First-principles Study On Defects And Structure Of Single Layer PbI₂

Perovskite materials have been highly recognized by the scientific community because of their excellent photoelectric conversion efficiency.Currently,the certified power conversion efficiency(PCE)of single-junction solar cells has reached 25.5%,but this is far from the limit of perovskite solar cells.Consequently,scientists have discovered a variety of perovskite based on different composition,including MAPbI₃,which is also considered to be a candidate with the greatest potential to improve efficiency.However,the complex structure of MAPbI₃ results in unstable at room temperature and is easily decomposed into MAI and PbI₂,which is deleterious and difficult to stabilize the device of perovskite solar cells.In recent years,abundant researchers have been conducted to study how to improve the conversion efficiency and structural stability,but the results are hard to meet expectations.PbI₂ has extensive application value in optoelectronics,especially as a vital precursor for the preparation of hybrid perovskite.PbI₂ exists widely in hybrid perovskite,and they can transform into each other under certain conditions.PbI₂ and MAPbI₃ possess some common properties,which had been proved that the existence of part PbI₂ can enhance the optoelectrical efficiency.Now,more and more researchers have transferred the simple study from how to improve the performance and stability of hybrid perovskite to study the influence of precursor PbI₂ on the performance and stability of hybrid perovskite.Noting that defects have a very crucial influence on the stability of materials,the existence of defects in materials is inevitable yet.The intrinsic defects of hybrid perovskite have been widely studied,but PbI₂ coexisting with hybrid perovskite for a long time and can transform with it,has received little attention.The band gap has a critical impact on photoelectric materials.PbI₂ refers as a semiconductor with a wide-band gap while MAPbI₃ is a conventional semiconductor.When they coexist for a long time,PbI₂ will inevitably lead to an increase in the overall band gap of the hybrid perovskite material,which is not conducive to the improvement of the performance of hybrid perovskite.Therefore,in this paper,we use the first-principles calculation method based on density functional theory to study PbI₂ from two aspects.On the one hand,we introduce chemical potential to calculate the formation energy and energy band of neutral and charged defects of PbI₂ in view of the perspective of defects;On the other hand,we design the structure of PbI₂ to find a new PbI₂ structure with a band gap close to MAPbI₃.Throughout our research,we have obtained the following results:1.According to the structural characteristics of 1T and 1H phase PbI₂,a large number of defect structures were designed,and a variety of defect types that could exist stably were obtained.Likely defect structures in both 1T and 1H phases were predicted and the corresponding formation energies at the neutral and charged states based on the pre-estimated chemical potential of I/Pb were both calculated.At the neutral states,the formation energy of V_I relatively low,which is consistent with the experimental observation.While at the charged states,V_Pb seems to be dominated regardless of the chemical potential and contribute more to the conductivity.Meanwhile,we also find out the chemical potential conditions for the formation of these defects.These results can be used to guide the experimental adjustment variables and reduce the defect concentration in the system from the root,so as to improve the stability of the hybrid perovskite.2.Two new PbI₂ structures are designed and proved to be very stable.In the transition-state theory calculation,we found that two new PbI₂ structures can be developed from the known PbI₂ structures,more importantly,the band gaps of the two new PbI₂ structures are significantly lower than those of the existing PbI₂ structures,which are close to the band gaps of MAPbI₃.If the new PbI₂ structure is applied to the preparation of hybrid perovskite,the adverse effect of PbI₂ band gap on the performance of hybrid perovskite will be greatly reduced.Hence,the photoelectric conversion ability of hybrid perovskite might be improved.