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

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 PEA(2)MA(2)Pb(3)I(10) perovskite (PEA = phenylethylammonium) has been gradually substituted by the Gua cation to synthesize thin films of the mixed-cation PEA(2)(MA(1-x)Gua(x))(2)Pb3I10 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 PEA(2)MA(2)Pb(3)I(10) 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 PEA(2)(MA(1-x)Gua(x))(2)Pb3I10 perovskites with power conversion efficiency (PCE) values similar to those of the reference cell (similar to 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.