Charge-induced luminescence quenching in organic light-emitting diodes

We use charge modulation spectroscopy (CMS) in model holes-only devices to address the reason for decreasing quantum efficiency with increasing drive current in organic light-emitting diodes based on Alq doped with the red dye DCJTB. We are able to measure the density of DCJTB and correlate it with the diminution of the photoluminescence. The magnitude and DCJTB density dependence of the photoluminescence reduction are roughly consistent with quenching of the singlet-excited-state of DCJTB via Förster energy transfer to DCJTB. At low DCJTB densities, the quenching is stronger than theory would predict and we propose that singlet energy transport among DCJTB molecules extends the distance over which quenching by DCJTB is effective. Since the Förster mechanism can be responsible for charge-induced PL quenching as large as 70% even though DCJTB is very weakly absorbing where DCJTB singlets fluoresce, it may be difficult to avoid quenching by electrically generated species.