Examination of transient carrier behaviors in organic field-effect devices via displacement current measurement.

Abstract

Organic field-effect transistors (OFETs) are one of the key components of the ubiquitous flexible electronics in the near future. Since the first report in the mid-1980s, OFETs have been intensively studied for more than 20 years. However, most of these studies are based on steady state or quasi steady state DC measurements, which are not sensitive to transient processes including the formation and depletion of the conducting channel. These transient processes are essential parts of the device operation, and determine the response frequency of OFETs. For these reasons, the transient carrier behaviors during these processes warrant examination. This thesis develops displacement current measurement (DCM) as a technique to probe transient carrier behaviors. Instead of using OFETs for measuring displacement current, long-channel capacitors (LCCs) are used. A LCC can be viewed as a simplified OFET with only one channel contact to limit the carrier injection/extraction to one direction only. The channel of a LCC is elongated to millimeter range to increase the transient time and the displacement current associated with charging/discharging the channel. Displacement current has been measured from LCCs under cyclic gate voltage sweeps. The number of the injected, extracted, and trapped carriers can be calculated by integrating the displacement current with respect to time. A current peak has always been observed in the charging sweep, and it is attributed to the transient process of conducting channel formation. Analytical and numerical device models have been developed to understand the transient carrier behaviors. It is found that carrier mobility can be calculated from the slope of the displacement current peak. In addition, the evolution of the carrier distribution in the long channel during the conducting channel formation and depletion are presented in detail. Further more, the effects of carrier traps on the transient carrier behaviors are discussed. DCM has also been used to study the contact effects at metal-pentacene interfaces. It is found that the carrier trapping in the long channel of the LCCs with Au contacts is indirectly caused by the deep trap states at the pentacene-dielectric interface in the contact region generated by Au penetration. Low trapped carrier density is found in the LCCs with Cu contacts due to the shallow penetration of Cu atoms. In addition, ambipolar injection and transport are observed in a LCC with Al contact and a PMMA buffer layer between pentacene and SiO2. Thus DCM can be used to characterize the quality of metal-organic contacts. The conducting channel depletion dynamics under constant gate voltages has also been examined. It is found that the discharging displacement current can either follow an exponential decay or a power law decay, depending on the discharging gate voltage. A simple RC model has been given to explain these different decay behaviors. For the power law decay, the decay exponent measured from the experimental data is around 1.2 to 1.3, while the exponent predicted by the RC model is 2. The smaller exponent observed in experiment might be attributed to the effect of carrier traps.