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 ind ${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ ) and direct ( E 2 d ${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ ) bandgaps, with phonon-assisted processes playing a significant role in the material's optoelectronic behaviors. The E 1 ind ${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ and E 2 d ${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition E 1 ind ${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ is confirmed by transmittance (T) measurement, while the direct transition E 2 d ${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ is simultaneously detected through micro-photoluminescence (µPL) and micro-thermoreflectance (µTR) measurements. A stacked p-Ga0.5In0.5Se/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.