Negative magnetic efficiency induced by Dexter energy transfer in coexistence system of exciplex and electroplex

Abstract

Exciplex-type organic light-emitting diodes (OLEDs) are research focus at present because of the high-efficiency luminescence at low cost due to the reverse intersystem crossing (RISC, EX<sub>1</sub> ← EX<sub>3</sub>). Their microscopic processes usually exhibit intersystem crossing (ISC, PP<sub>1</sub> → PP<sub>3</sub>) process dominated by polar pairs, resulting in the magneto-electroluminescence[MEL, MEL=(DEL)/EL×100%] effect values and the magneto-conductance[MC, MC=(D<i>I</i>)/<i>I</i>×100%] effect values are all positive, and the amplitude of MEL is greater than that of MC at the same current; the corresponding magnetic efficiency[M<i>η</i>, M<i>η</i>=(D<i>η</i>)/<i>η</i>×100%] values are also positive due to the linear relationship EL ∝ <i>η</i>×<i>I</i> within general current (<i>I</i>) range. Surprisingly, although the MEL values of the device coexisting with exciplex and electroplex are also greater than the MC values at low current, MEL values are less than MC values at high current. In other words, M<i>η</i> values of this device undergo a conversion from positive to negative with increasing current. In this work, to find out the reason why M<i>η</i> values of exciplex-type OLED formed by TAPC and TPBi show negative under high current and study the micro-dynamic evolution mechanisms of spin-pair states in this device, three OLEDs were fabricated and their luminescence spectra and organic magnetic field effects were measured. The results indicate that the electroplex is produced in the exciplex-type OLED formed by TAPC and TPBi. Since the triplet exciton energies of monomer TAPC and TPBi are higher than the energy of triplet charge-transfer states of exciplex (CT<sub>3</sub><sup>ex</sup>), and the CT<sub>3</sub><sup>ex</sup> energy is greater than the energy of triplet charge-transfer states of electroplex (CT<sub>3</sub><sup>el</sup>), the CT<sub>3</sub><sup>ex</sup> energy can only be transferred to CT<sub>3</sub><sup>el</sup> through Dexter energy transfer (DET) process without other loss channels. The electroluminescence (EL) spectrum of this device shows the luminescence intensity of exciplex is greater than that of electroplex, which indicates that the amount of exciplex is more than that of electroplex. Besides, EL spectra at different currents prove that the formation rate of exciplex is faster than that of electroplex with increasing current. Less quantity of exciplex at low current, so the DET process from CT<sub>3</sub><sup>ex</sup> to CT<sub>3</sub><sup>el</sup> is too weak to facilitate the RISC process of charge-transfer states of electroplex (CT<sup>el</sup>). Therefore, the low field amplitude of M<i>η</i> curve is positive at low current. The number of spin-pair states of exciplex increases with increasing current, which enhances the DET process. These processes of direct charge carriers trapped and energy transfer critically increases the number of CT<sub>3</sub><sup>el</sup> at high current, which strongly boosts the RISC process of CT<sup>el</sup>. Therefore, the low field amplitude of M<i>η</i> curve changes from positive to negative with increasing current. Furthermore, the M<i>η</i> curves of this device were measured when only exciplex exist and only electroplex exist employing filter, respectively. As expected, the results confirm the accuracy of the mechanisms of the negative value of the total M<i>η</i> for this device. Obviously, this work contributes to the comprehension of the internal micro-physical mechanisms in OLEDs and the law of interactions of excited states.