Power-efficiency trade-off due to density of states (DOS) distortion in a molecular thermoelectric system

The issue of how a distortion in the electronic DOS affects nanoscale thermoelectric performance is addressed within an ``electrical engineering'' perspective. This view point is based on the direct evaluation of the overall efficiency and power from device current-voltage characteristics and gives a more complete picture of the thermoelectric performance in comparison to the traditional ``figure of merit'' based material science approach. We use representative examples from molecular conduction to study the trade-off between maximum efficiency and the maximum power generated within the set up. The trade-off is maximum for the well known example of a sharply resonant molecular level which represents the ultimate distortion in the electronic density of states. As the distortion is reduced via contact induced broadening, we obtain a smaller trade-off between maximum power and efficiency. We then present the effects of self consistent charging, contact induced asymmetry and the HOMO-LUMO gap on the thermoelectric performance. In all cases we compare our non-equilibrium calculations with zT calculations, and our results depict that zT is not the sole metric for the assessment of nanoscale thermoelectric performance.