6 research outputs found
Apical Functionalization of Chiral Heterohelicenes
We describe a synthetic protocol to selectively functionalize
chiral
bridged triarylamines at the apical position using regioselective
copper-catalyzed amination reaction. This protocol allows the coupling
of diphenylamines with a sterically hindered but electronically activated
aryl–Br bond in the presence of a sterically unhindered but
electronically unactivated aryl–Br bond. The unactivated aryl–Br
bond was utilized further to synthesize a chiral heterohelicene homodimer
using Stille coupling
Solution-Processed Photovoltaics with a 3,6-Bis(diarylamino)fluoren-9-ylidene Malononitrile
3,6-BisÂ(<i>N,N</i>-dianisylamino)-fluoren-9-ylidene
malononitrile (FMBDAA36) was used as an electron donor material in
solution-processed organic photovoltaic devices with configuration
ITO/PEDOT:PSS/(1:3Â[w/w] FMBDAA36:PC<sub>71</sub>BM)/LiF/Al to give
power conversion efficiencies up to 4.1% with open circuit voltage <i>V</i><sub>OC</sub> = 0.89 V, short circuit current <i>J</i><sub>SC</sub> = 10.35 mA cm<sup>–2</sup>, and fill factor
FF = 44.8%. Conductive atomic force microscopy of the active layer
showed granular separation of regions exhibiting easy versus difficult
hole transport, consistent with bulk heterojunction type phase separation
of FMBDAA36 and PC<sub>71</sub>BM, respectively. Single-crystal X-ray
diffraction analysis showed pure FMBDAA36 to form columnar π-stacks
with a 3.3 Ã… intermolecular spacing
Tunable Percolation in Semiconducting Binary Polymer Nanoparticle Glasses
Binary polymer nanoparticle glasses
provide opportunities to realize
the facile assembly of disparate components, with control over nanoscale
and mesoscale domains, for the development of functional materials.
This work demonstrates that tunable electrical percolation can be
achieved through semiconducting/insulating polymer nanoparticle glasses
by varying the relative percentages of equal-sized nanoparticle constituents
of the binary assembly. Using time-of-flight charge carrier mobility
measurements and conducting atomic force microscopy, we show that
these systems exhibit power law scaling percolation behavior with
percolation thresholds of ∼24–30%. We develop a simple
resistor network model, which can reproduce the experimental data,
and can be used to predict percolation trends in binary polymer nanoparticle
glasses. Finally, we analyze the cluster statistics of simulated binary
nanoparticle glasses, and characterize them according to their predominant
local motifs as (<i>p</i><sub><i>i</i></sub>, <i>p</i><sub>1‑<i>i</i></sub>)-connected networks
that can be used as a supramolecular toolbox for rational material
design based on polymer nanoparticles
Interplay between Ion Transport, Applied Bias, and Degradation under Illumination in Hybrid Perovskite p‑i‑n Devices
We
studied ion transport in hybrid organic–inorganic perovskite
p-i-n devices as a function of applied bias under device operating
conditions. Using electrochemical impedance spectroscopy (EIS) and
equivalent circuit modeling, we elucidated various resistive and capacitive
elements in the device. We show that ion migration is predictably
influenced by a low applied forward bias, characterized by an increased
capacitance at the hole-transporting (HTM) and electron-transporting
material (ETM) interfaces, as well as in bulk. However, unlike observations
in n-i-p devices, we found that there is a capacitive discharge leading
to ion redistribution in the bulk at high forward biases. Furthermore,
we show that a chemical double-layer capacitance buildup as a result
of ion accumulation impacts the electronic properties of the device,
likely by inducing either charge pinning or charge screening, depending
on the direction of the ion-induced field. Lastly, we extrapolate
ion diffusion coefficients (∼10<sup>–7</sup> cm<sup>2</sup> s<sup>–1</sup>) and ionic conductivities (∼10<sup>–7</sup> S cm<sup>–1</sup>) from the Warburg mass (ion)
diffusion response and show that, as the device degrades, there is
an overall depletion of capacitive effects coupled with increased
ion mobility
High Efficiency Tandem Thin-Perovskite/Polymer Solar Cells with a Graded Recombination Layer
Perovskite-containing tandem solar
cells are attracting attention
for their potential to achieve high efficiencies. We demonstrate a
series connection of a ∼90 nm thick perovskite front subcell
and a ∼100 nm thick polymer:fullerene blend back subcell that
benefits from an efficient graded recombination layer containing a
zwitterionic fullerene, silver (Ag), and molybdenum trioxide (MoO<sub>3</sub>). This methodology eliminates the adverse effects of thermal
annealing or chemical treatment that occurs during perovskite fabrication
on polymer-based front subcells. The record tandem perovskite/polymer
solar cell efficiency of 16.0%, with low hysteresis, is 75% greater
than that of the corresponding ∼90 nm thick perovskite single-junction
device and 65% greater than that of the polymer single-junction device.
The high efficiency of this hybrid tandem device, achieved using only
a ∼90 nm thick perovskite layer, provides an opportunity to
substantially reduce the lead content in the device, while maintaining
the high performance derived from perovskites
Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability
Solar
cells fabricated using alkyl ammonium metal halides as light
absorbers have the right combination of high power conversion efficiency
and ease of fabrication to realize inexpensive but efficient thin
film solar cells. However, they degrade under prolonged exposure to
sunlight. Herein, we show that this degradation is quasi-reversible,
and that it can be greatly lessened by simple modifications of the
solar cell operating conditions. We studied perovskite devices using
electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-,
formamidinium (FA)-, and MA<sub><i>x</i></sub>FA<sub>1–<i>x</i></sub> lead triiodide as active layers. From variable temperature
EIS studies, we found that the diffusion coefficient using MA ions
was greater than when using FA ions. Structural studies using powder
X-ray diffraction (PXRD) show that for MAPbI<sub>3</sub> a structural
change and lattice expansion occurs at device operating temperatures.
On the basis of EIS and PXRD studies, we postulate that in MAPbI<sub>3</sub> the predominant mechanism of accelerated device degradation
under sunlight involves thermally activated fast ion transport coupled
with a lattice-expanding phase transition, both of which are facilitated
by absorption of the infrared component of the solar spectrum. Using
these findings, we show that the devices show greatly improved operation
lifetimes and stability under white-light emitting diodes, or under
a solar simulator with an infrared cutoff filter or with cooling