From thermoelectricity to phonoelectricity

Here, we review the history and progress of solid-state heat harvesting. Efficiency models, material metrics, and current research are discussed for thermoelectrics, thermionics, laser-coolers, and modern technologies. Then, we discuss nonequilibrium optical phonon harvest in the phonovoltaic cell. We make an effort to distinguish between the harvest of equilibrium heat, or thermal-electricity, and the targetted harvest of a particular nonequilibrium phonon mode, or phonoelectricity. We survey the state of phonovoltaic research, focusing on phonovoltaic materials, the electron-phonon coupling, and entropy production in a phonovoltaic cell. Throughout this review, discussions are connected to the electron and phonon structures, interactions, and transport. The modern thermal-electric harvesters are shown to reshape broad-spectrum, high-entropy heat into a narrow-spectrum of low-entropy emissions in order to efficiently generate thermal-electricity. Phonoelectricity, in contrast, intervenes before a low-entropy population of nonequilibrium optical phonons becomes high-entropy heat. In particular, the phonovoltaic cell generates phonoelectricity by utilizing the nonequilibrium, low-entropy, and high temperature optical phonon population produced by, e.g., the relaxation of electrons excited by an electric field. A phonovoltaic material has an ultranarrow electronic bandgap, such that the hot optical phonon population can relax by producing electron-hole pairs (and power) instead of multiple acoustic phonons (and entropy). The phonovoltaic device has an electric diode, e.g., a p-n junction, such that the internal electric field of the diode splits the electrons and holes in order to produce an electric current, and thus, power. The low entropy and high-temperature of the nonequilibrium optical phonon population enable efficient, in-situ heat harvesting. Bilayer graphene, for example, can theoretically convert the nonequilibrium population of optical phonons generated in a field-effect transistor into electricity with an efficiency exceeding 70% of the Carnot limit. The thermal-electric devices, in contrast, are either inefficient or restricted to ex-situ power generation. Furthermore, as the phonoelectric Carnot limit is defined by the local nonequilibrium between electron and optical phonon populations, rather than the spatial nonequilibrium across the device, the Carnot limit can be very large without inducing melting.Here, we review the history and progress of solid-state heat harvesting. Efficiency models, material metrics, and current research are discussed for thermoelectrics, thermionics, laser-coolers, and modern technologies. Then, we discuss nonequilibrium optical phonon harvest in the phonovoltaic cell. We make an effort to distinguish between the harvest of equilibrium heat, or thermal-electricity, and the targetted harvest of a particular nonequilibrium phonon mode, or phonoelectricity. We survey the state of phonovoltaic research, focusing on phonovoltaic materials, the electron-phonon coupling, and entropy production in a phonovoltaic cell. Throughout this review, discussions are connected to the electron and phonon structures, interactions, and transport. The modern thermal-electric harvesters are shown to reshape broad-spectrum, high-entropy heat into a narrow-spectrum of low-entropy emissions in order to efficiently generate thermal-electricity. Phonoelectricity, in contrast, intervenes before a low-ent...