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

It is shown that when multilayer organic light-emitting devices (OLEDs) con taining hole (h(+)) and electron (e(-)) transporting layers (HTLs and ETLs, respectively) are biased with microsecond to millisecond voltage pulses hi gher than a threshold value V-th, the electroluminescence (EL) intensity in creases dramatically to a peak value which then relaxes to the lower dc val ue; 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 c oincides with the bias for maximum dc efficiency typically observed when et a is monitored vs V. This relation and the apparent absence of the transien t peak in single-layer OLEDs suggest that it is due either to internal fiel d redistribution processes in the ETL and HTL or to space charges, e.g., tr apped polarons which accumulate at the HTL/ETL interface, and quench the em itting singlet excitons. It is concluded that highly efficient OLED operati on may be achieved at high brightness by pulsed bias at an optimized duty c ycle. (C) 2000 American Institute of Physics. [S0003-6951(00)05616-3].