Molecular Control of Charge Carrier and Seebeck Coefficient in Hybrid Two-Dimensional Nanoparticle Superlattices

The development of high efficiency thermoelectric materials would revolutionize energy harvesting capabilities and be useful for a large number of applications. Hybrid organic-inorganic nanostructured materials are an intriguing platform for developing efficient thermoelectric systems due to the possibility of independently controlling the thermal and electrical conductivity of the material with intelligent choices for material components. In this study we control the thermopower of hybrid 2-dimensional (2D) nanoparticle-molecule superlattices by systematically modifying molecular properties. Five conjugated, ladder-type, heteroacene molecules are used to interlink gold nanoparticles and control the thin films' properties. Interestingly, we measure a change in the sign of the Seebeck coefficient, corresponding to a crossover of the majority charge carrier (from hole to electron). Hall-effect measurements are used to confirm the change in dominant carrier for these systems. And density functional theory (DFT) is used in combination with Green's function-based transport calculations to examine the energy level-alignments in the system. The single-molecule Seebeck coefficient predictions from these results compare favorably with the experimental results of the molecular arrays and provide potential insights into the origins of the sign-change of the carriers in the system. In addition, the thermoelectric power factor sigma S-2 is found to range above predicted values for hybrid systems and to deviate from optimization strategies for conventional materials. A simple strategy to further increase sigma S-2 is highlighted. Limitations of the model and sources of variability in the experimental results are discussed. Our findings develop a stronger understanding of charge transport in molecule-nanoparticle hybrid films; demonstrate that these hybrid materials allow facile control over both the carrier type and the power factor of the material, both of which are important for maximizing the efficiency of functional thermoelectric devices; and establish a framework for continuing to maximize the thermoelectric efficiency of these materials.