The Two Charge Transfer States Solution

Bharati Neelamraju, a PhD student from The University of Arizona presented her work on organic semiconductors (OSCs) titled "Doping in Organic Thermoelectrics—The Tale of Two Charge Transfer States" as a part of EP13: Thermoelectrics - Materials, Methods and Devices. This research was carried out in collaboration with Kristen Watts, Erin Ratcliff, and Jeanne Pemberton, also from The University of Arizona. 

Neelamraju's research focuses on studying the change in electrical conductivity as dopants interact with their OCS hosts. This is especially important because improving the performance of thermoelectric devices relies critically on enhancing and understanding their electrical conductivities, especially when a dopant is added. Neelamraju studied the system of regiregular (rr) and regioirregular (ri) P3HT that is doped with F4TCNQ which is a p-type dopant. Additionally, she studied the two types of charge transfer states that occur in doped systems: (i) Integral charge transfer (ICT) where one electron is transferred. This is an ideal charge transfer state and is preferable since it results in a completely free charge carrier, and (ii) Partial charge transfer (CPX) where less than one electron is available, and instead there is a hybridization of the molecular orbitals. This results in the electron being in a trapped state, and hence is not a preferable state. Neelamraju observed that for the P3HT-F4TCNQ system, there is a movement between the ICT and CPX states, in case of the rr-P3HT. This is directly correlated to the microstructure of the system, and hence changing this can change the charge transfer state. When it comes to ri-P3HT which is completely amorphous, the CPX state is dominant at high dopant mole ratios. This was an interesting observation, and the research group is still studying the reasons for this observation and how to further characterize it. They are also trying to understand if there is a direct correlation between the microstructures and charge transfer states even in the case of ri-P3HT-F4TCNQ systems.