Two different implementation strategies for highly efficient non-doped fluorescent organic light-emitting diodes based on benzothiadiazole derivatives

Abstract

Organic light-emitting diodes (OLEDs) have been widely used in flat-panel displays and solid-state lightings because of their unique advantages. The non-doped red OLEDs with satisfactory external quantum efficiency (EQE) still remains challenging. In this work, a series of red emissive materials are successfully designed and synthesized by adjusting the spatial structure and the property of excited state. PyBZTPA is constructed by the combination of electron-withdrawing benzothiadiazole (BZ), bipolar pyrene[4,5-d]imidazole (PyI) and electron-donating triphenylamine (TPA) groups, which exhibits a red emission peaking at 609 nm. On the basis of molecular structure of PyBZTPA, PyBZmTPA is obtained by introducing two methyl substituents on TPA unit, which leads to an increased charge transfer (CT) component in the molecule with a weaker exciton binding energy, facilitating the enhancement of upper-level reverse intersystem crossing (RISC). Further attaching TPA with big steric hinderance at the N1 position of PyI affords compound TPyBZTPA, which aims to increase the molecular distortion degree and reduce the non-radiative transition to improve the photoluminescence quantum yield (PLQY). The results show that both strategies can effectively improve the performance of non-doped devices. As compared with the maximum EQE of 5.3% for PyBZTPA-based non-doped OLED, the maximum EQEs of non-doped devices based on PyBZmTPA and TPyBZTPA go up to 6.8% and 7.3%, respectively. Especially, the maximum current efficiency (CE), power efficiency (PE) and brightness of TPyBZTPA device are corresponding to 12.6 cd A−1, 10.4 lm W−1 and 22682 cd m−2. To the best of our knowledge, the non-doped device performance of TPyBZTPA is fairly good among emitters containing BZ unit with CIEy ≈ 0.4.