Work Function Tuning in Hydrothermally Synthesized Vanadium-Doped MoO3 and Co3O4 Mesostructures for Energy Conversion Devices

Pietro Dalle Feste,Filippo Drago,Matteo Crisci,Silvia Gross,Mirko Prato,Francesca Tajoli,Federico Barbon,Marco Salerno,Teresa Gatti,Francesco Lamberti

doi:10.3390/app11052016

Abstract

The wide interest in developing green energy technologies stimulates the scientific community to seek, for devices, new substitute material platforms with a low environmental impact, ease of production and processing and long-term stability. The synthesis of metal oxide (MO) semiconductors fulfils these requirements and efforts are addressed towards optimizing their functional properties through the improvement of charge mobility or energy level alignment. Two MOs have rising perspectives for application in light harvesting devices, mainly for the role of charge selective layers but also as light absorbers, namely MoO3 (an electron blocking layer) and Co3O4 (a small band gap semiconductor). The need to achieve better charge transport has prompted us to explore strategies for the doping of MoO3 and Co3O4 with vanadium (V) ions that, when combined with oxygen in V2O5, produce a high work function MO. We report on subcritical hydrothermal synthesis of V-doped mesostructures of MoO3 and of Co3O4, in which a tight control of the doping is exerted by tuning the relative amounts of reactants. We accomplished a full analytical characterization of these V-doped MOs that unambiguously demonstrates the incorporation of the vanadium ions in the host material, as well as the effects on the optical properties and work function. We foresee a promising future use of these materials as charge selective materials in energy devices based on multilayer structures.

Highlights

Transition metal oxides are a multifaceted and diversified class of inorganic materials whose chemico-physical, structural and functional properties encompass a very wide range of different features [1,2,3,4]
We describe the subcritical hydrothermal synthesis of V-doped Co3O4 micro-wires and MoO3 micro-lamellae starting from water soluble precursors, with the aim of finely tuning the electronic properties of these metal oxide (MO) semiconductors to improve their potential for exploitation in energy-related devices
An additional phase is detected only at the highest nominal concentration of dopant (i.e., 20% at.), as shown in Figure 2b, where weak reflections appear at 2θ = 20.3◦, 26.2◦. They are ascribable to the lattice planes (0 0 1), (1 1 0) of crystalline orthorhombic V2O5 (PDF 41-1426) [54], indicating the formation of a second and separated MO phase in the synthetized material when a relatively high amount of V-precursor is introduced in the reaction mixture

Summary

Introduction

Transition metal oxides are a multifaceted and diversified class of inorganic materials whose chemico-physical, structural and functional properties encompass a very wide range of different features [1,2,3,4]. From the structural point of view, depending on their stoichiometry, they display very different structures, such as, for instance, (the most common) NaCl, rutile, corundum, fluorite, spinel and cuprite structures As far as their electric behavior is concerned, it ranges from superconductors (e.g., high-Tc copper oxides), to good metallic conductors (e.g., V2O3, ReO3) through to semiconductors (e.g., NiO, ZnO, TiO2, VO2) and they can display interesting magnetic (e.g., Fe3O4), and electrochromic (e.g., WO3) characters [2,3]. From the chemical point of view, their properties span the full range, from acidic through amphoteric to basic These chameleonic features allow these materials to be widely used in commercial products combined to their thermal and chemical stability, ease of processing, and excellent mechanical robustness [5,6,7,8,9].

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