Photoemission study of the electronic structures of tris-(8-hydroxyquinoline) aluminum/Li2O∕Al interfaces

Abstract

The electronic structures of tris-(8-hydroquinoline) aluminum (Alq3)∕Li2O∕Al interfaces were studied using in situ ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS). The UPS and XPS spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interfaces. Inserting Li2O between Al and Alq3 led to the highest occupied molecular orbital (HOMO) of Alq3 shifting to a higher binding energy compared to that without Li2O, which resulted in an improved electron injection. We also observed that the magnitude of the secondary cutoff shift was almost identical to that of the HOMO shift with the insertion of Li2O. This implies that the energy level alignment depends on the interface dipole and ionization energy of the adsorbate. Additionally, a gap state was observed in the gap of Alq3, which is related to the interfacial reaction. The N 1s spectra revealed that there were destructive chemical reactions between Alq3 and Al, which could be prevented by inserting Li2O between them.

P‐213: Decomposable Precursors as Electron Injection Materials for High Performance Organic Light‐Emitting Devices

“…The higher brightness is attributed to the improved electron injection of electron and more balanced recombination of electrons and holes. The peak efficiency for LiBH 4 device was also about 10% higher the LiF device as shown in Fig. 1(c).…”

“…1(a), the IV curve of the LiBH 4 based device has a steeper rise than the control device with LiF/Al cathode. At 10V, the current densities were 6089 A/m 2 and 3556 A/m 2 for LiBH 4 and LiF device, respectively. The LiBH 4 based device also showed higher brightness at the same driving voltage.…”

Strong interfacial dipole formation with thermal evaporation of lithium cobalt oxide for efficient electron injections

“…15 In addition, we also performed in situ PES experiments to check if both lithium oxides, which are formed from the source of Li 2 O and LiCoO 2 , show identical electronic structures. Actually, they show identical electronic structures and the strong dipole on ITO substrate (not shown here because the electronic structures of Li 2 O are already well known 15,16 ). Therefore, we expect the same result if we use Li 2 O itself as the source of lithium oxide instead of LiCoO 2 .…”

“…With the strong chemical interaction between Alq 3 and alkali metals, a significant gap state emerges, which could deteriorate the device stability. [16][17][18] However, the gap state was hardly seen at the interface of Li 2 O/Alq 3 . Therefore, this weak interaction between Li 2 O and Alq 3 would also contribute to the extension of the device lifetime as well as the eD formation.…”

Power Conversion Efficiency Improvement of Planar Organic Photovoltaic Cells Using an Original Hybrid Electron-Transporting Layer

“…30 This lifetime increase is attributed to the fact that during deposition there is LiCoO 2 decomposition and formation of LiO, which has been reported to reduce the destructive interaction between the cathode and Alq 3 . 31 In the case of OPVs, similar lifetime improvement was obtained by substituting CsCO 3 with LiF, which delayed oxygen contamination of the organic materials. 32 It is concluded that oxides, such as LiO, CsCO 3 , etc., allow for the improvement of the lifetime of the devices.…”