Pure phase synthesis of Cu3PS4 and Cu6PS5Cl for semiconductor applications

We have achieved the first reported pure phase synthesis of two new nanoparticle materials, Cu₃PS₄ and Cu₆PS₅Cl. We have achieved this through learning about the potential reaction pathways that CuCl₂, P₂S₅, and 1-dodecanethiol can take. This study has shown that the key variable to control is the state of the phosphorus source when the CuCl₂ is added. If P₂S₅ is added together with the CuCl₂ to dodecanethiol then the reaction will follow a path to Cu₃PS₄, but if it is dissolved in dodecanethiol prior to the addition to CuCl₂ then the reaction will produce Cu₆PS₅Cl. The formation of these two different phases can occur simultaneously, yet we have found sets of conditions that manipulate the reaction system to form each phase exclusively. These nanoparticles could have broad semiconductor or solid electrolyte applications.

We have achieved the first reported pure phase synthesis of two new nanoparticle materials, Cu₃PS₄ and Cu₆PS₅Cl.

There is a need to explore new thin film photovoltaic absorbers, as many of the current thin film technologies have challenges associated with them. The high efficiency materials such as CuIn_xGa_1−xSe₂ (ref. 1–5) and CdTe,^6–8 require the use of the less-abundant elements indium and tellurium. To rectify this short coming, materials that use earth abundant elements such as Cu₂ZnSnSe₄ (CZTSe)^9,10 and amorphous-Si^11–13 have been explored. This class of materials has been unable to reach the efficiencies of the CuIn_xGa_1−xSe₂ and CdTe cells that are necessary to become an economic alternative to fossil fuel based energy. Specifically in the case for CZTSe, the issue is caused by intrinsic defect formation, leading to band tails in the material.^14–17 This defect is caused by the zinc on copper site (Cu_Zn) and the accompanying copper on zinc (Zn_Cu) site.^18,19 This is due to the similar sizes of the Cu¹⁺ and the Zn²⁺ ions.Because of the uncertainties regarding the limitations and future of previously developed earth abundant materials for solar cells, it is necessary to investigate new materials that avoid the pitfalls that have hampered the previous technologies. It has been proposed to use a Cu₃–V–VI₄ (V = P, As, Sb; VI = S, Se) structured material to address these issues.^20–28 This class of materials uses earth abundant cations to allow for production on a terawatt scale. They also avoid the cation switching that has hampered the efficiencies of CZTSe devices, due to the mismatch between the sizes of V⁵⁺ and Cu¹⁺ cations.Some work examining the phosphorus member of the Cu₃–V–VI₄ material family and its potential use as a solar absorber material has been reported in the literature. The reported calculations have estimated that the band gap of the selenide material is within the ideal range of 1.0–1.5 eV, and they have potential for the power conversion efficiencies to be greater than that of CuInSe₂. Experimental studies have confirmed the band gap of Cu₃PSe₄ to be 1.35 eV.²² On the other hand, Cu₃PS₄ with a higher band gap is a potential candidate for a top cell in a tandem cell. Both of the materials have shown a photoelectric response,^22,23 and could be attractive materials for photovoltaic devices.In the past, crystals of Cu₃PS₄ have been synthesized either using chemical vapor transport and temperatures in excess of 850 °C for long periods of time such as 24 hours^20,29 or heating elemental powders of copper, phosphorus and sulfur in sealed evacuated fused silica tubes at high temperatures for extended time periods.^22,27,30 While these techniques produce crystals of Cu₃PS₄ that could be used for fundamental characterization, they are not suitable for fabrication of thin films of Cu₃PS₄. There is a need to pursue and develop new solution based techniques for the synthesis of Cu₃PS₄, if it is to be competitive with other thin film technologies. Using nanoparticles as a method for forming thin films has been employed for a variety of other materials for PV applications.^1,9,31,32The previous solution-based method, to synthesize Cu₃PS₄ nanoparticles has faced significant obstacles.²³ This method is based on reducing both copper and phosphorus to a neutral state and reacting them together to form Cu₃P nanoparticles. These nanoparticles are then reacted with thiourea in a separate reaction. While this procedure does produce Cu₃PS₄ nanoparticles, they are not pure phase. There is the presence of a phosphorus rich phase that is altering the composition and effecting the photoluminescence. If Cu₃PS₄ nanoparticles are to be used a precursor to a solar absorber, they will need to be free of any contaminants that could adversely affect a final film.For this contribution we have examined copper–phosphorus–sulfide system. This material can occur in two main phases, the Cu₃PS₄ enargite phase and the Cu₇PS₆ argyrodite phase. The argyrodite structure also has a chloride phase compound Cu₆PS₅Cl. The enargite phase is of more interest for photovoltaic applications, as either a top material for a multi-junction device or for use in high band gap electronic devices.Argyrodites, while they may not be useful as solar absorbers, have been explored for use as solid electrolytes.^33–35 Cu₆PS₅Cl has been of particular interest due to its high performance and copper mobility.^36,37 This material has shown better conductivities than other materials in the same family. In a similar case to the enargite materials, synthesis of the argyrodites is done in sealed ampule or vacuum based methods.^34–37