Enhanced Electrical Properties and Air Stability of Amorphous Organic Thin Films by Engineering Film Density

The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (alpha-NPD) were investigated. The substrate temperature (T-sub) during vacuum deposition had different effects on the film density and molecular orientation of alpha-NPD. Film density was a concave function of T-sub; maximum density was attained at T-sub, = 270-300 K. alpha-NPD molecules were randomly oriented at T-sub, = 342 K, and their horizontal orientation on the substrate became dominant as T-sub:, decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to carrier hopping between neighboring alpha-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with alpha-NPD films than control of molecular orientation.