Suppressing the efficiency roll-off in thermally-activated-delayed-fluorescence—sensitized fluorescent OLEDs by triplet management under pulsed operation

Thermally-activated-delayed-fluorescence (TADF)—sensitized fluorescent organic light-emitting diodes (TSF OLEDs) can simultaneously exhibit high quantum efficiency and high color purity. However, the long-lived nature of excited states during a reverse intersystem-crossing (RISC) process may result in the accumulation and annihilation of triplets and consequently the efficiency roll-off in TSF OLEDs under high current density. In this study, we demonstrate that the pulsed operation of TSF OLEDs can suppress the efficiency roll-off by manipulating exciton dynamics to reduce the accumulation of triplets. Under pulsed operation, the on-cycle luminance of 5CzBN-based OLEDs is increased from 31 370 to 55 760 cd ${\mathrm{m}}^{-2}$ at an amplitude of 10 V, accompanied by an enhancement on on-cycle external quantum efficiency by approximately 100% at a high current density of 1 A ${\mathrm{cm}}^{-2}$. Exciton dynamics analysis on transient EL curves reveals that the triplet annihilation becomes predominated over the RISC process after the time delay of 1–2 µs, offering a time window for pulse operation. The competition between triplet annihilation and RISC is further investigated by the magnetoelectroluminescence measurement, which confirms that the pulsed operation suppresses the triplet annihilation and thereby improves the conversion from triplets to singlets through RISC. These results show that the on-cycle electroluminescence performance of various TSF OLEDs can be optimized by tuning the pulsed operation parameters according to exciton dynamics, and provide an alternative way to address the issue of efficiency roll-off in next-generation OLEDs.

Figure 1

(a) The device architecture and the energy-level diagram of micro TSF OLEDs. (b) Optical microscopic images of TSF OLEDs with various active areas. Scale bars are 10 µm. (c) The EL intensity profile of the TSF OLED at a current density of 1 A cm⁻². (d) The transient current and EL responses of TSF OLEDs under a pulse with an amplitude of 9 V. Pulse excitation: pulse width of 5 µs, repetition frequency of 10 kHz, duty ratio of 5%. The dots and lines represent the experimental data points and fitting curves, respectively. (e) J-V-L curves measured from TSF OLEDs under dc and pulsed operation. (f) Plots of EQE versus on-cycle luminance in TSF OLEDs under dc and pulsed operation. Pulse excitation in (e),(f): pulse width of 0.5 µs, repetition frequency of 100 kHz, duty ratio of 5%.

Figure 2

(a) Transient EL spectra of TSF OLEDs operated under pulsed operation with a width of 5 µs, a repetition rate of 10 kHz, a duty ratio of 5%, and a current density of 1 A cm⁻². (b) The EL spectrum of TSF OLEDs under pulsed operation at various times. (c) Simulated temporal density of polarons, singlets, and triplets in TSF OLEDs at 1 A cm⁻². (d) The rate of RISC and triplet annihilation in TSF OLEDs.

Figure 3

(a) MEL curves measured from the TSF OLEDs at various current densities. (b) The EL intensity and MEL curves of TSF OLEDs at various current densities. The MEL curve is measured at 300 mT. Pulse excitation in (a),(b): pulse width of 0.5 µs, repetition frequency of 100 kHz, duty ratio of 5%. (c) Time-resolved MEL of the EL response of TSF OLEDs at a current density of 1 A cm⁻². (d) Time-resolved MEL of the EL decay of TSF OLEDs at a current density of 1 A cm⁻². Pulse excitation in (c),(d): pulse width of 5 µs, repetition frequency of 10 kHz, duty ratio of 5%. (e) The MEL dependence on pulse widths of TSF OLEDs under electrical pumping at a current density of 1 A cm⁻². (f) Pulse-width-dependent MEL and EL intensity of TSF OLEDs at a current density of 1 A cm⁻².

Figure 4

(a) The current and EL responses of 2CzPN-sensitzing OLEDs under a pulse with an intensity of 9 V. Pulse excitation: pulse width of 5 µs, repetition frequency of 10 kHz, duty ratio of 5%. (b) Transient EL responses of 5CzBN- and 2CzPN-sensitizing OLEDs. (c) Pulse width versus current density, as a function of RISC rate simulated by exciton dynamics. (d) Plots of EQE versus on-cycle luminance in 2CzPN-sensitizing OLEDs under dc and pulsed operation. Pulse excitation: pulse width of 0.5 µs, repetition frequency of 100 kHz, duty ratio of 5%. (e) Plots of on-cycle luminance versus current density in 5CzBN- and 2CzPN-sensitizing OLEDs under dc and pulsed operation. (f) The improvement of on-cycle EL intensity under pulsed operation simulated by exciton dynamics.

Figure 5

The energy levels and energy transfer process in TSF OLEDs.