Luminescence Dynamics of Alq 3 -Based Multilayer Structures in Terms of HOMO and LUMO Energy Discontinuity
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EXPERIMENTAL A s multilayer structures with type-II energy lineup with respect to HOMO and LUMO energy discontinuities , the combination of Alq 3 and cyclopentadiene (PPCP; 1,2,3,4,5-pentaphenyl-1,3-cyclopentadiene) or Alq 3 and diamine (TPD; N,N'-diphenyl-N,N'-bis -(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine) were taken as samples . The molecular structures and the energy diagrams of the organic materials used in the present work are shown in figure 1. The sample layers were fabricated by vacuum evaporation, or molecular beam deposition, at a back-pressure of 3×10-7 Torr. The deposition sources were set in separate quartz crucibles whose temperatures were independently controlled by coil heaters . The thickness of each organic layer was controlled by monitoring a quartz oscillator thickness monitor. A s a substrate, a glass coated with indium-tin-oxide (ITO) was used. It was kept at room temperature during deposition of the organic layers. In the most of experiments , the thickness of Alq 3 is kept at 5 nm while that of other materials (PBD, PPCP and TPD) was varied between 5 and 20 nm. Optical properties were investigated by conventional time-integrated PL and time-res olved PL (TRPL) measurements . Here, the TRPL was performed with a fast-scan streak camera in conjunction with a 25 cm monochromator. Pulsed excitation was provided by a beam from an optical parametric amplifier (OPA ), using a mode-locked Ti-sapphire laser as a seed laser and a yttrium-lithium-fluoride (YLF) laser as a pump laser. The wavelength and pulse width were 320 nm and 150 fs, respectively. The sample was kept in vacuum at room temperature for the measurement.
N O
2.3eV
2.7eV
2.8eV
HOMO 5.7eV
5.4eV
6.1eV
5.6eV
Alq 3
TPD
PBD
PPCP
O Al
N
LUMO 3.0eV A l q3
N
O
C H3
C H3 N
N
TPD N N H 3C H 3C
O
t-BuPBD
C
H 3C
PPCP
(a)
(b)
Figure 1. The molecular structures and the energy diagrams of the organic materials.
RESULTS AND DISCUSSIONS For the TRPL measurement of the type-I Alq 3(5 nm)/PBD(20 nm) ×20 multilayer sample , the decay curves of PL emission at the PL peak wavelength of Alq3 (510 nm) and PBD (390 nm) are shown in figure 2 by closed and open circles , respectively. Immediately after excitation, the PL emission from PBD is dominant compared to that from Alq 3. Then the PL emission from PBD decays and that from Alq 3 becomes stronger and dominant. Comparing the PL emission from PBD in the multilayer to that in the single layer of PBD, also shown in figure 2 by open triangles , it is clear that the former decays much faster than the latter. This implies rapid energy transfer of the excited states of PBD molecules to Alq 3, resulting in the enhanced PL from the Alq3 layer as we have reported in ref.[4]. The decay curves were approximated by a single exponential term, and the decay times for the emission from Alq 3 and PBD were calculated. The similar experiments were made for the type-II Alq 3(5 nm)/PPCP(5-20 nm) samples . The decay times are summarized in table I, together with those for Alq3/PBD. It is seen th
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