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 (h(+)) and electron (e(-)) transporting layers (HTLs and ETLs, respectively) are biased with microsecond to millisecond voltage pulses higher than a threshold value V-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 eta. The value of V-th coincides with the bias for maximum dc efficiency typically observed when eta is monitored vs 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. (C) 2000 American Institute of Physics. [S0003-6951(00)05616-3].