Dynamics Performance And Electronic Structure Of Superatom Perovskites:First-principles Study

With the growth of population and the aggravation of pollution,the exploration and demand of the clean energy are also increased.Among the clean energy sources,such as wind,hydrogen and solar energy,the solar energy undoubtedly is one of the most clean energy sources.Among the many conversion ways of solar energy,the solar cell is a kind of optoelectronic device which can directly convert solar energy into electric energy.In order to achieve the high energy conversion efficiency and wide commercial application of solar cells,it is essential to select the excellent light absorbing materials.ABX₃ hybrid perovskites,such as CH₃NH₃PbI₃,has been regarded as the superior photovoltaic materials because of the low cost,high power conversion efficiency,large carrier mobility,tunable direct bandgap and easy solution-based synthesis,and so on.The power conversion efficiency?PCE?of 24.2% have been obtained for the single-junction solar cells based these hybrid perovskites so far.Unfortunately,the single-junction perovskite solar cells with high efficiency usually have the issue of toxic lead or instability,which seriously restricts the industrialization of the solar cell.Therefore,it is imminent to explore the new,stable and lead-free perovskite materials,which has triggered an upsurge for the study of Ge-/Sn-based perovskites,double perovskites,superatom perovskites,and so on.Among these new perovskites,the study on superatom perovskites is particularly scarce.Based on that,many superatom perovskites in this paper have been studied by using density functional theory based first principles and ab initio molecular dynamics simulation.The main work is as follows:?1?The theoretical study of single-layer?C₄H₉NH₃?₂Sn?BH₄?₄ hybrid perovskites with the different strains:Firstly,92 initial structures of single-layer?C₄H₉NH₃?₂Sn?BH₄?₄ hybrid perovskite are constructed and optimizied.Then the ground state structure of the single-layer?C₄H₉NH₃?₂Sn?BH₄?₄ hybrid perovskite with a=8.38?and b=8.25?is obtained.Secondly,the strain from-5% to 10%is imposed along the plane of this single-layer hybrid perovskite.The calculation results show that,for this single layer perovskite,its direct band gap can be tuned from 2.92 to 3.60 eV under this strain range,and the effective electron?hole?mass can be tuned from 0.29?0.26?to 0.72?0.55?m_e.In addition,the single layer material has the small cleavage energy of 0.30 J/m².It is worth mentioning that when the strain is tuned from-5% to 3%,the little change are found for the effective masses of electrons and holes as well as the direct band gap of the monolayer material,which suggests that the monolayer material has potential application advantages in optoelectronic devices.?2?The theoretical study for the dynamical stability and the power conversion efficiency of monolayer[C₆H₅?CH₂?₂NH₃]₂MSn₂Br₇?M=Li₂Cl,Li₃O,Li₂F and H₅O₂;PEA=C₆H₅?CH₂?₂NH₃?perovskites:Firstly,the structure of monolayer PEA₂MSn₂Br₇perovskites were constructed by referring to the structure of PEA₂CsPb₂Br₇ perovskite.The ground state structures of PEA₂MSn₂Br₇ have been obtained by changing the lattice constants?a and b?and optimizing their structures.Secondly,ab initio molecular dynamics simulation results of 2 ps show that PEA₂MSn₂Br₇ perovskites have stable dynamics performance under room temperature and pressure,and Li₃O and H₅O₂super-alkalis are not gathered,while Li₂Cl and Li₂F super-alkalis show the gather.The calculated results also show that the monolayer PEA₂MSn₂Br₇ perovskites have negative formation energy.The calculation of electronic structures shows that monolayer PEA₂Li₃OSn₂Br₇,PEA₂Li₂ClSn₂Br₇,PEA₂Li₂FSn₂Br₇ and PEA₂H₅O₂Sn₂Br₇perovskites have the direct band gaps of 0.67,1.48,1.49 and 1.51 eV,respectively.For the four perovskites,the PCE of 8.52%,22.63%,22.55% and 22.45%are obtained respectively.The calculation for the effective electron and hole masses show that only monolayer PEA₂Li₂FSn₂Br₇ and PEA₂H₅O₂Sn₂Br₇ perovskites have the effective mass less than 0.2 m_e.In addition,the band edge components and band gaps of the monolayer[C₆H₅?CH₂?₂NH₃]₂MSn₂Br₇ perovskites show well tolerance after the ab initio molecular dynamics simulation of 2 ps.These results suggest that monolayer perovskite of PEA₂Li₂ClSn₂Br₇,PEA₂Li₂FSn₂Br₇ and PEA₂H₅O₂Sn₂Br₇ can be used as the light-absorbing materials of perovskite solar cells.?3?The theoretical study of the electronic structures and power conversion efficiency of cubic?Li₃O?M?BH₄?_3-xBr_x?M=Ge,Sn,Pb;x=0-3?superatom perovskites:Firstly,the A,B and X site atoms of the cubic CH₃NH₃PbBr₃ hybridized perovskite are replaced with Li₃O super-alkali atoms,Ge/Sn atoms and BH₄superhalogen atoms,respectively.Then the structures of cubic Li₃O?BH₄?_3-xBr_x perovskite with different lattice constants are optimized,and their the lowest energy structures are obtained.Secondly,the electronic structures and optical absorption properties have been systematically studied by using HSE06.The calculated results show that the cubic perovskites have the tunable direct band gap from 0.97 to 2.42 eV,negative formation energy,small effective masses of electrons and the different distribution for the charge densities of conduction band minimum and valence band maximum.Finally,for the?Li₃O?M?BH₄?_3-xBr_x perovskites,their power conversion efficiency are evaluated by their band gap.The results show that the single-junction solar cells based the cubic?Li₃O?Ge?BH₄?Br₂ and?Li₃O?Pb?BH₄?₂Br perovskites show the power conversion efficiency of 23.12%.Interestingly,with the increase of Br,PCEs of the single-junction solar cells based?Li₃O?M?BH₄?_3-xBr_x perovskites show the trend of first increase and then decreases.These results show the reference significance for the theoretical design and experimental preparation of the superatom perovskites with high efficiency.?4?The theoretical study for the dynamical stability and the power conversion efficiency of ABX₃ type super-alkali perovskite:According to the tolerance factor of0.80¹.00 suitable for the formation of the stable perovskites,Li₃O⁺,Li₃S⁺,Li₂F⁺,Li₂Cl⁺,H₅O₂⁺,NLi₄⁺super-alkali ions are selected to replace the CH₃NH₃⁺ion of cubic CH₃NH₃PbX₃?X=Cl,Br and I?.Then,the dynamic stability of cubic Li₃OMBr₃,H₅O₂MBr₃,Li₂FMBr₃?M=Ge,Sn,and Pb?,Li₃OPbI₃,Li₃SPbI₃,H₅O₂PbI₃,Li₂ClPbI₃and NLi₄PbBr₃ super-alkali perovskites are studied by ab initio molecular dynamics simulation.The ab initio molecular dynamics simulation results of 5 ps show that only cubic H₅O₂MBr₃ and H₅O₂PbI₃ super-alkali perovskites can exhibit stable dynamics performance under room temperature and pressure,and H₅O₂⁺super-alkali ions are not gathered.On the contrary,other super-alkali perovskites are transformed into non-perovskite after the simulation,and the A-site super-alkali ions are gathered.Finally,by using hybrid density functional theory,the electronic structures of H₅O₂MBr₃ and H₅O₂PbI₃ super-alkali perovskites with dynamics performance have been explored.The results show that they have negative formation energy,tunable direct band gap and small carrier effective masses.In addition,the study results suggest that the single junction solar cells based H₅O₂SnBr₃ and H₅O₂PbBr₃ perovskites can achieve the power conversion efficiency of 23.17%and 22.83%,respectively.These results can offer the reference value for the experimental preparation of the lead-free single-junction perovskite solar cells with high efficiency.