Synthesis of atomically thin transition metal disulfides for photovoltaics and water splitting catalysts

Abstract

As a new class of material, two-dimensional (2D) atomic crystals have attracted enormous research interest in the last decade that has led to a number of breakthroughs in physics owing to the confined charge, spin and heat transport within the 2D planes. The most outstanding one of these materials is graphene, as its exceptional electronic, optical and mechanical properties may hold great promise for a variety of future applications. The chapter 3 will cover the application of 2D transition metal disulfide thin films to charge transport layers and p-n junction material with p-Si wafer. Transition metal disulfides (MeS2) such as MoS2 and WS2 were used as charge transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells in order to enhance the stability in air comparing to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). MeS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. The ultraviolet-ozone (UV-O3) treatment on MeS2 leads to the removal of the surface contaminants produced by the transfer process, resulting in a uniform surface and an increase of the work function. Furthermore, the strong light absoption of single-layer of MoS2 could be utilized by light absorption layer in p-type Si-based photovoltaic cells. Specifically, a single semiconducting 0.6-nm-thick MoS2 can absorb as much sunlight as 50 nm of Si and generate photocurrents as similar as 12-nm-thick GaAs semiconductor. The MoS2 thin films could be the one of promising light absorption layer and have potential to make p-n heterojunction photovoltaic cells with p-Si substrate. We synthesize the wafer-scale molybdenum disulfide thin-films by thermolysis of solution precursor based method. After that, the thin films are transferred to p-Si and formed a heterojunction with p-Si. In order to maximize and fully utilize the excellent property of the n-MoS2, Transparent Au nanomesh electrode (Sheet resistance ≈ 6 Ω/sq. at 90% transmittance) fabricated from UV-O3 treated polymeric nanofiber templates is integrated to n-MoS2/p-Si heterojunction. The n-MoS2/p-Si heterojunction with Au nanomesh electrodes exhibit a power conversion efficiency of 4.69%. After antireflection coating, the device shows the efficiency of 5.96% at 0.44 cm2 of the active area. Hydrogen appears as a next-generation clean energy source to replace fossil fuels. One of the most promising ways to produce hydrogen is photoelectrochemical (PEC) water splitting. However, the existing photoelectrodes such as Si with noble metal catalysts still suffer from low efficiency and poor stability and the extremely high cost of the noble metal catalysts limits the wide use of water splitting photoelectrodes. Therefore, a novel approach is necessary to make a breakthrough for highly efficient PEC water splitting. This thesis contains that the demonstration of wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide the low onset potential of 1 mA/cm2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA/cm2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown by a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction. Moreover, in chapter 4.3, the anion-engineered MoS2 thin-films display the higher catalytic activity compared to the partially vertical-aligned MoS2 thin films, due to its many inherent dangling bonds on their surface and the metallic nature. The transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts synthesized through simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with different sulfur/phosphorus weight ratios. The synthesized sulfur-doped molybdenum phosphide (S:MoP) thin film changed from two-dimensional van der Waals structure to three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS2 thin film. The S:MoP thin film catalyst, which is composed of cheap and earth abundant elements, could provide the lowest onset potential and the highest photocurrent density for planar p-type Si photocathode. The density functional theory calculations indicate that the surface of S:MoP thin film absorb hydrogen better than that of MoS2 thin film. The structurally engineered thin film catalyst facilitates the easy transfer of photogenerated electrons from p-Si light absorber to electrolyte. Anion-engineering of MoS2 thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.