Yohai Roichman

Generalized Einstein relation for disordered semiconductors—implications for device performance

The ratio between mobility and diffusion parameters is derived for a Gaussian-like density of states. This steady-state analysis is expected to be applicable to a wide range of organic materials (polymers or small molecules) as it relies on the existence of quasiequilibrium only. Our analysis shows that there is an inherent dependence of the transport in trap-free disordered organic materials on the charge density. The implications for the contact phenomena and exciton generation rate in light emitting diodes as well as channel width in field-effect transistors is discussed.

Structures of polymer field-effect transistor: Experimental and numerical analyses

We compare two basic organic field-effect transistor structures both experimentally and theoretically. By using time-resolved analysis, we gain insight into the mechanisms affecting the performance of these structures. Using a two-dimensional numerical model, we focus on the top contact structure and analyze the difference between the two structures.

Two-dimensional simulation of polymer field-effect transistor

A two-dimensional simulation of intrinsic top-contact field-effect transistor is presented. The simulated structure is unique to organic transistors and hence is most relevant. By time resolving the operation of such a transistor, the mechanisms underlying its operation are resolved. The effect of this device configuration on the measured “intrinsic” material properties is also discussed and shown to explain previously reported features.

Threshold voltage as a measure of molecular level shift in organic thin-film transistors

The potential across an organic thin-film transistor is measured by Kelvin probe force microscopy and is used to determine directly the pinch-off voltage at different gate voltages. These measurements lead to the determination of a generalized threshold voltage, which corresponds to molecular level shift as a function of the gate voltage. A comparison between measured and calculated threshold voltage reveals a deviation from a simple Gaussian distribution of the transport density of states available for holes.

Amorphous organic devices—degenerate semiconductors

The device-physics features of organic materials are presented from an engineering point of view. By treating the organic material and the device in a self-consistent manner the unique features of organic devices are revealed. We discuss charge injection and transport relevant to (polymer/small molecule) light-emitting diodes and field-effect transistors.

Turn-on and Charge Build-up Dynamics in Polymer Field Effect Transistors

Turn-on dynamics of polymer field effect transistors were examined experimentally over a wide timescale. We found that the source current dependence on time following switch on of the gate bias exhibits a power law at the short time range, and an exponential decay at the intermediate to long time range. We demonstrate that the transistor dynamic behavior is governed by the channel charge build-up, and can be described accurately by a simple capacitorresistor distributed line model.