Chemical Doping of Organic and Coordination Polymers for Thermoelectric and Spintronic Applications: A Theoretical Understanding

Conspectus The controlled doping of organicsemiconductors(OSCs) is crucialnot only for improving the performance of electronic and optoelectronicdevices but also for enabling efficient thermoelectric conversionand spintronic applications. The mechanism of doping for OSCs is fundamentallydifferent from that of their inorganic counterparts. In particular,the interplay between dopants and host materials is complicated consideringthe low dielectric constant, strong lattice-charge interaction, andflexible nature of materials. Recent experimental breakthroughs inthe molecular design of dopants and the precise doping with high spatialresolution call for more profound understandings as to how the dopantinteracts with the charge introduced to OSCs and how the admixtureof dopants alters the electronic properties of host materials beforeone can exploit controllable doping to realize desired functionalities. By employing state-of-the-art computational tools, we revealedthe effects of doping in representative and emerging organic and coordinationpolymers aiming toward thermoelectric and spintronic applications.We showed that dopants and hosts should be taken as an integratedsystem, and the type of charge-transfer interaction between them isthe key for spin polarization. First, we found doping-induced modificationsto the electronic band in a potassium-doped coordination polymer,an n-type thermoelectric material. The charge localization due tothe Coulomb interaction between the completely ionized dopant andthe injected charge on the polymer backbone and also the polaron bandformation at low doping levels are responsible for the nonmonotonictemperature dependence of the conductivity and Seebeck coefficientobserved in recent experiments. The mechanistic insights gained fromthese results have provided important guidelines on how to controlthe doping level and working temperature to achieve a high thermoelectricconversion efficiency. Next, we demonstrated that the ionized dopantsscatter charge carriers via screened Coulomb interactions, and itmay become a dominant scattering mechanism in doped polymers. Afterincorporating the ionized dopant scattering mechanism in PEDOT:Tos,a p-type thermoelectric polymer, we were able to reproduce the measuredSeebeck coefficient-electrical conductivity relationship spanning a wide range of doping levels, highlighting the importance of ionizeddopant scattering in charge transport. In the two cases describedabove, charge injection is enabledby integral charge transfer between the dopant and host polymers.In a third example, we showed that a novel type of stacked two-dimensionalpolymer, conjugated covalent organic frameworks (COFs) with closed-shellelectronic structures, can be spin polarized by iodine doping viafractional charge transfer even at high doping levels. We then manifestedthat magnetization can be attained in nonmagnetic materials lackingmetal d electrons and further designed two new COFs with tunable spintronicstructure and magnetic interactions after the iodine doping. Thesefindings have suggested a practical route to enable spin polarizationin nonradical materials by chemical doping via orbital hybridization,which holds great promise for flexible spintronic applications.

Automated summary

Controlled doping of organic semiconductors (OSCs) is crucial for improving electronic and optoelectronic device performance and enabling applications such as thermoelectric conversion and spintronics. Recent experimental breakthroughs in molecular design and precise doping call for a deeper understanding of the dopant-host interactions. Computational tools were used to study doping effects in organic and coordination polymers. The results provided insights into how to control doping levels and working temperature for high thermoelectric conversion efficiency, highlighted the importance of ionized dopant scattering in charge transport, and demonstrated a practical route for spin polarization in nonradical materials via chemical doping.