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

AgBiP₂Se₆ 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) AgBiP₂Se₆ 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 ( $E_1^{\mathrm{ind}}$ ) and direct ( $E_2^d$ ) bandgaps, with phonon-assisted processes playing a significant role in the material's optoelectronic behaviors. The $E_1^{\mathrm{ind}}$ and $E_2^d$ transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition $E_1^{\mathrm{ind}}$ is confirmed by transmittance (T) measurement, while the direct transition $E_2^d$ is simultaneously detected through micro-photoluminescence (µPL) and micro-thermoreflectance (µTR) measurements. A stacked p-Ga_0.5In_0.5Se/n-AgBiP₂Se₆ heterojunction solar cell is successfully fabricated, achieving a photoelectric conversion efficiency (PCE) up to ≈0.583%. Furthermore, AgBiP₂Se₆ is demonstrated as a promising photocatalyst, exhibiting a high degradation efficiency to organic dyes. ML-AgBiP₂Se₆ 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.