Solubility of N,N′-Di(1-naphthyl)-N,N′-diphenyl Benzidine (NPB) in Various Organic Solvents: Measurement and Correlation with the Hansen Solubility Parameter

Solubilities of organic semiconductor compounds are of great importance in controlling the coating, crystallization, and phase-separation processes in the fabrication of organic thin-film devices. Here we measured the solubility of N,N′-di(1-naphthyl)-N,N′-diphenyl benzidine (NPB), a typical hole-transport material, in 23 organic solvents by UV absorption spectroscopy. The solubility of NPB in mole fraction unit x2 ranged widely from 2.4 × 10–7 in methanol to 2.8 × 10–3 in chloroform. The solubility data was correlated with the Hansen solubility parameter (HSP) as a function of the solute–solvent distance Ra in the 3D HSP space. A least-squares regression to the equation −log x2 = aRa2 + b, based on the Scatchard–Hildebrand regular solution theory, was carried out to determine the coefficients (a = 0.0102 MPa–1 and b = 2.16) and the HSP of NPB (δD = 22.6 MPa1/2, δP = 6.8 MPa1/2, and δH = 5.8 MPa1/2 for the dispersion, polarity, and hydrogen-bonding interactions, respectively). The correlation enabled us t...