, the application of more voltage may actually enhance diffusion further and also real diffusion values as opposed to the extrapolated values maybe greater than those shown in the figure. Much sodium can be extracted from the CIGS layers, even at temperatures less than 400°C. The graph shows that doping the multilayer at low temperatures is indeed possible in reasonable lengths of time. The

extrapolated data shown here indicates that doping at levels of lxlO¹⁹ sodium atoms/cm³ (-95 ng sodium/cm² cell for 2.5 urn thick CIGS layer) can be accomplished in less than a day at temperatures of 250°C or more. Most of the sodium would be expected to populate the grain boundaries.

[0047] Alkali profiles in the glass can be customized by applying positive and then no or even negative voltages for different lengths of time and temperatures such as

schematically depicted in Figures 4A, 4B, and 4C, or by applying an alkali containing film or alkali containing bath and subsequently a voltage with or without heat.

[0048] Figures 4A, 4B, and 4C are schematics of exemplary steps of the method, according to some embodiments. The method steps can be used to create an enhanced alkali layer at or near the surface of the glass substrate. Figure 4A shows that the as-made glass, prior to any treatment, has more or less a uniform sodium level, Line 21, throughout the surface and the bulk. Figure 4B shows that during exemplary step 1, the surface 23 of the glass substrate is enriched with alkali and the bulk starts to deplete of alkali as the sample is heated under the presence of a voltage. Figure 4C shows that during exemplary step 2, reversing the bias or removing the bias, with heat drives the alkali into the surface, Line 25.

[0049] In one embodiment, as shown in Figure 14, features 101 of a photovoltaic device comprise the glass substrate 86

having an intrinsic alkali barrier layer or an enhanced alkali layer 90 made according to methods described herein. The photovoltaic device can comprise more than one of the glass substrates, for example, as a substrate and/or as a

superstrate. In one embodiment, the photovoltaic device 101 comprises the glass sheet as a substrate and/or superstrate 86, a conductive material 88 adjacent to the substrate, and an active photovoltaic medium 92 adjacent to the conductive material. In one embodiment, the active photovoltaic medium comprises a CIGS layer. In one embodiment, the active

photovoltaic medium comprises a cadmium telluride (CdTe) layer. In one embodiment, the photovoltaic device comprises a functional layer comprising copper indium gallium diselenide or cadmium telluride. In one embodiment, the photovoltaic device the functional layer is copper indium gallium diselenide. In one embodiment, the functional layer is cadmium telluride.

[0050] In CIGS, a molybdenum back contact conducting layer may be deposited directly onto the glass surface (adjacent to the barrier layer) and in between the CIGS functional layer. This moly film would be layer 88 in Figure 14.

[0051] In one embodiment, the barrier layer is adjacent to a transparent conductive oxide (TCO) layer, wherein the TCO layer is disposed between or adjacent to the functional layer and the barrier layer. A TCO may be present in a photovoltaic device comprising a CdTe functional layer.

[0052] In one embodiment, in Figure 15, features 201 of a glass substrate 86 are shown. The glass substrate can have a first region 94 depleted in alkali metal ions, alkaline earth metal ions, or a combination thereof; and a second region 96 having alkali metal ions, alkaline earth metal ions, or a combination thereof in a substantially uniform concentration in physical contact with the first region, wherein the glass substrate does not have a third region having an enhanced concentration of alkali metal ions, alkaline earth metal ions, or a

combination thereof. The concentration in the second region can be equal to that found in the bulk glass or as made glass prior to application of voltage.

[0053] In another embodiment, the glass substrate can further comprise another region 98, having an enhanced concentration of alkali metal ions, alkaline earth metal ions, or a

combination thereof, wherein the enhanced layer is in physical contact with the second region and can have a concentration equal to that found in the bulk glass or as made glass prior to application of voltage. In this embodiment, the first region may be absent. [0054] In one embodiment, the glass sheet is optically transparent. In one embodiment, the glass sheet as the substrate and/or superstrate is optically transparent.

[0055] According to some embodiments, the glass sheet has a thickness of 4.0mm or less, for example, 3.5mm or less, for example, 3.2mm or less, for example, 3.0mm or less, for example, 2.5mm or less, for example, 2.0mm or less, for example, 1.9mm or less, for example, 1.8mm or less, for example, 1.5mm or less, for example, 1.1mm or less, for example, 0.5mm to 2.0mm, for example, 0.5mm to 1.1mm, for example, 0.7mm to 1.1mm. Although these are exemplary

thicknesses, the glass sheet can have a thickness of any numerical value including decimal places in the range of from 0.1mm up to and including 4.0mm.

[0056] Embodiments described herein may provide one or more of the following advantages: can improve efficiency of CIGS cells by controlling alkali levels in the cells, or controlling where and when alkali is present in multilayered cells. For example, the process enables the presence of sodium during growth or after growth of the CIGS film. To date, it is not explicitly known if efficiency is dependent on when the sodium is present during forming of the CIGS cell; the improved efficiency can be obtained for glasses of various

compositions. Thus allowing more inexpensive glasses to be used or glasses with higher strain points which may allow processing at higher temperatures. This may facilitate processing speed or a preferred temperature dependent

microstructure in the CIGS and or Mo films to be obtained; may facilitate processing speed by increasing diffusion rates of alkali such as sodium and potassium; the use of voltage will make for a more robust, repeatable process, less dependent on prior sample history such as aging effects of the glass or impact of cleaning, washing, grinding or polishing procedures on the glass; lower alkali content and or less mobile alkali content glasses will improve the shelf life of the glass prior to processing, thus alleviating inventory and allowing a better supply chain; and/or the alkali content can be tailored to different device manufacturing processes in order to better maximize cell efficiency for specific processing histories. One example would be a 2 minute process vs a 1 hour process vs. a process which desires very low level alkali.

[0057] An advantage of the current disclosure over sputter- deposited barrier layers is that the depleted layer is intrinsic to the glass as an extension of the bulk glass structure. This eliminates any issues of adhesion or

delamination due to the absence of a sharp interface between the surface layer and bulk glass. An additional advantage of the depleted surface layer is that it significantly improves chemical durability of that glass surface and prevents or minimizes alkali and/or alkaline-earth egress over long exposure times to the environment (particularly water vapor) . This protection scheme will improve electrical reliability of PV modules and enable better retention of photoconversion efficiency over the module's lifetime. Modification of the glass surface via applied voltage also provides a methodology for engineering the glass surface to control alkali delivery and removes this constraint on the bulk glass composition. This tremendously increases the glass compositional space that can be employed to optimize other bulk glass attributes such as CTE, strain point, melting, forming, cost, etc.

Examples

[0058] Example 1: A soda lime silica glass (SLG) whose two surfaces were sputtered with gold was heated to 400°C in air for 20 minutes and held at temperature for 10 minutes. Then a DC voltage of 50 V was applied across the two faces with the positive lead on the non-tin side of the glass. After 10 minutes with voltage at 400°C, the samples was cooled to 100°C in 30 minutes with the voltage maintained at 50 volts. Then the voltage was removed and the sample was allowed to cool to room temperature. The current behavior from the point at which the voltage was applied for this sample is shown in

Figure 5, Line 26. Figure 5 is a graph of current as function of time for a SLG type glass. From this data, the total coulomb flow through the sample was calculated to be 0.042C/cm^' which then allows an estimate of the depletion depth to be made. Based on the glass composition, a depletion depth of 388 nm was estimated assuming a square sodium profile, shown by area 28 under Line 26. Point 30 on Line 26 shows the point at which the glass was beginning to cool.

[0059] A SIMS sodium profile in the glass was taken with these measurements shown in Figure 6. Figure 6 is a graph of a sodium profile taken on the non-tin side of SLG glass by secondary ion mass spectrometry (SIMS) . Line 32 shows the Na profile on the fracture surface of the glass, Line 34 shows the Na profile after voltage treatment on the positively biased glass surface, and Line 35 shows the hydrogen profile after voltage treatment on the positively biased glass

surface. A depletion depth of 125 nm was achieved with a then gradual increase of sodium back to bulk levels.

[0060] The gold was removed from a similar sample. The sample was then washed and had Mo sputter coated onto the depleted surface followed by co-evaporative deposition of CIGS on top of the Mo. SIMS measurements were then made on the thin film with results shown in Figure 7. Figure 7 is a graph of sodium profiles in CIGS and Mo films by SIMS for Example 1. A comparison is made with a standard piece of non-voltage treated SLG glass, Line 42. The data shows that the voltage depleted sodium in the surface of the SLG glass, Line 44, has resulted in reduced levels of sodium in both the Mo, Line 36, and the CIGS films. Line 38 represents In in the CIGS film and Line 40 represents 0 from the glass surface.

[0061] Example 2: A sodium free, potassium rich silicate glass whose two surfaces were sputtered with gold was heated to 400°C in air for 20 minutes and held at temperature for 10 minutes. Then a DC voltage of 50 V was applied across the two faces. After 10 minutes with voltage at 400°C, the samples was cooled to 100°C in 30 minutes with the voltage maintained at 50 volts. Then the voltage was removed and the sample was allowed to cool to room temperature. The current behavior from the point at which the voltage was applied for this sample is shown in Figure 8. Figure 8 is a graph of current behavior with a 50 volt applied field for examples 1 and 2, Lines 46 and 48, respectively. From this data, the total coulomb flow through the sample was calculated to be 0.071cm² which then allows an estimate of the depletion depth to be made. Based on the glass composition, a depletion depth of 388 nm was estimated assuming a square sodium profile, shown by area 50 under Line 46. Based on the glass composition, a depletion depth of 733 nm was estimated assuming a square potassium profile, area 52 under Line 48. A SIMS potassium profile in the glass was taken with these measurements and is shown in Figure 9. Line 54 shows the K fracture and Line 56 shows the potassium

profile on the positive voltage side. A depletion depth of 600 nm was achieved with a then gradual increase of potassium back to bulk levels.

[0062] The gold was removed from a similar sample, the sample was then washed and had Mo sputter coated onto the depleted surface followed by co-evaporative deposition of CIGS on top of the Mo. SIMS measurements were then made on the thin film with these results shown in Figure 10. Figure 10 is a graph of potassium profiles in CIGS and Mo films by SIMS for Example 2. A comparison is made with a standard piece of untreated glass, Line 64. The data shows that the voltage depleted potassium in the surface of the glass, Line 60 has resulted in significantly reduced levels of potassium in both the Mo, Line 58, and the CIGS films, In, Line 66, 0, Line 62.

[0063] Example 3: A soda lime silica glass (SLG) whose two surfaces were sputtered with gold was heated to 425°C in air in 1.5 hours and held at temperature for 10 minutes. Then a DC voltage of 25 V was applied across the two faces. After 10 minutes with voltage at 425°C, the voltage was turned off and the sample was cooled to 100°C in 8 hours with no applied voltage. The application of the voltage is believed to have pulled sodium to the surface of the (-) electrode region between the gold and the glass. The slow cooling was expected to allow diffusion of the sodium back into the glass in order to enrich the surface of the glass.

[0064] SIMS was then performed on the glass sample, both the (+) and the (-) lead sides of the glass, Lines 72 and 68, respectively, and the results are shown in Figure 11 along with that of an untreated piece of glass, Line 70. The positive lead side of the glass is still depleted in sodium and the (-) lead side of the glass has become enriched in sodium .

[0065] Example 4: A sodium rich silica glass whose two surfaces were sputtered with gold was heated to 425°C in air in 1.5 hours and held at temperature for 10 minutes. Then a DC voltage of 25 V was applied across the two faces. After 10 minutes with voltage at 425°C, the voltage was turned off and the sample was cooled quickly to 100°C in 10 minutes with no applied voltage. The application of the voltage was found to have pulled sodium to the surface of the (-) electrode region between the gold and the glass. The sodium in the glass itself at the (-) electrode side appeared to have sodium levels approximately equal to that of the bulk glass at the near surface region. The positive electrode side of the glass has a sodium depleted region to a depth of about 200 nm.

[0066] Figure 12 is a graph of a sodium profile for Example 4 by SIMS. SIMS was then performed on the glass sample, both the (+) and the (-) lead sides of the glass, Lines 78 and 74, respectively, and the results are shown in Figure 12 along with that of an untreated piece of glass, Line 76. The positive lead side of the glass is depleted in sodium.

[0067] Example 5: A sodium rich silica glass whose two surfaces were sputtered with gold was heated to 425°C in air in 1.5 hours and held at temperature for 10 minutes. Then a DC voltage of 5 V was applied across the two faces. After 10 minutes with voltage at 425°C, the voltage was turned off and the sample was cooled quickly to 100°C in 10 minutes with no applied voltage. The application of the voltage was found to have pulled sodium to the surface of the (-) electrode region between the gold and the glass. The sodium in the glass itself at the (-) electrode side appeared to have sodium levels approximately equal to that of the bulk glass at the near surface region. The positive electrode side of the glass has a sodium depleted region to a depth of about 100 nm.

[0068] Figure 13 is a graph of a sodium profile for Example 5 by SIMS. SIMS was then performed on the glass sample, both the (+) and the (-) lead sides of the glass, Lines 84 and 80, respectively, and the results are shown in Figure 12 along with that of an untreated piece of glass, Line 82. The positive lead side of the glass is depleted in sodium.

[0069] It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims

CLAIMS What is claimed is:

1. A method of moving alkali ions in a glass substrate to form a glass substrate having an intrinsic alkali barrier layer, the method comprising:

alkaline earth metal ions, or the combination thereof in at least one surface move into the thickness to form the glass substrate having the intrinsic alkali barrier layer.

2. The method according to claim 1, further comprising heating the glass substrate prior to the applying, during the applying, or both.

3. The method according to claim 2, wherein the heating comprises heating with a heat source that is at a temperature in the range of from 20°C to 600°C.

4. The method according to claim 1, wherein the applying comprises applying the voltage on a Mo film during CIGS deposition .

5. The method according to claim 1, wherein the applying comprises applying the voltage on a CIGS or a Mo film after CIGS deposition.

6. The method according to claim 5, wherein the applying comprises applying the voltage to the Mo film and then heating the glass substrate and the Mo film to a temperature of 50°C or more .

7. The method according to claim 1, wherein the applying comprises applying the voltage to the glass substrate as the substrate is heated.

8. The method according to claim 1, wherein the intrinsic alkali barrier layer has a thickness of from 3 nanometers to 10 microns from at least one surface of the glass substrate into the bulk of the glass substrate.

9. An article comprising the glass substrate having an intrinsic alkali barrier layer made according to claim 1.

10. A photovoltaic device comprising the glass substrate having an intrinsic alkali barrier layer made according to claim 1.

11. The photovoltaic device according to claim 9, comprising a functional layer comprising copper indium gallium diselenide or cadmium telluride, wherein the functional layer is adjacent to the barrier layer.

12. A method of moving alkali ions in a glass substrate to form a glass substrate having an enhanced alkali layer, the method comprising:

13. The method according to claim 12, further comprising heating the glass substrate prior to the applying, during the applying, or both.

14. The method according to claim 13, wherein the heating comprises heating with a heat source that is at a temperature in the range of from 20°C to 600°C.

15. The method according to claim 12, wherein the applying comprises applying the voltage on a Mo film during CIGS deposition .

16. The method according to claim 12, wherein the applying comprises applying the voltage on a CIGS or a Mo film after CIGS deposition.

17. The method according to claim 16, wherein the applying comprises applying the voltage to the Mo film and then heating the glass substrate and the Mo film to a temperature of 50°C or more .

18. The method according to claim 12, wherein the applying comprises applying the voltage to the glass substrate as the substrate is heated.

19. The method according to claim 12, wherein the enhanced alkali layer has a thickness of from 3 nanometers to 10 microns from at least one surface of the glass substrate into the bulk of the glass substrate.

20. An article comprising the glass substrate having an enhanced alkali layer made according to claim 12.

21. A photovoltaic device comprising the glass substrate having an enhanced alkali layer made according to claim 12.

22. The photovoltaic device according to claim 21, comprising a functional layer comprising copper indium gallium diselenide or cadmium telluride, wherein the functional layer is adjacent to the barrier layer.

23. An article comprising a glass substrate having a first region depleted in alkali metal ions, alkaline earth metal ions, or a combination thereof; and a second region having alkali metal ions, alkaline earth metal ions, or a combination thereof in a substantially uniform concentration in physical contact with the first region, wherein the glass substrate does not have a third region having an enhanced concentration of alkali metal ions, alkaline earth metal ions, or a

combination thereof.