Mediation of energy transfer on magnetic field effects in 4CzTPN-Ph-based light-emitting diodes

Abstract

4CzTPN-Ph is a kind of typical thermally activated delayed fluorescence (TADF) material with a small energy gap between its singlet and triplet levels of the charge-transfer states (CTs). In this paper, we used 4CzTPN-Ph as dopant and materials with different triplet exciton (T1) energy as host, hole-transporting layer (HTL) or electron-transporting layer (ETL) to fabricate a series of 4CzTPN-Ph-doped organic light-emitting diodes (OLEDs). The magneto-electroluminescence (MEL) and magneto-conductance (MC) from these devices were measured with different bias currents at room temperature, and with various temperatures at a fixed injection current. The experimental results demonstrated that when the materials with different T1 energy are selected as the HTL, ETL and doping host, the MEL and MC of these devices exhibited various line-shapes with different variation tendency within low magnetic fields (| B | 20 mT), respectively. When each functional layer of the OLEDs with high T1 energy, the amplitude of MEL curves showed abnormal behavior that reduced with decreasing injection currents at low magnetic field, and could convert from positive to negative. At the same time, the MC traces showed typical RISC process where the MC value was negative and rapidly enhanced with increasing magnetic field. However, when the HTL, ETL or host were selected with low T1 energy materials, the MEL and MC of devices showed weakened RISC process. Especially, when Alq3, whose T1 energy is close to that of 4CzTPN-Ph, was used as the ETL or host, the MEL and MC traces of devices directly presented the line-shape that was similar to that of pure Alq3-based fluorescent devices. By analyzing the energy transfer process of spin-pair states in these studied OLEDs, we found that the HTL, ETL and host materials with different T1 energy could limit the T1 energy of 4CzTPN-Ph with different intensity, causing different energy transfer channel and energy loss of T1 in devices. Consequently, MEL and MC of these devices were diverse from each other in low magnetic fields. Not only could this work enrich the understanding of the intrinsic physical mechanisms of 4CzTPN-Ph-based OLEDs, but also provide a theoretical reference for the controllable applications of T1 in TADF devices.