First-principles Calculation Of Photoelectric Properties Of Cs₂TiBr₆ Doped Structures

All-inorganic lead-free perovskite Cs₂TiBr₆ is a potential material for the light absorption layer of perovskite solar cells due to its excellent photoelectric properties,stability and environmental friendliness.However,the intrinsic Cs₂TiBr₆ system has a narrow optical absorption range and low photoelectric conversion efficiency due to its quasi-direct band gap structure and wide band gap value,which limits its wide application.In order to explore an effective methods about improving the optical and electrical properties of Cs₂TiBr₆,it is considered to use Cl,I,Si,Ge,Te for substitutional doping,because of the similarity of halogen elements and the electronic distribution characteristics of semimetal subshells,hoping to explore a method to effectively regulate the electronic structure of the original structure and improve the optical properties.In this thesis,the density functional theory（DFT）based on the first principle is used to calculate and study the effects of these five elements on the optical and electrical properties of Cs₂TiBr₆ under different doping concentrations.The main research results are as follows:（1）The electronic structures regulation of halogen doping are studied.The results show that Cl and I prefer to replace the Br element on the same Ti when conducting multi-atomic doping.Both Cl and I doping can adjust the band gap to direct band gap.With the increase of Cl doping concentration,the conductivity decreases,the Fermi level shifts down,and the band gap value increases.In the I-doped structure,the Valence Band Maximum（VBM）is redefined by the I-5p orbital.With the increase of I doping concentration,the conductivity increases and the Fermi level shifts up the band gap value to decrease.Among all halogen-doped structures,the 25%concentration I-doped structure has the most suitable band gap value of 1.29e V.（2）The calculation of the optical properties of halogen doping shows that:due to the widening of the band gap of Cl doping,the light absorption spectrum,refractive index,reflectivity,and energy loss function are all blue-shifted,the absorption range of sunlight is reduced,and the photoelectric conversion ability is weakened.In the I-doped structure,the band gap value decreases with the increase of concentration,the light absorption spectrum,refractive index,reflectivity,and energy loss function are all red-shift,and the absorption range of visible light and near-infrared light of sunlight becomes larger.（3）The effects of semi-metallic elements Si,Ge and Te doping at different concentrations on Cs₂TiBr₆ were investigated,and the results showed that Ge and Si doping enhance the stability of the material.Si doping reduces the band gap value slightly.The new Conduction Band Minimum（CBM）is contributed by Ti-3d,Br-4p and a small amount of Si-3s orbitals.The band types of the doping structures with 25% and 50%concentrations are adjusted to direct band gaps.After Ge doping,the impurity band formed by the hybridization of Ge-4s orbital and Br-4p orbital becomes a new Conduction Band Minimum.With the increase of Ge concentration,the impurity band broadening gradually increases,reducing the band gap.The band structure is adjusted to direct band gap when Ge doping at 50%and75%concentration.The best band gap value is 1.33e V when Ge doping with 50%concentration.In the Te-doped structure,the valence band top is composed of the anti-bonding state of Te-5s and Br-4p orbital hybridization,which decreases the stability of the system.（4）The calculation results of semi-metallic optics show that Si doping causes the light absorption ability to weaken.Ge doping reduces the band gap and enhances the light absorption ability of the solar wavelength greater than 545 nm.Te doping enhances the light absorption capacity,but due to the excessive localization of electrons around Te,the blue shift of the absorption spectrum leads to the decrease of the solar absorption range.