Study On The Thermodynamic Stability Of Cd_1-xZn_xS And ZnO_1-xS_x Alloys

With the progress of human society, a series of environmental problems are produced such as the conventional energy is reduced or even exhausted,environmental pollution and ecological destruction. Exploitation for clean energy is an important development direction for many countries around the world. Solar energy has become the first choice for many people as a renewable green energy.Cu In_1-x Gax S₂（CIGS） thin-film solar cell is recognized as the most promising thin-film solar cell materials due to its excellent optoelectronic properties and low cost.CIGS thin-film solar cell is layer structure with Cd S as the most used buffer layer material which not only has a unique advantage but also has a fatal flaw that Cd is a toxic element in Cd S. Improper handling of Cd element can do harm to the environment as well as the human health. In order to reduce cadmium usage, people improve the buffer layer material from two aspects. On one hand, Cd can be replaced by other elements（Zn for example） with no effect or even can improve the cell efficiency. On the other hand, other materials（such as Zn S, Zn Se, Zn O,（Zn, Mg）O,In₂S₃, In（OH）₃） are adopted to replace Cd S.The first-principles calculations were used to calculate the lattice structure, band structure and density of states of both wurtzite（WZ） Cd S、Zn S、Zn O and zinc-blende（ZB） Cd S、Zn S. The results agree well with other experimental or calculated results,which show that our calculated parameters are correct. For the first time, the binodal phase diagrams of wurtzite（WZ） and zinc-blende（ZB） Cd_1-x Znx S and WZ ZnO_xS_1-x solid solutions are calculated by first-principles calculations combined with the cluster expansion and Monte Carlo simulations. All formation energies of these alloy systems are positive indicating that they have a tendency to phase separation. For（WZ and ZB） Cd_1-x Znx S and WZ ZnO_xS_1-x alloys, the consolute temperature Tc is 655 K, 604 K and 3168 K, corresponding concentration xc is 0.695, 0.646 and 0.5,respectively. And both of the former two have an asymmetric miscibility gap while the latter one has a centrosymmetric shape. We also presented the spatial distribution of Cd and Zn atoms in both WZ and ZB phases Cd_0.5Zn_0.5S alloys as well as O and S atoms in WZ Zn O_x S_1-x alloys. It is found that the several kinds of alloy systems exhibit a phase segregation at low temperature, which is consistent with the calculated phase diagrams. It also indicated that the alloy systems becoming more and morehomogeneous with the temperature increase under some extent.