6 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
Synthesis and Reactivity of Triazaphenanthrenes
Pyridonaphthyridines
(triazaphenanthrenes) were prepared in 4 steps and in 13–52%
overall yield using Negishi cross-couplings between iodopicolines
and 2-chloro-pyridylzinc derivatives. After chlorination, Gabriel
amination and spontaneous ring-closure, the final aromatization leading
to the triazaphenanthrenes was achieved with chloranil. This heterocyclic
scaffold underwent a nucleophilic addition at position 6 leading to
further functionalized pyridonaphthyridines. The influence of these
chemical modifications on the optical properties was studied by steady-state
and time-resolved optical spectroscopy. While the thiophene-substituted
heterocycles exhibited the most extended absorption, the phenyl- and
furan-substituted compounds showed a stronger photoluminescence, reaching
above 20% quantum yield and lifetimes of several nanoseconds
Oligothiophene-Bridged Conjugated Covalent Organic Frameworks
Two-dimensional covalent organic
frameworks (2D-COFs) are crystalline,
porous materials comprising aligned columns of π-stacked building
blocks. With a view toward the application of these materials in organic
electronics and optoelectronics, the construction of oligothiophene-based
COFs would be highly desirable. The realization of such materials,
however, has remained a challenge, in particular with respect to laterally
conjugated imine-linked COFs. We have developed a new building block
design employing an asymmetric modification on an otherwise symmetric
backbone that allows us to construct a series of highly crystalline
quaterthiophene-derived COFs with tunable electronic properties. Studying
the optical response of these materials, we have observed for the
first time the formation of a charge transfer state between the COF
subunits across the imine bond. We believe that our new building block
design provides a general strategy for the construction of well-ordered
COFs from various extended building blocks, thus greatly expanding
the range of applicable molecules
Preparation of Single-Phase Films of CH<sub>3</sub>NH<sub>3</sub>Pb(I<sub>1–<i>x</i></sub>Br<sub><i>x</i></sub>)<sub>3</sub> with Sharp Optical Band Edges
Organometallic lead-halide perovskite-based
solar cells now approach
18% efficiency. Introducing a mixture of bromide and iodide in the
halide composition allows tuning of the optical bandgap. We prepare
mixed bromide–iodide lead perovskite films CH<sub>3</sub>NH<sub>3</sub>PbÂ(I<sub>1–<i>x</i></sub>Br<sub><i>x</i></sub>)<sub>3</sub> (0 ≤ <i>x</i> ≤ 1) by
spin-coating from solution and obtain films with monotonically varying
bandgaps across the full composition range. Photothermal deflection
spectroscopy, photoluminescence, and X-ray diffraction show that following
suitable fabrication protocols these mixed lead-halide perovskite
films form a single phase. The optical absorption edge of the pure
triiodide and tribromide perovskites is sharp with Urbach energies
of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>1.2</sub>Br<sub>1.8</sub>. We
demonstrate a bromide–iodide lead perovskite film (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>1.2</sub>Br<sub>1.8</sub>) with an optical
bandgap of 1.94 eV, which is optimal for tandem cells of these materials
with crystalline silicon devices
Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks
Covalent
organic frameworks (COFs), formed by reversible condensation
of rigid organic building blocks, are crystalline and porous materials
of great potential for catalysis and organic electronics. Particularly
with a view of organic electronics, achieving a maximum degree of
crystallinity and large domain sizes while allowing for a tightly
Ï€-stacked topology would be highly desirable. We present a design
concept that uses the 3D geometry of the building blocks to generate
a lattice of uniquely defined docking sites for the attachment of
consecutive layers, thus allowing us to achieve a greatly improved
degree of order within a given average number of attachment and detachment
cycles during COF growth. Synchronization of the molecular geometry
across several hundred nanometers promotes the growth of highly crystalline
frameworks with unprecedented domain sizes. Spectroscopic data indicate
considerable delocalization of excitations along the π-stacked
columns and the feasibility of donor–acceptor excitations across
the imine bonds. The frameworks developed in this study can serve
as a blueprint for the design of a broad range of tailor-made 2D COFs
with extended π-conjugated building blocks for applications
in photocatalysis and optoelectronics
Synchronized Offset Stacking: A Concept for Growing Large-Domain and Highly Crystalline 2D Covalent Organic Frameworks
Covalent
organic frameworks (COFs), formed by reversible condensation
of rigid organic building blocks, are crystalline and porous materials
of great potential for catalysis and organic electronics. Particularly
with a view of organic electronics, achieving a maximum degree of
crystallinity and large domain sizes while allowing for a tightly
Ï€-stacked topology would be highly desirable. We present a design
concept that uses the 3D geometry of the building blocks to generate
a lattice of uniquely defined docking sites for the attachment of
consecutive layers, thus allowing us to achieve a greatly improved
degree of order within a given average number of attachment and detachment
cycles during COF growth. Synchronization of the molecular geometry
across several hundred nanometers promotes the growth of highly crystalline
frameworks with unprecedented domain sizes. Spectroscopic data indicate
considerable delocalization of excitations along the π-stacked
columns and the feasibility of donor–acceptor excitations across
the imine bonds. The frameworks developed in this study can serve
as a blueprint for the design of a broad range of tailor-made 2D COFs
with extended π-conjugated building blocks for applications
in photocatalysis and optoelectronics