Regulation of cobalt phthalocyanine spin-state transition by magnetic field and temperature for exciton manipulation in OLEDs

This study focuses on the regulatory mechanism of magnetic field and temperature on the spin state transition of cobalt phthalocyanine (CoPc) molecules and its impact on the performance of organic electroluminescent devices (OLEDs). By constructing an OLED device with the structure ITO/MoO3/NPB/Alq3:CoPc/CsCl/Al, and combining optoelectromagnetic integrated characterization techniques, the regulatory mechanism of CoPc spin state transition on exciton distribution was systematically investigated within the range of 0–300 mT magnetic field and 145–295 K temperature. The experiment reveals that the magnetic field induces the transition of Co2+ ions from the low-spin state (LS, t2g5eg2) to the high-spin state (HS, t2g4eg3), which significantly enhances the exchange interaction (EI). This enhancement increases the generation proportion of singlet excitons from 25.00% to approximately 25.17% and effectively promotes the triplet-triplet annihilation (TTA) process. Magnetic electroluminescence (MEL) tests show that the MEL amplitude raises rapidly at low magnetic fields, tends to saturate at high magnetic fields, and decreases with the increase of voltage. Device A doped with CoPc enters the saturation state more easily than the undoped control device B. Electroluminescence spectra confirm that TTA is the dominant mechanism for exciton conversion in the device, and the magnetic field improves the luminous efficiency by enhancing TTA. The study of temperature effects indicates that at low temperature (145 K), the probability of exciton generation in the high spin state increases due to the difference in Boltzmann distribution, and the MEL amplitude increases by 43%. Meanwhile, the weakening of carrier thermal motion prolongs the spin coherence time, but the space charge effect at high voltage leads to the saturation and weakening of the MEL signal. This research provides a theoretical basis for the performance optimization of organic spintronic devices and reveals the influence mechanism of tunable spin states of CoPc on OLED performance.