Relaxing the Goldschmidt Tolerance Factor: Sizable Incorporation of the Guanidinium Cation into aTwo-dimensional Ruddlesden-Popper Perovskite
Susana Ramos Terrón a, Alexander Davis Jodlowski a, Cristóbal Verdugo Escamilla b, Luis Camacho a, Gustavo de Miguel a
a Universidad de Córdoba, Instituto Universitario de Investigación en Química Fina y Nanoquímica, IUNAN, Spain, Córdoba, Spain
b Laboratorio de Estudios Cristalográficos, IACT, CSIC-UGR. Avda. Las Palmeras, n° 4, E-18100 Armilla (Granada), Spain
nanoGe Perovskite Conferences
Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO20)
Sevilla, Spain, 2020 February 23rd - 25th
Organizer: Hernán Míguez
Poster, Susana Ramos Terrón, 010
Publication date: 25th November 2019

The incorporation of large organic cations in between “perovskite slabs” to form two-dimensional (2D) hybrid perovskites has been reported to mitigate the degradation against the environmental agents of the three-dimensional (3D) perovskites widely employed in optoelectronic devices. The 2D hybrid perovskites, in particular the Ruddlesden-Popper (RP) phase, exhibit excellent optoelectronic properties with a wide flexibility in the type of large organic cations that can be employed. However, the small organic cations inserted in 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 that may open the way to the insertion of other cations. In this study, the incorporation into the octahedral sites of the “perovskite slabs” of a large ammonium cation neglecting the tolerance factor (Guanidinium, Gua) 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-xGuax)2Pb3I10 perovskite. X-ray Diffraction (XRD) and GrazingIncidence 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. Scanning Electron Microscopy (SEM) has also displayed a densely packed grain morphology that is not modified due to the addition of Gua. Importantly, the combined analysis of the absorption and photoluminescence (PL) spectra have revealed a change in the distribution of the n-members of the 2D RP perovskites towards phases with low n values upon increasing the Gua content. In particular, a sudden change is observed at 30% Gua content which is related to the impossibility of the phases with high n values to incorporate more than 25% Gua in their structure. Thus, the addition of a large organic cation that substitutes the small MA cation plays a key role to control the distribution of n-members in the 2D RP perovskite films. With this strategy, it is possible to fine tune the position and shape of the absorption and PL spectra and to significantly increase 25 times the PL intensity of the films. Theoretical calculations on the 2D RP perovskites have proven that the incorporation of the Gua cation to the low dimensional perovskite (n = 3) is energetically more favorable with respect to the insertion in the 3D material. Finally, the stability against the environmental factors of the 2D RP perovskite films without encapsulation has been significantly improved even at low percentages of Gua.

G. d. M. thanks the Ministry of Economy and Competitiveness for a “Ramón y Cajal” contract (RYC-2013-12772). S. R-T. thanks the Ministry of Science, Innovation and Universities for a FPU fellowship (FPU18/04452). This work was financially supported by the Ministerio de Economía y Competitividad (MINECO) through project CTQ2017-84221-R, cofinanced with the FEDER funds. We are grateful to Dr. Eduardo Espinosa Victor and Prof. Alejandro Rodríguez Pascual for his assistance with the Infrared Spectroscopy

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