Negative magnetic efficiency induced by Dexter energy transfer in coexistence system of exciplex and electroplex
Authors: Wu Yu-Ting , Zhu Hong-Qiang , Wei Fu-Xian , Wang Hui-Yao , Chen Jing , Ning Ya-Ru , Wu Feng-Jiao , Chen Xiao-Li , Xiong Zu-Hong ,   School of Physical Science and Technology, Chongqing Key Laboratory of Micro & nano Structure Optoelectronics, Southwest University, Chongqing 400715, China,   School of Physics and Electronic Engineering, Chongqing Key Laboratory of Optoelectronic Functional Materials, Chongqing Normal University, Chongqing 401331, China
Published: August 2022

Exciplex-type organic light-emitting diodes (OLEDs) are research focus at present, because of their 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, leading the magneto-electroluminescence [MEL, MEL = (ΔEL)/EL × 100%] effect values and the magneto-conductance [MC, MC = (Δ<i>I</i>)/<i>I</i> × 100%] effect values to be both positive, the amplitude of MEL to be greater than that of MC at the same current, and the corresponding magnetic efficiency [M<i>η</i>, M<i>η</i> = (Δ<i>η</i>)/<i>η</i> × 100%] values to be also positive due to the linear relationship EL <inline-formula><tex-math id="Z-20221116105031-1">\begin{document}$ \propto \eta\cdot I $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221116105031-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221116105031-1.png"/></alternatives></inline-formula> within general current (<i>I</i>) range. Surprisingly, although the MEL value of the device coexisting with exciplex and electroplex is also greater than the MC value at low current, MEL value is less than MC value at high current. In other words, M<i>η</i> value of this device undergoes a conversion from positive to negative with current increasing. In this work, to find out the reason why M<i>η</i> value of exciplex-type OLED formed by TAPC and TPBi shows a negative value under high current and also to study the micro-dynamic evolution mechanism of spin-pair states in this device, three OLEDs are fabricated and their luminescence spectra and organic magnetic field effect curves are measured. The results indicate that the electroplex is produced in the exciplex-type OLED formed by TAPC and TPBi. Since the triplet exciton energy of monomers TAPC and TPBi is higher than those of triplet charge-transfer states of exciplex (CT<inline-formula><tex-math id="Z-20221107140615">\begin{document}${}_3^{\rm{ex}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140615.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140615.png"/></alternatives></inline-formula>), and the CT<inline-formula><tex-math id="Z-20221107140631">\begin{document}${}_3^{\rm{ex}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140631.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140631.png"/></alternatives></inline-formula> energy is greater than the energy of triplet charge-transfer states of electroplex (CT<inline-formula><tex-math id="Z-20221107140638">\begin{document}${}_3^{\rm{el}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140638.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140638.png"/></alternatives></inline-formula>), the CT<inline-formula><tex-math id="Z-20221107140644">\begin{document}${}_3^{\rm{ex}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140644.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140644.png"/></alternatives></inline-formula> energy can only be transferred to CT<inline-formula><tex-math id="Z-20221107140650">\begin{document}${}_3^{\rm{el}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140650.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140650.png"/></alternatives></inline-formula> through Dexter energy transfer (DET) process without other loss channels. The electroluminescence (EL) spectrum of this device shows that the luminescence intensity of exciplex is greater than that of electroplex, which indicates that the quantity of exciplex is more than that of electroplex. Besides, EL spectra at different currents prove that the formation rate of exciplex is faster than that of electroplex with current increasing. Owing to less quantity of exciplex at low current, the DET process from CT<inline-formula><tex-math id="Z-20221107140657">\begin{document}${}_3^{\rm{ex}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140657.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140657.png"/></alternatives></inline-formula> to CT<inline-formula><tex-math id="Z-20221107140702">\begin{document}${}_3^{\rm{el}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140702.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140702.png"/></alternatives></inline-formula> is too weak to facilitate the RISC process of charge-transfer states of electroplex (CT<sup>el</sup>). Therefore, the low field amplitude of M<i>η</i> curve is positive at low current. The number of spin-pair states of exciplex increases with current increasing, which enhances the DET process. These processes of direct charge carriers trapped and energy transferred critically increase the number of CT<inline-formula><tex-math id="Z-20221107140712">\begin{document}${}_3^{\rm{el}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140712.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20221288_Z-20221107140712.png"/></alternatives></inline-formula> at high current, which greatly strengthens 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 current increasing. Furthermore, the M<i>η</i> curves of this device are measured when only exciplex exists and only electroplex exists in the employing filter, respectively. As expected, the results confirm the accuracy of the mechanism 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 mechanism in OLEDs and the law of interactions between excited states.