Studies Of Some Cu Based Chalcogenides Solar Cells Through Density Functional Theory Calculations

Solar cell materials attract significant attention in order to solve the big issue of energy source.In this thesis,the Cu-based quaternary compound Cu2ZnSiSe4 is selected as host material for solar cells.An intermediate band(IB)is induced when group-VA element(N,P,As and Sb)is doped into the Cu2ZnSiSe4 alloy at Si site.Three photo processes due to the appearance of IB greatly enhance the absorption coefficient.On the other hand,a tunable bandgap can be obtained in a wide energy range through the doping of group-IV element(Ge,Sn)in the Cu2ZnSiSe4 alloy.Both the induced IB and the tunable bandgap bring huge potential applications for the Cu2ZnSiSe4 alloy in solar cell field.The background and research progress of solar cell materials are summarized in the first chapter.The density functional theory and computational approach are introduced in the second chapter.The third chapter is about the electronic structures and optical properties of the group-VA(N,P,As and Sb)-doped Cu2ZnSiSe4 alloys using density functional theory calculations through hybrid functional.The minor lattice distortion and small formation energy indicate that the synthesis of these alloys is highly possible in experiment.For each doped alloy,an isolated and partial filled intermediate band(IB)appears in its band structure.The doping induced IB is mainly contributed by s states of the doped group-VA atom and p states of four neighboring Se atoms,slightly by d states of eight Cu atoms.The existence of IB brings obvious enhancement of the absorption coefficient with two additional absorption peaks at the visible light range.For the P,As and Sb-doped Cu2ZnSiSe4 alloys,not only the bandgap between valence band maximum(VBM)and conduction band minimum(CBM)but also the sub bandgap between VBM and IB are very close to the optimal values of IB solar cell(IBSC).Therefore,these alloys are recommended as good candidates for IBSC materials.In the fourth chapter,we have studied the geometric structures,electronic states and optical properties of Cu2ZnSixSn1-xSe4 and Cu2ZnSixGe1-xSe4 alloys through hybrid functional.The lattice constants linearly decrease with the increment of the Si concentration and the mean atomic weight.The formation enthalpies are positive but with low values for the Cu2ZnSixSn1-xSe4 alloys,indicating that the synthesis of these alloys is easy in experiment.For Si-doped Cu2ZnSixGe1-xSe4 alloys,the formation enthalpies have a negative value,meaning that the synthesis of these alloys is exothermic and can be realized at any temperature.Most importantly,through the doping of Sn or Ge,tunable bandgaps are obtained in a wide energy range,namely 1.06-2.26 eV for Cu2ZnSixSn1-xSe4 and 1.27～2.26 eV for Cu2ZnSixGe1-xSe4,respectively.The variation of the bandgap is primarily due to the upward shift of the conduction band minimum.The bandgaps are 1.49 eV and 1.45 eV,respectively,for the half-doped Cu2ZnSi0.5Sn0.5Se4 and the quarter-doped Cu2ZnSi0.25Ge0.75Se4 alloys.These two values are very close to the optimal bandgap(1.50 eV)for solar cell materials.Their optical absorption coefficients are higher than that of the well-studied Cu2ZnSnS4,which has optimal bandgap too.This study indicates that the bandgap-tunable Cu2ZnSixSn1-xSe4 and Cu2ZnSixGe1-xSe4 alloys can be promising for their photovoltaic applications.A summary of this thesis is provided in the fifth chapter.