Achieving high-efficiency non-doped blue organic light-emitting diodes: charge-balance control of bipolar blue fluorescent materials with reduced hole-mobility

Abstract

We found an unusual way in improving electroluminescence efficiency of blue organic light-emitting diodes (OLEDs). Two electron deficient 4,5-diazafluorene- or di(2,2′-pyridyl)-containing blue fluorophores, PhSP_N2N2DPV (4,5-diaza-2′-diphenylamino-7′-(2,2″-diphenylvinyl)-9,9′-spirobifluorene) and PhF_py2py2DPV (N-[7-(2,2-diphenylvinyl)-9,9′-di(2,2″-pyridyl)-2-fluorenyl]-N,N-diphenylamine), were synthesized and characterized for non-doped blue OLEDs. Whereas PhF_py2py2DPVOLED performs ordinarily, PhSP_N2N2DPVOLED outperforms previously known PhSPDPV (2-diphenylamino-7-diphenylvinyl-9,9′-spirobifluorene) OLED significantly: maximum external quantum efficiency of ∼5% (4.6% at 20 mA cm⁻²) and the peak electroluminance of 60510 cd m⁻² (1810 cd m⁻² at 20 mA cm⁻²) versus 3.4% (2.9% at 20 mA cm⁻²) and 33020 cd m⁻² (910 cd m⁻² at 20 mA cm⁻²) ofPhSPDPVOLED. We attribute the superior performance of PhSP_N2N2DPVOLED to the good charge balancing, which is in turn due to the very low hole mobility of PhSP_N2N2DPV. The experimental results reveal that the electron-deficient moiety, 4,5-diazafluorene or di(2,2′-dipyridyl), increases electron affinity but reduces the hole mobility. Electron mobility, determined by time-of-flight (TOF) method, is 5 × 10⁻⁵ and 5 × 10⁻⁴ cm²V⁻¹ s⁻¹ (at an electric field of 4.9 × 10⁵ V cm⁻¹) for PhSP_N2N2DPV and PhF_py2py2DPV, respectively. Surprisingly, they are not higher than 8 × 10⁻⁴ cm²V⁻¹ s⁻¹ of nonpolar PhSPDPV. On the other hand, hole mobility is 2 × 10⁻⁶ and 2 × 10⁻⁴ cm²V⁻¹ s⁻¹ for PhSP_N2N2DPV and PhF_py2py2DPV, respectively, and they are both significantly lower than 6 × 10⁻³ cm²V⁻¹ s⁻¹ of PhSPDPV. For PhSP_N2N2DPV and PhF_py2py2DPV bipolar blue fluorophores, we have demonstrated that electron-transporting and light-emitting functions involve different molecular halves. The design of such molecular halves greatly facilitates the optical and electronic optimizations of fluorophores for high-performance OLEDs.