Abstract:
A flexible electronic display device is provided comprising a substrate; an imaging layer zone; a transparent superstrate; and a thermal control layer. The device is able to resist thermal deformation caused by the heating generated by the operation of the display.

Description:
FIELD OF THE INVENTION 
     The present invention is in the field of flexible coated electronic devices and, more particularly, relates to a method of eliminating thermally induced deformations in such a structure. 
     BACKGROUND OF THE INVENTION 
     There is substantial and growing interest in the development of flexible electronic circuitry for applications that range from intelligent labels for inventory control, to large format flexible displays. This technology has great potential for many such applications due to the inherent low costs and high throughput of the manufacturing process. 
     From a structural perspective, flexible electronic circuits are essentially a multilayer stack of thin film laminates. These laminates can range in thickness from a few nanometers, to hundreds of microns. When these structures carry an electrical current, joule heating takes place, and there is a potential for deleterious structural deformation due to the mismatch of thermal expansion coefficients from one layer to the next. The prior art has attempted to address the aforementioned drawbacks and disadvantages, but has achieved mixed results. 
     For example, in order to redistribute thermal stress, the use of a spacer layer between the thin film and a more rigid layer of a multilayer flexible electronic device has been devised. Although this technique is applied in U.S. Pat. Nos. 6,281,452B1 and 6,678,949 in order to minimize thermal stress, it is nonetheless characterized by drawbacks. This method is generally less than ideal, since it adds unnecessary thickness to a device that is required to be sufficiently thin. Additionally, such thickness restrictions hinder the possibility of employing additional layers that may be needed to minimize thermal stress. 
     U.S. Pat. No. 5,319,479 discloses a multilayer device, comprised of an electronic element, a plastic substrate, and a thin film, wherein the thermal deformation of the thin film is minimized by plastic substrate and the electronic element. This method has a distinct disadvantage in that it does not provide flexibility in adjusting the coefficient of thermal expansion and the thickness of the respective layers. 
     PROBLEM TO BE SOLVED BY THE INVENTION 
     The invention addresses the continuing need for a method to prevent deformation due to thermal heating effects in flexible electronic structures 
     SUMMARY OF THE INVENTION 
     In answer to the aforementioned and other problems of the prior art the invention provides a display device comprising: a substrate; an imaging layer zone; a transparent superstrate; and a thermal control layer. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention provides a comprehensive method of eliminating thermally induced deformation in flexible electronic structures. The method is applicable to multi-layer electronic structures constructed from a variety of flexible materials. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the detailed description of the preferred embodiments of the invention presented below, reference is made to the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a generalized laminated multi-layer electronic structure; 
         FIG. 2  illustrates a cross-sectional diagram of a simple multi-layer flexible electronic structure; 
         FIG. 3  is a graph showing the thermal deformation predicted for the multi-layer structure of  FIG. 2 ; 
         FIG. 4  illustrates a cross-sectional diagram of a multi-layer flexible electronic structure made in accordance with an embodiment of the present invention; 
         FIG. 5  is a graph showing the thermal deformation predicted for the electronic structure of  FIG. 4 ; 
         FIG. 6  illustrates a cross-sectional diagram of a multi-layer flexible electronic structure made in accordance with an embodiment of the present invention; 
         FIG. 7  is a graph showing the thermal deformation predicted for the electronic structure of  FIG. 6 ; 
         FIG. 8  illustrates a cross-sectional diagram of a multi-layer flexible electronic structure made in accordance with yet an embodiment of the present invention; 
         FIG. 9  is a graph showing the thermal deformation predicted for the electronic structure of  FIG. 8 ; and 
         FIG. 10  is a graph showing the effect of varying the coefficient of thermal expansion of the control layer in the electronic structure of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the process of utilizing flexible coated electronic devices, deformations are observed when current is applied to these devices, due to differential thermal expansion of the coated layers. The invention teaches a comprehensive approach to solve this problem by the use of a thermal control layer. 
     A general theoretical model for predicting the thermal deformation in an N-layer laminated structure, expressed in terms of the thicknesses and material properties of the layers in the laminate is provided below. 
     The thermoelastic deformation of an N-layer laminated structure shown in  FIG. 1  is governed by the following thermoelastic equation: 
                       (       BD   -     C   2       B     )     ⁢       ∇   4     ⁢     w   ⁡     (     x   ,   y     )           =       ∇   2     ⁢     [         C   B     ⁢       N   T     ⁡     (     x   ,   y     )         -       M   T     ⁡     (     x   ,   y     )         ]               (   1   )               
where w(x,y) is the vertical deflection at point (x,y) of the structure,
 
                         ∇   4     ⁢   w     =           ∂   4     ⁢   w       ∂     x   4         +     2   ⁢         ∂   4     ⁢   w         ∂     y   2       ⁢     ∂     x   2             +         ∂   4     ⁢   w       ∂     y   4             ,         ∇   2     ⁢   w     =           ∂   2     ⁢   w       ∂     x   2         +         ∂   2     ⁢   w       ∂     y   2             ,     
     ⁢   and           (   2   )                   N   T     ⁡     (     x   ,   y     )       =       ∑     k   =   1     N     ⁢           E   k     ⁢     α   k         1   -     v   k         ⁢       ∫     h     k   -   1         h   k       ⁢         T   k     ⁡     (     x   ,   y     )       ⁢           ⁢     ⅆ   z                     (   3   )                     M   T     ⁡     (     x   ,   y     )       =       ∑     k   =   1     N     ⁢           E   k     ⁢     α   k         1   -     v   k         ⁢       ∫     h     k   -   1         h   k       ⁢         T   k     ⁡     (     x   ,   y     )       ⁢   z   ⁢           ⁢     ⅆ   z               ⁢     
     ⁢   where           (   4   )                 B   =       ∑     k   =   1     N     ⁢         E   k       1   -     v   k   2         ⁢     (         h   k     -     h     k   -   1         2     )           ,     C   =       ∑     k   =   1     N     ⁢         E   k       1   -     v   k   2         ⁢     (         h   k   2     -     h     k   -   1     2       2     )           ,     
     ⁢   and           (   5   )               D   =       ∑     k   =   1     N     ⁢         E   k       1   -     v   k   2         ⁢       (         h   k   2     -     h     k   -   1     2       2     )     .                 (   6   )               
In these expressions E i , ν i , and α i  are the Young&#39;s Modulus, Poisson&#39;s Ratio, and coefficient of thermal expansion of the i&#39;th layer of the laminated structure, h i  is the distance from the surface of the top layer (z=0) to the bottom of the i&#39;th layer, and T i (x,y) is the temperature rise at point (x,y) in the i&#39;th layer ( FIG. 1 ).
 
It follows from Eq. (1) that the source of the thermal deformation is the
 
                 C   B     ⁢       N   T     ⁡     (     x   ,   y     )         -       M   T     ⁡     (     x   ,   y     )             
term on the right hand side of the equation. One can eliminate any deformation by adjusting E i , ν i , h i  and α i  so that
 
     
       
         
           
             
               
                 
                   
                     
                       
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     Use of the application of this general model for predicting thermal deformation in specific flexible electronic multilayer laminates will now be illustrated. 
     Referring now to  FIG. 2 , a simple multi-layer flexible electronic structure  10  is shown. Structure  10  comprises a glass superstrate layer  20 , imaging layer zone  30 , and substrate layer  40  comprised of polyethylene terphthalate (PET). Imaging layer zone  30  typically comprises a plurality of very thin layers (not shown) and it is in this imaging zone  30  where current passes and where joule heating takes place. However, because the imaging zone  30  is much thinner and much more flexible than either superstrate  20  or substrate  4 , the contribution of imaging zone  30  to thermal deformation may be ignored, and structure  10  may be treated essentially as a bilayer with respect to the predictive model of equations 1-7. 
     According to the aforementioned equations 1-7, the peak thermal deformation of the device  10  can be modeled with respect to the impact of the various layer thicknesses and their material and thermal properties. Continuing with the example of  FIG. 2 , glass layer  20  has a thickness of 60 μm, a thermal coefficient of expansion of 7×10 −6 /C and a Young&#39;s Modulus of 50 GPa. The PET layer  40  has a thermal coefficient of expansion of 80×10 −6 /C, a Young&#39;s Modulus of 4 GPa, and a thickness of 1000 μm. As previously mentioned, imaging layer zone  30  is much thinner (typically less than 1 μm) and much more flexible than either layers  20  or  40  and may be ignored.  FIG. 3  shows the thermal deformation expected for structure  10  when the thermal deformation model described by equations 1-7 is applied. Referring to  FIG. 3 , it can be seen that when a structure such as  10  with layer properties and dimensions as described above experiences uniform heating in the imaging zone  30  sufficient to raise the temperature of the device 20 degs C., deformation will be imminent. Furthermore, a structure such as that depicted in  FIG. 2 , which has essentially only two layers which impact thermal deformation, will experience deformation regardless of thickness variations in the layers. In the results of the model calculations shown in  FIG. 3 , the thickness of glass layer  20  was continuously varied from 100 to 1000 μm without the value of the peak thermal deformation reaching a value of zero. One way to eliminate thermal deformation in such a device is to introduce an additional thermal balancing layer with appropriate thickness and physical and thermal expansion properties into the structure. 
     One example of a display structure  50  with such a thermal balancing layer is illustrated in  FIG. 4 . The layers shown in structure  50  comprise aluminum substrate layer  90 , imaging layer zone  80 , and transparent superstrate glass layer  70 . Transparent superstrate glass layer  70  may also serve as an oxygen and moisture diffusion barrier in certain types of display devices. A thermal control layer  60  comprised of PET polymer has been incorporated on top of layer  70 . 
     In the example modeled, for a display of overall dimensions 1 meter×0.5 meter the aluminum substrate layer  90  has a thickness of 500 μm, a thermal coefficient of expansion of 23×10 −6 /C and a Young&#39;s Modulus of 70 GPa. The glass layer  70  has a thickness of 60 μm, a thermal coefficient of expansion of 7×10 −6 /C, and a Young&#39;s Modulus of 50 GPa. The polymer superstrate PET layer  60  has a thermal coefficient of expansion of 70×10 −6 /C and a Young&#39;s Modulus of 4 GPa. As in the previous example of  FIG. 2 , imaging zone  80  is comprised of layers which are much thinner and more flexible than the aforementioned layers and contain light-emissive or light modulating components and/or electronic control layers. Again, as in the previous example, the layers in the imaging zone  80  do not control thermal deformation in the overall device, but instead it is the behavior of the layers  60 ,  70 ,  90  illustrated which, when heated, control this deformation. For the purposes of this example, it is again assumed that the device experiences an overall temperature increase of 20 deg C. at equilibrium when operated. 
     For this example, the effect of varying the thickness of the PET polymer layer  60  was modeled in accord with equations 1-7.  FIG. 5  shows the results of the model calculations and it can be seen that a laminated structure  50  with a PET layer  60  of approximately 145 μm thickness will experience zero peak deformation. 
     Alternative materials would also be expected to be useful in a display structure such as structure  50 . In addition to aluminum, substrate layer  90  may also comprise aluminum alloy, anodized aluminum, stainless steel, titanium, molybdenum, or copper. In addition to glass, transparent superstrate layer  70  may also comprise crystalline inorganic oxides such as quartz, polyolefin, polymer-inorganic composites or barrier layers such as Vitex™. Alternatively, transparent superstrate layer  70  may also contain particulate or other optical scattering materials such as titanium dioxide to improve light yield from the display device. Balancing layer  60  may comprise any of a number of film-forming polymeric materials with appropriate properties including polycarbonate or polyolefin materials and their derivatives. 
     Imaging layers contained in imaging layer zone  80  which would be useful in flexible electronic display devices include, for example, the layers required for an organic light emitting diode (OLED) display device. Typically the imaging layers in an OLED display comprise (in order) a transparent cathode, an electron injection layer, an emitter layer, a hole injection layer, and a transparent anode. Electrical current is applied to the anode and cathode. The current flows in the form of holes from the anode and electrons in the cathode. The holes and electrons subsequently meet and recombine in the emitter layer causing the emission of photons (light). For a typical passive matrix OLED display the anode and cathode layers may also be patterned in orthogonal arrays to form pixels, while in active-matrix displays each pixel is controlled independently, for example, with thin film transistors (TFTs). 
     Other examples of layers useful expected to be useful in imaging layer zone  80  include the layers that would be associated with a flexible liquid crystal display (LCD). Typically, the imaging layers in an LCD display comprise (in order) a light polarizing layer, a transparent anode, a liquid crystal layer, a transparent cathode, and another light polarizing layer. When electrical current is applied to the electrodes, the liquid crystal layer changes state and prevents light from passing out of the display. As described for the OLED display above, the anode and cathode will typically be patterned to form pixels and both active and passive matrix architectures may be used. 
     A flexible cholesteric LCD display would have layers very similar to the layers required for an LCD display described above, with the exception that the liquid crystal layer comprises a liquid crystal material in the cholesteric phase. This cholesteric liquid crystal layer changes state when current is applied to the electrodes, but remains in the changed state when the current is turned off. 
     Additionally, the model of equations 1-7 can be used to predict the thermal deformation behavior of other display structures and to design improved structures where thermal deformation is reduced or eliminated. Another example of a multilayer, flexible electronic structure is shown in  FIG. 6 . The device  100  comprises a polymer thermal control layer  110 , glass layers  120  and  140 , imaging layer zone  130 , and PET substrate layer  150 . Again, as in the previous examples given, because of thickness and flexibility considerations, the imaging layer zone  130  does not contribute significantly to thermal deformation, and the device  100  may be treated essentially as a trilayer. As before, the thermal properties and thickness of the polymer thermal control layer  110  can selected to minimize thermal deformation.  FIG. 7  shows the results of modeling, according to equations 1-7, the thermal deformation behavior of device  100  where the overall dimensions of the display are 1 meter×0.5 meters with a polymer substrate  150  of TCE=80×10 −6 /C and E=4 GPa and glass midsections  120  and  140  of TCE=7×10 −6 /C and E=2.4 GPa and polymer thermal control layer  110  of TCE=200×10 −6 /C and E=4 GPa with a temperature change of 20 deg C. . It can be seen from the data in  FIG. 7 , that a polymer thermal control layer  110  of approximately 430 μm and a hypothetical thermal coefficient of expansion (TCE) of 200×10 −6 /C will control the thermal deformation of the device to essentially zero. 
     In  FIG. 8 , still another example of a multilayer, flexible electronic structure is provided. The device  160  of dimensions 1 meter×0.5 meter subjected to a temperature change of 20 deg C., is comprised of a polymer PET thermal control layer  170  with TCE=80×10 −6 /C and E=4 GPa, glass layers  180  and  200  with TCE=7×10 6 /C and E=2.4 GPa, imaging layer zone  190 , and PET substrate layer  210  with TCE=80×10 −6 /C and E=4 GPa.  FIG. 9  shows the results of modeling the thermal deformation behavior of device  160  under the temperature, layer material type, and layer property conditions as described above. Structure  160  experiences zero peak deformation when the PET thermal control layer  170  has a thickness of approximately 1000 μm as can be seen in  FIG. 9 . Thus, employing polymer superstrate  210  with a lower TCE of 80×10 −6 /C requires a greater superstrate thickness of 1000 μm, as seen in  FIG. 9 , than with employing polymer superstrate  110  with a higher TCE of 200×10 −6 /C which requires a lower thickness of 430 μm, as seen in  FIG. 7 . 
     The aforementioned theoretical model of equations 1-7 can also be used to predict the peak thermal deformation of a flexible display device based on an assumed or required thermal control layer thickness. This allows the thermal coefficient of expansion to be computed, which can, in turn, be used to determine a different polymer material. The results for modeling the thermal deformation in a device  160  where the polymer thermal control layer  170  is constrained to be 300 μm are shown in  FIG. 10  (same layer dimensions and properties as described for  FIG. 9  except that the TCE for the thermal control layer  170  is allowed to change Under this constraint, the results in  FIG. 10  show that thermal deformation is minimized when a control layer material of TCE=285×10 −6 /C is selected. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.