Absorption spectra of field-generated cation radical in triphenyldiamine film: Lack of intervalence-charge transfer band

Charge-modulation spectra of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) films were measured in the metal/insulator/semiconductor structure with Parylene-C as insulator. Signal strengths and phases were investigated as applied voltage and modulation frequency as parameters. The spectra observed in the forward bias exhibited a clear peak at 480 nm, in agreement with one of the two absorption bands of TPD monocation radical in solution. However, the rest of the bands in the near infrared, assignable to the intervalence-charge transfer, was missing. The reason for this is explained by the difference in conformations of a TPD cation in solution and solid film.