10 research outputs found
CaCO<sub>3</sub> Precipitation and Polymorph Forms During CO<sub>2</sub> Sequestration from the Atmosphere: Effects of the Basic Buffer Components
CO<sub>2</sub> sequestration and
polymorph selection was achieved
by CaCO<sub>3</sub> precipitation via the reaction of calcium ions
and atmospheric CO<sub>2</sub> in a basic buffer, in a process that
mimicked geological sedimentation. Precipitation proceeded in yield
exceeding 80% in the presence of basic buffers at room temperature
over 10 h. Calcite formed mainly during the early stages of precipitation,
within less than 5 h, followed by needle-like aragonite precipitation
between 5 and 10 h of aging. The aragonite polymorph selection increased
in the presence of carbonic anhydrase and at high solution temperatures.
We found that the deposited CaCO<sub>3</sub> polymorphs depended on
the rate of calcium ion consumption and precipitation as well as the
ionic strength of the basic buffer and the solution pH. We developed
a method for depositing high-purity aragonitic CaCO<sub>3</sub> crystals
in solutions with temperatures exceeding 60 °C in the presence
of basic buffer, using CO<sub>2</sub> from the atmosphere without
the need for seed crystals or metal ions
Predicting the Morphology of Perovskite Thin Films Produced by Sequential Deposition Method: A Crystal Growth Dynamics Study
We
performed a kinetic analysis of the sequential deposition method
(SDM) to investigate how to form perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) phases, and the effects of processing conditions
on the final perovskite morphology. The reaction was found to consist
of two periods with distinct kinetics. During the first period, perovskite
crystals nucleated on the lead iodide (PbI<sub>2</sub>) surface, and
the reaction proceeded until the surface was completely converted
to perovskites. The reaction during this period determined the surface
morphology of the perovskites. We were able to extract the value of
the rate of the phase transformation during the first period by applying
the Johnson–Mehl–Avrami–Kolmogorov model, in
which the rate <i>r</i> is related to the average grain
size <i>R</i> by <i>R</i> ∝ <i>r</i><sup>–1/3</sup>. In this way, <i>r</i> was used
to predict the surface morphology of the perovskite under certain
processing conditions. During the second period, the remaining lead
iodide under the top perovskite layer was converted. Methylammonium
iodide (CH<sub>3</sub>NH<sub>3</sub>I, MAI) molecules apparently diffused
into the buried PbI<sub>2</sub> through intergrain gaps of the top
perovskite layers. Added MAI molecules reacted with PbI<sub>2</sub> but also generated single-crystal perovskite nanorods, nanoplates,
and nanocubes. The current study has furthered the understanding of
detailed features of the SDM, enabled a reliable prediction of the
final perovskite morphology resulting from specified processing conditions,
and contributed to a reproducible fabrication of high-quality perovskite
films
Nonfullerene/Fullerene Acceptor Blend with a Tunable Energy State for High-Performance Ternary Organic Solar Cells
Ternary
blending is an effective strategy for broadening the absorption
range of the active layer in bulk heterojunction polymer solar cells
and for constructing an efficient cascade energy landscape at the
donor/acceptor interface to achieve high efficiencies. In this study,
we report efficient ternary blend solar cells containing an acceptor
alloy consisting of the indacenodithiophene-based nonfullerene material,
IDT2BR, and the fullerene material, phenyl-C<sub>71</sub>-butyric
acid methyl ester (PC<sub>71</sub>BM). The IDT2BR materials mix fully
with PC<sub>71</sub>BM materials, and the energy state of this phase
can be tuned by varying the blending ratio. We performed photoluminescence
and external quantum efficiency studies and found that the ternary
charge cascade structure efficiently transfers the photogenerated
charges from the polymer to IDT2BR and finally to PC<sub>71</sub>BM
materials. Ternary blend devices containing the IDT2BR:PC<sub>71</sub>BM acceptor blend and various types of donor polymers were found
to exhibit power conversion efficiencies (PCEs) improved by more than
10% over the PCEs of the binary blend devices
Decoupling Charge Transfer and Transport at Polymeric Hole Transport Layer in Perovskite Solar Cells
Tailoring charge
extraction interfaces in perovskite solar cells (PeSCs) critically
determines the photovoltaic performance of PeSCs. Here, we investigated
the decoupling of two major determinants of the efficient charge extraction,
the charge transport and interfacial charge transfer properties at
hole transport layers (HTLs). A simple physical tuning of a representative
polymeric HTL, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate),
provided a wide range of charge conductivities from 10<sup>–4</sup> to 10<sup>3</sup> S cm<sup>–1</sup> without significant modulations
in their energy levels, thereby enabling the decoupling of charge
transport and transfer properties at HTLs. The transient photovoltaic
response measurement revealed that the facilitation of hole transport
through the highly conductive HTL promoted the elongation of charge
carrier lifetimes within the PeSCs up to 3 times, leading to enhanced
photocurrent extraction and finally 25% higher power conversion efficiency
Enhancing the Durability and Carrier Selectivity of Perovskite Solar Cells Using a Blend Interlayer
A mechanically
and thermally stable and electron-selective ZnO/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> interface is created via hybridization of
a polar insulating polymer, poly(ethylene glycol) (PEG), into ZnO
nanoparticles (NPs). PEG successfully passivates the oxygen defects
on ZnO and prevents direct contact between CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and defects on ZnO. A uniform CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> film is formed on a soft ZnO:PEG layer after
dispersion of the residual stress from the volume expansion during
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> conversion. PEG also increases
the work of adhesion of the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> film on the ZnO:PEG layer and holds the CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> film with hydrogen bonding. Furthermore, PEG tailors
the interfacial electronic structure of ZnO, reducing the electron
affinity of ZnO. As a result, a selective electron-collection cathode
is formed with a reduced electron affinity and a deep-lying valence
band of ZnO, which significantly enhances the carrier lifetime (473
μs) and photovoltaic performance (15.5%). The mechanically and
electrically durable ZnO:PEG/CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> interface maintains the sustainable performance of the solar cells
over 1 year. A soft and durable cathodic interface via PEG hybridization
in a ZnO layer is an effective strategy toward flexible electronics
and commercialization of the perovskite solar cells
Improved Charge Transport and Reduced Non-Geminate Recombination in Organic Solar Cells by Adding Size-Selected Graphene Oxide Nanosheets
Size-selected
graphene oxide (GO) nanosheets were used to modify
the bulk heterojunction (BHJ) morphology and electrical properties
of organic photovoltaic (OPV) devices. The GO nanosheets were prepared
with sizes ranging from several hundreds of nanometers to micrometers
by using a physical sonication process and were then incorporated
into PTB7:PC<sub>71</sub>BM photoactive layers. Different GO sizes
provide varied portions of the basal plane where aromatic sp<sup>2</sup>-hybridized regions are dominant and edges where oxygenated functional
groups are located; thus, GO size distributions affect the GO dispersion
stability and morphological aggregation of the BHJ layer. Electron
delocalization by sp<sup>2</sup>-hybridization and the electron-withdrawing
characteristics of functional groups p-dope the photoactive layer,
giving rise to increasing carrier mobilities. Hole and electron mobilities
are maximized at GO sizes of several hundreds of nanometers. Consequently,
non-geminate recombination is significantly reduced by these facilitated
hole and electron transports. The addition of GO nanosheets decreases
the recombination order of non-geminate recombination and increases
the generated carrier density. This reduction in the non-geminate
recombination contributes to an increased power conversion efficiency
of PTB7:PC<sub>71</sub>BM OPV devices as high as 9.21%, particularly,
by increasing the fill factor to 70.5% in normal devices and 69.4%
in inverted devices
Energy Level Engineering of Donor Polymers via Inductive and Resonance Effects for Polymer Solar Cells: Effects of Cyano and Alkoxy Substituents
Fine tuning the energy levels of
donor polymers is a critically
important step toward achieving high power conversion efficiencies
in polymer solar cells (PSCs). We systematically controlled the energy
levels of donor polymers by introducing cyano (CN) and alkoxy (OR)
groups into the 4,4′-didodecyl-2,2′-bithiophene (BT)
unit in a step-by-step fashion, thereby varying the inductive and
resonance effects. The three monomer units (BT, BTC, and BTCox) were
polymerized with benzo[1,2-b:4,5-<i>b</i>′]dithiophene
(BDT) as a counter unit to afford three polymers (PBDT-BT, PBDT-BTC,
and PBDT-BTCox). The highest occupied molecular orbital and lowest
unoccupied molecular orbital energy levels decreased significantly
upon the introduction of CN groups, and these levels increased slightly
upon attachment of the OR groups, in good agreement with the measured
open-circuit voltages of the three polymer devices. The strong inductive
and resonance effects present in PBDT-BTCox narrowed the polymer band
gap to 1.74 eV to afford a power conversion efficiency of 5.06%, the
highest value achieved among the three polymers
Atomically Thin Epitaxial Template for Organic Crystal Growth Using Graphene with Controlled Surface Wettability
A two-dimensional
epitaxial growth template for organic semiconductors was developed
using a new method for transferring clean graphene sheets onto a substrate
with controlled surface wettability. The introduction of a sacrificial
graphene layer between a patterned polymeric supporting layer and
a monolayer graphene sheet enabled the crack-free and residue-free
transfer of free-standing monolayer graphene onto arbitrary substrates.
The clean graphene template clearly induced the quasi-epitaxial growth
of crystalline organic semiconductors with lying-down molecular orientation
while maintaining the “wetting transparency”, which
allowed the transmission of the interaction between organic molecules
and the underlying substrate. Consequently, the growth mode and corresponding
morphology of the organic semiconductors on graphene templates exhibited
distinctive dependence on the substrate hydrophobicity with clear
transition from lateral to vertical growth mode on hydrophilic substrates,
which originated from the high surface energy of the exposed crystallographic
planes of the organic semiconductors on graphene. The optical properties
of the pentacene layer, especially the diffusion of the exciton, also
showed a strong dependency on the corresponding morphological evolution.
Furthermore, the effect of pentacene–substrate interaction
was systematically investigated by gradually increasing the number
of graphene layers. These results suggested that the combination of
a clean graphene surface and a suitable underlying substrate could
serve as an atomically thin growth template to engineer the interaction
between organic molecules and aromatic graphene network, thereby paving
the way for effectively and conveniently tuning the semiconductor
layer morphologies in devices prepared using graphene
Medium-Bandgap Conjugated Polymers Containing Fused Dithienobenzochalcogenadiazoles: Chalcogen Atom Effects on Organic Photovoltaics
We
designed, synthesized, and characterized a series of three medium-bandgap
conjugated polymers (PBDTfDTBO, PBDTfDTBT, and PBDTfDTBS)
consisting of fused dithienobenzochalcogenadiazole (fDTBX)-based
weak electron-deficient and planar building blocks, which possess
bandgaps of ∼2.01 eV. The fDTBX-based medium-bandgap polymers
exhibit deep-lying HOMO levels (∼5.51 eV), which is beneficial
for use in multijunction polymer solar cell applications. The resulting
polymers with chalcogen atomic substitutions revealed that the difference
in the electron negativity and atomic size of heavy atoms highly affects
an intrinsic property, morphological feature, and photovoltaic property
in polymer solar cells. The polymer solar cells based on sulfur-substituted
medium-bandgap polymer showed power conversion efficiencies above
6% when blended with [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl
ester in a typical bulk-heterojunction single cell. These results
suggest that the fDTBX-based medium-bandgap polymer is a promising
alternative material for P3HT in tandem polymer solar cells for achieving
high efficiency
Singlet Exciton Delocalization in Gold Nanoparticle-Tethered Poly(3-hexylthiophene) Nanofibers with Enhanced Intrachain Ordering
We
fabricated hybrid poly(3-hexylthiophene) nanofibers (P3HT NFs)
with rigid backbone organization through the self-assembly of P3HT
tethered to gold NPs (P3HT-Au NPs) in an azeotropic mixture of tetrahydrofuran
and chloroform. We found that the rigidity of the P3HT chains derives
from the tethering of the P3HT chains to the Au NPs and the control
of the solubility of P3HT in the solvent. This unique nanostructure
of hybrid P3HT NFs self-assembled in an azeotropic mixture exhibits
significantly increased delocalization of singlet (S<sub>1</sub>)
excitons compared to those of pristine and hybrid P3HT NFs self-assembled
in a poor solvent for P3HT. This strategy for the self-assembly of
P3HT-Au NPs that generate long-lived S<sub>1</sub> excitons can also
be applied to other crystalline conjugated polymers and NPs in various
solvents and thus enables improvements in the efficiency of optoelectronic
devices