The thermal stability and decomposition pathway of formamidinium iodide (FAI, HC(NH2)2I) in contact with NiO and TiO2 are investigated by combined experimental studies and density functional theory (DFT) calculations. Based...

The toxicity of lead in metal halide perovskite solar cells is considered the most critical technical barrier for their commercialization. Among the candidates to replace Pb, trivalent cations (e.g., Bi3+ and Sb3+) are attracting considerable interest owing to their excellent reliability and favorable optical performance. Trivalent cation-based lead-free halide perovskites have two polymorphs: 0D phase (dimer, P63/mmc) and 2D phase (layered, P m1). The 2D phase has a lower band gap and is preferred for carrier transport; however, its phase stability at room temperature remains a challenge. This study investigated the effects of dual-site compositional mixing on the stabilization of the 2D phase of A3Bi2X9 (A = Cs, methylammonium (MA), formamidinium (FA); X = I, Br, Cl) and electronic structure modulation. Cl mixing was noted to be essential in creating a stable 2D phase. MA mixing can be applied to reduce the band gap widening for the Cl-mixed perovskites. We suggest the optimal criterion as Cs3−χMAχBi2I9–y Cl y , with χ < 0.1 and y > 0.3.

Metal-halide perovskites (MHPs) have attracted tremendous attention as active materials in optoelectronic devices. For light-emitting diode (LED) applications, nanostructuring of MHPs is considered to be inevitable, but its light-enhancement mechanism is still elusive because the particle (or grain) size is often beyond the quantum confinement regime. As motivated by the experimental finding that the nanostructuring can change the preferred crystalline symmetry from the orthorhombic phase to the high-symmetric cubic phase, we here investigated the carrier dynamics in various polymorphic phases of CsPbBr3 using ab initio quantum dynamics simulation. We found that the cubic phase shows a smaller inelastic phonon scattering than the orthorhombic phase; the suppression of the octahedral tilt minimizes the longitudinal Br fluctuation and helps disentangle the A-site cation dynamics from the nonadiabatic carrier dynamics. We thus anticipate that our present work will offer a material design principle to enhance the quantum yield of MHPs via symmetry engineering, which will help develop highly luminescent LED technology based on MHPs.

Despite the optoelectronic similarities between tin and lead halide perovskites, the performance of tin‐based perovskite solar cells remains far behind, with the highest reported efficiency to date being ≈14%. This is highly correlated to the instability of tin halide perovskite, as well as the rapid crystallization behavior in perovskite film formation. In this work, l‐Asparagine as a zwitterion plays a dual role in controlling the nucleation/crystallization process and improving the morphology of perovskite film. Furthermore, tin perovskites with l‐Asparagine show more favorable energy‐level matching, enhancing the charge extraction and minimizing the charge recombination, leading to an enhanced power conversion efficiency of 13.31% (from 10.54% without l‐Asparagine) with remarkable stability. These results are also in good agreement with the density functional theory calculations. This work not only provides a facile and efficient approach to controlling the crystallization and morphology of perovskite film but also offers guidelines for further improved performance of tin‐based perovskite electronic devices.