Seven-member-ring-based electron-transporting materials for high-efficiency OLEDs

Abstract

Organic light-emitting diodes (OLEDs) have become the mainstream display technology and show potential in lighting. With a constant demand of higher efficiency and longer lifetime, the design and synthesis of better OLED materials are essential. Amongst these, electron-transporting materials (ETMs) consume about 30% of the applied energy. Developing ETMs with a high electron mobility and a high thermal stability is hence crucial. Seven-member-ring-based molecules are known to be highly thermally stable. They could be suitable for serving as ETMs if they also show good optoelectric characteristics. Herein, we have synthesized four seven-member-ring-based ETMs. A high efficiency was achieved for a green OLED by using 2′,12-di(pyridin-4-yl)spiro[dibenzo[3,4 : 5,6]cyclohepta[1,2-b]pyridine-9,9′-fluorene] (DPP) as ETM. At 1000 cd m−2, for example, the power efficacy (PE) was increased from 23 to 32 lm W−1, an increase of 39%, when the typical 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) was replaced by DPP. The corresponding device lifetime at 5000 cd m−2 was increased from 1.4 to 2.0 h, an increase of 43%. The high efficiency may be attributed to the facts that DPP has an electron mobility (7.2 × 10−5 cm2 V−1 s−1) higher than that of TPBi (2.2 × 10−5 cm2 V−1 s−1) and has a highest occupied molecular orbital (HOMO) level (−6.3 eV) deeper than that of TPBi (−6.2 eV). Electrical simulation confirmed that the higher electron mobility could lead to a 5-fold increase in carrier recombination rate and the deeper HOMO level would lead to an increase of 15% in carrier recombination rate. The lifetime enhancement may be attributed to the high glass transition temperature of DPP (181 °C), being 124 °C for TPBi. However, DPP did not work for blue phosphorescent emitters, such as bis[2-(4,6-difluorophenyl)pyridinato-C2,N](picolinato)iridium (FIrpic), because its triplet energy (2.54 eV) is lower than that of FIrpic (2.65 eV). However, a high-efficiency blue OLED was achieved by using 4-(spiro[dibenzo[3,4 : 5,6]cyclohepta[1,2-b]pyridine-9,9′-fluoren]-12-yl)benzonitrile (PC) with a higher triplet energy (2.67 eV). At 100 cd m−2, for example, its PE was increased from 13 to 19 lm W−1, an increase of 46%, when TPBi was replaced by PC.