The Phonovoltaic Cell: Harvesting Optical Phonons in Order to Approach the Carnot Limit

Abstract

A new energy conversion device, the phonovoltaic (pV) cell, is proposed. In this cell, a non-equilibrium (hot) population of optical phonons more energetic than the bandgap produces electron-hole pairs in a p-n junction, which separates them to produce power. That is, it harvests optical phonons like a photovoltaic harvests photons. In this thesis, the function of the pV cell is modeled, suitable materials for the device are investigated, and a possible application is discussed. In particular, a physically intuitive efficiency equation and material figure of merit are analytically derived in order to describe the function and performance of a pV cell. Then, ensemble Monte Carlo and hydrodynamic transport simulations are used to validate the predictions of this model. In combination, these modeling efforts show that a pV cell can reach the Carnot efficiency, unlike the thermoelectric generator (which is limited in practice by the achievable material figure of merit and theoretically by the coupled electronic and thermal transport). To reach this promising result, the pV requires a material with a hot optical phonon mode more energetic than its bandgap and much more energetic than the thermal energy. Moreover, the hot phonon mode must relax by generating electrons and power rather than acoustic phonons and heat. By surveying many semiconductors and semimetals and by discussing the relationship between the bandgap and optical phonon energy in typical materials, graphene is shown to be uniquely suited to meet these requirements. Indeed, it has an energetic phonon mode (200 meV), a tunable bandgap, and suitable kinetics. Opening and tuning the bandgap in graphene typically requires altering its time reversal symmetry or sp2 hybridization. However, an ab initio investigation of partially hydrogenated graphene suggests that altering the hybridization of graphene weakens its electron-phonon coupling and ensures poor kinetics. Conversely, an ab initio investigation of graphene alloyed with or deposited on boron nitride (h-C:BN or h-C/BN) shows that changing the symmetry of graphene preserves its electron-phonon coupling. Indeed, a h-C:BN pV cell can theoretically exceed 20% efficiency at 300 K at a Carnot limit of 50%, double the efficiency of a typical TE. Even more impressively, ab initio simulations of bilayer graphene under a strong field (FE-BG) show that a FE-BG pV cell can exceed 60% of the Carnot limit at 300 K. This indicates that a combined graphene field-effect transistor and pV (FET-pV) could greatly reduce heat generation in the transistor while simultaneously generating power. Monte-Carlo simulations are performed to investigate the suitability of large electric fields as a source of optical phonons in support of the combined graphene FET-pV. Results indicate that a field can excite a suitable hot, non-equilibrium population of optical phonons.