Patent Application
20090001878
This application claims priority to China Patent Application Serial No. 200710065095.5 filed on Apr. 3, 2007 and China Patent Application Serial No. 200710177325.7 filed on Nov. 14, 2007, the contents of which are incorporated herein by reference.
1. Field of Invention
The present invention relates to an organic electroluminescent device, and particularly relates to an organic electroluminescent device in which at least one of hole injection layer, hole transport layer and electron transport layer is doped with an inorganic inactive material.
2. Background of the Invention
An organic electroluminescence flat display has many significant advantages, such as initiative light-emitting, light, thin, good contrast, independence of an angle, low power consumption and the like. In 1963, an organic electroluminescence device was fabricated by Pope et al with an anthracene single crystal. However, the first high efficient organic light-emitting diode (OLED) fabricated by vacuum evaporation was an OLED developed by C. W. Tang et al in 1987, wherein aniline-TPD was used as a hole transport layer (HTL), and a complex of aluminium and 8-hydroxyquinoline-ALQ was used as a light-emitting layer (EML). Its operating voltage was less than 10V, and its luminance was up to 1000 cd/m2. The light-emitting wavelength of organic electroluminescence materials developed later could cover the whole range of visible light. This breakthrough development made the field becoming a currently research hotspot. After entering 1990s, organic high molecular optical-electric functional materials entered a new development stage.
The structure of an organic electroluminescent device usually includes: substrate, anode, organic layer and cathode. An organic layer therein includes emitting layer (EML), hole injection layer (HIL) and/or hole transport layer (HTL) between anode and EML, electron transport layer (ETL) and/or electron injection layer (EIL) between EML and cathode, and also hole block layer between EML and ETL, and so on.
The mechanism of an organic electroluminescent device is like this:
When the electric field is on the anode and cathode, hole is injected into EML from anode through HIL and HTL, and electron is injected into EML from cathode through EIL and ETL. The hole and electron recombine and become exciton in EML. The exciton emits light from excitated state to ground state.
In the conditional devices of double layer or multilayer, HTL is absolutely necessary, which possess of good ability of charge transport and play a role of hole transport through proper energy level and structure design. However, the ability of hole transport is usually much better than electron transport. The difference of carrier mobility between hole and electron can be up to 10˜1000, which will impact the device on efficiency and lifetime severely. To obtain higher luminous efficiency, it is necessary to balance the hole and electron.
Now, the normally used hole transport materials are aromatic triamine derivatives, such as NPB, TPD and so on. However, the thermal stability of these materials are very poor, for example, the glass transition temperature (Tg) of NPB is 96° C. and TPD is only 65° C. As a result of the poor stability, the device has a shorter lifetime.
In order to overcome the above problems, there have been activities, in recent years, to develop organic electroluminescent devices using doping technology in HIL, HTL and ETL.
There have been a report about rubrene doped in HTL by Z. L. Zhang et al. (J. Phys. D: Appl. Phys., 31, 32-35, 1998). The doping of rubrene in HTL can facilitate hole and electron injection at the interface of ITO/HTL and Alq3/HTL because of the lower HOMO (−5.5 eV) and higher LUMO (−2.9 eV) of rubrene. The doping of rubrene in HTL can improve the device stability due to the reduction of Joule heat in device working and suppression of molecular aggregation and crystallization at interface. But the dopant of rubrene have an unfavorable impact on the device spectra because of the emission of rubrene itself.
As usual, the thickness of HIL must be thick enough to cover the merits on ITO anode surface to improve the quality of ITO surface. It is also important to introduce dopant into HIL to reduce the driven voltage and improve the power consumption. The dopant in HIL is called p-type dopant. The p-type dopant and HIL host will form charge transfer complexes (CT), which can favor hole injection and so reduce voltage and power consumption. F4-TCNQ and oxide of metal, etc., are the most used p-tpye dopants. However, the disadvantages of F4-TCNQ are its volatility to easily pollute the deposition chamber and poor thermal stability, which will unfavor storage and use at high temperature.
According to one aspect of the present invention, there is provided an organic electroluminescent device comprising
an anode;
an cathode; and
an organic functional layer between the anode and the cathode;
wherein the organic functional layer comprises at least one of light emission layer, hole injection layer, hole transport layer, electron transport layer, electron injection layer and hole blocking layer, and at least one of the hole injection layer (HIL), hole transport layer (HTL) and electron transport layer (ETL) comprises a host material and an inorganic inactive material doped in the host material.
The term of “inorganic inactive material” used herein may refer to an inorganic material that does not emit light and has electrical and chemical stability in an organic electroluminescent device of the present invention under common conditions.
In some embodiments of the present invention, the inorganic inactive material may be doped in the whole host material uniformly, or in the partial or whole host material in a gradient manner, or in at least one zone of the host material. In the case that the inorganic inactive material is doped in zones of the host material, the number of said zones can be 1˜5. In some cases, zones of the host material and zones of the host material doped with the inorganic inactive material may be disposed together alternatively.
In some embodiments of the present invention, the concentration of said inorganic inactive material doped in the host material may be within a range of: 1˜99 wt %, 4˜80 wt %, 10˜50 wt %, 30˜40 wt %, for example, may be 4wt %, 10 wt %, 30 wt %, 40 wt %, 50 wt % or 80 wt %.
In some embodiments of the present invention, the inorganic inactive material can be a halide, oxide, sulfide, carbide, nitride or carbonate of a metal, or a mixture thereof. The halide, oxide, sulfide, carbide, nitride or carbonate of metal can be a halide, oxide, sulfide, carbide, nitride or carbonate of a transition metal, or a halide, oxide, sulfide, carbide, nitride or carbonate of a Group 5A metal of the Periodic Table. The halide, oxide, sulfide, carbide, nitride or carbonate of transition metal can be a halide, oxide, sulfide, carbide, nitride or carbonate of a metal of lanthanide series of the Periodic Table, and the halide, oxide, sulfide, carbide, nitride or carbonate of Group 5A metal can be a halide, oxide, sulfide, carbide, nitride or carbonate of bismuth. The halide, oxide, sulfide, carbide, nitride or carbonate of metal of lanthanide series can be a halide, oxide, sulfide, carbide, nitride or carbonate of neodymium, samarium, praseodymium or holmium.
In some certain embodiments of the present invention, the inorganic inactive material can be selected from BiF3, BiCl3, BiBr3, BiI3, Bi2O3, YbF3, YbF2, YbCl3, YbCl2, YbBr3, YbBr2, Yb2O3, Yb2(CO3)3, LiF, MgF2, CaF2, AIF3, rubidium fluoride, molybdenum oxide, tungsten oxide, titanium oxide, rhenium oxide, tantalum oxide, lithium nitride, and mixtures thereof. In some particular embodiments of the present invention, the inorganic inactive material can be BiF3 or YbF3, and the concentration of the inorganic inactive material in the host material can be 30˜40 wt %.
In some other embodiments of the present invention the inorganic inactive material doped in the host material may have a thickness of 10˜200 nm in the HIL, or a thickness of 5˜20 nm in the HTL, or a thickness of 5˜20 nm in the ETL.
According to another aspect of the present invention, there is provided a method for preparing an organic electroluminescent device comprising an anode, an cathode, and an organic functional layer between the anode and the cathode, in which the organic functional layer comprises at least one of light emission layer, hole injection layer, hole transport layer, electron transport layer, electron injection layer and hole blocking layer, wherein an inorganic inactive material is doped in the host material of at least one of the hole injection layer (HIL), hole transport layer (HTL) and electron transport layer (ETL). The inorganic inactive material can be a halide, oxide, sulfide, carbide, nitride or carbonate of a metal, or a mixture thereof as shown above.
Without being limited to any theory, we believe that it will control the concentration of charge carrier and make a better balance between hole and electron by doping of inorganic inactive materials in HIL, HTL and ETL. The balance of hole and electron can lead to effective recombination of carriers and enhance the luminous efficiency. If hole is blocked, the probability of Alq3 cation can be reduced effectively. The injection and transport of electron could be enhanced by the interaction between inactive materials and EIL, ETL materials. The device stability also could be improved by crystallization suppression of organic layers due to higher stability of dopant materials. On the other hand, the film growth mode of organic materials is usually island-like. The doping of inactive material could fill the space of organic host and make the organic film more uniform and smooth. The inactive material is equal to parallel capacitance when the device is put on electric field. This can reduce the resistance of organic layers and enhance the charge concentration and finally improve the driven voltage of devices.
According to certain embodiments of the present invention, the host material of HTL can be aromatic amine derivatives, for example, aromatic diamine, aromatic triamine compound, amine with starburst and spire structure and so on, such as TPD, NPB, m-MTDATA, TCTA and spiro-NPB etc. The host material of HIL can be phthalocyanine and triphenylamine derivatives, such as CuPc, m-MTDATA and TNATA etc.
The following merits may be observed in some embodiments of the present invention:
1. The luminous efficiency could be improved effectively by the better balance between hole and electron, which may from the higher recombination efficiency of charge carrier due to the control of carrier concentration by doping with inorganic inactive materials.
2. The resistance of organic layers could be improved by doping with inorganic inactive materials to enhance conductance of organic layers. This leads to the increase of charge concentration and the increase of driven voltage.
3. The blocking of hole transport by doping could reduce the probability of Alq3 cation and slow the attenuation of device operation.
4. The crystallization of organic materials could be suppressed effectively by doping with higher thermal stable inorganic materials. Then, the stability of organic film could be improved obviously, which is one of the key factors to decide the temperature range and thermal stability of a device.
5. The doping of inorganic inactive materials cannot impact on the device electroluminescent spectra.
Now, some embodiments of organic electroluminescent device of the present invention are described with reference to the accompanying drawings in which:
According to some embodiments of the present invention, the basic structure of organic electroluminescent device includes: transparent substrate, which may be glass or flexible substrate. The flexible substrate may be one of polyester or polyimide compound. The first electrode (anode), which may be inorganic material or organic conductive polymer. The inorganic material is usually oxide of metal, such as indium tin oxide (ITO), zinc oxide and tin zinc oxide and so on, or metal with high work function, such as gold, copper and silver, etc. The optimization is ITO. The organic conductive polymer may be PEDOT:PSS, polyaniline. The second electrode (cathode), which may be metal with low work function, such as lithium, magnesium, calcium, strontium, aluminum, indium, etc. or alloy of them and copper, gold and silver, or alternate layers of metal and fluoride of metal. The optimization in present invention is MgAg alloy/Ag and LiF/Al.
The host of HIL may be CuPc, m-MTDATA and 2-TNATA.
The host of HTL may be aromatic amine derivatives, especially, NPB.
The materials of EML may be commonly selected from small molecules, such as fluorescent and phosphorescent materials. The fluorescence may be formed from metal complexes (such as Alq3, Gaq3, Al(Saph-q) or Ga(Saph-q)) and dyes (such as rubrene, DMQA, C545T, DCJTB or DCM). The concentration of dye in EML is 0.01%˜20% by weight. The phosphorescence is from carbazole derivatives (such as CBP) or polyethylene carbazole compound (such as PVK). The phosphorescent dyes may, for example, be Ir(ppy)3, Ir(ppy)2(acac), PtOEP, etc.
The materials used in ETL may be sma1 molecular capable of electron transporting, such as metal complexes (such as Alq3, Gaq3, Al(Saph-q) or Ga(Saph-q)), fused-ring aromatic compounds (such as pentacene, perylene), or phenanthroline compounds (such as Bphen, BCP), etc.
Now, the present invention will be illustrated in further detail with reference to the following Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples.
Device Structure:
Glass/ITO/m-MTDATA(120 nm):BiF3[40%]/NPB(30 nm)/Alq3(30 nm):C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
An organic electroluminescent device having the structure above is prepared by the following method.
The glass substrate is cleaned by thermal detergent ultrasonic and deionized water ultrasonic methods, and then dried under an infrared lamp. Then, the dried glass substrate is preprocessed by ultraviolet ozone cleaning and low energy oxygen ion beam bombardment, wherein the indium tin oxide (ITO) film on the substrate is used as an anode layer. The Sheet Resistance of the ITO film is 50 Ω, and its thickness is 150 nm.
The preprocessed glass substrate is placed in a vacuum chamber which is pumped to 1×10−5 Pa. A hole injection layer is deposited on the ITO anode by co-evaporating of m-MTDATA and BiF3 from separated crucible at an evaporation rate of 0.1 nm/s. The film thickness of the HIL is about 120 nm and the concentration of BiF3 is 40%.
A hole transport layer of NPB is deposited on the HIL without disrupting the vacuum. The evaporation rate of NPB is 0.2 nm/s and the film thickness is 30 nm.
Then, an emitting layer of Alq3 doping with C545T is vapor-deposited onto the HTL by co-evaporation. The layer thickness is 20 nm. The concentration of C545T is 1%.
The electron transport layer is Alq3, which is deposited onto the emitting layer. The evaporation rate of Alq3 is 0.2 nm/s and the layer thickness is 20 nm.
At last, LiF is vapor-deposited thereon as a electron injection layer in a thickness of 0.5 nm and aluminum as a cathode in a thickness of 200 nm with evaporation rate of 0.05 nm/s and 2.0 nm/s, respectively.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): Bi2O3[40%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to Bi2O3.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): Sm2(CO3)3[40%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to Sm2(CO3)3.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): YbF3[40%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to YbF3.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): YbCl3[40%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to YbCl3.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): WO3[33%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to WO3 and the concentration is 33%.
Device Structure:
Glass/ITO/m-MTDATA(120 nm)/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that there is no dopant material in HIL.
As shown in
Device Structure:
Glass/ITO/m-MTDATA(120 nm): YbCl3[50%]: F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to YbCl3 and F4-TCNQ and the concentration of F4-TCNQ in HIL is 2%.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): Bi2O3[50%]: F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 6 except that the dopant material of YbCl3 in HIL is changed to Bi2O3 and the concentration of Bi2O3 in HIL is 50 wt %.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): F4-TCNQ[2%]/NPB(10 nm)/NPB(5 nm): Bi2O3[20%]/NPB(10 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material of in HIL is changed to F4-TCNQ and the concentration of F4-TCNQ in HIL is 2 wt %.
The HTL of the device is firstly evaporated a 10 nm thick NPB layer, and then co-evaporated NPB and Bi2O3. The doping layer thickness is 5 nm and the concentration of Bi2O3 in the doping layer is 20 wt %. At last, a NPB layer of 10 nm thick is deposited onto the doping layer.
Device Structure:
Glass/ITO/m-MTDATA(200 nm): BiF3[50%]: F4-TCNQ[2%]/NPB(10 nm)/NPB(15 nm): YbCl3[30%]/NPB(10 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 7 except that the dopant material of Bi2O3 in HIL is changed to BiF3 and the total thickness of doping film is 200 nm.
The HTL of the device is firstly evaporated a 10 nm thick NPB layer, and then co-evaporated NPB and YbCl3. The doping layer thickness is 15 nm and the concentration of YbCl3 in the doping layer is 30 wt %. At last, a NPB layer of 10 nm thick is deposited onto the doping layer.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the dopant material in HIL is changed to F4-TCNQ and the concentration of F4-TCNQ is 2%.
The both doping of inorganic inactive material and F4-TCNQ can improve the device voltage effectively as listed on Table 2 and depicted in
d) is a graph of brightness as function of aging time of Exam.-8 and Comp. Exam.-3. Both devices are tested at a high temperature of 90° C. and the initial brightness is about 1000 cd/m2. It is obvious that there is 4 times of improvement in Exam.-8, which demonstrated that the thermal stability of the doping device have been improved largely due to the high stable material of Bi2O3.
Device Structure:
Glass/ITO/2-TNATA (120 nm): BiF3[x %]: F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the host material in HIL is changed to 2-TNANA and the dopant material changed to BiF3 and F4-TCNQ. The concentration of F4-TCNQ in HIL is 2% and that of BiF3 is x, where x is 4, 10, 20, 40, 50, respectively.
The luminous efficiencies of all the devices doped with BiF3 are higher than Comp. Exam.-3 obviously, as shown in Table 3 and
Device Structure:
Glass/ITO/2-TNATA(80 nm): Sm2(CO3)3[12%]: WO3[17%]/2-TNATA(20 nm)/NPB(10 nm)/NPB(5 nm): NdF3[50%]/NPB(10 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner of EML, ETL, EIL and cathode as in Example 1 except that the HIL and HTL.
The HIL of the device is firstly co-evaporated by 2-TNATA, Sm2(CO3)3 and WO3 from separated crucible. The concentration of Sm2(CO3)3 and WO3 is 12 wt % and 17 wt %, respectively. The film thickness is 80 nm. Then, a 20 nm thick layer of 2-TNATA is deposited on the top of the doping layer.
The HTL of the device is firstly evaporated a 10 nm thick NPB layer, and then co-evaporated NPB and NdF3. The doping layer thickness is 5 nm and the concentration of NdF3 in doping layer is 50%. At last, a NPB layer of 10 nm thick is deposited onto the doping layer.
Device Structure:
Glass/ITO/m-MTDATA(100 nm): WO3[20%]/2-TNATA(50 nm): PrF3[30%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the HIL.
The HIL of the device is made of two layers. One is co-evaporated by m-MTDATA and WO3 onto the ITO anode. This layer is 100 nm thick and then concentration of WO3 is 20%. The other layer is also co-evaporated by 2-TNATA and PrF3 on the top of first layer. The layer thickness is 50 nm and the concentration of PrF3 is 30%.
Device Structure:
Glass/ITO/m-MTDATA(40 nm): F4-TCNQ[2%]/m-MTDATA(30 nm): Ho2(CO3)3[80%]/m-MTDATA(40 nm): F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the HIL.
The HIL of the device is made of three layers. The first one is co-evaporated by m-MTDATA and F4-TCNQ onto the ITO anode. This layer is 40 nm thick and the concentration of F4-TCNQ is 2%. The second layer is also co-evaporated by m-MTDATA and Ho2(CO3)3 on the top of first layer. The layer thickness is 30 nm and the concentration of Ho2(CO3)3 is 80%. The third layer is the same as the first layer.
Device Structure:
Glass/ITO/2-TNATA(10 nm): Nd2O3[4%]/2-TNATA(100 nm): V2O5[10%]/NPB(15 nm): NdF3[50%]/NPB(15 nm)/Alq3(30 nm): C545T[1%]/Alq3(20 nm)/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 1 except that the HIL and HTL.
The HIL of the device is made of two layers. The first one is co-evaporated by 2-TNATA and Nd2O3 onto the ITO anode. This layer is 10 nm thick and the concentration of Nd2O3 is 4%. The second layer is also co-evaporated by 2-TNATA and V2O5 on the top of first layer. The layer thickness is 100 nm and the concentration of V2O5 is 10%.
The HTL of the device is firstly evaporated a 15 nm thick co-evaporation layer of NPB and NdF3. The concentration of NdF3 in doping layer is 50%. At last, a NPB layer of 15 nm thick is deposited onto the doping layer.
The doping position of dopant materials in HIL and HTL is adjusted in Exam.-15˜-Exam.-18. From the data listed on Table 4, these doping devices have similar performance compared to Comp. Exam.-3, more particularly, Exam.-18 have the best characteristics. The control of concentration of hole and electron zonely by change the doping position can facilitate the balance of charge carrier and reach an excellent performance.
Device Structure:
Glass/ITO/2-TNATA(10 nm): Nd2O3[4%]/2-TNATA(100 nm): V2O5[10%]/NPB(15 nm): NdF3[50%]/NPB(15 nm)/Alq3(30 nm): C545T[1%]/Alq3(10 nm)/Alq3(10 nm): BiF3[20%]/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Example 18 except that the ETL.
The ETL of the device is a 10 nm thick Alq3 layer and a 10 nm thick doping layer of Alq3 and BiF3. The concentration of BiF3 in doping layer is 20 wt %.
Device Structure:
Glass/ITO/m-MTDATA(120 nm): F4-TCNQ[2%]/NPB(30 nm)/Alq3(30 nm): C545T[1%]/Alq3(5 nm)/Alq3(20 nm): Bi2O3[10%]/LiF(0.5 nm)/Al(200 nm)
A device is prepared in the same manner as in Comparative Example 3 except that the ETL.
The ETL of the device is a 5 nm thick Alq3 layer and a 20 nm thick doping layer of Alq3 and Bi2O3. The concentration of Bi2O3 in doping layer is 10 wt %.
The doping of inorganic materials in HIL, HTL and ETL have been applied in Exam.-19 and Exam.-20. Comparing with Comp. Exam.-2 and Comp. Exam.-3, Exam.-19 has a better performance and Exam.-20 is similar to Comp. Exam.-3. The doping of inorganic inactive materials in HIL, HTL and ETL simultaneity can favor the balance of hole and electron and get an expected device.
| Number | Date | Country | Kind |
|---|---|---|---|
| 200710065095.5 | Apr 2007 | CN | national |
| 200710177325.7 | Nov 2007 | CN | national |
