Theoretical Study On Electronic And Optical Properties Of Inorganic Solar Cell Materials

Halide perovskites have recently emerged as promising materials for low-cost,high-efficiency solar cells.The efficiency of perovskite-based solar cells has increased rapidly,from 3.8%in 2009 to 19.3%in 2014,by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites.The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency,but also flexibility in material growth and architecture.The superior performance of the perovskite solar cell suggested that perovskite materials possess intrinsically unique properties.Inorganic halide perovskites(IHPs)are promising candidates for applications in solar cell devices.However,the band gaps of most IHPs are too large,so that the energy conversion efficiency is limited.In this work,we proposed a donor-acceptor pair cooping scheme to reduce the bandgaps Sn-and Pb based IHPs,based on first principles calculations.Interestingly,the Sb-Se pair co-doping in CsSnBr3 can reduce the band gap from 1.8 eV to?1.2 eV which is very close to the optimal band gap for solar cell materials.The band gap of CsPbI3 could also be tuned close to the optimal value with smaller concentration of the Bi-Te pairs.The absorption coefficient of donor-acceptor pair co-doped CsSnBr3 and CsPbI3 in the visible light region are large,which indicates that they are good light absorber for applications in solar cell devices.Density functional theory(DFT)calculations were carried out with the Vienna ab-initio simulation package.The interaction between valence electrons and ionic cores was described within the framework of the projector augmented wave(PAW)method.The energy cut off for the plane wave basis expansion was set to 400 eV.The spin-polarized local density approximation(LDA)and generalized-gradient approximation(GGA)were employed to optimize atomic structures.To obtain accurate bandgaps,the hybrid functional HSE06 and meta-GGA functional in the modified Becke Johnson(MBJ)scheme were used for further calculations of the electronic structures.For cubic and tetragonal phases,we used 2×2×2 and(?)×1 supercells,respectively,so that both of them contain 40 atoms.The Brillouin zone was sampled by 6 × 6 and 12 × 12 k-grid meshes,respectively for structural relaxations and electronic calculations.The spin orbit coupling(SOC)was considered to reveal the relativistic effect on the electronic structuresIn the past few years,significant attention has been paid to graphene and hexagonal boron nitride(h-BN)because of their unique electronic,optical,and mechanical properties Even though graphene and h-BN have similar two-dimensional(2D)structures,they show very different electronic properties in band structures,piezoelectricity,etc.Recently,layered metal di-chalcogenides(LMDs)such as MoS2,WS2,MoSe2,and WSe2 have been extensively studied.Such a surge of interest in 2D LMDs is partly due to their electronic properties controlled by the number of layers and external stress and electric fields.In order for photovoltaic(PV)technology to provide a significant fraction of the world's energy demands,devices must be composed of cheap and abundant materials.Tin mono sulfide(SnS)is increasingly being investigated as a photo converter because it is neither scarce,expensive,nor toxic,although various theoretical and experimental studies on bulk SnS2 and SnSe2 and monolayer SnS2 have been reported till now a few theoretical studies have been carried out on monolayer and multilayer SnSe2.Based on density functional theory,we systematically study the structural,mechanical,and electronic properties of SnS2 and SnSe2 in monolayer and bilayer structures.Density functional theory was used for the structural stability,electrical and optical absorption of SnSe2(1-x)S2x and Sn2(1-x)Se2x(x=0.05,0.125,0.11,0.16 and 0.33)single-layer alloys by using ab initio method.It is found that random alloys come to emerged when SnSe2 and SnS2 is mixed.The computed negative change of S at Se of SnSe2 can provide a rare tool for the synthesis of alloys,i.e.substituting S in place of Se in SnSe2 monolayer.The results also show that,the band gap of the alloy deviate greatly because of the lattice incompatibility and the obvious charge transformation between SnSe2 and SnS2.In addition,the optical properties of the alloys are anisotropic,and alloying improves the absorption intensity in the visible spectrum region.We hope that it can contribute well to the future application of SnSe2(1-x)S2x andSn2(1-x)Se2x alloy.