Effect of triplet multiple quantum well structures on the performance of blue phosphorescent organic light-emitting diodes
- Seokjae Lee†1,
- Jaryong Koo†1,
- Gunwoo Hyung†1,
- Donghwan Lim1,
- Donghyung Lee1,
- Kumhee Lee†2,
- Seungsoo Yoon2Email author,
- Wooyoung Kim3 and
- Youngkwan Kim1Email author
© Lee et al; licensee Springer. 2012
Received: 16 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
We investigate multiple quantum well [MQW] structures with charge control layers [CCLs] to produce highly efficient blue phosphorescent organic light-emitting diodes [PHOLEDs]. Four types of devices from one to four quantum wells are fabricated following the number of CCLs which are mixed p- and n-type materials, maintaining the thickness of the emitting layer [EML]. Remarkably, such PHOLED with an optimized triplet MQW structure achieves maximum luminous and external quantum efficiency values of 19.95 cd/A and 10.05%, respectively. We attribute this improvement to the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation which occurs within each EML. It also shows a reduction in the turn-on voltage from 3.5 V (reference device) to 2.5 V by the bipolar property of the CCLs.
Due to their high efficiency, phosphorescent organic light-emitting diodes [PHOLEDs] are promising light-emitting materials in organic light-emitting diodes [OLEDs]. An internal quantum efficiency of 100% could be realized in red and green PHOLEDs [1, 2]. However, the performance of blue PHOLEDs still needs to be improved for lighting applications. Light emission in PHOLEDs depends on the properties of the organic material in the devices [3, 4]. In particular, the energy level of the charge transport, host, and emitter materials influences the light-emitting efficiency. Besides, many different device architectures have attempted to improve the light-emitting efficiency of PHOLEDs. Hole and electron blocking layers or triplet exciton blocking layers [TEBLs] in PHOLEDs were introduced to confine both carriers and excitons within emitting layers [EMLs] . A double emitting layer structure was also employed in OLEDs by utilizing phosphorescent materials doped in two different hosts. As a result, these ways were effective in providing higher efficiency in PHOLEDs .
Another way to achieve high efficiency in OLEDs is to confine excitons inside the EML using the multiple quantum well [MQW] structure . Only a few reports concerning the MQW structure with good carrier and exciton confinement ability have been presented on OLEDs until quite recently. For example, Qiu et al.  improved the charge balance by utilizing an organic MQW structure to decelerate hole transportation. Huang et al.  used MQW structures to increase the carrier recombination efficiency, where both charges and excitons were confined to the EMLs. Park et al.  and Kim et al.  also reported similar triplet MQW structures. Recently, Liu et al.  proposed a non-doping EML method, instead of a host-emitter doping method, to improve the efficient triplet exciton confinement effect and the suppression of triplet-triplet annihilation that occurs via a single-step long range (Forster-type) energy transfer between excited molecules.
In this paper, we demonstrate efficient blue PHOLEDs by using iridium(III) bis[(4, 6-di-fluorophenyl)-pyridinato-N,C2']picolinate [FIrpic] doped in N,N'-dicarbazolyl-3, 5-benzene [mCP] with charge control layers [CCLs] as an MQW structure. The device architecture was developed to confine excitons inside each EML and to manage triplet excitons by controlling the charge injection. A stacked recombination zone structure, which can prevent triplet quenching processes and triplet exciton confinement within recombination region, was designed, and its performance was compared with that of blue devices. In addition, a mixed CCL, which has a p-type mCP and an n-type 2, 2',2"-(1, 3, 5-benzenetryl)tris(1-phenyl)-1H-benzimidazol [TPBi], reduced the turn-on voltage and enhanced efficiencies by prohibiting triplet exciton diffusion out of each EML.
A glass substrate coated with a 180-nm-thick indium tin oxide [ITO] layer has a sheet resistance of 12 Ω/sq. The ITO was cleaned with acetone, methanol, distilled water, and isopropyl alcohol in an ultrasonic bath. The pre-cleaned ITO was then treated with O2 plasma with the conditions of 2 × 10-2 Torr, 125 W, and 2 min. All organic layers were sequentially deposited onto the substrate without breaking the vacuum at a pressure of 5 × 10-7 Torr, using thermal evaporation equipment. The deposition rates were 1.0 to 1.1 Å/s for organic materials and 0.1 Å/s for lithium quinolate [Liq]. Finally, the aluminum cathode was deposited at a rate of 10 Å/s. The deposition rates were controlled with a quartz crystal monitor, and the doping concentrations of the emitters were optimized. After the organic and metal depositions, the devices were encapsulated in a glove box with O2 and H2O at concentrations below 1 ppm. A desiccant material consisting of barium oxide powder was used to absorb the residue moisture and oxygen in the encapsulated device. The devices had emission areas of 3 × 3 mm. The voltage, luminance, luminous efficiency, external quantum efficiency, power efficiency, and current density characteristics were measured and immediately recorded with a Chroma meter CS-1000A (Konica Minolta Holdings, Inc., Chiyoda, Tokyo, Japan). The current and voltage were controlled with a measurement unit (model 236, Keithely Instruments Inc., Cleveland, OH, USA).
Results and discussion
The electrical characteristics for blue PHOLEDs
Current density (mA/cm2)
Turn-on voltage (V)
Luminous efficiency max. (cd/A)
Quantum efficiency max. (%)
In conclusion, the present study reports on the high efficiency blue PHOLEDs based on a carrier and triplet exciton confinement inside recombination zones by using a triplet multiple quantum structure. The triplet energies of mCP and TPBi are higher than those of FIrpic. Therefore, triplet multiple quantum structures with CCLs exhibited efficient carrier and triplet exciton confinement within each EML. Moreover, CCLs can play a role in carrier movement with ease and triplet exciton blocking as expected from high triplet energy levels. In the electrical characteristic results of blue devices, the properties of device C with three recombination zones were found to be superior to the properties of devices A, B, and D. We attribute such high efficiencies and reduced turn-on voltage to the following two advantages caused by the triplet MQW structure: (1) efficient charge and exciton confinement effect by CCLs and TEBLs with high triplet energy level and (2) charge transportation balance in each EML by CCLs with bipolar property. The described MQW device concept may be useful to fabricate highly efficient devices for future OLED display and lighting applications.
This work was supported by the ERC program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Ministry of Education, Science and Technology (MEST; no. 20100009882).
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