A New Class of Bifunctional Perovskites BaMX₄ (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I): An n-Type Semiconductor with Combined Multiferroic and Photovoltaic Properties

We design a new class of bifunctional materials, namely, BaMX₄ (M = Co, Ni, Fe, Mn; X = F, Cl, Br, I), with corner-shared octahedral layered perovskite structures. Our joint experimental and theoretical study demonstrates that BaCoF₄, a prototype in this new class, possesses a unique combination of multiferroic and photovoltaic properties. Moreover, BaCoF₄ exhibits both an antiferromagnetic spin structure and ferroelectric polarization along the (001) direction. These combined electronic and optical features render BaCoF₄ a promising bifunctional material for application in spintronic or photovoltaic devices. Density functional theory calculations suggest that the dominant point defects in BaCoF₄ are mostly shallow-level donor defects, leading to fascinating n-type self-doping. As such, a BaCoF₄ layer may be exploited for electron transport and light absorption altogether, which may enhance the photovoltaic performance in solar cells. To confirm this predicted feature, we incorporate BaCoF₄ as an electron transport layer and fabricated a BaCoF₄/Cs₀.₀₅MA₀.₁₄FA₀.₈₁PbI₂.₅₅Br₀.₄₅-based solar cell device. Notably, the solar cell devices yield the champion power conversion efficiency of ca. 13.14%. We also investigate photovoltaic properties of other analogous materials and find that BaNiBr₄ and BaMnCl₄ also possess both multiferroic and photovoltaic bifunctionalities. Hence, this new class of BaMX₄ offers diverse and tunable capability for various applications.