Diffusion-enhanced hole transport in thin polymer light-emitting diodes

The transport of holes in polymer light-emitting diodes (PLEDs) based on poly(2-methoxy, 5-(2' ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV) is investigated as a function of layer thickness. For thicknesses smaller than 100 nm, the current in these thin PLEDs is strongly enhanced as compared to the expected space-charge limited (SCL) current. Applying the standard SCL model to measurements on a PLED with a thickness of only 40 nm results in an apparent increase of the hole mobility of a factor of 40. We show that this strong increase of the hole transport properties in these thin devices originates from the presence of an Ohmic hole contact. For Fermi-level alignment, holes diffuse from the contact into the MEH-PPV, forming an accumulation layer with a width of a few tens of nanometers. Due to the density dependence of the mobility, the hole transport in this accumulation region is strongly enhanced. For the analysis of thin PLEDs, it is therefore essential that both drift and diffusion of charge carriers are taken into account.