Bulk photovoltaic effect in 2D materials from density functional theory and real-time dynamics

The bulk photovoltaic effect (BPVE) is a phenomenon that consists in the generation of a DC current in a material under ilumination with strong enough oscillating electric fields. This effect is intrinsic and occurs in non-centrosymmetric materials without the need of heterostructures or interfaces. The second-order current that contributes to the BPVE is known as the shift-current. First-principles Density Functional Theory (DFT) calculations of this quantity have been carried out during the last decade, seeking efficient materials that can be used for solar energy conversion. In this talk, we present two DFT-based methodologies to evaluate the first and second-order (DC) responses based on (i) evaluating the frequency-dependent perturbative expressions and (ii) soving the time-dynamical equations for the density matrix incuding an electric pulse. The use of Gaussian basis sets saves computational effort, where typical expressions containing expensive “sums over bands” become expeditious. More remarkably, our time-dynamics approach allows to include electron-hole effects in the calculation of shift currents, a theoretical challenge with almost no general understanding yet. We apply our methodology to several 2D crystals that elucidate the enormous changes in the shift current, inlcuding both a huge redshift in energy and qualitative changes in its shape, when one goes beyond a single-particle DFT approach.