Organic light-emitting devices with a hole-blocking layer inserted between the hole-injection layer and hole-transporting layer
Y. Divayana, B. J. Chen, and X. W. Suna兲
School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798
K. S. Sarma
Aerospace Electronic Systems, Honeywell, 21111 N.19thAvenue, Phoenix, Arizona 85027
共Received 9 October 2005; accepted 30 January 2006; published online 23 February 2006兲 A hole-blocking layer 共HBL兲, 2,9-dimethyl-4, 7-diphenylphenanthroline 共BCP兲, was incorporated between the hole-transporting layer 共HTL兲and hole-injection layer for a tris-共8-hydroxyqunoline兲 aluminum-based organic light-emitting device. Such a structure helps to reduce the hole-leakage to the cathode resulting in improved current efficiency. Optimum BCP thickness of around 3 nm was observed to produce a current efficiency of 3.25 cd/ A, which corresponds to a 30% improvement compared to that of the standard device without BCP共2.5 cd/ A兲. Low operating voltage was also achieved by minimizing the thickness of the HTL. Both operating voltage and efficiency can be tuned by varying the thickness of HTL and HBL, respectively. © 2006 American Institute of Physics.关DOI:10.1063/1.2178581兴
High-performance organic light-emitting devices 共OLEDs兲 should have a low operating voltage, high effi- ciency and relatively good stability. For undoped OLEDs, various techniques are available to improve the efficiency, such as anode1,2 or cathode3 modification, hole blocking,4 doping,5 annealing6 and optical coupling.7 Cathode modifi- cation and doping, for example, have been shown to increase electron 共minority carrier兲 injection, so as to improve the electron-hole balance. As a result, the total current and effi- ciency can be improved simultaneously. On the other hand, modification of the anode, for example, has been shown to improve the current injection but sacrifices the efficiency, or vice versa.8 Insertion of a hole-blocking layer 共HBL兲 be- tween the hole-transporting layer 共HTL兲 and electron- transporting layer共ETL兲has also been shown to improve the efficiency but not the current.9In this letter, we shall report insertion of a HBL between the hole-injection layer 共HIL兲 and HTL to increase device efficiency without much sacrifice on the current injection.
In a typical OLED, the hole experiences a smaller barrier compared to the electron,10 moreover hole mobility in HTL is an order of magnitude higher than electron mobility in ETL.11 For example, in a typical indium tin oxide 共ITO兲/
NPB/Alq3/ Mg: Ag structure, where NPB is N,N’-Di共naphth- 2-yl兲-N,N’-diphenyl-benzidine with hole mobility of 10−5cm2/ Vs, and Alq3 is tris-共8-hydroxyquinoline兲 alumi- num with electron mobility of 10−6cm2/ Vs, the hole in- jected from the ITO side, experiences an overall energy bar- rier of around 1.7 eV, while electron needs to overcome a larger barrier of 2 eV before recombination. Such a device suffers hole leakage to the cathode, which decreases the ef- ficiency, while electron is more and less consumed before reaching HTL. One way to decrease hole leakage is by in- serting a HBL between the emission layer and ETL. How- ever, because the electron mobility in HBL is much lower
than that in ETL, addition of this layer in the ETL side has been shown to increase the operating voltage significantly.9
To increase device efficiency without increasing the volt- age significantly, we blocked the holes at the HTL side. The inset in Fig. 1共a兲shows the band diagram of our improved design. The accumulation of holes at the interface of HBL and HIL increases the electric field in HTL. To minimize the increase of the total voltage, a thin HTL was used. The thin HTL should not affect the efficiency, as luminescence oc- curred at the ETL side. This design improves hole-electron balance at the HTL-ETL interface, without significant volt- age shift. Tuning of current efficiency is possible by varying the thickness of HBL, which controls the hole injection, while voltage tuning can be realized by varying of the thick- ness of HTL, which drops the largest electric field. To obtain the same optical coupling, the total thickness of the organic layers was more or less kept constant in our experiment.
In this experiment, we used 4 , 4⬘, 4⬙-tris兵N,共3-meth- ylphenyl兲-Nphenylamino其-triphenylamine 共m-MTDATA兲 as the HIL with the highest-occupied molecular orbital 共HOMO兲 level of 5.11 eV,12 2,9-dimethyl-4,7-diphenyl- phenanthroline 共BCP兲 as HBL with HOMO level of 6.7 eV,13 NPB as HTL with the HOMO level and lowest- unoccupied molecular orbital 共LUMO兲 level of 5.6 eV and 2.4 eV,14 respectively, Alq3 as ETL with the HOMO and LUMO level of 5.7 eV and 3 eV,15 respectively, and trans- parent ITO and magnesium共Mg兲: silver共Ag兲as anode and cathode, respectively. Four devices with a structure of ITO/ m-MTDATA共80 nm兲/ BCP共X nm兲/ NPB共20 nm兲/Alq3 共100 nm兲/ Mg: Ag共300 nm兲, where X was chosen as 0, 3, 5, and 10 nm, were fabricated in the same runs.
The routine cleaning procedure, including ultrasonica- tion in acetone, ethanol, and rinsing in deionized water, was firstly carried out to clean ITO glass共50⍀/ square兲. Before deposition, the ITO was treated by oxygen plasma at 10 Pa for 2.5 mins. The deposition was carried out in a high vacuum condition of about 2⫻10−4 Pa. Except for HBL, all other materials were deposited simultaneously for four samples 共the same evaporation process兲.16 This avoids the
a兲Author to whom correspondence should be addressed; electronic mail:
APPLIED PHYSICS LETTERS88, 083508共2006兲
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uncertainties in comparing devices fabricated with different evaporation processes. Electroluminescence spectra of the fabricated devices were measured with a PR650 Spectra Scan spectrometer. Luminescence-current density-voltage characteristics were recorded simultaneously with the mea- surements of the electroluminescence共EL兲spectra by attach- ing the spectrometer to a programmable Keithley 236 source measurement unit. All measurements were carried out at room temperature under ambient atmosphere without any encapsulation of the OLED.
Figures 1共a兲 and 1共b兲 show the current density-voltage 共J-V兲 and luminescence-voltage 共L-V兲 curves, respectively, for devices with various BCP thicknesses and fixed thickness of other layers. Both the current density and luminescence shift toward higher voltage as the BCP thickness increases.
This is understood because the BCP layer blocks the hole and increases the voltage-drop across the BCP-NPB layers.
The inset in Fig. 1共b兲shows the corresponding current effi- ciency curves for devices in Fig. 1共b兲. The efficiency is maximized with a 3 nm thick BCP layer. This clearly shows that BCP controls the hole injection to the NBP/ Alq3 inter- face, and for a 3 nm thick BCP, holes should be in perfect balance with electrons. With 3 nm thick BCP layer, the cur- rent efficiency is around 3.25 cd/ A, corresponding to a 30%
increase compared to the standard device without BCP 共2.5 cd/ A兲. The current efficiency drops with further in- crease of BCP layer thickness to 5 and 10 nm. For these
thicknesses, most of the holes are blocked by BCP at the m-MTDATA and BCP interface, resulting in lesser holes at the NPB/ Alq3interface.
Figure 2 shows the operating-voltage as a function of BCP thickness at a fixed current density of 100 mA/ cm2. The inset in Fig. 2 shows the corresponding EL spectrum.
The voltage monotonously increases with the BCP thickness.
The nonlinear voltage-thickness relation indicates that the hole experiences a trapezoid barrier in BCP as shown in the inset in Fig. 1共a兲. For this type of barrier, the current injec- tion is not only dependent on the electric field, but also the thickness of the BCP layer.17It was reported previously that control of hole current could shift the emitting region throughout the organic layers.18 However, for all devices with less than 10 nm BCP in our experiment, the emission always originated from the Alq3layer共inset in Fig. 2兲.
In addition to the variation of BCP thickness, we also varied the thickness of the NPB layer while fixing the thick- ness of all the other layers. The structure used was ITO/
m-MTDATA共80 nm兲/ BCP共3 nm兲/ NPB共X nm兲/ Alq3 共100 nm兲/Mg: Ag共300 nm兲, where X was chosen as 5, 10, 15, and 30 nm. Figures 3共a兲and 3共b兲show theJ-V andL-Vcurves, respectively, for devices with various NPB thicknesses. It can be seen that, the current and luminescence curves shift toward lower voltage as the NPB thickness reduces. The de- vice with a 5 nm thick NPB even shows a better current profile compared to the standard one without BCP layer, as shown by Fig. 3共a兲. However, the current efficiency is inde- pendent on the NPB thickness, as shown in the inset in Fig.
3共b兲. This indicates that NPB layer does not reduce the hole current in the device. As a result, it can be realized that lower operating voltage can be obtained with a thinner NPB layer without affecting the current efficiency.
The inset in Fig. 3共a兲shows the operating voltage as a function of NPB thickness at a current density of 100 mA/ cm2. The voltage is linearly dependent on the NPB thickness, indicating holes in NPB layer experience a trian- gular barrier, instead of a trapezoid one as in BCP. For trian- gular barrier, current injection depends only on the electric field.17Based from the slope of the graph, the average elec- tric field in the NPB layer is about 0.11 V / nm at a current density of 100 mA/ cm2. This value is higher than the one in m-MTDATA and Alq3layers of about 0.058 V / nm, which is
FIG. 1. 共Color online兲Current density共a兲and luminance共b兲 vs voltage characteristics for devices with various thicknesses of BCP. The insets in共a兲 and共b兲show the band diagram of an OLED with a HBL inserted between HIL and HTL, and the current efficiency vs current density curves, respec- tively. The symbols in共a兲apply to共b兲as well.
FIG. 2. The operating voltage as a function of BCP thickness at a current density of 100 mA/ cm2for all four devices in Fig. 1. The insert shows the EL spectrum for all devices.
083508-2 Divayanaet al. Appl. Phys. Lett.88, 083508共2006兲
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estimated by dividing the total voltage drop with the total thickness of both layers.19 Obviously, large bandbending indeed occurred in NPB.
It is worth mentioning that further improvement can be achieved by optimizing the thickness of the m-MTDATA and Alq3layers. The improvement achieved by this technique is attributed to reduced surface-plasmon loss20 and optical in-
terference effect,21but not the electron-hole balance. So it is expected that the optimum thickness of the BCP layer re- mains to be 3 nm, independent on the thickness variations of the m-MTDATA and Alq3layers, although such optimization is believed to further improve the performance.
In conclusion, we have introduced a HBL between HIL and HTL to balance the electron and hole injection in OLED.
The efficiency and operating voltage can be controlled sepa- rately by varying the HBL and HTL thickness, respectively.
For the optimized structure, the current efficiency has been improved by 30% without any significant voltage shift compared to the standard device.
Financial support from Honeywell Foundation is gratefully acknowledged.
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FIG. 3. 共Color online兲Current density共a兲and luminance共b兲 vs voltage characteristics for devices with various thicknesses of NPB. The device without BCP layer is also shown for comparison共represented by star兲. The inserts in 共a兲 and 共b兲show the operating voltage as a function of NPB thickness at a current density of 100 mA/ cm2, and the current efficiency versus current density curves, respectively. The symbols in共a兲apply to共b兲 as well.
083508-3 Divayanaet al. Appl. Phys. Lett.88, 083508共2006兲
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