posted on 2020-03-05, 18:21authored byDaniel
B. Straus, Sebastian Hurtado Parra, Natasha Iotov, Qinghua Zhao, Michael R. Gau, Patrick J. Carroll, James M. Kikkawa, Cherie R. Kagan
We report a family
of two-dimensional hybrid perovskites (2DHPs)
based on phenethylammonium lead iodide ((PEA)2PbI4) that show complex structure in their low-temperature excitonic
absorption and photoluminescence (PL) spectra as well as hot exciton
PL. We replace the 2-position (ortho) H on the phenyl
group of the PEA cation with F, Cl, or Br to systematically increase
the cation’s cross-sectional area and mass and study changes
in the excitonic structure. These single atom substitutions substantially
change the observable number of and spacing between discrete resonances
in the excitonic absorption and PL spectra and drastically increase
the amount of hot exciton PL that violates Kasha’s rule by
over an order of magnitude. To fit the progressively larger cations,
the inorganic framework distorts and is strained, reducing the Pb–I–Pb
bond angles and increasing the 2DHP band gap. Correlation between
the 2DHP structure and steady-state and time-resolved spectra suggests
the complex structure of resonances arises from one or two manifolds
of states, depending on the 2DHP Pb–I–Pb bond angle
(as)symmetry, and the resonances within a manifold are regularly spaced
with an energy separation that decreases as the mass of the cation
increases. The uniform separation between resonances and the dynamics
that show excitons can only relax to the next-lowest state are consistent
with a vibronic progression caused by a vibrational mode on the cation.
These results demonstrate that simple changes to the cation can be
used to tailor the properties and dynamics of the confined excitons
without directly modifying the inorganic framework.