Thermoelectric Efficiency of a Quantum Dot in the Single-Electron Transistor Configuration

The thermoelectric efficiency of a single-dot (e.g., nanocrystal or molecule) device has been studied theoretically. The transport coefficients, the power factor, and the figure of merit (ZT) of the quantum dot in the single-electron transistor configuration have been calculated in the sequential tunneling regime when Coulomb blockade is important. Very high values of ZT have been found in the quantum regime, and they have been explained by quantum confinement. An approximate analytical formalism has been derived for ZT as a function of the separation between the energy levels of the dot, ΔE, and the thermal energy, kBT. The electron–phonon coupling has been included in the calculations, and it is shown that it is important for realistic prediction of the thermoelectric efficiency of a quantum-dot device. It has been found that the thermoelectric efficiency decreases due to electron–phonon coupling. Distinct behavior has been obtained for weak and for strong electron–phonon coupling. Quantum confinement is the dominant mechanism determining the magnitude of ZT for weak electron–phonon coupling. For strong electron–phonon coupling, the energy conversion efficiency is determined by the phonon spectrum. In all cases, it has been found that the thermoelectric efficiency decreases rapidly with increasing temperature.