Transient Electroluminescence of Phosphorescent OLEDs with Mixed Host System

Hee-Kyung Kim*, Chang-Hee Lee** *Samsung Advanced Institute of Technology, Yongin, Korea

**Seoul National University, Seoul, Korea

Abstract The transient EL profiles for a electrophosphorescent OLEDs with the solution-processed and mixed-host emissive layer featured prominent overshoot peaks depending on the host compositions. These overshoot peaks showed exceptionally high intensity ratio and duration time ratio over the steady-state EL emission, reflecting the interfacial charge accumulation and the phosphorescent emission nature of devices. We propose that these intense overshoot phenomena can be utilized to increase the device efficiency and stability of OLEDs.

Author Keywords OLED; Transient Electroluminescence; Mixed Host; Solution Process; Overshoot; Phosphorescence

1. Objective and Background In OLEDs (Organic Light Emitting Devices), the

understanding of the carrier distribution and the recombination process in the emissive layer is critically important to get efficient recombination of electrons and holes, and to achieve the high luminous efficiency. The transient electroluminescence (EL) measurement has been widely used to investigate the luminescence mechanism of OLEDs, since it enables us to analyze the carrier transport process and recombination process separately in the whole luminescence process [1], [2].

In this letter, we have studied transient EL responses of phosphorescent OLEDs adopting the mixed-host emissive layer in order to investigate the carrier recombination mechanism. The use of mixed host system composed of electron transporting materials (ETMs) and hole transporting materials (HTMs) is a powerful approach to get balanced carrier distribution in the emitting layer (EML) [3],[4]. In the mixed-host-based OLEDs, we observed a prominent overshoot peak in the transient EL profiles, depending on the composition ratio of the HTM and the ETM, when the applied bias voltage decreased abruptly to zero. We propose that these large overshoot phenomena should be effectively utilized to get enhanced light efficiency of OLEDs, and we suggest the basic mechanism of these phenomena.

2. Methods The solution-processed OLEDs were fabricated and measured

following the procedure that we reported earlier [5]. Our devices have the structure of indium tin oxide (ITO)/hole transporting layer (HTL, 140 nm)/emitting layer (EML, 106 nm)/electron transporting layer (ETL, 58 nm)/LiF (0.8 nm) /Al(150 nm).

Fig.1 Device structure of OLEDs with mixed host. The HTL, poly (3,4-ethylenedioxythiophene): polystyrene

sulfonate (PEDOT:PSS) (CH8000, Baytron Co.) was spin coated on the ITO surface. An emissive layer comprising a red phosophorescent emitter, tris-(1-phenylisoquinolinato) iridium (III) (Ir(piq)3) (5 wt %) and a combined host system of 1,3,5-tris-(N –phenylbenzimidazol-2,yl) benzene (TPBI) and N,N’ -bis(1-naphtalenyl)-N-N’-bis(phenyl benzidine) (NPB) dissolved in dichloroethane was spin coated on the HTL. The mixing ratio of TPBI and NPB was varied as 10:0, 8:2, 7:3, 5:5, 3:7 and 2:8. The TPBI (ETL), LiF and Al were thermally evaporated on the EML consecutively. After encapsulation, we measured the EL performances of each device with a Spectra Colorimeter (PR 650, Photo Research Co.) and a source-measure unit (model 238, Keithley).

The transient EL characteristics were measured at room temperature with an oscilloscope under the application of square-wave voltage pulse (pulse width of 20 μs and a repetition rate of 10 Hz) from a pulse generator. The applied electric fields were set as bipolar type and unipolar type, in which the voltage amplitude ranges from -10V to +10V and from 0V to +10V, respectively.

3. Results and Analysis Fig. 2 shows luminous efficiency variation measured from the

devices with different host compositions. The device with TPBi-only host shows high turn-on voltage and low EL emission due to poor injection of holes and charge unbalance. As the concentration of NPB increases in the TPBi:NPB mixed host, the turn-on voltage decreases and the luminescence efficiency increases due to better hole injection and improved charge balance.

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Fig.2 LE variations depending on host compositions Fig. 3 shows the transient EL responses, normalized at the time

when the applied voltage decreases abruptly from the forward bias (+10 V) to the reverse bias (-10 V) for the devices with various TPBi:NPB composition ratios. The transient EL responses exhibit a large overshoot peak followed by the residual EL intensity which lasts for about 10s. Fig. 3 shows that the EL overshoot increases as the NPB concentration increases up to 3:7 TPBi:NPB ratio.

Under this bipolar voltage pulses, the EL overshoot intensity increases rapidly with increasing NPB concentration and shows a maximum (~3.1 times higher than the steady-state EL intensity) at the 3:7 TPBi:NPB ratio. Moreover, as the NPB concentration increases, the EL overshoot decay time decreases significantly.

Even though our devices are designed to facilitate charge injection into EML, there exists energy difference between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of EML and HTL or ETL. The overshoot effects in our devices can be attributed to the enhanced recombination of electrons and holes in the EML and back drifted charges once accumulated at interfaces.

Fig.3 Overshoot Peaks in Transient EL profiles

4. Impacts We have discovered that the mixed host based phosphorescent

OLEDs showed the transient EL overshoot peaks featuring exceptionally higher intensity ratio and duration time over the steady state EL emission than previous reports for the fluorescent OLEDs [6][7]. This prominent overshoot behavior including the host composition dependent intensity variation can be interpreted in terms of interfacial charge accumulation at the EML and the phosphorescent emission from emitter, Irpiq3 of devices. These overshoot peaks observed at decreased voltage periods implies that we can get enhanced power efficiency by optimizing the driving schemes of OLEDs. It is also known that reducing the space charge formation in the mixed EML devices enhances the device stability [8].

Consistently with this finding, if we adopt the driving scheme of utilizing the overshoot effect in our mixed host OLEDs, we can expect increased device life time by avoiding the harmful effect of space charges on the device stability as well as getting higher device efficiency. The use of mixed host system composed of electron transporting materials and hole transporting materials is a powerful approach to get balanced carrier distribution in the emitting layer. Especially these considerations should be quite interesting, since the mixed host system is a strong candidate for the commercial application of efficient organic electrophosphorescent devices. Therefore, the mechanism of this EL overshoot behavior in OLEDs with mixed host system should be studied more extensively to devise a novel driving scheme for OLEDs with increased efficiency and stability.

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[7] Z. Gan, R. Liu, R. Shinar, and J. Shinar, Appl. Physics. Lett. 97 113301 (2010).

[8] Y. Luo and H. Aziz, Appl. Physics. Lett. 95 073304 (2009).

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