Hole mobility measurement of 4,4′-Bis[N-(1-naphthyl)-N-phenylamino]-biphenyl by dark injection method

The hole carrier mobility of deposited thin film composed of 4,4′-Bis[N-(1-naphthyl)-N-phenylamino]-biphenyl (NPD) was measured using the dark injection method. An improved bridge circuit achieved faster response time, which enabled the evaluation of practical hole transporting material with high hole mobility, using thin vacuum deposited film. The mobility basically agreed with that measured by the time-of-flight method. We also evaluated the effect of a hole injection barrier on the estimated mobility, using several surface treatments of indium–tin oxide anode. The results indicated that increasing barrier height decreases the estimated mobility; however, its effect was relatively small.