Fluorescence Lifetime of Organic Thin Films Alternately Deposited with Diamine Derivative and Aluminum Quinoline

Abstract

Fluorescence lifetimes of organic thin films alternately deposited with diamine derivative (TPD) and aluminium quinoline (Alq3) were measured. The alternating deposition structure was shown to enhance the emission from Alq3 in spite of imperfect film structures. Energy transfer from TPD to Alq3 was evidenced by the correlation between lifetime and period number. It was found to be a competitive process with the TPD radiation process, and its rate of TPD radiation was estimated to be 1.2×1010 s-1. The authors suggested that the presence of the region mixed with TPD and Alq3 at the TPD/Alq3 interface plays an important role in the electroluminescence (EL) process. The Alq3 exciton confinement was clarified experimentally since the component of the faster fluorescence lifetime of Alq3 increased in the short-wavelength region. In addition, the authors proposed a possible EL model based on the suppression of the thermal inactivation of Alq3 excitons during their diffusion.

Delayed fluorescence and phosphorescence of tris-(8-hydroxyquinoline)aluminum (Alq3) and their temperature dependence

“…After the laser is turned off, the intensity of spontaneous PL from the singlet states decreases very fast due to their short lifetime of about 10 ns [1,13,22,23] and only the triplet state whose lifetime is often in the range of several milliseconds, is still populated; thus after just 1 ms no spontaneous fluorescence is present any more. However, even after a delay time Dt of 4 ms, a weak PL is still observed.…”

Electroluminescence in organics

“…This should produce a blue shift of the absorption and emission maxima of the structure. Indeed such a shift has been observed in a system of alternating layers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and 3,4,7,8-naphthalenetetracarboxylic dianhydride (NTCDA) [133], and N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (TPD) and Alq 3 [134]. Figure 9 shows the basic characteristics of a PTCDA/NTCDA multilayer structure.…”

“…As a rule the multilayer EL cells exhibit the narrower and blue-shifted emission spectrum compared with that for the DL cell ( figure 42(a)), the EL intensity peaking at a layer thickness of about 10 nm (figure 42(b)). The spectral shift was originally interpreted as being due to the confinement of charge carriers and excitons within a narrow emitter layer according to equation (41) [46,57] although the thickness-controlled relaxation time of the primary excited Alq 3 molecules is suggested to account for the blue shift in thinner Alq 3 layers in multilayer structures [134]. Both explanations seem to be unclear and poorly documented.…”

“…The superlattice effect on the exciton energy should thus be considered as quite a probable explanation for the spectral shift in the multilayer EL structures. Although a quantitative evaluation of the shift as a function of thickness and number of junctions await exact elaboration, generally, a few (two to three) nearinterface molecular layers undergo a noticeable energy shift [134]; copyright publication board Japan. J. Appl.…”

Thermal and Structural Properties of the Organic Semiconductor Alq 3 and Characterization of Its Excited Electronic Triplet State

“…As in most cases the S0-S1 transition is an allowed transition, the lifetime of the S1-state is very short. For Alq3 it was measured to be about 12ns [3,29,53,54]. On the other hand, the S0-T1 transition is a socalled forbidden transition and thus the lifetime of the T1 state is expected to be several orders of magnitude larger.…”