Tailored exciton diffusion in organic photovoltaic cells for enhanced power conversion efficiency.

Photoconversion in planar-heterojunction organic photovoltaic cells (OPVs) is limited by a short exciton diffusion length (L(D)) that restricts migration to the dissociating electron donor/acceptor interface. Consequently, bulk heterojunctions are often used to realize high efficiency as these structures reduce the distance an exciton must travel to be dissociated. Here, we present an alternative approach that seeks to directly engineer L(D) by optimizing the intermolecular separation and consequently, the photophysical parameters responsible for excitonic energy transfer. By diluting the electron donor boron subphthalocyanine chloride into a wide-energy-gap host material, we optimize the degree of interaction between donor molecules and observe a ~50% increase in L(D). Using this approach, we construct planar-heterojunction OPVs with a power conversion efficiency of (4.4 ± 0.3)%, > 30% larger than the case of optimized devices containing an undiluted donor layer. The underlying correlation between L(D) and the degree of molecular interaction has wide implications for the design of both OPV active materials and device architectures.

Promoting social inclusion through Unified Sports for youth with intellectual disabilities: a five-nation study.

BACKGROUND: Although the promotion of social inclusion through sports has received increased attention with other disadvantaged groups, this is not the case for children and adults with intellectual disability who experience marked social isolation. The study evaluated the outcomes from one sports programme with particular reference to the processes that were perceived to enhance social inclusion. METHOD: The Youth Unified Sports programme of Special Olympics combines players with intellectual disabilities (called athletes) and those without intellectual disabilities (called partners) of similar skill level in the same sports teams for training and competition. Alongside the development of sporting skills, the programme offers athletes a platform to socialise with peers and to take part in the life of their community. Unified football and basketball teams from five countries--Germany, Hungary, Poland, Serbia and Ukraine--participated. Individual and group interviews were held with athletes, partners, coaches, parents and community leaders: totalling around 40 informants per country. RESULTS: Qualitative data analysis identified four thematic processes that were perceived by informants across all countries and the two sports to facilitate social inclusion of athletes. These were: (1) the personal development of athletes and partners; (2) the creation of inclusive and equal bonds; (3) the promotion of positive perceptions of athletes; and (4) building alliances within local communities. CONCLUSIONS: Unified Sports does provide a vehicle for promoting the social inclusion of people with intellectual disabilities that is theoretically credible in terms of social capital scholarship and which contains lessons for advancing social inclusion in other contexts. Nonetheless, certain limitations are identified that require further consideration to enhance athletes' social inclusion in the wider community.

Short contacts between chains enhancing luminescence quantum yields and carrier mobilities in conjugated copolymers.

Efficient conjugated polymer optoelectronic devices benefit from concomitantly high luminescence and high charge carrier mobility. This is difficult to achieve, as interchain interactions, which are needed to ensure efficient charge transport, tend also to reduce radiative recombination and lead to solid-state quenching effects. Many studies detail strategies for reducing these interactions to increase luminescence, or modifying chain packing motifs to improve percolation charge transport; however achieving these properties together has proved elusive. Here, we show that properly designed amorphous donor-alt-acceptor conjugated polymers can circumvent this problem; combining a tuneable energy gap, fast radiative recombination rates and luminescence quantum efficiencies >15% with high carrier mobilities exceeding 2.4 cm2/Vs. We use photoluminescence from exciton states pinned to close-crossing points to study the interplay between mobility and luminescence. These materials show promise towards realising advanced optoelectronic devices based on conjugated polymers, including electrically-driven polymer lasers.

Chain Coupling and Luminescence in High-Mobility, Low-Disorder Conjugated Polymers.

Optoelectronic devices based on conjugated polymers often rely on multilayer device architectures, as it is difficult to design all the different functional requirements, in particular the need for efficient luminescence and fast carrier transport, into a single polymer. Here we study the photophysics of a recently discovered class of conjugated polymers with high charge carrier mobility and low degree of energetic disorder and investigate whether it is possible in this system to achieve by molecular design a high photoluminescence quantum yield without sacrificing carrier mobility. Tracing exciton dynamics over femtosecond to microsecond time scales, we show that nearly all nonradiative exciton recombination arises from interactions between chromophores on different chains. We evaluate the temperature dependence and role of electron-phonon coupling leading to fast internal conversion in systems with strong interchain coupling and the extent to which this can be turned off by varying side chain substitution. By sterically decreasing interchain interaction, we present an effective approach to increase the fluorescence quantum yield of low-energy gap polymers. We present a red-NIR-emitting amorphous polymer with the highest reported film luminescence quantum efficiency of 18% whose mobility concurrently exceeds that of amorphous-Si. This is a key result toward the development of single-layer optoelectronic devices that require both properties.

Long-range exciton transport in conjugated polymer nanofibers prepared by seeded growth.

Easily processed materials with the ability to transport excitons over length scales of more than 100 nanometers are highly desirable for a range of light-harvesting and optoelectronic devices. We describe the preparation of organic semiconducting nanofibers comprising a crystalline poly(di-n-hexylfluorene) core and a solvated, segmented corona consisting of polyethylene glycol in the center and polythiophene at the ends. These nanofibers exhibit exciton transfer from the core to the lower-energy polythiophene coronas in the end blocks, which occurs in the direction of the interchain π-π stacking with very long diffusion lengths (>200 nanometers) and a large diffusion coefficient (0.5 square centimeters per second). This is made possible by the uniform exciton energetic landscape created by the well-ordered, crystalline nanofiber core.

Efficient Vacuum-Processed Light-Emitting Diodes Based on Carbene-Metal-Amides.

Efficient vacuum-processed organic light-emitting diodes are fabricated using a carbene-metal-amide material, CMA1. An electroluminescence (EL) external quantum efficiency of 23% is achieved in a host-free emissive layer comprising pure CMA1. Furthermore external quantum efficiencies of up to 26.9% are achieved in host-guest emissive layers. EL spectra are found to depend on both the emissive-layer doping concentration and the choice of host material, enabling tuning of emission color from mid-green (Commission Internationale de l'Éclairage co-ordinates [0.24, 0.46]) to sky blue ([0.22 0.35]) without changing dopant. This tuning is achieved without compromising luminescence efficiency (>80%) while maintaining a short radiative lifetime of triplets (<1 µs).

Vibrationally Assisted Intersystem Crossing in Benchmark Thermally Activated Delayed Fluorescence Molecules.

Electrically injected charge carriers in organic light-emitting devices (OLEDs) undergo recombination events to form singlet and triplet states in a 1:3 ratio, representing a fundamental hurdle for achieving high quantum efficiency. Dopants based on thermally activated delayed fluorescence (TADF) have emerged as promising candidates for addressing the spin statistics issue in OLEDs. In these materials, reverse singlet-triplet intersystem crossing (rISC) becomes efficient, thereby activating luminescence pathways for weakly emissive triplet states. However, despite a growing consensus that torsional vibrations facilitate spin-orbit-coupling- (SOC-) driven ISC in these molecules, there is a shortage of experimental evidence. We use transient electron spin resonance and theory to show unambiguously that SOC interactions drive spin conversion and that ISC is a dynamic process gated by conformational fluctuations for benchmark carbazolyl-dicyanobenzene TADF emitters.

Order enables efficient electron-hole separation at an organic heterojunction with a small energy loss.

Donor-acceptor organic solar cells often show low open-circuit voltages (V OC) relative to their optical energy gap (E g) that limit power conversion efficiencies to ~12%. This energy loss is partly attributed to the offset between E g and that of intermolecular charge transfer (CT) states at the donor-acceptor interface. Here we study charge generation occurring in PIPCP:PC61BM, a system with a very low driving energy for initial charge separation (E g-E CT ~ 50 meV) and a high internal quantum efficiency (η IQE ~ 80%). We track the strength of the electric field generated between the separating electron-hole pair by following the transient electroabsorption optical response, and find that while localised CT states are formed rapidly (<100 fs) after photoexcitation, free charges are not generated until 5 ps after photogeneration. In PIPCP:PC61BM, electronic disorder is low (Urbach energy <27 meV) and we consider that free charge separation is able to outcompete trap-assisted non-radiative recombination of the CT state.

Biotic and abiotic controls of Argentine ant invasion success at local and landscape scales.

Although the ecological success of introduced species hinges on biotic interactions and physical conditions, few experimental studies--especially on animals--have simultaneously investigated the relative importance of both types of factors. The lack of such research may stem from the common assumption that native and introduced species exhibit similar environmental tolerances. Here we combine experimental and spatial modeling approaches (1) to determine the relative importance of biotic and abiotic controls of Argentine ant (Linepithema humile) invasion success, (2) to examine how the importance of these factors changes with spatial scale in southern California (USA), and (3) to assess how Argentine ants differ from native ants in their environmental tolerances. A factorial field experiment that combined native ant removal with irrigation revealed that Argentine ants failed to invade any dry plots (even those lacking native ants) but readily invaded all moist plots. Native ants slowed the spread of Argentine ants into irrigated plots but did not prevent invasion. In areas without Argentine ants, native ant species showed variable responses to irrigation. At the landscape scale, Argentine ant occurrence was positively correlated with minimum winter temperature (but not precipitation), whereas native ant diversity increased with precipitation and was negatively correlated with minimum winter temperature. These results are of interest for several reasons. First, they demonstrate that fine-scale differences in the physical environment can eclipse biotic resistance from native competitors in determining community susceptibility to invasion. Second, our results illustrate surprising complexities with respect to how the abiotic factors limiting invasion can change with spatial scale, and third, how native and invasive species can differ in their responses to the physical environment. Idiosyncratic and scale-dependent processes complicate attempts to forecast where introduced species will occur and how their range limits may shift as a result of climate change.

Ultrafast Long-Range Charge Separation in Nonfullerene Organic Solar Cells.

Rapid, long-range charge separation in polymer-fullerene organic solar cells (OSCs) enables electrons and holes to move beyond their Coulomb capture radius and overcome geminate recombination. Understanding the nature of charge generation and recombination mechanisms in efficient, nonfullerene-acceptor-based OSCs are critical to further improve device performance. Here we report charge dynamics in an OSC using a perylene diimide (PDI) dimer acceptor. We use transient absorption spectroscopy to track the time evolution of electroabsorption caused by the dipolar electric field generated between electron-hole pairs as they separate after ionization at the donor-acceptor interface. We show that charges separate rapidly (<1 ps) and that free charge carriers are generated very efficiently (∼90% quantum yield). However, in the PDI-based OSC, external charge extraction is impaired by faster nongeminate decay to the ground state and to lower-lying triplet states.

Photon Reabsorption in Mixed CsPbCl3:CsPbI3 Perovskite Nanocrystal Films for Light-Emitting Diodes.

Cesium lead halide nanocrystals, CsPbX3 (X = Cl, Br, I), exhibit photoluminescence quantum efficiencies approaching 100% without the core-shell structures usually used in conventional semiconductor nanocrystals. These high photoluminescence efficiencies make these crystals ideal candidates for light-emitting diodes (LEDs). However, because of the large surface area to volume ratio, halogen exchange between perovskite nanocrystals of different compositions occurs rapidly, which is one of the limiting factors for white-light applications requiring a mixture of different crystal compositions to achieve a broad emission spectrum. Here, we use mixtures of chloride and iodide CsPbX3 (X = Cl, I) perovskite nanocrystals where anion exchange is significantly reduced. We investigate samples containing mixtures of perovskite nanocrystals with different compositions and study the resulting optical and electrical interactions. We report excitation transfer from CsPbCl3 to CsPbI3 in solution and within a poly(methyl methacrylate) matrix via photon reabsorption, which also occurs in electrically excited crystals in bulk heterojunction LEDs.

Limits for Recombination in a Low Energy Loss Organic Heterojunction.

Donor-acceptor organic solar cells often show high quantum yields for charge collection, but relatively low open-circuit voltages (VOC) limit power conversion efficiencies to around 12%. We report here the behavior of a system, PIPCP:PC61BM, that exhibits very low electronic disorder (Urbach energy less than 27 meV), very high carrier mobilities in the blend (field-effect mobility for holes >10-2 cm2 V-1 s-1), and a very low driving energy for initial charge separation (50 meV). These characteristics should give excellent performance, and indeed, the VOC is high relative to the donor energy gap. However, we find the overall performance is limited by recombination, with formation of lower-lying triplet excitons on the donor accounting for 90% of the recombination. We find this is a bimolecular process that happens on time scales as short as 100 ps. Thus, although the absence of disorder and the associated high carrier mobility speeds up charge diffusion and extraction at the electrodes, which we measure as early as 1 ns, this also speeds up the recombination channel, giving overall a modest quantum yield of around 60%. We discuss strategies to remove the triplet exciton recombination channel.

Inhibition of myogenic differentiation by the H-ras oncogene is associated with the down regulation of the MyoD1 gene.

Skeletal muscle development is regulated by a complex series of genetic and environmental cues that control the establishment of the myogenic lineage and the differentiation of determined myoblasts. Numerous agents, including growth factors and oncogene products, have been shown to inhibit skeletal muscle development, possibly by affecting the pattern of signal transduction that is required for myogenesis. Among the eukaryotic G proteins that have been implicated as mediators of signal transduction are the protein products of the mammalian ras genes (p21s). In this study, we demonstrate that expression of a transfected, oncogenic, human H-ras gene in C3H10T1/2-derived myoblasts has dramatic, yet varied, effects on skeletal myogenesis. While some H-ras transformed myoblast clones are differentiation-defective, other clones are inhibited from morphologically differentiating but retain a limited ability to biochemically differentiate. The H-ras induced inhibition of differentiation usually is associated with a decreased expression of the myogenic determination gene, MyoD1. Introduction of a MyoD1 cDNA expression vector into differentiation-defective H-ras expressing myoblasts partially restores the myogenic potential in these cells. Our results suggest that activated H-ras p21 inhibits the terminal differentiation of myoblasts by producing a general reduction in the differentiation competence of cells which, in the most extreme case, is a consequence of the down-regulation of the MyoD1 determination gene.

Energy-cascade organic photovoltaic devices incorporating a host-guest architecture.

In planar heterojunction organic photovoltaic devices (OPVs), broad spectral coverage can be realized by incorporating multiple molecular absorbers in an energy-cascade architecture. Here, this approach is combined with a host-guest donor layer architecture previously shown to optimize exciton transport for the fluorescent organic semiconductor boron subphthalocyanine chloride (SubPc) when diluted in an optically transparent host. In order to maximize the absorption efficiency, energy-cascade OPVs that utilize both photoactive host and guest donor materials are examined using the pairing of SubPc and boron subnaphthalocyanine chloride (SubNc), respectively. In a planar heterojunction architecture, excitons generated on the SubPc host rapidly energy transfer to the SubNc guest, where they may migrate toward the dissociating, donor-acceptor interface. Overall, the incorporation of a photoactive host leads to a 13% enhancement in the short-circuit current density and a 20% enhancement in the power conversion efficiency relative to an optimized host-guest OPV combining SubNc with a nonabsorbing host. This work underscores the potential for further design refinements in planar heterojunction OPVs and demonstrates progress toward the effective separation of functionality between constituent OPV materials.

Directing energy transport in organic photovoltaic cells using interfacial exciton gates.

Exciton transport in organic semiconductors is a critical, mediating process in many optoelectronic devices. Often, the diffusive and subdiffusive nature of excitons in these systems can limit device performance, motivating the development of strategies to direct exciton transport. In this work, directed exciton transport is achieved with the incorporation of exciton permeable interfaces. These interfaces introduce a symmetry-breaking imbalance in exciton energy transfer, leading to directed motion. Despite their obvious utility for enhanced exciton harvesting in organic photovoltaic cells (OPVs), the emergent properties of these interfaces are as yet uncharacterized. Here, directed exciton transport is conclusively demonstrated in both dilute donor and energy-cascade OPVs where judicious optimization of the interface allows exciton transport to the donor-acceptor heterojunction to occur considerably faster than when relying on simple diffusion. Generalized systems incorporating multiple exciton permeable interfaces are also explored, demonstrating the ability to further harness this phenomenon and expeditiously direct exciton motion, overcoming the diffusive limit.

Cooperation of oncogenes in the multistep transformation of established fibroblasts in culture.

The multistage nature of carcinogenesis observed in a variety of systems has been linked experimentally to the sequential activation and subsequent cooperation of oncogene proteins. Cellular transformation resulting from cooperative interactions between activated oncogenes has been described previously for cultured primary cells and certain established cell lines. Our laboratory is using the mouse embryonic cell line C3H10T1/2 to investigate cooperative transformation mediated by the activated human H-ras gene and several nuclear oncogenes from both viral and cellular sources. Oncogene cooperation in C3H10T1/2 is marked by an increase in focus number and an alteration in focus morphology. Although ras cooperates with myc and fos oncogenes to transform C3H10T1/2 fibroblasts, cooperative transformation is not observed in cells similarly co-transfected with the H-ras and E1A oncogenes. This result appears to be due to a growth inhibitory effect of the EIA gene product on C3H10T1/2 cells. The observed cooperation between the H-ras and MC29 viral gag-myc oncogenes has been used along with site-directed mutagenesis of the gag-myc gene to identify two structural regions within the gag-myc protein that mediate ras/myc cooperativity. In addition, the gag-myc mutants generated for these cooperation studies have been used to map the nuclear localization signal of the gag-myc protein and relate the proper cellular location of the protein to its activity in several transformation assays.

Experimentally induced Mycobacterium avium serotype 8 infection in swine.

Five of 6 swine experimentally inoculated with Mycobacterium avium serotype 8 had microgranulomas in the cervical or mesenteric lymph nodes at necropsy 92 days later. In vivo tuberculin skin reactivity and in vitro lymphocyte immunostimulation responses were evaluated at 10 and 12 weeks after the pigs were inoculated. Positive responses were obtained on both tests in inoculated pigs, whereas test results in noninoculated pigs and pigs given killed bacterial cells were negative. Mycobacterium avium serotype 8 was isolated at necropsy from the cervical or mesenteric lymph nodes of each of the pigs inoculated with viable microorganisms and from the 2 pigs kept in the pen with inoculated swine. Mycobacteria were not isolated from tissues of the noninoculated swine or those given killed cells.

[Patient questionnaire for an epidemiologic cancer registry--results of an initial trial]. / Patientenfragebogen für ein epidemiologisches Krebsregister-Ergebnisse eines erstmaligen Einsatzes.

For the first time a patient questionnaire was employed for an epidemiologic cancer registry to complete special history data routinely. After a successful pretest in 1995 it is being employed in the region Weser-Ems (Lower Saxony) since March 1996. The quality criterion completeness increased compared to the previously used sheet for history data, which had be filled in by the doctor. The quota of missing questionnaires is 10% and may be lowered by efficient measures. The occupational data should be used critically for comparison with other data sources (e.g. Bundesanstalt für Arbeit [BfA]) or studies, because the code used by the BfA has been designed for other purposes and cannot be applied to investigations of occupational cancer risks. The geographical relation to cancer will be difficult to estimate because of the obvious mobility of the people.

[Evaluation of anamnestic data of cancer patients (comprehensiveness of the "Oldenburg Anamnesis Questionnaire")]. / Auswertung anamnestischer Daten von Krebspatienten (Auswertbarkeit des "Oldenburger Anamnesebogens").

An evaluation of case history data of cancer patients is presented. These data were recorded by means of a case history form in the cancer registry of Weser-Ems between 1988 and 1993 in the Weser-Ems region (Lower Saxony). Completeness and rate of errors were the quality criteria. The completeness was insufficient (23-92%), whereas the rate of errors was low (1%). In the course of years the completeness of the clinically relevant items increased, probably because the form had to be completed by the physician. About 99% of the cancer patients were of German nationality. The cause of registration was mainly the appearance of symptoms or a pathological finding while investigating for another reason. In respect of correlations between anamnestic data and carcinomas smoking or/and alcohol-abusus were significant. There was an association in men between these factors and carcinomas of the oropharynx and oesophagus, respectively of the lung; 85 resp. 75% of the men with theses carcinomas had smoked excersively and/or had abused alcohol. Data concerning the profession of the patients were irrelevant. Gynaecological ananestic data did not yield any new knowledge, which is why these and other items may be ignored. This resulted in a new anamnestic and patient questionnaire sheet were drawn up. These are now being tested in the Weser-Ems region since October 1995.