Exciplex Formations between Tris(8-hydoxyquinolate)aluminum and Hole Transport Materials and Their Photoluminescence and Electroluminescence Characteristics

Abstract

We investigated exciplex formations between tris(8-hydoxyquinolate)aluminum (Alq3) and hole transport materials (HTMs) that have various highest occupied molecular orbital (HOMO) levels (−6.0 to −5.1 eV). The investigation focused on the photoluminescence and electroluminescence characteristics of Alq3:HTM codeposited films. Intermolecular interaction between Alq3 and the HTMs correlated with the HOMO levels of the HTMs. In the HTMs with HOMO levels ranging between −5.3 and −5.1 eV, the exciplex formation was clearly observed. No appreciable exciplex formation was observed in the HTMs with HOMO levels between −6.0 and −5.4 eV. The HTMs with −5.5 and −5.4 eV of HOMO levels, which are in an equivocal gray zone, showed weak intermolecular interaction, leading to a slight decrease in Alq3 fluorescence efficiency without a significant spectral shift. The quenching of Alq3 fluorescence by such weak intermolecular interaction was signaled by an external electric field. We observed that this type of weak exciplex formation also affected the electroluminescence quantum efficiency of HTM/Alq3-based organic light emitting diodes (OLEDs); therefore, it is necessary to control this delicate interface interaction to maximize OLED performance.

White organic light emitting diodes based on DCM dye sandwiched in 2-methyl-8-hydroxyquinolinolatolithium

“…It can be seen that there is a red shift in the PL spectrum of the mixture in comparison to pure LiMeq PL spectrum, which can be attributed to the formation of exciplex between a-NPD and LiMeq. This type of red shift in EL in comparison to PL has also been observed and explained by Naoki Matsumoto et al in different HTL with Alq 3 based interface [30].…”

Solution‐Processable Thermally Activated Delayed Fluorescence White OLEDs Based on Dual‐Emission Polymers with Tunable Emission Colors and Aggregation‐Enhanced Emission Properties

“…The emerging 580 nm emission could be caused by exciplex excitons formed between P3 and TmPyPB. However, the PL spectrum of P3/TmPyPB (1:1) (Figure S12, Supporting Information) shows no 580 nm emission, which indicates that an exciplex is not formed between these two materials, nor between P3 and TCTA or TAPC . Therefore, we tentatively assign the 580 nm peak in EL to the effect of the electric field on the randomly oriented dipole moments of the TADF units, i.e., electroplex emission, or most probably due to the formation of an electromer, which cannot be observed in PL spectra.…”

Organic Light-Emitting Diodes with Heterojunction of Thiophene/Phenylene Co-Oligomer Derivatives

“…This peak is assigned to the (013) plane of the BP2T-CN crystal in which the molecular axis of BP2T-CN makes an angle of 25 against the substrate surface. Although possibility of a non-emissive exciplex formation 18 and a charge separation at the BP2T/BP2T-CN interface are also considered, this oblique orientation of BP2T-CN in the film deposited on the BP2T film results in lower PL intensity than in the single-layered BP2T-CN film in which the molecules are lying with smaller angles. Nevertheless, the obliquely lying orientation of BP2T-CN in the deposited on the BP2T film results in the EL spectrum resembling to the PL spectrum of the of BP2T-CN film due to higher surface-emitting intensity from the BP2T-CN layer than from the BP2T layer.…”