Relaxing the Goldschmidt Tolerance Factor: Sizable Incorporation of the Guanidinium Cation into a Two-Dimensional Ruddlesden–Popper Perovskite

Abstract

The two-dimensional (2D) hybrid perovskites, in particular the Ruddlesden–Popper (RP) phase, exhibit excellent optoelectronic properties, higher flexibility in the employed large organic cations, and an enhanced stability against the environmental agents compared to the three-dimensional (3D) perovskites. However, the small organic cations inserted into the octahedral voids have been limited so far to those three fulfilling the Goldschmidt tolerance factor (t) despite the relaxed structure of the 2D RP perovskites. In this work, the incorporation of the large guanidinium (Gua) cation into the octahedral sites of the “perovskite slabs” has been explored for the first time in 2D RP perovskites. Thus, the methylammonium (MA) cation in the PEA2MA2Pb3I10 perovskite (PEA = phenylethylammonium) has been gradually substituted by the Gua cation to synthesize thin films of the mixed-cation PEA2(MA1–x Gua x )2Pb3I10 perovskite. X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements have revealed a regular expansion of the unit cell when increasing the Gua content up to 90%, proving the sequential insertion into the lattice of the Gua having a larger ionic radius than that of the MA cation. Furthermore, the preferential orientation of the PEA2MA2Pb3I10 perovskite films with the (hk0) planes parallel to the substrate is maintained up to a limit value of 60% Gua content. Importantly, the combined analysis of the steady-state and time-resolved absorption and photoluminescence (PL) spectra has revealed a change in the distribution of the n-members of the 2D RP perovskites toward phases with lower n values upon increasing the Gua content. The position and intensity of the photoluminescence can be modulated within the low-dimensional perovskites (n = 2, 3, 4, and 5) at high Gua content (≥70%). We have fabricated solar cells based on the mixed-cation PEA2(MA1–x Gua x )2Pb3I10 perovskites with power conversion efficiency (PCE) values similar to those of the reference cell (∼2.5%) up to percentages of Gua of 20%. The unencapsulated devices have shown a significant enhancement in the stability after 750 h, demonstrating the positive effect of the Gua cation on the degradation of the 2D RP perovskites.

A‐Site Cation Engineering in 2D Ruddlesden–Popper (BA)2(MA1‐xAx)2Pb3I10 Perovskite Films

“…The larger exciton quantum confinement effect in the 2D RP perovskites with lower n values accounts for the shift of the PL peak to lower wavelengths. [ 34,46 ] In the films with EA, the blue shift of the PL peak from 20% to 40% could be additionally associated with the reduction of the bandgap observed in the absorption measurements which explains the lower threshold value measured in the PL compared to that found in the XRD experiments. In the films with Cs, the minor blue shift of the PL peak up to 60% is explained due to the increase of the octahedral tilting by the inclusion of the Cs cation in the lattice provoking a reduction of the bandgap.…”

“…[ 37 ] Regarding the preparation of films, we have recently reported the formation of mixed A‐cation (PEA) 2 (MA x Gua 1‐ x ) 2 Pb 3 I 10 2D RP perovskites in a wide range of Gua percentage and with a notable impact on the distribution of the n ‐members and, therefore on the optoelectronic properties of the films. [ 34 ] Thus, there is a lack of a general study to investigate the sort of A‐cations that can be incorporated in 2D RP films and their impact on the optoelectronic properties.…”

High‐Quality Ruddlesden–Popper Perovskite Film Formation for High‐Performance Perovskite Solar Cells

“…Luckily, both large cations, which were beyond the tolerance factor range, can still be inserted into A‐site in RP perovskites, opening the way to the introduction of a wider variety of A‐site cations and enriching the family of RP perovskites. [ 143,144 ]…”

Chalcogenide Perovskites and Perovskite-Based Chalcohalide as Photoabsorbers: A Study of Their Properties, and Potential Photovoltaic Applications

“…Theoretically, by cutting the 3D parent compounds along with a particular crystallography plane, the 2D layered perovskites are acquired, forming "perovskite slabs" that could be linked to one another by a broad range of organic positively charged ions as the restriction of size is more versatile compared to that of 3D equivalents. The responsibility of these organic cations are based on the compositional reliability of the 2D layered perovskite by electrostatic attraction in the middle of the negative charges of the halide negatively charged ions in the molecular geometry with eight faces and the positive amounts of the ammonium positively charged ions; notwithstanding the improved steady state of the 2D layered perovskite, however, further adjustments to the composition are required to evade the ultimate deterioration of the 2D compounds [39].…”