Electrically conductive polymers have the potential to transform the form factor of current electronic devices and enable novel applications, such as mechanically flexible and stretchable wearable electronic devices. Conductive polymers could even be incorporated into flexible thermoelectric devices to power these wearable devices based on the user’s body heat. In the paper led by Zhiming Liang of the Graham group entitled “n-type charge transport in heavily p-doped polymers” (available here: https://rdcu.be/cc2C5) the Graham, Risko, Strachan, Mei (Purdue), and Podzorov (Rutgers) groups uncover how the charge-carrier polarity can change in heavily doped π-conjugated polymers to enable both n-type and p-type thermoelectric materials to be made from the same polymer-dopant combination.

Electrical conductivity in conjugated polymers has been researched for decades, in part led by the pioneering work of the 2000 Chemistry Nobel Prize winners Heeger, MacDiarmid, and Shirakawa. However, many details of charge-carrier transport in these materials remain elusive and as such they have remained a subject of intense 2interest over the past half century. In “n-type charge transport in heavily p-doped polymers” the researchers show that even the polarity of the charge-carriers is not always as it seems. Here, the authors show that doping conjugated polymers with oxidizing dopants that introduce holes into the conjugated polymers can eventually lead to electrons being the dominant charge-carriers at high doping concentrations. This discovery has potentially transformative consequences for the field, such as enabling the development of n-type thermoelectric materials through heavy p-type doping.

Thermoelectric devices based on electrically conductive polymers are particularly attractive for converting waste heat, such as body heat, unused heat from a coal-fired power plant, or heat from a car’s engine, into useful electrical energy. Conductive polymers are advantageous due to their potential for integration into low-cost devices and their inherent mechanical flexibility; however, they currently display lower performance than necessary for most applications. This lower performance stems in part from the n-type materials. Thermoelectric modules require both p-type and n-type materials, with both materials ideally showing equally high performance. Currently, n-type polymers are significantly lagging their p-type counterparts, but here the authors show how traditionally p-type materials can be converted to n-type materials simply by increasing the doping ratio to beyond typical levels. This new strategy greatly increases the library of potential n-type thermoelectric polymers and may lead to the jump needed to make polymer-based thermoelectric modules a reality.

Fundamentally, a change in the polarity of charge carriers was hinted at in publications from the 1980s and 1990s, but never confirmed. In this recent publication the authors use a combination of Seebeck coefficient measurements and Hall effect measurements to unambiguously show for the first time that highly mobile electrons contribute significantly to charge-carrier transport in heavily p-doped materials. Through combining ultraviolet photoelectron spectroscopy, inverse photoelectron spectroscopy, Hall effect measurements, density functional theory calculations, electron paramagnetic resonance, and ultraviolet-visible-near infrared (UV–vis–NIR) absorbance the authors develop a model whereby both electrons and holes are present in the heavily-doped conducting polymers. In this model the mobile electrons are shown to have more delocalized character and reside primarily in the more crystalline regions whereas more localized holes that move through hopping type transport are present in the more disordered amorphous regions. The sign of the Seebeck coefficient, which indicates whether the material can serve as the n-type or p-type material in a thermoelectric module, is thereby determined by the relative contributions of the electrons and holes. Through varying the doping ratio this balance can be the shifted to switch the material from p-type to n-type.

For more information on the Graham Group, see https://graham.as.uky.edu/