Monitoring the Transition from Molecular Surface Passivation to 2D Layer Formation on 3D Perovskite Films

While organic-inorganic (hybrid) perovskites achieved competitive power conversion efficiency within an impressive short time [1], they still suffer from insufficient stability [2]. Instability of hybrid perovskite thin films under thermal stress and/or in humid air is one of the major obstacles for the commercialization of perovskite-based solar modules [2,3]. One of the most promising approaches to improve the long-term stability is the incorporation of bulky, hydrophobic molecules into the perovskite layer [4]. While the beneficial effects of these post-deposition treatments are generally accepted, there is an ongoing discussion about the mechanism of this passivation effect. In particular, it is under debate when optimum surface passivation is reached using different concentrations of bulky molecules where a transition is supposed to happen from the surface attached bulky molecule itself to a 2D layer formation with general structure R₂A_n−1B_nX_3n+1 [5], which forms via intermixing of the bulky molecules and PbI₂ from the 3D perovskite. Here, A denotes the cation, B the metal ion, and X the halogen anion of the 3D perovskite, R is the bulky organic cation.<br/><br/>In this study, we use <i>in situ</i> photoluminescence (PL) measurements during spin-coating and annealing to probe the dynamic deposition of 2-thiophenemethylammonium iodide (2-TMAI) and phenylethylammonium (PEAI) with varied concentration on 3D triple cation perovskites. For both molecules, we find the transition from molecular passivation to the formation of an R₂A_n−1B_nX_3n+1 layer at concentrations around 4-10 mmol/L. Using higher concentrations, we see the formation of a distinct surface layer. During spin-coating and annealing of these higher concentrated solutions, we furthermore monitor the transition from a single inorganic layer spaced by the bulky cations (n=1) to mixed 2D layers (n=2 and higher) and, in the case of 2-TMAI, to the formation of a mixed disordered phase [6]. The latter may negatively affect the electronic properties of the perovskite layer [7]. Our results illustrate how <i>in situ</i> PL can be used to gain mechanistic understanding on the 2D layer formation, its interaction with the 3D perovskite, and its transformation to the disordered phase. Therefore, it can be utilized to deliberately optimize the annealing sequence targeting an ideal 2D/3D interface satisfying enhanced charge transport and stability. <br/><br/>Combining the abovementioned results with <i>in situ</i> XRD measurements, we propose a model for the surface passivation mechanism either via molecular passivation or 2D layer formation, compare the mechanisms for both investigated systems, and ultimately correlate them with device stability to propose an optimized stabilization treatment. <br/><br/>References:<br/>[1] M. A. Green et al., Prog. In Photov.: Res. & Appl. 29(7), 2021.<br/>[2] J. A. Christians et al., J. Am. Chem. Soc. 137(4), 2015.<br/>[3] B. Conings et al., Adv. Energ. Mat. 5(15), 2015.<br/>[4] C. Ma et al., Nanoscale 8, 18309, 2016.<br/>[5] Q. Jiang et al., Nature Photonics 13, 2019.<br/>[6] A. A. Sutanto et al., Nano Letters 20, 2020.<br/>[7] S. G. Motti, Nano Letters 19, 2019.