Shift and Ballistic Currents from First Principles

As the need for clean, safe, and sustainable energy increases, renewed focus on alternative energy sources such as photovoltaics and next-generation computing have become vital. This motivates study of the bulk photovoltaic effect (BPVE), a nonlinear optoelectronic property that can generate electricity without a p-n junction. To demonstrate the capability of first-principles BPVE theories to guide materials design, we outline an automated method to design distortions that enhance the shift current of monolayer MoS₂ and use it to uncover a polar distortion that increases the integrated shift current more than ten-fold. Because the distortion can be driven by a static electric field via the converse piezoelectric effect, this finding shows that electric fields can be used to engineer the shift current response of a material and complements previous work showing that mechanical strain can also modulate the shift current response.

The ab initio methods commonly used to model the BPVE include only the shift current contribution to the BPVE. They have enabled significant increase in our understanding of the BPVE, but only explain part of the experimentally observed photocurrent. To overcome this deficiency, we present a method that enables the ballistic current—a current resulting from asymmetric scattering—from first principles. We use a perturbative approach to express the ballistic current due to electron-phonon and electron-hole scattering in a form amenable for ab initio calculation, and then calculate the ballistic current for BaTiO₃ from first principles. The current due to electron-phonon scattering is comparable to the shift current, and is therefore experimentally relevant, while the current due to electron-hole scattering is much smaller in magnitude. This methodological development enables closer agreement between theory and experiments and lays the groundwork for further prediction and design of materials with large BPVE.

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