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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>1</mml:mn> <mml:mi>ind</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$</mml:annotation></mml:semantics> </mml:math> ) and direct ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>2</mml:mn> <mml:mi>d</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$</mml:annotation></mml:semantics> </mml:math> ) bandgaps, with phonon-assisted processes playing a significant role in the material's optoelectronic behaviors. The <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>1</mml:mn> <mml:mi>ind</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$</mml:annotation></mml:semantics> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>2</mml:mn> <mml:mi>d</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$</mml:annotation></mml:semantics> </mml:math> transitions at 300 K are determined to be 1.46 and 1.535 eV, respectively. The indirect transition <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>1</mml:mn> <mml:mi>ind</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{1}}^{{\mathrm{ind}}}$</mml:annotation></mml:semantics> </mml:math> is confirmed by transmittance (T) measurement, while the direct transition <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>E</mml:mi> <mml:mn>2</mml:mn> <mml:mi>d</mml:mi></mml:msubsup> <mml:annotation>${\mathrm{E}}_{\mathrm{2}}^{\mathrm{d}}$</mml:annotation></mml:semantics> </mml:math> 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.