Electronic Structures And Optical Properties Of Copper-based Multinary Semiconductors

Adamantine Cu-based ternary semiconductors Cu-III-VI₂（with III=Al, Ga,and In; VI=S, Se, and Te）, quaternary semiconductors Cu₂-II-IV-VI₄（with II=Znand Cd; IV=Ge and Sn; VI=S and Se）, as well as their solid solutions (e.g.,CuIn_xGa_1-xS_2ySe_2（1-y）and Cu₂ZnSnS_4xSe_4（1-x）) have been intensively studied owing totheir desired optical properties for photovoltaic application. These materials arecomposed of multinary elements. As a result, although it is widely recognized that thechemical flexibility of these systems provide a vast space for materials design andoptimization, this flexibility inevitably comes at the cost of complexity.The first principles calculation methods that based on the local densityapproximation （LDA） and generalized gradient approximation （GGA） functionalsare widely used for electronic properties of solids. There are also lots of reports thatstudying the electronic and optical properties Cu-based semiconductors. However,these methods usually seriously underestimate the band gap of a semiconductor,which is one of the most important parameters for photovoltaic applications. Eventhe advanced many-body method of GW fails to produce reliable band gaps ofCu-based ternary semiconductors. So, it is of great importance to investigate theseunresolved theoretical issues and to investigate the electronic properties of Cu-basedquaternary semiconductors in order to find new candidates for photovoltaicapplications. Besides the electronic properties, the investigation of optical propertiesare further complicated because of the computation methods, especially when theexcitonic effect is involved. So, reliable information of their electronic and opticalproperties from theoretical investigation is strongly needed for experiments.This thesis deals with structural, electronic, and optical properties of Cu-basedmultinary semiconductors using the GW and Bethe-Salpeter Equation （BSE）.Because of the similarity, we also compare with binary analogy of ZnS. We find that:（1） the difficulties for theoretical research come from the dual nature of Cu d states.Cu d states are only slightly shallower than VI p and contribute to the strong pdhybridization, leading to the interplay of strong pd hybridization and intrinsicallylocalization of Cu d states;（2） The established procedure that firstly relax the crystalstructures in the Heyd-Scuseria-Ernzerhof （HSE） functional and then calculate theband structures in the methods of LDA+U （U=4eV） and GW （i.e., LDA+U+G₀W₀）can give reasonable anion displacement μ and electronic band structures,respectively. The error bar for band gaps of ternary semiconductors are within±0.2eV compared with measured values and the calculated density of states agree wellwith X-ray photoelectron spectroscopy;（3） The structural and band structures of quaternary semiconductors are also investigated. The band gaps of both the stanniteand kesterite structures are calculated, which show that Cu₂ZnSnS₄、Cu₂CdSnS₄,Cu₂ZnGeSe₄, and Cu₂CdGeSe₄have suitable band gaps for photovoltaic applications;（4） Taking CuGaS₂and CuInS₂as examples, we study the optical properties ofCu-based semiconcutors. Results show that the excitonic effect is effectivelysupressed in real experiments. As a consequence, the dielectric function is welldescriped in independent partical approximation. The differences of opticalproperties of Cu-based semiconductors are induced by the near-edge conductionstates. Cu₂GeS₃, Cu₂GeSe₃, and Cu₂SnS₃are potential candidates for photovoltaicapplications because of their excellent properties for the sun irradiation.The optical absorption coefficient of Cu-based semiconductors are much higherthan other thin film materials （e.g., GaAs and CdTe）, which is of great advantage toimprove the photocurrent and thus the efficiency of solar cell. By impurity doping,intermediate bands which are partially occupied by electrons can be introduced intothe intrinsic band gap of the host material. As a result, the photons with energiessmaller than the intrinsic band gap can also be absorbed via two-step transition. Theresearch of intermediate band solar cell （IBSC） materials is in its infancy stagewhich calls for theoretical studies to guide the experiments.Based on the above obtained information about electronic and opticalproperties and the established calculating methods, we design two IBSC materials.Intermediate bands are observed by doping Sn or Fe element into host CuGaS₂material. Our results manifest that Cu₈Ga₇SnS₁₆is a suitable candidate as an IBSCmaterial. The absorption from the valence band to the intermediate band is quitestrong in Cu₈Ga₇FeS₁₆. However, it is rather weak for absorption from theintermediate band to the conduction band which limits the potential usage.In summary, we theoretically study several properties of Cu-based multinarysemiconductors, which are found to be sensitively influenceed by dual nature of Cud states, i.e., hybridization and localization. This thesis also establises a procedurethat using the HSE functional, LDA+U+G₀W₀（U=4eV） method, and independentpartical approximation for the reliable structural, electronic, and optical properties.The structural and electronic band structructure of Cu-based quaternarysemiconductors are investigated, which provided useful and informative guidancefor future material design and selection. Based on above findings, we design twoIBSC materials, i.e., Cu₈Ga₇SnS₁₆and Cu₈Ga₇FeS₁₆, which show that Cu₈Ga₇SnS₁₆is a suitable candidate as an IBSC material and that the weak absorption from theintermediate band to the conduction band limits the potential usage of Cu₈Ga₇FeS₁₆.