GeSe: Optical Spectroscopy and Theoretical Study of a van der Waals Solar Absorber

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Van der Waals materials like Germanium Selenide (GeSe) are explored for solar energy applications.
• The precise optoelectronic properties of GeSe, crucial for solar absorbers, require further elucidation.
• Previous studies reported conflicting values for its fundamental band gap and transition type.

Purpose of the Study:

• To comprehensively determine the optoelectronic and structural properties of GeSe.
• To resolve discrepancies in reported band gap values and transition types.
• To validate theoretical methods for predicting properties of van der Waals materials.

Main Methods:

• Combined theoretical (first-principles calculations) and experimental (optical absorption) approaches.
• Quasiparticle self-consistent GW method and Bethe-Salpeter equation for band gap calculations.
• Temperature-dependent optical absorption measurements and structural analysis using hybrid functionals.

Main Results:

• A fundamental absorption onset of 1.30 eV at room temperature, aligning with the Shockley-Queisser limit.
• Experimental absorption spectra and theoretical joint density of states show agreement, with an additional onset ~0.3 eV above the fundamental edge.
• First-principles calculations converge to a 1.33 eV band gap (0 K), consistent with experimental data.
• Structural analysis indicates a direct fundamental transition, contradicting earlier reports of an indirect transition.

Conclusions:

• Germanium Selenide (GeSe) is characterized as a direct semiconductor with a 1.30 eV room-temperature band gap.
• The findings provide accurate optoelectronic properties for GeSe, supporting its potential as a solar absorber.
• The strong agreement between theory and experiment validates the computational methodology for similar van der Waals materials.