Abstract
This study presented the effects of carrier-transporting layer (CTL) on electroluminescence (EL) performance of a blue phosphorescent organic light-emitting diodes (PHOLEDs) with electron transporting host based on three kinds of electron-transporting layers (ETLs) including 3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ), diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) and 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TmPyPB) and two kinds of hole-transporting layers (HTLs) such as 4,4′-bis[N-1-naphthyl-N-phenyl-amino]biphenyl (NPB), 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The carrier recombination and exciton formation zones in blue PHOLEDs strongly depend on the carrier mobility of CTLs and the layer thickness, especially the carrier mobility. Between ETLs and HTLs, the high electron mobility of ETL results in a lower driving voltage in blue PHOLEDs than the high hole mobility of HTL did. In addition, layer thickness modulation is an effective approach to precisely control carriers and restrict carriers within the EML and avoid a leakage emission of CTL. For CTL pairs in OLEDs using the electron transporting host system, ETLs with low mobility and also HTLs with high hole mobility are key points to confine the charge in EML for efficient photon emission. These findings show that appropriate CTL pairs and good layer thickness are essential for efficient OLEDs.
Highlights
Organic light-emitting diodes (OLEDs) consist of several organic stacks, such as holetransporting layer (HTL), emitting layer (EML), and electron-transporting layer (ETL), which are sandwiched by electrodes [1,2,3,4,5]
Numerous efforts have been invested on the effects of ETLs and HTLs on device characteristics, such as device efficiency and operational lifetime, these papers reported the individual effect from either ETLs or HTLs
Liu et al reported the impact of ETLs on device stability under high current stressing [13], and indicated LUMO level and electron mobility as two other factors accounting for the degradation rate of a device
Summary
Organic light-emitting diodes (OLEDs) consist of several organic stacks, such as holetransporting layer (HTL), emitting layer (EML), and electron-transporting layer (ETL), which are sandwiched by electrodes [1,2,3,4,5]. Some scarce papers stressed on electron transporting host system as well as investigated both the HTLs and ETLs simultaneously, and compared their effects on device characteristics to figure out the crucial parameters of them to determine a high-efficiency OLED device with a low driving voltage. Two HTLs, N, N-bis-(1-naphthyl)-N, N’-diphenyl1,1 -biphenyl-4,4 diamine (NPB) [18,19], 1,1-Bis[(di-4-tolylamino) phenyl] cyclohexane (TAPC) [20,21] and three ETLs, TAZ, diphenyl-bis[4-(pyridin-3-yl)phenyl]silane (DPPS) [22,23,24]; and 1,3,5-tri(m -pyrid-3-yl-phenyl)benzene (TmPyPB) [25,26,27,28] were studied in this work This is respective of TAZ, DPPS, and TmPyPB, whose electron mobility are approximately ~10−5 (cm2/Vs) [29], ~
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