Abstract:
Varying the optical absorption of an electrochromic device in situ allows optimal control over the depth and quality of laser patterning lines when patterning electrochromic devices. Accordingly, an electrochromic device comprises a target conductive layer, an absorbing electrochromic layer formed below the target layer, and an electrolyte layer formed below the absorbing electrochromic layer. The absorbing electrochromic layer is placed in a darkened state, and the target layer is laser ablated using a wavelength that is minimally absorbed in the target layer and a fluence level that does not ablate layers of the electrochromic device that are below the absorbing electrochromic layer. The absorbing electrochromic layer is placed in the darkened state by applying a predetermined control voltage to the electrochromic device, forming the electrochromic device by dark-state deposition, or forming an electrochromic device that is in its darkened state in an equilibrium state.

Description:
BACKGROUND 
     The field of electrochromics is extensive and has been developing over about the last forty years. In one application, an electrochromic coating is used for controlling the amount of light and heat passing through the window based on a user-controlled electrical potential that is applied across the optical stack of the electrochromic coating. An electrochromic coating can reduce the amount of energy used for room heating and/or air conditioning, however this energy benefit is dependent on the efficiency of the electrochromic device. By switching between a clear state having an optical transmission of about 60-80% and a colored state having an optical transmission of between 0.1-10%, both energy flow into a room through a window and privacy provided by the window can be controlled. The amount of glass used for various types of windows, such as skylights, aircraft windows, residential and commercial building windows, and automobile windows, is on the order of one billion square meters per year. Accordingly, the potential energy saving provided by electrochromic glazing is substantial. See, for example, Solar Energy Materials and Solar Cells, (1994) pp. 307-321. 
     A conventional electrochromic cell generally is structured as follows: a substrate, a transparent conductive layer, a counter electrode, an electrolyte, an electrochromic layer, and a transparent conductive layer. Conventional cathodic materials, commonly referred to as “electrochromic electrodes,” have included tungsten oxide WO 3  (most common), molybdenum oxide MoO 3 , niobium oxide Nb 2 O 5 , among others. Anodic materials, commonly referred to as “counter electrodes,” include nickel oxide NiO, tungsten-doped nickel oxide, and vanadium oxide (V 2 O 5 ), among others. The electrolyte materials exhibit poor electron conduction, but good ion conduction. Examples of ion layer materials include silicon dioxide SiO 2 , titanium oxide TiO 2 , aluminum oxide Al 2 O 3 , and tantalum oxide Ta 2 O 5 . 
     Various types of transparent conducting thin films have been employed for the first and second transparent conducting layers, such as, indium tin oxide (ITO), which is the most commonly used material. Other thin oxide layers have also been used, such as fluorine-doped tin oxide, antimony-doped tin oxide, aluminum-doped zinc oxide, and fluorine-doped zinc oxide. Regardless which thin film is used, conductivities of less than about 20 Ohms/□ are needed in order to produce a uniform voltage between the two conductive layers across the conductive layers. 
     At the anode, the following reaction takes place:
 
Metal Oxide or Polymer or Organic Molecule (Colored)+xM + +xe − ⇄Metal Oxide or Polymer or Organic Molecule (Transparent).
 
     At the cathode, the following reaction takes place:
 
Metal Oxide or Polymer or Organic Molecule (Transparent)+xM + +xe − ⇄Metal Oxide or Polymer or Organic Molecule (Colored).
 
in which M is H + , Li +  or Na + , e is an electron, and x is an integer.
 
     In operation, a voltage of between −5 and +5 volts is applied between the first and second conducting layers and causes reversible migration of ions between the anode and cathode resulting in the above oxidation-reduction reactions that change the absorption of the cathode, anode, or both. The electrolyte conducts ions between the materials. In a device without shorting in the electrolyte between the two conductive layers, the device resistance is high and is primarily due to the resistance of the electrolyte. 
     Laser irradiation is capable of ablating materials by several mechanisms, with the primary mechanism being thermal vaporization. Each material has a different ablation threshold for each wavelength of laser light. The ablation threshold is a function of material properties and absorption of the wavelength used. Ablation threshold and fluence are described in units of energy per unit area. In order to adjust the fluence, the power output of the laser, the duration of the exposure, and/or the spot size of the incident light can be controlled. To adjust the duration of exposure, the speed of the laser scribe process can be adjusted. In the case of pulsed lasers, the repetition rate and pulse width can also control the duration of laser light exposure. Due to these attributes of laser processing, reducing the ablation threshold reduces the fluence for a desired process, thereby permitting the process to be performed using less energy, at a faster scribe speed, or both. This is a desirable situation for processing speed and process costs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter disclosed herein is illustrated by way of example and not by limitation in the accompanying figures in which like reference numerals indicate similar elements and in which: 
         FIGS. 1A and 1B  depict the mechanism that can cause shorting of the layers of an electrochromic device; 
         FIG. 2  depicts a cross-sectional elevation schematic view of two portions of an exemplary embodiment of an electrochromic device according to the subject matter disclosed herein for a high-transmission state and a low-transmission state; 
         FIG. 3  depicts a cross-sectional schematic diagram of an exemplary electrochromic device that is configured to have an absorbing electrochromic layer that is suitable for patterning according to the subject matter disclosed herein; 
         FIG. 4  depicts a cross-sectional elevation schematic view of a laser ablation scribe line of two portions of an exemplary embodiment of an electrochromic device according to the subject matter disclosed herein for a high-transmission state and a low-transmission state; 
         FIG. 5  is an electron micrograph of a cross-sectional view of an exemplary electrochromic device in which the pictured electrochromic device corresponds to the left side of  FIG. 3 ; and 
         FIG. 6  provides a pictorial summary of the benefits of the subject matter disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not to be construed as necessarily preferred or advantageous over other embodiments. 
     Patterning the materials of an electrochromic device increases the efficiency of the device. Moreover, patterning the transparent conductors in an electrochromic device is the most cost-effective way for preventing shorts between conductive layers, and for maintaining efficiency in areas where the first and second conductive layers may come in contact, or where the electrolyte is ineffective. 
       FIGS. 1A and 1B  depict the mechanism that can cause shorting of the layers of an electrochromic device. In particular,  FIG. 1  depicts an exemplary embodiment of an electrochromic device  100 , such as a smart window, that comprises a substrate  101 , a conductor  102  and an electrochromic stack  103 .  FIG. 1B  depicts an enlarged view of electrochromic device  100  in a region that electrolyte  104  exhibits coating roll-off and can be insufficient for preventing conductor  102  and a conductive layer  105  from coming into contact and shorting in the region indicated by  106 . The coating roll-off of electrolyte  104  can also cause an unpredictable resistance across the edge of electrochromic device  100 . Accordingly, it is necessary to pattern conductive layer  105  to prevent shorts and to provide a predictable resistance across the edge of an electrochromic device. 
     Patterning of conductive layer  105  has been accomplished by physical patterning techniques and by laser-ablation techniques that use laser light of many different wavelengths. Three problems occur when patterning an electrochromic device using laser ablation. First, there is an inability to control the depth of the ablation due to the transmission of the laser light of commercially available lasers through the layers of the electrochromic device. Second, when using a laser having a wavelength within the transmission range of an electrochromic device, transmission of lower energy light (i.e., laser light having an energy below the ablation threshold) to lower layers in an electrochromic stack produces large amounts of slag around an ablation scribe line. Third, the capital and maintenance costs are high for commercially available lasers that can efficiently scribe materials that are transparent at visible and near infrared (IR) wavelengths. 
     Additionally, when patterning an electrochromic device by laser ablation, the melt of materials for incident energy that is below the ablation threshold can be distributed to the surrounding areas by vaporization of other materials. Laser light transmitted through the conducting layer of the electrochromic device can cause melting or vaporization of layers under the ablation target layer and/or damage to the substrate, thereby interrupting device operation by damaging both conductors. Additionally, a large kerf of undesirable material could be formed on the sides of the ablation scribe. 
     According to the subject matter disclosed herein, patterning an electrochromic device while an absorbing electrochromic layer of the electrochromic device is in a darkened state can increase the temperature-to-fluence efficiency and protect layers that are not targeted for ablation in the process, thereby allowing laser-ablation scribing to a specific depth in an electrochromic stack. This approach also provides a lower energy cost for the patterning process, which in turn minimizes the amount of melt at an ablation site and reduces the eject and slag in the area around a laser ablation scribe line. Further, a larger process window is enabled. 
     Moreover, the subject matter disclosed herein pertains to use of any laser having a peak wavelength in the region that an electrochromic device is capable of variable absorption. For window construction, this wavelength range is from visible light to near infrared. Additionally, the subject matter disclosed herein pertains to any technique of darkening an absorption layer in an electrochromic device. 
       FIG. 2  depicts a cross-sectional elevation schematic view of two portions of an exemplary embodiment of an electrochromic device  200  according to the subject matter disclosed herein for a high-transmission state and a low-transmission state. In  FIG. 2 , electrochromic device  200  comprises a substrate  201 , one or more lower layers  202 , an absorbing (or absorber) electrochromic layer  203 , and a target layer  204 . In one exemplary embodiment, target layer  204  is a transparent conductive oxide (TCO). When variable absorbing layer  203  is in a high-transmission state  210  (i.e., a bleached state, as depicted on the left side of  FIG. 2 ), laser light  205  is transmitted through electrochromic device  200 . When electrochromic device  200  is in a low-transmission state  220  (i.e., a darkened state, as depicted on the right side of  FIG. 2 ), laser light  205  is not transmitted through electrochromic device  200 , but is absorbed by absorbing electrochromic layer  203 . 
     Thus, when absorbing electrochromic layer  203  is in a darkened state, incident laser light  205  is absorbed by absorbing electrochromic layer  203 , thereby preventing laser light energy from being incident on underlying layers  202  and substrate  201 . Additionally, for the case the low-transmission state, as the absorbing electrochromic layer  203  absorbs energy, the temperature realized for a given amount of incident energy is greater because the energy is concentrated in a smaller area. 
       FIG. 3  depicts a cross-sectional schematic diagram of an exemplary electrochromic device  300  that is configured to have an absorbing electrochromic layer that is suitable for patterning according to the subject matter disclosed herein. Electrochromic device  300  comprises conductors  1  and  2 , electrodes  1  and  2 , and an electrolyte. Conductors  1  and  2  are selected from the following exemplary transparent conductive materials that include indium tin oxide (ITO), fluorine-doped tin oxide (FTO), antimony-doped tin oxide, aluminum-doped zinc oxide, and fluorine-doped zinc oxide. Conductors  1  and  2  can be selected to be the same material or can be different materials. Electrodes  1  and  2 , as a pair, are selected so that one is a cathodic and/or electrochrome material, and the other is an anodic and/or electrochrome material. Electrode  2  must be an electrochromic material in order to make the device relevant for the patterning technique disclosed herein. Suitable cathodic and/or electrochrome materials include tungsten oxide WO 3 , molybdenum oxide MoO 3 , niobium oxide Nb 2 O 3 , among others, and should be reduced by intercalation with a mobile ion such as Li+, H+, or Na+ in order to be absorbing. Suitable anodic and/or electrochrome materials include nickel oxide NiO X , tungsten-doped nickel oxide NiWO, vanadium oxide V 2 O 5 , among others, and these materials must be made devoid of intercalation ions in order to act as the absorbing layer. Suitable electrolyte materials include tantalum oxide Ta 2 O 5 , silica dioxide SiO 2 , aluminum oxide Al 2 O 3 , and titanium oxide TiO 2 . Thus, there are a number of suitable combinations of materials that can be selected to form the five primary layers of an electrochromic device that can be patterned in accordance with the subject matter disclosed herein. One exemplary embodiment comprises a first conductor formed from fluorine-doped tin oxide (FTO), a first electrode formed from tungsten oxide WO 3 , an electrolyte formed from tantalum oxide Ta 2 O 5  or silicon dioxide SiO 2 , a second electrode formed from tungsten-doped nickel oxide, and a second conductor formed from indium tin oxide (ITO). 
       FIG. 4  depicts a cross-sectional elevation schematic view of a laser ablation scribe line of two portions of an exemplary embodiment of an electrochromic device  400  according to the subject matter disclosed herein for a high-transmission state and a low-transmission state. In  FIG. 4 , electrochromic device  400  comprises a substrate  401 , one or more lower layers  402 , an absorbing electrochromic layer  403 , and a target layer  404 . In one exemplary embodiment, target layer  404  is a transparent conductive oxide (TCO). On the left side of  FIG. 4 , a laser-ablation scribe line  430  is depicted in which the incident laser light fluence was sufficient to ablate target layer  404 . Because there was transmitted energy through electrochromic device  400 , the incident laser light also had sufficient energy to melt layers  402  and  403  below target layer  404 . The violent thermal vaporization of the target layer  404  and thermal expansion of the underlying layers  402  and  403  cause the melt to push out of the trough of laser-ablation scribe line  430  and form slag (or kerf)  435 . Additionally, there may be cavitations (not shown) in kerf  435  that contributes to kerf  435  further expanding and pushing out of the trough. The cavities may remain after the melt re-solidifies, as shown in  FIG. 5  by cavity  501 .  FIG. 5 , in particular, is an electron micrograph of a cross-sectional view of an exemplary electrochromic device  500  in which the left side of electrochromic device  500  corresponds to the left side of  FIG. 4 . A cavity  501  is shown in kerf  502 . Additionally, the change in density and the intermixing of materials from the various layers of an electrochromic a stack make kerf undesirable. 
     On the right side of  FIG. 4 , a laser-ablation scribe line  440  is depicted in which the incident laser light fluence was a lower fluence than for scribe line  430  because layer  303  sufficiently absorbed the lower laser fluence, thereby ablating target layer  404 , which lies over the top of layer  403 . To ablate layer  404  by itself would require a higher laser fluence because the absorption coefficient for layer  404  is very low at the laser wavelength. In this case, however, absorbing electrochromic layer  403  protects lower layers  402 , as well as transfers heat to target layer  404 . Laser-ablation scribe line  440  requires significantly less energy than laser-ablation scribe line  430 , and produces significantly less kerf, if any kerf is produced at all. 
       FIG. 6  provides a pictorial summary of the subject matter disclosed herein in contrast with previous patterning techniques. In particular,  FIG. 6  depicts a cross-sectional view of an exemplary embodiment of an electrochromic device  600  in which three laser-ablation scribe lines  610 ,  620  and  630  are shown. Each laser-ablation scribe line is formed by a different technique with the intention of scribing the top transparent conductive oxide (TCO) layer (i.e., the target layer). Electrochromic device  600  comprises a substrate  601 , a bottom transparent conductive oxide (TCO)  602 , one or more additional layers  603 , an electrolyte layer  604 , an absorbing layer  605  and a top transparent conductive oxide (TCO)  606 . 
     Laser-ablation scribe line  610  represents a scribe line that is formed using a technique that utilizes a laser having sufficient fluence to ablate completely through all layers of electrochromic device  600 . Such a technique results in failure of electrochromic device  600  because the scribe interrupts the conductivity of both conductors. 
     Laser-ablation scribe line  620  represents a scribe line that is formed when electrochromic device is in a bleached state and the laser light is transmitted to lower layers of electrochromic device  600 . The fluence of the laser light is less than that used for scribe line  610 , but the fact that the laser light is transmitted to layers below target layer  606 , scribe line  620  cuts across electrolyte layer  604 , which is the highest resistance layer. Any conductive melt (kerf) from top transparent conductive layer  606  that spans the high resistance electrolyte layer  604  greatly reduces the resistance of the electrochromic device and, consequently, the efficiency of the electrochromic device. Thus, a new low-resistance short has been established in the effort to isolate resistor  607 . Scribe line  620  corresponds to scribe line  430  in  FIG. 4  and the scribe line shown on the left side of  FIG. 5 . For this case, resistor  607  represents an unpredictable edge resistor for scribe line  620 , and/or a short between the target layer and bottom TCO layer  602 . 
     Laser-ablation scribe line  630  is formed in accordance with the subject matter disclosed herein. The ablation target layer  606  (top transparent conductive oxide (TCO)  606  in exemplary electrochromic device  600 ) is semi-transparent to the wavelength of the laser used. By increasing the absorption of a layer underneath the target layer in the electrochromic stack, the temperature-to-fluence efficiency is increased; and the fluence needed for laser ablation is decreased, thereby protecting underlying layers from damage. There is no rule of thumb; however, the ablation of a material requires a specific minimum temperature. Energy absorbed is based on the transparency and reflectivity of the material to be ablated as well as the incident energy from the laser source. The temperature is dependent on the energy absorbed as defined by the following equation of energy (simplified for the relevant physical system): 
                     ρ   ⁢           ⁢       C   p     ⁡     (       ∂   T       ∂   t       )         =       ∇     (     K   ⁢     ∇   T       )       +     Q   ab               (   1   )               
in which ρ is the density of the film, C p  is the constant pressure heat capacity of the material, K is the thermal conductivity, and Q ab  is a term that represents the energy of absorption. Minimizing the incidence of energy on the underlying layers also produces a higher quality laser scribe line with less eject, kerf, and scribe dimension variation. Resistor  608  represents a predictable edge resistance that is associated with scribe line  630 .
 
     The electrochromic material layer that is closest to the incident laser light is the absorbing layer of the electrochromic device. Laser ablation is carried out when the electrochromic device is in its darkened state. Several different techniques can be used for placing the electrochromic device in its darkened state. In exemplary embodiment, the electrochromic device is placed in the darkened state using an electronic controller that applies appropriate control voltages to the electrochromic device. In another exemplary embodiment, the darkened state is obtained by employing dark-state deposition. That is, the electrochromic device is manufactured in the darkened state and laser ablation is performed without switching the state of the device. For anodic materials the dark state is the default state of deposition. A third exemplary embodiment provides that an electrochromic device is produced that is in its darkened state in its relaxed or equilibrium state. 
     Although the foregoing disclosed subject matter has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced that are within the scope of the appended claims. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the subject matter disclosed herein is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.