Engineering the figure of merit and thermopower in single-molecule devices connected to semiconducting electrodes

We propose a possible route to achieve high thermoelectric efficiency in molecular junctions by combining a local chemical tuning of the molecular electronic states with the use of semiconducting electrodes. The former allows to control the position of the highest-occupied molecular orbital (HOMO) transmission resonance with respect to the Fermi energy while the latter fulfills a twofold purpose: the suppression of electronlike contributions to the thermopower and the cutoff of the HOMO transmission tails into the semiconductor band gap. As a result a large thermopower can be obtained. Our results strongly suggest that large figures of merit in such molecular junctions can be achieved.

Figure 1

(Color online) A typical molecular junction investigated in this study. Two silicon electrodes terminated at the (111) facet are bridged by a molecule. In the calculations, periodic boundary conditions in the lateral directions have been used. The electrode surface is passivated with hydrogen atoms to avoid strong structural distortions related to surface reconstruction effects.

Figure 2

Top panel: energy profile of the model semiconductor-molecule-semiconductor junction. $E_{gap}$ is the gap between the valence and conduction bands, $\Delta =E_{\text{HOMO}}-E_F$ denotes the position of the HOMO with respect to $E_F$. The molecular gap is chosen so as to have the LUMO level placed inside the semiconducting gap. Lower panel: (b) Seebeck coefficient $S$ and (c) the electronic part of the thermoelectric figure of merit $\left(Z{T}_{el}\right)$ as a function of $\Delta$ and the effective coupling strength to the electrodes $\Gamma$ at $T=300\text{K}$. Notice that the largest thermopower could be achieved by having a weak coupling to the electrodes (large tunnel barriers) and a molecular resonance close to the Fermi level (large derivative). Since there is no upper bound for $Z{T}_{el}$, very large values can be formally attained within these model calculations.

Figure 3

(Color online) (Top panel) Zero-temperature transmission spectra of a benzene molecule sandwiched between Si electrodes via different linker groups. The position of the HOMO level is modified due to charge-transfer effects. (Bottom panel) Temperature dependence of the thermopower $S$ (left) and corresponding figure of merit $Z{T}_{el}$ (right) for the different linker groups.

Figure 4

(Color online) (Top panel) Zero-temperature transmission spectra of three polyacene molecules (benzene, naphtalene, and anthracene) with ${\text{NH}}_2$ side groups and attached to Si electrodes via thiol linkers. For all three molecules the HOMO level lies almost at the corresponding Fermi energy. Notice also that for the benzene molecule the HOMO state moves into the semiconductor band gap. (Bottom panel) Temperature dependence of the thermopower $S$ (left) and figure of merit $Z{T}_{el}$ (right).

Figure 5

(Color online) (Left panel) Vibrational thermal conductance of the three polyacene molecules (benzene, naphtalene, and anthracene) with ${\text{NH}}_2$ side groups. (Right panel) The full figure of merit $ZT$ including the vibrational contribution of the thermal conductance.