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

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, PhSPN2DPV (4,5-diaza-2'-diphenylamino-7'-(2,2-diphenylvinyl)-9,9'-spirobifluorene) and PhFpy2DPV (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 PhFpy2DPV OLED performs ordinarily, PhSPN2DPV OLED outperforms previously known PhSPDPV (2-diphenylamino-7-diphenylvinyl-9,9'-spirobifluorene) OLED significantly: maximum external quantum efficiency of similar to 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)) of PhSPDPV OLED. We attribute the superior performance of PhSPN2DPV OLED to the good charge balancing, which is in turn due to the very low hole mobility of PhSPN2DPV. 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 x 10(-5) and 5 x 10(-4) cm(2) V-1 s(-1) (at an electric field of 4.9 x 10(5) V cm(-1)) for PhSPN2DPV and PhFpy2DPV, respectively. Surprisingly, they are not higher than 8 x 10(-4) cm(2) V-1 s(-1) of nonpolar PhSPDPV. On the other hand, hole mobility is 2 x 10(-6) and 2 x 10(-4) cm(2) V-1 s(-1) for PhSPN2DPV and PhFpy2DPV, respectively, and they are both significantly lower than 6 x 10(-3) cm(2) V-1 s(-1) of PhSPDPV. For PhSPN2DPV and PhFpy2DPV 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.