| As a renewable clean energy,solar energy has incomparable advantages to other energy sources.Currently,the commercial exploitation of solar energy in many countries around the world is an important development direction.CuInSe2(CIS)-based thin-film solar cell is recognized as the most industrialized and promising thin-film solar cell by industry insiders due to its excellent comprehensive performances.In June 2016,the record of the photoelectric conversion efficiency of CuIn1-xGaxSe2(CIGS)thin-film solar cell on a laboratory scale was broken again.Efficiency of 22.6%has been obtained,which further widens the gap of the photoelectric conversion efficiency between the CIGS thin-film solar cell and the polycrystalline silicon solar cell to 2.2%.The record of the photoelectric conversion efficiency for the polycrystalline silicon solar cell is as high as 20.4%.Hence,CIS-based thin-film solar cell is expected to play an important role in the next investment of solar power.CIS-based thin-film solar cell materials have thus been a research hotspot in the present photovoltaic field.In order to narrow the wide gap between the photoelectric conversion efficiency of laboratory products and that of industrial products,lots of experimental studies have been carried out.Moreover,using the computer simulation technology,one can study the CuInSe2-based thin-film solar cell materials theoretically on the atomic and electronic levels which are difficult for experiment to reach,which is an effective way and also an inevitable choice to prefect and deepen the basic theoretical studies of this class of solar cell materials.In this dissertation,we theoretically studied the lattice structures,electronic properties,thermodynamic phase diagrams,atomic distribution,inhomogeneity,intrinsically ductile-brittle property,and pressure-induced phase transition and so on under doping,deformation and pressure conditions for the light-absorbing materials of CIS-based thin-film solar cells.We summarized some valuable findings and explained some important experimental phenomena.We also provided some novel ideas and theoretical basis for the further design and preparation of these materials.The main contents and results of this dissertation are listed as follows:1)The phase diagram(binoal line and spinodal line)of Ga-doped CuInSe2 was determined by using a combination of ab initio density functional theory calculations and thermodynamic modeling.Our phase diagram is M-shape-like,which is obviously different from the inverted U-shape-like phase diagrams in literatures.It is a correction for the thermodynamic phase diagrams of CIGS calculated by others perviously.We found that there are two local maximum and a local minimum on both the binodal and the spinodal curves for the first time.i.e.,there is a consolute point in the poor-Ga and rich-Ga regions of CIGS phase diagram respectively,which is xc,0.174,Tc = 524K and xc = 0.818,Tc = 659K.Our phase diagram can well explain the result found by Gutay et al.as well as Ludwig et al.that the inhomogeneity of CuIn0.25Ga0.75Se2 is higher than that of CuIn0.75Ga0.25Se2 at the same temperature,while all of the previous phase diagrams can not explain it.The M-shape-like phase diagram calculated by us can provide a theoretical basis and direction for the further preparation of the homogeneous and high-performance CIGS absorbing materials.2)The miscibility gap(binodal curve)of S-doped CuInSe2 was studied by using a combination of the cluster expansion,first-principles calculations based on density functional theory,and Monte Carlo simulations.And the effect of lattice vibrations on the miscibility gap of CuIn(Se1-xSx)2(CISS)alloys was further studied.We have obtained the miscibility gap on the phase-separation of CISS alloys for the first time and found the mechanism of the low-temperature phase transition for CISS alloys.The consolute point of CISS alloys is Tc =170K,xc = 0.61.The miscibility gap of CISS is asymmetric.When the temperature is below 170 K,the CISS alloys are thermodynamically unstable and will decompose to the two isostructural phases with different compositions(i.e.Se-rich and S-rich CISS phases).It is found that the miscibility gap of CISS alloys has a significant change after we considered the contribution of lattice vibrations to the free energies of CISS alloys.The miscibility gap of CISS alloys exhibits a larger asymmetry with a consolute temperature of 130 K.The reduction of the consolute temperature is up to 26.7%,which is so big that is not in the normal range.The size mismatch mechanism was uesd to explain the large effect of lattice vibration.3)The variations of distribution morphologies and inhomogeneity degrees ? of both In-Ga atoms in CIGS and Se-S atoms in CISS alloys with temperature(T)were studied using Monte Carlo simulations based on the cluster expansion method and first-principles calculations.We found the Ga(In)atoms in CIGS alloys cluster in a form of sphere,rod,lamella and mass at low temperatures,respectively,as the Ga concentration increases.While the S(Se)atoms in CISS alloys only cluster in a form of ellipsoid or lamella at low temperatures when the concentration of S atoms rises.The Z-shape-like ?(T)curves for both CIGS and CISS alloys mean that three is a sharp decline of inhomogeneity degree within a certain temperature range where a phase transition from an inhomogeneous state to a homogeneous state occurs.We proposed and tested a new way to predict the phase transition temperature from inhomogeneous to homogeneous state for CIGS and CISS alloys.And this new method is proved to be promising to study the other similar doped alloys.4)The lattice structures,electronic structures and miscibility gap of Al-doped CuInSe2 systematically were studied from first-principles calculations.We found that the lattice parameters a and c of CuIn1-xAlxSe2(CIAS)vary linearly with the doping concentration x.The In-Se and Al-Se bond lengths decrease with the increasing x in CIAS alloys.And the angle of Se-Al-Se tends to 109.5° with the increasing x.The band structures of CIAS demonstrate that they have direct band gaps.The energy gap broadens in CIAS alloys as x increases.The miscibility gap calculated by us shows that the consolute temperature Tc is 480 K.When the temperature is below 480 K,the CIAS alloys have a tendency of phase separation thermodynamically.5)The generalized stacking fault(GSF)energies,cleavage energies,bonding characteristics and intrinsically ductile-brittle property of the chalcopyrite compounds CuAlSe2,CuGaSe2,CuInSe2,CuGaS2 and CuGaTe2 were studied systematically by first-principles calculations.We found that<110>(112)direction is the easiest slip direction for the five compounds.And CuInSe2 is the most probable one to undergo a dislocation-nucleation-induced plastic deformation<110>(112)slip direction.The(112)plane is the preferable plane to fracture for the five compounds.Based on the Rice-Thompson criterion,we found that all the compounds are brittle materials.And the ductility of these compounds can be improved with the increasing ionicity in these compounds at some extent,which points a way to improve the ductility of these light-absorbing materials and prevent them from cracks in flexible thin-film solar cells.6)First-principles calculations based on density functional theory have been carried out for CuGaSe2(CGS)over the pressure region of 0-100 GPa to clarify its structural phase transitions and electronic properties.We confirmed the experimental result theoretically that CGS transforms from the tetragonal I42d structure to the cubic Fm3m structure for the first time.Our results showed that this structural phase transition occurs at 11.87 GPa with a volume reduction of 13.33%.And we also found that under higher pressure CGS can transform from the cubic Fm3m structure to the orthorhombic Cmcm structure.This new phase transition occurs at 51.4 GPa with a volume reduction 0.49%.The energy gap of CGS with the I42d structure broadens as the pressure increases.The Fm3m and Cmcm phases are metallic nature due to the increasing iconicity of CGS at high pressure. |
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