Small molecule organic light-emitting diodes can exhibit high performance without conventional hole transport layers

It is generally accepted that hole transport layers (HTLs) with thicknesses on the order of tens of nm are indispensable to the function of small molecule organic light-emitting diodes (OLEDs) if high electroluminescence and quantum efficiencies are to be achieved. In the present letter, small molecule OLEDs with high luminance and external quantum efficiencies are fabricated in which the HTL is replaced solely by an ultrathin (1-2 nm) self-assembled, saturated hydrocarbon organosiloxane monolayer. These results require some reconsideration of conventional design criteria regarding the necessity of HTLs and argue that the role of the self-assembled monolayer here is to enhance hole injection and charge recombination efficiency, while blocking electron transport to the anode. These results therefore offer significantly simplified device fabrication.