Title: Phonovoltaic. III. Electron-phonon coupling and figure of merit of graphene:BN

Abstract

The phonovoltaic cell harvests optical phonons like a photovoltaic harvests photons, that is, a nonequilibrium (hot) population of optical phonons (at temperature $$T$$ _p,O ) more energetic than the band gap produces electron-hole pairs in a $$p$$-$$n$$ junction, which separates these pairs to produce power. A phonovoltaic material requires an optical phonon mode more energetic than its band gap and much more energetic than the thermal energy ($$E$$_p,O $ΔE$_e,g≫ $$k$$_B$$T$$ ) , which relaxes by generating electrons and power (at rate $$\dot{γ}$$ _e-p) rather than acoustic phonons and heat (at rate $$\dot{γ}$$ _p-p ). Graphene (h-C) is the most promising material candidate: when its band gap is tuned to its optical phonon energy without greatly reducing the electron-phonon (e-p) coupling, it reaches a substantial figure of merit [ $$Z$$_pV= $ΔE$_e,g $$\dot{γ}$$ _e-p/ $$E$$_p,O($$\dot{γ}$$ _e-p + $$\dot{γ}$$ _p-p) ≈ 0.8 ] . A simple tight-binding (TB) model presented here predicts that lifting the sublattice symmetry of graphene in order to open a band gap proscribes the e-p interaction at the band edge, such that $$\dot{γ}$$ _e-p→ 0 as $ΔE$_e,g → $$E$$_p,O. However, ab initio (DFT-LDA) simulations of layered h-C/BN and substitutional h-C:BN reveal that the e-p coupling remains substantial in these asymmetric crystals. Indeed, h-C:BN achieves a high figure of merit ( $$Z$$_pV ≈ 0.6 ) . At 300 K and for a Carnot limit of 0.5 ( $$T$$ _p,O = 600 K ) , a h-C:BN phonovoltaic can reach an efficiency of η_pV ≈ 0.2 , double the thermoelectric efficiency ( $ZT$ ≈ 1) under similar conditions.

Authors:

Melnick, Corey ^[1]; Kaviany, Massoud ^[1]

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+ Show Author Affiliations

1. Univ. of Michigan, Ann Arbor, MI (United States). Dept. of Mechanical Engineering

Publication Date:

Fri Dec 09 00:00:00 EST 2016

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC)

Sponsoring Org.:

USDOE Office of Science (SC); National Science Foundation (NSF)

OSTI Identifier:

1544415

Alternate Identifier(s):

OSTI ID: 1335056

Grant/Contract Number:  

AC02-05CH11231; CBET1332807

Resource Type:

Accepted Manuscript

Journal Name:

Physical Review B

Additional Journal Information:

Journal Volume: 94; Journal Issue: 24; Journal ID: ISSN 2469-9950

Publisher:

American Physical Society (APS)

Country of Publication:

United States

Language:

English

Subject:

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY