Light-Induced Halide Segregation in Mixed-Halide Ruddlesden-Popper Quasi-2D Perovskites

Layered Ruddlesden-Popper perovskite phases have emerged as a promising class of optoelectronic materials due to their high defect tolerance, structural tunability between a variety of lower-dimensional phases, and compositional versatility on account of a growing library of spacer cations. However, the photostability of mixed-halide compositions, relevant for light harvesting as well as emission applications, has not yet been comprehensively explored in such materials compared to in three-dimensional (3D) analogs.<br/> <br/>This work characterizes light-induced halide segregation in lower-dimensional Ruddlesden-Popper perovskites.¹ Using <i>in-situ</i> time- and temperature-dependent photoluminescence (PL) spectroscopy, the optoelectronic behaviour of such mixed-halide layers is tracked.² The observations are compared to compositional and structural data, generated using solid state nuclear magnetic resonance (ssNMR) spectroscopy, density functional theory (DFT), and grazing incidence wide angle X-ray scattering (GIWAXS) tools, and a correspondence between optoelectronic instability upon illumination and structural nature of the layer is established.<br/> <br/>Phenethylammonium (PEA)-based Ruddlesden-Popper two-dimensional (2D) mixed-halide perovskite thin films (<i>n</i> = 1) are first characterized for their photostability and are found to be largely resilient to halide segregation over prolonged exposure to visible light. ssNMR measurements show that this resilience is likely due to the presence of nanoscale iodide-rich sites that limit ion migration pathways which typically drives halide segregation in 3D perovskites.<br/> <br/>Quasi-2D perovskite layers are then developed using a co-solvent engineering approach.³ By tuning the crystallization rate of different <i>n</i>-phases, structurally stratified films with lower-dimensional (<i>n</i> = 1,2) phases at the substrate interface and higher-dimensional (3D-like) phases at the air interface can be formed. Here, a quasi-2D phase (<i>n</i> = 2) is found to be vulnerable to halide segregation, as also observed in 3D-like phases, showing the commonly observed PL red-shift. However, while entropic remixing of halide ions in dark conditions occurs in 3D-like phases, with a blue-shift of the PL peak, the process is absent in the <i>n</i> = 2 phase at room temperature and only occurs at elevated temperatures to restore the mixed-phase. This is due to a higher activation barrier for ion migration in such quasi-2D phases.⁴<br/> <br/>These observations show that iodide-bromide phase stability in mixed-halide quasi-2D perovskites is strongly related to the structural phase of the layer (n = 1, 2, 3, …). The prevailing view that lower-dimensional phases are inherently stable to ion migration is challenged by observations that clearly show the vulnerability of the <i>n</i> = 2 phase to halide segregation. The structural nature of the perovskite is also found to not only determine the tendency of halides to segregate, but also control the halide remixing behaviour in dark conditions.<br/> <br/>References:<br/>1. E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa, M. D. McGehee. <i>Chem. Sci.</i> <b>2015</b>, <i>6</i> (1), 613–617.<br/>2. K. Datta, B. T. van Gorkom, Z. Chen, M. J. Dyson, T. P. A. van der Pol, S. C. J. Meskers, S. Tao, P. A. Bobbert, M. M. Wienk, R. A. J. Janssen. <i>ACS Appl. Energy Mater.</i> <b>2021</b>, <i>4</i> (7), 6650–6658.<br/>3. A. Caiazzo, K. Datta, J. Jiang, M. C. Gélvez-Rueda, J. Li, R. Ollearo, J. M. Vicent-Luna, S. Tao, F. C. Grozema, M. M. Wienk, R. A. J. Janssen. <i>Adv. Energy Mater.</i> <b>2021</b>, <i>11</i> (42), 2102144.<br/>4. X. Xiao, J. Dai, Y. Fang, J. Zhao, X. Zheng, S. Tang, P. N. Rudd, X. C. Zeng, J. Huang. <i>ACS Energy Lett.</i> <b>2018</b>, <i>3</i> (3), 684–688.