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

Abstract

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 (Voc) 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 Voc 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 Voc 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.

Recent Progress on Two-Dimensional Materials

“…are semiconductor with unique optoelectronic properties, such as layer-dependent bandgap and high charge mobility 2,1475 . When the layer number decreases to be monolayer, they exhibit strong photoluminescence due to large exciton binding energy by the nanoconfinement effect 1476 , thus many monolayer 2D materials are very attractive in optoelectronic devices, such as transistors 1477,1478 , photodetectors 1479,1480 , light emitting diodes 1481,1482 and solar cells 1483,1484 . MoS2 is the most studied 2D semiconducting material.…”

“…Overall, 2D materials for flexible electronic devices is a rapidly expanding area, widely applicable in wearable logic circuits, energy storage and healthcare 1443,1444,1447,1448,1455,1469,1472,1483,1484 . However, their future largely depends on the development of material synthesis and technological processing for 2D devices on plastic or elastomeric substrates, requiring a lot of scientific understanding and material engineering.…”

Optical Harmonic Generation in 2D Materials

“…[ 27–29 ] Recently, investigations into exploring the unique layer dependent optical properties [ 30 ] and phase matching attributes [ 31 ] in 2D materials has attracted large interest. 2D materials not only provide strong nonlinear optical properties compared to common bulk nonlinear materials, [ 32–40 ] but also the nonlinear properties in 2D materials can be tuned using various techniques such as wavelength/exciton tuning, [ 41 ] electrical tuning, [ 42 ] and thickness variation, [ 43 ] which can maximize the performance of nonlinear optical devices. Moreover, nonlinear interaction of light in 2D materials has the potential for advanced materials characterization, such as layer identification, [ 43 ] lattice orientation, [ 43 ] phase variation, [ 31 ] strain direction and intensity, [ 44,45 ] stacking angle determination, [ 46 ] etc., making OHG a powerful characterization tool for 2D materials.…”

Retracted: Emerging 2D MXene/Organic Heterostructures for Future Nanodevices

“…[ 1 ] Nowadays, 2D material families are regularly expanding and now cover materials ranging from metallic graphene to wide band‐gapped hexagonal boron nitride (h‐BN). However, recent research is more focused toward the semiconducting category of 2D materials, which has now widely expanded into transition metal dichalcogenides (TMDs), [ 2–5 ] black phosphorous (BP), [ 6–8 ] and its iso‐electronic group‐IV monochalcogenides (MNs) such as GeS, GeSe, SnS, SnSe, etc., [ 9–11 ] main group element compounds such as InSe, [ 12 ] In 2 Se 3 , [ 12,13 ] Bi 2 O 3, [ 14 ] Bi 2 S 3, [ 15 ] Sb 2 O 3 , [ 16 ] Sb 2 Se 3 , [ 17 ] SnP 3 , [ 18,19 ] etc., Xenes such as silicene, germanene, stanene, etc., [ 17 ] transition metal carbides and/or nitrides (MXenes), [ 20–22 ] perovskite, [ 23 ] and various 2D organic materials such as pentacene, and 2,7‐Dioctyl[1]benzothieno[3,2‐b][1] benzothiophene (C 8 ‐BTBT). [ 24,25 ] In particular, the past decade has shown tremendous success in expanding the 2D materials family, and demonstrating their potential nanotechnological applications.…”