Design And Physical Properties Research Of New Perovskite Photovoltaic Materials

Organic-inorganic halide perovskites have emerged as a promising solar cell material for advanced photovoltaic applications with high performances in recent years.The power conversion efficiency（PCE）of thin film solar cells based on this class of materials exceeding 20%have become competitive with record efficiency of the convertional thin film solar cells based on crystalline Si.However,two key issues including the poor device stabilities associated with intrinsic material instability and the toxicity due to water soluble Pb²⁺need to be resolved before large scale commercialization.Insightful understanding into structure-property relationships regarding defect property,ion migration and device hysteresis is extremely important.In this paper,we have performed systematic research on the halide perovskite photovoltaic materials combined with first principle calculations,and have achieved the following innovative results offering guidance for the development of perovskite solar cells:1.Significant ion diffusion and ionic conductivity in methylammonium lead iodide（MAPbI₃）have been reported.However,their nature remains controversial.The calculated diffusion barriers for native defects scatter in a wide range and the calculated formation energy also differ qualitatively.These controversies hinder the understanding and control of the ion migration in MAPbI₃.The calculated diffusion barrier and formation energies for native defects demonstrate iodine vacancy with high hopping rates is the dominant diffuser.2.A comprehensive study of formation energetics and diffusion dynamics of transition metal impurities（Au,Ag,Cu,Cr,Mo,W,Co,Ni,Pd）in MAPbI₃ based on first principle calculations is reported.We find that when the Fermi level is near midgap of MAPbI₃,the formation energy of the interstitial impurities is low except Mo_i and W_i.All metal impurities introduce detrimental deep levels in the band gap of MAPbI₃ except Cu and Ag.The diffusion barriers of the interstitial impurities tend to increase with the charge state of the impurity.Cu_i⁺,Ag_i⁺,Au_i⁺,Co_i⁺,Ni_i⁺andPd_i⁺have low diffusion barriers and can diffuse in MAPbI3 whileCr_i²⁺,Mo_i^2+/3+,_iW^2+/3+,Co_i²⁺,Ni_i²⁺andPd_i²⁺have much higher diffusion barrier,rendering them largely immobile at room temperature.The diffusion barrier and formation energy should be considered together with the resistivity and the work function of the metal electrode to identify the optimal metal electrodes for MAPbI₃ solar cells.3.Previous reports have suggested that the ionic radii and electronegativity of SCN^-is close to that of I^-.The SCN^-doped MAPbI₃ exhibited similar optical properties as pure MAPbI₃.Consequently,it was expected that doping of CsPbBr₃perovskite with SCN^-would result in band gap narrowing.Interestingly,the photoluminescent all-inorganic CsPbBr₃ perovskite nanocrystals exhibit an abnormal blue shift in optical properties and improvement of the crystallinity when successfully doped by SCN^-.Combined experimental and theoretical investigations revealed that doping of the CsPbBr₃ perovskite with the rod-like SCN^-anion introduced disorder in the crystal lattice,leading to its expansion,and impacted the electronic structure of the perovskite with band gap broadening.4.We apply a functionality-directed theoretical materials selection approach as a filter for initial screening of the compounds that satisfy the desired intrinsic photovoltaic functionalities.14 Ge-and Sn-based materials with potential superior bulk-material-intrinsic photovoltaic performance have been identified in this materials selection process.Furthermore,a distinct class of compounds containing NH₃COH⁺with the organic molecule derived states intriguingly emerging at band-edges is found.The results provide guidance for acceleration of experimental discovery of alternative solar absorbers to further boost the conversion efficiency.5.Low-dimensional Sn perovskites exhibiting markedly enhanced air stability in comparison with their 3D counterparts were reported.The solar cells based on perpendicular growth of the perovskite domain achieve power conversion efficiency（PCE）of 5.94%and enhanced stability sustained over 100h.With effective encapsulation,the long term stability of Sn perovskite solar cells can be realized.6.Through a joint theoretical-experimental study we have demonstrated that Cl-incorporation is an effective approach to suppress the formation of the 0D dimer phase and stabilize the 2D layered phase of Sb-based perovskites.The solar cells fabricated from 2D high-qualityMA₃Sb₂Cl_xI_-9xfilms show PCE of 2.19%.Moreover,theMA₃Sb₂Cl_xI_-9xfilms and solar cells exhibit good air stabilities.The results demonstrate the potential of Sb for the fabrication of lead-free halide perovskite solar cells.7.The idea of cation-transmutation is exploited to rationally design stable lead-free halide perovskite for photovoltaic applications.The idea is to convert two divalent Pb²⁺ions into one monovalent B⁺and one trivalent C³⁺ions,forming a rich class of quaternary halides in double-perovskite structure.We find through first-principles calculations this class of materials have good phase stability against decomposition and wide range tunable optoelectronic properties.With photovoltaic-functionality-directed materials screening,we identify 11 optimal materials with intrinsic thermodynamic stability,suitable band gaps,small carrier effective masses,and low exciton binding energies as promising candidates to replace Pb-based photovoltaic absorbers in perovskite solar cells.The chemical trends of phase stabilities and electronic properties are also established for this class of materials,offering useful guidance for the development of perovskite solar cells fabricated with them.