Quantifying Quasi-Fermi Level Splitting and Mapping its Heterogeneity in Atomically Thin Transition Metal...

Area of Science:

• Materials Science
• Condensed Matter Physics
• Renewable Energy

Background:

• Quasi-Fermi level splitting (∆µ) is crucial for determining the maximum open-circuit voltage (V_oc) in photovoltaic materials.
• Atomically thin 2D transition metal dichalcogenides (TMDs) are promising candidates for next-generation solar cells due to their unique electronic properties.

Purpose of the Study:

• To develop and apply a contactless, nondestructive method for quantifying ∆µ in 2D TMDs.
• To assess the theoretical V_oc limits for solar cells based on WS₂, MoS₂, WSe₂, and MoSe₂ monolayers.
• To investigate methods for improving ∆µ homogeneity in TMD monolayers.

Main Methods:

• A contactless, nondestructive optical technique was employed to measure ∆µ under illumination.
• Electrical gating in a metal-oxide-semiconductor structure was used to modulate the doping and ∆µ heterogeneity.
• The measured ∆µ values were used to predict potential V_oc for different TMD monolayers.

Main Results:

• The study quantified the upper limits of V_oc for WS₂ (≈1.4 V), MoS₂ (≈1.12 V), WSe₂ (≈1.06 V), and MoSe₂ (≈0.93 V) monolayers at 1 Sun.
• Inhomogeneity in ∆µ was observed across different regions of the TMD monolayers.
• Electrically gating the TMD monolayers demonstrated an improvement in ∆µ heterogeneity.

Conclusions:

• Atomically thin TMDs possess significant potential for realizing high-voltage, ultralight, flexible, and even eye-transparent solar cells.
• The developed method provides a critical tool for evaluating and optimizing TMD-based photovoltaic devices.
• Engineering ∆µ heterogeneity is a viable strategy for enhancing the performance of TMD solar cells.