First principles calculations of the Shift Current Bulk Photovoltaic Effect in Ferroelectrics

Abstract: We calculate the bulk photovoltaic response of the ferroelectrics BaTiO$_3$ and PbTiO$_3$ from first principles by applying "shift current" theory to the electronic structure from density functional theory. The first principles results for BaTiO$_3$ reproduce eperimental photocurrent direction and magnitude as a function of light frequency, as well as the dependence of current on light polarization, demonstrating that shift current is the dominant mechanism of the bulk photovoltaic effect in BaTiO$_3$. Additionally, we analyze the relationship between response and material properties in detail. The photocurrent does not depend simply or strongly on the magnitude of material polarization, as has been previously assumed; instead, electronic states with delocalized, covalent bonding that is highly asymmetric along the current direction are required for strong shift current enhancements. The complexity of the response dependence on both external and material parameters suggests applications not only in solar energy conversion, but to photocatalysis and sensor and switch type devices as well.

• The paper demonstrates that shift current is the primary mechanism behind the bulk photovoltaic effect in ferroelectrics, especially in BaTiO3.
• It reveals that the photocurrent relies on asymmetrical, delocalized electronic states rather than solely on the material's intrinsic polarization.
• The research validates theoretical predictions by showing that computed shift currents in BaTiO3 align well with experimental data through specific electronic transitions.

Overview of the Shift Current Photovoltaic Effect in Ferroelectrics

The paper "First Principles Calculation of the Shift Current Photovoltaic Effect in Ferroelectrics" by Steve M. Young and Andrew M. Rappe presents a detailed first-principles analysis of the bulk photovoltaic effect (BPVE) in ferroelectric materials, specifically barium titanate (BaTiO$_3$) and lead titanate (PbTiO$_3$). The authors employ density functional theory (DFT) to compute the electronic structure and apply shift current theory to model the photovoltaic response in these materials.

Key Findings

1. Shift Current as a Dominant Mechanism: The study confirms that the shift current is the primary mechanism behind the BPVE in BaTiO$_3$. The first-principles calculations reproduce the experimentally observed photocurrent's direction, magnitude, and polarization dependence, thus establishing the dominance of shift current in this material.
2. Photocurrent and Material Properties: Contrary to a common assumption in the literature, the authors demonstrate that the photocurrent is not strongly correlated with the material's intrinsic polarization magnitude. Instead, it is contingent upon asymmetrical electronic states with delocalized and covalent bonding characteristics along the current direction.
3. Spectral Analysis and Transition Intensities: The investigation reveals that significant spectral features and rises in current response do not solely depend on transition intensities or shift vector magnitudes. Instead, they arise from specific electronic transitions that exhibit both high intensity and substantial shift vector alignment, which are intricately related to the material's electronic band structure.
4. Comparison with Experimental Data: The computed shift current and associated Glass coefficients for BaTiO$_3$ align well with experimental data, reinforcing the theoretical predictions. The study highlights the complexity and subtlety in the interaction between atomic displacement, electronic structure, and photovoltaic response.

Implications and Future Directions

The findings have several implications for both theoretical understanding and practical applications of the BPVE in ferroelectrics:

• Theoretical Advancement: This work advances the understanding of how electronic structure influences the shift current, highlighting the nuanced role of covalent bonding and state asymmetry. This challenges the previously assumed linear relationship between material polarization and photovoltaic response, suggesting a need for refined models that accommodate electronic complexity.
• Material Design: The insights gained from this study could inform the design of new ferroelectric materials with optimized photovoltaic properties. Targeted modifications to electronic structures and symmetries might allow for enhanced response in desired spectral regions, particularly within the visible spectrum for photovoltaic applications.
• Broad Application Potential: Beyond solar energy conversion, the results suggest potential utility in photocatalysis and electronic devices such as sensors and switches, where polarization-dependent photovoltaic effects could be exploited.

The paper sets the groundwork for future research aimed at discovering and engineering ferroelectric materials with tailored shift current responses. Further studies might explore additional ferroelectric compounds and expand the computational methods to incorporate more advanced theories beyond DFT, potentially improving the accuracy and predictability of photovoltaic responses in complex systems.