Understanding and Controlling the Factors that Affect Carrier Mobility in Doped Conjugated Polymers

Nontechnical Abstract

Semiconductors are used in many types of devices, including computer chips, displays, solar cells, and thermoelectric devices that directly convert heat to electricity. Most semiconductors are made of hard materials, such as silicon, that are expensive to process and manufacture. This project explores the properties of plastic semiconductors that are lightweight, flexible and inexpensive to process into devices.

The main challenge facing the development of plastic semiconductors is that it is not as easy to control their electrical properties by doping (the addition or removal of electrons to the semiconductor) as with inorganic semiconductors. This is because the structure of the plastic materials changes upon doping. This project aims to understand how those structure changes control electrical conductivity, and to use that understanding to controllably produce plastic semiconductors with improved conductivity.

The efforts will include using new processing methods to allow dopant molecules to be precisely added to the plastic semiconductors at desired molecular locations, and using the structure of the polymer to drive the doping process. The project is a collaboration between researchers who have complimentary expertise in understanding the structure and physical properties of plastic semiconductors and the electronic and device properties of the materials.

In addition to scientific advances, this multidisciplinary project trains undergraduate and graduate students in areas of national need and helps bring experiments on related topics (harvesting energy from sunlight and nanoscale materials) to Los Angeles area high schools. Technical Abstract

Conjugated polymers have numerous potential uses because they combine the mechanical properties of plastics with the electrical properties of semiconductors. When doped by strong oxidizing agents, their conductivity can be changed by orders of magnitude, but interactions with the dopant counterion and dopant-induced changes in morphology can limit the final carrier mobility.

This project aims to understand how the choice of dopant counterion size and shape, along with changes to the degree of polymer crystallinity and crystal structure in the doped state, all work together to determine the electrical properties of doped conjugated polymer films. The project will accomplish this using both chemical and electrochemical doping with a variety of counterions and dopants.

Structural aspects of the doped films will be determined using synchrotron-based grazing-incidence wide-angle X-ray diffraction experiments. Electrical properties will be determined via temperature-dependent four-point probe electrical conductivity, Hall effect, and ultrafast transient absorption spectroscopy experiments. The project will also take advantage of rub-aligned films in which the polymer chains all point in the same direction, creating anisotropic conductivity.

The first specific question to be addressed by the project is the connection between counterion size and shape, and electrical properties. This will be accomplished by doping the same series of polymers using a series of counterions whose sizes vary by nearly an order of magnitude, directly elucidating correlations between the ion size, local structural change, and carrier mobility.

Another specific question to be answered is how the energetics of crystallization (either the crystal-to-crystal phase transformations or the gain or loss of crystallinity upon doping) alter the propensity of conjugated polymers to be doped. This will be accomplished by using blends of polymers that show different propensity for dopant-induced ordering or disordering.

A third main question that will be targeted by the project is the role of the local dielectric environment in doping. This will be accomplished by adding electrolytes to the solutions used for doping, with the aim of altering the propensity for doping.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.