Quasi-2D lead-free halide perovskite using superalkali cations for red-light-emitting diodes

Superalkali halide perovskites with suitable direct band gaps for photovoltaic applications

The construction of superalkali halide perovskites has attracted attention for the development of new photovoltaic materials, but stable superalkalis have not been found until now. Herein, to construct new three-dimensional superalkali halide perovskites with a MI3 frame (M = Sn and Pb), a new Li(H2O)3+ superalkali cation is designed and selected based on low vertical ionization potential, suitable tolerance factor, small ionic radius and large dissociation energy. High-throughput first-principles calculations show that superalkalis with lower vertical ionization potentials exhibit stronger interactions with the MI3 frame. The normal and cubic Li(H2O)3MI3 perovskites and cubic Li(H2O)4PbI3 perovskites have direct band gaps, s-p and p-p electron transitions, effective carrier masses of less than 0.45me and exciton binding energies of less than 291 meV. Moreover, the cubic Li(H2O)3PbI3 perovskite with a direct band gap of 1.40 eV can in theory show a power conversion efficiency of 33.49%. These results strongly suggest that superalkali cations with large dissociation energy can be used to develop stable superalkali perovskites for photovoltaic applications.

Atomically thin two-dimensional hybrid perovskites using hydrophobic superalkali cations with tunable electron transition type

The direct band gaps of two-dimensional (2D) metal halide perovskites can be tuned via component engineering, but their electron transition type hardly changes. Herein, atomically thin (C5NH6)2MX4 (M = Ge, Sn, Pb; X = Cl, Br, I) hybrid perovskites with hydrophobic superalkali cations were systematically explored using high-throughput hybrid density functional calculations and ab initio molecular dynamics simulations. We found that the electron transition between the M and X atoms was converted into that between the C5NH6 parts and X atoms via X change in the 2D (C5NH6)2MX4 perovskites. Negative formation energy, stable thermodynamic and kinetic properties, sharp valence bands, and tunable direct band gaps were obtained for the 2D perovskites. A power conversion efficiency (PCE) of 32.54% was obtained in theory for the passivated cubic NH2CHNH2PbI3 (FAPbI3) perovskite containing the 2D (C5NH6)2PbI4 perovskite. The hybrid Pb-free (C5NH6)2SnI4 perovskite with a direct bandgap of 1.56 eV may be viewed as a potential passivation material for perovskite devices. Moreover, the C5NH6 cations and X atoms show different hydrogen bonding interactions, which can be extended to other atomically thin organic-inorganic hybrid perovskites.