Carrier transport in multilayer organic photodetectors: II. Effects of anode preparation

Abstract

We address the effects of anode preparation on the performance of multilayer organic photodetectors introduced in the accompanying article (Part I) [J. Xue and S. R. Forrest, J. Appl. Phys. 95, 1859 (2004)]. Using atomic force microscopy and ultraviolet photoemission spectroscopy, we find that the surface morphology and electronic properties of indium–tin–oxide (ITO) anodes depend strongly on the anode preparation. ITO films deposited by rf magnetron sputtering at room temperature have smoother surfaces and 0.1–0.3 eV lower work functions than commercially obtained ITO films. Oxygen plasma and ultraviolet (UV)-ozone treatment can both increase the ITO work function, although the mechanisms leading to the increase have different origins. The dark current in a multilayer organic photodetector is reduced when the anode has a high work function, which agrees with our conclusion in Part I that electron tunneling injection from the anode into the organic active region dominates the dark current. The quantum efficiency of multilayer organic photodetectors is reduced by ∼10% for ITO anodes exposed to oxygen plasma or UV ozone, possibly due to an increased density of defect states at the anode/organic interface.