Design and synthesis of host materials based on spirofluorene and s-triazine moieties and the applications in organic light-emitting devices

Abstract

Organic light-emitting devices (OLEDs) attract intensive attentions from both fundamental and applied researchers due to their unique properties of flexibility, light-weight, color-tunability, high electroluminescent efficiency, and ease of device fabrication. The design of the emitting layer (EML) of OLEDs plays a dominant role in ruling the electroluminescent performances. Typically, the EML is composed of a host and an organic emitter. As the emerging emitters for OLEDs, thermally activated delayed fluorescence (TADF) materials are capable of utilizing 100% of the electrically generated excitons. However, the device performances are also limited by charge injection from the electrodes, charge balance in the EML, and the outcoupling schemes. In this contribution, we designed four host materials for the TADF emitter, which were synthesized based on spirofluorene and s-triazine moieties. The physical and chemical properties were characterized via thermogravimetric analysis, differential scanning calorimetry, UV-vis absorption, fluorescence, phosphorescence, cyclic voltammetry measurements. It is found that the four host materials exhibited excellent thermal stability and the peaks of the UV-vis absorption located between 280 and 355 nm, which led to freedom of self-absorption. From the low temperature phosphorescent spectra (77 K), we can determine that the peak wavelengths are 505, 519, 522, 525 nm respectively for DTSF-2-DP-TRZ, SF-2-DTDP-TRZ, SF-4-DTDP-TRZ, and SF-4-TDP-TRZ, which make them ideal as a host for green TADF emitters. To evaluate their performances in the electroluminescence, these host materials were blended with the green TADF emitter, i.e., 2,4,5,6-tetrakis(carbazole-9-yl)-1,3-dicyanobenzene (4CzIPN) with nearly 100% photoluminescence quantum yield, as the emitting layers processed by spin-coating. The used glass substrate was covered with pre-patterned indium tin oxide (ITO). After routine cleaning procedures, the ITO glass substrate was treated with UV-ozone for 20 min. And then the conducting polymer poly(styrene sulfonic acid)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS) was spin-coated onto the substrate, following with thermal anneal in the glove box. Later, the EML was spin-coated onto PEDOT:PSS from chlorobenzene solution. To improve the charge balance and thus high radiative recombination, we deposited the additional hole blocking layer, electron transporting layer, and electron injection layer onto the EML via vacuum process. By comparing the electroluminescent performances of the devices with the four hosts, we figure out the structure-property relationship. It is well known that the host materials played roles in ruling the exciton utilization. The electroluminescent spectra of the devices are similar with a slight deviation of the peak wavelengths, i.e., 516, 516, 502, and 506 nm respectively for the case using DTSF-2-DP-TRZ, SF-4-TDP-TRZ, SF-2-DTDP-TRZ, and SF-4-DTDP-TRZ as the hosts, which could be ascribed to the different polarity of the host-guest environment and the slightly incomplete of host-guest energy transfer. In our investigation, we found that the host DTSF-2-DP-TRZ exhibited high glass transition temperature, well-match energy levels, and bipolar charge injection. Therefore, the device with DTSF-2-DP-TRZ rendered a maximum brightness of 6600 cd/m2, a maximum current efficiency of 51.3 cd/A, and a maximum external quantum efficiency of 16.6%, which are superior to those of the devices with the other hosts. This work provides a guidance for exploring host materials for efficient OLEDs with TADF as the emitter. Further works on material and device optimization would lead to better electroluminescent performances to meet the requirements of mass production.