17 research outputs found
Highly Transparent Low Resistance ZnO/Ag Nanowire/ZnO Composite Electrode for Thin Film Solar Cells
We present an indium-free transparent conducting composite electrode composed of silver nanowires (AgNWs) and ZnO bilayers. The AgNWs form a random percolating network embedded between the ZnO layers. The unique structural features of our ZnO/AgNW/ZnO multilayered composite allow for a novel transparent conducting electrode with unprecedented excellent thermal stability (∼375 °C), adhesiveness, and flexibility as well as high electrical conductivity (∼8.0 Ω/sq) and good optical transparency (>91% at 550 nm). Cu(In,Ga)(S,Se)<sub>2</sub> (CIGSSe) thin film solar cells incorporating this composite electrode exhibited a 20% increase of the power conversion efficiency compared to a conventional sputtered indium tin oxide-based CIGSSe solar cell. The ZnO/AgNW/ZnO composite structure enables effective light transmission and current collection as well as a reduced leakage current, all of which lead to better cell performance
Enhanced Photocurrent of Transparent CuFeO<sub>2</sub> Photocathodes by Self-Light-Harvesting Architecture
Efficient
sunlight-driven water-splitting devices can be achieved by using an
optically and energetically well-matched pair of photoelectrodes in
a tandem configuration. The key for maximizing the photoelectrochemical
efficiency is the use of a highly transparent front photoelectrode
with a band gap below 2.0 eV. Herein, we propose two-dimensional (2D)
photonic crystal (PC) structures consisting of a CuFeO<sub>2</sub>-decorated microsphere monolayer, which serve as self-light-harvesting
architectures allowing for amplified light absorption and high transparency.
The photocurrent densities are evaluated for three CuFeO<sub>2</sub> 2D PC-based photoelectrodes with microspheres of different sizes.
The optical analysis confirmed the presence of a photonic stop band
that generates <i>slow light</i> and at the same time amplifies
the absorption of light. The 410 nm sized CuFeO<sub>2</sub>-decorated
microsphere 2D PC photocathode shows an exceptionally high visible
light transmittance of 76.4% and a relatively high photocurrent of
0.2 mA cm<sup>–2</sup> at 0.6 V vs a reversible hydrogen electrode.
The effect of the microsphere size on the carrier collection efficiency
was analyzed by in situ conductive atomic force microscopy observation
under illumination. Our novel synthetic method to produce self-light-harvesting
nanostructures provides a promising approach for the effective use
of solar energy by highly transparent photocathodes
Salami-like Electrospun Si Nanoparticle-ITO Composite Nanofibers with Internal Conductive Pathways for use as Anodes for Li-Ion Batteries
We report novel salami-like core–sheath
composites consisting
of Si nanoparticle assemblies coated with indium tin oxide (ITO) sheath
layers that are synthesized via coelectrospinning. Core–sheath
structured Si nanoparticles (NPs) in static ITO allow robust microstructures
to accommodate for mechanical stress induced by the repeated cyclical
volume changes of Si NPs. Conductive ITO sheaths can provide bulk
conduction paths for electrons. Distinct Si NP-based core structures,
in which the ITO phase coexists uniformly with electrochemically active
Si NPs, are capable of facilitating rapid charge transfer as well.
These engineered composites enabled the production of high-performance
anodes with an excellent capacity retention of 95.5% (677 and 1523
mAh g<sup>–1,</sup> which are based on the total weight of
Si-ITO fibers and Si NPs only, respectively), and an outstanding rate
capability with a retention of 75.3% from 1 to 12 C. The cycling performance
and rate capability of core–sheath-structured Si NP-ITO are
characterized in terms of charge-transfer kinetics
Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu<sub>2</sub>ZnSnS<sub>4</sub> Photocathodes for Photoelectrochemical Water Splitting
To realize economically competitive
hydrogen production through
photoelectrochemical (PEC) water splitting, it is essential to develop
an efficient photoelectrode consisting of earth-abundant constituents
in conjunction with low-cost solution processing. Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) has received significant attention as a promising
photocathode owing to its abundance and good absorption properties.
However, the efficiency of the solution-processed CZTS photocathode
is not yet comparable to its counterparts. Here, a hybrid ink, obtained
by careful control of precursor mixing order, was used to produce
a highly efficient CZTS photocathode. The molecular chemistry-controlled
hybrid ink formulation, particularly the roles of thiourea–Sn<sup>2+</sup> complexation, was elucidated by liquid Raman spectroscopy.
The hybrid ink-derived CZTS thin films modified with conformal coating
of an n-type TiO<sub>2</sub>/CdS double layer and a Pt electrocatalyst
achieved an exceptionally high photocurrent of 13 mA cm<sup>–2</sup> at −0.2 V versus a reversible hydrogen electrode
under 1 sun illumination. The modified photocathodes showed relatively
stable H<sub>2</sub> production with faradaic efficiency close to
unity
Bandgap-Graded Cu<sub>2</sub>Zn(Sn<sub>1–<i>x</i></sub>Ge<sub><i>x</i></sub>)S<sub>4</sub> Thin-Film Solar Cells Derived from Metal Chalcogenide Complex Ligand Capped Nanocrystals
We demonstrate organic residue free,
bandgap-graded Cu<sub>2</sub>ZnÂ(Sn<sub>1–<i>x</i></sub>Ge<sub><i>x</i></sub>)ÂS<sub>4</sub> (CZTGeS) thin-film
solar cells based on metal
chalcogenide complex (MCC) ligand capped nanocrystals (NCs). The bandgap
of the CZTGeS films is tuned by varying the amount of Sn<sub>2</sub>S<sub>6</sub><sup>4–</sup> MCC ligand absorbed on the surface
of the Cu<sub>2</sub>ZnGeS<sub>4</sub> (CZGeS) NCs, without an undesirable
postselenization process. Using CZGeS NCs inks with three different
Sn/(Ge+Sn) ratios, bandgap-graded CZTGeS thin films are obtained via
multicoating and annealing procedures. Compositional and spectroscopic
analyses along the film thickness confirm that the band-graded CZTGeS
absorber layer, with a gradually increasing bandgap from the back
contact to the <i>p</i>–<i>n</i> junction,
is successfully accomplished. Compared with an ungraded band structured
CZTGeS cell, this normal grading structure facilitates both higher
short circuit current and open-circuit voltage, facilitating a power
conversion efficiency of 6.3%
Retarding Crystallization during Facile Single Coating of NaCl-Incorporated Precursor Solution for Efficient Large-Area Uniform Perovskite Solar Cells
We demonstrated crystallization
retardation of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> thin film
during single coating of precursor solution by simple addition of
NaCl. NaCl was codissolved into a precursor mixture solution containing
PbI<sub>2</sub> and methylammonium iodide (MAI). Dissolved NaCl interacted
with the PbI<sub>2</sub> in solution and produced a stable intermediate
phase, which was converted to a full-coverage uniform perovskite absorber
layer via reaction with MAI during a single spin-coating. The resulting
planar-structure perovskite solar cell made from NaCl-supplemented
precursor solution showed a 48% improvement in power conversion efficiency
(PCE) (maximum value 15.16%) over the device fabricated without the
additive. Our NaCl-supplemented single coating represents an easy
approach to effectively obtain highly reproducible uniform performance
at an overall position in 5 cm × 5 cm sized cells (divided into
20 subcells with an active area of 0.06 cm<sup>2</sup>) with average
PCEs of 12.00 ± 0.48%
Facile Sol–Gel-Derived Craterlike Dual-Functioning TiO<sub>2</sub> Electron Transport Layer for High-Efficiency Perovskite Solar Cells
Organic–inorganic
hybrid perovskite solar cells (PSCs) are
considered promising materials for low-cost solar energy harvesting
technology. An electron transport layer (ETL), which facilitates the
extraction of photogenerated electrons and their transport to the
electrodes, is a key component in planar PSCs. In this study, a new
strategy to concurrently manipulate the electrical and optical properties
of ETLs to improve the performance of PSCs is demonstrated. A careful
control over the Ti alkoxide-based sol–gel chemistry leads
to a craterlike porous/blocking bilayer TiO<sub>2</sub> ETL with relatively
uniform surface pores of 220 nm diameter. Additionally, the phase
separation promoter added to the precursor solution enables nitrogen
doping in the TiO<sub>2</sub> lattice, thus generating oxygen vacancies.
The craterlike surface morphology allows for better light transmission
because of reduced reflection, and the electrically conductive craterlike
bilayer ETL enhances charge extraction and transport. Through these
synergetic improvements in both optical and electrical properties,
the power conversion efficiency of craterlike bilayer TiO<sub>2</sub> ETL-based PSCs could be increased from 13.7 to 16.0% as compared
to conventional dense TiO<sub>2</sub>-based PSCs
Chemically Driven Enhancement of Oxygen Reduction Electrocatalysis in Supported Perovskite Oxides
Perovskite oxides have the capacity
to efficiently catalyze the
oxygen reduction reaction (ORR), which is of fundamental importance
for electrochemical energy conversion. While the perovskite catalysts
have been generally utilized with a support, the role of the supports,
regarded as inert toward the ORR, has been emphasized mostly in terms
of the thermal stability of the catalyst system and as an ancillary
transport channel for oxygen ions during the ORR. We demonstrate a
novel approach to improving the catalytic activity of perovskite oxides
for solid oxide fuel cells by controlling the oxygen-ion conducting
oxide supports. Catalytic activities of (La<sub>0.8</sub>Sr<sub>0.2</sub>)<sub>0.95</sub>MnO<sub>3</sub> perovskite thin-film placed on different
oxide supports are characterized by electrochemical impedance spectroscopy
and X-ray absorption spectroscopy. These analyses confirm that the
strong atomic orbital interactions between the support and the perovskite
catalyst enhance the surface exchange kinetics by ∼2.4 times,
in turn, improving the overall ORR activity
Role of Anions in Aqueous Sol–Gel Process Enabling Flexible Cu(In,Ga)S<sub>2</sub> Thin-Film Solar Cells
Recently, environmental-friendly,
solution-processed, flexible
CuÂ(In,Ga)Â(S,Se)<sub>2</sub> devices have gained significant interest,
primarily because the solution deposition method enables large-scale
and low-cost production of photovoltaics, and a flexible substrate
can be implemented on uneven surfaces in various applications. Here,
we suggest a novel green-chemistry aqueous ink that is readily achievable
through the incorporation of molecular precursors in an aqueous medium.
A copper formate precursor was introduced to lower the fabrication
temperature, provide compatibility with a polyimide plastic substrate,
and allow for high photovoltaic performance. Through a comparative
spectroscopic study on temperature-dependent chemical/crystal structural
evolution, the chemical role of copper formate was elucidated, which
led to the chalcopyrite framework that was appropriate to low-temperature
annealed CuÂ(In,Ga)ÂS<sub>2</sub> absorber layers at 400 °C. This
CuÂ(In,Ga)ÂS<sub>2</sub> solar cell exhibited a power conversion efficiency
of 7.04% on a rigid substrate and 5.60% on a polymeric substrate.
Our cell on the polymeric substrate also demonstrated both acceptable
mechanical flexibility and durability throughout a repeated bending
test of 200 cycles
Investigating Recombination and Charge Carrier Dynamics in a One-Dimensional Nanopillared Perovskite Absorber
Organometal
halide perovskite materials have become an exciting
research topic as manifested by intense development of thin film solar
cells. Although high-performance solar-cell-based planar and mesoscopic
configurations have been reported, one-dimensional (1-D) nanostructured
perovskite solar cells are rarely investigated despite their expected
promising optoelectrical properties, such as enhanced charge transport/extraction.
Herein, we have analyzed the 1-D nanostructure effects of organometal
halide perovskite (CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub>) on recombination
and charge carrier dynamics by utilizing a nanoporous anodized alumina
oxide scaffold to fabricate a vertically aligned 1-D nanopillared
array with controllable diameters. It was observed that the 1-D perovskite
exhibits faster charge transport/extraction characteristics, lower
defect density, and lower bulk resistance than the planar counterpart.
As the aspect ratio increases in the 1-D structures, in addition,
the charge transport/extraction rate is enhanced and the resistance
further decreases. However, when the aspect ratio reaches 6.67 (diameter
∼30 nm), the recombination rate is aggravated due to high interface-to-volume
ratio-induced defect generation. To obtain the full benefits of 1-D
perovskite nanostructuring, our study provides a design rule to choose
the appropriate aspect ratio of 1-D perovskite structures for improved
photovoltaic and other optoelectrical applications