The First-Principle Investigation On Pb-Free Solar Cell Candidates

Organic-inorganic lead halide perovskite(e.g.,CH3NH3PbI3)was first used as visible-light sensitizer in photovoltaic cells with a power conversion efficiency(PCE)of 3.8%in 2009.Current champion cell could achieve a PCE of 22.7%,which is comparable to polycrystalline solar cells.Detailed density functional theory(DFT)analysis reveals that CH3NH3PbI3 exhibits following superior properties which are good for phtotvoltatic application:(1)corner-sharing perovskite octahedral unit;(2)high structural and electronic dimensionality;(3)Pb lone-pair s electron dominated valence band maximum(VBM)and(4)strong p-p transition.However,due to CH3NH3PbI3 intrinsic instability against temperature,moisture as well as light and toxic Pb inclusion,searching for new stable,environment-friendly and efficient solar cell candiates has attracted intense attentions.Keep above in mind,we first we show that the alloy perovskite system BaZr1-xTixS3(x<0.25)is a promising candidate for producing high power conversion efficiency(PCE)solar cells with ultrathin absorber layers.To maximize the minority carrier lifetime,which is important for achieving high PCE,the defect calculations show that BaZr1-xTixS3 films should be synthesized under moderate(i.e.,near stoichiometric)growth conditions to minimize the formation of deep-level defects.The perovskite BaZrS3 is also found to exhibit ambipolar self-doping properties,indicating the ability to form homo p-n junctions.However,our theoretical calculations and experimental solid-state reaction efforts indicate that the doped perovskite BaZr1-xTixS3(x>0)may not be stable under thermal equilibrium growth conditions.Calculations of decomposition energies suggest that introducing compressive strain may be a plausible approach to stabilize BaZr1-xTixS3 thin films.We then analyze the optical absorption properties of lead(Pb)-free metal halide perovskites(AB2+X3)and double perovskites(A2B+B3+X6)(A = Cs or monovalent organic ion,B2+ = non-Pb divalent metal,B+ = monovalent metal,B3+ = trivalent metal,X = halogen).We show that if B2+ is not Sn or Ge,Pb-free metal halide perovskites exhibit poor optical absorptions because of their indirect band gap nature.Among the nine possible types of Pb-free metal halide double perovskites,six have direct band gaps.Of these six types,four show inversion symmetry-induced parity-forbidden or weak transitions between band edges,making them not ideal for thin-film solar cell applications.Only one type of Pb-free double perovskite shows optical absorption and electronic properties suitable for solar cell applications,namely,those with B+ = In,T1 and B3+ = Sb,Bi.Our results provide important insights for designing new metal halide perovskites and double perovskites for optoelectronic applications.At last,we present density functional theory investigation of the stability,electronic and optical properties of M4M'X4.We find that M4M'X4 exhibit unique electronic properties.M elements lose partially both the outmost s and p electrons,while M' elements only lose a small fraction of the valence electrons.As a result,the conduction band edges of M4M'X4 consist of a large contribution from the M s orbitals,leading to rather small electron effective masses.The valence bands are derived from M,M' and X p orbitals.The bandgap of this family can be tuned by selecting the combination of M and X elements.Among these semiconductors,In4GeS4,In4GeSe4,In4SnS4 and In4SnSe4 are suitable for photovoltaic applications due to their stability and suitable bandgaps.However,the inclusion of scarce In may hinder their large scale application.