Doped Organic Semiconductors
Conjugated polymers are poor electronic conductors for two principal reasons: their amorphous structure has a significant disorder and they do not have free charge carriers. Therefore, to increase the concentration of free carriers, we need to dope the conjugated polymers. Many electronic and optoelectronic properties of conjugated polymers emanate from free-charge carriers. These properties are the basis for applications such as light-emitting diodes, field-effect, and electrochemical transistors, photovoltaics, redox batteries, thermoelectrics, and sensors.
Conjugated polymers are typically doped via electron transfer by exposing them to molecules—termed as dopants—that can either remove or add electrons. This simple process introduces complexities in the electronic structure because of the charge-charge Coulomb interactions between the dopant and the polymer. This long-range interaction is much more pronounced in polymers than inorganic semiconductors because of their low dielectric constants. Poor screening of the dopant-polymer interactions increases energetic disorder, alters the width and the shape of the distribution of the density of states (DOS), and suppresses the density of free charge carriers. These factors adversely affect charge transport and thus the electronic and optoelectronic properties of the polymer.
We recently demonstrated a method to measure electrical conductivity (σ ) and Seebeck coefficient (α) via a controlled de-doping of vapor-doped films of conjugated polymers. Our method is unique because it captures the trend over a four-orders of magnitude conductivity window using a single sample and without modulation doping. Our general approach, therefore, will be to measure the α vs. σ curve via a controlled de-doping of vapor-doped films of conjugated polymers over a broad range of carrier concentrations. We will fit the data to computational models to understand the effect of the dopants on the DOS using a previously reported phonon-assisted hopping model. Concurrently, we will use simulations to identify specific experimental parameters for altering dopant-DOS interactions. This tightly-coupled closed loop, where the experiments will guide simulation and vice versa, makes us perfectly positioned to identify levers to mitigate the impact of dopants on the DOS and thus the charge transport in doped conjugated polymers.
Boyle, C. J.; Upadhyaya, M.; Wang, P. J.; Renna, L. A.; Lu-Diaz, M.; Jeong, S. P.; Hight-Huf, N.; Korugic-Karasz, L.; Barnes, M. D.; Aksamija, Z.; Venkataraman, D. “Tuning Charge Transport Dynamics Via Clustering of Doping in Organic Semiconductor Thin Films”, Nat. Commun. 2019, 10, 2827 . DOI:10.1038/s41467-019-10567-5
Upadhyaya, M.; Boyle, C. J.; Venkataraman, D.; Aksamija, Z. “Effects of Disorder on Thermoelectric Properties of Semiconducting Polymers”, Sci .Rep. 2019, 9, 5820. DOI:10.1038/s41598-019-42265-z
Prof. Zlatan Aksamija and NET lab. Department of Electrical and Computer Engineering at UMass Amherst