The influence of doping concentration andtemperature on magnetic field effect of delayed fluorescence in organiclight-emitting diode

Up to date, both reverse intersystem crossing (RISC) and triplet–triplet annihilation (TTA) are effective approaches to improve the internal efficiency of fluorescent OLEDs, because they can make the best of triplet excitons to generate delayed fluorescence. The energy difference between singlet and triplet excited states ( D E S-T ) of special intramolecular charge transfer (CT) species is quite small, which make it posible for the participation of the CT species in RISC and TTA. When the RISC and TTA coexist in fluorescent OLEDs, the utilization of triplet excitons should be much better. The goal of this work was to demonstrate the feasibility of the coexistence. However, determining whether the coexistence happens in fluorescent OLEDs is not an easy task, because the general methods can hardly distinct the delayed fluorescence is originated from which approaches. Fortunately, both RISC and TTA are highly spin-dependent processes that can generate sizable magneto-electroluminescence (MEL) responses in OLEDs. Typically, the MEL response of RISC is a low-field effect, i.e., the MEL decreases rapid within the low-field regime (<5 mT) and then tends to saturation. Differently, the MEL of TTA exhibits a slight increase within the range of 20 mT but a remarkable decrease in the high-field regime (>20 mT). Hence, the MEL can be used as a novel tool to discern RISC and TTA in fluorescent OLEDs. Following such strategy, in this work, we firstly fabricated the devices by doping different concentrations of intramolecular CT states specie 4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-4-yl-vinyl)-4H-pyran (DCJTB) into phosphor host material 1,3-bis(9-carbazolyl)benzene (mCP) and then measured the MEL responses of these devices at room temperature. It exhibited that the sign of low-field (<5 mT) component turned from positive to negative as the doping concentration decreasing from 20% to 5%, this was same to the situation for the high-field ( B >20 mT) but was more prominent. These results are quite similar to the theoretical MEL curves of RISC in low-field (<5 mT) and TTA in high-field ( B >20 mT), respectively. Thus, we believe that the delayed fluorescence of DCJTB-doped devices is caused by RISC and TTA simultaneously. Moreover, the RISC and TTA rates increase as the doping concentration decreasing. Subsequently, more remarkable RISC-dominanted low-field and TTA-dominanted high-field decrease of MEL were observed with decreasing temperature from 300 to 20 K. It is attributed to the fact that low temperature can significantly enhance RISC and TTA. In summary, low doping concentration and low temperature are more favorable to the coexistence of RISC and TTA in the DCJTB-doped devices. This work has a certain reference value for the improvments of internal quantum efficiency of fluorescent OLEDs via enhancing the utilization efficiency of triplet.