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 Alq3 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-excitedstate of DCJTB via Forster 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 Forster 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.