Efficiency peaks in the transient electroluminescence of multilayer organic light-emitting devices

It is shown that when multilayer organic light-emitting devices (OLEDs) containing hole ${\mathrm{(h}}^{+})$ and electron ${\mathrm{(e}}^{-})$ transporting layers (HTLs and ETLs, respectively) are biased with microsecond to millisecond voltage pulses higher than a threshold value $V_{\mathrm{th}},$ the electroluminescence (EL) intensity increases dramatically to a peak value which then relaxes to the lower dc value; the relaxation time decreases strongly with increasing pulse amplitude. Since the current waveforms are essentially rectangular, the transient EL is proportional to the external quantum efficiency η. The value of $V_{\mathrm{th}}$ coincides with the bias for maximum dc efficiency typically observed when η is monitored vs $\mathrm{V.}$ This relation and the apparent absence of the transient peak in single-layer OLEDs suggest that it is due either to internal field redistribution processes in the ETL and HTL or to space charges, e.g., trapped polarons which accumulate at the HTL/ETL interface, and quench the emitting singlet excitons. It is concluded that highly efficient OLED operation may be achieved at high brightness by pulsed bias at an optimized duty cycle.