Enhanced hole injection in organic light-emitting diodes by optimized synthesis of self-assembled monolayer

The purpose of the present study is to demonstrate that formation conditions for a self-assembled monolayer (SAM) on indium tin oxide (ITO) highly influence the device performance of organic light-emitting diodes (OLEDs). An ITO substrate was modified with a silane modifier under systematically controlled conditions. Pentyltriethoxysilane (PTES) and tetrahydrofuran (THF) were used as a surface modifier and a solvent, respectively. The immobilization of PTES on the ITO substrate was performed under both the acidic and basic conditions with various H₂O/PTES ratios, r. The relationship between the resultant SAM structure and the hole-injection property was investigated by using hole only devices (HODs) fabricated on the SAM-modified ITO substrate. It was found that the catalytic condition, the value of r, and the concentration of PTES highly influence the structure of obtained SAM, and thus affect the hole-injection property of HOD. The SAM formation under the acidic conditions allows homogeneous coverage of a silane layer on the ITO surface, which leads to the improved hole-injection from the anode. On the other hand, the use of basic catalysts results in the inhomogeneous coverage of a silane layer on the ITO surface and the decreased hole-injection from the anode. The choice of r and the concentration of PTES also affect the kinetics of hydrolysis and condensation reactions of PTES, and hence affects the resultant SAM structure and hole-injection property. The results obtained here show that the device performance can be improved drastically by choosing the SAM formation conditions appropriately, even in the case that particular and novel modifiers are not employed.