Halide-Dependent Formation Dynamics of 2D Ruddlesden-Popper Phases on 3D Perovskites

While organic-inorganic perovskites achieved competitive power conversion efficiency within a short time¹, they still suffer from insufficient stability². A promising approach to improve their long-term stability is the incorporation of bulky, hydrophobic molecules into the perovskite layer³. The addition of these species onto the 3D perovskite induces the formation of 2D Ruddlesden-Popper (RP) phases, and their formation dynamics along with the effects of these 2D layers on the material’s optical/electrical response in solar cells are under intense debate. Recently, we showed that the formation dynamics of these RP phases strongly depend on the choice of the bulky molecules.⁴<br/>In this study, we investigate the role of the halide (Iodine, Bromine, and Chlorine) in Phenethylammonium halides (PEAX) on the formation dynamics of RP phases on triple cation, mixed halide perovskite thin films. Using a multimodal approach combining <i>in situ</i> photoluminescence (PL), and <i>in situ</i> grazing incidence wide-angle X-ray scattering (GIWAXS) measurements during spin-coating and annealing, we monitor the deposition of the PEAX molecules and investigate the formation mechanics of the subsequently forming RP-phases. We find that the formation dynamics strongly depend on the halide used for the RP phase: In the case of Iodine, we see clear evidence for a slow conversion from the PEAI salt to pure 2D (n = 1 in A’₂A_n−1B_nX_3n+1, where A’ is PEA, A is Cs,MA,FA, B is Pb, and X is the halide) and quasi-2D (n &gt; 1) phases. In the case of PEABr and PEACl, however, we observe a rather fast formation of solely n = 1 RP-phases indicating different reaction dynamics and phase stability depending on the halide used. Furthermore, we observe an intermixing of Br and I when PEAI or PEABr is deposited on the I- and Br-containing 3D perovskite but no significant intermixing in the case of the PEACl. The latter might suggest a further increase of the chemical stability of the 2D RP phase with decreasing size of the used halide.<br/>Our results illustrate how multimodal <i>in situ</i> characterization can provide mechanistic insights into the 2D layer formation and its interaction with the 3D perovskite. Therefore, it can be utilized to deliberately optimize the annealing sequence targeting an ideal 2D/3D interface satisfying enhanced charge transport and stability. We will further correlate these mechanistic insights with device performance and stability.<br/><br/> <br/><b>References:</b><br/>[1] M. A. Green et al., <i>Prog. In Photov.: Res. & Appl.</i> 29(7), 2021.<br/>[2] J. A. Christians et al., <i>J. Am. </i><i>Chem. Soc.</i> 137(4), 2015.<br/>[3] C. Ma et al., <i>Nanoscale </i>8, 18309, 2016.<br/>[4] T. Kodalle et al. <i>Advanced Energy Materials</i>, Accepted for Publication, 2022. [DOI: 10.1002/aenm.202201490]