Enhanced hole injection in organic light-emitting diodes consisting of self-assembled monolayer of tripod-shaped π-conjugated thiols

Self-assembled monolayers (SAMs) of two tripod-shaped π-conjugated thiols and a disulfide have been grafted onto Au(111)/mica substrates. These SAMs exhibit oxidation and reduction peaks in the cyclic voltammograms arising from the electrochemical responses of the conjugated oligothiophene parts in the molecules. Organic light-emitting devices have been fabricated by using the SAM-coated Au electrode as the anode. In comparison with the device consisting of Au/TPD/Alq3/Mg–Ag, the electroluminescence performance of the device using the SAM grafted Au electrode is much improved for the SAM of the tripod-shaped thiols and worsened for the SAM of the disulfide. The different influences of the SAMs on the performances are explained by considering structures of the molecules and the SAMs, and the energy level alignment at the related interfaces in the devices.