Journal
PHYSICAL REVIEW B
Volume 84, Issue 11, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.84.115208
Keywords
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Funding
- German Federal Ministry for Education and Research (BMBF) [FKZ 13N9279]
- German Research Foundation (DFG) [LE 747/37-1, LE 747/41-1]
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We investigate organic light-emitting diodes (OLEDs) comprising the singlet emitter system 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) doped into aluminium tris(8-hydroxyquinoline) (Alq(3)) at high excitation densities. With the OLED active area reduced to 100 x 100 mu m(2), current densities up to 800 A/cm(2) are achieved in pulsed operation. These devices exhibit an intense electroluminescence (EL) turn-on peak on the nanosecond time scale. With the help of streak camera measurements, we prove that the steady state EL of the fluorescent OLEDs is reduced due to singlet-triplet quenching. We demonstrate that short electrical pulses with a rise time of 10 ns make the separation of singlet emission and singlet-triplet quenching in time domain possible. By modeling the singlet and triplet population dynamics in the emission layer, we find that the triplet-triplet annihilation-rate coefficient in doped fluorescent materials is triplet-density dependent at high excitation density. The increased triplet lifetime usually observed in host: guest systems due to triplet trapping on guest molecules vanishes at high current densities. An increase in current density leads to an increased triplet-triplet annihilation rate, while the triplet density in the emission layer stays constant.
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