Theoretical Investigations On The Absorption Of Materials For Intermediate Band Solar Cell:based On First Principles

With the increase of energy consumption and environment problems caused by fossil fuels,it is urgent to explore and utilize the clean and renewable energy in our modern society.Solar energy with the advantages as non-pollution,wide-distribution and renewable resource has been regarded as the ultimate approach to solve the energy and environment problems.Solar cell is the most effective and direct way to utilize solar energy.Nowadays solar cell has evolved from crystalline silicon solar cell to thin film solar cell and then to third generation solar cell.Inserting an intermediate band in the main band gap of the absorber has been proposed to exceed the Shockley-Queisser limit on single band solar cell.The specific research contents in this dissertation are as follows:1.Group-IV elements Si,Ge and Sn substituting at A1 site in AgAlSe2 to form an intermediate band in the main band gap has been studied by first-principles calculations.The half-filled intermediate bands from the antibonding state of group-IV s-state and Se-p state show delocalized charactistics and just shift from each other in Si,Ge and Sn doped AgAlSe2.Based on the analysis on the position of the intermediate band and defect formation energy,Si-doped AgAlSe2 has been excluded and Ge and Sn doped AgAlSe2 have been suggested as the promising absorber for the intermediate band solar cell.A heterojunction from CuAlTe2,AgAlSe2:Sn and CdS has been proposed as the device for the intermediate band solar cell after considering the lattice mismatch and band alignment.2.We employed a screened-exchange hybrid density functional Heyd-Scuseria-Emzerhof(HSE06)to calculate the electronic structure,band structure,optical properties and formation energy of all quatematy compounds.Our calculated results demonstrated that the band gap of Cu2ZnSnS4,Cu2ZnGeS4,Ag2ZnSnS4 and Ag2ZnGeS4 are 1.77 eV,2.38 eV,1.99 eV and 2.49 eV,respectively.After doping a Sb in a supercell with 32 atoms,an isolated half-filled intermediate band has been inserted,which consists of the Cu-d state,S-p state and Sb-p state.By comparing the calculated optical properties of the pure and doped compounds,we find that the absorption efficiency is greatly improved and the absorption range is also expanded owing to the generation of the intermediate band.Meanwhile,we have calculated the defect formation energies to evaluate the possibility of a large concentration doping.The results show that under Sn-poor or Ge-poor condition,Sb doping in Cu2ZnSnS4,Cu2ZnGeS4,Ag2ZnSnS4 and Ag2ZnGeS4 has a low formation energy and can induce the ideal sub-band gap for IBCSs.3.We have investigated Sb-doped Cu2BaSnS4 as the candidates for intermediate band solar cell.We calculated that Cu2BaSnS4 has a indirect bandgap of 2.08 eV.After doping,an isolated half-filled intermediate band has been inserted,which consists of the Cu-d state,S-p state and Sb-p state.And the absorption efficiency is greatly improved,and the absorption range is also expanded.The defect formation energies show that under Cu-moderate,Ba-poor,Sn-poor and S-rich condition,Sb doping in Cu2BaSnS4 has a low formation energy.Furthermore,we have demonstrated that equivalent cation alloying is a viable method of adjusting the two sub-band gaps to the desired width.Over all,our results suggest Sb-doped Cu2BaSnS4 as the promising absorber candidates for IBSCs.