Enhanced thermoelectric performance by resonant vibrational mode-selective density-of-states distortions. Issue 28 (5th July 2022) - Birch, Shantonio W.
Abstract : As the number of photons ( N ph ) is increased to augment the coupling of IR light to C–C stretching vibrations, a resonant four-step process is shown to improve power factor by maximizing asymmetries in the tail distribution of conducting carriers. 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 ∼2.4 k B T 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.