Thin film deposition of organic hole transporting materials: optical, thermodynamic and morphological properties of naphthyl-substituted benzidines

Aromatic diamines and naphthyl-substituted benzidines (BDB, TPB, TPD, NPB, α-NPD, β-NPB, TNB) are listed as one of the best series available of hole transport materials used as thin films in organic electronics (OLEDs, OPVs). High-quality, homogeneous and compact thin films (≈ 300 nm of thickness) of this compound series were prepared by a physical vapor deposition procedure. SEM and XRD characterizations evidence the amorphous nature of the thin films of NPB, α-NPD, β-NPB and TNB, prepared onto ITO and gold surfaces by a controlling mass flow rate. The semiconducting behavior of this class of π-conjugated materials was investigated through UV–vis characterization by the determination of optical band gaps (≈ 3 eV). According to DSC, SEM and XRD analyses, the materials evidenced an amorphous structure and high thermal stability in the glassy state. Analyzing the melting properties, the ratio T_g/T_m = 2/3 was observed for TPB and NPB, which have a higher molecular symmetry, while T_g/T_m = 3/4 was observed for the asymmetric β-NPB and TPD. The first accurate measurements of the vapor pressures and thermodynamic properties of phase transition were obtained for the most common hole transport material (NPB) in OLEDs. The relative stability of the crystalline phases of the diamine derivatives (BDB, TPB, NPB) was found to be enthalpically driven, increasing linearly with the molar volume of the compound.