摘要
Thermoelectric (TE) devices have been attracting increasing attention because of their ability to convert heat directly to electricity. To date, improving the TE figure of merit remains the key challenge. The advent of the topological insulator and the emerging nanotechnology open a new way to design high-performance TE devices. By combining first-principles calculations with Boltzmann transport theory, we demonstrate for epitaxial Bi 2 Se 3 thin films with thickness slightly larger than six quintuple layers, the relaxation time of the in-gap topological surface states can reach hundreds of femtoseconds, which is 2 orders of magnitude larger than that of the bulk states. Such a strong relaxation time enhancement achieves an approximately 3 times larger electrical- to thermal-conductance ratio than the value predicted by the Wiedemann-Franz law. This condition also enhances the Seebeck coefficient, and consequently leads to the excellent TE figure of merit zT ∼ 2.1 at room temperature with high TE efficiency over a wide temperature range. The TE performance can be further improved by introducing defects in the bulk-like middle layers of the thin film. The improvement is significant at room temperature and can be even better at a higher temperature. Similar strong enhancement of TE performance is expected in other topological insulator thin films.