Organic electroluminescent devices: enhanced carrier injection using SAM derivatized ITO electrodes
Taking as a device model ITO|TPD|Alq3|Al (where TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine and Alq3 is tris(quinolin-8-olato)aluminium) it is shown that control and improvement of carrier injection may be achieved using self-assembled monolayers (SAMs) to manipulate the Schottky energy barrier at the ITO–TPD interface. By using polar adsorbate molecules with the dipole oriented outward from the surface an artificial dipolar layer is formed and the work function is increased, and viceversa. With this method the threshold voltage for light emission (turn-on) can 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 Probe (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.