12 research outputs found
Efficient All-Printable Solid-State Dye-Sensitized Solar Cell Based on a Low-Resistivity Carbon Composite Counter Electrode and Highly Doped Hole Transport Material
Monolithic device architectures provide
a route to large-area mesoporous
solar cell manufacture through scalable solution-processed fabrication.
A limiting factor in device scale-up is availability of low-resistivity
printable counter electrode materials and reliable doped charge transport
materials. We report an efficient all-printable monolithic solid-state
dye-sensitized solar cell (ss-DSC) based on a high-conductivity porous
carbon counter electrode and a highly doped 2,2â˛,7,7â˛-tetrakisÂ(<i>N</i>,<i>N</i>-di-4-methoxyÂphenylÂamino)-9,9â˛-spiroÂbiÂfluorene
(spiro-OMeTAD) hole transport material (HTM). A review of current
state-of-the-art printable porous counter electrodes in DSC literature
was conducted and identified blends of graphite/carbon black as promising
composites for high-conductivity electrodes. Direct ex situ oxidation
of spiro-OMeTAD produced a stable HTM dopant, and its incorporation
with one of the lowest-resistivity graphite/carbon black composite
materials reported to date drastically decreases device series resistance,
particularly that of the porous insulating spacer. Doping improved
all performance parameters, and following optimization we demonstrate
scaled-up 1.21 cm<sup>2</sup> (1.01 cm<sup>2</sup> masked) devices
achieving a maximum efficiency of 3.34% (average, 3.05 Âą 0.23%)
Near-Infrared Absorbing Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> and Cu<sub>3</sub>SbS<sub>4</sub> Nanocrystals: Synthesis, Characterization, and Photoelectrochemistry
Herein, we present the novel synthesis
of tetrahedrite copper antimony
sulfide (CAS) nanocrystals (Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub>), which display strong absorptions in the visible and NIR. Through
ligand tuning, the size of the Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub> NCs may be increased from 6 to 18 nm. Phase purity is achieved
through optimizing the ligand chemistry and maximizing the reactivity
of the antimony precursor. We provide a detailed investigation of
the optical and photoelectrical properties of both tetrahedrite (Cu<sub>12</sub>Sb<sub>4</sub>S<sub>13</sub>) and famatinite (Cu<sub>3</sub>SbS<sub>4</sub>) NCs. These NCs were found to have very high absorption
coefficients reaching 10<sup>5</sup> cm<sup>â1</sup> and band
gaps of 1.7 and 1 eV for tetrahedrite and famatinite NCs, respectively.
Ultraviolet photoelectron spectroscopy was employed to determine the
band positions. In each case, the Fermi energies reside close to the
valence band, indicative of a p-type semiconductor. Annealing of tetrahedrite
CAS NC films in sulfur vapor at 350 °C was found to result in
pure famatinite NC films, opening the possibility to tune the crystal
structure within thin films of these NCs. Photoelectrochemistry of
hydrazine free unannealed films displays a strong p-type photoresponse,
with up to 0.1 mA/cm<sup>2</sup> measured under mild illumination.
Collectively these optical properties make CAS NCs an excellent new
candidate for both thin film and hybrid solar cells and as strong
NIR absorbers in general
Surface State Recombination and Passivation in Nanocrystalline TiO<sub>2</sub> Dye-Sensitized Solar Cells
The
relative role of surface state recombination in dye-sensitized solar
cells is not fully understood, yet reductions in the recombination
rate are frequently attributed to the passivation of surface states.
We have investigated reports of trap state passivation using an Al<sub>2</sub>O<sub>3</sub>-coated TiO<sub>2</sub> photoanode achieved through
atomic layer deposition (ALD). Electrochemical characterization, performed
through impedance measurements and intensity modulated photovoltage
spectroscopy (IMVS), data showed that the Al<sub>2</sub>O<sub>3</sub> deposition successfully blocked electron recombination and that
the chemical capacitance of the film was unchanged after the ALD treatment.
A theoretical model outlining the recombination kinetics was applied
to the experimental data to obtain charge transfer rates from conduction
band states, exponentially distributed traps, and monoenergetic traps.
The determined electron transfer rates showed that the deposited Al<sub>2</sub>O<sub>3</sub> coating did not selectively passivate trap states
at the nanoparticle surface but reduced recombination rates equally
from both conduction band states and surface states. These results
imply that the reduction in the recombination rates reported in coreâshell
structured photoanodes cannot be attributed to a modification of surface
traps, but rather to the weakened electronic coupling between electrons
in the film and the electrolyte species
Mimicry of Sputtered <i>i-</i>ZnO Thin Films Using Chemical Bath Deposition for Solution-Processed Solar Cells
Solution processing provides a versatile
and inexpensive means to prepare functional materials with specifically
designed properties. The current challenge is to mimic the structural,
optical, and/or chemical properties of thin films fabricated by vacuum-based
techniques using solution-based approaches. In this work we focus
on ZnO to show that thin films grown using a simple, aqueous-based,
chemical bath deposition (CBD) method can mimic the properties of
sputtered coatings, provided that the kinetic and thermodynamic reaction
parameters are carefully tuned. The role of these parameters toward
growing highly oriented and dense ZnO thin films is fully elucidated
through detailed microscopic and spectroscopic investigations. The
prepared samples exhibit bulk-like optical properties, are intrinsic
in their electronic characteristics, and possess negligible organic
contaminants, especially when compared to ZnO layers deposited by
solâgel or from nanocrystal inks. The efficacy of our CBD-grown
ZnO thin films is demonstrated through the effective replacement of
sputtered ZnO buffer layers within high efficiency solution processed
Cu<sub>2</sub>ZnSnS<sub>4<i>x</i></sub>Se<sub>4(1â<i>x</i>)</sub> solar cells
In Situ Formation of Reactive Sulfide Precursors in the One-Pot, Multigram Synthesis of Cu<sub>2</sub>ZnSnS<sub>4</sub> Nanocrystals
Herein we outline a general one-pot
method to produce large quantities
of compositionally tunable, kesterite Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) nanocrystals (NCs) through the decomposition of in situ generated
metal sulfide precursors. This method uses air stable precursors and
should be applicable to the synthesis of a range of metal sulfides.
We examine the formation of the ligands, precursors, and particles
in turn. Direct reaction of CS<sub>2</sub> with the aliphatic primary
amines and thiols that already constitute the reaction mixture is
used to produce ligands in situ. Through the use of <sup>1</sup>H
and <sup>13</sup>C nuclear magnetic resonance, Fourier transform infrared
spectroscopy, and optical absorption spectroscopy, we elucidate the
formation of the resulting oleyldithiocarbamate and dodecyltrithiocarbonate
ligands. The decomposition of their corresponding metal complexes
at temperatures of âź100 °C yields nuclei with a size of
1â2 nm, with further growth facilitated by the decomposition
of dodecanethiol. In this way the nucleation and growth stages of
the reaction are decoupled, allowing for the generation of NCs at
high concentrations. Using in situ X-ray diffraction, we monitor the
evolution of our reactions, thus enabling a real-time glimpse into
the formation of Cu<sub>2</sub>ZnSnS<sub>4</sub> NCs. For completeness,
the surface chemistry and the electronic structure of the resulting
CZTS NCs are studied
Cu<sub>2</sub>ZnGeS<sub>4</sub> Nanocrystals from Air-Stable Precursors for Sintered Thin Film Alloys
The synthesis of
an air and moisture stable germanium complex and
its use in the synthesis of ternary and quaternary copper containing
nanocrystals (NCs) is described. Through the use of <sup>1</sup>H-/<sup>13</sup>C nuclear magnetic resonance and Fourier transform infrared
spectroscopies, thermogravimetric analysis, and powder X-ray diffraction,
the speciation and chemistry of this precursor is elucidated. This
germanium source is employed in the gram scale, noninjection synthesis
of Cu<sub>2</sub>ZnGeS<sub>4</sub> (CZGeS) and Cu<sub>2</sub>GeS<sub>3</sub> (CGeS) NCs using a binary sulfide precursor approach. To
demonstrate the versatility of such NCs for fabricating thin films
suitable for high-efficiency optoelectronic devices, they are blended
with Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) NCs and selenized to form
homogeneously alloyed Cu<sub>2</sub>ZnSn<sub><i>x</i></sub>Ge<sub>1â<i>x</i></sub>S<sub><i>y</i></sub>Se<sub>4â<i>y</i></sub> (CZTGeSSe) thin films. The
structural, optical, and electronic properties of such thin films
are studied using X-ray diffraction, scanning electron microscopy,
UVâvisâNIR spectroscopy, and photoelectron spectroscopy
in air. These measurements demonstrate collectively that incorporating
Ge into micrometer-sized, tetragonal Cu<sub>2</sub>ZnSnS<sub><i>x</i></sub>Se<sub>4â<i>x</i></sub> (CZTSSe)
provides a facile manner in which the conduction band energy can be
readily tuned. The strategy developed herein provides a pathway to
controlled levels of Ge incorporation in a single step process, thus
avoiding the need for intra-alloyed Cu<sub>2</sub>ZnSn<sub><i>x</i></sub>Ge<sub>1â<i>x</i></sub>S<sub>4</sub> nanocrystals
Probing the Interaction between Individual Metal Nanocrystals and Two-Dimensional Metal Oxides via Electron Energy Loss Spectroscopy
Metal nanoparticles can photosensitize
two-dimensional metal oxides,
facilitating their electrical connection to devices and enhancing
their abilities in catalysis and sensing. In this study, we investigated
how individual silver nanoparticles interact with two-dimensional
tin oxide and antimony-doped indium oxide using electron energy loss
spectroscopy (EELS). The measurement of the spectral line width of
the longitudinal plasmon resonance of the nanoparticles in absence
and presence of 2D materials allowed us to quantify the contribution
of chemical interface damping to the line width. Our analysis reveals
that a stronger interaction (damping) occurs with 2D antimony-doped
indium oxide due to its highly homogeneous surface. The results of
this study offer new insight into the interaction between metal nanoparticles
and 2D materials
A New Direction in Dye-Sensitized Solar Cells Redox Mediator Development: In Situ Fine-Tuning of the Cobalt(II)/(III) Redox Potential through Lewis Base Interactions
Dye-sensitized solar cells (DSCs) are an attractive renewable
energy
technology currently under intense investigation. In recent years,
one area of major interest has been the exploration of alternatives
to the classical iodide/triiodide redox shuttle, with particular attention
focused on cobalt complexes with the general formula [CoÂ(L)<sub><i>n</i></sub>]<sup>2+/3+</sup>. We introduce a new approach to
designing redox mediators that involves the application of [CoÂ(PY5Me<sub>2</sub>)Â(MeCN)]<sup>2+/3+</sup> complexes, where PY5Me<sub>2</sub> is the pentadentate ligand, 2,6-bisÂ(1,1-bisÂ(2-pyridyl)Âethyl)Âpyridine.
It is shown, by X-ray crystallography, that the axial acetonitrile
(MeCN) ligand can be replaced by more strongly coordinating Lewis
bases (B) to give complexes with the general formula [CoÂ(PY5Me<sub>2</sub>)Â(B)]<sup>2+/3+</sup>, where B = 4-<i>tert-</i>butylpyridine
(tBP) or <i>N</i>-methylbenzimidazole (NMBI). These commonly
applied DSC electrolyte components are used for the first time to
fine-tune the potential of the redox couple to the requirements of
the dye through coordinative interactions with the Co<sup>II/III</sup> centers. Application of electrolytes based on the [CoÂ(PY5Me<sub>2</sub>)Â(NMBI)]<sup>2+/3+</sup> complex in combination with a commercially
available organic sensitizer has enabled us to attain DSC efficiencies
of 8.4% and 9.2% at a simulated light intensity of 100% sun (1000
W m<sup>â2</sup> AM1.5 G) and at 10% sun, respectively, higher
than analogous devices applying the [CoÂ(bpy)<sub>3</sub>]<sup>2+/3+</sup> redox couple, and an open circuit voltage (<i>V</i><sub>oc</sub>) of almost 1.0 V at 100% sun for devices constructed with
the tBP complex
Tunable Crystallization and Nucleation of Planar CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> through Solvent-Modified Interdiffusion
A smooth
and compact light absorption perovskite layer is a highly desirable
prerequisite for efficient planar perovskite solar cells. However,
the rapid reaction between CH<sub>3</sub>NH<sub>3</sub>I methylammonium
iodide (MAI) and PbI<sub>2</sub> often leads to an inconsistent CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> crystal nucleation and growth
rate along the film depth during the two-step sequential deposition
process. Herein, a facile solvent additive strategy is reported to
retard the crystallization kinetics of perovskite formation and accelerate
the MAI diffusion across the PbI<sub>2</sub> layer. It was found that
the ultrasmooth perovskite thin film with narrow crystallite size
variation can be achieved by introducing favorable solvent additives
into the MAI solution. The effects of dimethylformamide, dimethyl
sulfoxide, Îł-butyrolactone, chlorobenzene, and diethyl ether
additives on the morphological properties and cross-sectional crystallite
size distribution were investigated using atomic force microscopy,
X-ray diffraction, and scanning electron microscopy. Furthermore,
the light absorption and band structure of the as-prepared CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> films were investigated and
correlated with the photovoltaic performance of the equivalent solar
cell devices. Details of perovskite nucleation and crystal growth
processes are presented, which opens new avenues for the fabrication
of more efficient planar solar cell devices with these ultrasmooth
perovskite layers
Toward Rollable Printed Perovskite Solar Cells for Deployment in Low-Earth Orbit Space Applications
The thin physical
profile of perovskite-based solar cells (PSCs)
fabricated on flexible substrates provides the prospect of a disruptive
increase in specific power (power-to-mass ratio), an important figure-of-merit
for solar cells to be used in space applications. In contrast to recent
reports on space applications of PSCs which focus on rigid glass-based
devices, in this work we investigate the suitability of flexible PSCs
for low-earth orbit (LEO) applications, where the perovskite layer
in the PSCs was prepared using either a RuddlesdenâPopper precursor
composition (BA2MA3Pb4I13; BA = butylammonium, MA = methylammonium) or a mixed-cation precursor
composition (Cs0.05FA0.81MA0.14Pb2.55Br0.45; FA = formamidinium). The flexible PSC
devices display a tolerance to high-energy proton (14 MeV) and electron
(>1 MeV) radiation comparable with, or superior to, equivalent
glass-based
PSC devices. The photovoltaic performance of the PSCs is found to
be significantly less dependent on angle-of-incidence than GaAs-based
triple-junction solar cells commonly used for space applications.
Results from a preliminary test of the robustness of the perovskite
film when subjected to LEO-like thermal environments are also reported.
In addition, a unique deployment concept integrating printed flexible
solar cells with titaniumânickel based shape memory alloy ribbons
is presented for thermally actuated deployment of flexible solar cells
from a rolled state