7 research outputs found
Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Nanocrystals
Photoinduced
degradation of individual methylammonium lead triiodide
(MAPbI<sub>3</sub>) perovskite nanocrystals was studied using super-resolution
luminescence microspectroscopy under intense light excitation. The
photoluminescence (PL) intensity decrease and blue-shift of the PL
spectrum up to 60 nm together with spatial shifts in the emission
localization position up to a few hundred nanometers were visualized
in real time. PL blinking was found to temporarily suspend the degradation
process, indicating that the degradation needs a high concentration
of mobile photogenerated charges to occur. We propose that the mechanistic
process of degradation occurs as the three-dimensional MAPbI<sub>3</sub> crystal structure smoothly collapses to the two-dimensional layered
PbI<sub>2</sub> structure. The degradation starts locally and then
spreads over the whole crystal. The structural collapse is primarily
due to migration of methylammonium ions (MA<sup>+</sup>), which distorts
the lattice structure causing alterations to the Pb–I–Pb
bond angle and in turn changes the effective band gap
Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Nanocrystals
Photoinduced
degradation of individual methylammonium lead triiodide
(MAPbI<sub>3</sub>) perovskite nanocrystals was studied using super-resolution
luminescence microspectroscopy under intense light excitation. The
photoluminescence (PL) intensity decrease and blue-shift of the PL
spectrum up to 60 nm together with spatial shifts in the emission
localization position up to a few hundred nanometers were visualized
in real time. PL blinking was found to temporarily suspend the degradation
process, indicating that the degradation needs a high concentration
of mobile photogenerated charges to occur. We propose that the mechanistic
process of degradation occurs as the three-dimensional MAPbI<sub>3</sub> crystal structure smoothly collapses to the two-dimensional layered
PbI<sub>2</sub> structure. The degradation starts locally and then
spreads over the whole crystal. The structural collapse is primarily
due to migration of methylammonium ions (MA<sup>+</sup>), which distorts
the lattice structure causing alterations to the Pb–I–Pb
bond angle and in turn changes the effective band gap
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
Multiscale Active Layer Morphologies for Organic Photovoltaics Through Self-Assembly of Nanospheres
We address here the need for a general
strategy to control molecular
assembly over multiple length scales. Efficient organic photovoltaics
require an active layer comprised of a mesoscale interconnected networks
of nanoscale aggregates of semiconductors. We demonstrate a method,
using principles of molecular self-assembly and geometric packing,
for controlled assembly of semiconductors at the nanoscale and mesoscale.
Nanoparticles of polyÂ(3-hexylthiophene) (P3HT) or [6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) were fabricated with targeted
sizes. Nanoparticles containing a blend of both P3HT and PCBM were
also fabricated. The active layer morphology was tuned by the changing
particle composition, particle radii, and the ratios of P3HT:PCBM
particles. Photovoltaic devices were fabricated from these aqueous
nanoparticle dispersions with comparable device performance to typical
bulk-heterojunction devices. Our strategy opens a revolutionary pathway
to study and tune the active layer morphology systematically while
exercising control of the component assembly at multiple length scales
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