Photocurrent Spectroscopy of Pentacene Thin Film Transistors

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Date
2006-09-25Author
Breban, Mihaela
Advisor
Breban, Mihaela
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We demonstrate the application of photocurrent modulation
spectroscopy in characterizing the performance of organic thin-film
transistors. A parallel analysis of the direct current and
photocurrent voltage characteristics provides a model-free
determination of the field-effect mobility and the density of free
carriers in the transistor channel as a function of the applied gate
voltage. Applying this technique to pentacene thin-film transistors
demonstrates that the mobility increases as V$_g^{1/3}$. The
free-carrier density is approximately 1/10 of the expected
capacitive charge, and the mobility increases monotonically with the
free carrier density, consistent with the trap and release model of
transport.
Also, the modulated photocurrent spectroscopy can be used as a probe
of defect states in pentacene thin film transistors, measuring
simultaneously the magnitude and the phase of the photocurrent as a
function of the modulation frequency. This is accomplished by
modeling the photo-carrier generation process as exciton
dissociation via interaction with localized traps. Experimental data
reveal a Gaussian distribution of localized states centered around
0.3 eV above the highest occupied molecular orbital. We also
investigated the effect of the gate dielectric material with our
probe and found that the position of the extracted gaussian slightly
shifts, consistent with the expected image charge effect for Pn
through the dielectric substrate. Also shifts in the gaussian
position for samples fabricated with variable deposition conditions
are correlated with changes in Pn morphology. The morphological
differences between Pn films were also detected in current-voltage
characteristics and photocurrent spectra. However, the origin of the
ubiquitous 0.3 eV defect in Pn seems to be unrelated to structural
differences in Pn films.