17 research outputs found
Formation Mechanism and Control of Perovskite Films from Solution to Crystalline Phase Studied by in Situ Synchrotron Scattering
Controlling
the crystallization and morphology of perovskite films is crucial
for the fabrication of high-efficiency perovskite solar cells. For
the first time, we investigate the formation mechanism of the drop-cast
perovskite film from its precursor solution, PbCl<sub>2</sub> and
CH<sub>3</sub>NH<sub>3</sub>I in <i>N</i>,<i>N</i>-dimethylformamide, to a crystalline CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> film at different substrate temperatures from 70 to 180 °C
in ambient air and humidity. We employed an in situ grazing-incidence
wide-angle X-ray scattering (GIWAXS) technique for this study. When
the substrate temperature is at or below 100 °C, the perovskite
film is formed in three stages: the initial solution stage, transition-to-solid
film stage, and transformation stage from intermediates into a crystalline
perovskite film. In each stage, the multiple routes for phase transformations
are preceded concurrently. However, when the substrate temperature
is increased from 100 to 180 °C, the formation mechanism of the
perovskite film is changed from the “multistage formation mechanism”
to the “direct formation mechanism”. The proposed mechanism
has been applied to understand the formation of a perovskite film
containing an additive. The result of this study provides a fundamental
understanding of the functions of the solvent and additive in the
solution and transition states to the crystalline film. It provides
useful knowledge to design and fabricate crystalline perovskite films
for high-efficiency solar cells
Using an Airbrush Pen for Layer-by-Layer Growth of Continuous Perovskite Thin Films for Hybrid Solar Cells
In this manuscript we describe hybrid
heterojunction solar cells, having the device architecture glass/indium
tin oxide/poly(3,4-ethylenedioxythiopene)/poly(styrenesulfonic acid)/perovskite/[6,6]-phenyl-C<sub>61</sub>-butyric acid methyl ester/C<sub>60</sub>/2,9-dimethyl- 4,7-diphenyl-1,10-phenanthroline/Al,
fabricated using lead halide perovskite obtained through spray-coating
at a low precursor concentration. To study the relationship between
the morphology and device performance, we recorded scanning electron
microscopy images of perovskite films prepared at various precursor
ratios, spray volumes, substrate temperatures, and postspray annealing
temperatures. Optimization of the spray conditions ensured uniform
film growth and high surface area coverage at low substrate temperatures.
Lead halide perovskite solar cells prepared under the optimal conditions
displayed an average power conversion efficiency (PCE) of approximately
9.2%, with 85% of such devices having efficiencies of greater than
8.3%. The best-performing device exhibited a short-circuit current
density of 17.3 mA cm<sup>–2</sup>, a fill factor of 0.63,
and an open-circuit voltage of 0.93 V, resulting in a PCE of 10.2%.
Because spray-coating technology allows large-area deposition, we
also fabricated devices having areas of 60 and 342 mm<sup>2</sup>,
achieving PCEs with these devices of 6.88 and 4.66%, respectively
Insights into the Morphological Instability of Bulk Heterojunction PTB7-Th/PCBM Solar Cells upon High-Temperature Aging
The impact of the
morphological stability of the donor/acceptor mixture under thermal
stress on the photovoltaic properties of bulk heterojunction (BHJ)
solar cells based on the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-<i>b</i>;4,5-<i>b</i>′]-dithiophene-2,6-diyl-<i>alt</i>-(4-(2-ethylhexyl)-3-fluorothieno[3,4-<i>b</i>]-thiophene)-2-carboxylate-2,6-diyl]/phenyl-C<sub>61</sub>-butyric
acid methyl ester (PTB7-Th/PC<sub>61</sub>BM) blend is extensively
investigated. Both optical microscopy and transmission electron microscopy
micrographs show that long-term high-temperature aging stimulates
the formation of microscale clusters, the size of which, however,
is about 1 order of magnitude smaller than those observed in thermally
annealed poly(3-hexylthiophene)/PC<sub>61</sub>BM composite film.
The multilength-scale evolution of the morphology of PTB7-Th/PC<sub>61</sub>BM film from the scattering profiles of grazing incidence
small-angle and wide-angle X-ray scattering indicates the PC<sub>61</sub>BM molecules spatially confine the self-organization of polymer chains
into large domains during cast drying and upon thermal activation.
Moreover, some PC<sub>61</sub>BM molecules accumulate into ∼30–40
nm clusters, the number of which increases with heating time. Therefore,
the hole mobility in the active layer decays much more rapidly than
the electron mobility, leading to unbalanced charge transport and
degraded cell performance. Importantly, the three-component blend
that is formed by replacing a small amount of PC<sub>61</sub>BM in
the active layer with the bis-adduct of PC<sub>61</sub>BM (bis-PC<sub>61</sub>BM) exhibits robust morphology against thermal stress. Accordingly,
the PTB7-Th/PC<sub>61</sub>BM:bis-PC<sub>61</sub>BM (8 wt %) device
has an extremely stable power conversion efficiency
Neutron Scattering Methodology for Absolute Measurement of Room-Temperature Hydrogen Storage Capacity and Evidence for Spillover Effect in a Pt-Doped Activated Carbon
A neutron scattering methodology is proposed to simultaneously determine the total hydrogen adsorption, the excess hydrogen adsorption, and hydrogen gas confined in the porous sample. This method is capable of an absolute measurement of the hydrogen content without need for any calibration. It involves the least amount of corrections and is not likely to be affected by the instrumental factors compared to the traditional gravimetric and volumetric methods. We used this method to study the physisorption behavior at room-temperature (RT) of a Pt-doped activated carbon sample as a function of hydrogen pressure. This method will become a simple and important tool for solving various problems arising from the traditional measurements of RT hydrogen storage capacities. It can be combined with an in situ small-angle neutron scattering to study the hydrogen spillover effect in the kinetic adsorption process. Storage capacity and spatial distribution of the hydrogen adsorbed due to spillover are concurrently revealed
Structural Evolution of Crystalline Conjugated Polymer/Fullerene Domains from Solution to the Solid State in the Presence and Absence of an Additive
The power conversion efficiencies
of polymer/fullerene solar cells
are critically dependent on the nanometer-scale morphologies of their
active layers, which are typically processed from solution. Using
synchrotron wide- and small-angle X-ray scattering, we have elucidated
the intricate mechanism of the structural transitions from solutions
to solid films of the crystalline polymer poly[bis(dodecyl)thiophene-thieno[3,4-<i>c</i>]pyrrole-4,6-dione] (PBTTPD) and [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl ester (PC<sub>71</sub>BM), including the effect
of the solvent additive 1,6-diiodohexane (DIH). We found that the
local assembly of rigid-rod PBTTPD segments that formed in solution
instantly and then relaxed within several hundred seconds upon cooling
to room temperature from 90 °C could re-emerge and develop into
seeds for subsequent crystallization of the polymer in the solid films.
At room temperature (25 °C), the presence of DIH in chlorobenzene
slightly enhanced the formation of local assembly PBTTPD segments
in the supersaturated PBTTPD in PBTTPD/PC<sub>71</sub>BM blend solution.
Two cases of films were subsequently developed from these blend solutions
with drop-casted and spin-coated methods. For spin-coated thin films
(90 nm thick), which evolve quickly, polymer’s crystallinity
and the fullerene packing in the solid-state thin films were enhanced
in the case of involving DIH. Regarding the effect of DIH for processing
the drop-casted thick films (2.5 μm thick), which evolve slowly,
DIH has no observable effect on PBTTPD/PC<sub>71</sub>BM structure.
Our results provide some understanding of the mechanism behind the
structural development of polymer/fullerene blends upon their transitions
from solution to the solid state, as well as the key functions of
the additive
Revealing Ordered Polymer Packing during Freeze-Drying Fabrication of a Bulk Heterojunction Poly(3-hexylthiophene-2,5-diyl):[6,6]-Phenyl-C61-butyric Acid Methyl Ester Layer: In Situ Optical Spectroscopy, Molecular Dynamics Simulation, and X‑ray Diffraction
Formation of ordered
poly(3-hexylthiophene-2,5-diyl) (P3HT) molecular
stacking during the freeze-drying process is tracked with in situ
spectroscopy of Raman scattering, absorption, and photoluminescence.
Raman spectra of pristine P3HT dissolved in 1,2-dichlorobenzene show
that P3HT polymers undergo drastic ordered aggregation upon being
lower than 0 °C, at which the solubility of P3HT is reached,
as evidenced by the emergence of pronounced red-shifted, narrow Raman
peaks (1422 and 1435 cm<sup>–1</sup>) caused by intermolecular
coupling. The absorption and photoluminescence spectra bear similar
temperature dependence as the results of Raman. Aggregation of P3HT
is further confirmed by coarse-grained molecular dynamics simulation
showing the enhanced order parameters of distance and orientation
between P3HT chains upon cooling. The incorporation of [6,6]-phenyl-C61-butyric
acid methyl ester (PCBM) does not significantly alter the P3HT packing
configuration, as verified by nearly identical Raman features observed
in P3HT:PCBM mixing solution upon cooling. While optical spectroscopy
and MD simulation portrayed the short-range order of P3HT aggregates,
grazing-incident X-ray diffraction exposed the long-range order by
the pronounced diffraction spots corresponding to the lamellar stacking
of P3HT. This study demonstrates the ability of Raman spectroscopy
to reveal the short-range order of polymer packing, while the in situ
monitoring illustrates that the ability of freeze-drying to separate
molecular aggregation from solvent removal thus is advantageous for
photovoltaic device fabrication without resorting to trial and error
Distribution of Crystalline Polymer and Fullerene Clusters in Both Horizontal and Vertical Directions of High-Efficiency Bulk Heterojunction Solar Cells
In this study, we used (i) synchrotron
grazing-incidence small-/wide-angle X-ray scattering to elucidate
the crystallinity of the polymer PBTC<sub>12</sub>TPD and the sizes
of the clusters of the fullerenes PC<sub>61</sub>BM and ThC<sub>61</sub>BM and (ii) transmission electron microscopy/electron energy loss
spectroscopy to decipher both horizontal and vertical distributions
of fullerenes in PBTC<sub>12</sub>TPD/fullerene films processed with
chloroform, chlorobenzene and dichlorobezene. We found that the crystallinity
of the polymer and the sizes along with the distributions of the fullerene
clusters were critically dependent on the solubility of the polymer
in the processing solvent when the solubility of fullerenes is much
higher than that of the polymer in the solvent. In particular, with
chloroform (CF) as the processing solvent, the polymer and fullerene
units in the PBTC<sub>12</sub>TPD/ThC<sub>61</sub>BM layer not only
give rise to higher crystallinity and a more uniform and finer fullerene
cluster dispersion but also formed nanometer scale interpenetrating
network structures and presented a gradient in the distribution of
the fullerene clusters and polymer, with a higher polymer density
near the anode and a higher fullerene density near the cathode. As
a result of combined contributions from the enhanced polymer crystallinity,
finer and more uniform fullerene dispersion and gradient distributions,
both the short current density and the fill factor for the device
incorporating the CF-processed active layer increase substantially
over that of the device incorporating a dichlorobenzene-processed
active layer; the resulting power conversion efficiency of the device
incorporating the CF-processed active layer was enhanced by 46% relative
to that of the device incorporating a dichlorobenzene-processed active
layer
Effect of Catalyst Size on Hydrogen Storage Capacity of Pt-Impregnated Active Carbon via Spillover
There are two regimes that exhibit two distinctive behaviors of spillover. The present study used small-angle X-ray scattering (SAXS) to measure size distribution of Pt nanoparticles in the bulk Pt-impregnated active carbon sample. The peak position of the size distribution as determined by SAXS turns out to be at ∼1 nm, which is rarely discussed in this field. SAXS technique is complementary to the other characterization methods. The experimental clue coming from SAXS measurement and our hydrogen storage capacity study shows that the impregnated Pt nanoparticles of ∼1 nm in size can enhance the hydrogen spillover effect. It can significantly increase the room temperature hydrogen uptake compared to currently studied similar systems. The mass loading of catalyst is not a critical factor. Tuning the pore-confined Pt sizes (<2 nm) in combination with an optimum activation method should play an effective role in further enhancement via the spillover effect
Reaction Kinetics and Formation Mechanism of TiO<sub>2</sub> Nanorods in Solution: An Insight into Oriented Attachment
The reaction kinetics and formation
mechanism of oriented attachment
for shaped nanoparticles in solution are not well-understood. We present
the reaction kinetics and formation mechanism of organic-capped anatase
TiO<sub>2</sub> nanorods in solution as a case study for the oriented
attachment process using small-angle X-ray scattering (SAXS) and transmission
electronic microscopy. The SAXS analysis qualitatively and quantitatively
provides in-depth understanding of the mechanism, including the structural
evolution, interparticle interaction, and spatial orientation of nanoparticles
developed from nanodots to nanorods during the nucleation, isotropic,
and anisotropic growth steps. The present study demonstrates the growth
details of oriented attachment of nanoparticles in solution. An ordered
lamellar structure in the solution is constructed by the balance of
interaction forces among surface ligands, functional groups, and solvent
molecules serving as a natural template. The template allows the alignment
of spherical nanoparticles into ordered chain arrays and facilitates
simultaneous transformation from spherical to rod shape via proximity
attachment. The proposed model reveals an insight into the oriented
attachment mechanism. This multistep formation mechanism of TiO<sub>2</sub> nanorods in solution can provide the fundamental understanding
of how to tune the shape of nanoparticles and further control the
aggregation of spatial nanorod networks in solution
Hydrogen Spillover Effect of Pt-Doped Activated Carbon Studied by Inelastic Neutron Scattering
We employed the inelastic neutron scattering (INS) method to directly monitor the change of molecular hydrogen in the Pt-doped activated carbon (Pt/AC) samples and provide very conclusive evidence that significant hydrogen atoms can diffuse to the carbon surface at room temperature during the spillover process. The INS method is uniquely capable of revealing the state of the hydrogen (either the atomic or molecular form). The INS result shows a direct quantitative evaluation of the amount of hydrogen adsorbed on AC in an atomic form via spillover. Two Pt/AC samples with different spillover effects were studied herein. The spillover behavior related to dissociation, diffusion, and adsorption of hydrogen in the Pt/AC samples at the temperature cycling from 4 up to 300 K was investigated by this INS study. The present study proposes the concept of diffusion length and hydrogen-rich domain around a Pt cluster center in this system based on INS data