Low temperature transport measurements of classical semiconductors are a well-defined method to determine the physics of transport behavior. These measurements are also used to evaluate organic semiconductors, though physical interpretation is not yet fully developed. The similar energy ranges of the various processes involved in charge transport in organic semiconductors, including excitonic coupling, charge-phonon coupling, and trap distributions, result in ambiguity in the interpretation of temperature dependent electrical measurements. The wide variety of organic semiconductors, ranging from well-ordered small molecule crystals to disordered polymers, manifest varying degrees of “ideal” device behavior and require intensive studies in order to capture the full range of physical mechanisms involved in electronic transport in this class of materials. In addition, the physics at electrical contacts and dielectric material interfaces strongly affect device characteristics and results in temperature dependent behavior that is unrelated to the semiconductor itself. In light of these complications, our group is working toward understanding the origins of temperature dependent transport in single crystal, small molecule organic semiconductors with ordered packing. In order to disentangle competing physical effects on device characterization at low temperature, we use TEM and Raman spectroscopy to track changes in the structure and thermal molecular motion, correlated with density functional theory calculations. We perform electrical characterization, including DC current-voltage, AC impedance, and displacement current measurements, on transistors built with a variety of contact and dielectric materials in order to fully understand the origin of the transport behavior. Results of tetracene on silicon dioxide and Cytop dielectrics will be discussed.
We investigate charge injection and transport in organic field-effect transistors fabricated by using poly(2,5-bis(3-tetradecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT-C14) or poly(3-hexylthiophene) (P3HT) as the active polymer
layer. We show that in high mobility devices where the channel resistances are low compared to the contact resistances,
the device performance can be dominated by the metal/organic semiconductor (OSC) contacts. However, in sets of
devices where the channel resistance is dominant over the contacts (usually the lower mobility devices), we see
pronounced field dependence in the saturation regime mobilities consistent with a Poole-Frenkel model of charge
transport within the channel. The field-dependent mobility in short-channel devices produces nonlinear output current-voltage
characteristics which can be modeled consistently in the Poole-Frenkel framework.
n-channel organic thin film transistors (OTFTs) with field-effect mobility comparable to that typically reported for p-channel pentacene TFTs were fabricated on oxidized silicon wafers using N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C<sub>13</sub>H<sub>27</sub>) as the semiconductor. Au, Cr, Al, and LiF/Al source and drain contacts were studied. Accumulation mode n-channel transistor operation was demonstrated for all contact metals despite the large differences in their work functions. High field effect mobility near 0.6 cm<sup>2</sup>/Vs and large <i>I<sub>on</sub></i>/<i>I<sub>off</sub></i> of 10<sup>7</sup> were achieved for the best device which compares favorably with the best reported performance for OTFTs fabricated using this class of material. Despite the impressive performance significant device instability was observed. n-channel TFT performance was sufficient to demonstrate pentacene/PTCDI-C<sub>13</sub>H<sub>27</sub> TFT complementary inverters with a high gain of 140.
We report on an empirically based physical model developed for small-molecule organic thin film transistors (OTFTs). The model is an extension of an adapted MOSFET model for hydrogenated amorphous silicon TFTs accounting for an arbitrary energy distribution of mobile and trap states and allows the extraction of the parameters from the measured device characteristics. Ideally all parameters can be derived from the material properties of the organic semiconductor, but often those are masked by extrinsic effects. To provide model input and validation data sets we fabricated top contact pentacene TFTs on heavily doped and thermally oxidized wafers. The device structure allows the systematic study of the influence of the source and drain contacts and the properties of the semiconductor/metal interface on the device characteristics by varying the contact metal, deposition parameters, or the silane coupling agent used to treat the gate dielectric prior to deposition. From this a functional dependence of the contact and interface effects is incorporated into the model. The subthreshold regime is mainly used to test the charge trapping and charge-configuration model because charge-configuration related effects are usually more exposed in this regime. Currently the model routinely exhibits > 90% accuracy for most devices. Further insight into the actual physical mechanisms is expected from comparing the extracted trap state distributions with those extracted with other methods.
We report on the use of silicon dioxide gate dielectric chemically-modified with vapor-deposited octadecyltrichlorosilane (OTS) monolayers for improved organic thin film transistor (OTFT) performance. To date, silicon dioxide gate dielectric chemically-modified with OTS monolayers deposited from solvent solution have demonstrated the highest reported OTFT performance using the small-molecule organic semiconductor pentacene as the active layer. Vapor treatment is an attractive alternative, especially for polymeric substrates that may be degraded by solvent exposure. Using our OTS vapor treatment we have fabricated photolithographically defined pentacene OTFTs on flexible polymeric substrates with field-effect mobility greater than 1.5 cm2/V-s. We find the performance of pentacene as well as several other small-molecule organic active layer materials can be significantly improved using silicon dioxide gate dielectric chemically-modified with vacuum vapor prime OTS. Pentacene, naphthacene, Cu-phthalocyanine, and alpha-sexithienyl OTFTs fabricated on thermally oxidized silicon substrates with photolithographically defined bottom contacts typically show a factor of 2 to 5 improvement in field-effect mobility and reduced subthreshold slope when using silicon dioxide gate dielectric vacuum vapor treated with OTS compared to OTFTs on untreated gate dielectric.