Geometry Optimization for Miniaturized Thermoelectric Generators

Thermoelectric materials capable of converting heat into electrical energy are used in sustainable electric generators, whose efficiency has been normally increased with incorporation of new materials with high figure of merit (ZT) values. Because the performance of these thermoelectric generators (TEGs) also depends on device geometry, in this study we employ the finite element method to determine optimized geometries for highly efficient miniaturized TEGs. We investigated devices with similar fill factors but with different thermoelectric leg geometries (filled and hollow). Our results show that devices with legs of hollow geometry are more efficient than those with filled geometry for the same length and cross-sectional area of thermoelectric legs. This behavior was observed for thermoelectric leg lengths smaller than 0.1 mm, where the leg shape causes a significant difference in temperature distribution along the device. It was found that for reaching highly efficient miniaturized TEGs, one has to consider the leg geometry in addition to the thermal conductivity

Improving the electroluminescence of [Zn(salophen)(OH2)] in polyfluorene-based light-emitting diode: the role of energy transfer and charge recombination

Combining two or more different compounds with advantageous properties has been a useful and straightforward strategy in achieving a new class of materials with improved physical properties. This has been especially true for electronic polymers, whose optoelectronic properties can be completely tuned, and even improved, when mixed with other polymeric materials, dye molecules and guest coordination compounds. Here, a light-emitting diode prepared with the conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(9,9-di-{5'-pentanyl}-fluorenyl-2,7-diyl)] (PFOFPen) as the host material and aquo[N,N'-bis(salicylidene)-o-phenylenediamine]zinc(II) ([Zn(salophen)(OH2)]) as the guest molecule was studied in terms of its photo and electroluminescence properties. The role of the ZnII coordination compound as a guest in the electroluminescence is discussed as a strategy for the improvement of the electroluminescence performance of coordination compounds using conjugated polymers as matrices. An additional advantage of these composites is that they are solution processable, a low-cost and time efficient alternative to vacuum vapor deposition. Additionally, the photophysical processes involved in both electroluminescence and photoluminescence emissions are discussed because they are markedly different.Combining two or more different compounds with advantageous properties has been a useful and straightforward strategy in achieving a new class of materials with improved physical properties. This has been especially true for electronic polymers, whose opt272295302FAPESP - FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULOCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO2013/16245-2470529/2012-1The authors acknowledge FAPESP (2013/16245-2), CNPq (470529/2012-1), National Institute of Organic Electronics (INEO) (MCT/CNPq/FAPESP) and UNICAMP/FAEPEX for financial support and fellowship

Polyfluorene based blends for white light emission

Single layer white light emitting diodes were constructed with blends containing as blue emitter and matrix host poly(9,9-dihexyl-2,7-fluorene) (LaPPS10), and the green and red emitters poly[(9,9-dihexyl-9H-fluorene-2,7-diyl)-1,2-ethenediyl-1,4-phenylene-1,2-ethenediyl] (LaPPS16) and poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), respectively, as guests components. Two blends were studied: LaPPS10:LaPPS16:MEH-PPV = 100:0.01:0.20 (w/w) (JF14) and another with the same components and a second blue emitter: a copolymer of poly(methyl methacrylate-co-methyl antracenyl methacrylate), P(MMA-co-MMAnt), with the composition LaPPS10:P(MMA-co-MMAnt):LaPPS16:MEH-PPV = 100:40:0.01:0.20 (w/w) (JF17). The PL spectra of JF14 and JF17 in the higher energy edge are characteristic of the matrix (LaPPS 10). The morphological characterization showed the formation of a biphasic structure with a matrix of LaPPS10 and segregated domains of MEH-PPV in the case of JF14 and of P(MMA-co-MMAnt) in the case of JF17. Significant differences between EL and PL spectra were observed for the blends indicating that the mechanism for the excited states or quenching processes are very different in both cases. In addition, the EL emissions suggested that the cascade mechanism for the charge migration or charge recombination or energy transfer processes are also incomplete allowing more than one type of emission. EL spectra were red-shifted in relation to PL, which was partly attributed to the trapping of energy carriers that may occur in EL, apart from energy transfer. The CIE coordinates for EL emission of JF14 blend the coordinates are (0.30, 0.32) and for JF17 blend are (0.29, 0.38) corresponding to white light emission. Spectroscopic data brought evidence of the formation of the β phase in the polyfluorene matrix, and due to differences in band gap, it was concluded that hole carriers tend to be injected into that phase rather than into the disordered one, leading to the proposal that EL emission was predominantly originated from the β phase.CNPqFAPESPINCT-INE

Dynamics of energy level alignment at ITO/organic semiconductor interfaces

Time-dependence of space-charge built-up at the ITO/organic semiconductor interface has been investigated for the first time using the charge extraction by linearly increasing voltage ( CELIV) technique in the dark. It will be demonstrated that space-charge next to an ITO/P3HT interface is extracted during the initial CELIV ramp establishing a non-equilibrium situation whose temporal recovery can be probed by utilizing sequentially prolonged delay times for successive voltage ramps. It will be reported that the buildup time constant for space charge at the ITO/P3HT interface amounts to 230 mu s. The dynamics of the relatively slow charge transfer at the interface of the transparent conducting oxide and the organic semiconductor as well as time-limiting processes will be discussed by considering tunneling, thermal detrapping and diffusion of the involved electrons and holes. It will be shown that thermal detrapping of electrons from molecules with their ground state close the center of the HOMO-DOS to vibronic states within the same molecule followed by tunneling to neighboring molecules with their not occupied ground state within the tail states of the DOS is the time limiting process and responsible for the long build-up time. (C) 2015 Published by Elsevier B.V

Geometry Optimization for Miniaturized Thermoelectric Generators [Dataset]

9 pages. -- Table S1. Physical properties of all materials at 300K used in the finite element method simulations. -- Table S2. Internal resistance (R) of devices with hollow and filled thermoelectric legs assuming the temperature gradient (Tvarying) and without the temperature gradient (Tfixed) as well as the percentage between the two values. -- Figure S1. Relative percentage increase for G, R, and S as a function of the thermoelectric length. -- Figure S2. The (a) G, (b) R, and (c) S with filled and hollow thermoelectric leg geometries as a function of the thermoelectric leg cross-sectional area. -- Figure S3. Temperature profile along the TEG with filled and hollow thermoelectric legs geometries when a temperature difference of 100 K is applied on the devices. -- Figure S4. Temperature profile along the TEG with length of the thermoelectric legs at 0.2 mm for filled and hollow thermoelectric legs geometries cases. -- Figure S5. Open circuit voltage and output power as a function of the electrical current for the TEG with filled and hollow thermoelectric legs. -- Figure S6. Temperature profile along the TEG with hollow thermoelectric legs geometries measured at two different points.Thermoelectric materials capable of converting heat into electrical energy are used in sustainable electric generators, whose efficiency has been normally increased with incorporation of new materials with high figure of merit (ZT) values. Because the performance of these thermoelectric generators (TEGs) also depends on device geometry, in this study we employ the finite element method to determine optimized geometries for highly efficient miniaturized TEGs. We investigated devices with similar fill factors but with different thermoelectric leg geometries (filled and hollow). Our results show that devices with legs of hollow geometry are more efficient than those with filled geometry for the same length and cross-sectional area of thermoelectric legs. This behavior was observed for thermoelectric leg lengths smaller than 0.1 mm, where the leg shape causes a significant difference in temperature distribution along the device. It was found that for reaching highly efficient miniaturized TEGs, one has to consider the leg geometry in addition to the thermal conductivity.Peer reviewe

Enhancing fullerene-based solar cell lifetimes by addition of a fullerene dumbbell.

Cost-effective, solution-processable organic photovoltaics (OPV) present an interesting alternative to inorganic silicon-based solar cells. However, one of the major remaining challenges of OPV devices is their lack of long-term operational stability, especially at elevated temperatures. The synthesis of a fullerene dumbbell and its use as an additive in the active layer of a PCDTBT:PCBM-based OPV device is reported. The addition of only 20 % of this novel fullerene not only leads to improved device efficiencies, but more importantly also to a dramatic increase in morphological stability under simulated operating conditions. Dynamic secondary ion mass spectrometry (DSIMS) and TEM are used, amongst other techniques, to elucidate the origins of the improved morphological stability

Lisímetro de pesagem de grande porte.: desenvolvimento e calibração A large-scale weighting lysimeter. part I: development and calibration

A evapotranspiração de plantas frutíferas foi medida a partir da instalação e calibração de um lisímetro de pesagem de grande porte. O equipamento, composto por caçamba metálica em aço carbono, possui arquitetura circular com área de 5,72 m² e massa total de 10,5 toneladas. Uma muda de coqueiro anão-verde foi transplantada para o lisímetro e mantida sob as mesmas condições de solo e adubação do campo experimental. Na automação do processo foi utilizado o mecanismo de alívio de carga baseado em sistemas de alavancas e contrapesos, apoiado em uma célula de carga com capacidade de 200 kg e precisão de 0,02% de sua capacidade. Os dados de saída da célula de carga foram armazenados em um coletor automático de dados tipo data logger. A relação entre a variação de massa e a voltagem da célula de carga do lisímetro é descrita adequadamente por modelo linear com mínima histerese. A metodologia empregada na instalação e na calibração do lisímetro foi adequada, obtendo-se um coeficiente de determinação de 0,99 na equação de calibração. Os resultados ainda indicam que o lisímetro de pesagem pode ser utilizado com êxito na determinação de evapotranspiração de plantas frutíferas de grande porte.<br>The objectives of present study were to develop and to calibrate a large-scale weighing lysimeter in order to estimate evapotranspiration of fruit plants. The equipment consisted of a metallic bucket (steel carbon) of circular design with total area of 5.72 m² and total mass of 10.5 t. A seedling of dwarf-green coconut palm was transplanted in lysimeter and maintained under same fertilization conditions of experimental field. For automation of the process, a mechanism of load relief based on systems of levers and counterbalances was used, leaning in a load cell with capacity of 200 kg and precision of 0.02% of its capacity. The output data of load cell were stored in an automatic data acquisition system. The relationship between mass variation (mm) and voltage (mV) of the load-cell lysimeter described a linear model with minimum hysterisis. Also, it can be concluded that the methodology used for the calibration of the weighing lysimeter was appropriate, showing a coefficient of determination of 0.99 for the calibration equation. Results also indicated that the large-scale weighing lysimeter can be used with success in the determination of evapotranspiration of fruit plants