Santanu Saha

Abstract:

We incorporate Se into the 3D halide perovskite framework using the zwitterionic ligand: SeCYS (+NH3(CH2)2Se-), which occupies both the X- and A+ sites in the prototypical ABX3 perovskite. The new organoselenide-halide perovskites: (SeCYS)PbX2 (X = Cl, Br) expand upon the recently discovered organosulfide-halide perovskites. Single-crystal X-ray diffraction and pair distribution function analysis reveal the average structures of the organoselenide-halide perovskites, whereas the local lead coordination environments and their distributions were probed through solid-state 77Se and 207Pb NMR, complemented by theoretical simulations. Density functional theory calculations illustrate that the band structures of (SeCYS)PbX2 largely resemble those of their S analogs, with similar band dispersion patterns, yet with a considerable bandgap decrease. Optical absorbance measurements indeed show bandgaps of 2.07 and 1.86 eV for (SeCYS)PbX2 with X = Cl and Br, respectively. We further demonstrate routes to alloying the halides (Cl, Br) and chalcogenides (S, Se) continuously tuning the bandgap from 1.86 to 2.31 eV-straddling the ideal range for tandem solar cells or visible-light photocatalysis. The comprehensive description of the average and local structures, and how they can fine-tune the bandgap and potential trap states, respectively, establishes the foundation for understanding this new perovskite family, which combines solid-state and organo-main-group chemistry.

Abstract:

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements.