The Intermolecular Interaction In Organic Electroluminescence And Up-conversion Luminescence

Organic electroluminescence (EL) and photoluminescence (PL) have attracted much attention because of their particular advantages. Today, Organic light-emitting diode (OLED) has gradually realized industrialization as one novel display application, but some mechanisms are still existed and need further to be researched. The effects of intermolecular interaction on the efficiency of OLEDs and organic up-conversion luminescence were studied, respectively.Excimer is a compound between a excited molecule and a ground homogeneous molecule due to intermolecular interaction. In the emission spectrum, excimer will display a new, strong and wide, redshift emission band and generally will produce negative effect. We made use of excimers formed between DMQA molecules due to intermolecular interaction in DMQA doped OLEDs with NPB/AlQ double layers, and found that the external quantum efficiencies of heavily doped devices (doped concentration>0.8 wt% ) increase with the rise of current density.We compared EL spectra of doped devices with various weight ratio and EL spectra of doped device in different driving current density. With the rise of doped concentration, the intermolecular interaction between DMQA molecules will increase and produce a great deal of excimers. Simultaneously, the emission of excimers of heavily doped devices increases with the rise of current density, but the excimer emission of devices with low doped concentration keeps almost constant with current density. We can infer that the EQE increase is attributed to the dissociation of excimer species formed by interaction of higher concentration DMQA molecules into DMQA monomer excitons under high current density.In addition, the heaviy doped device behaves good temperature stability. EQE of 5.0 wt% DMQA doped device at 60℃not only increases with rising of current density but also is higher than that at room temperature in the whole current intensity range. Thus it could be concluded that the higher resistibility of Joule heat resulted from local larger current density could be another reason for the increase in efficiencies of DMQA doped devices with current density. The smaller effect of Joule heat at larger current density on EQE of DMQA doped devices is perhaps attributed to weaker singlet-heat annihilation.We found the up-conversion phenomenon of crystalline Rubrene in absence of any sensitizers. Under excitation of 980 nm laser, crystalline Rubrene shows the up-conversion luminescence (UCL) at 610 nm, which is different from the emission wavelength (550 nm) of Rubrene solution or film, while amorphous rubrene sample, such as, both polymethylmethacrylate (PMMA) film dispersed with rubrene and Rubrene chloroform solution, do not exhibit the UCL. The UCL intensity depends on the proportion of orthorhombic crystal phase, particle size, structural defects, and reabsorption of fluorescence in all samples.We think that UCL emission level is attributed to the excitonic state due to strong intermolecular interaction in rubrene crystal. The intermolecular interaction of rubrene crystal is more stronger than amorphous rubrene solution or film, and thus excitonic state can be formed. Simultaneously, rubrene crystal phase will affect the intensity of intermolecular interaction, therefore, the UCL intensity of orthorhombic rubrene is stronger than rubrene samples with other crystal phase. The mechanism of UCL process was also discussed in detail. According to the dependence of the integral UCL intensity on the excitation power density of 980 nm laser, a two-photon absorption feature in our UCL system was obtained.To modify the photoluminescence properties the Eu(DBM)3bath complexes were encapsulated into sub-nanometer pores of aluminosilicates zeolites L and Y using a combined method of pressure difference and wet impregnation. The thermostability and luminescence efficiency of the composite Eu-L are improved considerably and the lifetime becomes longer in comparison to the pure complexes because it contains less water and reduce the annihilation due to the intermolecular interaction between H2O and Eu(DBM)3bath molecules. Overall, zeolite L is a more ideal host material for modification of lanthanide complexes.