Enhanced efficiency and stability of organic light-emitting diodes via binary self-assembled monolayers of aromatic and aliphatic compounds on indium tin oxide

摘要

Regulating hole injection via self-assembled monolayers (SAMs) modification on indium tin oxide (ITO) is significantly favourable for achieving high efficiency and stability of organic light-emitting diodes (OLEDs). In this work, ITO was modified by pentafluorobenzylphosphonic acid (F_5B_nPA) and heneicosa-fluorododecylphosphonic acid (HF_(21)DPA) in a binary SAMs way. By altering mole ratios of these two materials, ITO work function can be achieved from 5.20 to 5.82 eV and the roles of F_5B_nPA and HF_(21)DPA in affecting hole injection and device performances were also explored. The results demonstrate a synergistic effect that HF_(21)DPA improves ITO work function efficiently for its fluorinated alkyl chain and F_5B_nPA enhances the charge transfer directly due to its aromatic group gathering HOMO level. When α-naphthylphenylbiphenyldiamine (NPB) and 4,4',4"-tris(carbazol-9-yl) triphenylamine (TCTA) were used as hole transport layers (HTLs) in OLEDs with a structure of ITO/SAM/HTL/tris(8-hydroxyquinolino) aluminum (Ⅲ) (Alq_3)/lithium fluoride (LiF)/Al, brightness up to 36839 and 58189 cd/m2 and current efficiency up to 5.75 and 7.35 cd/A were obtained, respectively. Furthermore, the influence of binary SAMs on the stability of NPB-based OLEDs was studied. The maximum half-lifetime at the initial brightness of 2200 cd/m~2 reaches 368.0 h, which is 75.1 times that of the poly(3, 4-eth-ylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)-containing device. The high durability is attributed to higher hydrophobicity and lower hole injection barrier caused by SAMs. This is the first report on the synergistic effect of aromatic and aliphatic compounds in binary SAMs on regulating hole injection, showing a new approach of obtaining high efficiency and stability of OLEDs.