Study Of Defect Physics For Perovskites And Silver Orthophosphate

Photovoltaic absorbers and photocatalyst are discussed in this thesis.We first have made a qualitative analysis of the stability of a typical absorber CH₃NH₃PbI₃ through the calculation of band structures of monohydrate CH₃NH₃PbI₃·H₂O and dihydrate?CH₃NH₃?₄PbI₆·2H₂O.We have considered two kinds of materials in order to resolve the stability of CH₃NH₃PbI₃:quasi-two-dimensional layered organic perovskite CH₃NH₃Pb?SCN?₂I₂ and inorganic perovskite CsPbI₃.Based on the GW method with high accuracy?G is the one-electron Green's function,W is the screened Coulomb interaction?and spin-orbit coupling effect?SOC?,this article calculated a direct band gap of CH₃NH₃Pb?SCN?₂I₂ is 2.32 eV,which does not belong to the ideal band gap range?1.10-1.55 eV?as promising absorber for efficient single-junction solar cell applications.This results is consistent with recent experiments,but it is much larger than 1.50-1.60 eV previously reported both in experiment and theory.Based on the results of GW+SOC,this article further combined the Bethr-Salpeter equation?GW+SOC+BSE?to calculate the exciton binding energy.The results show that the first absorption peak is located at 1.92 eV,therefore,the exciton binding energy is 0.40 eV.This indicates that photovoltaic solar cell based on absorber CH₃NH₃Pb?SCN?₂I₂ is a typical excitonic solar cell which first need to dissociate excitons in order to generate free carriers.Replacing the hygroscopic CH₃NH₃ cation with robust inorganic cations can be considered as a promising approach to overcome the instability of CH₃NH₃PbI₃.Recently,CsPbI₃-based solar cells has shown enhanced resistance to moisture and improved thermal stability.Here,the formation energies and transition energy levels of intrinsic point defects in y and 8 phase have been studied systematically by first-principles calculations.It is found that CsPbI₃ exhibits a unipolar self-doping behavior?p-type conductivity?,in contrast to CH₃NH₃PbI₃.Most of the intrinsic defects induce deep transition energy levels in 8 phase than y phase,this is possibly due to the small Pb-I-Pb bond angles in 8 phase which can resulting the valence band maximum?VBM?with weak antibonding character,however,the strong antibonding character of the VBM play critical role in keeping defect tolerance in semiconductors.Therefore,these results once again shows the importance of the large metal-halide-metal bond angle to the performance of perovskite solar cells.For photocatalyst,the transition energy levels and formation energies of N,C,F,Cl,and S as substitutional dopants in Ag₃PO₄ are studied by using first-principles calculations,which based on the hybrid Hartree-Fock density functional.And the functional correctly reproduces the band gap and thus provides accurate description of defect states.The calculation results show that No and Co act as deep acceptors,Fo,Clo,and Sp act as shallow donors.They are both have high formation energies,thus,they are not possiblely lead to n-or p-type conductivity in Ag₃PO₄.Based on the results of No,this article further investigate the effects of defects and defect complexes related with Cd,Li,and N impurities on the atomic and electronic properties of Ag₃PO₄.It was found that substitutional Cd on Ag lattice site?CdAg?contributes to the n-type conductivity of Ag₃PO₄,for substitutional Cd on P?or O?lattice site?Cdp??or Cdo?,it is not expected that Cd will incorporate into the P?or O?site due to the strong covalent interactions in the PO4 structural units.The interstitial Cd?Cdi?acts as a shallow donor,but its formation energy is relatively high compared with that of CdAg.For the?CdAg-2No?complex,the formation of this inactive complex generates a fully occupied impurity band just above the?VBM?of Ag₃PO₄,which significantly reduces the acceptor?CdAg-2No?complex transition energy level.But the formation energy of the?CdAg-2No?complex is even higher than that of the corresponding single point defect?such as No?.Unlike Lip and Lio which has relatively high formation energy,interstitial Li?conversional configuration Lii or splitting configuration Lii?s??with an appreciable solubility is likely to be the n-type dopant under O-poor condition,but its possiblely compensated by the acceptor?LiAg-VAg?complex.