Exfoliated Molybdenum Disulfide for TiO2 Based Dye Sensitized Solar Cells
Authors: Ortiz-Quiles Edwin O. , Cabrera Carlos R 
Published: February 2020

Photochemical properties of transition metal dichalcogenides have been studied for more than 30 years, providing promising results as photocatalysts for energy applications. Recent works on these layered materials, particularly on molybdenum disulfide (MoS2), demonstrate that structural changes provoke different electron transitions in the system.  MoS2 is a nonprecious metal, relatively earth abundant, and stable, which possess three main crystalline structure types: 2H, 3R and 1T; with hexagonal, rhombohedral, and trigonal unit cells, respectively. The thermodynamically stable 2H phase can be affected by chemical reactions such as the intercalation of small molecules. A common reactive used to complete this intercalation is n-butyllithium (n-BuLi), since lithium ions strongly interact with MoS2 layers. The quantity of lithium ions inside the layers may fluctuate depending on the duration of the reaction and the concentration of n-BuLi. This interaction produce a LixMoS2 specie that can react with water to exfoliate the MoS2 layers.

MoS2 chemical treatments can affect the oxidation state of MoS2 and its photoabsorption behavior, hence altering its physicochemical properties. These changes provide the opportunity to produce new materials and interfaces, improving properties such as radiation absorption and electron mobility range. These two characteristics make exfoliated MoS2 an excellent material to be tested in dye sensitized solar cells (DSC), especially for the anode of this device. Solar cells normally use expensive organometallic ruthenium based dyes, which decompose under high temperature. In contrast, exfoliated MoS2 (E-MoS2) is a stable inorganic compound, which can also absorb in the visible region where a relatively high concentration of sunlight photons are available. Two approaches are studied in this work; 1) the use of exfoliated MoS2 as enhancer for electron transfer reactions and 2) as a substitute of ruthenium based dyes. Exhaustive physicochemical characterization of the MoS2 during the intercalation and exfoliation process was accomplished using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Raman Spectroscopy and Ultraviolet and Visible Absorption (UV/Vis). The E-MoS2 was further tested in a dye sensitized solar cell. Finally, Electrochemical Impedance Spectroscopy (EIS) was done to obtain an insight of the electron process occurring in these two approaches. Previous results provide evidence of the potential use of exfoliated MoS2 in this photovoltaic devices 

E-MoS2 layers were produced after a lithium intercalation, exfoliation and purification synthesis, and an exhaustive physicochemical characterization was completed. The characterization demonstrated structural and chemical modifications, which affect electron transitions in the E-MoS2 product. DSCs devices were employed to test this E-MoS2 in two different setups: 1) as a layered formation with TiO2 thin film and 2) as a dye-substitute in a hybrid form with TiO2, both at the anode. In the first approach, an increase in photovoltage was detected on the DSC device during testing. Between a 10 fold increase at low wavelength (e.g. 330 nm) and two-fold at higher wavelength (e.g. 370nm). EIS results show lower resistances and higher effective lifetime. In this layered structured thin film between E-MoS2 and TiO2 a MoS2 optical-reflection effect may have caused an increases in the number of electron-hole pairs produced in the cell. Improvements in the electron transfer processes between the iodide/E-MoS2/TiO2 interphase are also a possible explanation of these results. Surprisingly, EIS shows higher resistances compared with the device having an E-MoS2-TiO2 layered structured thin film DSC. A possible explanation is a low E-MoS2 chemisorb process on TiO2, where the nanocomposite may be degraded while the photochemistry develops. We can conclude that E-MoS2 could offer an alternative material in DSC by enhancing the photochemical behavior, yet further studies are needed to attend stability concerns of the material.