Defect states in pentacene thin films prepared by thermal evaporation and Langmuir-Blodgett technique

We investigate defect states in pentacene thin films (PTFs) prepared by two different deposition techniques: standard thermal evaporation in high vacuum and the Langmuir-Blodgett (LB) technique. Time domain capacitance-voltage (C-V) and charge deep-level transient spectroscopy (DLTS) techniques were employed. C-V measurements indicate that Schottky barrier structures on both types of PTFs exhibit a common behavior: the thinner the PTF, the closer the measured C-V curve to the ideal shape for the Schottky barrier. Charge DLTS measurements show a peak with an activation energy of about 0.70 eV above E-v and an attempt-to-escape frequency of 3 x 10(14) s(-1) for PTFs deposited by thermal evaporation. For PTFs prepared by LB technique two deep-levels with activation energies of 0.20 eV and 0.21 eV, and an attempt-to-escape frequencies of 2 x 10(7) s(-1) and 2 x 10(6) s(-1), respectively, are found. The amplitude of the charge DLTS signals increases with decreasing PTF sample thickness for both deposition techniques. The observed defects can not be removed by annealing in vacuum. The origin of defects responsible for the measured charge DLTS spectra is discussed. (C) 2008 Elsevier B.V. All rights reserved.