Improving the design and performance of polymer thin-film transistors for circuit applications.

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Abstract Title: Improving the design and performance of polymer thin-film transistors for circuit applications.Nontechnical Description: Polymer thin-film transistors are circuit components in which the semiconductor material is not silicon or amorphous silicon, but a polymer. Such components are being actively pursued for a variety of applications including flexible displays, biocompatible electronics, and large-area electronics. The attractiveness of polymer transistors for these applications, many of which are flexible, stem from simple fabrication methods and low costs. It is possible to deposit a polymer semiconductor by a simple process such as inkjet printing, in which the ink contains the polymer. The printer defines the area where the semiconductor is deposited. It is also necessary to deposit insulating layers and conductors to complete the fabrication process. The aim of the proposed research is to improve the design of polymer transistors so that they perform better: possess higher speed and better switching characteristics. To do this, we will study how charges move in such polymers so that we can devise ways to increase their speeds so that the transistor will switch faster. We will develop new and improved measurements so we can get a deeper understanding of transistor operation. These improved polymer transistors will be used to build logic circuits such as flip-flops and shift registers to see how they perform in a circuit. This is important since most practical applications involve the construction of circuits involving many components. The results of this research will impact the fields of flexible and printable electronics.Technical Description: This project will analyze new field-effect transistor device structures that will employ polymer semiconductors with the goal of realizing vastly improved device and circuit performance. In addition to increasing the mobility and speed, attention will be paid to device designs aimed at lowering the operating voltage. This project will address in detail the measurement of Hall mobility in donor-acceptor polymers and use the Hall Effect along with time-resolved current-voltage measurements to get a detailed characterization and understanding of charge transport in these polymeric semiconductors that show much promise. We have developed methods to characterize devices at timescales ranging from 100 ns to many seconds which will be employed along with Hall Effect measurements. Some of the highest mobilities that have been hitherto achieved in polymer semiconductors are in the 3-15 cm2/V-s range, with higher mobilities possible. These relatively large mobilities will result in enhanced carrier delocalization, which will be characterized in detail, leading to an improved theoretical understanding of transport and device physics. We will also develop advanced device structures that will involve new gate insulator combinations and source/drain materials, the use of optimized process conditions to improve molecular ordering and therefore mobilities and operating lifetime. We will demonstrate low-voltage, high speed clocked sequential circuits such as D-Flip Flops and shift registers. These new and improved devices and the circuits that will be constructed with them will help the field of printed electronics.