Hole-transport comparison between solution-processed and vacuum-deposited organic semiconductors

Charge transport in the amorphous organic small molecules α-NPD (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine) and Spiro-TAD (2,2′,7,7′-tetrakis(N,N-diphenylamino)-9,9-spirobifluorene) is investigated in solution-processed films and compared to charge transport in vacuum-deposited films of the same molecule. By optimizing the solution-deposition conditions, such as solvent and concentration, equal charge-transport parameters for solution-processed and vacuum-deposited films are demonstrated. Modeling of the charge carrier transport characteristics was performed by drift-diffusion simulations. The dependence of the charge carrier mobility on temperature, carrier density, and electric field was found to be the same for vacuum deposition and solution processing. In both material processing cases, hole mobilities of 4 × 10−8 m2 V−1 s−1 for spiro-TAD and 0.9 × 10−8 m2 V−1 s−1 for α-NPD are obtained, demonstrating that solution processing can be a viable alternative to vacuum deposition in terms of charge transport.