Research Project Highlight

Enhancing Polymer Thermoelectric Efficiency

An n-type pyromellitic diimide polymer composite with in situ microstructure growth of the common element compound SnCl2 reaches the highest purely n-type polymer composite power factor yet reported.

An n-type pyromellitic diimide polymer composite with in situ microstructure growth of the common element compound SnCl2 reaches the highest purely n-type polymer composite power factor yet reported.

Polymer thermoelectrics offer a number of advantages, including printability and the avoidance of rare or toxic elements. Now, Johns Hopkins University engineers have demonstrated a record increase in efficiency for an n-type polymer semiconductor through the combination of a new polymer electron-transporting semiconductor and a solid additive.

The research team—which includes researchers from the Department of Materials Science and Engineering at Johns Hopkins University, the University of Colorado, and a high school student from Western High School in Baltimore—published their findings in the June 2015 issue of Advanced Science. Their article reported that the figure of merit (ZT), which projects thermoelectric efficiency, is greater than 0.1 in their n-type polymer, an important milestone for a new class of materials as well as for medical and mobile applications.

“It was particularly gratifying to observe the increased electrical conductivity from a relatively air-stable additive, a longstanding challenge for n-type polymers,” said Howard Katz, Professor of Materials Science and Engineering at Johns Hopkins University.

Varying percentages of tin chloride (SnCl2) were mixed with a synthesized polymer. Subsequent drop-casting or spin-casting on glass followed by heating resulted in the formation of tin-based microcrystals. Thermoelectric measurement testing demonstrated a strong increase in electrical conductivity, which appears to be correlated with the overlapping of elongated tin-containing crystals, accompanied by an increase in the voltage generated through temperature differences, without undue increases in heat conduction.

“I was ecstatic when I looked under the microscope and saw beautiful crystals for the first time. Since then I have tried to control the distribution and size of crystals, quickly learning how sensitive these composite materials are to processing and how variable the morphology, and therefore the physical properties, can be,” said Robert Ireland, first author on the paper and a graduate student in Prof. Katz’s lab.

The principles which enable the increase in efficiency in n-type semiconductors have also been shown to be effective in a companion hole-carrying (p-type) polymer semiconductor.

“This provides a route to complementary efficiencies for n- and p-type polymer semiconductors, needed to construct full thermoelectric modules,” said Katz.

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