Energy and charge transfer in organic light-emitting diodes: A soluble quinacridone study

A soluble derivative of quinacridone, ${\mathrm{N,N}}^{\prime }$ -di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Förster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline $({\mathrm{Alq}}_3\mathrm{).}$ Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with ${\mathrm{Alq}}_3$ and DIQA. Both PL and EL showed an efficiency enhancement in films of ${\mathrm{PVK:Alq}}_{\mathrm{3}}\mathrm{:DIQA}$ compared to films of ${\mathrm{PVK:Alq}}_3.$ However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Förster energy transfer from ${\mathrm{Alq}}_3$ to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that electron transfer from ${\mathrm{Alq}}_3$ to DIQA, hole transfer from PVK to DIQA, and hole transfer from ${\mathrm{Alq}}_3$ to DIQA are all energetically favorable processes. These results suggest that charge transfer is an important mechanism in the efficiency enhancement seen in OLEDs based on a host–dopant scheme, and that both the electronic properties and the optical properties of the dopant material are important parameters for device optimization.