High-efficiency blue fluorescent devices with simple structure

Abstract

Thermally activated delayed fluorescence (TADF) materials can harvest both singlet and triplet excitons for light emission without the utilization of heavy metal elements such as iridium and platinum and become the third generation organic optoelectronic materials after fluorescent and phosphorescent materials. TADF material bis[4-(9,9-dimethyl-9, 10-dihydroacridine)phenyl]sulfone (DMAC-DPS) shows high photoluminescence quantum yield in solid films, rendering it as a suitable emitter for non-doped organic light emitting diodes. We optimize the hole transport layer, electron transport layer and emission layer thickness for the none-doped DMAC-DPS devices to pursuit high luminance efficiency. We compare the properties of the devices using indium tin oxide (ITO)/MoO3 hole injection contact with either a 40 nm mCP or 30 nm NPB/10 nm mCP hole transport layer and the results indicate (1) the former devices show larger current than the latter devices for there is a large energy barrier of ca. 0.7 eV for hole injection from NPB into mCP in the latter devices; (2) Both of the devices show similar luminance efficiency and the underlying reason is that the small modulation of the dominant hole current slightly affects the device efficiency. In order to investigate the influence of different electron transport layers on the device performance, we have prepared the devices with either 10 nm DPEPO/40 nm TmPyPB, 50 nm TmPyPB or 50 nm SPPO13 electron transport layer and find out (1) the devices with DPEPO electron transport layer show the highest EQE of 16.2% due to the fact that DPEPO with triplet energy of 3.3 eV can confine excitons inside the emisson layer effectively; (2) The devices with SPPO13 possess the highest current density at certain voltage among the devices and the maximum external quantum efficiency and power efficiency for the SPPO13 based devices are 14.3% and 26.8 lm W - 1, which are higher than 13.8% and 22.2 lm W - 1 for the TmPyPB based devices. The possible reason is that highly polarized phosphorus oxygen groups of SPPO13 greatly improve the electron injection and as a result improve the carrier balance in the devices. Moreover, we compare the properties of the devices with a 15 and 30 nm emission layer and find that the devices with a 30 nm emission layer have slightly better luminance efficiency. High efficiency of the DMAC-DPS devices can be attributed to the effective conversion of triplet excited states to singlet excited states via reverse intersystem crossing (RISC) process. TADF sensitized fluorescence devices can achieve high luminance efficiency by using the well-established fluorescent materials with the advantages such as the superior stability, versatility and abundance. We report highly efficient blue TADF sensitized devices using DMAC-DPS as the host material and the traditional fluorescence blue-emitting material 2,5,8,11-tetra-tert-butylperylene (TBPe) as the guest material. The devices with 1.0% TBPe show the maximum external quantum efficiency and power efficiency of 12.7% and 22.9 lm W - 1, which are about 3 times higher than those of the conventional 2-methyl-9,10-bis(naphthalen- 2-yl)anthracene (MADN): TPBe devices. This can be attributed to the effective use of triplet excited states for light emission via RISC process and highly efficient Forster energy transfer process from DMAC-DPS to TBPe. Utilization of a TADF material as host material and sensitizer and a traditional fluorescence material as emitting dopant serves as a novel approach to achieve high-efficiency organic light emitting diodes.