43 research outputs found
A phase-field model for the evaporation of thin film mixtures
The performance of solution-processed solar cells strongly depends on the
geometrical structure and roughness of the photovoltaic layers formed during
film drying. During the drying process, the interplay of crystallization and
liquid-liquid demixing leads to the structure formation on the nano- and
microscale and to the final rough film. In order to better understand how the
film structure can be improved by process engineering, we aim at theoretically
investigating these systems by means of phase-field simulations. We introduce
an evaporation model based on the Cahn-Hilliard equation for the evolution of
the fluid concentrations coupled to the Allen-Cahn equation for the
liquid-vapour phase transformation. We demonstrate its ability to match the
experimentally measured drying kinetics and study the impact of the parameters
of our model. Furthermore, the evaporation of solvent blends and solvent-vapour
annealing are investigated. The dry film roughness emerges naturally from our
set of equations, as illustrated through preliminary simulations of spinodal
decomposition and film drying on structured substrates
Phase-field simulation of liquid-vapor equilibrium and evaporation of fluid mixtures
In solution-processing of thin films, the material layer is deposited from a
solution composed of several solutes and solvents. The final morphology and
hence the properties of the film often depend on the time needed for the
evaporation of the solvents. This is typically the case for organic photoactive
or electronic layers. Therefore, it is important to be able to predict the
evaporation kinetics of such mixtures. We propose here a new phase-field model
for the simulation of evaporating fluid mixtures and simulate their evaporation
kinetics. Similar to the Hertz-Knudsen theory, the local liquid-vapor
equilibrium is assumed to be reached at the film surface and evaporation is
driven by diffusion away from this gas layer. In the situation where the
evaporation is purely driven by the liquid-vapor equilibrium, the simulations
match the behavior expected theoretically from the free energy: for evaporation
of pure solvents, the evaporation rate is constant and proportional to the
vaporpressure. For mixtures, the evaporation rate is in general strongly
time-dependent because of the changing composition of the film. Nevertheless,
for highly non-ideal mixtures, such as poorly compatible fluids or polymer
solutions, the evaporation rate becomes almost constant in the limit of low
Biot numbers. The results of the simulation have been successfully compared to
experiments on a polystyrene-toluene mixture. The model allows to take into
account deformations of the liquid-vapor interface and therefore to simulate
film roughness or dewetting
Guidelines for Material Design in Semitransparent Organic Solar Cells
Organic solar cells (OSCs) are uniquely suited for semitransparent
applications due to their adjustable absorption spectrum. However, most
high-performance semitransparent cells reported to date are based on materials
that have shown high power conversion efficiency for opaque devices. We
therefore present a model to assess the optimum efficiency and transparency for
a specific donor and acceptor band gap. The absorption characteristics of both
donor and acceptor are modeled with spectral data of typical absorber materials
from the literature which are adjusted to achieve the desired band gap value.
The results show three distinct regions of high light utilization efficiency if
the photopic curve is employed as a weighting function (corresponding to window
applications), and a broad maximum for the plant action spectrum as a weighting
function (corresponding to greenhouse applications). When comparing these
findings to reported experimental values, it is evident that the band gaps of
the materials used for the experimental studies do not correspond to the maxima
identified by our simulation model. The analysis of the energy levels of
molecules recorded in the literature confirms that all band gaps and therefore
all LUE maxima are chemically feasible so that the performance of
semitransparent OSCs can be further improved by designing materials with
optimized absorption spectra.Comment: 20 pages and 10 figure in the main manuscript, aditionally 6 pages
and 4 figures in the supporting informatio
Matching the photocurrent of perovskite/organic tandem solar modules by varying the cell width
Photocurrent matching in conventional monolithic tandem solar cells is
achieved by choosing semiconductors with complementary absorption spectra and
by carefully adjusting the optical properties of the complete top and bottom
stacks. However, for thin film photovoltaic technologies at the module level,
another design variable significantly alleviates the task of photocurrent
matching, namely the cell width, whose modification can be readily realized by
the adjustment of the module layout. Herein we demonstrate this concept at the
experimental level for the first time for a 2T-mechanically stacked perovskite
(FAPbBr3)/organic (PM6:Y6:PCBM) tandem mini-module, an unprecedented approach
for these emergent photovoltaic technologies fabricated in an independent
manner. An excellent Isc matching is achieved by tuning the cell widths of the
perovskite and organic modules to 7.22 mm (PCEPVKT-mod= 6.69%) and 3.19 mm
(PCEOPV-mod= 12.46%), respectively, leading to a champion efficiency of 14.94%
for the tandem module interconnected in series with an aperture area of 20.25
cm2. Rather than demonstrating high efficiencies at the level of small lab
cells, our successful experimental proof-of-concept at the module level proves
to be particularly useful to couple devices with non-complementary
semiconductors, either in series or in parallel electrical connection, hence
overcoming the limitations imposed by the monolithic structure
The 2021 flexible and printed electronics roadmap
This roadmap includes the perspectives and visions of leading researchers in the key areas of flexible and printable electronics. The covered topics are broadly organized by the device technologies (sections 1–9), fabrication techniques (sections 10–12), and design and modeling approaches (sections 13 and 14) essential to the future development of new applications leveraging flexible electronics (FE). The interdisciplinary nature of this field involves everything from fundamental scientific discoveries to engineering challenges; from design and synthesis of new materials via novel device design to modelling and digital manufacturing of integrated systems. As such, this roadmap aims to serve as a resource on the current status and future challenges in the areas covered by the roadmap and to highlight the breadth and wide-ranging opportunities made available by FE technologies
Gaze Strategy in the Free Flying Zebra Finch (Taeniopygia guttata)
Fast moving animals depend on cues derived from the optic flow on their retina. Optic flow from translational locomotion includes information about the three-dimensional composition of the environment, while optic flow experienced during a rotational self motion does not. Thus, a saccadic gaze strategy that segregates rotations from translational movements during locomotion will facilitate extraction of spatial information from the visual input. We analysed whether birds use such a strategy by highspeed video recording zebra finches from two directions during an obstacle avoidance task. Each frame of the recording was examined to derive position and orientation of the beak in three-dimensional space. The data show that in all flights the head orientation was shifted in a saccadic fashion and was kept straight between saccades. Therefore, birds use a gaze strategy that actively stabilizes their gaze during translation to simplify optic flow based navigation. This is the first evidence of birds actively optimizing optic flow during flight
Organic photovoltaic modules with new world record efficiencies
During the last years, the development of new active materials has led to constant improvement in the power conversion efficiency (PCE) of solution‐processed organic photovoltaics (OPV) to nowadays record values above 17% on small lab cells. In this work, we show the developments and results of a successful upscaling of such highly efficient OPV systems to the module level on large areas, which yielded two new certified world record efficiencies, namely, 12.6% on a module area of 26 cm2 and 11.7% on a module area of 204 cm2. The decisive developments leading to this achievement include the optimization of the module layout as well as the high‐resolution short‐pulse (nanosecond) laser structuring processes involved in the manufacturing of such modules. By minimizing the inactive areas within the total module area that are used for interconnecting the individual solar cells of the module in series, geometric fill factors of over 95% have been achieved. A production yield of 100% working modules during the manufacturing of these modules and an extremely narrow distribution of the final PCE values underline the excellent process control and reproducibility of the results. The new developments and their implementation into the production process of the record OPV modules are described in detail, along with the challenges that arose during this development. Finally, dark lock‐in thermography (DLIT), electroluminescence (EL), and photoluminescence (PL) measurements of the record module are presented
Automatized analysis of IR-images of photovoltaic modules and its use for quality control of solar cells
It is well known that the performance of solar cells may significantly suffer from local electric defects. Accordingly, infrared thermography (i.p. lock-in thermography) has been intensely applied to identify such defects as hot spots. As an imaging method, this is a fast way of module characterization. However, imaging leads to a huge amount of data, which needs to be investigated. An automatized image analysis would be a very beneficial tool but has not been suggested so far for lock-in thermography images. In this manuscript, we describe such an automatized analysis of solar cells. We first established a robust algorithm for segmentation (or recognition) for both, the PV-module and the defects (hot spots). With this information, we then calculated a parameter from the IR-images, which could be well correlated with the maximal power (Pmpp) of the modules. The proposed automatized method serves as a very useful foundation for faster and more thorough analyses of IR-images and stimulates the further development of quality control on solar modules
A phase-field model for the evaporation of thin film mixtures
The performance of solution-processed solar cells strongly depends on the geometrical structure and roughness of the photovoltaic layers formed during film drying. During the drying process, the interplay of crystallization and liquid-liquid demixing leads to structure formation on the nano- and microscale and to the final rough film. In order to better understand how the film structure can be improved by process engineering, we aim at theoretically investigating these systems by means of phase-field simulations. We introduce an evaporation model based on the Cahn-Hilliard equation for the evolution of the fluid concentrations coupled to the Allen-Cahn equation for the liquid-vapour phase transformation. We demonstrate its ability to match the experimentally measured drying kinetics and study the impact of the parameters of our model. Furthermore, the evaporation of solvent blends and solvent-vapour annealing are investigated. The dry film roughness emerges naturally from our set of equations, as illustrated through preliminary simulations of spinodal decomposition and film drying on structured substrates
Fully solution-processed, light-weight, and ultraflexible organic solar cells
Organic photovoltaic (OPV) devices have the potential to be superior to other PV technologies forthe use in applications that require very high flexibility or maximum specific power(power-per-weight ratio), such as textile integration, wearable electronics, or outer spaceapplications. However, OPV devices also require encapsulation by barrier films to reduce thedegradation driven by extrinsic factors, which in turn limits their flexibility and leads to lowerspecific power values. In this work, fully solution-processed (including both electrodes)semitransparent organic solar cells (OSCs) with performance comparable with conventionalindium tin oxide-based devices are processed directly onto different barrier films of varyingthicknesses. Direct cell fabrication onto barrier films leads to the elimination of the additionalpolyethylene terephthalate substrate and one of the two adhesive layers in the final stack of anencapsulated OPV device by replacing the industrial state-of-the-art sandwich encapsulation witha top-only encapsulation process, which yields significantly thinner and lighter ‘product-relevant’PV devices. In addition to the increase of the specific power to 0.38 W g−1, which is more than fourtimes higher than sandwich-encapsulated devices, these novel OSCs exhibit better flexibility andsurvive 5000 bending cycles with 4.5 mm bending radius. Moreover, the devices show comparablestability as conventionally encapsulated devices under constant illumination (1 sun) in ambient airfor 1000 h. Finally, degradation under damp heat conditions (65 ◦C, 85% rh) was investigated andfound to be determined by a combination of different factors, namely (UV) light soaking, intrinsicbarrier properties, and potential damaging of the barriers during (laser) processing