Toward ultra-high efficiency tandem OLEDs: Benzothiazole-based bipolar hosts and specific device architectures

Tandem organic light-emitting devices (TOLEDs) satisfy the requirements of various applications owing to their high luminance, high efficiency, and long lifetime. A crucial factor in achieving optimal electroluminescence (EL) performance is the host material used for the emitting layer. In this study, a series of benzothiazole (BTZ)-based materials were linked via a phenyl bridge to three different electron-donor moieties, i.e., phenothiazine (PTZ), dimethylacridine (DMAC), and carbazole (CZ), named BTZ-PTZ, BTZ-DMAC, and BTZ-CZ, as bipolar hosts for green phosphors to realize ultra-high efficiency organic light-emitting devices (OLEDs). A Ir(ppy)3-based device based on a BTZ-CZ host is shown to exhibit a satisfactory peak efficiency of 28.0 % (98.8 cd A-1 and 111.6 lm W-1). Additionally, highly efficient green TOLEDs based on a BTZ-CZ host are demonstrated, and they are constructed with a specific charge generation layer and a TiO2 nanoparticle-based internal light extraction structure. The proposed TOLEDs achieved a maximum efficiency of 64.4 % (223.6 cd A-1 and 138.1 lm W-1) and a high maximum luminance of 24.9 x 104 cd m- 2, one of the highest levels of efficiency and luminance reported in the literature for Ir(ppy)3-based tandem devices. These results show that the ingenious BTZ-based bipolar hosts exhibit immense potential for the development of tandem OLEDs, thus facilitating the realization of various highperformance EL applications.