Organic Light Emitting Diode Comprising an Organic Semiconductor Layer

Abstract

The present invention relates to an organic light emitting diode including an anode electrode, a cathode electrode, at least one emission layer and at least one organic semiconductor layer, wherein the at least one emission layer and the at least one organic semiconductor layer are arranged between the anode electrode and the cathode electrode and the organic semiconductor layer includes a substantially metallic rare earth metal dopant and a first matrix compound, the first matrix compound including at least two phenanthrolinyl groups as well as to a method for preparing the same.

Claims

1. Organic light emitting diode comprising an anode electrode, a cathode electrode, at least one emission layer and at least one organic semiconductor layer, wherein the at least one emission layer and the at least one organic semiconductor layer are arranged between the anode electrode and the cathode electrode and the at least one organic semiconductor layer comprises a substantially metallic rare earth metal dopant and a first matrix compound, the first matrix compound comprising at least two phenanthrolinyl groups.

2. Organic light emitting diode according to claim 1, wherein the first matrix compound is a compound of Formula 1 ##STR00029## wherein R.sup.1 to R.sup.7 are each independently selected from the group consisting of hydrogen, substituted or unsubstituted C.sub.6 to C.sub.18 aryl group, substituted or unsubstituted pyridyl group, substituted or unsubstituted quinolyl group, substituted or unsubstituted C.sub.1 to C.sub.16 alkyl group, substituted or unsubstituted C.sub.1 to C.sub.16 alkoxy group, hydroxyl group or carboxyl group, and/or wherein adjacent groups of the respective R.sup.1 to R.sup.7 may be bonded to each other to form a ring; L.sup.1 is a single bond or selected from a group consisting of a C.sub.6 to C.sub.30 arylene group, a C.sub.5 to C.sub.30 heteroarylene group, a C.sub.1 to C.sub.8 alkylene group or a C.sub.1 to C.sub.8 alkoxyalkylene group; Ar.sup.1 is a substituted or unsubstituted C.sub.6 to C.sub.18 aryl group or a pyridyl group; and n is an integer from 2 to 4, wherein each of the n phenanthrolinyl groups within the parentheses may be the same or different from each other.

3. Organic light emitting diode according to claim 1, wherein the organic semiconductor layer is arranged between the emission layer and the cathode electrode.

4. Organic light emitting diode according to claim 1, wherein the organic semiconductor layer is in direct contact with the cathode electrode.

5. Organic light emitting diode according to claim 1, wherein the organic light emitting diode comprises a first emission layer and a second emission layer, wherein the organic semiconductor layer is arranged between the first emission layer and the second emission layer.

6. Organic light emitting diode according to claim 5, wherein the organic light emitting diode further comprises a p-type charge generation layer, wherein the organic semiconductor layer is arranged between the first emission layer and the p-type charge generation layer.

7. Organic light emitting diode according to claim 6, wherein the organic semiconductor layer is in direct contact with the p-type charge generation layer.

8. Organic light emitting diode according to claim 5, wherein the organic light emitting diode comprises a first organic semiconductor layer and a second organic semiconductor layer, wherein the first organic semiconductor layer is arranged between the first emission layer and the second emission layer and the second organic semiconductor layer is arranged between the cathode electrode and the emission layer closest to the cathode electrode.

9. Organic light emitting diode according to any claim 1, further comprising an electron transport layer which is arranged between the at least one emission layer and the at least one organic semiconductor layer.

10. Organic light emitting diode according to claim 1 further comprising a p-type charge generation layer, wherein the p-type charge generation layer is arranged between the organic semiconductor layer and the cathode electrode.

11. Organic light emitting diode according to claim 1, wherein the cathode electrode is transparent to visible light emission.

12. Organic light emitting diode according to claim 1, wherein the cathode electrode comprises a first cathode electrode layer and a second cathode electrode layer.

13. Organic light emitting diode according to claim 1, wherein the substantially metallic rare earth metal dopant is a zero-valent metal dopant.

14. Organic light emitting diode according to claim 1, wherein n is 2 or 3.

15. Organic light emitting diode according to claim 1, wherein L.sup.1 is a single bond.

16. Organic light emitting diode according to claim 1, wherein Ar.sup.1 is phenylene.

17. Organic light emitting diode according to claim 1, wherein R.sup.1 to R.sup.7 are independently selected from the group consisting of hydrogen, C.sub.1 to C.sub.4 alkyl group, C.sub.1 to C.sub.4 alkoxy group, C.sub.6 to C.sub.12 aryl group and C.sub.5 to C.sub.12 heteroaryl group.

18. A method of manufacturing an organic light emitting diode according to claim 1, comprising the steps of sequentially forming an anode electrode, at least one emission layer, at least one organic semiconductor layer, and a cathode electrode on a substrate, and forming the at least one organic semiconductor layer by co-depositing a substantially metallic rare earth metal dopant together with a first matrix compound comprising at least two phenanthrolinyl groups.

19. Organic light emitting diode according to claim 13, wherein the zero-valent metal dopant is selected from Sm, Eu, or Yb.

20. Organic light emitting diode according to claim 17, wherein R.sup.1 to R.sup.7 are independently selected from the group consisting of hydrogen, C.sub.1 to C.sub.4 alkyl group and phenyl.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0257] These and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, of which:

[0258] FIG. 1 is a schematic sectional view of an organic light-emitting diode (OLED), according to an exemplary embodiment of the present invention;

[0259] FIG. 2 is a schematic sectional view of an OLED, according to another exemplary embodiment of the present invention.

[0260] FIG. 3 is a schematic sectional view of an OLED, according to another exemplary embodiment of the present invention.

[0261] FIG. 4 is a schematic sectional view of a tandem OLED comprising a charge generation layer, according to an exemplary embodiment of the present invention.

[0262] FIG. 5 is a schematic sectional view of an OLED comprising a charge generation layer in direct contact with the cathode electrode, according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0263] Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The exemplary embodiments are described below, in order to explain the aspects of the present invention, by referring to the figures.

[0264] Herein, when a first element is referred to as being formed or disposed “on” a second element, the first element can be disposed directly on the second element, or one or more other elements may be disposed there between. When a first element is referred to as being formed or disposed “directly on” a second element, no other elements are disposed there between.

[0265] FIG. 1 is a schematic sectional view of an organic light-emitting diode (OLED) 100, according to an exemplary embodiment of the present invention. The OLED 100 includes a substrate 110, an anode electrode 120, a hole injection layer (HIL) 130, a hole transport layer (HTL) 140, an emission layer (EML) 150. Onto the emission layer (EML) 150 the organic semiconductor layer 170 is disposed. The organic semiconductor layer 170 comprising or consisting of a substantially metallic rare earth metal dopant and a first matrix compound comprising at least two phenanthrolinyl groups, preferably comprising formula 1, is formed directly on the EML 150. The cathode electrode layer 190 is disposed directly onto the organic semiconductor layer 170.

[0266] FIG. 2 is a schematic sectional view of an OLED 100, according to another exemplary embodiment of the present invention. FIG. 2 differs from FIG. 1 in that the OLED 100 of FIG. 2 comprises an electron transport layer 160.

[0267] Referring to FIG. 2 the OLED 100 includes a substrate 110, an anode electrode 120, a hole injection layer (HIL) 130, a hole transport layer (HTL) 140, an emission layer (EML) 150. Onto the emission layer (EML) 150 an electron transport layer (ETL) 160 is disposed. Onto the electron transport layer (ETL) 160 the organic semiconductor layer 170 is disposed. The organic semi-conductor layer 170 comprising or consisting of a substantially metallic rare earth metal dopant and a first matrix compound comprising at least two phenanthrolinyl groups, preferably comprising of formula 1 is formed directly on the ETL 160. The cathode electrode layer 190 is disposed directly onto the organic semiconductor layer 170.

[0268] FIG. 3 is a schematic sectional view of an OLED 100, according to another exemplary embodiment of the present invention. FIG. 3 differs from FIG. 2 in that the OLED 100 of FIG. 3 comprises an electron blocking layer (EBL) 145 and a cathode electrode 190 comprising a first cathode layer 191 and a second cathode layer 192.

[0269] Referring to FIG. 3 the OLED 100 includes a substrate 110, an anode electrode 120, a hole injection layer (HIL) 130, a hole transport layer (HTL) 140, an electron blocking layer (EBL) 145 and an emission layer (EML) 150. Onto the emission layer (EML) 150 an electron transport layer (ETL) 160 is disposed. Onto the electron transport layer (ETL) 160 the organic semi-conductor layer 170 is disposed. The organic semiconductor layer 170 comprising or consisting of a substantially metallic rare earth metal dopant and a first matrix compound comprising at least two phenanthrolinyl groups, preferably comprising of formula 1 is formed directly on the ETL 160. The cathode electrode layer 190 comprises of a first cathode layer 191 and a second cathode layer 191. The first cathode layer 191 is a substantially metallic layer and it is disposed directly onto the organic semiconductor layer 170. The second cathode layer 192 is disposed directly onto the first cathode layer 191.

[0270] FIG. 4 is a schematic sectional view of a tandem OLED 100, according to another exemplary embodiment of the present invention. FIG. 4 differs from FIG. 2 in that the OLED 100 of FIG. 4 further comprises a charge generation layer and a second emission layer.

[0271] Referring to FIG. 4 the OLED 100 includes a substrate 110, an anode electrode 120, a first hole injection layer (HIL) 130, a first hole transport layer (HTL) 140, a first electron blocking layer (EBL) 145, a first emission layer (EML) 150, a first hole blocking layer (HBL) 155, a first electron transport layer (ETL) 160, an n-type charge generation layer (n-type CGL) 185, a p-type charge generation layer (p-type GCL) 135, a second hole transport layer (HTL) 141, a second electron blocking layer (EBL) 146, a second emission layer (EML) 151, a second hole blocking layer (EBL) 156, a second electron transport layer (ETL) 161, the organic semiconductor layer 170, a first cathode electrode layer 191 and a second cathode electrode layer 192. The organic semiconductor layer 170 comprising or consisting of a substantially metallic rare earth metal dopant and a first matrix compound comprising at least two phenanthrolinyl groups, preferably comprising of formula 1, is disposed directly onto the second electron transport layer 161 and the first cathode electrode layer 191 is disposed directly onto the organic semiconductor layer 170.

[0272] The second cathode electrode layer 192 is disposed directly onto the first cathode electrode layer 191. Optionally, the n-type charge generation layer (n-type CGL) 185 may be the organic semi-conductor layer of the present invention.

[0273] FIG. 5 is a schematic sectional view of an OLED 100, according to another exemplary embodiment of the present invention. FIG. 5 differs from FIG. 1 in that the OLED 100 of FIG. 5 further comprises a p-type charge generation layer in direct contact with the cathode electrode.

[0274] Referring to FIG. 5, the OLED 100 includes a substrate 110, an anode electrode 120, a hole injection layer (HIL) 130, a hole transport layer (HTL) 140, an emission layer (EML) 150. Onto the emission layer (EML) 150 the organic semiconductor layer 170 is disposed. The organic semi-conductor layer 170 comprising or consisting of a substantially metallic rare earth metal dopant and a first matrix compound comprising at least two phenanthrolinyl groups, preferably comprising formula 1, is formed directly on the EML 150. The p-type charge generation layer (p-type CGL) 135 is formed directly on the organic semiconductor layer of the present invention 170. The cathode electrode layer 190 is disposed directly onto the p-type charge generation layer 135.

[0275] In the description above the method of manufacture an OLED 100 of the present invention is started with a substrate 110 onto which an anode electrode 120 is formed, on the anode electrode 120, a first hole injection layer 130, first hole transport layer 140, optional a first electron blocking layer 145, a first emission layer 150, optional a first hole blocking layer 155, optional an ETL 160, an n-type CGL 185, a p-type CGL 135, a second hole transport layer 141, optional a second electron blocking layer 146, a second emission layer 151, an optional second hole blocking layer 156, an optional at least one second electron transport layer 161, the organic semiconductor layer 170, a first cathode electrode layer 191 and an optional second cathode electrode layer 192 are formed, in that order or the other way around.

[0276] While not shown in FIG. 1, FIG. 2, FIG. 3, FIG. 4 and FIG. 5, a sealing layer may further be formed on the cathode electrodes 190, in order to seal the OLEDs 100. In addition, various other modifications may be applied thereto.

Examples

[0277] First matrix compounds comprising at least two phenanthrolinyl groups can be synthesized as described in JP2002352961.

[0278] Bottom Emission Devices with an Evaporated Emission Layer

[0279] For bottom emission devices—Examples 1 to 3 and comparative examples 1 to 5, a 15 Ω/cm2 glass substrate with 90 nm ITO (available from Corning Co.) was cut to a size of 50 mm×50 mm×0.7 mm, ultrasonically cleaned with isopropyl alcohol for 5 minutes and then with pure water for 5 minutes, and cleaned again with UV ozone for 30 minutes, to prepare a first electrode.

[0280] Then, 97 wt.-% of Biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine (CAS 1242056-42-3) and 3 wt.-% of 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2-(p-cyanotetrafluorophenyl)acetonitrile) was vacuum deposited on the ITO electrode, to form a HIL having a thickness of 10 nm. Then Biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine was vacuum deposited on the HIL, to form a HTL having a thickness of 130 nm. 97 wt.-% of ABH113 (Sun Fine Chemicals) as a host and 3 wt.-% of NUBD370 (Sun Fine Chemicals) as a dopant were deposited on the HTL, to form a blue-emitting EML with a thickness of 20 nm.

[0281] Then, the organic semiconductor layer is formed by deposing a matrix compound and metal dopant according to examples 1 to 3 and comparative example 1 and 5 by deposing the matrix compound from a first deposition source and rare earth metal dopant from a second deposition source directly on the EML. The composition of the organic semiconductor layer can be seen in Table 1. In examples 1 to 3 the matrix compound is a compound of formula 1. The thickness of the organic semiconductor layer is 36 nm.

[0282] Then, the cathode electrode layer is formed by evaporating and/or sputtering the cathode material at ultra-high vacuum of 10.sup.−7 bar and deposing the cathode layer directly on the organic semi-conductor layer. A thermal single co-evaporation or sputtering process of one or several metals is performed with a rate of 0, 1 to 10 nm/s (0.01 to 1 Å/s) in order to generate a homogeneous cathode electrode with a thickness of 5 to 1000 nm. The thickness of the cathode electrode layer is 100 nm. The composition of the cathode electrode can be seen in Table 1. Al and Ag are evaporated while ITO is sputtered onto the organic semiconductor layer using a RF magnetron sputtering process.

[0283] Bottom Emission Devices with a Solution-Processed Emission Layer

[0284] For bottom emission devices, a 15 Ω/cm2 glass substrate with 90 nm ITO (available from Corning Co.) was cut to a size of 50 mm×50 mm×0.7 mm, ultrasonically cleaned with isopropyl alcohol for 5 minutes and then with pure water for 5 minutes, and cleaned again with UV ozone for 30 minutes, to prepare a first electrode.

[0285] Then, PEDOT:PSS (Clevios P VP AI 4083) is spin-coated directly on top of the first electrode to form a 55 nm thick HIL. The HIL is baked on hotplate at 150° C. for 5 min. Then, a light-emitting polymer, for example MEH-PPV, is spin-coated directly on top of the HIL to form a 40 nm thick EML. The EML is baked on a hotplate at 80° C. for 10 min. The device is transferred to an evaporation chamber and the following layers are deposited in high vacuum.

[0286] First matrix compound comprising at least two phenanthrolinyl groups and rare earth metal dopant are deposed directly on top of the EML to form the organic semiconductor layer with a thickness of 4 nm. A cathode electrode layer is formed by deposing a 100 nm thick layer of aluminium directly on top of the organic semiconductor layer.

[0287] Top Emission Devices

[0288] For top emission devices—Examples 2 and 3, the anode electrode was formed from 100 nm silver on a glass substrate. The glass substrate was prepared by the same methods as described above for bottom emission devices.

[0289] The HIL, HTL, EML and organic semiconductor layer are deposed as described above for bottom emission devices.

[0290] Then the cathode is deposited. In example 2, a layer of 13 nm Ag is formed in high vacuum as described for bottom emission devices above. In example 3, a layer of 100 nm ITO is formed using a sputtering process. 60 nm biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine (CAS 1242056-42-3) is deposed directly on top of the cathode electrode layer.

[0291] The OLED stack is protected from ambient conditions by encapsulation of the device with a glass slide. Thereby, a cavity is formed, which includes a getter material for further protection.

[0292] Pn Junction Device as Model for an OLED Comprising at Least Two Emission Layers

[0293] The fabrication of OLEDs comprising at least two emission layers is time-consuming and expensive. Therefore, the effectiveness of the organic semiconductor layer of the present invention in a pn junction was tested without emission layers. In this arrangement, the organic semi-conductor layer functions as n-type charge generation layer (CGL) and is arranged between the anode electrode and the cathode electrode and is in direct contact with the p-type CGL.

[0294] For pn junction devices—Examples 4 to 5 and comparative example 6, a 15 Ω/cm2 glass substrate with 90 nm ITO (available from Corning Co.) was cut to a size of 50 mm×50 mm×0.7 mm, ultrasonically cleaned with isopropyl alcohol for 5 minutes and then with pure water for 5 minutes, and cleaned again with UV ozone for 30 minutes, to prepare a first electrode.

[0295] Then, 97 wt.-% of Biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine (CAS 1242056-42-3) and 3 wt.-% of 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2-(p-cyanotetrafluorophenyl)acetonitrile) was vacuum deposited on the ITO electrode, to form a HIL having a thickness of 10 nm. Then 2,4-diphenyl-6-(3′-(triphenylen-2-yl)-[1,1′-biphenyl]-3-yl)-1,3,5-triazine (CAS 1638271-85-8) was vacuum deposited on the HIL, to form an electron blocking layer (EBL) having a thickness of 130 nm.

[0296] Then, the organic semiconductor layer is formed by deposing a matrix compound and metal dopant according to examples 4 and 5 and comparative example 6 by deposing the matrix compound from a first deposition source and rare earth metal dopant from a second deposition source directly on the EBL. The composition of the organic semiconductor layer can be seen in Table 2. In examples 4 and 5 the matrix compound is a compound of formula 1. The thickness of the organic semi-conductor layer is 10 nm.

[0297] Then, the p-type CGL is formed by deposing the host and p-type dopant directly onto the organic semiconductor layer. The composition of the p-type CGL can be seen in Table 2. In comparative example 6, a layer of 10 nm formula (17) was deposited. In examples 4 and 5, 97 wt.-% of Biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine, referred to as HT-1, and 3 wt.-% of 2,2′,2″-(cyclopropane-1,2,3-triylidene)tris(2-(pcyanotetrafluorophenyl)acetonitrile), referred to as Dopant 1, was vacuum deposited to form a p-type CGL having a thickness of 10 nm.

[0298] Then, a layer of 30 nm Biphenyl-4-yl(9,9-diphenyl-9H-fluoren-2-yl)-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-amine is deposed directly on the p-type CGL to form a hole blocking layer (HBL).

[0299] Then, the cathode electrode layer is formed by evaporating aluminium at ultra-high vacuum of 10.sup.−7 bar and deposing the aluminium layer directly on the HBL. A thermal single co-evaporation of one or several metals is performed with a rate of 0, 1 to 10 nm/s (0.01 to 1 Å/s) in order to generate a homogeneous cathode electrode with a thickness of 5 to 1000 nm. The thickness of the cathode electrode layer is 100 nm.

[0300] The pn junction device is protected from ambient conditions by encapsulation of the device with a glass slide. Thereby, a cavity is formed, which includes a getter material for further protection.

[0301] To assess the performance of the inventive examples compared to the prior art, the current efficiency is measured under ambient conditions (20° C.). Current voltage measurements are performed using a Keithley 2400 sourcemeter, and recorded in V. At 10 mA/cm.sup.2 for bottom emission and 10 mA/cm.sup.2 for top emission devices, a calibrated spectrometer CAS 140 from Instrument Systems is used for measurement of CIE coordinates and brightness in Candela. Lifetime LT of bottom emission device is measured at ambient conditions (20° C.) and 10 mA/cm.sup.2, using a Keithley 2400 sourcemeter, and recorded in hours. Lifetime LT of top emission device is measured at ambient conditions (20° C.) and 8 mA/cm.sup.2. The brightness of the device is measured using a calibrated photo diode. The lifetime LT is defined as the time till the brightness of the device is reduced to 97% of its initial value.

[0302] In bottom emission devices, the emission is predominately Lambertian and quantified in percent external quantum efficiency (EQE). To determine the efficiency EQE in % the light output of the device is measured using a calibrated photodiode at 10 mA/cm2.

[0303] In top emission devices, the emission is forward directed, non-Lambertian and also highly dependent on the mirco-cavity. Therefore, the efficiency EQE will be higher compared to bottom emission devices. To determine the efficiency EQE in % the light output of the device is measured using a calibrated photodiode at 10 mA/cm2.

[0304] The voltage rise over time is measured at a current density of 30 mA/cm.sup.2 and 85° C. over 100 hours. The voltage rise is recorded in Volt (V).

[0305] In pn junction devices, the operating voltage is determined at 10 mA/cm.sup.2 as described for OLEDs above.

Technical Effect of the Invention

[0306] 1. Organic Semiconductor Layer in Direct Contact with the Cathode Electrode

[0307] In Table 1, operating voltage, external quantum efficiency and voltage rise over time are shown for OLEDs comprising a fluorescent blue emission layer, an organic semiconductor layer comprising a first matrix compound and a metal dopant and various cathode electrodes.

[0308] In comparative examples 1 to 3, ETM-1 is used as first matrix compound

##STR00028##

[0309] ETM-1 comprises a single phenanthrolinyl group. Various metal dopants have been tested and the operating voltage is between 4.4 and 6.7 V and the external quantum efficiency is between 1.9 and 3.6% EQE.

[0310] In comparative examples 4 and 5, MX1 is used as first matrix compound. MX 1 comprises two phenanthrolinyl groups. In comparative example 4, Li is used as metal dopant. The operating voltage is 3.3 V and the external quantum efficiency is 5.2% EQE. The voltage rise over time at 85° C. is 0.18 V. In comparative example 5, Mg is used as metal dopant. The operative voltage is 6 V and the external quantum efficiency is 4.3% EQE. To check reproducibility of the metal doping concentration over several fabrication runs, the standard deviation for the actual concentration of metal dopant in the organic semiconductor layer and its impact on the operating voltage have been determined. In comparative example 4, the doping concentration varies by 0.06 mol.-% and the operating voltage varies by 0.09 V. This is a substantial variation in operating voltage which may result in a large number of devices not meeting the product specification.

[0311] In example 1, MX1 is used a first matrix compound and Yb as metal dopant. The operating voltage is very low at 3.8 V and the efficiency is further improved to 5.6% EQE. Yb is significantly less hazardous to use than alkali metals and alkaline earth metals. Additionally, the voltage rise over time is significantly lower at 0.04 V compared to 0.18 V in comparative example 4. A further benefit of rare earth metal dopants is that a higher doping concentration can be used compared to Li. In comparative example 4, which is closest in operating voltage, 0.6 wt.-% Li is used. In example 1, 11.1 wt.-% Yb is used. The standard deviation for the actual Yb concentration in the organic semiconductor layer is 0.04 and the standard deviation for the operating voltage is 0.02. In summary, external quantum efficiency, voltage rise over time and standard deviation in operating voltage have been significantly improved.

[0312] In example 2, MX1 is used as first matrix compound and Yb as metal dopant. The anode electrode is formed from 100 nm Ag and the cathode electrode is formed from 13 nm Ag. As the cathode electrode is very thin, it is transparent to visible light emission. The efficiency is increased further to 7% EQE.

[0313] In example 3, the same composition is used in the organic semiconductor layer as in example 2. The cathode electrode is formed from 100 nm ITO which is transparent to visible light emission. The efficiency is still very high at 6.6% EQE and the operating voltage is low at 3.9 V.

[0314] In summary, a significant improvement in external quantum efficiency, reproducibility of metal doping concentration and voltage stability over time at elevated temperature has been achieved. Additionally, the operating voltage is still low while allowing safe handling of the rare earth metal dopants while loading the VTE source and reduced safety concerns during maintenance of the evaporation tool.

[0315] 2. Organic Semiconductor Layer in Direct Contact with the p-Type CGL

[0316] In Table 2, operating voltages are shown for pn junction devices comprising a p-type CGL and an organic semiconductor layer comprising a first matrix compound and a metal dopant and various cathode electrodes.

[0317] In comparative example 6, formula (17) is used as p-type CGL. The organic semiconductor layer comprises ETM-1 and Yb metal dopant. ETM-1 comprises a single phenanthrolinyl group. The operating voltage is 7.2 V.

[0318] In example 4, again formula (17) is used as p-type CGL. The organic semiconductor layer comprised MX1 and Yb metal dopant. MX1 comprises two phenanthrolinyl groups. The operating voltage is significantly improved to 4.9 V.

[0319] In example 5, HT-1 and Dopant 1 are co-deposited to form the p-type CGL. The organic semi-conductor layer comprises MX1 and Yb metal dopant. The operating voltage is improved further to 4.8 V.

[0320] A lower operating voltage offers the benefit of lower power consumption and longer battery life in mobile devices.

[0321] The features disclosed in the foregoing description, in the claims and the accompanying drawings may, both separately or in any combination thereof be material for realizing the invention in diverse forms thereof.

TABLE-US-00001 TABLE 1 Device performance of organic light emitting diodes comprising the organic semiconductor layer of the present invention in direct contact with the cathode electrode Thickness V rise at First wt.-% mol.-% cathode Voltage at EQE at 30 mA/cm.sup.2 Anode matrix Metal metal metal Cathode electrode 10 mA/cm.sup.2 10 mA/cm.sup.2 at 85° C. electrode compound dopant dopant dopant electrode [nm] [V] [%] [V] Comparative ITO ETM-1 Li 0.6 34 Al 100 4.4 3.4 — example 1 Comparative ITO ETM-1 Mg 1.6 30 Al 100 6.7 1.9 — example 2 Comparative ITO ETM-1 Yb 10.5 30 Al 100 5.0 3.6 — example 3 Comparative ITO MX1 Li 0.6 33 Al 100 3.3 5.2 0.18 example 4 Comparative ITO MX1 Mg 1.7 30 Al 100 6.0 4.3 — example 5 Example 1 ITO MX1 Yb 11.1 30 Al 100 3.8 5.6 0.04 Example 2 Ag MX1 Yb 4.9 15 Ag 13 4.2 7.0 — Example 3 Ag MX1 Yb 4.9 15 ITO 100 3.9 6.6 —

TABLE-US-00002 TABLE 2 Device performance of pn junction devices comprising the organic semiconductor layer of the present invention in direct contact with the p-type charge generation layer (CGL) First wt.-% mol.-% Voltage at matrix Metal metal metal 10 mA/cm.sup.2 p-type CGL compound dopant dopant dopant [V] Comparative Formula (17) ETM-1 Yb 9.5 28 7.2 example 6 Example 4 Formula (17) MX1 Yb 11.4 30 4.9 Example 5 HT-1: Dopant 1 MX1 Yb 11.2 30 4.8