Estudio introductorio al modelado y simulación de redes de nanotubos de carbono aleatorios, como electrodo transparente para celdas solares orgánicas

En el presente informe se describe los resultados obtenidos durante las pasantías de investigación realizadas en el Grupo de investigación en Automatización Industrial y Control “GAICO” con asesoramiento del Centro de Micro y Nanotecnología de la Universidad de los Andes. En dicha pasantía se realizó el estudio del modelamiento de electrodos de nanotubos de carbono (CNT) para celdas solares orgánicas, el cual será la base de partida para el posterior modelamiento de la interfaz de la celda solar y así determinar los parámetros indispensables para el mejoramiento de la eficiencia de conversión de energía. Además, se conocieron los simuladores aptos para dicho modelamiento tales como Medici, Comsol, Silvaco y Nanohub.Incluye bibliografía, anexo

Introducción al modelado y simulación del electrodo transparente con CNTs en celdas solares orgánicas

Abstract: La fabricación de las celdas fotovoltaicas inorgánicas son aquellas cuyo semiconductor es a base de material inorgánico (Si, AsGa, CuInS2, CIS, CdTe), son poco económicas, por ende, es necesario hallar una manera de producir celdas solares sin que la inversión económica sea costosa. Debido a esta dificultad tecnológica, se ha optado por usar semiconductores orgánicos en la fabricación de las celdas. Los paneles solares orgánicos no se contemplan como un sustituto a los de silicio como material semiconductor, sino un complemento para crear mejores fuentes de energía. En este caso, se plantea el uso de los nanotubos de carbono (CNT) como material semiconductor en la fabricación de celdas fotovoltaicas orgánicas

Life cycle assessment of perovskite on silicon tandem PV modules at industrial scale

Abstract: For the first time a comprehensive life cycle assessment (LCA) is reported of a large-area perovskite/Si tandem photovoltaic (PV) module of about 2 m², with an assumed power conversion efficiency of 30%, 30 years lifetime and an annual degradation of 0.5%, manufactured in a 1 GW production plant. By means of the LCA method, we calculate the contribution to the International Reference Life Cycle Data System (ILCD) impact categories associated with a perovskite/Si tandem PV system compared with a passivated emitter and rear cell (PERC) used as reference. We consider a functional unit of 1 kWh of generated direct current electrical energy from the tandem module over its lifetime. It was found that the most significant environmental hotspots are the silicon wafer production and the balance of system (BOS), especially the mounting system, inverter and electric installation. Overall, the impact contributions to the environment caused by the perovskite sub-module are below 1% throughout all the ILCD categories in focus: climate change, freshwater ecotoxicity, water resource depletion and human toxicity. Even the contribution of lead to human toxicity is just about 0.01%. For all the impacts assessed, the perovskite/Si tandem shows a decrease of between 17-20% of the contributions to each impact category compared to the PERC-module based reference PV system, if the Si bottom sub-module has an initial cell efficiency of at least 22% with 1% cell-to-module (CTM) loss, and the perovskite top sub-module has a minimum initial cell efficiency of 18% with at most 5% CTM loss and a transmittance near IR region larger than 80%. The key result found in this work is the relevance of the performance and reliability of the PV system to achieve lower environmental impacts. A comparison between optimistic, realistic, and pessimistic scenarios was performed to investigate this matter: in the worst-case scenario, the environmental impact of the PV tandem system would have a general 50% increase throughout all ILCD impact categories against the realistic case, whereas in the optimistic one, the results suggest a reduction of 46% to the global impact contributions, compared to the realistic scenario

Transistores basados en nanotubos de carbono

Resumen: Las aplicaciones de la electrónica exigen mayor eficiencia de los dispositivos electrónicos que se utilizan hoy en día. Entre los aspectos mas importantes esta el área ocupada por un circuito: Se busca que los dispositivos electrónicos ocupen el menor espacio posible. La tecnología que se utiliza hoy en día para fabricar dispositivos semiconductores está llegando a un punto limite, donde cada vez es mas difícil obtener reducción en el tamaño sin afectar otras características del dispositivo. La nanoelectrónica está evolucionando a un punto donde se hace posible la construcción de dispositivos semiconductores que presenten mejores características que los fabricados en la actualidad. Este documento muestra una revisión bibliográfica sobre nanotecnología, con el fin de presentar una propuesta de investigación para implementar transistores basados en nanotubos de carbono. Este trabajo está en su fase inicial, se espera que a finales del 2009 se tenga una metodología de implementación de transistores utilizando nanotecnología

Biluminescence of purely organic materials: fundamentals and applications in optical sensing

Abstract: The property of an organic molecule able to emit light efficiently from both their singlet and triplet excited states is called biluminescence. This dual state emission, particularly at room temperature, is difficult to achieve by purely organic molecules. It is possible only if the competitive thermal decay is suppressed effectively, enhancing the yield from the triplet state (i.e. phosphorescence) in addition to the conventional fluorescence. Biluminescence was identified in a simple host:guest system in which the biluminophore (i.e. an organic molecule showing biluminescence) NPB [N,N’-di(naphtha-1-yl)-N,N’- diphenyl-benzidine] is embedded in a rigid matrix, for example, the polymer PMMA [poly(methyl methacrylate)]. This system is unique not only due to the dual state emission, having fluorescence and phosphorescence emissions at 425nm and 530nm, respectively, but also because the large range of exciton lifetimes is extended up to nine orders of magnitude between nanosecond fluorescence and second phosphorescence. Up to date, efforts have been placed exclusively in the research of room temperature phosphorescence (RTP), although fluorescence can be observed from these systems as well. Therefore, the aim of this thesis was the investigation of the fundamental photophysical processes and characteristics of biluminescent organic molecules, in addition to their usage in promising applications as it will be described next. For instance, the most used approach for oxygen sensing is based on quenching of luminescent excited states, following a scheme of optical monitoring, i.e. correlating luminescence over-time data with oxygen concentrations. However, regardless whether one pursues fluorescence or phosphorescence quenching, they share a common drawback: They cannot monitor the effect of sample degradation due to extensive light exposure. In fact, one cannot distinguish between oxygen having penetrated the film and quenched the luminescence, or the sample being degraded over time due to e.g. photobleaching of the emitting molecules. The former represents an issue, especially for build-in applications. Now herein, it was proposed to use biluminescence to exclude degradation processes from oxygen sensing data evaluation, through a simple fluorescence-to-phosphorescence intensity ratio. This can be achieved, because it was proven here that the fluorescence is not directly quenched by oxygen, whereas the long-lived RTP is sensitive to it. Therefore, the fluorescence acts as self-referencing in a biluminescent sensor, which serves as an alternative solution to monitor oxygen concentrations. Additionally, the integration of the sensor into optoelectronic devices is considered as a potential future direction. Furthermore, it was revealed in this thesis that the difference in exciton lifetime yields to a significant limitation of luminescence efficiency, because of exciton accumulation under continuous illumination, causing bimolecular processes like singlet-triplet and triplet-triplet annihilation (STA and TTA, respectively), as well as optical saturation. All those processes occur already at moderate excitation intensities. It was demonstrated through sample engineering and oxygen quenching experiments, that the triplet exciton density can be controlled over several orders of magnitude, allowing to study exciton interactions between singlet and triplet manifolds. Consequently, the presented biluminescent system represents an illustrative role model to study excitonic effects in organic molecules. For example, in organic light-emitting diode materials, where the aforementioned effects i.e. STA, TTA and saturation, are application relevant but much more difficult to investigate. Mainly because light emission can be observed only from one state, instead of both states like in biluminescence

Simultaneous fluorescence and phosphorescence from organic molecules

Diluting organic molecules in a polymer matrix material suppresses non-radiative behavior, leading to biluminescence, wherein light is emitted efficiently from both singlet and triplet states. In summary, biluminescence has developed into a thoroughly optimized and unique emissive system. Among the characteristics obtained through this system, the most significant is the ability of biluminophores to offer direct access to the radiative states of both spin manifolds (singlets and triplets) in organic molecules without the requirement for special or complex material combinations. As a result, an exciton is given the chance to re-emit its energy, regardless of its initial state. In future work, we intend to increase the toolset of organic biluminophores by screening materials to determine the structure-property relationships behind persistent phosphorescence, and to identify molecular building blocks that are suitable for the development of future materials. In addition, we will intensify our exploration of biluminescence for future applications (e.g., sensing)

Organic photodetectors = Fotodetectores organicos

Organic semiconductors are used in many fields of photonics. Displays fabricated using organic light emitting diodes (OLED) can be found in modern smartphones and tablets, whereas organic solar cells (OPV - organic photovoltaics) are emerging with demonstrated efficiencies above 10%. Organic photodetectors (OPD) are another very interesting domain, with ultrathin active layers (order of tens of nanometers) providing performance comparable to bulk inorganic devices. Thanks to a multitude of possible compounds, parameters such as response spectrum, cut-off wavelength etc. can be easily tuned. Because of very high absorption coefficient and low refractive index, issues such as crosstalk or reflection can be minimized. Another exciting feature is the low processing temperature and thus feasibility of using a flexible foil as substrate, leading to rollable or curved photodetector arrays.The performance of organic based photodetectors has grown considerably in the recent years in terms of high speed, high frequency response and detectivity so as to match the required custom specifications for practical applications such as imaging. However, further attention needs to go towards the understanding of the effects of scaling during miniaturization of these devices, while looking for the preservation and/or optimization of their opto-electrical properties. Therefore, in this thesis, emphasis is given on the photodetectors based on organic conjugated polymer materials for imaging applications. These organic photodetector devices normally suffer from a perimeter-to-area dark current density scaling, hence, special attention is on the understanding of this scaling effect as well as optimization of the fabrication process in order to get low leakage (dark) current, and therefore, large dynamic range of the OPDs. Photodetectors with different geometries and sizes from 0.08 cm2 down to 1.95E ?5 cm2 were fabricated, using two types of edge cover layer. Isolated islands of two different organic films were patterned with photolithography in order to study the dark current mechanisms. Electrical, optical and morphological properties were characterized for both patterned and non-patterned devices. The patterning process used to fabricate the isolated devices does not affect the optoelectrical and morphological characteristics of the photodetectors, and therefore their performance. Moreover, it does not induce or accelerate degradation of the organic films. The dark current density further decreases after full patterning is performed, while preserving all device characteristics. It is proposed to replace the semitransparent edge cover layer by the opaque, if thicker films can be achieved, as it might be possible to decrease the contribution of the perimeter leakage. Once the patterning process is performed, it is possible to implement these isolated devices as photodetectors arrays for imaging applications with an Ion/Ioff ratio of 6 orders of magnitude. It is proposed to perform a theoretical study of the dark current mechanism at the interface between the polymer and the metallic contact edges to model and simulate the perimeter recombination at reverse bias conditions. Moreover, further measurements such as electroluminescent and spectroscopy might aid to characterize the carrier transport in the OPDs. The project was aimed within the R&D department of large area electronics. Moreover, all the fabrication and characterization were performed in the facilities of IMEC

Interplay of Fluorescence and Phosphorescence in Organic Biluminescent Emitters

Abstract: Biluminescent organic emitters show simultaneous fluorescence and phosphorescence at room temperature. So far, the optimization of the room-temperature phosphorescence in these materials has drawn the attention of research. However, the continuous-wave operation of these emitters will consequently turn them into systems with vastly imbalanced singlet and triplet populations, which is due to the respective excited-state lifetimes. This study reports on the exciton dynamics of the biluminophore NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1-biphenyl)-4,4-diamine). In the extreme case, the singlet and triplet exciton lifetimes stretch from 3 ns to 300 ms, respectively. Through sample engineering and oxygen quenching experiments, the triplet exciton density can be controlled over several orders of magnitude, allowing us to study exciton interactions between singlet and triplet manifolds. The results show that singlet−triplet annihilation reduces the overall biluminescence efficiency already at moderate excitation levels. Additionally, the presented system represents an illustrative role model to study excitonic effects in organic materials