Organic light-emitting diodes under high currents explored by transient electroluminescence on the nanosecond scale

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Organic light-emitting diodes under high currents explored by transient electroluminescence on the nanosecond scale

D. Kasemann, R. Brückner, H. Fröb, and K. Leo

Phys. Rev. B 84, 115208 – Published 20 September 2011

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 $_{3}$ ) at high excitation densities. With the OLED active area reduced to $100 \times 100 μ$ 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.

Received 3 May 2011

DOI:https://doi.org/10.1103/PhysRevB.84.115208

Authors & Affiliations

D. Kasemann^*, R. Brückner, H. Fröb, and K. Leo^†

Institut für Angewandte Photophysik, George-Bähr-Strasse 1, D-01062 Dresden, Germany

^*Daniel.Kasemann@iapp.de
^†http://www.iapp.de

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Issue

Vol. 84, Iss. 11 — 15 September 2011

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Images

Figure 1
Singlet and triplet population dynamics for 40 nm Alq $_{3}$ doped with 2 wt% of DCM at a current density of 1 kA/cm $^{2}$ (rate coefficients are summarized in Table ). (a) The electron mobility in the emission layer is set to $μ_{e} = 1 \times 10^{- 5}$ cm $^{2}$ V $^{- 1}$ s $^{- 1}$ , as given in the literature. (b) An increased mobility of $μ_{e} = 2 \times 10^{- 4}$ cm $^{2}$ V $^{- 1}$ s $^{- 1}$ due to the increased applied field is assumed. The closed symbols indicate the dynamics for the low $κ_{T T}$ of the host:guest system. The open symbols (dashed lines) have been calculated with a TTA rate of $κ_{T T} = 5.0 \times 10^{- 13}$ cm $^{3}$ /s, a value much closer to the one for pristine Alq $_{3}$ .Reuse & Permissions
Figure 2
A 100 nm $p$ -transport layer (MeO-TPD doped by 4 wt% F4-TCNQ) between two metal electrodes excited at different pulse lengths and duty cycles.Reuse & Permissions
Figure 3
(a) Emission spectra of an Alq $_{3}$ :DCM (2 wt%) OLED under pulsed excitation at different current densities (50 ns pulses, 1 kHz repetition rate). The $J$ - $U$ characteristic is given in (b), and the integrated area under the spectrum is plotted vs the input power in (c). A reduced amount of emission spectra is depicted here; the corresponding points in the $J$ - $U$ characteristics are indicated by the open red symbols. The emission intensity continuously increases with current density up to 800 A/cm $^{2}$ and shows no shift in emission wavelength.Reuse & Permissions
Figure 4
(a) Time-resolved EL signal of an Alq $_{3}$ :DCM (2 wt%) OLED in response to a 100 ns rectangular voltage pulse at different current densities. The thin black lines indicate the calculated behavior. (b) Time-resolved EL signal of an OLED with undoped Alq $_{3}$ EML at similar current densities and slightly longer pulses for comparison. Details are given in Sec. 5.Reuse & Permissions
Figure 5
Streak camera measurements combining electrical excitation (150 ns pulses) with an optical probe. The current density within the EL pulse is varied [(a) no EL, (b) 12 A/cm $^{2}$ , (c) 75 A/cm $^{2}$ ] while the optical-pump intensity is kept constant. The plot (d) shows line cuts at the times indicated by the red boxes in (a)–(c). The PL intensity is significantly reduced after an electrical excitation.Reuse & Permissions
Figure 6
A double pulse with a width of 50 ns and varying double-pulse delay is applied to the sample ( $J = 490$ A/cm $^{2}$ ). The EL signal of the second pulse is depicted here. The turn-on overshoot probes the residual triplet population and increases with increasing delay. The inset gives a plot of unity-normalized intensity of the EL-turn-on overshoot of the second pulse (normalized to the overshoot intensity of the first pulse) vs the double-pulse delay.Reuse & Permissions

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