Publication date: 16th July 2025
Halide perovskites have attracted considerable interest over the past decade due to their exceptional applicability in optoelectronics. More recently, two-dimensional (2D) halide perovskites, characterised by the separation of lead-halide planes via large A-site cations, have received increasing attention. Previous studies have shown that the spacer molecule selection in 2D halide perovskites can have a critical influence on the optoelectronic properties, such as bandgap and charge carrier mobility. Illumination causes notorious iodide and bromide segregation in mixed-halide perovskites (MHP), compromising device stability. For 2D MHP, the existing literature lacks a systematic study how different spacers impact on the halide segregation.
In this work we present a comprehensive study towards the influence of different spacers molecules on the kinetics of halide segregation in n=1 2D MHP. Our analysis includes Ruddlesden-Popper (RP) (Phenethylammonium (PEA) and Butylammonium (BA)) as well as Dion-Jacobson (DJ) perovskites (Benzenediammonium (BEDA)) and similar derivatives. Initially, we confirm that the RP 2D MHP comprising alkyl spacers, such as BA show a severe segregation compared to PEA. The para-fluorination of the latter ones leads to a substantial mitigation of halide segregation.
Strikingly, compared to the RP 2D MHP the DJ analogues show a significantly enhanced resilience against light induced halide segregation. In optical absorbance spectroscopy, PEA exhibited a mixed-phase degradation of 1.5% after 30 minutes, whereas BEDA showed a lower degradation of 0.8% within the same timeframe. As several report link halide segregation to grain boundaries, the preparation of the 2D MHP using thiourea as crystallization additive leads to larger crystallites (>1 μm) and find a significantly suppressed degree of halide segregation.
Finally, we explored the impact of planar hot pressing (PHP), a variant of thermal imprint using a flat stamp, as a post-processing technique. While well established in 3D perovskite, studies of PHP on 2D halide perovskites are lacking so far. PHP results in 2D MHP layers with increased crystallite size and dramatically reduced layer roughness (<1.5 nm rms). Interestingly, the PHP process affords an overall better I:Br mixture of the 2D MHP as evidenced by Optical absorbance spectroscopy. Surprisingly, the halide segregation appears to be more pronounced in the PHP treated materials.
This research was supported through the Priority Program SPP2196 of the German Science Foundation (DFG; project 2D-FASOL, grant no. OL 462/6-1 and Rl 1551/22-1). Further we acknowledge support by the Federal Ministry of Education and Research via the grant NEPOMUQ (13N17113).
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