The electronic structure and optical properties of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7) from first-principle calculations

The electronic structure and optical properties of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7)(x¼0,1/8,2/8,3/8,4/8)were studied by first-principle calculations within the generalized gradient approximation approaches(GGA).The lattice constants of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7) increase with the increase of Ti^(4+)content caused by the substitution of Ti^(4+)with larger ionic radius for Mn^(4+).Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7) is a direct band gap semiconductor,and the band gap(Eg)increases with the increase of Ti^(4+)content.From the density of states,the introduction of Ti-3d states can weaken the effects of Mn-3d states on the bottom of conduction band and has little influence on O-2p states on the top of valence band.The introduction of nonmagnetic Ti^(4+)ions can weaken the magnetism of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7).According to the Mulliken population analysis,it is found that the introduction of Ti^(4+)enhances the electronic accepting capacity of oxygen ions and enhances the electronic losing capacity of manganese ions.The bond strength of Ti–O covalent bond is stronger than that of Mn–O covalent bond.Furthermore,the optical properties of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7) was calculated.As Ti^(4+)content increases,the absorption edge of Ca_(3)(Mn_(1-x)Ti_(x))_(2)O_(7) has a blue shift,the static refractive index n0 decreases,the static dielectric constant"1(0)decreases,the position of loss peak moves to higher energy.

Effect of Cl doping amount on the microstructure,photovoltaic properties and ferroelectric properties of Bi-based lead-free perovskite

Organic-inorganic hybrid perovskites with 3D perovskite structure have gained much attention as light harvesting materials in thinfilm photovoltaics.This is because of their outstanding light-absorption characteristics,charge-transport dynamics and their simple processability using lab-scale solution and vapor phase deposition techniques.However,the inherent instability and lead toxicity of lead-based PSCs are the major problems at present.Recent studies have shown that the(CH_(3)NH_(3)T_(3)Bi_(2)I_(9)(MBI)0D bismuth-based compound can be used as an optical absorption layer in solar cells.In this paper,the(CH_(3)NH_(3)T_(3)Bi_(2)I_(9)was doped with Cl
and a series of(CH_(3)NH_(3)T_(3)Bi_(2)I_(9-x)Cl_(x)films were prepared.The effects of different doping amounts on the microstructure,photovoltaic properties and ferroelectric properties were systematically investigated.Scanning electron microscope(SEM)and Atomic force microscope(AFM)analysis showed that with the increase of doping content,the density of the films increased and the roughness decreased.The photoelectric conversion efficiency of(CH_(3)NH_(3)T_(3)Bi_(2)I_(9-x)Cl_(x)raises with the increase of doping content.For example,the photoelectric conversion efficiency of(C_(H)3NH_(3)T_(3)Bi_(2)I_(3)Cl_(6)is 0.473%.We find that the leakage current descends into the increase in doping content,which may be due to the increase in the film density and the decrease of porosity.These research results have a positive effect on the development of Bi-based lead-free perovskite.