| It’s essential to develop new quantum energy materials so as to resolve the energy problems currently.The core of a photovoltaic device is the semiconductor p-n junction.The search for more efficient,inexpensive and longer-life photovoltaic semiconductor materials has received widespread attention from the academic community and the industry world.Many members of the ABC2 ternary semiconductors show good performance in the fields of nonlinear optics and photo-electric solar energy converters.In 2018,MgSiAs2,,a new member in the ABC2 family,has been synthesized for the first time by K.E.Woo et al.,and was found to have good nonlinear optical performance in the infrared region.Meanwhile,K.E.Woo et al.explored a new structure Mg3Si6As8 in experiment.The band structure of Mg3Si6As8 has pseudo-direct band gap with width 2.02 eV.The valence band top of Mg3Si6As8 has multiple local maxima with small energy differences between each other.The small energy differences between these local maxima provides multiple light absorption channels,therefore the light absorption coefficient of the material tend to be large.Accordingly,we can assume that the photovoltaic efficiency of Mg3Si6As8 is probably high.In this thesis,through substituting the Ⅳ and the Ⅴ elements in Mg3 Si6As8,we theoretically predicted a new class of pseudo-direct gap semiconductor denoted as Mg3IV6V8.The photovoltaic efficiencies of these solar cells are predicted to exceed that of silicon and most silicon containing tandem cells.The main contents of this thesis are summarized as follows:MgSiAs2 chalcopyrite crystallizes in a non-centrosymmetric structure with good second harmonic generation(SHG)performance,which poses its potential application as nonlinear optical material in the infrared region.However,the transparency window of IR-NLO materials can be limited by the photo-ionization of native defects,where electronic states arising from the native defects appear near the band edge or/and in band gap,and consequently the optical absorption behavior of the material is altered.In the third chapter,we systematically investigated the native point defects of MgSiAs2 including vacancies,interstitials and antisites using first-principles calculations with the hybrid functional.Of the thirteen different point defects studied,nine kinds of defects show deep transition levels,which might contribute to a limitation on the transparency spectrum of MgSiAs2 compounds.Our calculations show that changes of the growth conditions affects modestly the formation energy of defects or the concentration of charge carriers.Generally,photodetector and PV devices show improved efficiency when constructed from direct rather than indirect semiconductors.However,there are particular cases where the energy differences between the local maxima of the top valence band or the local minima of the bottom conductance band is very small,resulting in the so-called pseudo-direct band gaps.Due to expanded light absorption channels in the Brillouin zone,the light absorption coefficients of these materials are exceptionally large.In addition,researchers proposed that,the small difference between the indirect and direct transitions raises the possibility for "pseudo-direct" light emission/absorption through tweaking the band structure with strain or doping,which further increases the quantum efficiency of photodetectors.In the fourth chapter,through substituting the Ⅳ and the V elements in Mg3Si6As8,we theoretically predicted a new class of pseudo-direct gap semiconductor denoted as Mg3IV6V8.Nine stable crystals in this new class are theoretically predicted,and multilayer tandem solar cells consisting of these compounds are proposed.The photovoltaic efficiencies of these solar cells are predicted to reach 30%exceeding that of silicon and most silicon containing tandem cells.In the fifth chapter,we present a study of Co doping in the wide-band-gap semiconductor Mg3Si6As8.It is found that among all considered doping sites in the compound,the substitutional doping at the tetrahedral sites are by far the most stable,and the exchange interactions between substitutional Co ions at the tetrahedral sites yield antiferromagnetic order.The crystal field effect in the splitting of the Co-d orbitals as well as the nature in the magnetic couplings between Co atoms is revealed,and the optimal doping for the highest Neel temperature is found.At present,a large number of theoretical calculations in condensed matter physics and theoretical calculations in materials science are all based on density functional theory.From the perspective of physics-based modeling,the first-principles methods from quantum mechanics are the most accurate simulation methods.However,due to the high cost of first-principles calculations,the scope of application of these methods is limited.Over the past twenty years,machine-learning(ML)has grown to be a vital tool in the field of image and speech recognition,and the method has matured ever since.Now,the method is infiltrating nearly all traditional areas of science and engineering.When it comes to physical modeling,machine learning is not intrinsically competent.A physical system is characterized mostly by its symmetry.We categorized some known ML-based modeling techniques,and selected the deep potential method based on the better interpretability and the more flexible and primary framework of the algorithm.Using deep potential,we developed a model capable for the structural search problem of aluminum clusters.In the sixth chapter,a ML based deep potential model for Al clusters is developed through training with an extended database including ab initio data of both bulk and several clusters.Based on our developed DP model,the low-lying structures of 101 different sized Al clusters are extensively searched,among which the lowest energy candidates of 69 sized clusters are updated.Our calculations demonstrate that machinelearning is indeed powerful in generating potentials to describe the interaction of atoms in complex materials. |
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