36 research outputs found
Semitransparent Organic Light Emitting Diodes with Bidirectionally Controlled Emission
Semitransparent OLEDs
are a candidate for large-area eco-friendly light sources that can
be integrated into building facades, suggesting colorful windows that
become luminescent if the OLED is switched on. However, since the
light is emitted in two directions, smart light engineering has to
be implemented to direct the light into a preferred direction and
to prevent for instance huge energetic losses to the outside of a
building. We introduce an unprecedented device architecture, composed
of a dielectric mirror attached to a semitransparent OLED. Such a
system features a dual functionality that depends on the viewing direction:
changing the color perception and/or enhancing the light directionality
while still preserving a high overall device transparency. First,
we motivate the potential of this concept with a theoretical study,
showing that broad modifications in the color range can be realized
via device optimization and that the maximum possible emission enhancement
of the OLED is limited only by the transparency of the interfacial
layers and the electrodes. Then, experimental investigations with
a semitransparent yellow OLED (transparency = 58.2%) in combination
with six different dielectric mirrors validate the theoretical results.
Retaining the same color perception, up to 80% of the total emitted
light can be directed toward one side while the color is modified
at the other side of the device stack. Here, modifications from yellow
to purple to dark or light blue can be realized
Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells
The
efficiency of perovskite-based tandem solar cells and the respective
efficiency gain over the single-junction operation of the bottom cell
strongly depend on the performance of the component cells. Thus, a
fair comparison of reported top cells is difficult. We therefore compute
the tandem cell efficiency for the combination of several semitransparent
perovskite top solar cells and crystalline silicon or chalcopyrite
bottom cells from the literature. We focus on four-terminal configurations
but also estimate and discuss the differences between four- and two-terminal
configurations. For each top cell, we thereby determine the tandem
cell performance as a function of the bottom cell efficiency, which
results in a linear relationship. From these data, we extract two
parameters to quantify the suitability of the top cell: (i) the slope
of the tandem vs. bottom cell efficiency, which is the effective transparency
of the top cell, and (ii) the tandem cell efficiency for a targeted
bottom cell. These two figures of merit were calculated for a representative
set of bottom cells and may serve for comparison of semitransparent
perovskite top cells in the future
Performance Evaluation of Semitransparent Perovskite Solar Cells for Application in Four-Terminal Tandem Cells
The
efficiency of perovskite-based tandem solar cells and the respective
efficiency gain over the single-junction operation of the bottom cell
strongly depend on the performance of the component cells. Thus, a
fair comparison of reported top cells is difficult. We therefore compute
the tandem cell efficiency for the combination of several semitransparent
perovskite top solar cells and crystalline silicon or chalcopyrite
bottom cells from the literature. We focus on four-terminal configurations
but also estimate and discuss the differences between four- and two-terminal
configurations. For each top cell, we thereby determine the tandem
cell performance as a function of the bottom cell efficiency, which
results in a linear relationship. From these data, we extract two
parameters to quantify the suitability of the top cell: (i) the slope
of the tandem vs. bottom cell efficiency, which is the effective transparency
of the top cell, and (ii) the tandem cell efficiency for a targeted
bottom cell. These two figures of merit were calculated for a representative
set of bottom cells and may serve for comparison of semitransparent
perovskite top cells in the future
Assessing Temperature Dependence of Drift Mobility in Methylammonium Lead Iodide Perovskite Single Crystals
Hybrid organic–inorganic
perovskites have emerged as cost-effective
and high-performance semiconductors for optoelectronic applications.
Precise knowledge of charge carrier mobility and especially the temperature
dependence of mobility is therefore of utmost relevance for advancing
high-performance materials. Here, the charge carrier mobility in methylammonium
lead iodide single crystals is investigated with time of flight technique
from 290 to 100 K. A nondispersive transport with an electron mobility
of 135 (±20) cm<sup>2</sup>/V s and a hole mobility of 90 (±20)
cm<sup>2</sup>/V s is obtained at room temperature. A power-law temperature
dependence of mobility, μ ∝ <i>T</i><sup><i>m</i></sup>, with an exponent <i>m</i> = −2.8
and −2.0, is measured for electrons and holes in the tetragonal
phase. The highest electron and hole mobilities measured are 635 (±70)
and 415 (±20) cm<sup>2</sup>/V s, respectively. Our results indicate
that the scattering of charge carriers with phonons is the limiting
factor for carrier mobilities at room temperature
A Bayesian Approach to Predict Solubility Parameters
Solubility is a ubiquitous phenomenon in many aspects of material science. While solubility can be determined by considering the cohesive forces in a liquid via the Hansen solubility parameters (HSP), quantitative structure-property relationship models are often used for prediction, notably due to their low computational cost. Herein, we report gpHSP, an interpretable and versatile probabilistic approach to determining HSP. Our model is based on Gaussian processes (GP), a Bayesian machine learning approach that provides uncertainty bounds to prediction. gpHSP achieves its flexibility by leveraging a variety of input data, such as SMILES strings, COSMOtherm simulations, and quantum chemistry calculations. gpHSP is built on experimentally determined HSP, including a general solvents set aggregated from literature, and a polymer set experimentally characterized by this group of authors. In all sets, we obtained a high degree of agreement, surpassing well-established machine learning methods. We demonstrate the general applicability of gpHSP to miscibility of organic semiconductors, drug compounds and in general solvents, which can be further extended to other domains. gpHSP is a fast and accurate toolbox, which could be applied to molecular design for solution processing technologies.<br
Qualitative Analysis of Bulk-Heterojunction Solar Cells without Device Fabrication: An Elegant and Contactless Method
The enormous synthetic
efforts on novel solar cell materials require
a reliable and fast technique for the rapid screening of novel donor/acceptor
combinations in order to quickly and reliably estimate their optimized
parameters. Here, we report the applicability of such a versatile
and fast evaluation technique for bulk heterojunction (BHJ) organic
photovoltaics (OPV) by utilizing a steady-state photoluminescence
(PL) method confirmed by electroluminescence (EL) measurements. A
strong relation has been observed between the residual singlet emission
and the charge transfer state emission in the blend. Using this relation,
a figure of merit (FOM) is defined from photoluminescence and also
electroluminescence measurements for qualitative analysis and shown
to precisely anticipate the optimized blend parameters of bulk heterojunction
films. Photoluminescence allows contactless evaluation of the photoactive
layer and can be used to predict the optimized conditions for the
best polymer–fullerene combination. Most interestingly, the
contactless, PL-based FOM method has the potential to be integrated
as a fast and reliable inline tool for quality control and material
optimization
Improved High-Efficiency Perovskite Planar Heterojunction Solar Cells via Incorporation of a Polyelectrolyte Interlayer
Improved High-Efficiency Perovskite Planar Heterojunction
Solar Cells via Incorporation of a Polyelectrolyte Interlaye
Low-Temperature Solution-Processed Kesterite Solar Cell Based on in Situ Deposition of Ultrathin Absorber Layer
The production of high-performance,
solution-processed kesterite
Cu<sub>2</sub>ZnSn(S<sub><i>x</i></sub>,Se<sub>1–<i>x</i></sub>)<sub>4</sub> (CZTSSe) solar cells typically relies
on high-temperature crystallization processes in chalcogen-containing
atmosphere and often on the use of environmentally harmful solvents,
which could hinder the widespread adoption of this technology. We
report a method for processing selenium free Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS) solar cells based on a short annealing step at temperatures
as low as 350 °C using a molecular based precursor, fully avoiding
highly toxic solvents and high-temperature sulfurization. We show
that a simple device structure consisting of ITO/CZTS/CdS/Al and comprising
an extremely thin absorber layer (∼110 nm) achieves a current
density of 8.6 mA/cm<sup>2</sup>. Over the course of 400 days under
ambient conditions encapsulated devices retain close to 100% of their
original efficiency. Using impedance spectroscopy and photoinduced
charge carrier extraction by linearly increasing voltage (photo-CELIV),
we demonstrate that reduced charge carrier mobility is one limiting
parameter of low-temperature CZTS photovoltaics. These results may
inform less energy demanding strategies for the production of CZTS
optoelectronic layers compatible with large-scale processing techniques
Fully Solution-Processing Route toward Highly Transparent Polymer Solar Cells
We report highly transparent polymer
solar cells using metallic silver nanowires (AgNWs) as both the electron-
and hole-collecting electrodes. The entire stack of the devices is
processed from solution using a doctor blading technique. A thin layer
of zinc oxide nanoparticles is introduced between photoactive layer
and top AgNW electrode which plays decisive roles in device functionality:
it serves as a mechanical foundation which allows the solution-deposition
of top AgNWs, and more importantly it facilitates charge carriers
extraction due to the better energy level alignment and the formation
of ohmic contacts between the active layer/ZnO and ZnO/AgNWs. The
resulting semitransparent polymer:fullerene solar cells showed a power
conversion efficiency of 2.9%, which is 72% of the efficiency of an
opaque reference device. Moreover, an average transmittance of 41%
in the wavelength range of 400–800 nm is achieved, which is
of particular interest for applications in transparent architectures
Brightly Luminescent and Color-Tunable Formamidinium Lead Halide Perovskite FAPbX<sub>3</sub> (X = Cl, Br, I) Colloidal Nanocrystals
In
the past few years, hybrid organic–inorganic and all-inorganic
metal halide perovskite nanocrystals have become one of the most interesting
materials for optoelectronic applications. Here, we report a facile
and rapid room temperature synthesis of 15–25 nm formamidinium
CH(NH<sub>2</sub>)<sub>2</sub>PbX<sub>3</sub> (X = Cl, Br, I, or mixed
Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation
(LARP). The cubic and platelet-like nanocrystals with their emission
in the range of 415–740 nm, full width at half-maximum (fwhm)
of 20–44 nm, and radiative lifetimes of 5–166 ns enable
band gap tuning by halide composition as well as by their thickness
tailoring; they have a high photoluminescence quantum yield (up to
85%), colloidal and thermodynamic stability. Combined with surface
modification that prevents degradation by water, this nanocrystalline
material is an ideal candidate for optoelectronic devices and applications.
In addition, optoelectronic measurements verify that the photodetector
based on FAPbI<sub>3</sub> nanocrystals paves the way for perovskite
quantum dot photovoltaics