COMPLEX SYSTEMS AND BIOMEDICAL SCIENCES
coefficients, small exciton binding energies, and high charge carrier mobilities . However, much less is known about Sn-based perovskites, especially about their crystallisation mechanism from solution. Recently, it was shown that the addition of different amounts of low-dimensional PEA2SnI4 perovskite to formamidinium tin iodide (FASnI3) to form a 2D/3D composed
system increases stability against humidity and improves performances to a PCE as high as 9%, one of the highest reported for Sn-based devices . This is thanks to the highly aligned structure that occurs when processed in thin films. Thus, the motivation of this work is to understand the formation mechanism during spin coating of this aligned structure in the mixed 2D/3D systems (called Ruddlesden-Popper perovskites).
A specially designed sample environment was used for performing in-situ grazing incidence wide-angle X-ray scattering (GIWAXS) during the spin coating process at BM26 (Figure 55) . A home-built spin coater is equipped with an environmental chamber allowing flushing with inert N2 gas. Two remotely controlled micropipettes allow sequential deposition of the perovskite precursor solution and of an antisolvent. A laser and a photodiode are also coupled and mounted in reflection geometry to track the sample thickness evolution during drying via analysis of the interferometry curves. Figures 56a, 56c and 56e show the evolution of the systems from the solution state to the final thin film crystalline structure for three of the samples under study (3D, 2D and 2D/3D). After spin coating, colloidal precursor structures are formed, characterised by two broad GIWAXS peaks. Similar disordered precursors have already been observed for Pb-based perovskites . However, these precursors convert directly into the crystalline structure for Sn-based perovskites, without forming intermediate phases and without the need of any temperature annealing step. The GIWAXS patterns reveal that the 3D FASnI3 perovskite crystallises homogeneously within the wet layer, providing a random final distribution of the crystallites (Figure 56b). On the contrary, the 2D PEA2SnI4 perovskite crystallises anisotropically with the crystallisation starting at the air/solution interface and moving towards the substrate, leading to a highly oriented structure (Figure 56d). Addition of a small portion of PEA molecules to the 3D precursor solution dramatically alters the crystallisation behaviour. The in-situ data reveal an unexpected feature. In the 2D/3D samples, two main phases are formed: at first, crystallites with a 3D-like structure always form at the air/solution interface, followed by formation of a pseudo- 2D structure close to the substrate and with composition PEA2FASn2I7 (Figure 56e). The final structure of the 2D/3D hybrid samples is composed of an oriented 3D-like structure at the top of the film sitting on a highly oriented pseudo-2D structure at the bottom (Figure 56f).
In conclusion, in-situ GIWAXS reveals that addition of a small fraction of PEA+ molecules can suppress the homogeneous nucleation of
Fig. 55: Experimental setup used at BM26B to study in situ the crystallisation of perovskite solutions during spin coating.
Fig. 56: Evolution of (a,c,e) the GIWAXS intensity profiles and (b,d,f) final GIWAXS patterns for the 3D, 2D and 2D/3D systems. Chemical structures of the
phenylethylammonium cations, PEA+ (g) and the formamidinium cation, FA+ (h).