Atomic-Scale Tailoring of Organic Cation of Layered Ruddlesden–Popper Perovskite Compounds

Abstract

Layered Ruddlesden−Popper (RP) phase perovskite compounds have emerged as promising photovoltaic materials for solar cell applications, but they suffer from poor absorption and strong exciton-binding energy. Herein, fluoro-, chloro-, and bromo-substitutions on the 4-position of the phenyl group in the component C 6 H 5 CH 2 CH 2 NH 3 + (PEA + ) are designed and synthesized to investigate their effect on the layered RP type H-PEA 2 MA 2 Pb 3 I 10 (MA = CH 3 NH 3 ) perovskite as an example. Single-crystal X-ray diffraction and temperature-dependent photoluminescence spectroscopy characterization reveal the electron-withdrawing halogen in organic cations decreases the distortion of inorganic sheets and significantly reduces the impact of two-dimensional quantum and dielectric confinement. This is further verified with an increased visible absorption and lower excitonbinding energy for these new layered RP-type perovskite compounds. A planar structured perovskite solar cell using an F-PEA 2 MA 2 Pb 3 I 10 layer achieves a power conversion efficiency of 5.8%, which is better than that of the reference H-PEA 2 MA 2 Pb 3 I 10 .

Fluorinated Low‐Dimensional Ruddlesden–Popper Perovskite Solar Cells with over 17% Power Conversion Efficiency and Improved Stability

“…We believe that fluorine substitution on the para ‐position of the bulky PEA organic cation better retains the crystal symmetry with less structural distortion, and thus minimizes the unexpected structure disorder. We did not observe clear diffraction peaks at low diffraction angels (2θ < 10°) for both (PEA) 2 (MA) 4 Pb 5 I 16 and (4FPEA) 2 (MA) 4 Pb 5 I 16 films ( n = 5), consistent with the latest report . Therefore, we scraped the powders from the films, and measured the powder XRD from which a series of low‐diffraction peaks were observed, as seen in Figure S7 (Supporting Information).…”

“…As ε barrier increases, the large dipole moment of 4FPEA cations essentially decreased the dielectric confinement effect, and thereby reduced the exciton binding energy. Recent publications also found that introducing the polar organic substituents substantially reduces the exciton binding energy of 2D perovskite films . The reduced exciton binding energy can also be reflected by the corresponding redshift of excitonic peaks and onset of the absorption edge of the 4FPEA‐based RPP film shown earlier.…”

“…The theoretical calculation also predicted that a fluorine‐substituted organic spacer may distort the crystal structure of perovskite . Very recently, during our paper submission process, several latest publications also experimentally demonstrate that the fluorination of organic spacer cations can impact on the perovskite crystal structure and photovoltaic performance …”

Sensitive and Stable 2D Perovskite Single‐Crystal X‐ray Detectors Enabled by a Supramolecular Anchor

“…It is also reasonable that there is absence of this peak in counterpart PEAI sample. However, for (F‐PEA) 2 PbI 4 single crystal, the characteristic stretching vibration peak of CF bonds was shifted to smaller wavenumber of 1220 cm −1 (Figure 2b), [ 36 ] which illustrates that the deficient F atoms form strong electrostatic interaction with electron‐rich benzene rings in (F‐PEA) 2 PbI 4 . In addition, the supramolecular interaction is also confirmed by the improved thermal stability of (F‐PEA) 2 PbI 4 single crystals.…”

Stability Issue of Perovskite Solar Cells under Real‐World Operating Conditions

“…The crystalline orientation is one of the biggest obstacles for the 2D perovskite compounds in the photovoltaic application. Some researchers have used a thermal spin coating or added additive SCN − to control the vertical growth of 2D perovskite . Moreover, 2D perovskite materials possess a large bandgap at low n values, which may limit their development in the future.…”