Energy transfer from both triplet and singlet energy levels of PVK to DCM2 induced by heavy-ion

Abstract

The energy band of red light-emitting materials is usually very narrow, which easily results in non-radiative recombination of excited states. There also exists concentration quenching effect due to strong π-π interaction. To avoid this, host-guest doping system is mostly used. On the other hand, the ratio of singlet and triplet excited state caused by recombination is 1:3. In comparison with the fluorescence (singlet to singlet), phosphorescence (triplet to singlet, but spin-forbidden) is much weaker, and the quantum yield is much lower. To enhance it and make full use of triplet excited state energy, heavy atom effect is commonly used to induce strong spin-orbital coupling leading to mix of singlet and triplet and release the forbidden triplet energy. Based on this, we fabricated polymer light-emitting diodes adopting polyvinylcarbazole (PVK) as the host and a red fluorescent dye, 2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-pyrido[ 3,2,1-ij]quinolin-9-yl)-vinyl]-pyran-4-ylidene}-alononitrile (DCM2), as the dopant, and materials containing heavy-ion, kalium idode (KI) and bromo-carbazole, as energy transfer bridge to obtain complete energy transfer from excited states of both singlet and triplet energy level of PVK to ground state of singlet of DCM2. We found the current density of devices with heavy-ion materials were higher than device without it, and the weak blue emission from PVK host, existing in device of PVK:DCM2 device, can not be observed in electroluminescence spectra of device with heavy-ion materials, which indicates a complete energy transfer from both triplet and singlet energy levels.