First-principles Study On Defect Properties Of Novel Chalcogenide Perovskite Solar Cells

Organic-inorganic lead halide perovskites（OLHPs）have attracted much attention due to their high photovoltaic conversion efficiency and their simple and inexpensive preparation methods.However,the instability of OLHPs（influenced by humidity,oxygen,temperature）and the toxicity of lead limit their application in photovoltaic devices.Therefore,the research focus of perovskite materials has gradually shifted towards more environmentally friendly and structurally stable.Compared with OLHPs,chalcogenide perovskites,such as Ba Zr S₃,don’t contain lead elements,and their band edges delocalization characteristics confer benign defect properties and light absorption capability.Furthermore,with the oxidation state increases,their Coulomb interactions are four times stronger than those in OLHPs,resulting in a more stable structure,providing significant assistance in addressing the aforementioned issues.On the other hand,the A-site cation in the perovskite structure（ABX₃）has an important impact on the physical and chemical properties of the material.A-site cation with appropriate ion size and oxidation state helps stabilize the system structure and satisfy the electroneutrality condition.Among them,the divalent hydrazinium ion(NH₃NH₃²⁺,hydrazinium,denoted as Hz)has advantages such as low cost and appropriate ion size.Substituting the A-site cation in perovskite with Hzis a feasible method to improve the material properties.Considering the above two points,in order to construct an environmentally friendly,efficient and stable photovoltaic material,this study combined the advantages of Ba Zr S₃ and Hzfrom the structure,replaced the Ba in Ba Zr S₃ with Hz,and constructed six hydrazine perovskite structures HzBX₃（B=Ti,Zr,Hf;X=S,Se）.Here,based on first-principles calculations,the electronic structure,thermodynamic stability,and defect properties of the HzBX₃ perovskite were studied in detail,including the following five parts:（1）Tolerance factor and electronic structure calculations indicate that the HzBX₃ system has a cubic crystal structure and a band gap ranging from 0.75 e V to 1.88 e V.In addition,it is found that the band edges of the system have typical delocalization characteristics,the valence band maximum（VBM）is mainly contributed by X-np orbitals,while the conduction band minimum（CBM）is contributed by B-nd orbitals.And,due to the relatively delocalized nature of the B-nd orbitals with larger principal quantum numbers（n）,such as Zr-4d and Hf-5d,in the reciprocal space,the conduction band edge exhibits a significant downward curvature,which not only renders the system with a more appropriate optical absorption band gap,but also facilitates the formation and transport of electron.（2）The optical absorption coefficients of the HzBX₃ system were calculated.The results showed that the spectral range of light absorption and utilization of HzBX₃ is broader than that of MAPb I₃.Moreover,when Se is substituted for S,the band edges will be significantly redshifted.This is because the electronegativity of Se is lower than that of S,resulting in higher bonding and lower antibonding orbital energies of Se,which in turn leads to a reduction in the band gap.（3）The phase diagrams of representative HzZr S₃ and HzTi Se₃ perovskites in the HzBX₃ system were analyzed.The results show that these two kinds of perovskites can exist stably in nature within a certain range of chemical potential.Furthermore,the electronic properties of the intrinsic defects of HzZr S₃ and HzTi Se₃ are calculated,and it is found that the formation energy of the donor-type defects is lower than that of the acceptor defects in most chemical potential ranges,which has n-type characteristics.The calculation of defect transition energy levels shows that the transition energy levels of donor defects that play a dominant role in the n-type characteristics of the system are located near the bottom of the conduction band.A few acceptor defects with deeper transition energy levels(V_Zr and Se_i)have extremely low concentrations due to their high formation energy,indicating that the intrinsic defects of the system will not cause serious non-radiative recombination.（4）By self-consistently solving the charge neutrality equation of the semiconductor,the Fermi level,carrier concentration,and defect concentration of HzZr S₃ and HzTi Se₃ were obtained.The results show that the two perovskites exhibit n-type characteristics over most of the chemical potential range,and HzZr S₃ has a higher fermi level and electron concentration,which is consistent with the defect formation energy results.In addition,with the increase of cation chemical potential,the electron concentration of HzZr S₃ and HzTi Se₃will continue to rise,reaching the optimal range of carrier concentration(10¹⁵～10¹⁸ cm³)under the conditions of moderate cation and rich cation（poor anion）chemical potential respectively.The calculation results of defect concentration also confirm that the concentration of deep level defects is extremely low.（5）In order to reduce the band gap of HzTi S₃（1.88 e V）in HzBX₃ system,four hydrazinium double perovskite structures Hz₂BB’S₆（B=Bi,Sb;B’=V,Nb）were designed based on the structure of HzTi S₃.Calculations show that the band gap of Hz₂BB’S₆ ranges from 0.98 to 1.39 e V,and the band gap of the system is effectively reduced.Further thermodynamic analysis indicates that the decomposition energy（ΔH_D）of these double perovskites are all greater than 0,indicating that the system has a certain possibility of existence in thermodynamics,which provides a feasible approach to improve the band gap of perovskites.