First-principles Study Of Electronic And Defect Properties Of Halide Perovskites

Organic–inorganic hybrid halide perovskite solar cells have experienced remarkable progress since it was firstly reported in 2009.Currently,the perovskite solar cells can achieve high power conversion efficiencies（PCEs）of up to certified 23.7%.Because of their great photovoltaic performance,low cost and solution processability,they are expected to be next generation solar cell.Low long term stability and Pb toxicity,however,are main issues,to limit their commercial application.In order to address the issues,we have aimed to design halide perovskites with much better stability than mainstream halide perovskite MAPbI₃（MA=CH₃NH₃）,as well as free toxicity,followed by investigation of their electronic and intrinsic point defects properties by first-principles calculations.Beside,we have clarified whether the orientation of MA cations can influence the structural stability and the electronic properties of MAPbI₃ surfaces.In summary,the following statements are concluded.1.We have studied the electronic and intrinsic defect properties of AZPbI₃（AZ=（CH₂）₂NH₂）,which is much better than mainstream halide perovskite material MAPbI₃.We find that AZPbI₃ possesses a band gap of 1.31 eV,ideal for single-junction solar cells.Furthermore,its optical absorption is comparable with that of MAPbI₃（MA=CH₃NH₃）in the whole visible-light region.In addition,both the electron and hole effective masses of AZPbI₃are rather light as MAPbI₃.The conductivity of AZPbI₃ can be tuned from good p-type,intrinsic to good n-type by varying the growth conditions.Overall,we suggest that the halide perovskite AZPbI₃ has potential applications in high stability and performance photovoltaic devices.2.We have proposed lead-free halide perovskite AZSnI₃ as well alloys AZSnI_3-xBr_x,and then investigated their electronic and intrinsic defect properties.We find that the perovskite AZSnI₃ has much better stability than MAPbI₃ and possesses a band gap of 1.06 eV.Furthermore,I-poor/Sn-rich growth conditions may be preferred for synthesizing AZSnI₃,as the formation of deep-level defects is suppressed,and the carrier density can be maintained at moderate levels.In addition,the alloy AZSnI₂Br possesses an ideal band gap 1.31 eV,while AZSnI_3-xBr_x and AZSnI₃ have higher optical absorption than MAPbI₃ in general.Consequently,we suggest that the lead-free perovskite AZSnI₃ and alloys AZSnI_3-x-x Br_x are excellent candidate as a stable and high performance photovoltaic absorber materials.3.We have proposed lead-free double perovskite Rb₂SnI₆,and investigated their electronic and intrinsic defect properties.They are compared with those of Cs₂SnI₆,which is a promising photovoltaic material.Compared with the Cs₂SnI₆（with band gap of¹.60 eV）,Rb₂SnI₆ possesses a band gap of 1.48 eV,which is close to the optimal value for single junction solar cell.The fact that the band gap of Rb₂SnI₆ is lower than Cs₂SnI₆ can be explained by the fact that the distance between neighboring I sites in Rb₂SnI₆ is 4.29?,much smaller than that in Cs₂SnI₆,4.38?.As a result,we can change the distance between halide atoms by introducing compressive strain to tune the band gaps of the perovskites for this type perovskites.Besides,I-rich/Sn-poor growth conditions may be preferred for synthesizing AZSnI₃.4.We have investigated the structural stability and electronic properties of surfaces of MAPbI₃ with considering the orientation of MA cations,by first principles calculations.We find that the orientation of the organic cations MA has profound consequences on the structural stability and the electronic properties of the surfaces,although it does not contribute to the band edge directly for the surfaces,much like the performance in the bulk system.These are attributed to the fact that the bond angle of Pb-I-Pb in octahedral[PbI₆]in MAPbI₃,which plays a key role to the structural and electronic properties of MAPbI₃,can be affected by the orientation of the organic cations.Thus,we suggest that the photovoltaic performances of ABX₃ type perovskite materials can be enhanced by tuning the bond angle of Pb-I-Pb in octahedral[PbI₆].Our study may provide helps to experimentalists for designing high stability and performance solar cells.