Organic electroluminescent devices: enhanced carrier injection using SAM derivatized ITO electrodes

Taking as a device model ITO\TPD\Alq(3)\Al (where TPD is N,N'-bis(3-methylp henyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine and Alq(3) is tris(quinolin -8-olato)aluminium) it is shown that control and improvement of carrier inj ection may be achieved using self-assembled monolayers (SAMs) to manipulate the Schottky energy barrier at the ITO-TPD interface. By using polar adsor bate molecules with the dipole oriented outward from the surface an artific ial dipolar layer is formed and the work function is increased, and vice ve rsa. With this method the threshold voltage for light emission (turn-on) ca n be reduced by 4 V and the maximum luminance increased by a factor of 3.5, giving an overall performance superior to that using the more stable Ag/Mg counter electrode. The SAMs effect is confirmed using a Scanning Kelvin Pr obe (SKP) to profile the relative work function of half-coated ITO samples. Increases in work function in excess of 0.3 eV are observed, in line with predictions using the calculated molecular dipoles of the SAM molecules.