The Association of Antarctic Krill Euphausia superba with the Under-Ice Habitat

The association of Antarctic krill Euphausia superba with the under-ice habitat was investigated in the Lazarev Sea (Southern Ocean) during austral summer, autumn and winter. Data were obtained using novel Surface and Under Ice Trawls (SUIT), which sampled the 0–2 m surface layer both under sea ice and in open water. Average surface layer densities ranged between 0.8 individuals m−2 in summer and autumn, and 2.7 individuals m−2 in winter. In summer, under-ice densities of Antarctic krill were significantly higher than in open waters. In autumn, the opposite pattern was observed. Under winter sea ice, densities were often low, but repeatedly far exceeded summer and autumn maxima. Statistical models showed that during summer high densities of Antarctic krill in the 0–2 m layer were associated with high ice coverage and shallow mixed layer depths, among other factors. In autumn and winter, density was related to hydrographical parameters. Average under-ice densities from the 0–2 m layer were higher than corresponding values from the 0–200 m layer collected with Rectangular Midwater Trawls (RMT) in summer. In winter, under-ice densities far surpassed maximum 0–200 m densities on several occasions. This indicates that the importance of the ice-water interface layer may be under-estimated by the pelagic nets and sonars commonly used to estimate the population size of Antarctic krill for management purposes, due to their limited ability to sample this habitat. Our results provide evidence for an almost year-round association of Antarctic krill with the under-ice habitat, hundreds of kilometres into the ice-covered area of the Lazarev Sea. Local concentrations of postlarval Antarctic krill under winter sea ice suggest that sea ice biota are important for their winter survival. These findings emphasise the susceptibility of an ecological key species to changing sea ice habitats, suggesting potential ramifications on Antarctic ecosystems induced by climate change

Energy level tuning of Poly(phenylene-alt-dithienobenzothiadiazole)s for low photon energy loss solar cells

\u3cp\u3eSix poly(phenylene-alt-dithienobenzothiadiazole)-based polymers have been synthesized for application in polymer–fullerene solar cells. Hydrogen, fluorine, or nitrile substitution on benzo­thiadiazole and alkoxy or ester substitution on the phenylene moiety are investigated to reduce the energy loss per converted photon. Power conversion efficiencies (PCEs) up to 6.6% have been obtained. The best performance is found for the polymer–fullerene combination with distinct phase separation and crystalline domains. This improves the maximum external quantum efficiency for charge formation and collection to 66%. The resulting higher photocurrent compensates for the relatively large energy loss per photon (E \u3csub\u3eloss\u3c/sub\u3e = 0.97 eV) in achieving a high PCE. By contrast, the poly­mer that provides a reduced energy loss (E \u3csub\u3eloss\u3c/sub\u3e = 0.49 eV) gives a lower photocurrent and a reduced PCE of 1.8% because the external quantum efficiency of 17% is limited by a suboptimal morphology and a reduced driving force for charge transfer. (Figure presented.).\u3c/p\u3

Simulating phase separation during spin coating of a polymer–fullerene blend:a joint computational and experimental investigation

During spin coating of the photoactive layer of a bulk heterojunction organic solar cell, phase separation between the donor (D) and acceptor (A) components is triggered by solvent evaporation. The morphology of the resulting layer is one of the main determinants of the device efficiency and critically depends on processing conditions such as the spinning speed, D–A mixing ratio, and choice of solvents. It is crucial to understand how these conditions influence the nanostructure of the photoactive layer. Optical experiments have a limited spatial resolution and cannot probe the short length scales of phase separation. In this work, we present three-dimensional simulations of evaporation-induced phase separation in a diketopyrrolopyrrole–fullerene D–A blend, where we derive the simulation parameters from in situ laser interference and contact angle experiments. Depending on the drying rate, phase separation initiates in different regions of the thinning film. From a linear stability analysis, we estimate the early stage length scale of phase separation and compare it with simulations. The normalized drying rate is shown to be the key parameter. The experimentally found power law dependence of the characteristic length scale of phase separation on this parameter is reproduced with a matching exponent

Dichotomous role of exciting the donor or the acceptor on charge generation in organic solar cells

\u3cp\u3eIn organic solar cells, photoexcitation of the donor or acceptor phase can result in different efficiencies for charge generation. We investigate this difference for four different 2-pyridyl diketopyrrolopyrrole (DPP) polymer-fullerene solar cells. By comparing the external quantum efficiency spectra of the polymer solar cells fabricated with either [60]PCBM or [70]PCBM fullerene derivatives as acceptor, the efficiency of charge generation via donor excitation and acceptor excitation can both be quantified. Surprisingly, we find that to make charge transfer efficient, the offset in energy between the HOMO levels of donor and acceptor that govern charge transfer after excitation of the acceptor must be larger by ∼0.3 eV than the offset between the corresponding two LUMO levels when the donor is excited. As a consequence, the driving force required for efficient charge generation is significantly higher for excitation of the acceptor than for excitation of the donor. By comparing charge generation for a total of 16 different DPP polymers, we confirm that the minimal driving force, expressed as the photon energy loss, differs by about 0.3 eV for exciting the donor and exciting the acceptor. Marcus theory may explain the dichotomous role of exciting the donor or the acceptor on charge generation in these solar cells.\u3c/p\u3

Stochastic modeling and predictive simulations for the microstructure of organic semiconductor films processed with different spin coating velocities

A parametric stochastic model of the morphology of thin polymer:fullerene films is developed. This model uses a number of tools from stochastic geometry and spatial statistics. The fullerene-rich phase is represented by random closed sets and the polymer-rich phase is given by their complement. The model has three stages. First, a point pattern is used to model the locations of fullerene-rich domains. Second, domains are formed at these points. Third, the domains are rearranged to ensure a realistic configuration. The model is fitted to polymer:fullerene films produced using seven different spin coating velocities and validated using a variety of morphological characteristics. The model is then used to simulate morphologies corresponding to spin velocities for which no empirical data exists. The viability of this approach is demonstrated using cross-validation

Structure-property relationships for bis-diketopyrrolopyrrole molecules in organic photovoltaics

\u3cp\u3eThe design of small organic molecules for efficient solution-processed organic solar cells is hampered by the absence of relationships that connect molecular structure via processing to blend morphology and power conversion efficiency. Here we study a series of bis-diketopyrrolopyrrole molecules in which we systematically vary the aromatic core, the solubilizing side chains, and the end groups to achieve power conversion efficiencies of 4.4%. By comparing the morphology and performance we attempt to identify and rationalize the structure-property relationships. We find that the tendency to aggregate or crystallize are important factors to control and that these require a subtle balance.\u3c/p\u3

A real-time study of the benefits of co-solvents in polymer solar cell processing

The photoactive layer of organic solar cells consists of a nanoscale blend of electron-donating and electron-accepting organic semiconductors. Controlling the degree of phase separation between these components is crucial to reach efficient solar cells. In solution-processed polymer–fullerene solar cells, small amounts of co-solvents are commonly used to avoid the formation of undesired large fullerene domains that reduce performance. There is an ongoing discussion about the origin of this effect. To clarify the role of co-solvents, we combine three optical measurements to investigate layer thickness, phase separation and polymer aggregation in real time during solvent evaporation under realistic processing conditions. Without co-solvent, large ¿fullerene-rich domains form via liquid–liquid phase separation at ~20 vol% solid content. Under such supersaturated conditions, co-solvents induce polymer aggregation below 20 vol% solids and prevent the formation of large domains. This rationalizes the formation of intimately mixed films that give high-efficient solar cells for the materials studie

Sub-micrometer structure formation during spin coating revealed by time-resolved in situ laser and X-ray scattering

\u3cp\u3eSolution-processed thin polymer films have many applications, such as organic electronics and block-copolymer nanofabrication. These films are often made by spin coating a solution that contains one or more solids and can show different phase-separated structures. The formation mechanism of the droplet-like morphology is studied here by processing polystyrene (PS) and a fullerene derivative ([6,6]-phenyl-C\u3csub\u3e71\u3c/sub\u3e-butyric acid methyl ester, [70]PCBM) from o-xylene. The final structure consists of [70]PCBM droplets partially embedded in a PS-rich matrix showing interdomain distance of 100–1000 nm as determined from transmission electron microscopy and grazing incidence small angle X-ray scattering (GISAXS). To elucidate the formation of these morphologies in real time, ultrafast in situ GISAXS coupled with laser interferometry and laser scattering is performed during spin coating. In situ thickness measurements and laser scattering show that liquid–liquid phase separation occurs at ≈70 vol% solvent. Subsequently, in only 100–400 ms, almost dry [70]PCBM domains start to protrude from the swollen PS-rich matrix. These results are used to verify the ternary phase diagram calculated using Flory–Huggins theory. The discussed multitechnique approach can be applied to study fundamental aspects in soft matter such as phase separation in thin films occurring at very short time scales.\u3c/p\u3