Understanding the
sub-band gap luminescence in Ruddlesden–Popper
2D metal halide hybrid perovskites (2D HaPs) is essential for efficient
charge injection and collection in optoelectronic devices. Still,
its origins are still under debate with respect to the role of self-trapped
excitons or radiative recombination via defect states. In this study,
we characterized charge separation, recombination, and transport in
single crystals, exfoliated layers, and polycrystalline thin films
of butylammonium lead iodide (BA2PbI4), one
of the most prominent 2D HaPs. We combined complementary defect- and
exciton-sensitive methods such as photoluminescence (PL) spectroscopy,
modulated and time-resolved surface photovoltage (SPV) spectroscopy,
constant final state photoelectron yield spectroscopy (CFSYS), and
constant light-induced magneto transport (CLIMAT), to demonstrate
striking differences between charge separation induced by dissociation
of excitons and by excitation of mobile charge carriers from defect
states. Our results suggest that the broad sub-band gap emission in
BA2PbI4 and other 2D HaPs is caused by radiative
recombination via defect states (shallow as well as midgap states)
rather than self-trapped excitons. Density functional theory (DFT)
results show that common defects can readily occur and produce an
energetic profile that agrees well with the experimental results.
The DFT results suggest that the formation of iodine interstitials
is the initial process leading to degradation, responsible for the
emergence of midgap states, and that defect engineering will play
a key role in enhancing the optoelectronic properties of 2D HaPs in
the future.