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Conjugated Polymers Having High Charge-Carrier Mobilities

Progress has been made toward realization of practical polymer semiconductor devices.

A three-year research project encompassed multiple studies of (1) polymer semiconductors that exhibit relatively high electric-charge- carrier mobilities and (2) applications of these polymers in electronic (including optoelectronic and nanoelectronic) devices. Although these polymers are of broad importance to all polymer semiconductor devices — including light-emitting diodes, photovoltaic cells, photodetectors, and electrophotographic imaging devices — the focus in this project was largely upon the use of these polymers in thin-film transistors, organic light-emitting diodes, and related light-emitting transistors.

This Complementary Inverter was fabricated by spin-coating of p-type poly(3hexylthiophene) [P3HT] and n-type BBL onto gold source and drain electrodes that had been patterned on a heavily doped silicon substrate with thermally grown oxide.
A fundamental challenge encountered in the development of all polymer semiconductor devices is to achieve charge-carrier mobilities high enough to improve the performances of the devices sufficiently to move the development toward practical applications. This research project addressed the challenge through exploration of previously known and new ladder polymer semiconductors having high (>0.01 cm2/(V•s))charge-carrier mobilities. The main objectives of the research were to (1) develop new conjugated polymer semiconductors having high field-effect mobility of electrons; (2) establish the relationships among charge-carrier mobility, molecular structure, and solid-state morphological characteristics of polymer semiconductors; (3) develop high-performance n-channel polymer field-effect transistors as suitable switching devices and building blocks for all polymer integrated circuits for logic, memory, and other functions; and (4) explore polymer semiconductors that combine efficient light emission with usefully high charge-carrier mobilities as precursors to light-emitting transistors and, ultimately, electrically pumped diode lasers.

The research included studies of synthesis of new polymer and oligomer semiconductors; investigation of novel processing techniques for realizing nanocrystalline, microcrystalline, and single crystalline polymer semiconductor thin films; measurement of charge-carrier mobilities; and design, fabrication, and evaluation of high-performance field effect transistors, organic light-emitting diodes, and light-emitting transistors. The major accomplishments of this research were the following:

  • Field-effect electron mobilities as high as 0.1 cm2/(V•s) were observed in thin films of ladder poly(benzobisimidazobenzophenanthroline) [BBL] that had been formed by spin coating. This level of electron mobility is the highest observed to date in a conjugated polymer semiconductor, was found to vary strongly with the intrinsic viscosity (or molecular weight) of the polymer, and was found to be very stable in air and oxygen.
  • The first-ever polymer-based complementary inverter was fabricated (see figure) and demonstrated to function substantially as intended. (As used here "complementary" signifies that the device is analogous to a complementary metal oxide/semiconductor inverter).
  • A new class of ladder-type bisindoloquinoline semiconductors exhibiting a mobility of 1.0 cm2/(V•s) was synthesized.
  • Ambipolar organic field-effect transistors were made from blends of p- and n-type polymers.
  • The field-effect mobility of holes in regioregular poly(3 alkylthiophene)s was found to exhibit a non-monotonic dependence on alkyl chain length, showing a maximum mobility with hexyl. Fundamental insights into the structural factors that govern high-mobility charge transport and recombination in polymer semiconductors were also achieved.

This work was done by Samson A. Jenekhe, John D. Wind, Amit Babel, Yan Zhu, Christopher Tonzola, Jessica Hancock, Pei Tzu Wu, and Jessica Lembong of the University of Washington for the Air Force Office of Scientific Research.

AFRL-0082

Topics:
Electronic equipment Electronics & Computers Imaging and visualization Light emitting diodes (LEDs) Materials properties Off-board energy sources Optics Polymers Semiconductor devices Semiconductors Solar energy Transistors
This Brief includes a Technical Support Package (TSP).
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Conjugated Polymers Having High Charge-Carrier Mobilities

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 2).

    Read more articles from the archives here.

    Overview

    The document is a research report focused on high mobility conjugated polymers, addressing the limitations of current polymer electronic devices, such as organic field-effect transistors (OFETs), photovoltaic cells, and photodetectors, which are primarily hindered by low charge carrier mobilities. The research aims to investigate various polymer semiconductors that exhibit high carrier mobilities and explore their potential applications in electronics, optoelectronics, and nanoelectronics.

    The report outlines several key areas of research, including the development of ladder polymer semiconductors, which are noted for their high electron mobility. It discusses the molecular weight dependence of electron mobility, indicating that variations in molecular weight can significantly impact the performance of these materials. Additionally, the report highlights the creation of all-polymer complementary inverters, which integrate p-channel and n-channel organic field-effect transistors (OFETs) to achieve functional electronic circuits. This integration is crucial for advancing low-cost plastic electronics and realizing complementary metal-oxide-semiconductor (CMOS)-like polymer inverters that offer low power dissipation and high performance.

    The document also delves into the exploration of ambipolar OFETs, which can operate in both p- and n-channel modes. This characteristic is essential for developing low-power complementary integrated circuits suitable for logic and memory applications. The research investigates polymer blends that combine p-type and n-type components to create ambipolar charge transport properties, which are currently lacking in existing organic and polymer semiconductors.

    Figures and diagrams within the report illustrate the chemical structures of the polymers studied, as well as the output characteristics of devices such as P3HT/20 wt% P4HQ nanowire FETs. The findings are positioned as significant contributions to the fields of polymer science and supramolecular chemistry, with implications for the development of advanced electronic materials.

    Overall, the report emphasizes the importance of high mobility conjugated polymers in overcoming the limitations of current technologies and advancing the field of organic electronics. It presents a comprehensive overview of the research objectives, methodologies, and accomplishments, showcasing the potential for these materials to enhance the performance of electronic devices and contribute to the future of low-cost, high-performance plastic electronics.

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