Conjugated Polymers Having High Charge-Carrier Mobilities

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

April 1, 2008

Air Force Office of Scientific Research

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