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 (α-NPD) were investigated. The substrate temperature (T_sub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of T_sub; maximum density was attained at T_sub = 270-300 K. α-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 enhanced carrier hopping between neighboring α-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 α-NPD films than control of molecular orientation.