Organic light-emitting diodes under high currents explored by transient electroluminescence on the nanosecond scale
- 20 September 2011
- journal article
- research article
- Published by American Physical Society (APS) in Physical Review B
- Vol. 84 (11)
- https://doi.org/10.1103/physrevb.84.115208
Abstract
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-hydro-xyquinoline) (Alq) at high excitation densities. With the OLED active area reduced to m, current densities up to 800 A/cm 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. DOI: http://dx.doi.org/10.1103/PhysRevB.84.115208 ©2011 American Physical Society
Keywords
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