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Development of laser-based diffusion process in germanium for the manufacturing of low-cost thermophotovoltaic cells
No full textLa termofotovoltaica (TPV) se viene desarrollando desde finales de la década de 1950 para la conversión de energía térmica en eléctrica en una amplia gama de aplicaciones, como la producción combinada de calor y electricad, el almacenamiento de energía térmica o la recuperación de calor residual. A pesar de que los últimos avances, que han llevado a eficiencias de conversión de entre el 30-40%, la tecnología TPV sigue siendo cara. Esto se debe principalmente al elevado coste por unidad de superficie (€/cm2) de los semiconductores III-V y a los procesos de crecimiento epitaxial, que siguen siendo los que más contribuyen al coste total de los generadores TPV. El germanio -Ge- es considerado como una atractiva alternativa de bajo coste para el desarrollo de células TPV. Sin embargo, los trabajos anteriores sobre dispositivos TPV basados en Ge son escasos y su eficiencia récord sigue siendo relativamente baja (16,5%, lo que supone sólo el 27% de su límite termodinámico). Además, los dispositivos TPV de Ge más eficientes han sido desarrollados utilizando técnicas de alto coste como epitaxia metalorgánica en fase de vapor. Por tanto, hay una falta de dispositivos de alta eficiencia basados en Ge con técnicas de fabricación de bajo coste. Para abordar simultáneamente los retos de conseguir dispositivos TPV de bajo coste y alta eficiencia, el objetivo general de esta tesis doctoral es poner en marcha el desarrollo de un proceso de fabricación de bajo coste que sirva para la fabricación de células de contacto posterior interdigitado (IBC) de Ge. El diseño de célula IBC permite en teoría alcanzar mayores eficiencias de conversión con respecto a células TPV contactadas a doble cara, ya que elimina el compromiso entre la absorción de luz y las pérdidas óhmicas que sufren estas últimas. Además, la ausencia de electrodos frontales puede suponer una gran ventaja en el contexto de la TPV, donde la irradiancia incidente es mucho mayor que en los sistemas solares convencionales, lo que hace que la minimización de las pérdidas óhmicas sea de suma importancia. Esta tesis doctoral explora la fabricación de células TPV IBC mediante la difusión en estado líquido por irradiación con láser pulsado (PLM) en combinación con fuentes de dopantes sólidas. Esta combinación hace posible la apertura simultánea de capas dieléctricas y la creación de una región altamente dopada debajo, dejando el contacto listo para la metalización. De este modo, el número de pasos fotolitográficos se puede reducir en gran medida, permitiendo un proceso escalable de fabricación de bajo coste. El uso de PLM permite además un proceso flexible, al aire y con un bajo presupuesto térmico, lo que reduce el consumo de energía y los costes de procesamiento en comparación con el dopado basado en técnicas epitaxiales o de difusión en horno. Como primer paso hacia el desarrollo de dispositivos IBC-TPV en Ge se ha llevado a cabo un estudio de pasivación de la superficie de c-Ge basado en una limpieza con plasma de H2 in situ seguida del depósito de un stack de carburo de silicio amorfo por deposición química en fase vapor asistida por plasma. Se han obtenido resultados reseñables, alcanzando tiempos de vida efectivos de portadores minoritarios superiores a 600 μs y 1400 μs, para obleas de Ge tipo-n poco dopadas (1 cm) y Ge intrínseco, respectivamente, encontrándose estos resultados en el rango de los mejores alcanzados en el estado del arte para los mismos niveles de dopado. La estimación del límite superior de la velocidad de recombinación superficial en obleas de tipo-n, del orden de 14,5 cm/s, es otra prueba más de la buena calidad de la pasivación conseguida. Como segundo paso se ha desarrollado con éxito un método para el dopado de Ge tipo-n basado en PLM mediante irradiación con láser excímero a partir de dos fuentes alternativas de dopantes sólidas: disoluciones que contienen fósforo depósitadas por spinner (SoD-) y carburo de silicio amorfo dopado con fósforo (-PassDop-). El uso de SoD se ha evaluado en obleas de Ge con o sin capa de pasivación. El uso de SoD en obleas de Ge sin pasivar, ha resultado en niveles de dopante eléctricamente activo de casi el 100 % hasta concentraciones alrededor de 1 · 1019 cm-3, mediante la optimización de la fluencia del láser y el número de pulsos. Efectos perjudiciales para la fabricación de dispositivos debidos a posibles defectos inducidos por el láser parecen poder descartarse, ya que la movilidad hall, que oscila entre 346 y 463 cm2/V s, sigue los valores esperados para el nivel de activación conseguida. Por el contrario, el dopado a través de una capa pasivante de carburo de silicio, ya sea mediante SoD o PassDop, ha dado lugar a la creación de defectos inducidos por láser. Además, la concentración activa del dopante eléctrico es inferior a la química. No obstante, tanto la activación eléctrica como la calidad cristalina mejoran drásticamente cuando se aumenta el número de pulsos. Por lo tanto, la selección de las condiciones óptimas de parámetros láser ha demostrado ser más crítica cuando la irradiación láser se realiza en obleas pasivadas en comparación con las obleas sin pasivar. Un recrecimiento epitaxial efectivo del Ge puede verse obstaculizado a bajo número de pulsos en muestras pasivadas de Ge, debido principalmente a la elevada cantidad de Si y C incorporada procedente del stack de carburo de silicio, y/o a la incorporación de impurezas inicialmente en sitios intersticiales. Mediante la optimización de la fluencia del láser y el número de pulsos para las tres estrategias de dopado alternativas, se ha conseguido una dosis récord de P-eléctricamente activo de 1.5 · 1015 cm-2 para la tecnología PassDop a 750 mJ/cm2 y 100 pulsos, considerándose ésta como la fuente de dopante más prometedora para la fabricación de dispositivos TPV de Ge-IBC. Como paso final hacia la fabricación de células Ge-IBC, se ha investigado la formación de contactos locales selectivos de electrones y huecos usando los métodos PassDop y de contacto por láser firing a través de dieléctricos (LFC), respectivamente. Los experimentos se han realizado utilizando un sistema láser de Nd:YVO4 que emite a una longitud de onda de 355 nm con un spot de laser gaussiano de 6 μm. Se han investigado tanto los contactos selectivos de huecos como de electrones variando las condiciones del proceso láser (potencia y número de pulsos). El comportamiento eléctrico de ambos tipos de contactos se ha evaluado mediante la medición de curvas IV en oscuridad. En cuanto a los contactos selectivos de electrones sobre Ge tipo-p, se ha conseguido un comportamiento rectificante en todos los diodos fabricados con condiciones de potencia láser entre 3 y 22 mW y entre 3 y 100 pulsos, independientemente del espesor de SiC (45 y 180 nm) y del contacto metálico (PdTiPdAg o Al). Sin embargo, el comportamiento del diodo dista mucho de ser ideal, con problemas de shunting revelados a partir del ajuste de la curva IV en oscuridad. El uso de condiciones de baja potencia y alto número de pulsos (es decir, 3 mW y 64-100 pulsos) parecen mejorar las características del diodo en términos de recombinación y shunting para todos los conjuntos de experimentos. Sin embargo, la mejora se considera más bien moderada, por lo que es necesaria la implementación de nuevas estrategias para mejorar el comportamiento de los dispositivos. La formación de cracks o regiones no dopadas, o ligeramente dopadas, en el borde del spot de láser gaussiano se encuentran entre los motivos más plausibles para inducir los problemas de shunting observados durante el proceso posterior de metalización. En cuanto a la formación de contactos selectivos de huecos, se ha demostrado la formación de contactos óhmicos, independientemente del espesor de Al (1 y 2 μm), lo que sugiere que el dopado tipo-p de la capa de Al es capaz de sobrecompensar el dopado tipo-n proveniente de la capa de carburo de silicio inferior, creando con éxito contactos óhmicos con una resistencia específica de contacto mínima del orden de 2 · 10−4 cm2. El daño inducido por el láser debe analizarse todavía, ya que hay que alcanzar un compromiso entre la resistencia eléctrica y el daño inducido por el láser para establecer las condiciones óptimas de proceso. Los resultados de este trabajo pueden verse como una valiosa guía para afrontar el proceso de fabricación de células TPV de Ge-IBC de alta eficiencia y bajo coste. ----------ABSTRACT---------- Thermophotovoltaics (TPV) has been developed since the late 1950s for thermal-to-electric energy conversion in a wide range of applications, including combined heat and power, thermal energy storage, or waste-heat recovery. Despite recent developments that have led to conversion efficiencies in the range of 30-40%, TPV power is still expensive. This is mostly due to the high cost per unit area (€/cm2) associated with the use of III-V semiconductors and epitaxial processes, which remain the largest contributors to the total cost of a TPV generator. Germanium –Gehas been historically considered an appealing low-cost alternative for the development of TPV cells. However, previous works on Ge-based TPV devices are scarce and its record efficiency is still very low (i.e. 16.5%, which corresponds only to the 27% of its upper thermodynamic limit). Besides, the most efficient Ge TPV devices were developed using high-capital cost techniques such as metalorganic vapor phase epitaxy. Thus, the fabrication of high-efficiency Ge-based devices based on low-cost fabrication techniques is still missing. To address simultaneously the challenges of achieving low-cost and high-efficient TPV devices, the overall goal of this Ph.D. thesis is to kick-start the development of a low-cost cell manufacturing process toward the fabrication of interdigitated back contact (IBC)-Ge TPV cells. IBC cell designs theoretically allow higher conversion efficiencies than double-side TPV cells, since it gets rid of the trade-off between light absorption and ohmic losses that suffers the latter. The lack of front electrodes could entail a major advantage in the context of TPV, where the incident irradiance is much higher than for conventional solar-PV systems, making the minimization of ohmic losses of utmost importance. This Ph.D. thesis explores the fabrication of IBC TPV cells by means of pulsed laser melting (PLM) in combination with solid-dopant sources. This combination makes possible the simultaneous opening of the dielectric insulating layer and the creation of a heavily doped region underneath, leaving the contact ready for metallization. In this way, the number of photolithographic steps could be greatly reduced eventually enabling a low-cost and scalable manufacturing process. The use of PLM also enables a flexible, ambient air process with a low thermal budget, reducing energy consumption and processing costs as compared with doping based on furnace-diffusion or epitaxial techniques. As a first step toward the development of IBC Ge-based TPV devices, a study of c-Ge surface passivation based on an in situ H2 plasma cleaning followed by the deposition of an amorphous silicon carbide stack by plasma-enhanced chemical vapor deposition has been conducted. Promising results have been obtained, reaching effective minority carrier lifetimes higher than 600 μs and 1400 μs, for lowly doped n-type c-Ge (1 cm) and intrinsic c-Ge wafers, respectively, being these results in the range of the best ones achieved in the state-of-the-art for the same doping level. The estimation of the upper limit for the surface recombination velocity (SRV) in n-type wafers, indicates maximum SRV in the order of 14.5 cm/s, further confirming the high quality of the passivation. As a second step, a method for n-type doping of c-Ge has been developed based on pulsed laser melting by excimer laser irradiation based on two alternative solid dopant sources: phosphoruscontaining Spin on Dopant Solutions (-SoD-) and phosphorus-doped amorphous silicon carbide (-PassDop technology-). The use of SoD has been evaluated on bare-Ge and passivated-Ge wafers. The use of SoD on bare c-Ge wafers has led to electrical doping activation of nearly 100% up to a concentration of around 1 · 1019 cm-3, by optimization of the laser energy fluence and the number of pulses. Relevant detrimental effects for device fabrication due to laser-induced defects seem to be effectively prevented, since hall mobility, which ranges between 346 and 463 cm2/V s, follows the expected values for the level of activation achieved. In contrast, doping through a passivating silicon carbide layer, either using SoD or PassDop techniques, has led to laser-induced defects. Moreover, the electrically dopant active concentration was found to be lower than the chemical one. Nonetheless, both, electrical activation and crystalline quality drastically improve with the number of pulses. Thus, careful selection of the optimum laser conditions has been demonstrated to be more critical when laser irradiation is performed on passivated wafers as compared with bare wafers. An effective liquid phase epitaxial regrowth can be hindered at a low number of pulses for Ge-passivated samples, due to the high amount of Si and C incorporated during laser processing coming from the SiC stack, and/or to the incorporation of impurities initially at interstitial sites. By optimizing laser energy fluence and the number of pulses for the three alternative doping strategies, a record P-electrically active dose of 1.5 · 1015 cm-2 has been achieved for PassDop technique at 750 mJ/cm2 and 100 pulses, which has been considered as the most promising dopant source for the fabrication of Ge-IBC TPV devices. As a final step towards Ge-IBC TPV cell fabrication the formation of local electron- and holeselective contacts on p-type c-Ge has been investigated based on PassDop and laser-fired contact (LFC) methods, respectively. Experiments have been conducted using a Nd:YVO4 laser source emitting at 355 nm and with a gaussian spot of 6 μm. Both electron- and hole-selective contacts on p-type Ge have been investigated by varying the laser process conditions (power and number of pulses). The electrical behavior of both kinds of contacts has been evaluated through the measurement of I-V curves under dark conditions. Regarding electron-selective contacts on p-Ge, rectifying I-V characteristics has been achieved for all samples fabricated with laser power conditions between 3 to 22 mW and 3 to 100 pulses, regardless of the SiC thickness (45 and 180 nm) and metal contact (PdTiPdAg or Al). However, diode behavior is far from ideal, with shunting issues revealed by the fitting of the dark JV curve. Low laser power conditions and a high number of pulses (i.e., 3 mW and 64-100 pulses) seems to improve diode characteristics in terms of recombination and shunting for all set of experiments. However, the improvement is considered moderate, and new strategies to improve device performance are needed. It is believed that the formation of cracks and non-doped/slightly-doped regions at the edge of the gaussian laser spot are the most likely reasons for shunting during post-metallization. Regarding hole selective contacts, ohmic contact formation has been demonstrated, regardless of the Al thickness tested (1 and 2 μm) suggesting that p-type doping from the Al layer overcompensates n-type dopants from the underneath SiC stack, successfully creating ohmic contacts with minimum specific contact resistance in the order of 2 · 10−4 cm2. Laser-induced damage should still be analyzed since a trade-off between electrical resistance and laser-induced damage must be considered in order to determine optimum processing conditions. The results of this work could be a valuable guide to face the manufacturing process of costeffective highly-efficient Ge-IBC TPV cells
Development of solar thermophotovoltaic systems = Desarrollo de sistemas termofotovoltaicos solares
No full textEsta tesis aborda el análisis, tanto teórico como experimental, de los sitemas termofotovoltaicos solares. En estos sistemas, un material (emisor) se calienta hasta la incandescencia mediante radiaci ón solar. La radiación térmica emitida por dicho material se dirige hacia una célula fotovoltaica, que convierte dicha radiación en electricidad. En esta configuración, se pueden emplear elementos de control espectral para lograr que los fotones no útiles para el proceso de conversión fotovoltáica sean devueltos al emisor. En la primera parte de la tesis (capítulos 2-4) se presentan diversos modelos analíticos de sistemas termofofovoltaicos solares. En el capítulo 2 se analiza un sistema ideal con el fin de determinar la eficiencia límite máxima, que resulta ser del 85.4%. Desafortunadamente, dicha eficiencia sólo se logra para densidades de potencia eléctrica despreciables y bajo una serie de condiciones demasiado restrictivas. En el capítulo 3 se presenta un modelado multivariable más completo, basado en la teoría del balance detallado, en el que se contemplan (entre otras cosas) posibles pérdidas ópticas. Con dicho modelo se ha demostrado que se pueden alcanzar eficiencias y densidades de potencia eléctrica por encima de 30% y 50 W/cm2, repectivamente. Para ello se requiere de una elevada concentración solar (1000-2000 soles) y de un método para aumentar la transferencia de energía del emisor a las células (típicamente, mediante el aumento del área del emisor respecto al área del absorbente solar). Finalmente, en el capítulo 4 se presenta un modelo más realista de un sistema con geometría cilíndrica. Con esta configuración, se ha demostrado que se pueden lograr eficiencias y densidades de potencia electrica de 10-12% y 1-3 We/cm2 empleando componentes disponibles actualmente (células de germanio con reflectores traseros y emisores de tungsteno recubiertos con capas antireflectantes). En los capítulos 5-8 se describe el desarrollo y caraceterización de un sistema termofotovoltaico solar completo, que consta de un concentrador solar refractivo, un emisor cilíndrico de tungsteno recubierto con HfO2 y un array de células de Germanio. En esta tesis se presta especial atención a la carecterización del sistema de concentración y de los emisores (capítulo 6), a los procesos de fabricación de los arrays (capítulo 7) y a la caracterización del sistema completo en condiciones reales de operación (capítulo 8). Este sistema ha proporcionado densidades de corriente en las células de hasta 0.95 A/cm2, densidades de potencia eléctrica de 120-140 mW/cm2 y una eficiencia de conversión del 0.8%. La temperatura estimada de los emisores está en el rango de 1300-1500oC. La baja eficiencia se debe principalmente al calentamiento excesivo de las células (hasta 120oC) y a las pérdidas ópticas en el concentrador, que impiden alcanzar la temperatura de emisor óptima. Se ha estimado que mejorando ambos aspectos se podrían lograr eficiencias superiores al 5%. ABSTRACT This thesis presents a theoretical (Part I) and experimental (Part II) analysis of solar thermophotovoltaic systems. In these systems an intermediate material (emitter) is heated to incandescence by solar radiation. The thermal radiation emitted by that material is directed to a photovoltaic cell that converts this radiation into electricity. In this configuration, the photons that are not efficiently used by the cells can be redirected to the emitter using spectral control elements. The first part of the Thesis (chapters 2-4) present various analytical models of solar thermophotovoltaic systems. In chapter 2 an ideal system is analyzed and the maximum efficiency limit is determined, which turns out to be 85:4%. Unfortunately, such efficiency is achieved for negligible electric power densities and under severe restrictive conditions. Chapter 3 presents a more complete multivariable model, based on the detailed balance theory, which contemplates (among other things) possible optical losses. The model shows that efficiencies above 30% and electric power densities above 50 W/cm2 are achievable. This requires high solar concentration (above 1000 suns) and a method to enhance the energy transfer from the emitter to the cells (typically, by increasing the emitter area to the solar absorber area ratio). Finally, chapter 4 presents a more realistic model of a system with cylindrical geometry. With this configuration it was demonstrated that efficiencies of 10-15 % and electric power densities of 1-5 W/cm2 can be achieved using currently available components (Germanium cells with back-side reflectors and tungsten emitters coated with anti-reflective coatings). Chapters 5-8 present the development and characterization of a complete solar thermophotovoltaic system, consisting of a refractive solar concentrator, a cylindrical tungsten emitter coated with HfO2 and an array of germanium cells. In this thesis, special attention is paid to the characterization of the concentrator system and the emitters (chapter 6), to the manufacturing processes of the cell-modules (chapter 7) and to the characterization of the complete system under real operating conditions (chapter 8). This system has provided cell short-circuit current densities of 0.95 A/cm2, electric power densities of 120-140 mW/cm2 and a conversion efficiency of 0.8 %. The estimated temperature of the emitter is in the range of 1300-1500_C. The low efficiency is mainly due to overheating of the cells (up to 120oC) and to optical losses in the concentrator, which prevent the achievement of the optimum emitter temperature. It has been estimated that an improvement in both of these aspects would allow the achievement of efficiencies above 5%. The improvement of the photon recycling process between the emitter and the cells could lead to efficiencies close to 10%
Energía termofotovoltaica: la conversión fotovoltaica del calor y su aplicación en sistemas de almacenamiento de energía
No full textDel mismo modo que una célula solar convierte la luz del sol en electricidad, una célula termofotovoltaica convierte en electricidad la radiación térmica que emiten objetos incandescentes. Es decir, realizan una conversión directa del calor en electricidad, sin necesidad de emplear partes móviles ni fluidos. Entre las muchas aplicaciones de esta tecnología, en el Instituto de Energía Solar de la UPM estamos trabajando en un nuevo concepto de almacenamiento de energía térmica que utiliza silicio fundido, a unos 1400°C y células termofotovoltaicas para trasformar el calor almacenado en electricidad. De esta forma, es posible alcanzar densidades de energía de más de 1 MWh por metro cúbico, una de las mayores de entre todas las tecnologías de almacenamiento existentes
Optimum semiconductor bandgaps in single junction and multijunction thermophotovoltaic converters
Get PDFThe choice of the optimum semiconductor for manufacturing thermophotovoltaic (TPV) cells is not straightforward. In contrast to conventional solar photovoltaics (PV) where the optimum semiconductor bandgap is determined solely by the spectrum (and eventually the irradiance) of the incident solar light, in a TPV converter it depends on the emitter temperature and on the spectral control elements determining the net spectral power flux between the TPV cell and the emitter. Additionally, in TPV converters there is a tradeoff between power density and conversion efficiency that does not exist in conventional solar PV systems. Thus, the choice of the proper semiconductor compound in TPV converters requires a thorough analysis that has not been presented so far. This paper presents the optimum semiconductor bandgaps leading to the maximum efficiency and power density in TPV converters using both single junction and multijunction TPV cells. These results were obtained within the framework of the detailed balance theory and assuming only radiative recombination. Optimal bandgaps are provided as a function of the emitter and cell temperature, as well as the degree of spectral control. I show that multijunction TPV cells are excellent candidates to maximize both the efficiency and the power density simultaneously, eliminating the historical tradeoff between efficiency and power density of TPV converters. Finally, multijunction TPV cells are less sensitive to photon recycling losses, which suggest that they can be combined with relatively simple cut-off spectral control systems to provide practically-viable high performing TPV devices
Hybrid thermionic-photovoltaic converter
No full textA conceptual device for the direct conversion of heat into electricity is presented. This concept hybridizes thermionic (TI) and thermophotovoltaic (TPV) energy conversion in a single thermionic-photovoltaic (TIPV) solid-state device. This device transforms into electricity both the electron and photon fluxes emitted by an incandescent surface. This letter presents an idealized analysis of this device in order to determine its theoretical potential. According to this analysis, the key advantage of this converter, with respect to either TPV or TI, is the higher power density in an extended temperature range. For low temperatures, TIPV performs like TPV due to the negligible electron flux. On the contrary, for high temperatures, TIPV performs like TI due to the great enhancement of the electron flux, which overshadows the photon flux contribution. At the intermediate temperatures, 1650K in the case of this particular study, I show that the power density potential of TIPV converter is twice as great as that of TPV and TI. The greatest impact concerns applications in which the temperature varies in a relatively wide range, for which averaged power density enhancement above 500% is attainable. (C) 2016 AIP Publishing LLC
Thermionic-enhanced near-field thermophotovoltaics
No full textSolid-state heat-to-electrical power converters are thermodynamic engines that use fundamental particles, such as electrons or photons, as working fluids. Virtually all commercially available devices are thermoelectric generators, in which electrons flow through a solid driven by a temperature difference. Thermophotovoltaics and thermionics are highly efficient alternatives relying on the direct emission of photons and electrons. However, the low energy flux carried by the emitted particles significantly limits their generated electrical power density potential. Creating nanoscale vacuum gaps between the emitter and the receiver in thermionic and thermophotovoltaic devices enables a significant enhancement of the electron and photon energy fluxes, respectively, which in turn results in an increase of the generated electrical power density. Here we propose a thermionic-enhanced near-field thermophotovoltaic device that exploits the simultaneous emission of photons and electrons through nanoscale vacuum gaps. We present the theoretical analysis of a device in which photons and electrons travel from a hot LaB6-coated tungsten emitter to a closely spaced BaF2-coated InGaAs photovoltaic cell. Photon tunnelling and space charge removal across the nanoscale vacuum gap produce a drastic increase in flux of electrons and photons, and subsequently, of the generated electrical power density. We show that conversion efficiencies and electrical power densities of ∼ 30% and ∼ 70 W/cm2 are achievable at 2000 K for a practicable gap distance of 100 nm, and thus greatly enhance the performances of stand-alone near-field thermophotovoltaic devices (∼10% and ∼10 W/cm2). A key practical advantage of this nanoscale energy conversion device is the use of grid-less cell designs, eliminating the issue of series resistance and shadowing losses, which are unavoidable in conventional near-field thermophotovoltaic devices
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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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