Intrinsic Halide Immiscibility in 2D Mixed-Halide Ruddlesden–Popper Perovskites

Halide alloying of 3D and 2D Ruddlesden–Popper lead halide perovskites (RPPs) allows their bandgaps to be fine-tuned for optoelectronic applications. Many studies of mixed-halide RPPs assume that halide mixing yields homogeneous alloys like in 3D lead halide perovskites, with halide segregation only occurring under perturbations like light or heat. Here, we carefully investigate the mixed I/Br phases in solution-grown microplates of three representative n = 1 and n = 2 RPP phases to reveal heterogeneous halide microscale domains─even in absence of photoinduced phase separation. Such halide immiscibility is revealed by X-ray diffraction, and heterogeneity is confirmed by secondary ion mass spectrometry (SIMS) imaging; however, comparison with photoluminescence (PL) imaging results show how PL information alone can falsely imply homogeneous alloy formation in heterogeneous RPPs. We then demonstrate the use of spectral imaging as an alternative high-throughput tool for accurate halide phase characterization in mixed-halide RPPs.