Exploration of Quasi‐Direct Band Edge in a Multilayer Ferroelectric Semiconductor for Applications in Van Der Waals Stacked Heterojunction Solar Cell and Photocatalytic Devices

Abstract AgBiP ₂ Se ₆ is a ferroelectric semiconductor with a Curie temperature above 300 K which also possesses a variety of functional capabilities. In this work, it is demonstrated that despite its intrinsically indirect bandgap, multilayer (ML) AgBiP ₂ Se ₆ exhibits unexpectedly strong photoluminescence, attributed to its quasi‐direct band structure. The energy difference between the indirect and direct transitions is relatively small (≈0.075 eV), as confirmed by both theoretical calculations and experimental observations. Temperature‐dependent optical measurements, corroborated by electronic band structure analysis, reveal the coexistence of indirect () and direct () bandgaps, with phonon‐assisted processes playing a significant role in the material's optoelectronic behaviors. The and transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition is confirmed by transmittance (T) measurement, while the direct transition is simultaneously detected through micro‐photoluminescence (µPL) and micro‐thermoreflectance (µTR) measurements. A stacked p‐Ga _0.5 In _0.5 Se/n‐AgBiP ₂ Se ₆ heterojunction solar cell is successfully fabricated, achieving a photoelectric conversion efficiency (PCE) up to ≈0.583%. Furthermore, AgBiP ₂ Se ₆ is demonstrated as a promising photocatalyst, exhibiting a high degradation efficiency to organic dyes. ML‐AgBiP ₂ Se ₆ exhibits a quasi‐direct bandgap and ferroelectric behavior at room temperature, making it a strong candidate for next‐generation electronic, optoelectronic, and environment‐protection functions.