Patent Publication Number: US-10777768-B2

Title: Organic light emitting diode assembly, light source, and method of fabricating organic light emitting diode assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a national stage application under 35 U.S.C. § 371 of International Application No. PCT/CN2017/112550, filed Nov. 23, 2017, the contents of which are incorporated by reference in the entirety. 
     TECHNICAL FIELD 
     The present invention relates to display technology, more particularly, to an organic light emitting diode assembly, a light source, and a method of fabricating an organic light emitting diode assembly. 
     BACKGROUND 
     An organic light emitting diode (OLED) typically includes an anode, an organic layer including a light emitting layer, and a cathode. Optionally, the organic light emitting diode may further includes one or a combination of a hole transport layer, a hole injection layer, an electron transport layer, and an electron injection layer. In an organic light emitting diode, an electric field is applied between the anode and the cathode to inject electrons from the cathode into the light emitting layer and holes from the anode into the light-emitting layer. The electrons and the holes then recombine together in the light emitting layer to generate excitons. When the excitons return to the ground state, their energy is released in the form of light. 
     SUMMARY 
     In one aspect, the present invention provides an organic light emitting diode assembly comprising a first base substrate; an organic light emitting diode on the first base substrate; and a second base substrate on a side of the organic light emitting diode distal to the first base substrate; wherein the organic light emitting diode and the second base substrate are spaced apart from each other by a tunable gap; and a gap distance of the tunable gap is tunable such that a color of light emitted from the organic light emitting diode assembly is tunable in response to a change in the gap distance. 
     Optionally, the organic light emitting diode assembly comprises a microcavity; wherein a light path in the microcavity extends through the tunable gap; and the gap distance of the tunable gap is tunable such that a total light path distance of the microcavity is tunable in response to the change in the gap distance. 
     Optionally, a change in the gap distance is substantially linearly correlated with a change in a maximum-gain wavelength of light emitted from the organic light emitting diode assembly. 
     Optionally, the organic light emitting diode assembly further comprises an elastic spacer layer spacing apart the organic light emitting diode and the second base substrate by the tunable gap; wherein the elastic spacer layer is capable of undergoing deformation in response to an external force thereby actuating the gap distance. 
     Optionally, the organic light emitting diode assembly further comprises a rigid spacer layer on the first base substrate; wherein a height of the rigid spacer layer relative to the first base substrate is less than a height of the elastic spacer layer relative to the first base substrate when the gap distance is greater than zero. 
     Optionally, the organic light emitting diode comprises a first electrode on the first base substrate; an organic layer on a side of the first electrode distal to the first base substrate; and a second electrode on a side of the organic layer distal to the first electrode; wherein the organic layer comprises an organic light emitting layer. 
     Optionally, the second electrode is a substantially transparent electrode; and the second base substrate is a reflective substrate or a transflective substrate. 
     Optionally, the first electrode is a reflective electrode or a transflective electrode. 
     Optionally, the first base substrate is a reflective substrate or a transflective substrate. 
     In another aspect, the present invention provides a light source comprising the organic light emitting diode assembly described herein or fabricated by a method described herein. 
     Optionally, the light source further comprises an actuator configured to control the gap distance of the tunable gap. 
     Optionally, the actuator comprises a rigid support plate and a resilient support pad; the resilient support pad is configured to apply a compressive force on the organic light emitting diode assembly; and the rigid support plate is configured to apply a pulling force on the resilient support pad. 
     Optionally, the light source further comprises a transmission line operably coupled to the rigid support plate for providing the pulling force. 
     Optionally, the light source further comprises a change-speed gear box operably coupled to the transmission line and configured to change a transmission speed of the transmission line; and the change-speed gear box comprises a plurality of change-speed gears. 
     Optionally, the light source further comprises a driving pulley, around which the transmission line is wound for multiple turns; and the transmission line is operably connected to the plurality of change-speed gears in the change-speed gear box through one or more fixed pulleys. 
     Optionally, the light source further comprises a controller knob operably coupled to the driving pulley configured to control a magnitude of the pulling force applied on the resilient support pad. 
     Optionally, the actuator comprises a support plate and an electroactive layer sandwiched between the support plate and the organic light emitting diode assembly; and the electroactive layer is configured to exert a force to the elastic spacer layer in response to an electrical signal, thereby changing the gap distance of the tunable gap. 
     Optionally, the light source further comprises a controller knob operably coupled to the electroactive layer configured to controlling a magnitude of the electrical signal applied on the electroactive layer. 
     Optionally, the light source further comprises a base, a controller knob on the base, a lamp head for receiving the organic light emitting diode assembly, and one or more struts connecting the base to the lamp head. 
     In another aspect, the present invention provides a method of fabricating an organic light emitting diode assembly, comprising forming an organic light emitting diode on a first base substrate; and forming a second base substrate on a side of the organic light emitting diode distal to the first base substrate; wherein the organic light emitting diode and the second base substrate are formed to be spaced apart from each other by a tunable gap; and a gap distance of the tunable gap is formed to be tunable such that a color of light emitted from the organic light emitting diode assembly is tunable in response to a change in the gap distance. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The following drawings are merely examples for illustrative purposes according to various disclosed embodiments and are not intended to limit the scope of the present invention. 
         FIG. 1  is a diagram illustrating the structure of an organic light emitting diode assembly in some embodiments according to the present disclosure. 
         FIG. 2  a perspective view of an organic light emitting diode assembly in some embodiments according to the present disclosure. 
         FIG. 3  is a diagram illustrating the structure of an organic layer in some embodiments according to the present disclosure. 
         FIG. 4  is a diagram illustrating the working principle of an organic light emitting diode assembly in some embodiments according to the present disclosure. 
         FIG. 5  illustrates the correlation between a gap distance L of the tunable gap and a wavelength of light emitted from the organic light emitting diode assembly. 
         FIG. 6  illustrates the correlation between a change ΔL in a gap distance and a maximum-gain wavelength. 
         FIG. 7  is a side view of a light source in some embodiments according to the present disclosure. 
         FIG. 8  is a cross-sectional view of a light source in some embodiments according to the present disclosure. 
         FIG. 9  is a perspective view of a light source in some embodiments according to the present disclosure. 
         FIG. 10  is a diagram illustrating an actuator for controlling the gap distance of a tunable gap in an organic light emitting diode assembly in some embodiments according to the present disclosure. 
         FIG. 11  is a diagram illustrating an actuator for controlling the gap distance of a (unable gap in an organic light emitting diode assembly in some embodiments according to the present disclosure. 
         FIG. 12  is a flow chart illustrating a method of fabricating an organic light emitting diode assembly in some embodiments according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of some embodiments are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
     Conventional organic light emitting diode light sources typically have fixed specifications, for example, they are typically made for emitting a designated color of light. The color of light in the conventional organic light emitting diode light sources is typically not tunable. Personalized preferences of individual users are difficult to be satisfied using conventional organic light emitting diode light sources, either decorative light sources or light sources for illumination purpose. Color tunable light sources may be made, e.g., using a stacked organic light emitting diode in which both light intensity and color could be independently varied and controlled with external power supplies. The stacked organic light emitting diode light sources, however, require complicated circuits to control the color of emitted light, leading to high manufacturing costs and a complicated fabricating process. 
     Accordingly, the present disclosure provides, inter alia, an organic light emitting diode assembly, a light source, and a method of fabricating an organic light emitting diode assembly that substantially obviate one or more of the problems due to limitations and disadvantages of the related art. In one aspect, the present disclosure provides an organic light emitting diode assembly. In some embodiments, the organic light emitting diode assembly includes a first base substrate; an organic light emitting diode on the first base substrate; and a second base substrate on a side of the organic light emitting diode distal to the first base substrate. Optionally, the organic light emitting diode and the second base substrate are spaced apart from each other by a tunable gap. Optionally, a gap distance of the tunable gap is tunable such that a color of light emitted from the organic light emitting diode assembly changes in response to a change in the gap distance. The present disclosure provides a simple yet robust color tunable light source that obviates high manufacturing costs and the complicated fabricating process associated with the stacked organic light emitting diode light source. 
       FIG. 1  is a diagram illustrating the structure of an organic light emitting diode assembly in some embodiments according to the present disclosure.  FIG. 2  a perspective view of an organic light emitting diode assembly in some embodiments according to the present disclosure. Referring to  FIG. 1  and  FIG. 2 , the organic light emitting diode assembly  100  in some embodiments includes a first base substrate  60 ; an organic light emitting diode  90  on the first base substrate  60 ; and a second base substrate  70  on a side of the organic light emitting diode  90  distal to the first base substrate  60 . In the present organic light emitting diode assembly  100 , the organic light emitting diode  90  and the second base substrate  70  are spaced apart from each other by a tunable gap G. A gap distance of the tunable gap G can be actuated such that a color of light emitted from die organic light emitting diode assembly  100  changes in response to a change in the gap distance. Optionally, a change in the gap distance has a substantially linear correlation with a change in a maximum-gain wavelength of light emitted from the organic light emitting diode assembly  100 . By changing the gap distance, a full color tunable organic light emitting diode can be achieved to display a full spectrum of visible light. 
     The tunable gap G may be filled with any appropriate medium as long as it allows the gap distance to be tunable. Optionally, the tunable gap G is a vacuum. Optionally, the tunable gap G is filled with a gas such as an insert gas. Examples of insert gases include nitrogen, helium, argon, and the like. Optionally, the tunable gap G is filled with a transparent insulating liquid such as a transparent insulating dielectric liquid. Examples of insulating liquids include mineral oils, silicone oils, and hydrocarbons. Optionally, the tunable gap G is filled with an elastic, transparent, insulating solid such as a transparent lubricant. 
     Various appropriate mechanisms may be used for actuating the gap distance. The actuator may be mechanical or electrical. In some embodiments, the organic light emitting diode assembly  100  further includes an elastic spacer layer  40  spacing apart the organic light emitting diode  90  and the second base substrate  70  by the tunable gap G. The elastic spacer layer  40  is capable of undergoing deformation in response to an external force thereby actuating the gap distance G. As shown in  FIG. 1 , the elastic spacer layer  40  has a first surface  40   a  connected (directly or indirectly) to the first base substrate  60 , a second surface  40   b  connected (directly or indirectly) to the second base substrate  70 , and a side wall  40   c  connecting the first surface  40   a  and the second surface  40   b . Optionally, the first surface  40   a  is in contact with the first base substrate  60 . Optionally, the second surface  40   b  is in contact with the second base substrate  70 . The side wall  40   c  has a height h 1  defined by a distance between the first surface  40   a  and the second surface  40   b . The height h 1  changes in response to a deformation occurring in the elastic spacer layer  40 , thereby changing the gap distance of the tunable gap G. 
     In some embodiments, the height h 1  of the elastic spacer layer  40  may be actuated mechanically. In one example, the elastic spacer layer  40  is connected to a spring which, when controlled, may (directly or indirectly) exert a pressing force on one or a combination of the first surface  40   a  and the second surface  40   b . Optionally, the pressing force is applied on the second base substrate and is transmitted to the elastic spacer layer  40 . 
     In some embodiments, the height h 1  of the elastic spacer layer  40  may be actuated electrically. In one example, the organic light emitting diode assembly further includes an electroactive layer on a side of the first surface  40   a , or on a side of the second surface  40   b , or on both sides. In response to an electrical signal applied to the electroactive layer, the electroactive layer is configured to exert a pressing force (directly or indirectly) on one or a combination of the first surface  40   a  and the second surface  40   b.    
     Optionally, the height h 1  of the elastic spacer layer  40  is substantially the same as a sum of the gap distance and a thickness of the organic light emitting diode  90 . 
     Optionally, the elastic spacer layer  40  is made of a substantially transparent material. As used herein, the term “substantially transparent” means at least 70 percent (e.g., at least 75 percent, at least 80 percent, at least 85 percent, at least 90 percent, at least 95 percent and 100 percent) of light in the visible wavelength range transmitted therethrough. 
     Various appropriate elastic materials may be used for making the elastic spacer layer  40 . As used herein, the term “elastic” refers to a material that, upon application of a biasing force, can stretch to an elongated length of at least about 110% of its relaxed, original length (i.e. can stretch to 10 percent more than its original length), without rupture or breakage, and upon release of the applied force, recovers at least about 40% of its elongation. For example, a material that has an initial length of 100 mm can extend at least to 110 mm, and upon removal of the force would retract to a length of 106 mm (40% recovery). Examples of appropriate elastic materials for making the elastic spacer layer  40  includes elastomers (i.e., viscoelastic polymers) such as polyimides, polysilicones, polysiloxanes, rubbers, urethane polyimides, polyepoxides, and so on. Optionally, an elastic material is a material having a Young&#39;s modulus value less than 500 MPa, e.g., less than 400 MPa, less than 300 MPa, less than 200 MPa, less than 100 MPa, and less than 50 MPa. Optionally, an elastic material is a material having a Young&#39;s modulus value less than 20 MPa, e.g., less than 15 MPa, less than 10 MPa, less than 5 MPa, less than 2 MPa, and less than 1 MPa. 
     In some embodiments, the organic light emitting diode assembly  100  further includes a rigid spacer layer  50  on the first base substrate  60  for protecting the organic light emitting diode  90 . As shown in  FIG. 1 , the rigid spacer layer  50  has a first surface  50   a  connected (directly or indirectly) to the first base substrate  60 , a second surface  50   b  spaced apart from the second base substrate  70 , and a side wall  50   c  connecting the first surface  50   a  and the second surface  50   b . The second surface  50   b  is on a side of the first surface  50   a  distal to the first base substrate  60 . Optionally, the first surface  50   a  is in contact with the first base substrate  60 . The side wall  50   c  has a height h 2  defined by a distance between the first surface  50   a  and the second surface  50   b . The height h 2  of the rigid spacer layer  50  relative to the first base substrate  60  is less than the height h 1  of the elastic spacer layer  40  relative to the first base substrate  60  when the gap distance of the tunable gap G is greater than zero. 
     To protect the organic light emitting diode  90 , the height h 2  of the rigid spacer layer  50  relative to the first base substrate  60  is substantially equal to, or greater than, a height of the organic light emitting diode  90  relative to the first base substrate  60 . When the gap distance of the tunable gap G is actuated to zero, the rigid spacer layer  50  is in position to prevent the organic light emitting diode  90  from being pressed by the second base substrate  70 . 
     Various appropriate rigid materials may be used for making the rigid spacer layer  50 . As used herein, the term “rigid” refers to a material that does not flex or elastically deform readily upon application of a biasing force. Optionally, a rigid material is a material having a Young&#39;s modulus value greater than 500 MPa at room temperature. Optionally, the rigid spacer layer  50  has a Young&#39;s modules of at least 500 MPa, at least 1000 MPa, at least 5000 MPa, at least 10000 MPa, at least 100000 MPa, or at least 200000 MPa. Examples of appropriate rigid materials include polycarbonate, acrylic, polypropylene, and polystyrene. 
     Referring to  FIG. 1 , the organic light emitting diode assembly  100  in some embodiments includes a first electrode  10  on the first base substrate  60 ; an organic layer  20  on a side of the first electrode  10  distal to the first base substrate  60 ; and a second electrode  30  on a side of the organic layer  20  distal to the first electrode  10 . The organic light emitting diode assembly  100  in some embodiments further includes a plurality of conductive wires  80 . A cathode and an anode of the organic light emitting diode  90  are respectively connected to the plurality of conductive wires  80 . 
       FIG. 3  is a diagram illustrating the structure of an organic layer in some embodiments according to the present disclosure. Referring to  FIG. 3 , the organic layer  20  in some embodiments includes an organic light emitting layer  21 . Optionally, the organic layer  20  further includes other functional layers such as a hole transport layer, a hole injection layer, an electron transport layer, and an electron injection layer. Referring to  FIG. 1  and  FIG. 3 , the organic layer  20  in some embodiments includes a hole injection layer  24  on the first electrode  10 , a hole transport layer  22  on a side of the hole injection layer  24  distal to the first electrode  10 , the organic light emitting layer  21  on a side of the hole transport layer  22  distal to the hole injection layer  24 , an electron transport layer  23  on a side of the organic light emitting layer  21  distal to the hole transport layer  22 , and an electron injection layer  25  on a side of the electron transport layer  23  distal to the organic light emitting layer  21 . 
     Optionally, the first electrode  10  is an anode, and the second electrode  30  is a cathode. Optionally, the first electrode  10  is a cathode, and the second electrode  30  is an anode. 
     In some embodiments, a microcavity is formed between two reflective/transflective layers of the organic light emitting diode assembly. Optionally, the second electrode  30  is substantially transparent. Optionally, the first electrode  10  is a reflective electrode or a transflective electrode. Optionally, the second base substrate  70  is a reflective substrate or a transflective substrate. 
     As used herein, the term “reflective” refers to an object having a reflectivity of approximately 70% to approximately 100% relative to an incident light, the term “transflective” refers to an object having a reflectivity of approximately 30% to approximately 70% with respect to an incident light, and the term “substantially transparent” refers to an object having a reflectivity of approximately 0% to approximately 30% with respect to an incident light. 
     In one example, the first electrode  10  is a reflective electrode, the second electrode  30  is a substantially transparent electrode, and the second base substrate  70  is a transflective substrate. The microcavity is formed between the first electrode  10  which is reflective and the second base substrate  70  which is transflective. 
     In another example, the first electrode  10  is a transflective electrode, the second electrode  30  is a substantially transparent electrode, and the second base substrate  70  is a transflective substrate. Optionally, the first base substrate  60  is a substantially transparent substrate. The microcavity is formed between the first electrode  10  and the second base substrate  70 , both of which are transflective. 
     In another example, the first base substrate  60  is a reflective substrate, the first electrode  10  is a substantially transparent electrode, the second electrode  30  is a substantially transparent electrode, and the second base substrate  70  is a transflective substrate. The microcavity is formed between the first base substrate  60  which is reflective and the second base substrate  70  which is transflective. 
     In another example, the first base substrate  60  is a transflective substrate, the first electrode  10  is a substantially transparent electrode, the second electrode  30  is a substantially transparent electrode, and the second base substrate  70  is a transflective substrate. The microcavity is formed between the first base substrate  60  and the second base substrate  70 , both of which are transflective. 
     In another example, the first base substrate  60  is a transflective substrate, the first electrode  10  is a substantially transparent electrode, the second electrode  30  is a substantially transparent electrode, and the second base substrate  70  is a reflective substrate. The microcavity is formed between the first base substrate  60  which is transflective and the second base substrate  70  which is reflective. 
     Referring to  FIG. 1 , the organic light emitting diode assembly  100  in the example includes a microcavity M formed between a first electrode  10  which is reflective and a second base substrate  70  which is transflective. A light path in the microcavity M extends through the tunable gap G. The gap distance of the tunable gap G is tunable such that a total light path distance of the microcavity M is tunable in response to the change in the gap distance. 
       FIG. 4  is a diagram illustrating the working principle of an organic light emitting diode assembly in some embodiments according to the present disclosure. Referring to  FIG. 4 , the tunable gap G between the organic light emitting diode  90  and the second base substrate  70  has a gap distance L. The organic light emitting diode  90  has a thickness d. The point O is a point in the organic light emitting layer of the organic light emitting diode  90 , from which light emits, e.g., toward the first base substrate  60  (e.g., along O-B direction) or the second base substrate  70  (e.g., along O-A direction). A normal distance between the point O and the upper surface of the first base substrate  60  is indicated as “x”. 
     In the example depicted in  FIG. 4 , a microcavity is formed between the first base substrate  60  and the second base substrate  70 . Light emitted out of the organic light emitting diode assembly  100  is a light modulated by the microcavity effect. For example, light emitted from the point O includes light Ao 1  and light Bo 1  having a mirror symmetry with respect to the horizontal line. Light Ao 1  travels along O-A, reaches a point A on the upper surface of the organic light emitting diode  90 , and enters into the tunable gap G between the organic light: emitting diode  90  and the second base substrate  70 . At the interface between the organic light emitting diode  90  and the tunable gap G, light Ao 1  has an incent angle of θ, and an exit angle of θ′. Light Ao 1  then travels along the A-C line, reaches a point C on the second base substrate  70 , and emits out of the organic light emitting diode assembly  100  as light A 1 . Optionally, the tunable gap G is filled with nitrogen, which has a refractive index very similar to that of the air. Accordingly, at the interface between the second base substrate  70  and the air (e.g., at point C), light Ao 1  has an incident angle of θ′, and an exit angle of θ′. 
     Optionally, the second base substrate  70  is a transflective substrate. Accordingly, a portion of light Ao 1  is reflected by the second base substrate  70  at the interface between the second base substrate  70  and the air (e.g., at point C). The reflected light Ao 2  travels through the tunable gap G and the organic light emitting diode  90  along C-H-G line, and reaches an upper surface of the first base substrate  60 . Optionally, the first base substrate  60  is a reflective substrate, and light Ao 2  is reflected by the first base substrate  60 , travels along the G-F-E line. At the interface between the second base substrate  70  and the air, a portion of light Ao 2  emits out of the organic light emitting diode assembly as light A 2 , and a portion of light Ao 2  is again reflected by the transflective first base substrate  60 . The process is reiterated multiple times, thereby generating a series of emitted light A 1 , A 2 , A 3 , . . . , A n . 
     Similarly, light Bo 1  emitted from the point O undergoes the similar process in the organic light emitting diode assembly, and generates a series of emitted light B 1 , B 2 , B 3 , . . . , B n . An observer at a viewing angle θ′ observes a combined light of A 1 , A 2 , A 3 , . . . , A n , and B 1 , B 2 , B 3 , . . . , B n . 
     The microcavity effect of the organic light emitting diode assembly  100  depends on the gap distance L. A light path difference Δ between the emitted light A 1  and the emitted light A 2 , can be calculated based on the equation (1):
 
Δ=2 nd  cos θ+2 n′L  cos θ′  (1);
 
     wherein n stands for an average refractive index of the organic light emitting diode  90 ; n′ stands for a refractive index of the tunable gap G; d stands for a thickness of the organic light emitting diode  90 ; and L stands for a gap distance of the tunable gap G. 
     A phase difference Δφ between the emitted light A 1  and the emitted light A 2 , can be calculated based on the equation (2): 
     
       
         
           
             
               
                 
                   
                     
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     wherein λ stands for a wavelength of the incident light, φ stands for a phase shift due to light reflection at the first base substrate  60  and the second base substrate  70 . 
     The wave function of the light A 1  can be expressed using the equation (3):
 
Ψ( o,t ) A1   =t   US   Ae   −i(ωt−ko+ψ     0     )   =t   US Ψ( o,t ) 0   (3);
 
     wherein ω stands for an angular frequency of the emitted light A 1 ; (−i(ψ 0 −ko)) stands for a spatial phase factor; −iψt stands for a temporal phase factor; t US  stands for light transmittance of the second base substrate  70 , and Ψ(o, t) 0  stands for a wave function of light Ao 1 . 
     Base on the equation (3), the wave function of the light A 2  can be expressed using the equation (4):
 
Ψ( o,t ) A2   =t   US   r   US   r   BS   Ae   −i(ωt−ko+ψ     0     +Δφ)   =t   US Ψ( o,t ) 0   r   US   r   BS   e   −iΔφ   (4);
 
     wherein r US  stands for light reflectance of the second base substrate  70 ; and r bs  stands for light reflectance of the first base substrate  60 . 
     Base on the equations (3) and (4), a sum of the wave functions of the light A 1 , A 2 , A 3 , . . . , A n  can be expressed using the equation (4): 
     
       
         
           
             
               
                 
                   
                     
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     A light path difference Δ′ between the emitted light A 1  and the emitted light B 1 , i.e., a light path different between the emitted light A 1  at point J and the emitted light B 1  at point I, can be calculated based on the equation (6):
 
Δ′=2 nx  cos θ  (6);
 
     wherein x stands for a normal distance between the point O (light emitting point) and the first base substrate  60 . 
     Based on the equation (6), the phase difference between light A 1  and light B 1  can be expressed using the equation (7): 
     
       
         
           
             
               
                 
                   
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         φ 
                         ′ 
                       
                     
                     = 
                     
                       
                         
                           
                             4 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             π 
                           
                           λ 
                         
                         ⁢ 
                         nx 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         cos 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         θ 
                       
                       + 
                       
                         φ 
                         ′ 
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     wherein φ′ stands for a phase shift due to light reflection at the first base substrate  60 . 
     Based on the equations (3) and (7), the wave function alight B 1  can be expressed using the equation (8):
 
Ψ( o,t ) B1   =t   US Ψ( o,t ) 0   r   BS   e   −iΔφ′   (8).
 
     Based on the equation (8), the wave function of light B 2  can be expressed using the equation (9):
 
Ψ( o,t ) B2 =Ψ( o,t ) B1   r   US   r   BS   e   −iΔφ   (9).
 
     Based on the equation (9), a sum of the wave functions of the light B 1 , B 2 , B 3 , . . . , B n  can be expressed using the equation (10): 
     
       
         
           
             
               
                 
                   
                     
                       Ψ 
                       ⁡ 
                       
                         ( 
                         
                           o 
                           , 
                           t 
                         
                         ) 
                       
                     
                     B 
                   
                   = 
                   
                     
                       
                         
                           Ψ 
                           ⁡ 
                           
                             ( 
                             
                               o 
                               , 
                               t 
                             
                             ) 
                           
                         
                         
                           B 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       [ 
                       
                           
                       
                       ⁢ 
                       
                         1 
                         + 
                         
                           ( 
                           
                             
                               r 
                               US 
                             
                             ⁢ 
                             
                               r 
                               BS 
                             
                             ⁢ 
                             
                               e 
                               
                                 
                                   - 
                                   i 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 Δ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 φ 
                               
                             
                           
                           ) 
                         
                         + 
                         
                           
                             ( 
                             
                               
                                 r 
                                 US 
                               
                               ⁢ 
                               
                                 r 
                                 BS 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     i 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   φ 
                                 
                               
                             
                             ) 
                           
                           2 
                         
                         + 
                         … 
                         + 
                         
                           
                             ( 
                             
                               
                                 r 
                                 US 
                               
                               ⁢ 
                               
                                 r 
                                 BS 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     i 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   φ 
                                 
                               
                             
                             ) 
                           
                           n 
                         
                       
                       ] 
                     
                     = 
                     
                       
                         
                           Ψ 
                           ⁡ 
                           
                             ( 
                             
                               o 
                               , 
                               t 
                             
                             ) 
                           
                         
                         
                           B 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                       = 
                       
                         
                           1 
                           
                             1 
                             - 
                             
                               
                                 r 
                                 US 
                               
                               ⁢ 
                               
                                 r 
                                 BS 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     i 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   φ 
                                 
                               
                             
                           
                         
                         = 
                         
                           
                             t 
                             US 
                           
                           ⁢ 
                           
                             
                               Ψ 
                               ⁡ 
                               
                                 ( 
                                 
                                   o 
                                   , 
                                   t 
                                 
                                 ) 
                               
                             
                             0 
                           
                           ⁢ 
                           
                             
                               
                                 
                                   r 
                                   BS 
                                 
                                 ⁢ 
                                 
                                   e 
                                   
                                     
                                       - 
                                       i 
                                     
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       φ 
                                       ′ 
                                     
                                   
                                 
                               
                               
                                 1 
                                 - 
                                 
                                   
                                     r 
                                     US 
                                   
                                   ⁢ 
                                   
                                     r 
                                     BS 
                                   
                                   ⁢ 
                                   
                                     e 
                                     
                                       
                                         - 
                                         i 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       Δ 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       φ 
                                     
                                   
                                 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     Based on the equations (5) and (10), the wave function of the total emitted light at a viewing angle θ′ can be expressed using the equation (11): 
     
       
         
           
             
               
                 
                   
                     
                       Ψ 
                       ⁡ 
                       
                         ( 
                         
                           o 
                           , 
                           t 
                         
                         ) 
                       
                     
                     C 
                   
                   = 
                   
                     
                       
                         
                           Ψ 
                           ⁡ 
                           
                             ( 
                             
                               o 
                               , 
                               t 
                             
                             ) 
                           
                         
                         A 
                       
                       + 
                       
                         
                           Ψ 
                           ⁡ 
                           
                             ( 
                             
                               o 
                               , 
                               t 
                             
                             ) 
                           
                         
                         B 
                       
                     
                     = 
                     
                       
                         
                           t 
                           US 
                         
                         ⁢ 
                         
                           
                             Ψ 
                             ⁡ 
                             
                               ( 
                               
                                 o 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                           0 
                         
                         ⁢ 
                         
                           
                             1 
                             + 
                             
                               
                                 r 
                                 BS 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     i 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     φ 
                                     ′ 
                                   
                                 
                               
                             
                           
                           
                             1 
                             - 
                             
                               
                                 r 
                                 US 
                               
                               ⁢ 
                               
                                 r 
                                 BS 
                               
                               ⁢ 
                               
                                 e 
                                 
                                   
                                     - 
                                     i 
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   φ 
                                 
                               
                             
                           
                         
                       
                       = 
                       
                         
                           t 
                           US 
                         
                         ⁢ 
                         
                           
                             Ψ 
                             ⁡ 
                             
                               ( 
                               
                                 o 
                                 , 
                                 t 
                               
                               ) 
                             
                           
                           0 
                         
                         ⁢ 
                         
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     
                                       r 
                                       BS 
                                     
                                     ⁢ 
                                     cos 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       φ 
                                       ′ 
                                     
                                   
                                 
                                 ) 
                               
                               - 
                               
                                 i 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       r 
                                       BS 
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       φ 
                                       ′ 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     
                                       r 
                                       BS 
                                     
                                     ⁢ 
                                     
                                       r 
                                       US 
                                     
                                     ⁢ 
                                     cos 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     φ 
                                   
                                 
                                 ) 
                               
                               - 
                               
                                 i 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       r 
                                       1 
                                     
                                     ⁢ 
                                     
                                       r 
                                       2 
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     φ 
                                   
                                   ) 
                                 
                               
                             
                           
                           . 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     The gain coefficient of the light intensity due to the microcavity effect can be expressed using the equation (12): 
     
       
         
           
             
               
                 
                   
                     
                       
                         I 
                         C 
                       
                       ⁡ 
                       
                         ( 
                         λ 
                         ) 
                       
                     
                     
                       
                         I 
                         N 
                       
                       ⁡ 
                       
                         ( 
                         λ 
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                          
                         
                           
                             
                               Ψ 
                               ⁡ 
                               
                                 ( 
                                 
                                   o 
                                   , 
                                   t 
                                 
                                 ) 
                               
                             
                             C 
                           
                           
                             2 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               
                                 Ψ 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     o 
                                     , 
                                     t 
                                   
                                   ) 
                                 
                               
                               O 
                             
                           
                         
                          
                       
                       2 
                     
                     = 
                     
                       
                         
                            
                           
                             
                               
                                 
                                   t 
                                   US 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       
                                         r 
                                         BS 
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       Δ 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         φ 
                                         ′ 
                                       
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 i 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       r 
                                       BS 
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     
                                       φ 
                                       ′ 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               
                                 2 
                                 ⁢ 
                                 
                                   ( 
                                   
                                     1 
                                     - 
                                     
                                       
                                         r 
                                         BS 
                                       
                                       ⁢ 
                                       
                                         r 
                                         US 
                                       
                                       ⁢ 
                                       cos 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       Δ 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       φ 
                                     
                                   
                                   ) 
                                 
                               
                               - 
                               
                                 i 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       r 
                                       1 
                                     
                                     ⁢ 
                                     
                                       r 
                                       2 
                                     
                                     ⁢ 
                                     sin 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     Δ 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     φ 
                                   
                                   ) 
                                 
                               
                             
                           
                            
                         
                         2 
                       
                       = 
                       
                         
                           
                             
                               ( 
                               
                                 1 
                                 - 
                                 
                                   R 
                                   US 
                                 
                               
                               ) 
                             
                             ⁡ 
                             
                               [ 
                               
                                 1 
                                 + 
                                 
                                   R 
                                   BS 
                                 
                                 + 
                                 
                                   2 
                                   ⁢ 
                                   
                                     R 
                                     BS 
                                     0.5 
                                   
                                   ⁢ 
                                   
                                     cos 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         
                                           
                                             4 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             π 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             nx 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             cos 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             θ 
                                           
                                           λ 
                                         
                                         + 
                                         
                                           φ 
                                           ′ 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           
                             
                               
                                 
                                   4 
                                   ⁢ 
                                   
                                     ( 
                                     
                                       1 
                                       + 
                                       
                                         
                                           R 
                                           BS 
                                         
                                         ⁢ 
                                         
                                           R 
                                           US 
                                         
                                       
                                       - 
                                       
                                         2 
                                         ⁢ 
                                         
                                           
                                             ( 
                                             
                                               
                                                 R 
                                                 BS 
                                               
                                               ⁢ 
                                               
                                                 R 
                                                 US 
                                               
                                             
                                             ) 
                                           
                                           0.5 
                                         
                                       
                                     
                                   
                                 
                               
                             
                             
                               
                                 
                                   
                                     cos 
                                     ⁢ 
                                     
                                       ( 
                                       
                                         
                                           
                                             
                                               4 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               π 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               nd 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               cos 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               θ 
                                             
                                             + 
                                             
                                               4 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               π 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               
                                                 n 
                                                 ′ 
                                               
                                               ⁢ 
                                               L 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               cos 
                                               ⁢ 
                                               
                                                   
                                               
                                               ⁢ 
                                               
                                                 θ 
                                                 ′ 
                                               
                                             
                                           
                                           λ 
                                         
                                         + 
                                         φ 
                                       
                                       ) 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     In the equation (12), the relationship among the gain coefficient of the light intensity due to the microcavity effect, the gap distance L of the tunable gap G, and the wavelength of light in a free space is established. Further, optical loss due to the microcavity may be considered in calculating the gain coefficient. 
     An effect of the gap distance L of the tunable gap G on a maximum-gain wavelength λ max  can be expressed using the equation (13): 
     
       
         
           
             
               
                 
                   
                     
                       m 
                       ⁢ 
                       
                         
                           λ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           max 
                         
                         2 
                       
                     
                     = 
                     
                       
                         
                           
                             n 
                             ′ 
                           
                           ⁢ 
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             ′ 
                           
                         
                         + 
                         
                           
                             ∑ 
                             i 
                           
                           ⁢ 
                           
                             
                               n 
                               i 
                             
                             ⁢ 
                             
                               d 
                               i 
                             
                             ⁢ 
                             cos 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               θ 
                               i 
                             
                           
                         
                         + 
                         
                            
                           
                             
                               φ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               λ 
                             
                             
                               4 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               π 
                             
                           
                            
                         
                       
                       = 
                       
                         
                           
                             n 
                             ′ 
                           
                           ⁢ 
                           L 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             ′ 
                           
                         
                         + 
                         
                           nd 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           cos 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           θ 
                         
                         + 
                         
                            
                           
                             
                               φ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               λ 
                             
                             
                               4 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               π 
                             
                           
                            
                         
                       
                     
                   
                   ; 
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     wherein m stands for a number of modes; n i  stands for refractive indexes of various layers of the organic light emitting diode  90 ; d i  stands for thicknesses of various layers of the organic light emitting diode  90 ; and θi stands for angles of refraction of various layers of the organic light emitting diode  90 . 
       FIG. 5  illustrates the correlation between a gap distance L of the tunable gap and a wavelength of light emitted from the organic light emitting diode assembly. Referring to  FIG. 5 , AlQ3 stands for an organic light emitting diode assembly in which the gap distance is zero, e.g., a conventional organic light emitting diode assembly as a control; and L stands for the gap distance of the tunable gap in various organic light emitting diode assemblies in which the gap distance L is greater than zero and is denoted in  FIG. 5 . As shown in  FIG. 5 , by changing the gap distance L of the tunable gap, a color of light emitted from the organic light emitting diode assembly may be modulated in response to the change in the gap distance L. For example, when the gap distance L is 20 nm, the emitted light has a wavelength of 472 nm (blue light); when the gap distance L is 60 nm, the emitted light has a wavelength of 524 nm (green light); when the gap distance L is 90 nm, the emitted light has a wavelength of 564 nm (yellow light); when the gap distance L is 115 nm, the emitted light has a wavelength of 600 nm (orange light); and when the gap distance L is 130 nm, the emitted light has a wavelength of 620 nm (red light); 
       FIG. 6  illustrates the correlation between a change ΔL in a gap distance and a maximum-gain wavelength. Referring to  FIG. 6 , the change ΔL in a gap distance has a linear correlation with a change Δλ max  in the maximum-gain wavelength λ max . 
     In another aspect, the present disclosure provides a light source having an organic light emitting diode assembly described herein or fabricated by a method described herein. In some embodiments, the light source further includes an actuator for controlling the gap distance of the tunable gap.  FIG. 7  is a side view of a light source in some embodiments according to the present disclosure.  FIG. 8  is a cross-sectional view of a light source in some embodiments according to the present disclosure.  FIG. 9  is a perspective view of a light source in some embodiments according to the present disclosure. Referring to  FIGS. 7 to 9 , the light source  200  in some embodiments includes an organic light emitting diode assembly  100  as described herein. The light source  200  in some embodiments further includes a base  210 , a controller knob  220  on the base  210 , a first strut  230   a  connected to the base  210  through a first connecting joint  240   a , a second strut  230   b  connected to the first strut  230   a  through a second connecting joint  240   b , a change-speed gear box  250  connected to the second strut  230   b  through a third connecting joint  240   c , and a lamp head  260 . The base  210 , the first strut  230   a , the second strut  230   b , the first connecting joint  240   a , the second connecting joint  240   b , and the third connecting joint  240   c  serve as support for the light source  200 . Each of the first connecting joint  240   a , the second connecting joint  240   b , and the thud connecting joint  240   c  are capable of rotating within a certain angle, thereby adjusting the position of the lamp head  260 . The controller knob  220  is configured to control the gap distance of the tunable gap in the organic light emitting diode assembly  100 . The change-speed gear box  250  is configured to control the speed of the transmission line provided, by the controller knob  220 , thereby fine-tuning the gap distance of the tunable gap in the organic light emitting diode assembly  100 . 
     Referring to  FIG. 8 , the controller knob  220  is operably coupled to a driving pulley  221 , around which a transmission line  222  is wound for multiple turns. The transmission line  222  is operably connected to a plurality of change-speed gears  251  in the change-speed gear box  250  through a plurality of fixed pulleys  223 . In  FIG. 8 , each of the plurality of fixed pulleys  223  is positioned at one of the first connecting joint  240   a , the second connecting joint  240   b , and the third connecting joint  240   c . After the transmission speed of the transmission line  222  is reduced by four change-speed gears  251  in the change-speed gear box  250 , the transmission line  222  is operably coupled to a rigid support plate  262  in the lamp head  260 . The rigid support plate  262  is sandwiched between one or more resilient support pad  261  and the organic light emitting diode assembly  100 . The resilient support pad  261  applies a compressive force on the rigid support plate  262  along a direction substantially perpendicular to a surface of the organic light emitting diode assembly  100 . The transmission line  222  applies a pulling force on the rigid support plate  262  along a direction substantially opposite to the direction of the compressive force applied by the resilient support pad  261 . The magnitude of the compressive force applied to the organic light emitting diode assembly  100  by the resilient support pad  261  can be controlled by changing the magnitude of the pulling force applied to the rigid support plate  262 . Thus, the magnitude of the compressive force applied to the organic light emitting diode assembly  100  can be precisely controlled by the controller knob  220  through the transmission line  222 , thereby actuating the gap distance of the tunable gap in the organic light emitting diode assembly  100 . A color of light emitted from the organic light emitting diode assembly  100  can be fine-tuned in response to a change in the gap distance. 
     Accordingly, in some embodiments, the light source  200  includes an actuator configured to control the gap distance of the tunable gap in the organic light emitting diode assembly  100 . Optionally, the actuator is configured to control a magnitude of a compressive force applied on a surface of the organic light emitting diode assembly  100  (e.g., on a surface of the second base substrate  70  or on a surface of the first base substrate  60 ).  FIG. 10  is a diagram illustrating an actuator for controlling the gap distance of a tunable gap in an organic light emitting diode assembly in some embodiments according to the present disclosure. Referring to  FIG. 10  and  FIG. 8 , the actuator in some embodiments includes a rigid support plate  262  and a resilient support, pad  261 . The resilient support pad  261  is configured to apply a compressive force C on the organic light emitting diode assembly  100 . The compressive force C is in turn applied to the elastic spacer layer  40 , reducing the height h 1  of the elastic spacer layer  40 . The rigid support plate  262  is configured to apply a pulling force P on the resilient support pad  261 , compensating the compressive force C applied on the organic light emitting diode assembly  100  and the elastic spacer layer  40 . When the magnitude of the pulling force P increases, the height h 1  of the elastic spacer layer  40  increases, thereby increasing the gap distance of the tunable gap G. When the magnitude of the pulling force P decreases, the height h 1  of the elastic spacer layer  40  decreases, thereby decreasing the gap distance of the tunable gap G. 
     The gap distance of the tunable gap G may be actuated mechanically as shown in  FIG. 10 . In some embodiments, the gap distance of the tunable gap G may be actuated by an electrical signal.  FIG. 11  is a diagram illustrating an actuator for controlling the gap distance of a tunable gap in an organic light emitting diode assembly in some embodiments according to the present disclosure. Referring to  FIG. 11 , the actuator in some embodiments includes an electroactive layer  300  and a support plate  310 . The electroactive layer  300  is connected (directly or indirectly) to the support plate  310  and the organic light emitting diode assembly  100 . Optionally, the distance h 3  between the support plate  310  and the first base substrate is fixed, whereas the height h 1  of the elastic spacer layer  40  and the height h 4  of the electroactive layer  300  are variable. 
     In some embodiments, the height h 4  of the electroactive layer  300  increases when an electrical signal is applied to the electroactive layer  300 . Consequently, the electroactive layer  300  applies a compressive force on the organic light emitting diode assembly  100  (e.g., on a surface of the second base substrate  70 , the height h 1  of the elastic spacer layer  40  decreases whereas the distance h 3  remains substantially unchanged. The gap distance G of the tunable gap decreases as the height h 1  of the elastic spacer layer  40  decreases. 
     In some embodiments, the height h 4  of the electroactive layer  300  decreases when an electrical signal is applied to the electroactive layer  300 . The electroactive layer  300  applies a pulling force on the organic light emitting diode assembly  100  (e.g., on a surface of the second base substrate  70 ), Consequently, the height h 1  of the elastic spacer layer  40  increases whereas the distance h 3  remains substantially unchanged. The gap distance G of the tunable gap increases as the height h 1  of the elastic spacer layer  40  increases. 
     The electroactive layer  300  is made of an electroactive material. As used herein the term “electroactive material” refers to a material that reversibly changes one or more characteristic body dimension by an amount depending on an applied electrical voltage. As used herein, the term “electroactive layer” refers to a layer in the present light source that includes an electroactive material, and is capable of reversibly changing one or more characteristic body dimension by an amount depending on an applied electrical voltage. Optionally, the electroactive material is an electrostrictive material. Stress and strain response of the electrostrictive material to an electric field is proportional to the square of the electric field. Optionally, the electroactive material is a piezoelectric material. Stress and strain response of the piezoelectric material to an electric field is proportional to the electric field. 
     Any appropriate electrostrictive material may be used for making the electroactive layer, e.g., electrostrictive ceramics, electrostrictive polymers, electrostrictive valves, etc. Examples of appropriate electrostrictive materials include, but are not limited to, a polyurethane containing material (e.g., a doped polyurethane material), polyvinylidene fluoride, lead magnesium niobate, lead magnesium niobate-lead titanate, lanthanum doped lead zirconate titanate, barium doped lead zirconate titanate, and various substitutes and derivatives thereof (e.g., doped with one or more dopant). 
     Any appropriate piezoelectric material may be used for making the electroactive layer. Examples of appropriate piezoelectric materials include, but are not limited to, lead zirconium titanate, berlinite, zinc oxide, barium titanate, lead titanate, and various substitutes and derivatives thereof (e.g., doped with one or more dopant). 
     In some embodiments, the light source is a back light of a display apparatus. 
     In another aspect, the present disclosure provides a display apparatus having a back light described herein or fabricated by a method described herein. Examples of appropriate display apparatuses include, but are not limited to, an electronic paper, a mobile phone, a tablet computer, a television, a monitor, a notebook computer, a digital album, a GPS, etc. 
     In another aspect, the present disclosure provides a method of fabricating an organic light emitting diode assembly.  FIG. 12  is a flow chart illustrating a method of fabricating an organic light emitting diode assembly in some embodiments according to the present disclosure. Referring to  FIG. 12 , the method in some embodiments includes forming an organic light emitting diode on a first base substrate; and forming a second base substrate on a side of the organic light emitting diode distal to the first base substrate. The organic light emitting diode and the second base substrate are formed to be spaced apart from each other by a tunable gap. A gap distance of the tunable gap is formed to be tunable such that a color of light emitted from the organic light emitting diode assembly is tunable in response to a change in the gap distance. Optionally, a change in the gap distance is substantially linearly correlated with a change in a maximum-gain wavelength of light emitted from the organic light emitting diode assembly. 
     In some embodiments, the method includes forming a microcavity in the organic light emitting diode assembly. Optionally, the microcavity is formed so that a light path in the microcavity extends through the tunable gap; and the gap distance of the tunable gap is tunable such that a total light path distance of the microcavity is tunable in response to the change in the gap distance. Optionally, the second electrode is formed using a substantially transparent conductive material. Optionally, the second base substrate is formed using a reflective material or a transflective material. Optionally, the first electrode is formed using a reflective conductive material or a transflective conductive material. Optionally, the first base substrate is formed using a reflective material or a transflective material. 
     Optionally, the first electrode is formed using a reflective conductive material, the second electrode is formed using a substantially transparent conductive material, and the second base substrate is formed using a transflective material. The microcavity is formed between the first electrode which is reflective and the second base substrate which is transflective. 
     Optionally, the first electrode is formed using a transflective conductive material, the second electrode is formed using a substantially transparent conductive material, and the second base substrate is formed using a transflective material. Optionally, the first base substrate is formed using a substantially transparent material. The microcavity is formed between the first electrode and the second base substrate, both of which are transflective. 
     Optionally, the first base substrate is formed using a reflective material, the first electrode is formed using a substantially transparent conductive material, the second electrode is formed using a substantially transparent conductive material, and the second base substrate is formed using a transflective material. The microcavity is formed between the first base substrate which is reflective and the second base substrate which is transflective. 
     Optionally, the first base substrate is formed, using a transflective material, the first electrode is formed using a substantially transparent conductive material, the second electrode is formed using a substantially transparent conductive material, and the second base substrate is formed using a transflective material. The microcavity is formed between the first base substrate and the second base substrate, both of which are transflective. 
     Optionally, the first base substrate is formed using a transflective material, the first electrode is formed using a substantially transparent conductive material, the second electrode is formed using a substantially transparent conductive material, and the second base substrate is formed using a reflective material. The microcavity is formed between the first base substrate which is transflective and the second base substrate which is reflective. 
     In some embodiments, the method further includes thrilling an elastic spacer layer spacing apart the organic light emitting diode and the second base substrate by the tunable gap. Optionally, the elastic spacer layer is capable of undergoing deformation in response to an external force thereby actuating the gap distance. 
     In some embodiments, the method further includes forming a rigid spacer layer on the first base substrate. Optionally, a height of the rigid spacer layer relative to the first base substrate is less than a height of the elastic spacer layer relative to the first base substrate when the gap distance is greater than zero. 
     In some embodiments, the step of forming the organic light emitting diode includes forming a first electrode on the first base substrate; forming an organic layer on a side of the first electrode distal to the first base substrate; and forming a second electrode on a side of the organic layer distal to the first electrode. The organic layer is formed to include an organic light emitting layer. 
     The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like does not necessarily limit the claim scope to a specific embodiment, and the reference to exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. Moreover, these claims may refer to use “first”, “second”, etc. following with noun or element. Such terms should be understood as a nomenclature and should not be construed as giving the limitation on the number of the elements modified by such nomenclature unless specific number has been given. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.