19 research outputs found
Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide
Formamidinium lead iodide (FAPbI<sub>3</sub>) has the potential
to achieve higher performance than established perovskite solar cells
like methylammonium lead iodide (MAPbI<sub>3</sub>), while maintaining
a higher stability. The major drawback for the latter material is
that it can crystallize at room temperature in a wide bandgap hexagonal
symmetry (<i>P</i>6<sub>3</sub><i>mc</i>) instead
of the desired trigonal (<i>P</i>3<i>m</i>1) black
phase formed at a higher temperature (130 °C). Our results show
that employing a mixture of MAI and FAI in films deposited via a two-step
approach, where the MAI content is <20%, results in the exchange
of FA molecules with MA without any significant lattice shrinkage.
Additionally, we show with temperature-dependent X-ray diffraction
that the trigonal phase exhibits no phase changes in the temperature
range studied (25 to 250 °C). We attribute the stabilization
of the structure to stronger interactions between the MA cation and
the inorganic cage. Finally, we show that the inclusion of this small
amount of MA also has a positive effect on the lifetime of the photoexcited
species and results in more efficient devices
Charge Transport Limitations in Self-Assembled TiO<sub>2</sub> Photoanodes for Dye-Sensitized Solar Cells
Solid-state dye-sensitized solar cells offer the possibility
of
high-power conversion efficiencies due to theoretically lower fundamental
losses in dye regeneration. Despite continuous progress, limitations
in charge diffusion through the mesoporous photoanode still constrain
the device thickness and hence result in reduced light absorption
with the most common sensitizers. Here we examine block copolymer-assembled
photoanodes with similar surface area and morphology but a large variation
in crystal size. We observe that the crystal size has a profound effect
on the electron transport, which is not explicable by considering
solely the ratio between free and trapped electrons. Our results are
consistent with the long-range mobility of conduction band electrons
being strongly influenced by grain boundaries. Therefore, maximizing
the crystal size while maintaining high enough surface area will be
an important route forward
Guided in Situ Polymerization of MEH-PPV in Mesoporous Titania Photoanodes
Incorporation
of conjugated polymers into porous metal oxide networks is a challenging
task, which is being pursued via many different approaches. We have
developed the guided in situ polymerization of poly(2-methoxy-5-(2′-ethylhexyloxy)-<i>p</i>-phenylenevinylene) (MEH-PPV) in porous titania films by
means of surface functionalization. The controlled polymerization
via the Gilch route was induced by an alkoxide base and by increasing
the temperature. The selected and specially designed surface-functionalizing
linker molecules mimic the monomer or its activated form, respectively.
In this way, we drastically enhanced the amount of MEH-PPV incorporated
into the porous titania phase compared to nonfunctionalized samples
by a factor of 6. Additionally, photovoltaic measurements were performed.
The devices show shunting or series resistance limitations, depending
on the surface functionalization prior to in situ polymerization of
MEH-PPV. We suggest that the reason for this behavior can be found
in the orientation of the grown polymer chains with respect to the
titania surface. Therefore, the geometry of the anchoring via the
linker molecules is relevant for exploiting the full electronic potential
of the conjugated polymer in the resulting hybrid composite. This
observation will help to design future synthesis methods for new hybrid
materials from conjugated polymers and n-type semiconductors to take
full advantage of favorable electronic interactions between the two
phases
Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films
Organic–inorganic metal halide
perovskite solar cells have recently attracted considerable attention
with reported device efficiencies approaching those achieved in polycrystalline
silicon. Key for an efficient extraction of photogenerated carriers
is the combination of low nonradiative relaxation rates leading to
long carrier lifetimes and rapid diffusive transport. The latter,
however, is difficult to assess directly with reported values varying
widely. Here, we present an experimental approach for a contactless
visualization of the charge carrier diffusion length and velocity
in thin films based on time-resolved confocal detection of photoluminescence
at varying distances from the excitation position. Our measurements
on chloride-treated methylammonium lead iodide thin films, the material
for which the highest solar cell efficiencies have been reported,
reveal a charge carrier diffusion length of 5.5–7.7 μm
and a transport time of 100 ps for the first micrometer corresponding
to a diffusion constant of about 5–10 cm<sup>2</sup> s<sup>–1</sup>, similar to GaAs thin films
Recycling Perovskite Solar Cells To Avoid Lead Waste
Methylammonium
lead iodide (MAPbI<sub>3</sub>) perovskite based solar cells have
recently emerged as a serious competitor for large scale and low-cost
photovoltaic technologies. However, since these solar cells contain
toxic lead, a sustainable procedure for handling the cells after their
operational lifetime is required to prevent exposure of the environment
to lead and to comply with international electronic waste disposal
regulations. Herein, we report a procedure to remove every layer of
the solar cells separately, which gives the possibility to selectively
isolate the different materials. Besides isolating the toxic lead
iodide in high yield, we show that the PbI<sub>2</sub> can be reused
for the preparation of new solar cells with comparable performance
and in this way avoid lead waste. Furthermore, we show that the most
expensive part of the solar cell, the conductive glass (FTO), can
be reused several times without any reduction in the performance of
the devices. With our simple recycling procedure, we address both
the risk of contamination and the waste disposal of perovskite based
solar cells while further reducing the cost of the system. This brings
perovskite solar cells one step closer to their introduction into
commercial systems
Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films
Micro-
and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film
solar cells today reach power conversion efficiencies of over 20%.
We investigate the impact of grain boundaries on charge carrier transport
in large crystal MAPI thin films using time-resolved photoluminescence
(PL) microscopy and numerical model calculations. Crystal sizes in
the range of several tens of micrometers allow for the spatially and
time resolved study of boundary effects. Whereas long-ranged diffusive
charge carrier transport is observed within single crystals, no detectable
diffusive transport occurs across grain boundaries. The observed PL
transients are found to crucially depend on the microscopic geometry
of the crystal and the point of observation. In particular, spatially
restricted diffusion of charge carriers leads to slower PL decay near
crystal edges as compared to the crystal center. In contrast to many
reports in the literature, our experimental results show no quenching
or additional loss channels due to grain boundaries for the studied
material, which thus do not negatively affect the performance of the
derived thin-film devices
Synthesis of Perfectly Oriented and Micrometer-Sized MAPbBr<sub>3</sub> Perovskite Crystals for Thin-Film Photovoltaic Applications
Wide band gap perovskites
such as methylammonium lead bromide are
interesting materials for photovoltaic applications because of their
potentially high open-circuit voltage. However, the fabrication of
high-quality planar films has not been investigated in detail for
this material. We report a new synthesis approach for the fabrication
of bromide-based perovskite planar films based on the control of the
deposition environment. We achieve dense layers with large and perfectly
oriented crystallites 5–10 μm in size. Our results show
that large crystal sizes can be achieved only for smooth indium-doped
tin oxide substrates, whereas lateral perovskite crystal growth is
limited for the rougher fluorine-doped tin oxide substrates. We additionally
correlate photocurrent and perovskite crystal properties in photovoltaic
devices and find that this parameter is maximized for ordered systems,
with internal quantum efficiencies approaching unity. Hence, our work
not only gives a new pathway to tune morphology and crystal orientation
but also demonstrates its importance for planar perovskite solar cells
Unraveling the Function of an MgO Interlayer in Both Electrolyte and Solid-State SnO<sub>2</sub> Based Dye-Sensitized Solar Cells
The coating of n-type mesoporous metal oxides with nanometer
thick
dielectric shells is a route that has proven to be successful at enhancing
the efficiency of some families of dye-sensitized solar cells. The
primary intention is to introduce a “surface passivation layer”
to inhibit recombination between photoinduced electrons and holes
across the dye-sensitized interface. However, the precise function
of these dielectric interlayers is often ambiguous. Here, the role
of a thin MgO interlayer conformally deposited over mesoporous SnO<sub>2</sub> in liquid electrolyte and solid-state dye-sensitized solar
cells is investigated. For both families of devices the open-circuit
voltage is increased by over 200 mV; however, the short-circuit photocurrent
is increased for the solid-state cells, but reduced for the electrolyte
based devices. Through electronic and spectroscopic characterization
we deduce that there are four distinct influences of the MgO interlayer:
It increases dye-loading, slows down recombination, slows down photoinduced
electron transfer, and results in a greater than 200 mV shift in the
conduction band edge, with respect to the electrolyte redox potential.
The compilation of these four factors have differing effects and magnitudes
in the solid-state and electrolyte DSCs but quantitatively account
for the difference in device performances observed for both systems
with and without the MgO shells. To the best of our knowledge, this
is the most comprehensive account of the role of dielectric shells
in dye-sensitized solar cells and will enable much better interfacial
design of photoelectrodes for DSCs
Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers
Understanding
the charge transport characteristics and their limiting factors in
organolead halide perovskites is of great importance for the development
of competitive and economically advantageous photovoltaic systems
derived from these materials. In the present work, we examine the
charge carrier mobilities in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPI) thin films obtained from a one-step synthesis procedure and
in planar n–i–p devices based on these films. By performing
time-of-flight measurements, we find mobilities around 6 cm<sup>2</sup>/V s for electrons and holes in MAPI thin films, whereas in working
solar cells, the respective effective mobility values are reduced
by 3 orders of magnitude. From complementary experiments on devices
with varying thicknesses of electron and hole transport layers, we
identify the charge extraction layers and the associated interfaces
rather than the perovskite material itself as the major limiting factors
of the charge carrier transport time in working devices
Light-Emitting Electrochemical Cells Based on Hybrid Lead Halide Perovskite Nanoparticles
Methylammonium lead bromide (MAPbBr<sub>3</sub>) perovskite nanoparticles
(NPs) have been recently proposed as a new material for light-emitting
diodes, as well as a new paradigm to elucidate the operational mechanism
in perovskite solar cells. Here, we have expanded the synthesis concept
to fabricate NPs based on formamidinium lead bromide (FAPbBr<sub>3</sub>). Importantly, we have demonstrated that the photophysical features
of this novel material can be easily tuned by exchanging the organic
cation, achieving lower radiative bimolecular recombination rate for
FAPbBr<sub>3</sub> NPs. Additionally, we report for the first time
light-emitting electrochemical cells (LECs) based on perovskite NPs
by an easily up-scalable spray-coating technique. Stable luminance
of 1–2 cd/m<sup>2</sup> at low driving currents was achieved
for both types of materials. Overall, this work opens a new avenue
of research into the field of organic–inorganic metal halide
nanoparticles bearing different alkyl ammonium groups and their application
in the developing field of thin-film lighting devices