2 research outputs found
Electrochemical Impedance Spectroscopy of All-Perovskite Tandem Solar Cells
This work explores
electrochemical impedance spectroscopy to study
recombination and ionic processes in all-perovskite tandem solar cells.
We exploit selective excitation of each subcell to enhance or suppress
the impedance signal from each subcell, allowing study of individual
tandem subcells. We use this selective excitation methodology to show
that the recombination resistance and ionic time constants of the
wide gap subcell are increased with passivation. Furthermore, we investigate
subcell-dependent degradation during maximum power point tracking
and find an increase in recombination resistance and a decrease in
capacitance for both subcells. Complementary optical and external
quantum efficiency measurements indicate that the main driver for
performance loss is the reduced capacity of the recombination layer
to facilitate recombination due to the formation of a charge extraction
barrier. This methodology highlights electrochemical impedance spectroscopy
as a powerful tool to provide critical feedback to unlock the full
potential of perovskite tandem solar cells
Tailoring Interlayer Charge Transfer Dynamics in 2D Perovskites with Electroactive Spacer Molecules
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 the 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 to 5, as cations in the two-dimensional
(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 reveal 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
to 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 properties through the wide range of functionalities available
in the world of organics