Charge Injection Enhanced by Guest Material in Molecularly Doped Liquid Crystalline Thin Films

Abstract

We have investigated the charge injection into a host–guest type of molecularly doped liquid crystalline thin film in liquid crystal cells. A 2-phenylnaphthalene smectic liquid crystal of 2-(4'-octylphenyl)-6-dodecyloxynaphthalene was used as a host material and a diketopyrrolopyrrole derivative of 1,4-diketo-N,N '-dimethyl-3,6-bis(4-dodecyloxyphenyl) pyrrolo[3,4-c]pyrrole was used as a dopant material: the current–voltage characteristics of the thin films with Pt, indium–tin oxide (ITO), MgAg, and Al electrodes were measured and analyzed on the basis of the Schottky mechanism. By comparing the current–voltage characteristics, we determined the majority carriers dominating the current and estimated the barrier height for electrons and holes. We found that the barrier height was smaller than the energy difference between energy levels of the host liquid crystalline material and the work functions of electrode materials. We concluded that the current in the host–guest thin film was dominated by the guest material that has a narrow energy gap and is responsible for the charge injection into the host material.