Theoretical Exploration Of New Stable Perovskite Solar Cell Absorbers

The organic-inorganic hybrid perovskites(OIHP)have recently drawn tremendous interest because of their superior photovoltaic properties.However,the instability presents serious obstacles for large-scale commercialization.Currently,cognition of perovskite stability is based on tolerance factor(t)proposed by Switzerland scientist Goldschmidt.Tolerance factor(t)is recently provided qualitative guidance for experimentalists to engineer stable ABX3 perovskite by tuning effective ionic size with mixing cation or anions,and for theorists to search emerging perovskites.Through first-principles calculations,we have calculated decomposition energies(the measure of thermodynamic stability)of 138 perovskite compounds of potential solar cell applications.Instead of t,we have found that(u+t)η,where μ and η are the octahedral factor and the atomic packing fraction respectively,demonstrates a remarkably linear correlation with thermodynamic stability.As a stability descriptor,(μ+t)is able to predict the relative stability among any two perovskites with the accuracy～90%is much higher than descriptor,t(～70%).This trend is then used to predict decomposition energies of other 69 perovskites and the results are in excellent agreement with first-principles calculations,indicating the generalization of the trend.On the basis of this rule,chalcogenides are stable.Especially,oxide perovskites are most stable thermodynamic stability.Meanwhile,previous studies showed that the lone pair s electron plays a key role in superior photovoltaic properties of perovskite.Here,we propose stable lead-free oxide double perovskites through chemical mutation of the B site based on prototype CaTiO3 oxide perovskite,for photovoltaic application.Mutating Ti4+ ions to ns2 ions Bi3+ or Sb3+ and group VB elements forms a class of oxide double perovskites A2M3+M5+O6(A=Ca,Sr,Ba;M3+=Sb3+ or Bi3+;M5+=V5+,Nb5+,Ta5+).We find through first-principles calculations that these oxide double perovskites exhibit much better stability than halide perovskites.Moreover,differing from prevalent halide double perovskites such as Cs2AgBiBr6,which has an indirect band gap,A2M3+M5+O6 exhibit direct band gaps ranging from 0.3 to 3.8 eV.With a proper B site alloying,the band gap can be tuned within the range of 1.0-1.6 eV with optical absorptions as strong as CH3NH3PbI3,which are suitable for efficient single-junction thin-film solar cell application.Our work provides an alternative way to search for lead-free stable perovskite solar cell absorbers beyond halides.