Improving the Efficiency of Solution-Processed, Blue Fluorescent Organic Light-Emitting Diodes (OLEDs) by Employing TADF Exciplex Hosts
CUI Can1,2, SONG Dan-dan1,2, ZHAO Su-ling1,2, QIAO Bo1,2, XU Zheng1,2*
1. Key Laboratory of Luminescence and Optical Information，Beijing Jiaotong University, Ministry of Education，Beijing 100044，China
2. Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing 100044，China
Abstract：In order to improve the efficiency of solution-processed, blue fluorescent organic light emitting diodes (OLEDs), we propose the use of exciplex hosts with thermal-activated delayed fluorescence (TADF). The TADF exciplex hosts can utilize upconverted singlet excitons by reverse intersystem crossing and then transfer the energy to the guest to improve efficiency of the blue fluorescent OLEDs, which enables the full utilization of triplet and singlet excitons. Here, blue fluorescent material 1-4-Di-[4-(N,N-diphenyl) amino]styryl-benzene (DSA-ph) issued as the guest emitting material, 4,4′,4″-Tris(arbazol-9-yl)t-riphenylamine (TCTA) doped 1,3,5-Tri(1-phenyl-1H-cbenzo[d]imidazol-2-yl)phenyl) (TPBi) was used as the TADF exciplex hosts, and the emitting layer was fabricated by solution process. From the photoluminescence (PL) spectra of TCBi, TPBi and TCTA doped TPBi films, it was found that the emission peak of TCTA doped TPBi film was significantly red-shifted compared to that of pristine TCTA or TPBi films (peak wavelength changes to 437 nm). Meanwhile, the spectrum broadens, which proves the existence of exciplex. The PL spectra of DSA-ph doped exciplex hosts and DSA-ph doped poly(methyl methacrylate) (PMMA) films were found to be the same and both of the photol-uminescence peaks were derived from DSA-ph, which proved that exciplex hosts transfer energy to DSA-ph. The absorption spectrum of DSA-ph overlaped greatly with the PL spectrum of exciplex hosts, which also proved that the exciplex hosts transfer energy to DSA-ph effectively. Time-resolved PL measurements were performed on exciplex hosts doped with different concent-rations of the DSA-ph guest. It was found that the lifetime of the DSA-ph doped exciplex hosts becomes longer compared to that of the pure DSA-ph. The lifetime of pure DSA-ph is only 1.19 ns. The fluorescence decay curve of DSA-ph doped exciplex hosts is similar to that of exciplex hosts, which further demonstrates that the exciplex hosts transfer energy to the DSA-ph. We investigated the effects of the presence of TADF exciplex hosts and the DSA-ph con-centration on the device performance. The parameters such as brightness, current density, voltage, current efficiency, electroluminescence spectra of the devices were measured. The perfor-mance of the OLEDs using exciplex hosts were notably improved, compared to standard OLE-Ds without exciplex hosts. In the condition of 10% DSA-ph, the luminescence increased from 2 133.6 cd·m-2 (for pristine DSA-ph) to 3 597.6 cd·m-2, and the current efficiency increased from 1.44 cd·A-1 (for pristine DSA-ph) to 3.15 cd·A-1. All of the electroluminescence peaks were only derived from DSA-ph. The concept of using TADF exciplex hosts provided a facile way to achieve high performance solution-processed blue fluorescent OLEDs.
Key words：Organic light-emitting diodes; Exciplex; Blue emission; Fluorescent; Solution pro-cess
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