First-principles Study On Metal Ions Doped Perovskite MAPbI₃

MAPbI₃ is the core of the perovskite solar cells,having the important function in light absorption and charge transfer.However,due to problems of lead pollution and poor stability,its application is limited.For this reason,this paper uses the first-principles to study the crystal structures and photo-electric properties of MAPbI₃,MASnI₃,and MA₃Bi₂I₉.On this basis,inorganic metal（Rb/Cs,Sn/Bi）was used to dope into MAPbI₃,and the effects of doping on crystal structure,electronic structure and optical properties were calculated and analyzed.The results of study are as follows:1.The valence band maximum of MAPbI₃,MASnI₃,and MA₃Bi₂I₉ are mainly contributed by the I-5p states,while the conduction band minimum is mainly contributed by the metal outermost electronic states.Among them,the band gap of MASnI₃ is the narrowest,but the sharp band structure is not conducive to the transition of photons.The crystal structure of MA₃Bi₂I₉ is different from the typical perovskite,which is composed of coplanar octahedron.This crystal structure has the higher binding energy,so it is more suitable for practical applications.MAPbI₃ has a gentle band structure,maximum electron concentration,and MAPbI₃ has the highest light absorption in the same wavelength range.2.The band gap of Sn/Bi-doped MAPbI₃ decreased,but the mechanism is different.The impurity energy levels of Sn is located at the top of the valence band of MAPbI₃,which belongs hole doping.The concentration of holes in the MAPb_1-xSn_xI₃ system are the highest,which is 3.1259×10^-5 cm^-3.Furthermore,Sn-doping adjusts the band gap of MAPbI₃ more obviously,the band gap reduced by 0.31 eV,so it can absorb light of larger wavelengths.Moreover,the energy required for Sn-doping is lower than Bi-doping under the same conditions.The impurity energy levels of Bi is located at the bottom of the conduction band of MAPbI₃,which belongs electron doping.The concentration of electrons in the system is the highest,which is 3.0639×10^-5 cm^-3.Compared to MAPbI₃,the band structure of MAPb_1-xBi_xI₃ has shifted down to 2 eV,making more low-energy carriers able to complete the transition,and the system energy is lower and more stable.3.By analyzing the MAPb_1-xSn_xI₃ and MAPb_1-xBi_xI₃ systems at different replacement ratios（Sn/Bi:Pb）,it is found that when the ratio of Sn:Pb is less than 50%or the ratio of Bi:Pb is higher than 50%,the substitutional system easily generates a larger internal electric field,which is beneficial to the separation of the electron-hole pairs.As the ratio of Sn/Bi:Pb increases,the impurity energy levels increase,the band gap decreases and the absorption edge shows red shift.Compared to the MAPb_1-xBi_xI₃system,the optical properties of Sn-substitutional system appear more regularly changes.However,under the larger replacement ratio,MAPb_1-xBi_xI₃ has more obvious absorb for near-infrared light.In particular,the MABiI₃ system can significantly absorb the light that in the wavelength range of 400-800nm.4.Rb-/Cs-doping reduces the damage to the crystal structure of MAPbI₃ that caused by the torsion of organic cations.Compared with Sn/Bi-doping MAPbI₃,the energy of MA_1-xRb_xPbI₃ and MA_1-xCs_xPbI₃ systems is lower,and the energy was also lower than that of the pure MAPbI₃.Rb-doping not only reduces the band gap,but also changes the band structure from indirect bandgap to direct bandgap,broadens the optical absorption range and reduces the loss of photon transitions.Since Cs-doping produces the largest dipole moment,the carrier concentration is the highest in the all calculated systems.According to the partial density of states,the contribution of Cs states to the density of states is similar to that of organic cations,indicating that Cs is ideal inorganic doping element.