Development of a triple-cation Ruddlesden–Popper perovskite structure with various morphologies for solar cell applications

The present research sheds new light on the development of a triple-cation quasi-two-dimensional (2D) perovskite family with the general formula of (S 1− x S′ x ) 2 [Cs 0.05 (FA 1− x MA x ) 0.95 ] 3 Pb 4 (I 1− x Br x ) 13 , in which two spacers, namely 5-ammonium valeric acid iodide (S) and tetra- n -octylammonium bromide (S′) were simultaneously incorporated. Morphology, crystal structure, optical properties, photovoltaic performance, and internal resistances of such compound were systemically studied in comparison with an analogous single-cation 2D counterpart (i.e. (S) 2 (FA) 3 Pb 4 I 13 ) as a reference. X-ray diffraction set forth that the films deposited based upon these compounds had a 2D perovskite crystal structure owing to the pronounced (0 k 0) peak series at low angles. Field emission scanning electron microscopy propounded that the layered ultrathin 2D structures were stacked up to produce various 3D solids such as particles, flowers, needles, sheets, blades, and cubes, depending on the spacers atomic ratios (i.e. x = S′/S + S′) in the range of 0–0.05. Moreover, the photoluminescence spectra of the films exhibited that the triple-cation perovskites had lower bandgap of around 1.68 eV compared to the reference film (ca., 1.74 eV), confirming the formation of 2D perovskites. The device fabricated based on 97 at% S and 3 at% S′ (i.e. x = 0.03) showed the highest power conversion efficiency of 10.2% due mainly to its low series resistance (11.7 Ω), high charge recombination resistance (922.4 Ω), and long electron lifetime (8.0 µs) among the fabricated cells. The un-encapsulated x = 0.03 cell displayed a maximum external quantum efficiency of 82% and lost just 18% of its initial efficiency after 2500 h in ambient conditions.