1,721,590 research outputs found
Understanding exciton diffusion in organic solar cells
No full textOrganic solar cells are promising candidates for future energy production. However,
many challenges remain to optimise efficiency. One particular challenge lies in our understanding of the factors affecting exciton diffusion and the length scale of this process. This is particularly relevant to morphology optimisation for organic solar
cells. A powerful tool to gain insight into this is femtosecond photophysical measurements.
This thesis details work undertaken at the University of St Andrews between
September 2009 and December 2010 towards a Masters of Philosophy. It includes
an introduction to the field and a description of experimental methods used.
Experimental chapters discuss measuring the optical constants of P3HT, a study of
low concentration volume quenching, surface quenching measurements using
titanium dioxide, surface quenching measurements using other quenchers and an
investigation of the thickness dependence of the photoluminescence decay time in
thin polymer films
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
Photophysics of linear and star-shaped oligofluorenes and their application in lasers
No full textThis thesis presents a study of the photophysical properties of a number of fluorene molecules used for organic semiconductor lasers. These results are then combined with lasing results to assess what the important properties in an organic semiconductor laser material are.
Photophysical measurements were performed on a family of oligofluorenes; results show a redshift in the peak absorption and emission wavelengths with increasing length. There is also an increase in the molar extinction coefficient and photoluminescence quantum yields of the molecules. Transition dipole moments also increase with length, but fluorescence scales slower than absorption due to self-trapping occurring at longer molecular lengths.
This study was then expanded to two families of star-shaped molecules with fluorene arms and differing cores. These molecules have three arms connected to either a central benzene unit or a larger truxene core. These molecules show an increase in PLQY and roughly three times higher molar extinction coefficients than comparable linear oligofluorenes. The star-shaped molecules PLQY and transition dipole moments are both greater than their linear oligofluorene counterparts.
Energy transfer was then studied in the truxene-cored molecules, which showed that the symmetry of the molecule was broken due to interactions with the solvent. Energy transfer was observed on two timescales; a fast 500 fs process which is attributed to a localisation onto a single arm to emit, and a 3-10 ps second decay component, and was assigned to resonant energy transfer between the arms. Both decays were found to be wavelength dependent.
Lasing results were then obtained for the benzene cored molecules. It was found that star-shaped molecules present improved lasing characteristics with lower ASE and lasing thresholds. These results were compared with those obtained for truxene-cored molecules whose rigid core provides them with better lasing and ASE characteristics
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
Material and device design for organic optoelectronics
No full textThis thesis describes investigations into the photophysical properties of luminescent
materials and their application in optoelectronic devices such as light emitting diodes
and photodetectors. The materials used were all solution processable because of the
interest in low cost processing of organics.
I have investigated the photophysics of 1,4,5,8,9,12-hexamethyltriphenylene, a
triphenylene derivative which has its luminescence enhanced by the addition of
methyl groups. These groups change the planar shape of the triphenylene molecule
into a twisted one, changing the symmetry of the molecule and increasing its dipole
moment in absorption and emission by ~4 fold. This increased its rate of radiative deexcitation
by ~20 times. In addition, the twisted shape of the molecule prevents
intermolecular interactions and concentration effects from affecting the luminescence.
This results in an efficient solid-state photoluminescence quantum yield of 31%.
This thesis also includes an investigation into phosphorescent polymer dendrimers,
designed to have suitable viscosities in solution for inkjet printed OLED applications.
A photophysical study of the intra-chain aggregation effects on the luminescence was
undertaken in both homopolymers and copolymers with high energy gap spacer units.
Using double dendrons to increase the steric protection of the luminescent cores, the
best homopolymers achieved 12.1% external quantum efficiency (39.3 cd/A) at 100
cd/m² brightness and the best co-polymer achieved 14.7% EQE (48.3 cd/A) at 100
cd/m². This compares favourably with 11.8% EQE for the best phosphorescent
polymer and 16% for the best solution processed dendrimer OLED previously
reported.
Finally I have applied a solution processed enhancement layer to silicon photodiodes
to enhance their ultraviolet response. Using a blend of materials to give favourable
absorption and emission properties, 61% external quantum efficiency was achieved at
200 nm, which is better than the 20-30% typical for vacuum deposited lumogen
enhancement layers used commercially
Material and device design for organic photovoltaics
Full text linkThis thesis presents novel materials for photovoltaic conversion. The materials described are solution-processable organic semiconductors and have been used in the fabrication of organic photovoltaic cells (OPVs).
The widely used PEDOT:PSS layer was investigated in P3HT and PTB7 photovoltaics. By doping, the efficiencies recorded were amongst the highest reported in the field using a conventional architecture.
Two low band-gap BODIPY-based polymers were introduced and shown to have properties favourable for optoelectronics. Photovoltaics consisting solely of the polymers as the active component surpassed the performance expected without the use of an acceptor, indicating ambipolar behaviour, which was verified by charge carrier mobility measurements. When blended with an acceptor, the devices demonstrated a short-circuit current density similar to that of P3HT, a well-studied and successful OPV material. They also revealed a broad spectral response and were shown to operate as photodiodes.
Two small molecules containing diketopyrrolopyrrole (DPP) and BODIPY were introduced and characterised. The addition of thiophenes red shifted the absorption but did not result in a sufficient bathochromic shift. Instead, a propensity to aggregate limited the performance. PLQY measurements showed the aggregation to quench luminescence. The study demonstrated the importance of controlling aggregation for efficient devices.
Two solution-processable small molecules with a germanium-bridged spiro centre were investigated, and the molecular, electrochemical and optical properties discussed. The small molecule with shorter conjugation length exhibited an interesting packing motif shown to be favourable for charge transport. The mobility measurements were an order of magnitude higher than those reported for sexithiophene, a small molecule analogue, and the same order of magnitude as P3HT. The two-dimensional charge transporting nature of the material was verified with two independent techniques: time of flight (TOF) and organic field-effect transistor (OFET) measurements. The mobility of the material was found to vary with annealing, a result of morphological changes. These were studied with optical, electron and scanning probe microscopies. By controlling the morphology with the implementation of a well-defined annealing method, it was possible to improve the performance of OFETs and planar-heterojunction OPVs. Solution-processed bulk-heterojunction OPVs were fabricated, characterised and optimised with Ge spiro molecules. A PCE similar to that of P3HT, 2.66 %, was achieved for the one, whilst a PCE of 1.60 % was obtained for the other. The results are encouraging, and there is scope for improvement by increasing the overlap between the absorption and solar spectrum, for example
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