J. I. Lee

High efficiency organic light-emitting devices with Al/NaF cathode

An organic light-emitting device (OLED) with an Al/NaF (1.5 nm) cathode exhibited a highly enhanced performance comparable to the one with the best Al/LiF cathode. This suggests that NaF can be an alternative to commonly used LiF interlayer for improved OLED performance. Photoelectron spectroscopy studies revealed that the intensity of the gap states and the amount of the valence band shift at Al/NaF/Alq3 interface surpassed those of Al/LiF/Alq3 interface, suggesting that the observed performance improvement is directly related with these features.

Photoelectron spectroscopy study of the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline) aluminum interfaces

We have studied the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline)aluminum (Alq3) interface using UV and x-ray photoelectron spectroscopy [(UPS) and (XPS), respectively]. The UPS revealed that the valence peak shift occurred with MgF2 deposition before Al was deposited and was independent of the gap state formation. The XPS core level peaks indicated that the MgF2 strongly interacted with Al and O atoms in Alq3 even before Al was deposited, and the deposition of Al caused a slight change to the N 1s core level peak. These results indicate that the interaction mechanism in Al/MgF2/Alq3 is different from those found in Al/LiF/Alq3 and other metal/Alq3.

Energy level control for self-assembled InAs quantum dots utilizing a thin AlAs layer

Ground-state energy of InAs quantum dots (QDs) in the GaAs matrix can be changed significantly by introducing a thin AlAs layer (1 nm). The photoluminescence (PL) peak position of the QDs grown directly on the thin AlAs layer is blueshifted by 171 meV from that of the QDs grown without the AlAs layer. QDs grown on an additional GaAs thin layer on top of the AlAs layer have PL peaks systematically redshifted to lower energy as the GaAs layer becomes thicker. Time-resolved PL shows that the QDs have similar lifetimes, attesting to the fact that all the QDs grown in this way are of high quality, although the energy level change is large and a thin AlAs layer is introduced.

Photoelectron spectroscopy studies on the electronic structures of Al/RbF and Al/CaF2 cathodes for 8-hydroxyquinoline aluminium-based organic light-emitting devices

The electronic structures of Al/RbF/8-hydroxyquinoline aluminium (Alq3) and Al/CaF2/Alq3 interfaces were investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed that a significant valence band shift occurred by evaporating the thin fluoride layer on Alq3. However, the gap state formation and the evolution of N 1s core level spectra showed rather different trends, suggesting that the alkali fluoride and alkali–earth fluoride interlayer have different reaction mechanisms at the interface with the Al cathode. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF2. These results suggest that the complex charge transfer mechanism across the interface has considerably less effect on the enhancement of organic light-emitting device performance than the significant valence band shift as a result of deposition of interlayer materials.

Photoelectron spectroscopy studies of the electronic structures of Al/RbF and Al/CaF2 cathodes for Alq3‐based organic light‐emitting devices

Abstract The electronic structures of Al/RbF/tris‐(8‐hydroxyquinoline)aluminium (Alq3) and Al/CaF2/Alq3 interfaces were investigated using x‐ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). For both systems, the UPS showed a significant valence band shift following the deposition of the thin fluoride layers on Alq3. However, the formation of gap state in valence region and the extra peak N 1s core level spectra showed different trends, suggesting that the alkali fluoride and alkali‐earth fluoride interlayer have different reaction mechanisms at the interface between Al cathode and Alq3. In addition, the deposition of Al has considerably less effect on the valence band shift compared to the deposition of both RbF and CaF2. These results suggest that the charge transfer across the interface and the resulting gap state formation may have lesser effect on the enhancement of organic light‐emitting device performance than the observed valence band shift, which is thought to lower the electron injection barrier.

Recombination dynamics in n-AlxGa1−xAs/n-In0.5Ga0.5P type-II heterostructures

Recombination characteristics of n-AlxGa1−xAs/n-In0.5Ga0.5P type-II band line-up heterostructures are investigated using time-integrated and time-resolved photoluminescence (PL) measurements. It is observed that the decay time of the AlxGa1−xAs luminescence depends on whether or not the excitation photon energy, ℏωe, is larger than the In0.5Ga0.5P band-gap energy, Eg,InGaP. If ℏωe>Eg,InGaP, photoexcited holes in the AlxGa1−xAs and In0.5Ga0.5P layers are found to be in equilibrium within about 0.4 ns. The interface-related below-band-gap (BBG) PL shows a large blueshift as the excitation intensity is increased. The extremely long decay time of the BBG PL is attributed to the somewhat smaller wave function overlap between spatially separated, two-dimensional electrons and holes due mainly to the nonabrupt interfacial nature of the employed samples. The fast transient behavior of the BBG luminescence under high excitation intensity, as well as the peak energy blueshift, are explained by the band filling effect.