Organic light-emitting diode having doped layers

Abstract

Organic light-emitting diode comprising a lower electrode ( 2 ) and an upper electrode ( 8 ), an organic electroluminescent layer ( 5 ) and at least one doped organic layer ( 3; 7 ) in contact with one of said electrodes. According to the invention, the doping level of this organic layer is higher at the interface with the electrode than in the core of this layer ( 3 ). Thanks to the invention, the luminous efficiency of the diode is very substantially improved.

1 . An organic light-emitting diode comprising:
 a substrate,   a lower electrode of a first kind on the same side as the substrate and an upper electrode of a second kind on the opposite side to the substrate, the electrode kinds corresponding to anode and cathode,   an organic emissive electroluminescent layer that is inserted between the lower electrode and the upper electrode,   and at least one layer) made of doped organic material in contact with one of said electrodes, which layer is inserted between this electrode and said electroluminescent layer, said organic material being doped with a donor dopant if said electrode in contact with it is a cathode and/or with an accepter dopant if said electrode in contact with it is an anode,   
       characterized in that the level of doping of at least one doped organic layer is higher at the interface between this organic layer and the electrode with which this layer is in contact than in the core of this doped organic layer. 
     
     
         2 . The diode as claimed in  claim 1 , comprising, if said contacting electrode is a cathode, said donor dopant is capable of increasing the density of electron energy levels near the LUMO level of said doped material and in that, if said contacting electrode is an anode, said accepter dopant is capable of increasing the density of hole energy levels near the HOMO level of said doped material. 
     
     
         3 . The diode as claimed in  claim 1 , comprising, if the dopant concentration in the material of said doped organic layer is zero or practically zero at the boundary of this layer opposite said electrode with which it is in contact, the thickness of the slice with a zero or practically zero concentration in this layer is strictly less than the thickness of the slice with a non-zero dopant concentration. 
     
     
         4 . The diode as claimed in  claim 1 , comprising, if said contacting electrode is a cathode, the difference in absolute value between, on the one hand, the HOMO level energy or ionization potential of the donor dopant and, on the other hand, the LUMO level energy of said organic material is less than 5 eV, and/or in that, if said contacting electrode is an anode, the difference in absolute value between, on the one hand, the LUMO level energy or electron affinity of the acceptor dopant and, on the other hand, the HOMO level energy of said organic material is less than 5 eV. 
     
     
         5 . The diode as claimed in  claim 4 , comprising, if the electrode in contact with the doped layer is a cathode, the HOMO level energy or ionization potential of the donor dopant is higher than the LUMO level energy of the organic material of the doped layer and/or in that, if the electrode in contact with the doped layer is an anode, the LUMO level energy or electron affinity of the acceptor dopant is equal to or lower than the HOMO level energy of the organic material of the doped layer. 
     
     
         6 . The diode as claimed in  claim 4 , comprising a potential barrier of greater than 0.2 eV exists at said interface. 
     
     
         7 . The diode as claimed in  claim 1 , comprising the average conductivity of the material of said doped organic layer is at least three times higher in a 10 nm thick slice of this layer, located at the interface and in contact with said electrode, than in an at least 10 nm thick slice located in the core of this doped organic layer at more than 20 nm from said electrode. 
     
     
         8 . The diode as claimed in  claim 1 , comprising the average dopant concentration in the material of said doped organic layer is at least three times higher in a slice of this layer, located at the interface and in contact with said electrode, than in a slice located in the core of this doped organic layer at more than 10 nm from said electrode. 
     
     
         9 . The diode as claimed in  claim 4 , comprising it includes an organic blocking layer inserted between at least one doped organic layer and said electroluminescent layer, which blocks holes if said doped organic layer is in contact with a cathode and blocks electrons if said doped organic layer is in contact with an anode. 
     
     
         10 . An illuminating panel or image display screen comprising an array of diodes as claimed in  claim 4 , comprising the diodes of this array are supported by the same substrate. 
     
     
         11 . An organic light-emitting diode comprising:
 a substrate,   a lower electrode of a first kind on the same side as the substrate and an upper electrode of a second kind on the opposite side to the substrate, the electrode kinds corresponding to anode and cathode,   an organic emissive electroluminescent layer that is inserted between the lower electrode and the upper electrode,   and at least one layer made of doped organic material in contact with one of said electrodes, which layer is inserted between this electrode and said electroluminescent layer, said organic material being doped with a donor dopant if said electrode in contact with it is a cathode and/or with an accepter dopant if said electrode in contact with it is an anode, and the dopant concentration being zero or practically zero at the boundary of this layer opposite said electrode with which it is in contact, wherein:   the level of doping of at least one doped organic layer is higher at the interface between this organic layer and the electrode with which this layer is in contact than in the core of this doped organic layer; and   in this doped organic layer, the thickness of the slice with zero or practically zero dopant concentration is strictly less than the thickness of the slice with the non-zero dopant concentration.   
     
     
         12 . The diode as claimed in  claim 11 , wherein the average conductivity of the material of said doped organic layer is at least three times higher in a 10 nm thick slice of this layer, located at the interface and in contact with said electrode, than in an at least 10 nm thick slice located in the core of this doped organic layer at more than 20 nm from said electrode. 
     
     
         13 . The diode as claimed in  claim 11 , wherein the average dopant concentration in the material of said doped organic layer is at least three times higher in a slice of this layer, located at the interface and in contact with said electrode, than in a slice located in the core of this doped organic layer at more than 10 nm from said electrode. 
     
     
         14 . The diode as claimed in  claim 11 , wherein, if said contacting electrode is a cathode, said donor dopant is capable of increasing the density of electron energy levels near the LUMO level of said doped material and wherein, if said contacting electrode is an anode, said accepter dopant is capable of increasing the density of hole energy levels near the HOMO level of said doped material. 
     
     
         15 . The diode as claimed in  claim 11 , wherein, if said contacting electrode is a cathode, the difference in absolute value between, on the one hand, the HOMO level energy or ionization potential of the donor dopant and, on the other hand, the LUMO level energy of said organic material is less than 5 eV, and/or wherein, if said contacting electrode is an anode, the difference in absolute value between, on the one hand, the LUMO level energy or electron affinity of the acceptor dopant and, on the other hand, the HOMO level energy of said organic material is less than 5 eV. 
     
     
         16 . The diode as claimed in  claim 15 , wherein, if the electrode in contact with the doped layer is a cathode, the HOMO level energy or ionization potential of the donor dopant is higher than the LUMO level energy of the organic material of the doped layer and/or wherein, if the electrode in contact with the doped layer is an anode, the LUMO level energy or electron affinity of the acceptor dopant is equal to or lower than the HOMO level energy of the organic material of the doped layer. 
     
     
         17 . The diode as claimed in  claim 11 , wherein a potential barrier of greater than 0.2 eV exists at said interface. 
     
     
         18 . The diode as claimed in  claim 11 , wherein it includes an organic blocking layer inserted between at least one doped organic layer and said electroluminescent layer, which blocks holes if said doped organic layer is in contact with a cathode and blocks electrons if said doped organic layer is in contact with an anode. 
     
     
         19 . An illuminating panel or image display screen comprising an array of diodes as claimed in  claim 11 , wherein the diodes of this array are supported by the same substrate. 
     
     
         20 . The diode as claimed in  claim 1 , characterized in that, if energy levels are counted as positive in comparison with the energy level of an electron under vacuum :
 if said electrode at said interface is a cathode, if material of said electrode at this interface is a metal having a work function E M1 , if the material O 1  of said organic layer doped with a donor dopant at this interface has a Fermi energy level E 1  and a LUMO energy level E C1 , then |E C1 −E M1 |>0.2 eV et E M1 >E 1 ;   if said electrode at said interface is an anode, if material of said electrode at this interface is a metal having a work function E M2 , if the material O 2  of said organic layer doped with a acceptor dopant at this interface has a Fermi energy level E 2  and a HOMO energy level E V2 , then |E V2 −E M2 |>0.2 eV et E M2 <E 2 .   
     
     
         21 . An organic light emitting diode, comprising:
 an electroluminescent layer; and   a doped organic layer in contact at an interface with the electroluminescent layer and having a dopant gradient which varies from a first concentration at a first thickness at the interface with the electroluminescent layer to a second concentration at a second thickness opposite the interface, wherein the first thickness is less than the second thickness, and wherein the first concentration is less than the second concentration.   
     
     
         22 . An organic light emitting diode, comprising:
 a lower electrode;   a first organic layer in electrical contact with the lower electrode having a first conductivity type;   an electroluminescent layer in contact with the first organic layer;   a second organic layer in contact with the electroluminescent layer having a second conductivity type; and   an upper electrode in electrical contact with the second organic layer;   wherein one of the first and second organic layers is doped with a dopant corresponding to the conductivity type of this organic layer, comprises a dopant gradient that is decreasing from a high level at the interface between said organic doped layer and said electrode in contact with said doped layer, to a lower level of approximately zero opposite the interface,   wherein there is a potential barrier of at least 0.2 eV at the interface;   and wherein, said doped organic layer being divided into a low level sub-layer and a high level sub-layer, the gradient causes the average dopant concentration of the high level sub-layer to be at least three times higher than the average dopant concentration of the low level sub-layer, and causes the thickness of the low level sub-layer to be less than the thickness of the high level sub-layer;   thereby allowing both charge injection and transport of charge type corresponding to the conductivity type in the doped organic layer, such that luminosity of the diode is increased for a given voltage.