Voltage Reduction in Organic Light-Emitting Diodes by Sequential Deposition Doping

4,4'-N,N'-dicarbazolebiphenyl (CBP), a wide bandgap organicmaterial with a low glass transition temperature, was doped with WO3 or MoO3 through co-deposition and sequential deposition. The doping effects were studied via comparative characterization of hole-only devices and simple green organic light-emitting diodes. Doped films prepared by sequential deposition of alternative 0.5 nm oxide and 3-10 nm undoped CBP layers exhibited comparable or even higher conductivity than uniformly doped CBP prepared by co-deposition. Devices with a hole-transport layer of doped CBP had a turn-on voltage as low as 1 V and markedly improved lifetime compared to undoped devices. Secondary ionmass spectrometry revealed extensive intermixing in the sequentially deposited oxide/CBP structure, resulting in a nearly uniform doping profile with small concentration fluctuations. This paper demonstrated a simple and viable sequential deposition doping technique for organic semiconducting materials.