The Spin Mixing Process Between Singlet And Triplet In Exciton-based And Charge-transfer Organic Light Emitting Devices

Organic light-emitting diodes(OLEDs) have great potential in the flat-panel displays and solid-state lighting because of their distinctive advantages that is, low cost, ease of fabrication, and great mechanical flexibility. Although the research on OLED has gone through more than 20 years, the extensive commercial applications still have some unresolved issues, e.g., the low Electroluminescence(EL) efficiency and the short lifetime. However, recently, a research hotspot has been concerned to develop highly efficient OLEDs employing the reverse intersystem crossing(RISC) of thermally activated delayed fluorescence(TADF) emitters.Up to date, most of TADF emitters were small-molecular-weight materials and needed to be doped into a host material, containing spatially separated donor(D) and acceptor(A) moieties. But there are still some challenges when such TADF materials were used. For example, small-molecular-weight materials were commonly used as dopants in device fabrication. Thereby, it put difficulties on them to adapt mass production because of the reproducibility of the optimum doping level that requires careful manufacture control. In this M.S. Dissertation, the TADF process has been achieved by using the copolymer formed from a D-π-A configuration with two kinds of different groups. The advantage of this copolymer is containing two different excited emission states simultaneously: localized exciton(LE) states and charge transfer(CT) states, without doping. The main purpose of this work is to study the spin mixing process between singlet(S) and triplet(T) by means of the coexistence of LE and CT states in copolymer-based OLEDs.Nevertheless, it is actually difficult to observe the spin mixing in OLEDs directly. Organic magnetic field effects(OMFE), i.e., the relative variation of electroluminescence(EL) intensity and current when the organic optoelectronic devices are submitted to the external magnetic field, can become a promising method to unravel abundant spin-related transport and excited processes in organic semiconductors, such as inter-conversion between S and T, exciton- exciton annihilation and triplet-charge reaction. In this work, the factors affecting the spin mixing process between singlet and triplet has been systematically analyzed by utilizing the OMFE, and combining electroluminescence、photoluminescence and transient PL decay profiles. This work has demonstrated that RISC is significant to improve the EL efficiency of fluorescent organic light-emitting devices. The main contents are listed as follows:(1) At first, the properties and application prospect of organic optoelectronic devices are briefly introduced in this paper, meanwhile, the research backgrounds and the problem of low EL efficiency of OLEDs are also described. The contents pave the way for a novel method that is put forward to improve the external quantum efficiency of devices. In the second chapter, we mainly presented the fabrication and measurement of OLEDs.(2) Herein, we report a solution-processable polyfluorene-based copolymer exhibiting RISC characteristic, PFOPV. It is featured by a D-π-A configuration with PFO as the D component, MEH-PPV as the A component. LE and CT states are indeed co-existed in PFOPV, which was proved by the DFT optimized geometry and the solvatochromic effect of PFOPV, The absorption spectra and transient PL decay profiles of PFOPV, PFO and MEH-PPV.(3) polymer light-emitting devices based on PFOPV PFO and MEH-PPV were fabricated, the microscopic differences of the two emission processes i.e., the LE and CT states, were investigated via OMFE. For PFOPV devices, the MEL can be either positive or negative, the sign is closely related to the measuring temperature, the bias voltage and the balance degree of the charge carrier injection. Whereas, for PFO and MEH-PPV devices, the MEL value are positive under any circumstances. Analysis shows that OMFEs’ differences of the two emission processes are caused by the combined effects of intersystem crossing and reverse intersystem crossing induced by hyperfine interaction. This study reveals that charge-transfer states are important in improving the luminous efficiency of fluorescent organic light-emitting devices.(4) polymer light-emitting device with structure of ITO/PEDOT:PSS/PFOPV/CsF/Al was fabricated, and the magnetic field effects on current(magnetoconductance, MC) were measured at different temperatures and bias voltages. We found that positive-negative inversion of MC could be tuned via modifying the ratio of LE to CT states. I.e., when the relative proportion of CT states was quite high in the device, the MC at both high fields(>40mT) and low fields(<40mT) showed significant negative effect. What’s more, the MC displayed a positive effect in the opposite case. The quantitative analyses on MC effect through empirical formula, reveal that the inversion of MC from positive to negative at low fields is confirmed to be resulted from the intersystem crossing and reverse intersystem crossing processes. However, the transition at high magnetic fields can be attributed to the different reaction channels of triplets and charges. Our results demonstrate that the reaction paths of triplet-charge interactions are strongly dependent on electron-hole separation distance in triplets, moreover, enable a novel method to engineer the MC effect in organic semiconductors. Eventually, we successfully inhibited the TQA process by adjusting the balance of the device carrier injection, and improved the efficiency of the device.