Abstract:
An efficient organic light emitting diode (OLED) device—incorporating a stack of thin films comprising a cathode, electron transport layer, hole modulating conductive layer, host layer, electron modulating conductive layer, hole transport layer, hole injection layer and a transparent anode layer—has increased recombination efficiency to generate substantial amount of light. The electron modulating electrode and hole modulating electrode embedded in the stack is applied with appropriate electrical bias to contain the electrons and holes respectively inside the host layer to enhance recombination and consequently light output.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    Benefit of Provisional application No. 60/483,816 filed Jun. 30, 2003; Attorney file/docket No. 2502728.991130 (OLIT1130). U.S. Pat. No. 6,097,147-Baldo et.al, August 1, 200 U.S. Pat. No. 6,639,357-Parthasarathy et.al, Oct. 28, 2003 
     
    
     
       OTHER PUBLICATIONS  
         [0002]    “Fundamentals of OLED Displays”, Short Course S-2, Society for Information Display, SID&#39;03—by Eric Forsythe and Jianmin Shi  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0003]    Not Applicable  
         REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX  
         [0004]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0005]    Certain organic thin films emit light under the action of electric field. If such organic thin film whose thickness is of the order of wavelength of light is sandwiched between a cathode and an anode and applied with a voltage in the range of 5-7 V, substantial amount of light can be generated. Organic light emitting diodes (OLED) are fabricated exploiting this phenomenon of light emission, usually called Electro-luminescence. The basic mechanism, by which the light emission takes place in an organic layer, is through the recombination of electrons and holes in the organic layer. The electrons come in to the organic layer from the cathode that emits and injects electrons in to the organic layer with the help of externally applied electric field. Similarly holes come in to the organic layer from the anode which injects holes in to the organic layer under the action of externally applied electric field. On either side of the organic layer, that emits light, there are other organic layers that transport electrons from the cathode and holes from the anode. For efficient operation of OLED, all the electrons and holes that reach the emissive organic layer should recombine to produce light. Holes need to go up to the emissive organic layer and not beyond. Similarly electrons need to go up to the emissive organic layer and not beyond. Electrons and holes going beyond the emissive organic layer are considered to be a loss because they do not take part in recombination and subsequent emission of light.  
           [0006]    To prevent holes from going beyond the emissive organic layer, a ‘hole blocking’ organic layer is deposited adjacent to the emissive layer, as per the prior art. To prevent electrons going beyond the emissive layer, the hole transport layer, adjacent to the emissive layer towards the anode side, acts as a barrier to electrons. In a practical device, holes and electrons do leak towards the electrodes due to high fields (&gt;10 6  V/cm) existing in the device.  
           [0007]    This invention relates to the application of electrical bias to the interior of organic stack to repel the holes and electrons crossing the emissive layer and forcing them back to the emissive layer for effective recombination.  
           [0008]    Prior arts have dealt with the problem of leakage of holes and electrons beyond the emissive layer relying on the band-gap of hole-transport layer, for electrons, and hole blocking layer, for holes. In one prior art of 2000 (U.S. Pat. No. 6,097,147) Baldo et.al described a ‘hole blocking’ layer employing thin film of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthrolinr (BCP). This BCP layer had the band gap energy&gt;the energy level of ‘excitons’ (excited neutral molecule as a result of electron-hole recombination) formed in the emissive layer. This BCP layer was formed between the electron transport layer and emissive layer. However at high operating currents, needed for high brightness, the holes leak through and get collected by the cathode. Similarly the electrons leak through the barrier created by hole transporting layer on the anode side and get collected by anode. Hence barrier offered by the ‘blocking layer’ is not sufficient at high operating currents. In another invention by Parthasarathy et.al (U.S. Pat. No. 6,639,357) a similar ‘hole blocking’ layer which also functions as electron transport layer and electron emission layer was employed. The layer is the same as BCP but doped with Lithium (Li) for electron emission. This Li doped layer, in addition to having the same problem of leaking holes to the cathode at high operating currents, also has a problem of Li drift to the emissive layer during operation. Unlike the prior inventions, the present invention controls the loss of electrons and holes through an electrical bias.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    According to the present invention, the holes and electrons are forced to confine to the emissive layer through the establishment of an appropriate electric field across the emissive layer. The field can be varied through external bias control. To accomplish this, the present invention incorporates very thin and highly porous electrodes in the organic stack. Taking a conventional ‘down emitting’ OLED as an example, the anode is of transparent Indium Tin Oxide (ITO) formed on a glass substrate. Successive layers of hole injection, hole transport, light emission, hole blocking, electron transport and electron emissive layers are formed over ITO. There are two conductive layers in this configuration, one being the anode and the other being the cathode. Conductive connections to these layers are extended out of the device. The present invention brings about the elimination of ‘hole blocking’ layer in the device and instead two additional layers, fairly conductive and partly resistive, are formed on either side of the light emissive layer. These layers are nothing but porous thin metallic films that do not form a continuous sheet (incomplete film growth) but still remaining conductive enough to impress a uniform voltage across the layer. Conductive connections to these two layers are extended out of the device. Hence appropriate electrical bias can be applied to these electrodes to repel the electrons and holes crossing the light emissive layer on either side. The establishment of electric field across the light emissive host layer, through biasing of these layers, is analogous to the electric field across ‘depletion region’ in a conventional semiconductor Light Emitting Diode (LED). Since these layers are highly porous, the collection of primary electrons and holes by these layers is minimal. Organic materials employed in OLED are generally costly and in this context elimination of ‘hole blocking’ layer is economical in mass manufacturing. As these conductive layers are thin and porous, the optical transmission is high. It is also possible to have only one conductive layer and bias it relative to anode voltage to repel ‘holes’ back to the light emissive layer. In this case the optical transmission is higher.  
           [0010]    It is an object of this invention to provide electron and hole modulating electrodes that confine the holes and electrons to the light emissive layer by way of applying appropriate electrical bias.  
           [0011]    It is another object of this invention to employ the thin film mesh type modulating electrodes to optimize transmission and electrical conductivity.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is the cross section of OLED stack conventionally used in the prior art.  
         [0013]    [0013]FIG. 2 is the energy level diagram of the materials employed in FIG. 1  
         [0014]    [0014]FIG. 3 is the cross section of OLED stack used in another type of prior art.  
         [0015]    [0015]FIG. 4 is the energy level diagram of the materials employed in FIG. 3.  
         [0016]    [0016]FIG. 5 is the cross section of OLED stack incorporating electron and hole modulating electrodes.  
         [0017]    [0017]FIG. 6A is the plan view of the film structure of modulating electrode.  
         [0018]    [0018]FIG. 6B is the plan view of alternate film structure of modulating electrode.  
         [0019]    [0019]FIG. 7 is the individual biasing scheme for electron and hole modulating electrodes.  
         [0020]    [0020]FIG. 8 is another configuration of modulating electrodes.  
         [0021]    [0021]FIG. 9 is another configuration of the structure of OLED stack with modulating electrodes.  
     
    
     DETAILED DESCRIPTION  
       [0022]    [0022]FIG. 1 depicts the cross-section of thin film layers in a conventional OLED structure  100  that is applied with a voltage V2 to emit light. A metallic cathode layer  1  is applied with a small negative voltage and a transparent anode layer  6 , that is deposited usually on a glass substrate, not shown in FIG. 1, is applied with a small positive voltage. The organic layer  2  serves as electron transport layer for the electrons emitted from cathode layer  1 . Similarly the organic layer  4  functions as hole-transport layer for the holes injected from the organic layer  5 . The holes are injected from the anode layer  6 , which is transparent in this embodiment. Layer  5  is called hole injection layer and the layer  4  is called hole transport layer. In certain configurations, layer  4  can perform the function of hole-injection and hole transport. When a voltage of sufficient magnitude V2 is applied to the cathode and anode, the holes and electrons are transported to the layer  3 , which is called the host layer. The host layer  3  contains miniscule quantity of dopants, usually less than 5%. Total thickness of these layers is of the order of wavelength of light. The electrons and holes in layer  3  recombine to form ‘excitons’ which are excited molecules of the host material. The ‘excitons’ spontaneously transition to the ground state emitting radiation. When the host material is doped with dopants, the energy of the ‘excitons’ is coupled to the dopant material. This forms ‘excitons’ in the dopant material and these ‘excitons’ transition to ground state emitting radiation. The radiation travels in all directions. Since the anode is transparent and the cathode is reflective in the example shown in FIG. 1, the light rays  7  exit from the anode side in all angles. This is a&#39;down emitting OLED structure.  
         [0023]    [0023]FIG. 2 is the energy level diagram  200  of the conventional organic stack shown in FIG. 1, in equilibrium condition. If a voltage is applied to the stack, the band bending takes place and that is not shown in FIG. 2. The LUMO (lowest unoccupied molecular orbital) levels are shown as top horizontal steps  21  for various materials of the stack and the HOMO (highest occupied molecular orbital) are shown as bottom horizontal steps  22  for various materials of the stack. (The vacuum level is the top most stair case structure). Electron-transport takes place at LUMO levels and hole-transport takes place at HOMO levels. There is a barrier for electrons as could be seen from the step in LUMO levels between the cathode  23  and electron transport layer  28 . A similar barrier for electrons exists between the LUMO levels of host layer  27  and hole transport layer  26 . In the same way, there is a barrier for holes that exists between the HOMO levels of anode  24  and hole injection layer  25 . Holes also face a barrier between hole injection layer  25  and hole transport layer  26  and again between hole transport layer  26  and the host layer  27 . It can be noted that there is no barrier for holes between host layer  27  and the electron transport layer  28  like the one that exists for electrons between host layer  27  and hole transport layer  26 . Under the action of the electric field, substantial number of holes can leak through the host layer because of the absence of any barrier and thus not take part in useful recombination with electrons.  
         [0024]    [0024]FIG. 3 is the cross section of thin film organic stack  300  of prior art, which has an additional layer  33  to block the holes leaking towards the cathode  31 . With the bias V2 applied the holes are created by anode  37 , injected by hole injection layer  36  and transported by hole transport layer  35  to the host layer  34 . Similarly the electrons are emitted by cathode  31  and transported to the host layer  34  by both the electron transport layer  32  and hole blocking layer  33 . As a result of recombination of holes and electrons in the host layer, light  38  is emitted in all directions through the transparent anode  37 . Usually the transparent anode is deposited on a glass substrate, not shown in FIG. 3.  
         [0025]    [0025]FIG. 4 is the energy level diagram  400  of the material stack depicted in FIG. 3. The top horizontal steps  41 , next to vacuum level shows the LUMO levels of the materials. The bottom horizontal steps  42  are the HUMO levels. For electrons, there are energy barriers between cathode  43  and electron transport layer  48  and again between host  47  and hole transport layer  46 . For holes there are energy barriers between anode  44  and hole injection layer  45 , hole injection layer  45  and hole transport layer  46 , hole transport layer  46  and host layer  47  and finally between host layer  47  and hole blocking layer  48 . FIG. 4 distinctly differs from FIG. 3 with respect to the hole blocking layer and its barrier for holes crossing the host layer. However, if the current through the stack is increased, 20 mA/cm2 and beyond to obtain substantial brightness increase, the barrier offered by ‘hole blocking’ layer is not sufficient to prevent the leakage of holes towards the cathode side. Similar is the case for electrons leaking towards anode side.  
         [0026]    [0026]FIG. 5 illustrates the cross section of the stack  500  of thin film layers of the present invention. One distinction of this layer is that it is different from others shown in FIG. 1 and FIG. 3 due to the presence of two conductive layers  53  and  55  disposed on either side of host layer  54 . On top of the conductive layer  53  is the electron transport layer  52  followed by the cathode layer  51 . Towards the bottom side, following the conductive layer  55  is the hole transport layer  56 , hole injection layer  57  and anode  58 . The conductive layers  53  and  55  are highly porous but still conductive. The layers can also be of mesh structure. An electrical bias is applied to these thin film conductive layers, called hole and electron modulating electrodes. It is important to emphasize that these conductive layers have to be optically transparent by virtue of its porosity. The porosity and thin nature can also make these layers partially resistive.  
         [0027]    [0027]FIG. 6A is a plan view  600  of the structure of the conductive thin film layer of both the modulating electrodes. The thin film  61  has continuity but has plenty of pores  62  on its film surface. The film is with full of islands and is not fully grown to form a continuous film. Hence its sheet resistivity can be in the range of 1000 ohms/square. Due to the thinness and porosity, the film is optically transparent.  
         [0028]    [0028]FIG. 6B is an alternate plan view  601  of the structure of the conductive thin film layer of modulating electrodes. The film  63  has a mesh structure with openings  64  that make the film transparent. The mesh size and the pitch of the openings can be controlled with proper shadow mask while depositing the films during organic thin film processing. These modulating electrodes with high porosity and differing geometry of pores can be processed for very large area pixels of OLED. For example an OLED backlight device for Liquid Crystal Display or an OLED device for consumer lighting.  
         [0029]    [0029]FIG. 7 is the cross section of OLED structure  700 , incorporating modulating electrodes and the biasing scheme. A voltage of V2 is applied across cathode  71  and anode  78 . This voltage V2 is the operating voltage to obtain light emission from OLED. Between the cathode  71  and hole-modulating electrode  73  a voltage of V1 is applied making the hole-modulating electrode positive with respect to cathode. The magnitude of voltage V1 is such that it does not result in breakdown of the electron transport layer  72  or a high leakage through  72  or collection of electrons transported to the host layer  74 . The positive voltage, on hole modulating electrode  73 , repels the holes, which are positively charged and sends them back to the host layer  74  to take part in recombination to produce more light. A voltage of V3 is applied across anode  78  and electron modulating electrode  75 . The magnitude of voltage V3 is such that it does not result in breakdown of the hole transport layer  76  and hole injection layer  77  or high leakage through  76  and  77  or collection of holes transported to the host layer  74 . Additionally, the voltage between two modulating electrodes  73  and  75  should not result in breakdown of host layer  74  or high leakage through  74 . The negative voltage on the electron modulating electrode  75  repels the electrons and sends them back to the host layer  74  for taking part in recombination to produce more light. In this type of bias scheme a steep I-V characteristics of OLED can be obtained by properly adjusting V1 and V3. Instead of static bias a dynamic bias can also be employed to generate several brightness levels of OLED during operation.  
         [0030]    [0030]FIG. 8 is the cross section of alternative OLED structure  800  incorporating hole modulating electrode and its biasing scheme. A voltage of V2 is applied across cathode layer  81  and anode layer  87 . This is the main operating voltage for obtaining light emission from OLED. Between the cathode  81  and hole-modulating electrode  83  a voltage of V1 is applied to make hole-modulating electrode  83  positive with respect to cathode  81 . The magnitude of voltage V1 is such that it does not result in the breakdown of electron transport layer  82  or high leakage through  82  or collection of electrons being transported to host layer  84 . The positive voltage on  83  repels the holes, which are positively charged and sends them back to host layer  84  for taking part in recombination to generate more light. Unlike FIG. 7, in this OLED stack there is no electron-modulating electrode between host layer  84  and hole injection layer  85  that is adjacent to the hole injection layer  86 . This stack relies on the energy barrier offered to electrons by the hole-transport layer. The advantage of this stack is the increased optical transparency due to the absence of an electron modulating electrode and the absence of hole blocking layer made of costly organic material.  
         [0031]    [0031]FIG. 9 is the cross section of an alternative OLED stack  900  incorporating electron and hole modulating electrodes but omitting hole injection layer. The stack consists of cathode  91  followed by an electron transport layer  92  followed by a hole modulating electrode  93  followed by a host layer  94  followed by an electron modulating electrode  95  followed by a hole transport layer  96  and finally followed by an anode  97 .  
         [0032]    In all the foregoing embodiments of OLED stack, the electron and hole modulating electrodes can be manufactured by depositing, either by vacuum evaporation or by sputtering, a thin layer to a thickness in the range of 3 nm to 10 nm of metal or metal oxide. For example metals like, Aluminum or Nickel or Chromium or Titanium or Tantalum or combination of Magnesium and Silver or Molybdenum or oxides of metals like Indium Oxide or Zinc Oxide or Indium Tin Oxide or Chromic Oxide can be employed. The modulating electrodes can also be made by heavily doping an organic layer with metals.  
         [0033]    It will be apparent to those skilled in the art that various modifications and variations can be made in the construction, processing, configuration and/or operation and application of the present invention without departing from the scope or spirit of the invention. For example, in the embodiments mentioned above in FIG. 9, electron-modulating electrode can be omitted. The OLED stacks illustrated are multi-layer stacks with modulating electrodes. The same modulating electrodes can be employed in a ‘Bi-layer’ OLED stack. In the same manner, the illustration is for a large area single OLED pixel such as a backlight device for LCD or consumer lighting. With the advancement in patterning technology, the modulating electrodes can be employed in small area pixels as well. Thus it is intended that the present invention covers the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents.