Electron and hole transport in the organic small molecule α-NPD

Electron and hole transport properties of the organic small molecule N,N'-Di(1-naphthyl)-N,N'diphenyl-(1,1'-biphenyl)-4,4'-diamine are investigated by space-charge-limited current measurements. The hole transport shows trap-free behaviorwith a mobility of 2.3 x 10(-8) m(2)/Vs at vanishing carrier density and electric field. The electron transport, on the other hand, shows heavily trap-limited behavior, which leads to highly unbalanced transport. A trap concentration of 1.3 x 10(24) m(-3) was found by modeling the electron currents, similar to the universal trap concentration found in conjugated polymers. This indicates that electron trapping is a generic property of organic semiconductors, ranging from vacuum-deposited small-molecules to solution-processed conjugated polymers. Published by AIP Publishing.