Structural Dependence of Light-Induced Halide Segregation in Mixed-Halide Lower-Dimensional Perovskites

The advent of layered Ruddlesden-Popper phases promises the development of efficient lead-halide perovskite optoelectronic devices. Such lower-dimensional phases are more defect tolerant and resistant to environmental stressors, compared to three-dimensional (3D) analogs, leading to high operational stability.^[1] Mixed-halide compositions which are relevant for multijunction photovoltaic applications are, however, poorly characterized and as a result, the occurrence of light-induced instabilities is not well-understood in lower-dimensional phases. In 3D mixed-halide perovskites, such light-induced halide segregation leads to the formation of low-energy iodide-rich phases that act as trap sites, thereby limiting charge-carrier collection and limiting photovoltaic performance.^[2]<br/><br/>This work uses a model phenethylammonium-based series of lower-dimensional perovskites (<i>n</i> = 1, 2) and characterizes the occurrence of light-induced halide segregation through time- and temperature-dependent photoluminescence spectroscopy. It is found that the two-dimensional (2D) phase is largely resistant to halide segregation, even at high light intensity. Density functional theory calculations show that this is a consequence of preferential occupancy of halide (iodide and bromide) ions and halide vacancy defects between the axial and equatorial lattice sites which limits ion migration pathways in the 2D phase.^[3]<br/><br/>A co-solvent engineering approach is then used to develop a structurally stratified mixed-halide films with lower-dimensional (<i>n</i> = 1,2) phases at the substrate interface and higher-dimensional (approx. 3D) at the air interface, verified by grazing-incidence wide-angle X-ray scattering.^[4] An intermediate quasi-2D phase (n = 2) thus formed shows vulnerability to light-induced instability, similar to a 3D phase, with a characteristic red-shift related to the formation of iodide-rich phases. However, the entropic re-mixing of halide ions that occurs in 3D perovskites when stored in dark conditions is absent in quasi-2D phase; this is due to a higher activation barrier for ion migration in the quasi-2D phase which can occur at elevated temperatures to restore the well-mixed phase.<br/><br/>Collectively, these observations show that the structural nature (2D, quasi-2D or 3D) of mixed-halide perovskites plays a strong role in determining iodide-bromide phase stability in mixed-halide compositions. The contrast appears in both the occurrence of halide segregation under illumination and its reversal in the dark, guiding material design decisions for efficient and stable devices based on mixed-halide compositions.<br/><br/>[1] H. Tsai, W. Nie, J. C. Blancon, C. C. Stoumpos, R. Asadpour, B. Harutyunyan, A. J. Neukirch, R. Verduzco, J. J. Crochet, S. Tretiak, L. Pedesseau, J. Even, M. A. Alam, G. Gupta, J. Lou, P. M. Ajayan, M. J. Bedzyk, M. G. Kanatzidis, A. D. Mohite, <i>Nature</i> <b>2016</b>, <i>536</i>, 312.<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>, 6650.<br/>[3] N. E. Wright, X. Qin, J. Xu, L. L. Kelly, S. P. Harvey, M. F. Toney, V. Blum, A. D. Stiff-Roberts, <i>Chem. Mater.</i> <b>2022</b>, <i>34</i>, 3109.<br/>[4] 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>, 2102144.