High efficiency and long device lifetime of organic light-emitting diodes using new electron-transporting materials with spirobifluorene groups

In this study, novel electron transport layer (ETL) materials, 2-([1,1′-biphenyl]-2-yl)-4-(dibenzo [b,d]furan-4-yl)-6-(9,9-diphenyl-9H-fluoren-4-yl)-1,3,5-triazine (DFDDT) and 2-(9,9′- spirobi [fluoren]-4-yl)-4-([1,1′-biphenyl]-2-yl)-6-(dibenzo [b,d]furan-4-yl)-1,3,5-triazine (SFDDT), were synthesized and their electrical and optical properties were investigated for application in blue organic light emitting diodes devices. The triazine core, known for its strong electron-withdrawing properties, was utilized as the central framework. To enhance both thermal stability and electron-withdrawing capability, dibenzofuran (DBF) was incorporated, while an o-biphenyl group was introduced to regulate molecular morphology. By incorporating a spirobifluorene structure, SFDDT significantly enhanced thermal stability while preserving the optical properties of DFDDT, achieving a high glass transition temperature (T_g, 141 °C) and thermal decomposition temperature (T_d, 436 °C). The electron mobility of n-doped ETL (N-ETL) measurements showed that SFDDT exhibited a value of 6.329 × 10⁻⁵ cm²V⁻¹s⁻¹, approximately 1.9 times higher than that of the commercial ETL material 3,3'-(5'-(3-(pyridin-3-yl)phenyl)-[1,1':3′,1''-terphenyl]-3,3''-diyl)dipyridine (TmPyPB). When applied as an ETL, the device demonstrated a current efficiency (CE) of 5.47 cd/A and an external quantum efficiency (EQE) of 6.73 % at 10 mA/cm², along with a significantly extended lifetime (LT₉₅) of 135 h—more than 20 times longer than that of TmPyPB. The molecular design strategy incorporating spirobifluorene in ETL materials not only presents a new approach for ETL material development but also confirms its applicability in long-lifetime applications such as large-screen televisions (TVs), information technology (IT) devices, and automotive display systems.