  The family of hybrid organic-inorganic lead-halide perovskites are the
subject of intense interest for optoelectronic applications, from
light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert
nature of most organic molecules, the inorganic sublattice generally dominates
the electronic structure and therefore optoelectronic properties of
perovskites. Here, we use optically and electronically active carbazole-based
Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the
number of CH2 units in the chain, varying from 3-5, as cations in the 2D
perovskite structure. By investigating the photophysics and charge transport
characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling
between the inorganic lead-halide and organic layers. The strongest interlayer
electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection
spectroscopy results remarkably demonstrate an organic-inorganic charge
transfer state. Ultrafast transient absorption spectroscopy measurements
demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to
the Cz-Ci molecules, the efficiency of which increases by varying the chain
length from i=5 to i=3. The charge transfer results in long-lived carriers
(10-100 ns) and quenched emission, in stark contrast with the fast (sub-ns) and
efficient radiative decay of bound excitons in the more conventional 2D
perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert
spacer. Electrical charge transport measurements further support enhanced
interlayer coupling, showing increased out-of-plane carrier mobility from i=5
to i=3. This study paves the way for the rational design of 2D perovskites with
combined inorganic-organic electronic proper-ties through the wide range of
functionalities available in the world of organics.
