First-Principles Calculation of the Shift Current Bulk Photovoltaic Effect from Carrier Recombination

The bulk photovoltaic effect, which occurs in single-phase, noncentrosymmetric materials, is characterized by two primary mechanisms-the ballistic current and the shift current-that each generate dc current during carrier excitation. The excitation shift current in particular has intriguing properties, including ultrafast time evolution, a deep connection to wave function quantum geometry, and a well-established sensitivity to light frequency, optical polarization, material strain and strain gradients, coexisting dc electric fields, and more. Shift photocurrent from excitation has been explored in a variety of real materials using ab initio calculations, but shift current from carrier recombination, while predicted in model systems, has not been explored in real materials. To address this gap, we present a method to calculate the shift current due to carrier recombination under steady-state illumination using first-principles calculations. We then model the response of GaN and several other wurtzite semiconductors under realistic experimental conditions. These results show that recombination current may be comparable in magnitude to excitation shift current and demonstrate that even simple materials exhibit significant variations in recombination shift current that are jointly controlled by material properties and illumination conditions. As relaxation processes may depend on spin and valley degrees of freedom, this Letter enables new possibilities to understand and control the interplay between excitation and recombination shift currents that respond sensitively to electronic, magnetic, structural, and topological factors.