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

Multilayered AgBiP2Se6 has been systematically characterized to be a quasi‐direct semiconductor using theoretical calculations and optical experiments. It possesses an indirect bandgap of 1.46 eV and a direct gap of 1.535 eV for simultaneously absorbing the NIR to visible‐light photons. In combination with a p‐type Ga0.5In0.5Se, a high efficiency p‐Ga0.5In0.5Se/n‐AgBiP2Se6 stacked solar cell is thus fabricated and demonstrated in this work. Abstract 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 (E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$) and direct (E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$) bandgaps, with phonon‐assisted processes playing a significant role in the material's optoelectronic behaviors. The E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ and E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ is confirmed by transmittance (T) measurement, while the direct transition E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ 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. Multilayered AgBiP 2 Se 6 has been systematically characterized to be a quasi-direct semiconductor using theoretical calculations and optical experiments. It possesses an indirect bandgap of 1.46 eV and a direct gap of 1.535 eV for simultaneously absorbing the NIR to visible-light photons. In combination with a p-type Ga 0.5 In 0.5 Se, a high efficiency p-Ga 0.5 In 0.5 Se/n-AgBiP 2 Se 6 stacked solar cell is thus fabricated and demonstrated in this work. 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 (E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$) and direct (E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$) bandgaps, with phonon-assisted processes playing a significant role in the material's optoelectronic behaviors. The E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ and E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition E1ind${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$ is confirmed by transmittance (T) measurement, while the direct transition E2d${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$ 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. Advanced Science, Volume 13, Issue 2, 9 January 2026.