10 research outputs found
Dry-Stamping-Transferred PC<sub>71</sub>BM Charge Transport Layer via an Interface-Controlled Polyurethane Acrylate Mold Film for Efficient Planar-Type Perovskite Solar Cells
The study of interlayers
is important to enhance the performance
of inverted perovskite solar cells (PSCs) because interlayers in PSCs
align energy levels and improve charge transport. However, previous
research into applying interlayers for PSCs has focused only on wet-coated
methods, such as spin coating, to form the interlayer. Here, we fabricated
planar-type PSCs deposited with a 6,6-phenyl-C<sub>71</sub> butyric
acid methyl ester (PC<sub>71</sub>BM) layer onto a CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> (MAPbI<sub>3</sub>) layer by stamping transfer
through a relatively dry process condition. We demonstrated the effects
of a stamping-transferred PC<sub>71</sub>BM layer using polyurethane
acrylate (PUA), the surface energy of which was modified by 2-hydroxyethyl
methacrylate (HEMA) to increase the transfer reproducibility. In PSCs
with a stamping-transferred PC<sub>71</sub>BM layer, we observed an
enhanced <i>J</i><sub>SC</sub> and a comparable power conversion
efficiency
(PCE), which were caused by an enhanced coverage of the electron transport
layer onto the MAPbI<sub>3</sub> layer with preserved crystallinity,
which occurs owing to improved electron mobility and exciton dissociation.
The optimized device PCE through the dry-transferred PC<sub>71</sub>BM exhibited a <i>J</i><sub>SC</sub>, fill factor, and
PCE of 21.65 mA/cm<sup>2</sup>, 76.0%, and 15.46%, respectively. Moreover,
morphological analysis and electrical measurements confirmed the improved
durability of dry-stamping-transferred PSCs
Structure–Property Correlation: A Comparison of Charge Carrier Kinetics and Recombination Dynamics in All-Polymer Solar Cells
We
report a comparison of charge carrier kinetics and recombination
dynamics correlated with the device performances of <b>PBDTTT-C</b> and <b>PBDTTT-CT</b> in nonfullerene <b>PÂ(NDI2OD-T2)</b> solar cells. The nanoscale morphological characteristics are found
to be remarkably different for these two polymers in all-polymer bulk
heterojunction (BHJ) blends. Important insights into the carrier dynamics
and crystalline packing features of these different donor materials
are provided in detail. The higher device performance of the <b>PBDTTT-CT:PÂ(NDI2OD-T2)</b> device [2.78% compared to 1.56% for
the <b>PBDTTT-C:PÂ(NDI2OD-T2)</b> cell] is shown to result from
efficient charge generation and faster carrier separation, reducing
nongeminate recombination during charge collection, which is attributed
to the perfect intermixing between the donor and acceptor polymer
networks
Efficient Hole Extraction from Sb<sub>2</sub>S<sub>3</sub> Heterojunction Solar Cells by the Solid Transfer of Preformed PEDOT:PSS Film
Here,
we report significant improvements of <i>V</i><sub>oc</sub> and FF in Sb<sub>2</sub>S<sub>3</sub> quantum dot (QD)-based,
solid-state heterojunction solar cells prepared from the solid transfer
of preformed PEDOT:PSS hole extraction layers. Despite the moderate
optical properties of Sb<sub>2</sub>S<sub>3</sub> QDs, the solid state
QD solar cells suffer from poor power conversion efficiency (PCE)
resulting from the disappointing <i>V</i><sub>oc</sub> and
the high series resistance since there is inefficient charge extraction
from QDs to the metal top electrode. In order to improve the hole
extraction performance, a significantly uniform PEDOT:PSS (polyÂ(3,4-ethylenedioxythiophene)/polyÂ(styrenesulfonate))
layer was transferred on the hole transport layer (P3HT, polyÂ(3-hexylthiophene-2,5-diyl))
by using a simple solid-transfer method. In contrast with conventional
spin-cast methods, the hydrophilic PEDOT:PSS layer was uniformly coated
on the hydrophobic P3HT layer without any significant detriment to
P3HT film properties. Due to improved contact surface for the Au top
electrode and hole conductance resulting in significantly improved
charge extraction, the power conversion efficiency was dramatically
enhanced. Furthermore, the thickness of the PEDOT:PSS film was precisely
optimized by layer-by-layer solid transfer, and thereby the PCE of
the PEDOT:PSS solid-transfer device (30 nm) was improved by 25.7%
in comparison to the PEDOT:PSS spin-cast device and by 76% in comparison
to the PEDOT:PSS free device
Stamping Transfer of a Quantum Dot Interlayer for Organic Photovoltaic Cells
An organophilic cadmium selenide (CdSe) quantum dot (QD)
interlayer
was prepared on the active layer in organic solar cells by a stamping
transfer method. The mother substrate composed of a UV-cured film
on a polycarbonate film with strong solvent resistance makes it possible
to spin-coat QDs on it and dry transfer onto an active layer without
damaging the active layer. The QD interlayers have been optimized
by controlling the concentration of the QD solution. The coverage
of QD particles on the active layer was verified by TEM analysis and
fluorescence images. After insertion of the QD interlayer between
the active layer and metal cathode, the photovoltaic performances
of the organic solar cell were clearly enhanced. By ultraviolet photoelectron
spectroscopy of CdSe QDs, it can be anticipated that the CdSe QD interlayer
reduces charge recombination by blocking the holes moving to the cathode
from the active layer and facilitating efficient collection of the
electrons from the active layer to the cathode
Conflicted Effects of a Solvent Additive on PTB7:PC<sub>71</sub>BM Bulk Heterojunction Solar Cells
Recently, polymer–fullerene
based bulk heterojunction (BHJ)
solar cells, which contain blends of polyÂ({4,8-bisÂ[(2-ethylhexyl)Âoxy]ÂbenzoÂ[1,2-b:4,5-<i>b</i>′]Âdithiophene-2,6-diyl}Â{3-fluoro-2-[(2-ethylhexyl)Âcarbonyl]ÂthienoÂ[3,4-<i>b</i>]Âthiophenediyl}) (PTB7) and [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl ester (PC<sub>71</sub>BM), have been widely
studied due to exhibiting high power conversion efficiency (PCE) and
well-defined nanomorphology. Because of the short exciton diffusion
pathway (less than 10 nm) in organic thin films, the optimization
of PTB7:PC<sub>71</sub>BM BHJ with optimized morphology is very important
between the donor and acceptor. In order to increase nanoscale phase
separation, the chemical additives of 1,8-diiodooctane (DIO) have
been used in PTB7:PC<sub>71</sub>BM blend systems. However, the mechanism
studies of DIO in BHJ solar cells and its effectiveness on device
stability are unclear. In this study, we fabricated polymer solar
cells (PSCs) based on PTB7:PC<sub>71</sub>BM BHJ with various DIO
concentrations to investigate not only correlation between device
performances and different morphologies, but also the influence of
additives on device stabilities. Positive effects of DIO, which were
induced by efficient charge separation in BHJ at optimized blending
ratio, are proved by the results of time-resolved photoluminescence
(TRPL), and negative effects of DIO on a device stability have been
investigated according to the ISOS-D-1 protocol
Intensity Dependence of Current–Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells
Solution-processed small-molecule p-DTS(FBTTh<sub>2</sub>)<sub>2</sub>:PC<sub>71</sub>BM bulk heterojunction (BHJ) solar cells with power conversion efficiency of 8.01% are demonstrated. The fill factor (FF) is sensitive to the thickness of a calcium layer between the BHJ layer and the Al cathode; for 20 nm Ca thickness, the FF is 73%, the highest value reported for an organic solar cell. The maximum external quantum efficiency exceeds 80%. After correcting for the total absorption in the cell through normal incidence reflectance measurements, the internal quantum efficiency approaches 100% in the spectral range of 600–650 nm and well over 80% across the entire spectral range from 400 to 700 nm. Analysis of the current–voltage (<i>J–V</i>) characteristics at various light intensities provides information on the different recombination mechanisms in the BHJ solar cells with different thicknesses of the Ca layer. Our analysis reveals that the <i>J–V</i> curves are dominated by first-order recombination from the short-circuit condition to the maximum power point and evolve to bimolecular recombination in the range of voltage from the maximum power point to the open-circuit condition in the optimized device with a Ca thickness of 20 nm. In addition, the normalized photocurrent density curves reveal that the charge collection probability remains high; about 90% of charges are collected even at the maximum power point. The dominance of bimolecular recombination only when approaching open circuit, the lack of Shockley–Read–Hall recombination at open circuit, and the high charge collection probability (97.6% at the short circuit and constant over wide range of applied voltage) lead to the high fill factor
Discrepancy of Optimum Ratio in Bulk Heterojunction Photovoltaic Devices: Initial Cell Efficiency vs Long-Term Stability
Organic
photovoltaic devices are difficult to commercialize because
of their vulnerability to chemical degradation related with oxygen
and water and to physical degradation with aging at high temperatures.
We investigated the photophysical degradation behaviors of a series
of polyÂ(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl
ester (PC<sub>60</sub>BM) bulk heterojunctions (BHJs) as a model system
according to the donor–acceptor ratio. We found that the optimum
P3HT:PC<sub>60</sub>BM ratio in terms of long-term stability differs
from that in terms of initial cell efficiency. On the basis of cell
performance decays and time-resolved photoluminescence measurements,
we investigated the effects of oxygen and material self-aggregation
on the stability of an organic photovoltaic device. We also observed
the changes in morphological geometry and analyzed the surface elements
to verify the mechanisms of degradation
Improved Light Harvesting and Improved Efficiency by Insertion of an Optical Spacer (ZnO) in Solution-Processed Small-Molecule Solar Cells
We
demonstrate that the power conversion efficiency can be significantly
improved in solution-processed small-molecule solar cells by tuning
the thickness of the active layer and inserting an optical spacer
(ZnO) between the active layer and the Al electrode. The enhancement
in light absorption in the cell was measured with UV–vis absorption
spectroscopy and by measurements of the photoinduced carriers generation
rate. The ZnO layer used to improve the light-harvesting increases
the charge collection efficiency, serves as a blocking layer for holes,
and reduces the recombination rate. The combined optical and electrical
improvements raise the power conversion efficiency of solution-processed
small-molecule solar cells to 8.9%, that is, comparable to that of
polymer counterparts
Balancing Light Absorptivity and Carrier Conductivity of Graphene Quantum Dots for High-Efficiency Bulk Heterojunction Solar Cells
Graphene quantum dots (GQDs) have been considered as a novel material because their electronic and optoelectronic properties can be tuned by controlling the size and the functional groups of GQDs. Here we report the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency (PCE). Three different types of GQDsî—¸graphene oxide quantum dots (GOQDs), 5 h reduced GQDs, and 10 h reduced GQDsî—¸were tested in BHJ solar cells, and the results indicate that GQDs play an important role in increasing optical absorptivity and charge carrier extraction of the BHJ solar cells. The enhanced optical absorptivity by rich functional groups in GOQDs increases short-circuit current, while the improved conductivity of reduced GQDs leads to the increase of fill factors. Thus, the reduction level of GQDs needs to be intermediate to balance the absorptivity and conductivity. Indeed, the partially reduced GQDs yielded the outstandingly improved PCE of 7.60% in BHJ devices compared to a reference device without GQDs (6.70%)
Self-Position of Au NPs in Perovskite Solar Cells: Optical and Electrical Contribution
Metallic
nanoparticles (NPs) exhibit a localized surface plasmon resonance
(LSPR) and act as scattering centers and subwavelength antennas, so
metallic NPs can be incorporated into perovskite solar cells (PSCs)
to effectively improve the light absorption of light harvesting devices.
Here, we have embedded Au nanoparticles (NPs) into the hole transport
layer (HTL) of the PSCs to investigate the photovoltaic effect of
the PSCs with Au NPs. Interestingly, it was found that Au NPs dispersed
spiro-OMeTAD HTL solution could naturally end up located near the
perovskite layer as the result of the spin-coating step. Solar cell
performance observations indicate that the LSPR and electrical effects
of Au NPs enhance the photovoltaic response of PSCs, in spite of a
slight decrease in the open-circuit voltage (<i>V</i><sub>OC</sub>), by causing an incredible improvement in the photocurrent
density as a dominant factor