Suppressing X‐Migrations and Enhancing the Phase Stability of Cubic FAPbX₃ (X = Br, I)

Abstract

10 4 cm −1 in the visible-to-infrared region, bandgaps of 1.4-2.3 eV depending on A and X, form Wannier excitons of hundreds of Å diameter and tens of meV binding energy, and have high charge carrier diffusion lengths of up to µm. [1][2][3][4] The highest certified perovskite solar cell power conversion efficiency rapidly increased from 3.8% in 2009 to 22.1% in 2016 using hybrid FAPbI 3 doped with 5% MAPbBr 3[5] (and now stands at 24.2% [6] ). MA and FA stand for methylammonium and formamidinium, light polar organic cations formed by the simplest protonated amine NH 2 CH 3 and amidine NH 2 CHNH. Being incorporated into the lead halide crystal lattice, these organic cations participate not only in strong electrostatic and van der Waals interactions with the inorganic skeleton but also in hydrogen H···X bonding between H atoms (organic cation) and X atoms (inorganic skeleton) that is absent in all-inorganic perovskites. Being an electrondeficient π-conjugated system, FA also participates in π-anion π···X bonding with X atoms. [7] Though the organic cations do not directly determine the optoelectronic properties of hybrid perovskites, they do stabilize the inorganic skeleton of "3D" α-FAPbX 3 (cubic phase) and determine its defect properties, especially regarding X-defects. Goal of the present study therefore is to investigate the role of the FA bonding properties in α-FAPbX 3 and to propose promising organic cation doping strategies.The rising star of hybrid perovskite field of solar energy harvesting, α-FAPbI 3 , has a narrower bandgap than well-studied β-MAPbI 3 (tetragonal phase) and, more importantly, is more stable against decomposition and degradation. [8][9][10][11][12][13] Despite many advantages, β-MAPbI 3 has the serious problem that it is intrinsically thermodynamically unstable, decomposing into PbI 2 and MAI (and further into volatile amine and HI), and readily degrades even at moderate operating temperatures when exposed to water and/or oxygen. Lower affinity of α-FAPbI 3 to decomposition and degradation is related to weaker acidity (as a result of aromatic stabilization), larger volume, and stronger bonding with the halides of FA than MA, which manifests in experiment as suppressed hysteresis due to I-migrations. [5,14] The mobile I-defects in β-MAPbI 3 are detrimental not only to the structural stability of the material but also to its charge transport properties when acting as electron-hole recombination centers. [15][16][17] In this respect, it is crucial to understand the influence of FA on the X-migrations in α-FAPbX 3 and to identify ways to manipulate them by introducing stronger bonding organic cations as dopants.Chemical bonding of formamidinium (FA) with the inorganic perovskite skeleton of FAPbX 3 (X = Br, I) is studied with emphasis on the differences to methylammonium: stronger hydrogen bonding, the presence of π-anion bonding, and more sterically hindered motion inside the perovskite inorganic cage. Organic cation dopants fitting in the perovskite cubic cell and being capable of hydroge...

Molecular Bond Engineering and Feature Learning for the Design of Hybrid Organic-Inorganic Perovskites Solar Cells with Strong Non-Covalent Halogen-Cation Interactions

“…For example, at room temperature FAPbI 3 exist in a non-photoactive phase while when FA is mixed with MA or Cs, the photoactive phase becomes stable. 74,101 For what concerns the most common cations (MA and FA), our results generalize a previous study 35 where only the MA and FA cations were compared, showing higher binding strengths for the FA cation. Indeed, our findings generalize the higher E total of FA than MA to any type of halogen or metal, but by looking at E minX instead of only looking at E total , our work makes an even stronger case to use FA instead of MA, since MA binds some halogens particularly weakly.…”

“…In order to find the binding strengths, we combine the "Atoms in Molecules" theory 65 and the empirically established relationship between the properties of the BCPs. 35,67 Within this theory, a bonding interaction is defined by a maximum electron density path (bond path) connecting two interacting atoms. Critical points (CPs) are points where the gradient of the electron density, ∇ρ becomes zero.…”

“…The HSE06 functional is regularly used for HOIPs and is known to give good electronic and structural results although band gaps are slightly overestimated. 35,73,83 A 5 × 5 × 5 Monkhorst-Pack grid was used, centered on the Γ point. The electron densities were evaluated at a 400 × 400 × 400 grid and the critical points were obtained using the Critic2 program.…”

“…31 The instability is mainly caused by a relatively weak cohesion between the organic cation and the inorganic octahedra, and predominantly by the low energy barriers for the migration of halide anions and organic cations, with halide migration being the most prevalent. [33][34][35][36][37] Nevertheless, the (short-term) performance of HOIPs-based solar cells is considerably defect tolerant. Density Functional Theory (DFT) calculations based on hybrid functionals with spin orbit coupling have shown that the majority of the defects induce only shallow defect states near the band edges, 38,39 and that although iodine interstitials create deep levels in the gap of MAPbI 3 , 40,41 these only leave short-living hole traps.…”

Role of electrochemical reactions in the degradation of formamidinium lead halide hybrid perovskite quantum dots

“…28,29 One of the main reasons for inducing photochemical reactions is the high mobility of perovskite components inside the lattice. For example, the activation energy for the migration of halide ions in FAPbX 3 is very low (0.18 eV for bromide and 0.13 eV for iodide), 30 which makes hybrid perovskites “soft” materials compared with inorganic oxide perovskites such as BaTiO 3 , CaTiO 3 , and BaZrO 3 . Furthermore, the high polarity of perovskite ionic components 31 induces charge imbalance and defect-mediated ion migration, 28,31 representing one of the main reasons for the transport of halide ions from the CsPbX 3 perovskite structure to the electrolyte under an applied bias.…”