Abstract:
Various methods and systems are provided for related to organic light emitting diodes (OLEDs) having a microcavity. In one embodiment, a white-light source includes a first microcavity organic light emitting diode (OLED) configured to emit a narrow spectrum of blue light; a second microcavity OLED configured to emit a narrow spectrum of green light, and a third microcavity OLED configured to emit a narrow spectrum of red light. In another embodiment, a light source includes a plurality of OLEDs disposed on a glass substrate. Each of the OLEDs is configured to emit light in substantially orthogonal to the glass substrate in a predefined spectrum. Each of the OLEDs includes a semi-reflecting mirror; and an emitting layer, where the emitting layer in each OLED corresponds to a respective color of light emitted by the OLED.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. application Ser. No. 13/575,347, filed Jul. 26, 2012, which is the 35 U.S.C. §371 national stage of PCT application PCT/US2011/025667, filed Feb. 22, 2011, which claims priority to and the benefit of U.S. provisional application entitled “MICROCAVITY OLEDS FOR LIGHTING” having Ser. No. 61/307,191, filed Feb. 23, 2010, all of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    A broadband light source can be used to provide good quality lighting having a lighting spectrum that resembles natural sunlight. Light sources that do not provide light over the entire visible light spectrum can make the color of an object appear dull or even make the object appear to be a different color. For example, commercial fluorescent lights, which emit a limited amount of red light, can make an object appear to be dull red or even brown. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0004]      FIG. 1  is a graphical representation illustrating a non-limiting example of the transmission of emitted light through a plurality of layers of a white-emitting OLED in accordance with various embodiments of the present disclosure. 
           [0005]      FIG. 2  is a graphical representation illustrating of the various modes of the plurality of layers of the white-emitting OLED of  FIG. 1  in accordance with various embodiments of the present disclosure. 
           [0006]      FIGS. 3 and 4  are graphical representations of examples of microcavity organic light emitting diodes (OLEDs) in accordance with various embodiments of the present disclosure. 
           [0007]      FIGS. 5 and 6  are graphical representations of examples of semi-reflecting mirrors of the microcavity OLEDs of  FIGS. 3 and 4  in accordance with various embodiments of the present disclosure. 
           [0008]      FIGS. 7 and 8  are graphical representations illustrating non-limiting examples of the light intensity of a microcavity OLED of  FIGS. 3 and 4  and an OLED that lacks a microcavity in accordance with various embodiments of the present disclosure. 
           [0009]      FIG. 9  is a graphical representation of an example of a white-light emitting light source including a plurality of microcavity OLEDs of  FIGS. 3 and 4  in accordance with various embodiments of the present disclosure. 
           [0010]      FIG. 10  is a graphical representation illustrating a non-limiting example of the light intensity of the white-light emitting light source of  FIG. 9  in accordance with various embodiments of the present disclosure. 
           [0011]      FIG. 11  is a flow chart illustrating the fabrication of microcavity OLEDs of  FIGS. 3 and 4  in accordance with various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Disclosed herein are various embodiments of a light source including one or more organic light emitting diodes (OLEDs) having a microcavity and methods of fabricating the same. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views. 
         [0013]    A microcavity OLED emits light substantially orthogonal to the OLED substrate. The microcavity of the OLED allows the OLED to be highly efficient and produce intense light because light emitted by the OLED is directed out of the OLED instead of allowing the emitted light to be retained within the OLED. Additionally, the present application describes a white light source including a plurality of microcavity OLEDs. In some embodiments, the white light source includes a microcavity OLED that emits intense red light in a narrow spectrum, microcavity OLED that emits intense green light in a narrow spectrum, and a microcavity OLED that emits intense blue light in a narrow spectrum. Since each microcavity OLED intensely emits the specific colors in a narrow spectrum, when the white light source illuminates an object, the visible colors reflected by the object may be vibrant and warm due to the intensity and the selection of bands of light emitted by the white light source. 
         [0014]    A variety of light sources are available including luminaires using incandescent and/or fluorescent light bulbs. Luminaires are sometimes used in commercial, industrial, or office settings, and are often in the form of a light panel. Luminaires may lose 40-50% of the light they emit due to poor light extraction. Also, even if a light source such as a state of the art LED has a luminous efficacy of 100 lm/W (lumens per Watt), the efficacy of a luminaire may be as low as 40 lm/W. 
         [0015]    The broader the band of light that a light source emits, the more the light emitted by the light source resembles sunlight. A figure of merit used in lighting is color rendering index (CRI). CRI is a quantitative measure of the ability of a light source to reproduce the colors of various objects in comparison with a natural light source, such as the sun. A broadband light source covering the entire visible spectrum has a CRI larger than 90. In contrast, a commercial fluorescent light tube, which emits a small amount of red light, has a CRI as low as 50. Because of this lack of red light, a red object appears to be dull red or even brown when illuminated by a commercial fluorescent light tube. White-emitting OLEDs are useful for lighting because organic materials have wide emission spectra. Combining red, green and blue emitters in a single OLED panel yields an OLED that has a CRI higher than 80 depending on the emission spectrum. 
         [0016]    Some efficient white-emitting OLEDs have efficacies up to 100 lm/W. However, that requires exotic light extraction methods which are not practical for manufacturing.  FIG. 1  is a diagram of a non-limiting example of the transmission of emitted light  102  through a plurality of layers of a white-emitting OLED  100 . As can be seen in  FIG. 1 , because light emitted from the white-emitting OLED  100  is trapped due to refraction and reflection in an organic layer  104 , an Indium Tin Oxide (ITO) layer  106 , and/or a glass substrate  108 , only a small fraction of the emitted light  102  is extracted into air  110 . Examples of the indices of refraction (n) for the organic layers  104 , the ITO layer  106 , and the glass substrate  108  are also illustrated. 
         [0017]      FIG. 2  is a diagram of the various modes of the plurality of layers of the white-emitting OLED  100 . As can be seen in  FIG. 2 , thin film guided modes  202  (i.e., modes of the organic layer  104  and/or the ITO layer  106  of  FIG. 1 ) trap about 40-50% of the emitted light  102 , substrate modes  204  (i.e., modes of the glass substrate  108  of  FIG. 1 ) trap about 20-30% of the emitted light  102 , and only about 20-30% of the emitted light  102  reach the air modes  206 . While a glass substrate mode  204  may be eliminated using lens arrays or photonic crystals, a thin-film guided mode  202  is very difficult to eliminate because the organic/ITO layers  104 / 106  are inside the OLED  100  and are not accessible to the outside. 
         [0018]      FIG. 3  is a diagram of a non-limiting embodiment of a microcavity OLED  300 . The microcavity OLED  300  includes a glass substrate  302  and a semi-reflecting mirror  304  formed on the glass substrate  302 . In some embodiments, the semi-reflecting mirror  304  is a thin silver layer (e.g., about 10-20 nm thick), and in other embodiments the semi-reflecting mirror  304  is a quarter wave stack including stacks of silicon dioxide (SiO 2 ) and titanium dioxide (TiO 2 ), which will be discussed in further detail below. Further, an ITO layer  306  is formed on the semi-reflecting mirror  304 . In some embodiments, the ITO layer  306  is about 100 nm thick. 
         [0019]    A hole transport layer  308  is formed on the ITO layer  306 . In some embodiments, the hole transport layer  308  includes a 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) layer, which may be about 50 nm thick. An emitting layer  310  is formed on the hole transport layer  308 . The material included in the emitting layer  310  determines the color (or the spectral frequencies) of the light emitted by the microcavity OLED  300 . For example, for a microcavity OLED  300  that emits red light in the range of, e.g., about 585 nm to about 675 nm, the emitting layer  310  may include 3,5′-N,N′-dicarbazole-benzene (“mCP”) doped with tris(2-phenylisoquinoline)iridium (“Ir(pig) 3 ”). Similarly, for a microcavity OLED  300  that emits green light in the range of, e.g., about 525 nm to about 655 nm, the emitting layer  310  may include mCP doped with fac-tris(2-phenylpyridinato)iridium(III) (“Ir(ppy) 3 ”). Likewise, for a microcavity OLED  300  that emits blue light in the range of, e.g., about 435 nm to about 540 nm, the emitting layer  310  may include 3,5′-N,N′-dicarbazole-benzene (mCP):Iridium(III)bis[(4,6-di-flourophenyl)-pyridinato-N,C2′] picolinate (“Flrpic”). 
         [0020]    An electron transport layer  312  is formed on the emitting layer  310 . The electron transport layer  312  may include 2,9-dimethly-4,7-diphenyl-1, 10-phenanthroline (BCP) layer and/or a tris[3-(3-pyridyl)-mesityl]borane (“3TPYMB”) layer. Further, a cathode  314  is formed on the electron transport layer  312 . The cathode  314  includes a metal layer. For example, the cathode  314  may include cesium carbonate (CsCO 3 ) (about 1 nm thick) and aluminum (Al) (about 100 nm thick) or lithium fluoride (LiF) (about 1 nm thick) and Al (about 100 nm thick). 
         [0021]      FIG. 4  is a diagram of another non-limiting embodiment of a microcavity OLED  300  that emits blue light. In the embodiment illustrated in  FIG. 4 , the cathode  314  includes an aluminum (Al) layer  414   a,  which is about 100 nm thick, and a LiF layer  414   b  that is about 1 nm thick. The LiF layer  414   b  is deposited on an electron transport layer  312  including a BCP layer  412  that is about 40 nm thick. The BCP layer  412  is deposited on an emitting layer  310  including an mCP: Firpic layer  410  that is about 20 nm thick. Additionally, the emitting layer  310  is deposited on a hole transport layer  308  including a TAPC layer  408   a,  which is about 50 nm thick. Furthermore, the emitting layer  310  includes a PEDOT:PSS layer  408   b  that is about 25 nm thick and which is formed on an ITO layer  306  that is about 50 nm thick. 
         [0022]      FIG. 5  is a diagram of a non-limiting example of a semi-reflecting mirror  304  of the embodiment of a microcavity OLED  300  illustrated in  FIGS. 3 and 4 . The illustrated example of the semi-reflecting mirror  304  is a quarter wave stack that includes a silicon dioxide (SiO 2 ) layer  504   a,  which is formed on a titanium dioxide (TiO 2 ) layer  504   b.  The thicknesses of the SiO 2  layer  504   a  and the TiO 2  layer  504   b  each correspond to a quarter wavelength of light. Accordingly, the thickness of the layers  504   a  and  504   b  of the semi-reflecting mirror  304  depend upon the wavelength of light emitted by the microcavity OLED  300 . In one implementation, the silicon dioxide layer  504   a  is about 79 nm thick and the titanium dioxide layer  504   b  is about 48 nm thick. The semi-reflecting mirror  304  is formed on the glass substrate  302 , which may be about 1 mm thick. In some embodiments, the semi-reflecting mirror  304  has a width of about one inch. In some embodiments, the area of the semi-reflecting mirror  304  is about one inch by about one inch. Also, in some embodiments, the semi-reflecting mirror  304  has a reflectance (R) that is substantially equal to 0.39 at 475 nm. The reflectivity of the semi-reflecting mirror  304  may vary between about 40% and about 70%, and the reflection spectrum is broad. 
         [0023]      FIG. 6  is a diagram of another non-limiting example of a semi-reflecting mirror  304  of the embodiment of a microcavity OLED  300  illustrated in  FIGS. 3 and 4 . The example of the semi-reflecting mirror  304  illustrated in  FIG. 6  is similar to the example of the semi-reflecting mirror  304  illustrated in  FIG. 5  except that the semi-reflecting mirror  304  illustrated in  FIG. 6  includes two sets of silicon dioxide and titanium dioxide layers ( 504   a / 504   b  and  604   a / 604   b ) instead of one set ( 504   a / 504   b ) as illustrated in  FIG. 5 . In one implementation, the silicon dioxide layers  504   a  and  604   a  are about 79 nm thick and the titanium dioxide layer  504   b  and  604   b  are about 48 nm thick. In other implementations, the silicon dioxide layers  504   a  and  604   a  and/or the titanium dioxide layer  504   b  and  604   b  may have different thicknesses. Further, in other embodiments, the semi-reflecting mirror  304  may include three or more sets of silicon dioxide and titanium dioxide layers. In some embodiments, such as the one illustrated in  FIG. 6 , the semi-reflecting mirror  304  has an R that is substantially equal to 0.70 at 475 nm. Each of the semi-reflecting mirrors  304  illustrated in  FIGS. 5 and 6  include alternating layers of a material having a low refractive index (e.g., SiO 2 ) with a material having a high refractive index (e.g., TiO 2 ). 
         [0024]    Referring back to  FIG. 3 , the operation of the microcavity OLED  300  will now be described. The cathode  314  of the microcavity OLED  300  acts as a reflecting mirror and the semi-reflecting mirror  304  acts as a half mirror, thus forming a microcavity  320  between the cathode  314  and the semi-reflecting mirror  304 . The microcavity  320  has the properties of both low transmissivity and high reflectivity. In other words, the semi-reflecting mirror  304  is a partially transmissive and partially transparent layer. As photons are generated inside the microcavity  320 , they are reflected by the mirrors from both sides of the microcavity  320  and transmitted out of the half mirror provided by the semi-reflecting mirror  304 . Consequentially, the light  316  that is transmitted from the semi-reflecting mirror  304  through the glass substrate  302  is transmitted in a direction that is substantially orthogonal to the glass substrate  302  instead of in all directions. Because the microcavity  320  orients the emitted light in a particular direction, a considerable amount of the light that is emitted by the microcavity OLED  300  is also transmitted out of the microcavity OLED  300  and not retained within the microcavity OLED  300 . 
         [0025]    Because of the microcavity effects discussed above, a microcavity OLED  320  has very different emission characteristics from OLEDs that lack a microcavity  320 . An OLED that lacks a microcavity  320  is a Lambertian light source that emits light in all directions. A Lambertian light source is undesirable for lighting because a large amount of the emitted light is wasted (e.g., not directly illuminating the object or area to be illuminated). On the other hand, a microcavity OLED  300  is a directional emitter depending on the reflecting properties of the microcavity  320 . As a result, a microcavity OLED  300  can have efficiencies about three to four times the efficiencies of OLEDs that lack a microcavity  320 . 
         [0026]      FIG. 7  is a graph of the emission spectra of an embodiment of the microcativity OLED  300  illustrated in  FIG. 3  and an embodiment of an OLED that lacks a microcavity  320 . Specifically,  FIG. 7  is a graph of EL intensity versus wavelength for a microcavity OLED  320  versus a green-emitting OLED that lacks a microcavity  320 . The luminance of the microcavity OLED  320  is about 385 nits versus the luminance of the green-emitting OLED, which is about 108 nits, as can be seen in  FIG. 7 . 
         [0027]      FIG. 8  is a polar plot of the intensity of light versus the angle of the light for the embodiment of the microcativity OLED  300  of  FIG. 7  versus an OLED that lacks a microcavity  320 . As can be seen in  FIG. 8 , the OLED that lacks a microcavity  320  (marked “noncavity”  810 ) has a lower intensity than the microcavity OLED  300  (marked “cavity”  820 ), and the light is emitted from the light source at an angle primarily between −30 degrees and +30 degrees. Considering  FIGS. 7 and 8  together, it can be seen that for a microcavity OLED  300  both the spectrum of light emitted and the angle of emission are narrow. 
         [0028]      FIG. 9  is a diagram of a non-limiting embodiment of a white-light emitting light source  900  including a plurality of microcavity OLEDs  300   a,    300   b,    300   c.  The microcavity OLEDs  300   a,    300   b,    300   c  are phosphorescent OLEDs, which can be very efficient. For example, the luminous efficiency of a green light emitting, microcavity OLED  300   b  can be as high as 300 lm/W whereas a green light emitting OLED without a microcavity may have a luminous efficiency of only 100 lm/W. Further, the efficiency of a blue light emitting, microcavity OLED  300   a  and red light emitting, microcavity OLED  300   c  may each be over 60 lm/W. Accordingly, the white-emitting light source  900  including the plurality of microcavity OLEDs  300   a,    300   b,    300   c  may achieve an overall efficiency of about 150-200 lm/W. This efficiency may be three to four times greater than the efficiency of LEDs used in luminaries. 
         [0029]    White light generated from the red, green, and blue microcavity OLEDs  300   a,    300   b,    300   c  has an emission spectrum similar to the spectrum shown in  FIG. 10 . As can be seen in  FIG. 10 , the blue light  316   a  ( FIG. 9 ) may include an intensity  1010  corresponding to a wavelength substantially within a range of about 435 nm to about 540 nm, the green light  316   b  ( FIG. 9 ) may include an intensity  1020  corresponding to a wavelength substantially within a range of about 525 nm to about 655 nm, and the red light  316   c  ( FIG. 9 ) may include an intensity  1030  corresponding to a wavelength substantially within a range of about 585 nm to about 675 nm. Similarly, as can also be seen in  FIG. 10 , the blue light may include a peak intensity corresponding to a wavelength substantially within a range of about 450 nm to about 480 nm, the green light may include a peak intensity corresponding to a wavelength substantially within a range of about 530 nm to about 575 nm, and the red light may include a peak intensity corresponding to a wavelength substantially within a range of about 620 nm to about 650 nm. Accordingly, white light emitted by the white-light emitting light source  900  does not include the same intensity for all wavelengths, but rather the emission spectrum includes a narrow spectrum including a peak intensity for certain colors. When the light source  900  illuminates objects, the color that reflects off the objects appears to be saturated because the three narrow emission bands of the light emitted by microcavity OLEDs  300   a,    300   b,    300   c  peak at saturated RGB colors. When an embodiment of the white-light emitting light source  900  illuminates an object, the colors of the object appear warmer, more vibrant, and less dull as a result of the predefined bands of light emitted by the white-light emitting light source  900  and reflected off the object. For example, an object that is red appears to have more of a fire engine red color when illuminated by a white-light emitting light source  900  than a brick red or claret color that appears when the object is illuminated by an incandescent light source. Additionally, no external luminaries are needed since the light emitted by a white-light emitting light source  900  is highly directional. 
         [0030]    Referring next to  FIG. 11 , shown is a flow chart  1100  illustrating an example of a method of fabricating a microcavity OLED  300  ( FIG. 3 ). Beginning with block  1102 , a glass substrate  302  ( FIG. 3 ) is provided. In block  1104 , a semi-reflecting mirror  304  ( FIG. 3 ) is deposited on the glass substrate  302 . In the case of a semi-reflecting mirror  304  including a stack of SiO 2  and TiO 2  layers ( FIGS. 5 and 6 ), each layer of SiO 2  and TiO 2  may be deposited by sputtering. In the case of the semi-reflecting mirror  304  including a thin layer of silver, the silver can be deposited by vacuum evaporation. In some embodiments, the layer of silver is about 10-20 nm thick. 
         [0031]    Next, in block  1106 , an ITO layer  306  ( FIG. 3 ) is deposited by sputtering. In some embodiments, the ITO layer  306  is about 100 nm thick. A hole transport layer  308  ( FIG. 3 ) is then deposited in block  1108 . Vacuum evaporation may be used to deposit the hole transport layer  308  on the ITO layer  306 . Subsequently, an emitting layer  310  ( FIG. 3 ) is then deposited in block  1110  on the hole transport layer  308 . Vacuum evaporation may be used to deposit the emitting layer  310 . Further, an electron transport layer  312  ( FIG. 3 ) is deposited on the emitting layer  310  in block  1112 . Vacuum evaporation may be used to deposit the electron transport layer  312 . After the emitting layer  310  is deposited, a cathode  314  ( FIG. 3 ) is formed in block  1114 . In some implementations, the cathode  314  may be formed by depositing a layer of cesium carbonate (CsCO 3 ) (about 1 nm thick), aluminum (about 100 nm thick), and/or lithium fluoride (LiF) (about 1 nm thick) on the electron transport layer  312 . 
         [0032]    Referring to  FIG. 9 , a method of fabricating a white-light emitting light source  900  includes fabricating a plurality of microcavity OLEDs  300   a,    300   b,    300   c.  The plurality of microcavity OLEDs  300   a,    300   b,    300   c  may be fabricated on a common substrate  302  as illustrated in  FIG. 9 . Additionally, the method includes the steps described above with respect to fabricating microcavity OLED  300 , except that three different emitting layers  310   a,    310   b,  and  310   c  corresponding to blue, green, and red light are deposited for each microcavity OLEDs  300   a,    300   b,    300   c.  For example, the common substrate  302  may be provided in block  1102 . Each of the plurality of microcavity OLEDs  300   a,    300   b,    300   c  may then be fabricated as described with respect to blocks  1104  through  1114 . The plurality of microcavity OLEDs  300   a,    300   b,    300   c  may be concurrently or consecutively fabricated on the common substrate  302 . While  FIG. 9  depicts three microcavity OLEDs  300   a,    300   b,    300   c,  other embodiments of white-light emitting light sources  900  can include other multipes, combinations and/or configurations of microcavity OLEDs. 
         [0033]    It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications may be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims. 
         [0034]    It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt % to about 5 wt %, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term “about” can include traditional rounding according to significant figures of numerical values. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ‘y’”.