Defect-Engineered Reduction of the Carrier Multiplication Threshold in Monolayer WS₂ to 1.6E_g

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• Carrier multiplication (CM) generates multiple electron-hole pairs per photon, offering a route to surpass the Shockley-Queisser limit in solar cells.
• Conventional CM requires high photon energies (>2Eg), leaving significant energy from sub-2Eg photons unutilized.
• Developing methods for low-threshold CM is crucial for enhancing photovoltaic performance and energy conversion efficiency.

Purpose of the Study:

• To experimentally demonstrate and investigate low-threshold carrier multiplication in monolayer tungsten disulfide (WS2).
• To explore the role of electron-phonon coupling (EPC) and defect engineering in reducing the CM threshold.
• To provide a novel strategy for improving the efficiency of solar cells and optoelectronic devices.

Main Methods:

• Transient-absorption spectroscopy was employed to systematically study carrier multiplication dynamics.
• Steady-state spectroscopic techniques, including photoluminescence and Raman spectroscopy, were utilized.
• Density functional theory (DFT) calculations were performed to elucidate the underlying physical mechanisms.

Main Results:

• A low carrier multiplication threshold of 1.6Eg was experimentally achieved in monolayer WS2.
• Engineering high-symmetric disulfur vacancies and leveraging strong electron-phonon coupling were key to reducing the CM threshold.
• Spectroscopic data and theoretical calculations confirmed efficient phonon-assisted upconversion facilitating transitions from defect states.

Conclusions:

• The study presents a viable strategy for significantly lowering the carrier multiplication threshold in 2D materials.
• Strong EPC, enhanced by engineered vacancies, is identified as a critical factor for efficient low-threshold CM.
• This work contributes to the development of high-efficiency photovoltaics and advanced optoelectronic technologies.