Transparent metallic oxides are pivotal materials in information technology, photovoltaics, or even in architecture. They display the rare combination of metallicity and transparency in the visible range because of weak interband photon absorption and weak screening of free carriers to impinging light. However, the workhorse of current technology, indium tin oxide (ITO), is facing severe limitations and alternative approaches are needed. AMO3 perovskites, M being a nd1 transition metal, and A an alkaline earth, have a genuine metallic character and, in contrast to conventional metals, the electron–electron correlations within the nd1 band enhance the carriers effective mass (m*) and bring the transparency window limit (marked by the plasma frequency, ωp*) down to the infrared. Here, it is shown that epitaxial strain and carrier concentration allow fine tuning of optical properties (ωp*) of SrVO3 films by modulating m* due to strain‐induced selective symmetry breaking of 3d‐t2g(xy, yz, xz) orbitals. Interestingly, the DC electrical properties can be varied by a large extent depending on growth conditions whereas the optical transparency window in the visible is basically preserved. These observations suggest that the harsh conditions required to grow optimal SrVO3 films may not be a bottleneck for their future application.

Ferroelectric perovskite oxides are emerging as a promising photoactive layer for photovoltaic applications because of their very high stability and their alternative ferroelectricity-related mechanism for solar energy conversion that could lead to extraordinarily high efficiencies. One of the biggest challenges so far is to reduce their band gap toward the visible region while simultaneously retaining ferroelectricity. To address these two issues, herein an elemental composition engineering of BiFeO 3 is performed by substituting Fe by Co cations, as a means to tune the characteristics of the transition metal–oxygen bond. We demonstrate by solution processing the formation of epitaxial, pure phase, and stable BiFe 1– x Co x O 3 thin films for x ≤ 0.3 and film thickness up to 100 nm. Importantly, the band gap can be tuned from 2.7 to 2.3 eV upon cobalt substitution while simultaneously enhancing ferroelectricity. As a proof of concept, nonoptimized vertical devices have been fabricated and, reassuringly, the electrical photoresponse in the visible region of the Co-substituted phase is improved with respect to the unsubstituted oxide.

The interaction of BiFeO 3 and Co-doped BiFeO 3 thin film surfaces with water vapor is examined using photoelectron spectroscopy. Water exposure results in an upward shift of the Fermi energy, which is limited by the reduction of Bi and Fe in undoped BiFeO 3 and by the reduction of Co in oxidized Co-doped BiFeO 3 . The results highlight the importance of surface potential changes induced by the interaction of solid surfaces with water and the ability of photoelectron spectroscopy to quantitatively determine electrochemical reduction potentials and defect energy levels.

Compositional engineering of BiFeO3 can significantly boost its photovoltaic performance. Therefore, controlling site substitution and understanding how it affects the optical and electronic properties while achieving robust and stable phases...

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