Nanostructuring is one of the most promising ways to overcome the challenges to develop high-performance thermoelectric materials. Well-ordered tightly-bonded monolayers of solid assemblies of colloidal nanocrystals might be transparent for electrons to diffuse, but with a lot of boundaries for phonons to scatter. This would lead to the decoupling of electronic and thermal conductivity. So far the most common approach to utilizing colloidal nanocrystals for thermoelectricity is limited as material sources for hot-pressed doped nanocomposite pellets, but the merit of quantum confinement effect of the nanocrystals vanishes. The formation of quasi-atomic discrete energy levels resulting from the quantum confinement effect that leads to a sharp density of states is beneficial for thermoelectric (i.e. enhancing Seebeck coefficient) if we are able to fill them. Therefore we should explore different ways on how to enhance the electrical conductivity in colloidal nanocrystals assembly while preserving its quantum confinement properties and low thermal conductivity.

Here we show the importance of charge-density control on PbS and PbTe nanocrystal assemblies by field-induced doping utilizing electric-double-layer (EDL) gating. Through optimization of quantum dot size, interdot distance, as well as the utilized molecular crosslinkers, we can achieve improved orders of the nanocrystal assemblies and minimize the remaining trap densities in the system. The accumulation of high carrier density by EDL gating is not only enhanced the electronic conductivity of the nanocrystal assemblies by filling the carrier traps, but also allow us to observe the preservation of the discrete energy levels and to access them, despite the large-scale array of the assembly. This can be done because the relatively narrow bandgap of both PbS and PbTe QDs (< 1 eV) than the limit of the Fermi level shift that can be governed by the electrostatic process of EDLFET before the electrochemical process happens. Field-effect transistors with carrier mobility over 10 cm²/V.s and current modulation on/off ratio up to 10⁷ have been demonstrated using this technique. We show the demonstration of high room temperature Seebeck coefficient that was observed once we access or in the vicinity of this discrete energy levels of these two different nanocrystal systems. Furthermore, the optimized high electrical conductivity where the nanocrystal quantum capacitance significantly governs the carrier density can produce record power factor value from this material system. These findings provide a new approach to developing efficient thermoelectric materials by controlling the appropriate carrier density required to dope nanocrystal superlattices.