Investigation of charge-transporting layers for high-efficiency organic light-emitting diode

In the present work, we study a comprehensive model to quantitatively investigate the role of charge-transporting materials on exciton recombination and electric field distribution across the emissive and electron-transporting layers in organic light-emitting diode (OLED) devices via electrical simulation. The devices are composed of 4,4‧,4″-tris(carbazol-9-yl)triphenylamine (TCTA) and 4,4‧-bis(carbazol-9-yl) biphenyl (CBP) hosts, bathophenanthroline (Bphen) and 2,2‧,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as electron-transporting materials (ETMs), and 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) as a hole-transporting material. The outcomes reveal that the recombination rate across the emissive layer (EML) is highly influenced by the charge carrier mobility of ETMs. The Bphen-based device exhibits a maximum recombination rate of 3.5  ×  10⁶ cm^-3 s^-1, which is 10⁵ times higher than that of the TPBi-composed device. The recombination rate is increased by two orders of magnitude by incorporating a TAPC for the hole transport layer. It is increased from 1.9  ×  10⁶ to 3.5  ×  10⁶ cm^-3 s^-1 as the host is changed from TCTA to CBP. Meanwhile, the electric field distribution is decreased from 0.28 to 0.17 MV cm^-1 across the TCTA EML in the presence of the hole transport layer, but is increased 0.36 MV cm^-1 when TCTA EML is replaced with CBP. Experimentally, the Bphen-based device exhibits a power efficiency of 12.5 lm W^-1 and an external quantum efficiency of 5.1%, which are higher than those of the TPBi counterpart.