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Effects of solvent additive on "s-shaped" curves in solution-processed small molecule solar cells
(2016)
A novel molecular chromophore, p-SIDT(FBTThCA8)(2), is introduced as an electron-donor material for bulk heterojunction (BHJ) solar cells with broad absorption and near ideal energy levels for the use in combination with common acceptor materials. It is found that films cast from chlorobenzene yield devices with strongly s-shaped current-voltage curves, drastically limiting performance. We find that addition of the common solvent additive diiodooctane, in addition to facilitating crystallization, leads to improved vertical phase separation. This yields much better performing devices, with improved curve shape, demonstrating the importance of morphology control in BHJ devices and improving the understanding of the role of solvent additives.
Experimenting with Lurchi
(2019)
In organic solar cells, the resulting device efficiency depends strongly on the local morphology and intermolecular interactions of the blend film. Optical spectroscopy was used to identify the spectral signatures of interacting chromophores in blend films of the donor polymer PM6 with two state-of-the-art nonfullerene acceptors, Y6 and N4, which differ merely in the branching point of the side chain. From temperature-dependent absorption and luminescence spectroscopy in solution, it is inferred that both acceptor materials form two types of aggregates that differ in their interaction energy. Y6 forms an aggregate with a predominant J-type character in solution, while for N4 molecules the interaction is predominantly in a H-like manner in solution and freshly spin-cast film, yet the molecules reorient with respect to each other with time or thermal annealing to adopt a more J-type interaction. The different aggregation behavior of the acceptor materials is also reflected in the blend films and accounts for the different solar cell efficiencies reported with the two blends.
The authors present efficient all-polymer solar cells comprising two different low-bandgap naphthalenediimide (NDI)-based copolymers as acceptors and regioregular P3HT as the donor. It is shown that these naphthalene copolymers have a strong tendency to preaggregate in specific organic solvents, and that preaggregation can be completely suppressed when using suitable solvents with large and highly polarizable aromatic cores. Organic solar cells prepared from such nonaggregated polymer solutions show dramatically increased power conversion efficiencies of up to 1.4%, which is mainly due to a large increase of the short circuit current. In addition, optimized solar cells show remarkable high fill factors of up to 70%. The analysis of the blend absorbance spectra reveals a surprising anticorrelation between the degree of polymer aggregation in the solid P3HT:NDI copolymer blends and their photovoltaic performance. Scanning near-field optical microscopy (SNOM) and atomic force microscopy (AFM) measurements reveal important information on the blend morphology. It is shown that films with high degree of aggregation and low photocurrents exhibit large-scale phase-separation into rather pure donor and acceptor domains. It is proposed that, by suppressing the aggregation of NDI copolymers at the early stage of film formation, the intermixing of the donor and acceptor component is improved, thereby allowing efficient harvesting of photogenerated excitons at the donoracceptor heterojunction.
The macroscale function of multicomponent polymeric materials is dependent on their phase-morphology. Here, we investigate the morphological structure of a multiblock copolymer consisting of poly(L-lactide) and poly(epsilon-caprolactone) segments (PLLA-PCL), physically cross-linked by stereocomplexation with a low molecular weight poly(D-lactide) oligomer (PDLA). The effects of blend composition and PLLA-PCL molecular structure on the morphology are elucidated by AFM, TEM and SAXS. We identify the formation of a lattice pattern, composed of PLA domains within a PCL matrix, with an average domain spacing d0 = 12 - 19 nm. The size of the PLA domains were found to be proportional to the block length of the PCL segment of the copolymer and inversely proportional to the PDLA content of the blend. Changing the PLLA-PCL / PDLA ratio caused a shift in the melt transition Tm attributed to the PLA stereocomplex crystallites, indicating partial amorphous phase dilution of the PLA and PCL components within the semicrystalline material. By elucidating the phase structure and thermal character of multifunctional PLLA-PCL / PDLA blends, we illustrate how composition affects the internal structure and thermal properties of multicomponent polymeric materials. This study should facilitate the more effective incorporation of a variety of polymeric structural units capable of stimuli responsive phase transitions, where an understanding the phase-morphology of each component will enable the production of multifunctional soft-actuators with enhanced performance.
The genus Vanda and its affiliated taxa are a diverse group of horticulturally important species of orchids occurring mainly in South-East Asia, for which generic limits are poorly defined. Here, we present a molecular study using sequence data from three plastid DNA regions. It is shown that Vanda s.l. forms a clade containing approximately 73 species, including the previously accepted genera Ascocentrum, Euanthe, Christensonia, Neofinetia and Trudelia, and the species Aerides flabellata. Resolution of the phylogenetic relationships of species in Vanda s.l. is relatively poor, but existing morphological classifications for Vanda are incongruent with the results produced. Some novel species relationships are revealed, and a new morphological sectional classification is proposed based on support for these groupings and corresponding morphological characters shared by taxa and their geographical distributions. The putative occurrence of multiple pollination syndromes in this group of taxa, combined with complex biogeographical history of the South-East Asian region, is discussed in the context of these results.(c) 2013 The Linnean Society of London, Botanical Journal of the Linnean Society, 2013, 173, 549-572.
The present work investigated how morphological generalization, namely the way speakers extend their knowledge to novel complex words, is influenced by sources of variability in language and speaker properties. For this purpose, the study focused on a Semitic language (Hebrew), characterized by unique non-concatenative morphology, and native ( L1) as well as non-native (L2) speakers. Two elicited production tasks tested what information sources speakers employ in verbal inflectional class generalization, i.e., in forming complex novel verbs. Phonological similarity was tested in Experiment 1 and argument structure in Experiment 2. The analysis focused on the two most common Hebrew inflectional classes, Paal and Piel, which also constituted the vast majority of responses in the two tasks. Unlike the commonly found outcomes in Romance inflectional class generalization, the results yielded, solely for Piel, a graded phonological similarity effect and a robust argument structure effect, i.e., more Piel responses in a direct object context than without. The L2 pattern partially differed from the L1: (i) argument structure effect for L2 speakers was weaker, and (ii) L2 speakers produced more Paal than Piel responses. The results are discussed within the framework of rule-based and input-based accounts.