Globularity‐Selected Large Molecules for a New Generation of Multication Perovskites

Abstract

Perovskite solar cells (PSCs) use perovskites with an APbX structure, where A is a monovalent cation and X is a halide such as Cl, Br, and/or I. Currently, the cations for high-efficiency PSCs are Rb, Cs, methylammonium (MA), and/or formamidinium (FA). Molecules larger than FA, such as ethylammonium (EA), guanidinium (GA), and imidazolium (IA), are usually incompatible with photoactive "black"-phase perovskites. Here, novel molecular descriptors for larger molecular cations are introduced using a "globularity factor", i.e., the discrepancy of the molecular shape and an ideal sphere. These cationic radii differ significantly from previous reports, showing that especially ethylammonium (EA) is only slightly larger than FA. This makes EA a suitable candidate for multication 3D perovskites that have potential for unexpected and beneficial properties (suppressing halide segregation, stability). This approach is tested experimentally showing that surprisingly large quantities of EA get incorporated, in contrast to most previous reports where only small quantities of larger molecular cations can be tolerated as "additives". MA/EA perovskites are characterized experimentally with a band gap ranging from 1.59 to 2.78 eV, demonstrating some of the most blue-shifted PSCs reported to date. Furthermore, one of the compositions, MA EA PbBr , shows an open circuit voltage of 1.58 V, which is the highest to date with a conventional PSC architecture.

Engineering Multiphase Metal Halide Perovskites Thin Films for Stable and Efficient Solar Cells

“…In order to select suitable bulky spacers for high‐performance solar cells, we started by comparing four different ammoniums: ethyl ammonium (EA), butyl ammonium (BA), phenylethyl ammonium (PEA), and 5‐ammonium valeric acid (5AVA) presenting a larger and larger cation size ( Figure 1 a). The ionic sizes of all the four cations are too large to fit in the tolerance factor below 1, thus, in principle, they form 2D or 1D perovskite structure when they occupy the A site (Figure b) . We added these large cations into 3D‐forming perovskite solution, composed of FA and Cs (FA 0.85 Cs 0.15 PbX 3 with X = I 0.9 Br 0.1 ), and then we grew the polycrystalline thin film by using a conventional solvent‐quench method.…”

“…The BA cation has been used to produce 2D/3D perovskite structures, but there has been a broad statistic of PCEs depending on the chemical composition and processing methods of the thin films . The EA cation, with a shorter alkyl chain, has been reported as surface passivation molecule and directly within multication 3D perovskites . Nevertheless, the conditions which determine how this molecule should be used are yet to be studied more thoroughly.…”

Fabrication of Efficient and Stable Perovskite Solar Cells in High‐Humidity Environment through Trace‐Doping of Large‐Sized Cations

“…[24,33,34] The t-BA cation has quite as imilar chemical structure, [35] so it could occupy the Asite in the perovskite structure. The enlarged (110)a nd (2 20)l attice plane diffraction peaks show as ignificant shift to lower angles with increasinga mountso fa dditives compared with the pure MAPbI 3 samples.…”

Perovskites for Laser and Detector Applications

“…This size requirement, as well as structural stability, is captured by the so-called Goldschmidt's tolerance factor (GTF) (t ¼ rAþrX ffiffi 2 p ðrBþrXÞ ) [2] along with the octahedral factor (l ¼ rB rX ), [3] where r X simply refers to the radius of the ion X. The tolerance factors have been thus revised using Kieslich model of effective ionic radii, [7] and molecular globularity model by Gholipour et al [8] The ammonium halide-based hybrid perovskites mentioned above dispose into tunable structures, phases, dimensionalities, and morphologies (see Figure 1). For an ideal 3D perovskite structure, a t-value of 1 is desired and within the allowed range between 0.8 and 1, tilting of the BX 6 octahedra is noted , leading to a quasi-ideal perovskite structure.…”

“…To fit into an ideal cubic perovskite, the A-site cation has to be larger than B-site and thus specific size conditions need to be satisfied. [7,8] The pertinent reason in such cases is that r X cannot be clearly defined for the covalently bonded organic moieties in the perovskite framework. For an ideal cubic crystal structure, as in many 3D halide perovskites, the conditions 0.8 ≤ t ≤1 and 0.44 ≤ µ ≤ 0.90 need to be satisfied.…”