Electroluminescence spikes, turn-off dynamics, and charge traps in organic light-emitting devices

The strong electroluminescence (EL) spikes and long tails occurring at the turn-off of a bias pulse in high-quality guest-host small molecular organic light-emitting devices are described and analyzed. The observed behavior of the spikes is found to be in excellent agreement with a model based on correlated charge pairs, with average charge pair separation of 20-50 Angstrom. Immediately following the spike is a long EL tail which decays as 1/t. This tail is due to the recombination of initially independent charges. Hence the EL decay is determined by the dynamics of trapped charges in the recombination zone.