Enhanced thermoelectric performance by resonant vibrational mode-selective density-of-states distortions

Abstract

A method is developed that can emulate the dramatic enhancement of the thermoelectric power factor (PF) traditionally predicted to occur near the band edge of high-performance thermoelectric materials. The method uses photo-excitation of an infrared (IR)-active intramolecular vibration mode in a weakly-bonded (soft) organic material to couple high-mobility band states to tail states, creating sharply-peaked Dirac-delta-like resonant states within the tail density-of-states (DOS) that also enhance the carrier mobility (μ), the number of conducting carriers (N), and the asymmetry in the energy distribution of conducting carriers (σ(E)). The use of these resonant DOS distortions to optimize PF is explored as a function of the number of IR photons (N_ph). As N_ph is increased to augment the coupling between valence carriers and C–C stretching vibrations, a resonant four-step intramolecular charge transfer process is shown to shift the average energy of conducting carriers from band states to a position in the DOS tail near the intrinsic Fermi level. A critical photon number (N_ph = 35) is observed where DOS peaks merge to create a high mobility band of states on one side of the Fermi level and diverge to create a low mobility band of states on the other side. As a consequence, large asymmetries develop in σ(E), causing PF to attain a maximal value when the merged high-mobility DOS peak is located ∼2.4k_BT from the Fermi level. Importantly, these DOS distortions provide improvements in PF in the DOS tail and is therefore accessible to carrier concentrations achievable by traditional doping techniques.