Theoretical Study On Photoelectric Properties Of Light-absorbing Layer Materials And The Interfaces Of Perovskite Solar Cell

Organic-inorganic hybrid perovskite solar cells are widespreadly concerned by the academic and industrial society because they have shown excellent optoelectronic properties such as simple structure,high photoelectric conversion efficiency（Effi）,low preparation cost and so on.The laboratory Effi of perovskite solar cells has reached22%,which shows bright development prospects.Organic-inorganic perovskite material MAPbI₃,as the absorption layer,is the core of perovskite solar cells,and is favored by more researchers because of its long charge diffusion length,high carrier mobility,and high extinction coefficient.For perovskite solar cells,the control of crystallization and morphology of the interface layer is closely related to the long-term stability of the solar cells,and which have become a hot topic for current research.The thesis is mainly based on the first principles calculation method,to theoretically calculate the photoelectric properties of perovskite solar cell materials and their interfaces at the atomic microstructure level.In this work,the changes of the optical bandgap,electronic properties and optical properties of the doping perovskite solar cell materials were studied,and the stability of both side interfaces on the absorption layer material,the changes of the photoelectric properties.The effects of defect density on the photoelectric properties of perovskite solar cells were investigated based on device simulation.The main research contents and results of this thesis are as follows:（1）The optical and electronic properties of MAPb_1-xAg_x I₃ were studied by the first-principles calculations.The lattice models of MAPbI₃ and MAPb_1-xAg_xI₃ were constructed based on experimental lattice parameters,and optimized their cell structures.The band structures and density of states of MAPbI₃ and MAAgI₃ were investigated.The formation energy of the system is increasing with the increase of doping concentration.When the doping concentration reaches 5%,it can be found that the valence band shifts toward high energy levels,and the Fermi energy enters the valence band,and this indicates that MAPb_0.95Ag_0.05I₃ is a p-type conductor.Optical band gap of the MAPbI₃ is about 1.56 eV,the static dielectric constant of MAPbI₃ is 7.19.When the doping concentration is more than 5%,the optical band gap of MAPb_1-xAg_x I₃ is about3.1 eV.（2）The lattice structure,electronic properties of MAPbI₃（100）|Au（100）interface were studied with the first-principles calculations.It was confirmed that MAPbI₃（112）and Au（100）can form a good lattice match,and this lattice mismatch was calculated.On this basis,the electronic properties of the interface（MAPbI₃）|metal electrode（Au）were studied.It was found that the lattice mismatch of the MAPbI₃（100）|Au（100）interface is 3.48%and which belongs to a coherent interface.The interface binding energy is-0.248 J/m².There is almost no interface state nearby the interface through analyzing the density of states of the interface.In addition,the atoms orbitals have hybridization nearby the interface,and the orbital hybridization is advantageous for atomic bonding at the interface.Through analyzing charge density difference and Bader charges,it was found that there is obvious charge transfer at the interface.（3）Similar methods as in（2）were used to further investigate the interfacial lattice matching and the combining stability of the light absorbing layer material（MAPbI₃）|electron transporting layer material（WZ-ZnO）.The atomistic geometry,binding energy,Bader charge,charge density difference of interface,the local density of state and the partial density of state of the atoms on the interface were investigated,and the resulting data were analyzed.The stability of the interface,the bonding strength of the interface,interface states,the hybridization of the atomic orbitals and the charges transfer on the interface were studied emphatically.It was found that the lattice mismatch of this interface is 8.9%and the interface binding energy is-0.328 J/m².Interface states appear nearby the Fermi level,which come from the contribution of O-2p orbital,I-5p orbital and Pb-6s orbital.The appearance of interface states is detrimental to the photovoltaic performance of the solar cells.（4）Similarly,the lattice matching of the light-absorbing layer（MAPbI₃）|hole transporting layer（NiO）interface was investigated.MAPbI₃（100）|NiO（110）interface was established as a lattice matching interface,and the stability of their combination was studied.On this basis,the electronic properties of the interfacial layer were studied between the light-absorbing layer material（MAPbI₃）and the hole-transporting layer material（NiO）.Research shows that the MAPbI₃（100）|NiO（110）interfacial lattice mismatch is 7.3%.The binding energy of the MAPbI₃（100）|NiO（110）interface is-0.118 J/m²,and the atoms bonding is irregular at the interface.Compared with combination energy of MAPbI₃（112）|Au（110）and that of MAPbI₃|WZ-ZnO interface,the combination of this interface is fragiler.There are some interface states nearby the MAPbI₃（100）|NiO（110）interface.Interface states of the MAPbI₃（100）|NiO（110）interface mainly are attributed to I-5p,O-2p and Ni-3d orbitals on the first MAPbI₃（100）layer and the first NiO（110）layer.It was found that there is significant charge transfer on the interface by analyzing the charge density difference and Bader charges of the interface.（5）A typical and ideal perovskite solar cell model was built,and carrying on a theoretically research on the effects of defect states to its performance.The varieties of defect states（location in the band gap and densities）in the absorption layer MAPbI₃,in the interface between the buffer layer|MAPbI₃,and in the interface between the hole transport material（HTM）and MAPbI₃,were investigated.Quantitatively analyzing the performance parameters of perovskite solar cells of different defect states:Open-circuit voltage,Short-circuit current,Fill factor,and Photoelectric conversion efficiency.It can be found that defect states are always bad to the performance of perovskite solar cells,but when defect state density is less than 10¹⁶ cm^-3,defect states have little effect on the performance of perovskite solar cells.Defect states in the absorption layer have much larger effect than those in the adjacent interface layers.The perovskite solar cells have better performance as its working temperature is reduced.When the thickness of MAPbI₃ is about 0.3μm,the performance of perovskite solar cells is better,and the thickness 0.05μm for Spiro-OMeTAD is enough.Experimental methods can not or have difficult to obtain the results of above research.Our results are a useful complement to experimental research.It will be help to further understand the structures and properties of the light absorbing layer material and its related interfaces in the perovskite solar cells.