The bulk photovoltaic effect observed in ferroelectric materials can enable photovoltaic performance beyond the Shockley-Queisser limit of efficiency. This requires the use of ferroelectrics with strong polarization and low bandgap (Eg) that are typically contradictory in the common perovskite oxides ferroelectrics. Here, we use first-principles calculations to study the KNbO3-K(Ti0.5Mo0.5)O3 (KNTM) solid solutions as possible narrow-gap ferroelectric materials. KTM, the end-member of the recently discovered KNTM solid solution system, maintains a ferroelectric polarization similar to that of other K-based systems due to its d0 configuration at the B-site. The substitution of Nb in KTM reduces Eg from 2.9 of KTM to 1.83 eV for an unstrained system and 1.7 eV for a compressively strained system, while maintaining ferroelectricity. The combination of narrow Eg, strong ferroelectricity, low toxicity, and abundance of the constituent elements make Nb-substituted KTM a promising candidate material for photoferroelectric applications.
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