1,721,103 research outputs found
Nature of Charge Carriers in a High Electron Mobility Naphthalenediimide Based Semiconducting Copolymer
No full textThe nature of charge carriers in recently developed high mobility semiconducting donor-acceptor polymers is debated. Here, localization due to charge relaxation is investigated in a prototypal system, a good electron transporting naphthalenediimide based copolymer, by means of current-voltage I-V electrical characteristics and charge modulation spectroscopy (CMS) in top-gate field-effect transistors (FETs), combined with density functional theory (DFT) and time dependent DFT (TDDFT) calculations. In particular, pristine copolymer films are compared with films that underwent a melt-annealing process, the latter leading to a drastic change of the microstructure. Despite the packing modification, which involves also the channel region, both the electron mobility and the energy of polaronic transitions are substantially unchanged upon melt-annealing. The polaron absorption features can be rationalized and reproduced by TDDFT calculations for isolated charged oligomers. Therefore, it is concluded that in such a high electron mobility copolymer the charge transport process involves polaronic species which are intramolecular in nature and, from a more general point of view, that interchain delocalization of the polaron is not necessary to sustain charge mobilities in the 0.1 to 1 cm(2) V-1 s(-1) range. These findings contribute to the rationalization of the charge transport process in the recently developed class of donor-acceptor pi-conjugated copolymers featuring high charge mobilities and complex morphologies
Role of molecular thermodynamical processes at functionalized polymer/metaloxide interfaces for photovoltaics
No full textWe present a combined theoretical and experimental investigation of the role of pyridine-based functionalizers (IMs) at the hybrid titania (TiO 2)/poly(3-hexylthiophene) (P3HT) interface. We first used density functional theory to study the electronic structure and adsorption energy of several IMs, isolated and self-assembled, on a TiO2 anatase (101) surface. Small details in the molecular structure are found to induce strong differences in the morphology of the corresponding self-assembled monolayer. We then used model potential molecular dynamics simulations to study the translational and rotational diffusion of a P3HT oligomer on the naked and functionalized TiO2 surfaces. We correlate the photovoltaic performance measured on TiO2/IM/P3HT with the degree of order of the interface. In particular, we find that the thiol group of 4-mercaptopyridine is able to stabilize the corresponding self-assembled interlayer. This, in turn, increases the polymer mobility on the inorganic surface, yielding a larger polymer/substrate interface area
Role of the crystallization substrate on the photoluminescence properties of organolead mixed halides perovskites
No full textWe have fabricated CH3NH3PbI3-xCl x perovskite thin films crystallized in situ on substrates of different natures (e.g., porosity, wettability) and investigated their photoluminescence properties. We observe that the crystallization time and thin film structure are strongly influenced by the chemical nature and porosity of the substrate. Moreover, we find that the mesoporous scaffold can tune the emissive properties of the semiconducting compound both in terms of spectral region and dynamics. In particular, perovskite crystallites grown in the nanometre size porous scaffold present a shorter-living and blue-shifted emission with respect to the perovskite crystals which are free to grow without any constraints. © 2014 Author(s)
White light emission in low-dimensional perovskites
No full textLow-dimensional perovskites are rapidly emerging due to their distinctive emission properties, consisting of ultrabroad and highly Stokes shifted luminescence with pure white light chromaticity, which makes them very attractive for solution-processed light-emitting devices and scintillators. To foster the design of new materials and their device applications, it is timely to review the relation between perovskite structural properties and the photophysical phenomena underlying their unique light emission characteristics. From a number of recent studies, it has emerged that broadband emission properties in metal halide frameworks are very common, stemming from the self-localization of small polaron species at specific sites of the inorganic lattice, with a wide energy distribution. This review aims to provide an account of the current understanding of the photophysical processes underpinning luminescence broadening and highly efficient emission in various classes of low-dimensional metal-halide frameworks, and to highlight their potential for solution-processed optoelectronic device applications. The discussion will additionally establish a wider perspective on the role of intrinsic and extrinsic self-trapping, formation of polarons and their effect on charge generation and transport in low-dimensional perovskites
Phenotyping Key Fruit Quality Traits in Olive Using RGB Images and Back Propagation Neural Networks
Full text linkTo predict oil and phenol concentrations in olive fruit, the combination of back propagation neural networks (BPNNs) and contact-less plant phenotyping techniques was employed to retrieve RGB image-based digital proxies of oil and phenol concentrations. Fruits of cultivars (×3) differing in ripening time were sampled (∼10-day interval, ×2 years), pictured and analyzed for phenol and oil concentrations. Prior to this, fruit samples were pictured and images were segmented to extract the red (R), green (G), and blue (B) mean pixel values that were rearranged in 35 RGB-based colorimetric indexes. Three BPNNs were designed using as input variables (a) the original 35 RGB indexes, (b) the scores of principal components after a principal component analysis (PCA) pre-processing of those indexes, and (c) a reduced number (28) of the RGB indexes achieved after a sparse PCA. The results show that the predictions reached the highest mean R2 values ranging from 0.87 to 0.95 (oil) and from 0.81 to 0.90 (phenols) across the BPNNs. In addition to the R2, other performance metrics were calculated (root mean squared error and mean absolute error) and combined into a general performance indicator (GPI). The resulting rank of the GPI suggests that a BPNN with a specific topology might be designed for cultivars grouped according to their ripening period. The present study documented that an RGB-based image phenotyping can effectively predict key quality traits in olive fruit supporting the developing olive sector within a digital agriculture domain
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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