  The organic spacer cation plays a crucial role in determining the exciton
fine structure in two-dimensional (2D) perovskites. Here, we use
low-temperature magneto-optical spectroscopy to gain insight into the influence
of the organic spacer on dark excitons in Ruddlesden-Popper (RP) perovskites.
We show that by using modest magnetic field strengths (<1.5 T), the
spin-forbidden dark exciton state can be identified and its emission properties
significantly modulated through the application of in-plane magnetic fields, up
to temperatures of 15 K. At low temperatures, an increase in collected
photoluminescence efficiency of >30% is demonstrated, signifying the critical
role of the dark exciton state for light-emitting applications of 2D
perovskites. The exciton fine structure and the degree of
magnetic-field-induced mixing are impacted by the choice of organic spacer
cation, with 4-methoxyphenethylammonium (MeO-PEA) showing the largest effect
due to larger bright-dark exciton splitting. Our results suggest that dark
excitons preferentially form biexcitons depending on the choice of spacer. We
distinguish between interior (bulk) and surface dark-exciton emission, showing
that bright-dark exciton splitting differs between the interior and surface.
Our results emphasize the significance of the organic spacer cation in
controlling the exciton fine structure in 2D perovskites and have important
implications for the development of optoelectronic technology based on 2D
perovskites.
