Carrier Multiplication in Transition Metal Dichalcogenides Beyond Threshold Limit

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Carrier multiplication (CM) is crucial for enhancing photovoltaic efficiency by generating multiple excitons from a single photon.
• Conventional CM requires photon energies at least twice the material's bandgap (2Eg) due to energy conservation principles.

Purpose of the Study:

• To investigate and demonstrate sub-bandgap carrier multiplication in monolayer transition metal dichalcogenides (TMDCs).
• To explore the role of lattice vibrations and electron-phonon coupling in enabling CM below the conventional threshold.
• To identify TMDCs as promising materials for advanced optoelectronic applications.

Main Methods:

• Experimental observation of carrier multiplication in monolayer TMDCs using specific excitation energies.
• Theoretical analysis of electron-phonon coupling (EPC) effects on electronic structures.
• Investigation of defect engineering (chalcogen vacancies) to further reduce CM threshold energy.

Main Results:

• Carrier multiplication observed in monolayer TMDCs at excitation energies significantly below 2Eg, as low as 1.75Eg.
• Strong electron-phonon coupling in MoS2 monolayer correlates with the most efficient CM.
• Chalcogen vacancies in WS2 monolayer enable CM at an excitation energy as low as 1.51Eg.

Conclusions:

• Lattice vibrations and EPC are key mechanisms for achieving below-threshold CM in TMDCs.
• Monolayer TMDCs, particularly MoS2 and WS2 with defects, are excellent candidates for next-generation optoelectronic devices due to efficient CM and high photoconductivity.