25 research outputs found
Direct Comparison of Electron Transport and Recombination Behaviors of Dye-Sensitized Solar Cells Prepared Using Different Sintering Processes
Flexible dye-sensitized solar cells
on plastic substrates have
achieved a conversion efficiency of 8.6% with the hot compression
technique (<150 °C). However, the value of efficiency is only
70% of that achieved using glass substrates with high-temperature
sintering technique (500 °C). Investigating the origin of this
difference is a critical step for further improving the performance
of plastic dye-sensitized solar cells. In this study, an optimized
ternary viscous titania paste without the addition of organic binders
enables the fabrication of efficient dye-sensitized solar cells with
a low-temperature process. Therefore, the electron-transport behavior
of dye-sensitized solar cells can be directly compared with those
prepared with the high-temperature sintering technique. In addition
to the structural and optical differences, the hot compressed photoanode
of dye-sensitized solar cells have an electron diffusion coefficient
that is 2 times smaller and a recombination time that is 6 times shorter
than those of the high-temperature sintered cells, suggesting inadequate
interparticle connections and more recombination events. These results
indicate that electron transport and recombination are still the key
factors governing the performance of low-temperature fabricated dye-sensitized
solar cells. Eventually, the flexible cell with an efficiency of 6.81%
has been achieved on flexible indium tin oxide/polyethylene naphthalate
substrate. Further improvements in advanced low-temperature processing
or novel materials with minimized defect or grain boundaries are required
Development of Highly Crystalline DonorâAcceptor-Type Random Polymers for High Performance Large-Area Organic Solar Cells
We developed donorâacceptor
(DâA)-type random polymers
based on 3,3â˛-difluoro-2,2â˛-bithiophene with various
relative amounts of 5,6-difluoro-4,7-bisÂ(5-bromo-(2-decylÂtetradecyl)Âthiophen-2-yl)-2,1,3-benzothiadiazole
(2FBT) and 5,6-difluoro-4,7-bisÂ(5-bromo-(2-octyldodecyl)Âthiophen-2-yl)-2-(3,4-dichloroÂbenzyloxybutyl)-2<i>H</i>-benzoÂ[<i>d</i>]Â[1,2,3]Âtriazole (DCB-2FBTZ).
Introducing small relative amounts of DCB-2FBTZ into the polymer was
found to effectively enhance its solar cell performance, resulting
in a power conversion efficiency of 9.02%, greater than the 7.29%
that resulted from the PFBT-FTh copolymer. Moreover, when the active
area of the BHJ film was increased to 1 cm<sup>2</sup>, the solar
cell reproducibly showed a high performance, here with an efficiency
of 8.01% even when the thickness of the active layer was 313 nm. Our
studies revealed that including the DCB-2FBTZ group in the polymer
simultaneously improved the solution processability and crystallinity
of the polymer. These improvements resulted in the formation of highly
homogeneous BHJ films throughout large areas with only minor amounts
of defects resulting from overaggregation and hence with appropriate
morphologies for effective charge generation and transport
Enhancing Stability of Perovskite Solar Cells to Moisture by the Facile Hydrophobic Passivation
In this study, a novel and facile
passivation process for a perovskite solar cell is reported. Poor
stability in ambient atmosphere, which is the most critical demerit
of a perovskite solar cell, is overcome by a simple passivation process
using a hydrophobic polymer layer. Teflon, the hydrophobic polymer,
is deposited on the top of a perovskite solar cell by a spin-coating
method. With the hydrophobic passivation, the perovskite solar cell
shows negligible degradation after a 30 day storage in ambient atmosphere.
Suppressed degradation of the perovskite film is proved in various
ways: X-ray diffraction, light absorption spectrum, and quartz crystal
microbalance. This simple but effective passivation process suggests
new kind of approach to enhance stability of perovskite solar cells
to moisture
Unbiased Sunlight-Driven Artificial Photosynthesis of Carbon Monoxide from CO<sub>2</sub> Using a ZnTe-Based Photocathode and a Perovskite Solar Cell in Tandem
Solar
fuel production, mimicking natural photosynthesis of converting
CO<sub>2</sub> into useful fuels and storing solar energy as chemical
energy, has received great attention in recent years. Practical large-scale
fuel production needs a unique device capable of CO<sub>2</sub> reduction
using only solar energy and water as an electron source. Here we report
such a system composed of a gold-decorated triple-layered ZnO@ZnTe@CdTe
coreâshell nanorod array photocathode and a CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> perovskite solar cell in tandem. The assembly
allows effective light harvesting of higher energy photons (>2.14
eV) from the front-side photocathode and lower energy photons (>1.5
eV) from the back-side-positioned perovskite solar cell in a single-photon
excitation. This system represents an example of a photocathodeâphotovoltaic
tandem device operating under sunlight without external bias for selective
CO<sub>2</sub> conversion. It exhibited a steady solar-to-CO conversion
efficiency over 0.35% and a solar-to-fuel conversion efficiency exceeding
0.43% including H<sub>2</sub> as a minor product
Enhancing the Efficiency of Electron Conduction in Spray-Coated Anode of Photoelectrochemical Cell Using Oxygenated Multi-Walled Carbon Nanotubes
A multiwalled carbon nanotube (MWNT)
was physically cured with
oxygen plasma treatment, and the as-prepared oxygenated MWNT (OMWNT)
was incorporated into TiO<sub>2</sub> nanopowders to prepare a spray-coatable
OMWNTâTiO<sub>2</sub> composite suspension. The composite layer
was directly formed on a fluorinated tin oxide surface by spray coating
and served as a photoanode of a photoelectrochemical cell (PEC). The
cell performance was optimized in terms of the plasma treatment time
and compared with a conventional PEC, showing 37% increased energy
conversion efficiency. The efficiency improvement confirmed by the
electrochemical impedance spectra was related to the reduced charge-transfer
resistance and efficient electron transport through the OMWNT network
Synthesis and Charge Transport Properties of Conjugated Polymers Incorporating Difluorothiophene as a Building Block
A series of conjugated copolymers,
PDPPFT and PNDIFT, were developed
using difluoroterthiophene and DPP or NDI as the cobuilding block.
We obtained two different molecular weight polymers for each polymer
type by changing the conditions for the Stille coupling reaction and
studied their optoelectrochemical properties and charge-transport
behavior in organic field-effect transistors (OFETs). Both the lower
molecular weight polymers, PDPPFTÂ(L) and PNDIFTÂ(L), showed better
long-range ordered structures in films, whereas the polymers with
higher molecular weights were less long-range ordered and showed a
more preferential face-on orientation. By virtue of their favorable
polymer packing structures, PDPPFTÂ(L) and PNDIFTÂ(L) exhibited much
higher hole mobilities compared with their higher molecular weight
counterparts, PDPPFTÂ(H) and PNDIFTÂ(H). By contrast, both PDPPFT and
PNDIFT maintained good n-channel properties independent of their molecular
weights, thus their long-range ordering in a film. The strong electron-withdrawing
fluorine groups are favorable for stabilizing electrons on the polymer
chain and would enable the polymer to transport electrons efficiently
even in the case of a less-ordered packing structure with an unfavorable
face-on orientation
Highly Efficient Monolithic Dye-Sensitized Solar Cells
Monolithic
dye-sensitized solar cells (M-DSSCs) provide an effective way to reduce
the fabrication cost of general DSSCs since they do not require transparent
conducting oxide substrates for the counter electrode. However, conventional
monolithic devices have low efficiency because of the impediments
resulting from counter electrode materials and spacer layers. Here,
we demonstrate highly efficient M-DSSCs featuring a highly conductive
polymer combined with macroporous polymer spacer layers. With M-DSSCs
based on a PEDOT/polymer spacer layer, a power conversion efficiency
of 7.73% was achieved, which is, to the best of our knowledge, the
highest efficiency for M-DSSCs to date. Further, PEDOT/polymer spacer
layers were applied to flexible DSSCs and their cell performance was
investigated
Hierarchical Nanoflake Surface Driven by Spontaneous Wrinkling of Polyelectrolyte/Metal Complexed Films
A mechanical or physical change observed in nanocomposite thin films has recently offered new opportunities to generate intriguing nanostructures. In this study, we present a novel means of creating a hierarchically developed nanoflake structure by exploiting surface wrinkles that occur during the incorporation process of metallic nanoparticles into layer-by-layer assembled polyelectrolyte multilayer (PEM) thin films. The PEM film composed with linear polyethylenimine (LPEI) and poly(acrylic acid) (PAA) allows for facilitated cationic exchange reaction within the film even after the electrostatic complexation and chemical cross-linking reaction. The subsequent reduction process induces an <i>in situ</i> complexation of metallic nanoparticles with a PEM matrix, causing an accumulation of lateral compressive stress for surface wrinkling. The wrinkling characteristics of the complexed films can be theoretically interpreted by employing the gradationally swollen film model, whereby a gradual change in the elastic property along the axial direction of the film can be appropriately reflected. In addition, wrinkled surfaces are further processed to form vertically aligned and hierarchically ordered nanoflakes after selective removal of the PEM matrix with plasma ashing. Consequently, superhydrophobic surface properties (water contact angle = 170°, sliding angle <1°) can be attained from the hierarchical nanoflake structure. The method presented here is advantageous in that large-scale preparation can be readily implemented by a stepwise dipping process without resorting to specific patterning or a serially applied complex structuring process, which can provide a promising platform technique for various surface engineering applications
Highly Efficient CopperâIndiumâSelenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers
Copperâindiumâselenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO<sub>2</sub> electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date
Solution-Processed Ultrathin TiO<sub>2</sub> Compact Layer Hybridized with Mesoporous TiO<sub>2</sub> for High-Performance Perovskite Solar Cells
The electron transport layer (ETL)
is a key component of perovskite
solar cells (PSCs) and must provide efficient electron extraction
and collection while minimizing the charge recombination at interfaces
in order to ensure high performance. Conventional bilayered TiO<sub>2</sub> ETLs fabricated by depositing compact TiO<sub>2</sub> (c-TiO<sub>2</sub>) and mesoporous TiO<sub>2</sub> (mp-TiO<sub>2</sub>) in sequence
exhibit resistive losses due to the contact resistance at the c-TiO<sub>2</sub>/mp-TiO<sub>2</sub> interface and the series resistance arising
from the intrinsically low conductivity of TiO<sub>2</sub>. Herein,
to minimize such resistive losses, we developed a novel ETL consisting
of an ultrathin c-TiO<sub>2</sub> layer hybridized with mp-TiO<sub>2</sub>, which is fabricated by performing one-step spin-coating
of a mp-TiO<sub>2</sub> solution containing a small amount of titanium
diisopropoxide bisÂ(acetylacetonate) (TAA). By using electron microscopies
and elemental mapping analysis, we establish that the optimal concentration
of TAA produces an ultrathin blocking layer with a thickness of âź3
nm and ensures that the mp-TiO<sub>2</sub> layer has a suitable porosity
for efficient perovskite infiltration. We compare PSCs based on mesoscopic
ETLs with and without compact layers to determine the role of the
hole-blocking layer in their performances. The hybrid ETLs exhibit
enhanced electron extraction and reduced charge recombination, resulting
in better photovoltaic performances and reduced hysteresis of PSCs
compared to those with conventional bilayered ETLs