Solar Refrigeration Based on Impact Ionization in a Transition Metal Dichalcogenides Superlattice

Optical refrigeration of a semiconductor generally requires a laser excitation very close to its bandgap and a radiative efficiency close to 1. Under these two conditions, the material can be refrigerated by radiating more energy than it absorbs. In this theoretical work, we propose considering impact ionization, which appears to be predominant in transition metal dichalcogenides and evaporative cooling to overcome both requirements. With impact ionization, high-energy photons excite multiple low-energy electron-hole pairs rather than heating the material by emitting phonons when the high-energy carriers thermalize. Thanks to an evaporative cooling effect, such low-energy electron-hole pairs diffuse from a small bandgap absorber into a larger bandgap reservoir by absorbing phonons. This cooling process operates even in materials with a modest radiative efficiency. We propose a device based on a small bandgap absorber (a strain-balanced superlattice based on two-dimensional transition metal dichalcogenides) and a larger bandgap reservoir made of bulk MoS2, forming a type I heterojunction. With a detailed balance approach, parametrized with ab initio calculations, we demonstrate a net cooling of the absorber under solar irradiation above 25%, even considering low external radiative efficiency.