Strong Bulk Photovoltaic Effect in Planar Barium Titanate Thin Films

TL;DR

This study demonstrates a high bulk photovoltaic effect in planar BaTiO3 thin films by tuning ferroelectric properties through strain and orientation, revealing a new mechanism linking polarization and photocurrent.

Contribution

It introduces a novel mechanism explaining the relationship between ferroelectric polarization and photocurrent in thin films, surpassing previous BPE efficiency reports.

Findings

Achieved a high BPE coefficient (~10^{-2} 1/V) in BaTiO3 films.

Discovered a non-monotonic dependence of responsivity on polarization.

Proposed a new mechanism for BPE in ferroelectric thin films.

Abstract

The bulk photovoltaic effect (BPE) leads to the generation of a photocurrent from an asymmetric material. Despite drawing much attention due to its ability to generate photovoltages above the band gap ($E_g$), it is considered a weak effect due to the low generated photocurrents. Here, we show that a remarkably high photoresponse can be achieved by exploiting the BPE in simple planar BaTiO$_3$ (BTO) films, solely by tuning their fundamental ferroelectric properties via strain and growth orientation induced by epitaxial growth on different substrates. We find a non-monotonic dependence of the responsivity ($R_{\rm SC}$) on the ferroelectric polarization ($P$) and obtain a remarkably high BPE coefficient ($\beta$) of $\approx$10$^{-2}$ 1/V, which to the best of our knowledge is the highest reported to date for standard planar BTO thin films. We show that the standard first-principles-based descriptions of BPE in bulk materials cannot account for the photocurrent trends observed for our films and therefore propose a novel mechanism that elucidates the fundamental relationship between $P$ and responsivity in ferroelectric thin films. Our results suggest that practical applications of ferroelectric photovoltaics in standard planar film geometries can be achieved through careful joint optimization of the bulk structure, light absorption, and electrode-absorber interface properties.