A Comparison Study of the Organic Small Molecular Thin Films Prepared by Solution Process and Vacuum Deposition: Roughness, Hydrophilicity, Absorption, Photoluminescence, Density, Mobility, and Electroluminescence

Abstract

In this work, we report a systematic study of the properties of the solution-processed organic small molecular thin films in order to get an in-depth understanding of the distinctive nature of solution-processed thin films. N,N′-Di(3-methylphenyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4-diamine (TPD), a typical hole-transporting compound, was chosen to fabricate films by both solution process and vacuum deposition. The comparison study between the two kinds of TPD films shows that the surface of the solution-processed TPD films is more smooth and hydrophilic than that of the vacuum-deposited film. In addition, the density of the TPD films was measured by two different methods, and an increase in density was observed in solution-processed films in comparison with the vacuum-deposited film. The space-charge-limited current (SCLC) measurements indicate an increased hole transport mobility of the solution-processed TPD films over that of the vacuum-deposited film, which may result from the more compact intermolecular stacking in solution-processed TPD films. Finally, the light-emitting diodes with a solution-processed TPD layer as the hole-transport layer were fabricated and showed significantly enhanced injected current density and higher luminance relative to the device using the vacuum-deposited TPD film.