Triplet Energy Transfer between a Sacrificial PMP and Blue TPF2 Iridium Dopants Leading to Enhancement of OLED Device Performance

Abstract

In this study, we prepared phenylimidazole-based C^N-cyclometalated Ir­(III) complexes (DMP, TPF2) and a C^C-cyclometalated Ir­(III) complex (PMP), and investigated the energy transfer process by examining the intermolecular interactions between the two cyclometalated Ir­(III) complexes. In films doped with 3% Ir­(C^C)3 complex (PMP) and 15% Ir­(C^N)3 complex (DMP or TPF2), the PMP effectively induced energy transfer to the DMP or TPF2. This intermolecular energy transfer process was investigated using a picosecond time-resolved emission spectroscopic method. In the case of mixing PMP with DMP, where two types of luminescence were observed at 470 and 580 nm, the emission at 470 nm was due to DMP, while the emission at 580 nm can be assigned as the intermolecular exciplex emission. By contrast, in the case of mixing PMP with TPF2, the emission at 465 nm corresponding to the PMP emission region decreased for 18.5 ns, while the emission at 530 nm corresponding to TPF2 increased. This emission can be attributed to the energy transfer from PMP to TPF2. In addition, no change was observed in the longer wavelength region than the TPF2 emission region for 10 μs. We analyzed the energy transfer process when PMP was added to the dopant (DMP and TPF2) and found that TPF2 was more efficient than DMP in the device without PMP doping, but it showed performance deterioration in high current density (>1 mA/cm2) owing to activation of fluorinated ligands. Finally, it was confirmed that the operation lifetime and efficiency of the device were improved by doping 3% of PMP in emissive layer (EML)