• Advances in nano research
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• v.1 no.3
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• pp.125-131
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• 2013

This study is aimed to calculate the radiative lifetime of Wannier-Mott excitons in nanoclusters of a narrow-bandgap semiconductor embedded in a wide-bandgap one. The nanocluster linear dimensions are assumed to be much larger than the radius of the exciton so that the latter is not destructed by the confinement potential as it takes place in small quantum dots. The calculations were carried out for an example of InAs nanoclusters put into the GaAs matrix. It is shown that the radiative lifetime of Wannier-Mott excitons in such clusters increases with the decrease of the cluster dimensions, this tendency being more pronounced at low temperatures. So, the creation of excitons in nanoclusters of a narrow-bandgap material embedded in a wide-bandgap one can be used to significantly prolong their radiative lifetime in comparison with that of excitons in a bulk semiconductor.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2010.06a
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• pp.39-39
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• 2010

The authors have an extensive study of photoluminescences for Zn-polar and O-polar faces of single-crystalline ZnO bulks. In the photoluminescence (PL) spectra at 10 K, Zn-polar and O-polar faces show a common emission feature: neutral donor-bound excitons and their longitudinal-optical (LO) phonon replicas are strong, and free excitons are very weak. However, in the PL spectra at room temperature (RT), Zn-polar and O-polar faces show extremely different emission characteristics: the emission intensity of Zn-polar face is 30 times larger than that of O-polar face, and the band edge of Zn-polar face is 33 meV red-shifted from that of O-polar face. The temperature dependence of photoluminescence indicates that the PL spectra at RT are closely associated with free excitons and their phonon-assisted annihilation processes. As a result, it is found that the RT PL spectra of Zn-polar face is dominated by the first-order LO phonon replica of A free excitons, while that of O-polar face is determined by A free excitons. This is ascribed that Zn-polar face has larger exciton-phonon coupling strength than O-polar face.

• 한국정보디스플레이학회:학술대회논문집
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• 2009.10a
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• pp.1133-1135
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• 2009

The rolls of triplet excitons in a polymer based photovoltaic (PV) device are investigated for improving the efficiency of PV devices. Generally, the thick photo-absorbing layer can improve the PV device efficiency by increasing the photon absorption. However, in case of PV devices with singlet excitons, the efficiency is limited by the short exciton diffusion length, which depends on the mobility and lifetimes of excitons. Therefore, using the triplet excitons, which have a higher mobility and longer lifetime, can solve the problem of premature exciton dissociation caused by the shorter singlet exciton diffusion length in the thick photo-absorbing layer. In this study, the triplet exciton dynamics of a conjugated polymer in a phosphorescent dye blended polymer PV device is investigated by photo-induced absorption, and PV devices performance at various concentrations of phosphorescent dye are is also evaluated.

• Journal of the Korean Physical Society
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• v.73 no.11
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• pp.1735-1743
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• 2018

Excitons are the most prominent features of the optical properties of monolayer transition-metal dichalcogenides(TMDC). In view of optoelectronics it is very important to understand the decay mechanisms of the excitons of these materials. Auger recombination of excitons are regarded as one of the dominant decay processes. In this paper the Auger constant of recombination is computed based on the approach proposed by Kavoulakis and Baym. We obtain both temperature dependent (from type A, A' processes) and temperature independent (from type B, B' processes) contributions, and a numerical estimate of theoretical result yields the value of constant in the order of $10^{-2}cm^2s^{-1}$, being consistent with existing experimental data. This implies that Auger decay processes severely limit the photoluminescence yield of TMDC-based optoelectronic devices.

• Current Optics and Photonics
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• v.2 no.6
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• pp.589-594
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• 2018

We have investigated the dimensional nature of center-of-mass exciton confinement states in a CdMnTe/CdTe/CdMnTe heterostructure, where the CdTe well is too wide (144 nm) to confine both electrons and holes but able to confine whole excitons in the center-of-mass coordinate. Fine multiple photoluminescence spectra with a few meV separation were observed at 6 K. From the thickness dependence of the transition rate, they were attributed to even numbered center-of-mass exciton confinement states (N = 2, 4, 6, ${\cdots}$, 18). Dimensionality of the center-of-mass exciton confinement states was also investigated in terms of temperature dependence of radiative decay time. At low temperatures (${\leq}12K$), we found that the ground state excitons are likely localized possibly due to the barrier interface fluctuation, resulting in a constant decay time (~350 ps). With increased temperature (${\geq}12K$), localized excitons are thermally released, giving rise to a linear temperature dependence of radiative decay time as an evidence of two-dimensional nature.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2004.04b
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• pp.56-59
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• 2004

The stochiometric composition of $AgGaS_2$ polycrystal source materials for the $AgGaS_2/GaAs$ epilayer was prepared from horizontal furnace. From the extrapolation method of X-ray diffraction patterns it was found that the polycrystal $AgGaS_2$ has tetragonal structure of which lattice constant $a_0$ and $c_0$ were 5.756 ${\AA}$ and 10.305 ${\AA}$, respectively. $AgGaS_2/GaAs$ epilayer was deposited on throughly etched GaAs (100) substrate from mixed crystal $AgGaS_2$ by the Hot Wall Epitaxy (HWE) system. The source and substrate temperature were $590^{\circ}C$ and $440^{\circ}C$ respectively. The crystallinity of the grown $AgGaS_2/GaAs$ epilayer was investigated by the DCRC (double crystal X-ray diffraction rocking curve). The optical energy gaps were found to be 2.61 eV for $AgGaS_2/GaAs$ epilayer at room temperature. The temperature dependence of the photocurrent peak energy is well explained by the Varshni equation, then the constants in the Varshni equation are given by ${\alpha}=8.695{\times}10^{-4}eV/K$, and $\beta$=332 K. From the photocurrent spectra by illumination of polarized light of the $AgGaS_2/GaAs$ epilayer, we have found that crystal field splitting $\Delta$ Cr was 0.28 eV at 20 K. From the PL spectra at 20 K, the peaks corresponding to free and bound excitons and a broad emission band due to D-A pairs are identified. The binding energy of the free excitons are determined to be 0.2676 eV and 0.2430 eV and the dissociation energy of the bound excitons to be 0.4695 eV.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2002.07a
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• pp.214-217
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• 2002

The internal quantum efficiency of organic light emitting devices(OLEDs) using fluorescent organic materials is limited within 25% because of the triplet excitons which can hardly emit light. So there has been considerable interest in finding ways to obtain light emission from triplet excitons. One approach has been to add phosphorescent compounds to one of the layers in OLEDs. Then triplet excitons can transfer to these phosphorescent molecules and emit light. In this study, multilayer OLEDs with phosphorescent emitter, Iridium complexes were prepared. The devices with a structure of ITO/TPD/Ir complex doped in the host material/Alq3/Li:Al/Al were fabricated, and its electrical and optical characteristics were studied. Using various Ir complexes and the host materials, we fabricated several devices and investigated the device characteristics.

• Proceedings of the Optical Society of Korea Conference
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• 2001.02a
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• pp.50-51
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• 2001

We investigate the effect of resonantly excited excitons on the optical-absorption of the n=1hh and n=2hh exciton resonances in GaAs quantum wells. Under quasistationary excitation condition using the pump and probe beam at tow temperature, we report the first observation of long range Coulomb screening by two dimensional exciton-exciton interaction, and discriminate unambiguously exciton bleaching between by long range Coulomb screening and the Pauli exclusion principle. (omitted)

• Proceedings of the Korean Vacuum Society Conference
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• 2016.02a
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• pp.225-225
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• 2016

PHOLED devices which have the structure of ITO/HAT-CN(5nm)/NPB(50nm)/EML(47nm)/TPBi(10nm)/Alq3(20nm)/LiF(0.8nm)/Al(100nm) are fabricated to investigate the diffusion length of the triplet exciton by using double-quantum-well(DQE) EML structure. To fabricate DQW structures, Ir(ppy)3(2% wt) and Ir(btp)2(8% wt) are used as green and red emission zones, respectively. In DQW structured EML, as shown in Fig. 1, 1nm thick layers of green and red emission zones are located middle of the EML, and the distance between these wells(x) is changed from 0nm to 10nm. As shown in Fig. 2, the emission spectra from DQW PHOLED devices are changed with different x. The intensity of the green emission(520nm) is decreased when x is decreased, and it goes to near zero when x=0nm. This behavior can be identified as the diffusion of the triplet excitons from Ir(ppy)3 to Ir(btp)2 by the Dexter energy transfer(DET). From the external quantum efficiency(EQE) of the red emission, as shown in Fig. 3, the diffusion length of the triplet excitons can be determined by the equation of DET rate, R=A Exp(-2RDA/L), where RDA is donor-acceptor distance and L is the sum of the van der Wals radii. As a result, the measured data of the red EQEs with different x are identified to theoretical result from the equation of DET rate(Fig. 4). From this results, we could confirm that the diffusion length of the triplet excitons can be determined by using DQW structure and this method is very useful to investigate the behavior of the excitons in PHOLEDs.

• Proceedings of the KIEE Conference
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• 2000.11c
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• pp.437-439
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• 2000

The internal quantum efficiency of EL devices using fluorescent organic materials is limited within 25% because of the triplet excitons which can hardly emit light. So there has been considerable interest in finding ways to obtain light emission from triplet excitons. One approach has been to add phosphorescent compounds to one of the layers in an EL device. Then triplet excitons can transfer to these phosphorescent molecules and emit light. In this study, multilayer organic light-emitting devices with phosphorescent emitter, tris (2-phenylpyridine)iridium ($Ir(ppy)_3$) were prepared. The device exhibited power luminous efficiency of 1.07 1m/W at the luminance of $61.6\;cd/m^2$ diriven at the voltage of 9 V and current density of $1.9mA/cm^2$. At the luminance of $100\;cd/m^2$, the luminous efficiency was obtained 1.05 lm/W with the voltage of 9.5 V and the corrent density of $2.8\;mA/cm^2$.