Material And Divice Simulation For CIGS Thin-film Solar Cells

Solar energy is a renewable clean energy, which is inexhaustible without pollution to the environment and has attractive prospects. CuInGaSe2(CIGS) thin film solar cell material is a kind of ideal solar photoelectric materials. It is expected that CIGS solar cells will occupy more than40%share of the photovoltaic industry in2018. However, the main factors hindering CIGS thin film solar cell usage is the application of mostly toxic elements Cd and rare-precious elements In, Ga and Se. The main purposes of this thesis are to reduce the usage of toxic and rare-precious elements, to find their alternatives, and to improve the photovoltaic properties of CIGS solar cells.In this thesis, we used the first principle calculation, atomic simulatin and divice simulation to systemically investigate absorption layer materials and buffer layer materials of CIGS thin film solar cells. The main results are as follows.1) We studied the optical properties of S-doped CuInSe2using the first principle method within HSE06exchange-correlation functional. The lattice constant of CuIn(SxSe1_x)2vary linearly with the composition x as:a(x)=-0.2828*+5.8786A, c(x)=-0.5692x+11.834A. The complex dielectric functions, the refractive indices n, the extinction coefficients k and the absorption coefficient a(co) of S-doped CuInSe2vary very slightly. When the photon energy is less than4eV and larger than6eV, the imaginary part moves to the infrared region with the decrease of sulphur concentration. As x increases from0to1, the static dielectric constant of CuIn(SASe1-x)2decreases from7to5, the optical band gap increases from1.07eV to1.384eV.2) Device modeling had been theoretically carried out to investigate the effects of defect states on the performance of ideal CIGS thin film solar cells. The variety of defect states (location in the band gap and densities) in absorption layer CIGS and that in buffer layer CdS are examined. The performance parameters:open-circuit voltage, short-circuit current, fill-factor and photoelectric conversion efficiency for different defect states were quantitatively analyzed. We found that defect states always do harm to the performance of CIGS solar cells, but when defect state density is less than1014cm-3in CIGS or that is less than1018cm-3in CdS, defects states have little effects on the performances. When defect states are located in the middle of the band gap, they are more harmful. The effects of temperature and thickness are also considered. We found that CIGS solar cells have the optimal performance at about170K and that2um of CIGS is enough for solar light absorption.3) We studied the Cd1-xZnxS sphalerite crystal structure and optical properties. We calculated electronic and optical properties of Cd1-xZnxS at the doping concentration x=0,0.25,0.50,0.75,1.0. Optical properties (reflectivity, absorption coefficient, refractive index, dielectric function) and the electrical properties (band structure, electron density, etc.) are obtained including Zn-doing effects on the crystal structure. With the increase of doping concentration x, the lattice parameter reduces from5.91A to5.409A; as a direct wide band gap semiconductor, its band gap increased from1.15eV to2.22eV.4) Classical atomistic simulations based on lattice dynamics theory and Born core-shell model were performed to systematically study the crystal structure and thermal properties of Cd1-xZnxS (x=0,0.25,0.5,0.75and1). We calculated thermal properties such as the coefficient of thermal expansion, Gruneisen parameter, phonon density of states, specific heat, and Debye temperature at different temperatures and for different Zn-doping concentrations. It is found that the lattice constant decreases as Zn-doping concentration increases. Both the specific heat of constant volume and the coefficient of thermal expansion of Cd1-xZnxS increase and saturate as the temperature increases, and they also slightly inrease with the increasing of Zn-doping concentrations. With x increases from0to1, Gruneisen parameter inreases from0.78to0.94at0K, and the minimum of Debye temperature increases from197K to203K at70K. Some simulation results correspond with experimental data, and we anticipate our results will be helpful to select the base on which Cd1-xZnxS materials are prepared. We also calculated the thermal properties of Ga-dope CuInSe2.5) We studied the ZnO1-xSx sphalerite crystal structure and optical properties. We calculated the crystal structure, optical properties (reflectivity, absorption coefficient, refractive index, dielectric function) and the electrical Properties (band structure, electron density, etc.) of ZnO1_xSx (x=0,0.25,0.50,0.75,1.0). The impact of S doping concentration in ZnO1-xSx on the crystal structure, optical properties and electrical properties were obtained. It is shown that the zinc blend type compounds CdS and ZnO1-xSx are both direct band gap materials; the lattice constants increase with the quantity of S doping ZnO, the ZnO1-xSx has a wider band gap than CdS and can be expected to provide a high quality buffer layer for high efficiency (CIGSe) solar cells.