A voltage-controlled color tunable organic light emitting device has been demonstrated by using a co-evaporated emissive layer of NPB [(N,N'-bis(naphthalene-1-yl)-N,N'-bis(phenyl)benzidine)] and an organic Eu complex. As a result of minimized energy transfer from the host (NPB) to the guest emitter (Eu complex), this device can vary the light emission from vivid red to deep blue with increasing bias. Optimal device performance is obtained for a 20 wt.% Eu doping in NPB. This doping concentration is able to yield x and y chromaticity coordinates in the range of 0.61-0.17 and 0.31-0.12, respectively. (C) 2007 Elsevier Ltd. All rights reserved.