Chih-Hung Hsiao

Recombination zone in mixed-host organic light-emitting devices

In this letter, the authors had quantitatively investigated the recombination zone in the mixed-host (MH) emitting layer (EML) of an organic light-emitting device with different mixed ratios experimentally and theoretically. The MH-EML consisted of a hole-transport layer (HTL) and an electron-transport layer fabricated by coevaporation. When the mixed ratio of the HTL in the EML increases, the driving voltage increases then decreases; this can be well demonstrated by an electrical model with different carrier mobilities. A blueshift was also observed due to the solid state solvation effect combined with the exciton shift from the anode to the cathode side.

Blue phosphorescent organic light-emitting device with double emitting layer

In this paper, we demonstrated a blue phosphorescent organic light-emitting device (OLED) with a higher current-efficiency and a lower driving-voltage using conventional iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) doped in the double emitting layer (DEML), which consists of a hole- and an electron-transporting material, N,N′-dicarbazolyl-3,5-benzene (mCP) and 2,2′-bis[5-phenyl-2-(1,3,4)oxadazolyl]biphenyl (OXD), respectively. Compared to OLEDs with only single mCP- and OXD-EML, current-efficiency of the optimized DEML-OLED increases by 30.82% and 141.37%, combining with a voltage reduction of 0.34 and 0.59 V at 50 mA/cm2, which comes from the better charge balance in DEML.

Driving voltage reduction in white organic light-emitting devices from selectively doping in ambipolar blue-emitting layer

White organic light-emitting devices (OLEDs) consisting of ambipolar 9,10-bis(2′-naphthyl) anthracene (ADN) as a host of blue-emitting layer (EML) were investigated. A thin codoped layer of yellow 5,6,11,12-Tetraphenylnaphthacene (rubrene) served as a probe for detecting the position of maximum recombination rate in the 4,4′-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) doped-ADN EML. Due to the energy barrier and bipolar carrier transport, the maximum recombination rate was found to be close to but not exactly at the interface of the hole-transporting layer and the EML. With appropriate tuning in the thickness, position, and dopant concentrations of the codoped layer (rubrene:DPAVBi:ADN) in the EML, the device driving voltage decreased by 21.7%, nearly 2 V in reduction, due to the increased recombination current from the faster exciton relaxation induced by the yellow dopants. Among the advantages of introducing the codoped layer over conventional single-doped layers are the elimination of t...

Oxadiazole host for a phosphorescent organic light-emitting device

In this paper, we demonstrate a phosphorescent organic light-emitting device (OLED) with enhanced current efficiency (in terms of cd/A) based on an oxadiazole (OXD) derivative as the electron-transporting host of the emitting layer (EML) doped with a phosphorescent dopant, iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N, C2′] picolinate (FIrpic). The maximum current efficiency of OXD-based OLEDs was 13.0 cd/A. Compared to the phosphorescent OLED with a conventional hole-transporting host, 1,3-bis(carbazol-9-yl)benzene (mCP) with 11.1 cd/A in maximum current efficiency, 17.2% improvement was achieved. However, in terms of external quantum efficiency (EQE), the OXD- and mCP-based OLEDs were 4.01 and 4.66%, respectively, corresponding to a 13.9% decrease. Such a discrepancy can be understood from the electroluminescence (EL) variation. Contrary to the hole-transporting mCP, OXD exhibited electron transporting characteristics which shifted the recombination zone toward the anode. The optical interference e...

Study of the recombination zone of the NPB/Alq3 mixed layer organic light-emitting device

In this paper, we demonstrated methods for determining the recombination zone in a mixed-host (MH) organic light-emitting device (OLED). The host of the emitting layer material in this device consists of a hole transport layer and an electron transport layer fabricated by co-evaporation. By comparing the spectra shift between bilayer and MH OLEDs, the recombination position with different mixing concentration can be determined. It showed the recombination zone shifts from the anode to the cathode side with increasing NPB mixing ratio.

Aging-induced recombination zone shift in mixed-host organic light-emitting devices

In this paper, we have demonstrated the time-dependent distribution of recombination-rate of a mixed-host (MH) organic light-emitting devices (OLEDs) by co-evaporating an ultra thin red-emitting doped layer (probe). With various probe position, the intensity ratio of red to green directly indicates the exciton distribution in MH layer. If the position of probe insertion is that of maximum recombination-rate, the driving voltage is also reduced which can be explained by the increase of the recombination current. From spectral and J-V analyses, the maximum recombination-rate position is 10 nm to the hole transporting layer when MH-OLED is not aged. After 48 hours of the DC aging test, the changes in the red to green intensity ratio of different devices are different. After 96 hours aging, this ratio does not change further among all devices, indicative of the achievement of steady state of recombination-rate distribution. The organic materials degrade more when it locates near the maximum of the recombination-rate.

Blue phosphorescent organic light-emitting diode with triazole host achieving high current efficiency

We demonstrated a blue phosphorescent organic light-emitting diode with the maximum current efficiency and power efficiency of 57.4 cd/A and 51.6 lm/W, respectively, based on trizaole derivatives as the host of the emitting layer.

68.3: Achieving High Efficiency White Organic Light-emitting Diodes based on Transient Electroluminescence Analysis

In this paper, High efficiency white organic light emitting diodes (OLEDs) have been analyzed not only through steady-state characteristics but also transient measurement to understand the dynamics in OLEDs. After optimization, the quantum efficiency is reached to 12.88% at 1.04mA/cm2.

Degradation Induced Recombination-zone Shift in Mixed-host Organic Light-emitting Device

We investigate the degradation phenomena in a mixed-host organic light-emitting device by using an ultra thin red-emitting dopant as the probe for monitoring the recombination rate in the emitting layer. After the current stressing, the recombination peak shifts toward the hole-transport-layer side and the recombination zone becomes broader.

Recombination Zone Distribution and Current Enhancement in Amibipolar Organic Light-emitting Material

Recombination zone distribution can be determined by fitting the current-voltage characteristics and spectra of the ambipolar organic light-emitting device (OLED). Besides, a thin probe was inserted at different positions of the device to directly monitor the recombination distributions. Furthermore, it increases the recombination current and improves the power efficiency.