Decoupling Interlayer Spacing and Cation Dipole on Exciton Binding Energy in Layered Halide Perovskites

Layered
halide perovskites (LHPs) are emerging semiconductor materials
due to their superior environmental stability compared to that of
traditional halide perovskites. While LHPs have tunable optoelectronic
properties, quantum and dielectric confinement effects due to organic
spacer layers limit their application. Recent attempts to mitigate
the high exciton binding energy (Eb) of
LHPs by organic cation engineering have been demonstrated; however,
systematic studies to decouple the influence of interlayer spacing
and molecular dipole are very limited. Here, we designed a new class
of organic spacer employing a malononitrile (MN) functionality giving
a calculated dipole moment of 7.9 D. Malononitrile phenethylammonium
(MNPEA) was successfully incorporated into lead iodide-based LHPs
thin films and as single crystals. Comparing the MNPEA-based LHP to
phenethylammonium (PEA) and biphenethylammonium (BPEA), selected as
reference cations to elucidate the influence of increased dipole moment
while excluding the contribution of increased interlayer distance,
clarified the effect of the large organic dipole. Binding energies, Eb, estimated by temperature-dependent photoluminescence
spectroscopy for MNPEA2PbI4, PEA2PbI4, and BPEA2PbI4 were 122, 354,
and 183 meV, respectively. Moreover, the similar interlayer spacing
of BPEA2PbI4 and MNPEA2PbI4 (21.04 and 21.36 Å, respectively) confirms the importance of
dipole in tuning the optoelectronic properties. Photovoltaic devices
with n = 1 LHPs demonstrated a higher fill factor
and open circuit voltage with MNPEA2PbI4 compared
to the reference layered perovskites, likely due to the favored charge
dissociation and transport afforded by the malononitrile-based cation.