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

One of the most fundamental parameters of any photovoltaic material is its quasi-Fermi level splitting (∆µ) under illumination. This quantity represents the maximum open-circuit voltage (V_oc) that a solar cell fabricated from that material can achieve. Herein, a contactless, nondestructive method to quantify this parameter for atomically thin 2D transition metal dichalcogenides (TMDs) is reported. The technique is applied to quantify the upper limits of V_oc that can possibly be achieved from monolayer WS₂, MoS₂, WSe₂, and MoSe₂-based solar cells, and they are compared with state-of-the-art perovskites. These results show that V_oc values of ≈1.4, ≈1.12, ≈1.06, and ≈0.93 V can be potentially achieved from solar cells fabricated from WS₂, MoS₂, WSe₂, and MoSe₂ monolayers at 1 Sun illumination, respectively. It is also observed that ∆µ is inhomogeneous across different regions of these monolayers. Moreover, it is attempted to engineer the observed ∆µ heterogeneity by electrically gating the TMD monolayers in a metal-oxide-semiconductor structure that effectively changes the doping level of the monolayers electrostatically and improves their ∆µ heterogeneity. The values of ∆µ determined from this work reveal the potential of atomically thin TMDs for high-voltage, ultralight, flexible, and eye-transparent future solar cells.