Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

Compared with their 3D counterparts,
2D hybrid organic–inorganic
halide perovskites (HOIPs) exhibit enhanced chemical stabilities and
superior optoelectronic properties, which can be further tuned by
the application of external pressure. Here, we report the first high-pressure
study on CMA2PbI4 (CMA = cylcohexanemethylammonium),
a 2D HOIP with a soft organic spacer cation containing a flexible
cyclohexyl ring, using UV–visible absorption, photoluminescence
(PL) and vibrational spectroscopy, and synchrotron X-ray microdiffraction,
all aided with density functional theory (DFT) calculations. Substantial
anisotropic compression behavior is observed, as characterized by
unprecedented negative linear compressibility along the b axis. Moreover, the pressure dependence of optoelectronic properties
is found to be in strong contrast with those of 2D HOIPs with rigid
spacer cations. DFT calculations help to understand the compression
mechanisms that lead to pressure-induced bandgap narrowing. These
findings highlight the important role of soft spacer cations in the
pressure-tuned optoelectronic properties and provide guidance to the
design of new 2D HOIPs.