14 research outputs found
Preventing Hysteresis in Perovskite Solar Cells by Undoped Charge Blocking Layers
Preventing hysteresis
in lead halide perovskite solar cells remains one of the key challenges
hindering their integration into industrial applications. Herein,
we numerically study a model solar cell system that is based on a
mixed electronâion conducting perovskite active layer and vary
the configuration of undoped charge-blocking layers within the device.
We find that the use of undoped blocking layers significantly reduces
the potential drop across the perovskite active layer. This redistribution
of voltage across the device suppresses the ion accumulation, or deficiency,
which would otherwise develop at the two ends of the active layer.
The fill factor is not compromised, provided that the blocking layersâ
mobility value is not lower than 0.01 cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup>, which results in devices with power-conversion
efficiencies surpassing 20% and minimal hysteresis. We believe that
this method not only can suppress hysteresis effectively but could
also contribute to the long-term stability of such cells that have
been shown to be adversely affected by ion migration
Large-Scale Compositional and Electronic Inhomogeneities in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskites and Their Effect on Device Performance
Photovoltaic
devices based on lead halide perovskite materials are under extensive
investigation due to their remarkably high efficiencies. However,
it has become evident that the photovoltaic performance of these devices
shows wide distributions, even across individual perovskite samples,
resulting in the practice of presenting performance histograms, rather
than single values. We demonstrate that these variations in performance
are related to the large-scale compositional and electronic inhomogeneities
of perovskite films, which we characterize using X-ray and ultraviolet
photoemission spectroscopy mapping. These inhomogeneities are observed
for three different fabrication methods for the perovskite layers,
and while they are unaltered by storage in nitrogen and dry air, long-term
exposure to vacuum increases the homogeneity of the surface structure.
We demonstrate that perovskite films with a nonuniform surface structure
result in broadly varying photovoltaic performance despite seemingly
similar bulk properties such as absorption and microstructure
Improved Performance of ZnO/Polymer Hybrid Photovoltaic Devices by Combining Metal Oxide Doping and Interfacial Modification
Photoinduced
charge separation at hybrid organicâinorganic
interfaces is poorly understood and challenging to control. We investigate
charge separation at a model system of ZnO/polyÂ(3-hexylthiophene)
(P3HT) and employ Sr doping of ZnO and phenyl-C61-butyric acid (PCBA)
self-assembled modification to study and enhance the charge separation
efficiency. We find that doping alone lowers the efficiency of charge
separation due to the introduction of defect states at the oxide surface.
However, with the combination of doping and molecular modification,
charge separation efficiency is significantly enhanced due to the
passivation of interfacial traps and improved modifier coverage. This
demonstrates a complex noncumulative effect of doping and surface
modification and shows that with the correct choice of metal oxide
dopant and organic modifier, a poorly performing hybrid interface
can be turned into an efficient one
Triptycene-Based Porous Metal-Assisted Salphen Organic Frameworks: Influence of the Metal Ions on Formation and Gas Sorption
Porous
organic polymers (POPs) are chemically and thermally robust
materials and have been often investigated for their gas sorption
properties. From the related field of metalâorganic frameworks
(MOFs) it is known that open ligation sites at metal centers can enhance
the performance of gas sorption significantly, especially the selectivity
toward one gas of a binary mixture, such as CO<sub>2</sub>/N<sub>2</sub> or CO<sub>2</sub>/CH<sub>4</sub>. POPs that contain metal centers
are rarer. One possibility to introduce metals into POPs is by the
synthesis of metal-assisted salphen organic frameworks (MaSOFs), where
the framework development is associated with the formation of the
metalâsalphen pockets. Based on a hexakissalicylaldehyde, a
variety of three-dimensional isostructural porous MaSOFs with different
metal ions (Zn<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, Pd<sup>2+</sup>, and Pt<sup>2+</sup>) are introduced. All compounds show
a very similar pore structure and comparable specific surface areas,
which make these MaSOFs ideal candidates to study the influence of
the nature of the incorporated metal center on gas sorption selectivity.
Due to the environmental importance, the adsorption of CO<sub>2</sub> in comparison to N<sub>2</sub> and CH<sub>4</sub> was extensively
studied. Depending on the metal ions, the heat of adsorption was different
as well as the Henry and IAST selectivities. CuâMaSOF<sub>100</sub> for instance shows a high <i>Q</i><sub>st</sub> of 31.2
kJ mol<sup>â1</sup> for CO<sub>2</sub> and an uptake of 14.9
wt % at 1 bar and 273 K. The IAST selectivity of CO<sub>2</sub>/N<sub>2</sub> for an 80/20 mixture is with <i>S</i><sub>IAST</sub> = 52 very high for a metal containing POP and even comparable to
some of the best performing MOFs. The MaSOFs are stable even in boiling
water. This, as well as the simple synthesis, makes them potential
good candidates for CO<sub>2</sub> removal of binary mixtures
Triptycene-Based Porous Metal-Assisted Salphen Organic Frameworks: Influence of the Metal Ions on Formation and Gas Sorption
Porous
organic polymers (POPs) are chemically and thermally robust
materials and have been often investigated for their gas sorption
properties. From the related field of metalâorganic frameworks
(MOFs) it is known that open ligation sites at metal centers can enhance
the performance of gas sorption significantly, especially the selectivity
toward one gas of a binary mixture, such as CO<sub>2</sub>/N<sub>2</sub> or CO<sub>2</sub>/CH<sub>4</sub>. POPs that contain metal centers
are rarer. One possibility to introduce metals into POPs is by the
synthesis of metal-assisted salphen organic frameworks (MaSOFs), where
the framework development is associated with the formation of the
metalâsalphen pockets. Based on a hexakissalicylaldehyde, a
variety of three-dimensional isostructural porous MaSOFs with different
metal ions (Zn<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, Pd<sup>2+</sup>, and Pt<sup>2+</sup>) are introduced. All compounds show
a very similar pore structure and comparable specific surface areas,
which make these MaSOFs ideal candidates to study the influence of
the nature of the incorporated metal center on gas sorption selectivity.
Due to the environmental importance, the adsorption of CO<sub>2</sub> in comparison to N<sub>2</sub> and CH<sub>4</sub> was extensively
studied. Depending on the metal ions, the heat of adsorption was different
as well as the Henry and IAST selectivities. CuâMaSOF<sub>100</sub> for instance shows a high <i>Q</i><sub>st</sub> of 31.2
kJ mol<sup>â1</sup> for CO<sub>2</sub> and an uptake of 14.9
wt % at 1 bar and 273 K. The IAST selectivity of CO<sub>2</sub>/N<sub>2</sub> for an 80/20 mixture is with <i>S</i><sub>IAST</sub> = 52 very high for a metal containing POP and even comparable to
some of the best performing MOFs. The MaSOFs are stable even in boiling
water. This, as well as the simple synthesis, makes them potential
good candidates for CO<sub>2</sub> removal of binary mixtures
Charge Dynamics in Solution-Processed Nanocrystalline CuInS<sub>2</sub> Solar Cells
We investigate charge dynamics in solar cells constructed using solution-processed layers of CuInS<sub>2</sub> (CIS) nanocrystals (NCs) as the electron donor and CdS as the electron acceptor. By using time-resolved spectroscopic techniques, we are able to observe photoinduced absorptions that we attribute to the mobile hole carriers in the NC film. In combination with transient photocurrent and photovoltage measurements, we monitor charge dynamics on time scales from 300 fs to 1 ms. Carrier dynamics are investigated for devices with CIS layers composed of either colloidally synthesized 1,3-benzenedithiol-capped nanocrystals or <i>in situ</i> solâgel synthesized thin films as the active layer. We find that deep trapping of holes in the colloidal NC cells is responsible for decreases in the open-circuit voltage and fill factor as compared to those of the solâgel synthesized CIS/CdS cell
Synthesis and Modeling of Uniform Complex Metal Oxides by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition
A close-proximity atmospheric pressure
chemical vapor deposition
(AP-CVD) reactor is developed for synthesizing high quality multicomponent
metal oxides for electronics. This combines the advantages of a mechanically
controllable substrate-manifold spacing and vertical gas flows. As
a result, our AP-CVD reactor can rapidly grow uniform crystalline
films on a variety of substrate types at low temperatures without
requiring plasma enhancements or low pressures. To demonstrate this,
we take the zinc magnesium oxide (Zn<sub>1â<i>x</i></sub>Mg<sub><i>x</i></sub>O) system as an example. By
introducing the precursor gases vertically and uniformly to the substrate
across the gas manifold, we show that films can be produced with only
3% variation in thickness over a 375 mm<sup>2</sup> deposition area.
These thicknesses are significantly more uniform than for films from
previous AP-CVD reactors. Our films are also compact, pinhole-free,
and have a thickness that is linearly controllable by the number of
oscillations of the substrate beneath the gas manifold. Using photoluminescence
and X-ray diffraction measurements, we show that for Mg contents below
46 at. %, single phase Zn<sub>1â<i>x</i></sub>Mg<sub><i>x</i></sub>O was produced. To further optimize the growth
conditions, we developed a model relating the composition of a ternary
oxide with the bubbling rates through the metal precursors. We fitted
this model to the X-ray photoelectron spectroscopy measured compositions
with an error of Î<i>x</i> = 0.0005. This model showed
that the incorporation of Mg into ZnO can be maximized by using the
maximum bubbling rate through the Mg precursor for each bubbling rate
ratio. When applied to polyÂ(3-hexylthiophene-2,5-diyl) hybrid solar
cells, our films yielded an open-circuit voltage increase of over
100% by controlling the Mg content. Such films were deposited in short
times (under 2 min over 4 cm<sup>2</sup>)
Ultrafast Charge- and Energy-Transfer Dynamics in Conjugated Polymer: Cadmium Selenide Nanocrystal Blends
Hybrid nanocrystalâpolymer systems are promising candidates for photovoltaic applications, but the processes controlling charge generation are poorly understood. Here, we disentangle the energy- and charge-transfer processes occurring in a model system based on blends of cadmium selenide nanocrystals (CdSe-NC) with poly[2-methoxy-5-(3â˛,7â˛-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) using a combination of time-resolved absorption and luminescence measurements. The use of different capping ligands (<i>n</i>-butylamine, oleic acid) as well as thermal annealing allows tuning of the polymerânanocrystal interaction. We demonstrate that energy transfer from MDMO-PPV to CdSe-NCs is the dominant exciton quenching mechanism in nonannealed blends and occurs on ultrafast time scales (<1 ps). Upon thermal annealing electron transfer becomes competitive with energy transfer, with a transfer rate of 800 fs independent of the choice of the ligand. Interestingly, we find hole transfer to be much less efficient than electron transfer and to extend over several nanoseconds. Our results emphasize the importance of tuning the organicânanocrystal interaction to achieve efficient charge separation and highlight the unfavorable hole-transfer dynamics in these blends
Effect of Ozone on the Stability of Solution-Processed Anthradithiophene-Based Organic Field-Effect Transistors
We
have investigated the degradation effects of ozone exposure
on organic field-effect transistors based on 2,8-difluoro-5,11-bisÂ(triethylsilylethynyl)Âanthradithiophene
as the organic semiconducting channel layer, as well as on thin films
of this widely used, high-mobility, small molecule semiconductor.
Electrical <i>I</i>â<i>V</i> measurements
showed a loss of transistor characteristic behavior. We present <sup>1</sup>H Nuclear Magnetic Resonance (NMR) spectroscopy results as
well as X-ray Photoemission Spectroscopy (XPS) and Fourier Transform
Infrared (FTIR) spectroscopy measurements showing the oxidation of
the parent molecule, from which we suggest various possible reaction
paths
Structure Influence on Charge Transport in NaphthalenediimideâThiophene Copolymers
Reported here is a characterization
of a series of NDIâthiophene
copolymers with one, two, three, and four thiophene units synthesized
using Stille polycondensation of dibromo-naphthalene diimide and the
trimethylstannylthiophene monomers. The effect of extension of the
thiophene donor group is studied in terms of structure-charge transport
correlation. The influence of side chains located on the thiophene
units of copolymers with two and four thiophene units per monomer
is also investigated. Charge transport of both signs is studied experimentally
in field-effect transistors. Microstructural data obtained by near-edge
X-ray absorption fine structure (NEXAFS) and grazing incidence wide-angle
X-ray scattering (GIWAXS) is supported by AFM topography scans. Ultraviolet
photoelectron spectroscopy (UPS) and UVâvis spectroscopy data
are employed in the measurement of energy levels, and changes with
annealing temperature are also discussed. Most of the polymers reach
excellent electron and hole mobility with one copolymer (NDI-T4) exhibiting
an especially balanced ambipolar charge transport of 0.03 cm<sup>2</sup> V<sup>â1</sup> s<sup>â1</sup>. An oddâeven
effect in hole mobility is observed with higher values for polymers
with an even number of thiophene units. The reported findings indicate
that the final charge transport properties are a result of the interplay
of many factors, including crystallinity, planarity and linearity
of chain, spacing between acceptor units and packing of solubilizing branched side
chains