Efficient blue organic light-emitting device based on N, N ⬘ -di „ naphth-2-yl … - N, N ⬘ -diphenyl-benzidine with an exciton-confining structure
Y. Divayana, X. W. Sun,a兲 and B. J. Chen
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore
G. Q. Lo and C. Y. Jiang
Institute of Microelectronics, 11 Science Park Road, Singapore Science Park II, Singapore 117685, Singapore
K. R. Sarma
Honeywell, 21111 N. 19th Avenue, Phoenix, Arizona 85027-2708
共Received 12 July 2006; accepted 6 September 2006; published online 27 October 2006兲
A blue organic light-emitting device with improved efficiency and excellent color purity is reported 共Commission Internationale de’l Eclairage coordinates ofx= 0.1659 andy= 0.0772 at 5 V兲, where N, N⬘-di共naphth-2-yl兲-N, N⬘-diphenyl-benzidine共NPB兲, a traditional hole-transporting layer, was used as the emission layer. A significant increase in efficiency was achieved by confining the excitons within the NPB layer by two wide-band-gap hole-blocking layers sandwiching the NPB layer. This structure also increases the direct exciton formation at the NPB layer by promoting electrons to cross the NPB layer, responsible for further efficiency improvement. Optimized structure showed an external quantum efficiency of 1.38%, which accounts for a 25% increase compared to a standard device. © 2006 American Institute of Physics.关DOI:10.1063/1.2364161兴 Organic light emitting devices共OLEDs兲offer numerous
advantages such as low operating voltage, high luminance efficiency, excellent material properties共flexible兲, and most importantly simple fabrication on any substrates at room temperature. There are a lot of reports on green and red emitting OLEDs being optimized by various techniques, namely, the anode1or cathode2modification, organic-organic interface modification,3 doping,4 annealing,5 and optical coupling.6However, efficient saturated blue-emitting OLED, in particular, remains a challenge for many researchers. The main challenges faced in blue OLED are, among them,共1兲 suitable wide-band-gap materials with saturated blue color, 共2兲 confining the high-energy exciton, 共3兲 carrier confine- ment issues, and of course共4兲the weak photopic response in blue region. Recently, there are some reports on enhanced blue-emission OLED achieved by applying new emitting materials,7 wide-band-gap host for blue dopant,8 and exciton-confining structure;9 however, color purity still re- mains a problem.
N, N⬘-di共naphth-2-yl兲-N, N⬘-diphenyl-benzidine 共NPB兲 is normally used as a hole transporting layer共HTL兲. On the other hand, NPB is a deep blue material emitting at 420 nm with a full width at half maximum of 44 nm共the smallest among all blue materials兲.10,11 Recent reports showed that blue OLED utilizing NPB can be improved by adding a buffer such as 4, 4⬘, 4⬙-tris兵N 共3-methylphenyl兲- N-phenylamino其-triphenylamine 共m-MTDATA兲.12,13 More- over, the usage of hole-blocking materials, such as tetra 共b-naphthyl兲silane or boron-containing material, is effective in improving the efficiency up to 50%, mainly due to reduc- tion of exciplex emission.14,15In this letter, we shall report an exciton-confining structure for NPB-based blue OLED. Our
optimized structure showed a 25% improvement in external quantum efficiency共EQE兲.
The routine cleaning procedure, including ultrasonica- tion in acetone, ethanol, and rinsing in de-ionized water, was first carried out to clean indium tin oxide 共ITO兲 glass 共50⍀/ sq兲. Before deposition, the ITO was treated by oxy- gen plasma at 10 Pa for 2.5 min. Evaporation of organic ma- terials and metals was carried out in a high vacuum condition of about 2⫻10−4 Pa. Electroluminescence 共EL兲 spectra of the fabricated devices were measured with a PR650 Spectra Scan spectrometer. Luminance–current density–voltage 共L- J-V兲 characteristics were recorded simultaneously with the measurements of the EL spectra by attaching the spectrom- eter to a programmable Keithley 236 source measurement unit. We assumed the emission pattern was Lambertian, and calculated the EQE from the luminance, current density, and EL spectrum. All measurements were carried out at room temperature under ambient atmosphere without any encapsu- lation.
The structure of our improved device is shown in the inset of Fig. 1共a兲, with m-MTDATA as hole-injection layer 共HIL兲, 2,9-dimethyl-4, 7-diphenylphenanthroline 关BCP 共I兲兴 as the hole-blocking layer共HBL兲, NPB as the HTL and blue- emitting layer, BCP 共II兲 as the second HBL, and tris-共8- hydroxyquinoline兲 aluminum 共Alq3兲 as the electron- transporting layer. Carrier and exciton confinements were achieved by sandwiching the emission layer with two wide- band-gap HBLs. This structure minimizes diffusion of the high-energy exciton toward the electrodes, hence reducing the surface-plasmon absorption. Addition of a BCP 共I兲 in- creases the electric field at the HTL,3thereby increasing the probability of direct exciton formation at NPB layer.
To optimize the structure, two separate experiments were conducted. For the first study, the thickness of HBL near the cathode side关BCP共II兲兴was optimized with the HBL near the anode side关BCP共I兲兴 absent, while varying the thickness of BCP 共II兲. For the first set of experiment, the structure of
a兲Author to whom correspondence should be addressed; also at: Institute of Microelectronics, 11 Science Park II, Singapore 117685, Singapore; elec- tronic mail: [email protected]
APPLIED PHYSICS LETTERS89, 173511
共
2006兲
0003-6951/2006/89共17兲/173511/3/$23.00 89, 173511-1 © 2006 American Institute of Physics Downloaded 10 Nov 2010 to 155.69.4.4. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
ITO/m-MTDATA共30 nm兲/ NPB共20 nm兲/ BCP共II兲 共x兲/ Alq3 共50−xnm兲/ Mg: Ag was studied. The value ofxwas varied from 5, 10, 20, 30 to 40 nm. In the second study, we opti- mized the thickness of BCP共I兲 by keeping the thickness of BCP 共II兲 at 20 nm. Devices with structure of ITO/m-MTDATA共30 nm兲/ BCP 共I兲 共x兲/ NPB共20 nm兲/ BCP 共II兲 共20 nm兲/ Alq3 共30 nm兲/ Mg: Ag was studied, where x was varied from 0, 2, 5 to 10 nm.
For the first experiment without BCP共I兲, Figs.1共a兲and 1共b兲 show the current density versus voltage and emission spectra for devices with various BCP共II兲thicknesses, respec- tively. It can be seen that the voltage shifts monotonically toward higher value as the thickness of the BCP共II兲 layer increases. The increase in voltage is attributed to two factors:
共a兲 the much lower electron mobility of the BCP共II兲 com- pared to the Alq3 and 共b兲 the existence of small barrier for electron at the Alq3/ BCP 共II兲interface.12 From Fig. 1共b兲, it can be seen that the emission mainly takes place at the NPB layer 共NPB emission is peaking at 420 nm兲. With a thick BCP共II兲layer, excitons are well confined at the NPB. When the thickness of the BCP共II兲layer is reduced, Alq3emission starts to emerge. In the inset of Fig.1共b兲, we plotted the EQE for device with various BCP共II兲thicknesses. The efficiencies are almost fixed for a BCP共II兲thickness of 20 nm or above, and decreases quickly for thinner BCP共II兲layer. The appear- ance of Alq3 emission and reduction in EQE show that, for thin BCP共II兲layer, some excitons in the NPB layer transfer their energy to Alq3 molecules via Foster energy transfer.16 As Foster energy transfer is a nonradiative process, it will slow down the overall radiative-emission rate therefore in- troducing a loss 共reducing EQE兲.16 The EQE drops from 1.11% to 0.83% at current density of 10 mA/ cm2for device with 20 and 5 nm of BCP共II兲layer, respectively.
Figures2共a兲and2共b兲show the current density and lumi- nance versus voltage 关the second experiment with BCP 共II兲 thickness fixed at 20 nm兴 for devices with various BCP共I兲
thicknesses, respectively. Introduction of BCP共I兲 results in the increase in the operating voltage; this is understandable because holes are blocked at them-MTDATA/BCP共I兲inter- face and the electric field in NPB layer is increased accordingly.3 It can be seen from Fig. 2共b兲 that the device with 2 nm of BCP 共I兲 shows the best performance with a maximum luminance of 1070 cd/ m2at a current density of 265 mA/ cm2. The inset of Fig.2共a兲shows the EQE for de- vices with various BCP共I兲 thicknesses. It can be seen that, for the OLED with 2 nm of BCP共I兲 layer, there is a slight increase in the EQE from 1.11% to 1.38% at a current den- sity of 10 mA/ cm2, corresponding to a 25% improvement compared to the one without BCP 共I兲 layer. This modest increase can be attributed to two factors: better exciton con- finement and increase in direct excitons formation in NPB layer. In the absence of BCP共I兲layer, the high energy NPB excitons can easily diffuse to them-MTDATA layer, and ab- sorbed by the anode. This is becausem-MTDATA has a simi- lar band gap with the exciton energy. By introducing a 2 nm BCP 共I兲 layer, the diffusion is minimized and excitons are better confined in NPB layer. For our blue OLED, there are electrons and holes piling up on both sides of NPB/BCP共II兲 interface. Therefore, excitons can be formed in both BCP共II兲 and NPB layers. The excitons formed at the BCP共II兲 layer can transfer its energy to the NPB layer whereby final radia- tive emission occurred. The energy transfer process from ex- cited BCP共II兲 to NPB may reduce the device efficiency.16 The increase in electric field in NPB due to insertion of a 2 nm BCP共I兲enables more electrons to cross the NPB/BCP 共II兲 barrier, directly facilitates excitons formation in NPB 关excitons formed in BCP共II兲is reduced兴, therefore resulting in higher EQE. As the thickness of the BCP共I兲 is further increased, more holes are blocked at them-MTDATA/BCP 共I兲interface, increasing the electric field in NPB furthermore.
For devices with BCP共I兲 thicknesses of 5 and 10 nm, elec- trons start to leak out to them-MTDATA/BCP共I兲interface.
The presence of electron at this interface is evidenced from
FIG. 1.共a兲Current density vs voltage characteristics and共b兲EL spectra for devices with various thicknesses of BCP共II兲layer. The insets in共a兲show the proposed band diagram for the improved device. Inset in共b兲shows the EQE for devices with various BCP共II兲 thicknesses. The symbols in共a兲 apply to共b兲as well.
FIG. 2. Current density共a兲and luminance共b兲vs voltage characteristics for devices with various thicknesses of BCP共I兲layer. The insets in共a兲and共b兲 show the EQE vs current density and EL spectra curves, respectively. The symbols in共a兲apply to共b兲as well.
173511-2 Divayanaet al. Appl. Phys. Lett.89, 173511共2006兲
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the emission spectra, the inset in Fig.2共b兲, from which we can see that as the BCP共I兲thickness increases, a new peak appears at 504 nm. While the new peak resembles Alq3spec- trum, it cannot be attributed to the Alq3emission, as the BCP 共II兲layer near the cathode is very thick, which prevent any direct exciton formation or Foster energy transfer. It can be seen from Fig.2共a兲that EQE drops from 1.38% to 0.63% at a current density of 10 mA/ cm2 for devices with 2 and 10 nm of BCP 共I兲, respectively. The emerging peak at 504 nm and the reduction of the efficiency for a thick BCP 共I兲 are due to inefficient exciplex emission,17,18 which is originated from radiative transition of excited excitons in BCP共I兲 to the ground state of the m-MTDATA molecules.
The radiative rate of the exciplex is slow and inefficient be- cause of the weak overlap in the wave functions between the excited BCP共I兲and ground state m-MTDATA molecules.
Figure3 shows the change in emission spectra with ap- plied voltage for the optimized device with 2 nm BCP 共I兲 and 20 nm BCP共II兲. There is a slight increase in exciplex emission as the voltage increases, resulting in lower effi- ciency. The inset of Fig.3shows the change in the Commis- sion Internationale de’l Eclairage共CIE兲 coordinates for the optimized device with applied voltage. The optimized device shows a deep pure blue close to the deepest blue of the National Television System Committee 共NTSC兲 standard with CIE coordinates of x= 0.14 and y= 0.08.10 Moreover, there is only a slight variation in CIE coordinates when the applied voltage changes from 5 V共x= 0.1659, y
= 0.0772兲to 10 V共x= 0.1747, y= 0.1059兲.
To verify the exciplex emission at them-MTDATA/BCP 共I兲interface, we conducted another experiment with a struc- ture of ITO/m-MTDATA 共50 nm兲/ BCP 共50 nm兲/ Mg: Ag.
Figure 4 shows the photoluminescence 共PL兲 spectra of the m-MTDATA, BCP, and EL spectra of the device. It is clear that the EL spectrum does not resemble any of the constitu- ent materials, indicating that the emission occurred via exci- plex at them-MTDATA/BCP interface. The inset of Fig. 4 shows the EQE of the exciplex emission. Due to the weak overlap between the excited BCP exciton wave function with that of the ground statem-MTDATA molecule, exciplex ra- diative emission is inefficient, explaining the drops in effi- ciency for device with thick BCP layer共Fig. 2兲. The maxi- mum EQE for the exciplex emission is only 0.2%共inset in Fig.4兲.
In conclusion, we have introduced an exciton confining structure which was used to realize a more efficient blue emission from NPB. The device with 2 and 20 nm of BCP共I兲
and BCP共II兲, respectively, shows the best performance with improved efficiency and luminance. A significant improve- ment in EQE共25%兲was achieved. Further improvement can be done by utilizing a different cathode such as LiF / Al to inject more electrons, optimizing the thickness of the HIL and ETL for optimum output coupling, and incorporating better emitter, HBL, and ETL. The color coordinate of our OLED is close to that of the deepest blue specified by the NTSC standard and insensitive to applied voltage. The ap- pearance of exciplex emission was also observed at m-MTDATA/BCP interface. The exciplex emission is re- sponsible for the reduced efficiency in OLED with thick BCP共I兲layer.
Financial support from Honeywell is gratefully acknowl- edged.
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FIG. 4. Spectral ofm-MTDATA共PL兲, BCP共PL兲, andm-MTDAT共EL兲. The Inset shows the EQE vs current density for device with structure of m-MTDAT/BCP.
173511-3 Divayanaet al. Appl. Phys. Lett.89, 173511共2006兲
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