  The spectral density of localized states in the band gap of pentacene (trap
DOS) was determined with a pentacene-based thin-film transistor from
measurements of the temperature dependence and gate-voltage dependence of the
contact-corrected field-effect conductivity. Several analytical methods to
calculate the trap DOS from the measured data were used to clarify, if the
different methods lead to comparable results. We also used computer simulations
to further test the results from the analytical methods. Most methods predict a
trap DOS close to the valence band edge that can be very well approximated by a
single exponential function with a slope in the range of 50-60 meV and a trap
density at the valence band edge of approx. 2x10E21 eV-1cm-3. Interestingly,
the trap DOS is always slightly steeper than exponential. An important finding
is that the choice of the method to calculate the trap DOS from the measured
data can have a considerable effect on the final result. We identify two
specific simplifying assumptions that lead to significant errors in the trap
DOS. The temperature-dependence of the band mobility should generally not be
neglected. Moreover, the assumption of a constant effective accumulation layer
thickness leads to a significant underestimation of the slope of the trap DOS.

Materials Science