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A polymer light-emitting electrochemical cell (LEC) is a solid-state polymer device operating according to in situ electrochemical doping and the formation of a light-emitting polymer p-n junction. This operating mechanism, however, has been the subject of much debate. Planar LECs with millimeter scale interelectrode spacings offer great advantages for directly observing the electrochemical doping process. Photoluminescence quenching and the formation of a light-emitting junction have been observed in planar polymer LECs, demonstrating the existence of electrochemical doping. The chemical potential difference between the p- and n-doped regions creates a built-in potential/electric field in the junction region, which can be probed by measuring the optical beam induced current (OBIC). This study utilizes a versatile and easy-to-use method of performing OBIC analysis. The OBIC and photoluminescence profiles of LECs have been simultaneously measured by scanning a focused light beam across large planar LECs that have been turned on and cooled to freeze the doping profile. The photoluminescence intensity undergoes a sharp transition between the p- and n-doped regions. The OBIC photocurrent is only observed in the transition region that is narrower than the width of the excitation beam, which is about 35 μm. The results depict a static planar polymer p-n junction with a built-in electric field pointing from n to p. The electrode interfaces do not produce a measurable photocurrent indicating ohmic contact. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2013-05-28 12:52:14.171

Increased use of renewable energies that is taking place all over the world is having a very important impact on the photovoltaic solar energy industry. This means of obtaining electrical energy is one of the most promising ones nowadays, thanks to the fact that it is a technology of easy installation and maintenance. However, the number of hours that a photovoltaic system works at maximum power depends almost entirely on environmental conditions, mainly in terms of solar irradiance.Solar irradiance is a magnitude that measures the power released by sunlight per unit area; the higher it is, the more power the photovoltaic system will generate.Therefore, it is very important to measure this magnitude in order to obtain data that either can give information about which is the best place to install a photovoltaic system or expect the device performance.Unfortunately, sensors used nowadays to measure this magnitude are quite expensive. The most widely used are the so-called pyranometers, with an average cost of between 8000 SEK to 10000 SEK, and solar reference cells, which can be quite cheaper (1000 SEK), but also can be the most expensive devices on the market depending on the features they have (some reference cells cost 20000 SEK).In this thesis, a solar irradiance sensor based on the treatment of a current generated by a silicon photodiode has been designed, built and calibrated. The signal generated by the device is a voltage that has been obtained by means of a current-to-voltage converter amplifier stage. Once the construction of the circuit was completed, it was tested on the roof of Hall 45 located in the University of Gävle. The testing was carried out on 13, 14 and 15 May 2019, and it consisted in the comparison of the signal generated by the new device and the signals generated by a pyranometer and a solar cell.The result is a device priced at 200 SEK, which shows acceptable levels of accuracy during central daylight hours but shows a strong angular dependence on incident light during sunrise and sunset.

Esta tese de doutorado é resultado do estudo da fotocondutividade em materiais poliméricos, com particular enfoque em diodos poliméricos com camada ativa de derivados do poli (p-fenileno vinileno) (PPV): OC10-PPV, SY e MEH-PPV, em estrutura tipo sanduíche (ITO/polimérico/Metal) de camada única, utilizando eletrodos metálicos com diferentes funções trabalho depositados sobre o mesmo filme. Para isso, foi necessária a realização de todas as etapas de fabricação de dispositivos orgânicos, como descritas neste trabalho, para obter resultados com o máximo de reprodutibilidade, bem como o domínio das técnicas de caracterização. As propriedades ópticas dos filmes poliméricos foram analisadas através dos espetros de absorção e de fluorescência estacionária na região do visível. O comportamento fotocondutivo dos dispositivos foi realizado através de medidas de corrente-tensão (I-V) e da espectroscopia de fotocorrente no estado estacionário em regime dc sob incidência de luz com diferentes comprimentos de onda, ora através do ITO, ora através dos eletrodos metálicos semitransparente, para o dispositivo sob diferentes valores de tensões aplicadas. A variação desses parâmetros permitiu verificar como estes influenciam na fotogeração e no transporte de portadores de cargas e assim adaptar e aplicar um modelo teórico que levasse em consideração o campo elétrico aplicado para ajustar a fotocorrente obtida experimentalmente, obtendo informações sobre as propriedades de fotocondutividade dos materiais estudos. / This thesis is a study about photoconductivity in polymeric materials, in particular diodes with single-layer sandwich structure derivates of poly(para-phenylene vinylene) (PPV): OC10-PPV, SY and MEH-PPV. Different metallic electrodes were deposited on the same polymeric layer. Therefore, it was necessary to realize the devices fabrication process step by step as well to dominate the characterization techniques. The optical properties of polymeric film were analyzed through the absorption and emission spectrum in the visible region. The photoconductivity behavior was studied by dc current-voltage (I-V) and photocurrent action spectra through the absorption and emission spectrum in the visible region. The photoconductivity behavior was studied by dc current-voltage (I-V) and photocurrent action spectra through the both side of device under applied voltages. The variation of these parameters allowed verifying how are influencing in the charge carrier photo-generation and the charge transport and then to adapt and apply a theoretical model wich considers the electric field applied to fit the photocurrent action spectra of devices. The values of the parameters allow to survey important about photoconductivity properties of the materials

As células solares orgânicas, também conhecidas como (OPVs), fazem parte da terceira geração dos dispositivos fotovoltaicos. Entre outras tecnologias emergentes, a dos OPVs tem a vantagem de ser de fácil processamento e de baixo custo. Ou seja, uma tecnologia comercialmente promissora na área de conversão de energia solar em energia elétrica. No entanto, grandes desafios precisam ser superados para colocar estas células no mercado dos fotovoltaicos. Dentre esses desafios, pode estar incluído, inevitavelmente, a compreensão dos processos físicos envolvidos na fotogeração em OPVs, dentre os quais pode-se destacar o da recombinação de cargas fotogeradas. A recombinação é o principal responsável pela perda de eficiência em OPVs, uma vez que ela elimina uma fração relativamente grande de portadores de carga, diminuindo consideravelmente a potência de saída da célula. Para estudar este efeito indesejado em células orgânicas, desenvolvemos um modelo analítico para fotocorrente em OPVs do tipo bulk heterojunction (BHJ), assumindo uma recombinação bimolecular de cinética de segunda ordem. O modelo é representado por uma expressão analítica obtida a partir das equações fundamentais da eletrodinâmica clássica, onde despreza-se a contribuição da corrente de difusão e as mobilidades dos elétrons e dos buracos são consideradas iguais. Essa expressão foi de grande valia na análise dos resultados experimentais, sobretudo os de corrente-tensão (J-V) sob iluminação, e além disso, ela permitiu extrair parâmetros intrínsecos do transporte de carga, como mobilidade e coeficiente de recombinação. Neste sentido, foram fabricados dispositivos cuja estrutura foi ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al, e com eles foram realizados inúmeros experimentos. As técnicas usadas na parte experimental foram: medidas J-V, no escuro e sob iluminação, medidas de transiente de fotovoltagem (TPV), de transiente de fotocorrente (TPC), e de Foto-CELIV (Charge Extraction Linear Increasing Voltage). Usamos como parâmetros experimentais a temperatura e intensidade de iluminação. Das medidas J-V sob iluminação, foram extraídos os parâmetros essenciais da célula: corrente de curto (Jsc), potencial de circuito aberto (Voc), fator de preenchimento (FF) e a eficiência (PCE). A partir das abordagens experimental e teórica, exploramos a influência da recombinação bimolecular no comportamento fotovoltaico dos dispositivos. O desenvolvimento do modelo teve contribuição de trabalhos que se basearam em modelagem numérica a partir de condições físicas semelhantes às usadas em nosso tratamento e que foram levadas em consideração no processo de análise dos resultados experimentais. / Organic solar cells, also known as (OPVs), are part of the third generation of photovoltaic devices. Among other emerging technologies, OPVs have the advantage of being easy to process and exhibits low cost of production. That is, it is a promising commercial technology in the area of converting solar energy into electricity. However, major challenges need to be overcome to put these cells in the photovoltaic market. Among them, it can be included, inevitably, the comprehension of the physical processes involved in photogeneration in OPVs, of which, the recombination of photogenerated carriers is included. Recombination is primarily factor responsible for the loss of efficiency in OPVs, since recombination eliminates a large fraction of the carriers, considerably reducing the output power of the cell. To study this undesirable effect in organic cells, we developed an analytical model for the photocurrent in bulk heterojunction cells (BHJ), which assumes the bimolecular recombination of second order kinetics. The model is represented by an analytical expression obtained by the equations of the classical electrodynamics, where we neglected the contribution of the diffusion current and assumed that electrons and holes have equal mobilities. The expression was of great value for the analysis of the experimental results, especially the current-voltage (J-V) measurements under illumination, and it allowed to extract intrinsic parameters of charge transport effects, such as mobility and recombination coefficient. For this, it were fabricated devices whose structure was ITO/PEDOT:PSS/P3HT:PCBM/Ca-Al, and with them were performed numerous experiments. The techniques used in the experimental part were: J-V measurements, in the dark and under illumination, transient photovoltage (TPV), transient photocurrent (TPC), and of Charge Extraction Linear Increasing Voltage (Photo-CELIV). We used as experimental parameters the temperature and the intensity of. From J-V measurements under illumination we extracted the essential cell parameters: short current (Jsc), open circuit potential (Voc), fill factor (FF) and efficiency (PCE). From the experimental and theoretical approaches, we explored the influence of bimolecular recombination on the photovoltaic behavior of the devices. The development of the model had contributions of works based on numerical modelings from physical conditions similar to those used in our treatment and that were taken into account in the process of analysis of the experimental results.

Le champ électromagnétique térahertz (THz) se situe dans l'intervalle de fréquence entre l'infrarouge et les micro-ondes, à peu près entre 1 THz à 10 THz. Ce domaine est hautement souhaitable tant pour la recherche fondamentale que pour les applications. Pourtant des sources THz compacts et accordables ne sont pas encore disponibles. Depuis la première proposition en 1970, les superréseaux semiconducteurs, dans lequel deux couches semi-conductrices atomiques avec bande interdite différente sont disposés périodiquement, fournissent de nouvelles possibilités. De nouvelles techniques et de nouveaux dispositifs deviennent réalisables. Dans cette thèse, les oscillations de Bloch dans des mini-bandes électroniques d’un superréseau polarise et la dispersion du gain associée sont utilisées pour réaliser une source THz compacte et accordable : l’oscillateur de Bloch THz. Un premier ensemble de dispositifs utilisent des réseaux dopes spécifiquement conçus pour éviter la formation de domaine d’accumulation de charges. Ces dispositifs utilisent une surface semi-isolante ou deux surfaces métalliques permettant un guidage par plasmon de surface. Cependant, malgré la réalisation de couplage par les bords ou par un réseau diffractant en surface et des mesures directes ou avec un interféromètre a transformation de Fourrier (FTIR), l’électroluminescence a été observée dans le domaine térahertz, avec un gain qui n’a pas pu etre relie aux oscillations de Bloch. Avec des superréseaux non dope, l'émission THz des oscillations de Bloch a été détectée par spectroscopie dans le domaine temporel. La dépendance de la fréquence d’émission avec le champ électrique appliqué constitue une preuve directe des oscillations de Bloch. L’échelle de Wannier Stark des trous sous pompage optique continu a aussi été observe dans les superréseaux non dopes. Avec l’augmentation de la puissance de pompage optique, les pics du photocourant se décalent et leurs formes deviennent asymétriques. L’évolution est attribue a l’accumulation des porteurs photogénérés dans les deux couches encadrant le superréseau. En outre, pour une puissance de pompage élevée, la bistabilité du photocourant a été également observée. / Terahertz (THz) electromagnetic field, which lies in the frequency gap between the infrared and microwave, roughly between 1 THz to 10 THz, is highly desirable for both fundamental research and application. Yet tuneable compact THz sources are still not available. On the other hand, ever since first proposed in 1970, semiconductor superlattice provides new playground for various new technique and devices of tremendous research and application interest. In this thesis, an innovative theme, relying on Bloch oscillations in a dc biased semiconductor superlattice, is explored to realize tunable compact THz source THz Bloch oscillator. For doped superlattice Bloch oscillator, we designed quantum cascade super-superlattice structure to realize Bloch oscillations whilst prohibit electrical domain formation. The designed structures were processed into various waveguide and grating devices for electroluminescence detection using Fourier transform infrared spectroscopy (FTIR). The Bloch gain of semi-insulating surface plasmon waveguide device was also measured using THz time domain spectroscopy. Even though the electroluminescence and gain at THz regime were observed, no direct evidence of Bloch emission was confirmed. For undoped superlattice, the THz emission from Bloch oscillations was observed by time domain spectroscopy. At last, the photocurrent corresponding to heavy hole and Wannier Stark Ladder (WSL) states transitions in undoped superlattice was studied. Under CW laser pumping, the photocurrent as function of the applied voltage showed multiple WSL peaks, which indicated laser induced and controllable negative differential conductance (NDC). With increasing pumping power, the nonlinear NDC regime and bistable states were investigated as well.

Metal-halide perovskites show great promise as solution-processable semiconductors for efficient solar cells and LEDs. In particular, the diffusion range of photogenerated carriers is unexpectedly long and the luminescence yield is remarkably high. While much effort has been made to improve device performance, the barriers to improving charge transport and recombination properties remain unidentified. I first explore charge transport by investigating a back-contact architecture for measurement. In collaboration with the Snaith group at Oxford, we develop a new architecture to isolate charge carriers. We prepare thin films of perovskite semiconductors over laterally-separated electron- and hole-selective materials of SnOₓ and NiOₓ, respectively. Upon illumination, electrons (holes) generated over SnOₓ (NiOₓ) rapidly transfer to the buried collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. We characterise charge transport parameters of electrons and holes, separately, and demonstrate that grain boundaries do not prevent charge transport. Our results show that the low mobilities found in applied-field techniques do not reflect charge diffusivity in perovskite solar cells at operating conditions. We then use the back-contact architecture to investigate recombination under large excess of one charge carrier type. Recombination velocities under these conditions are found to be below 2 cm s⁻¹, approaching values of high quality silicon and an order of magnitude lower than under common bipolar conditions. Similarly, diffusion lengths of electrons and holes exceed 12 μm, an order of magnitude higher than reported in perovskite devices to date. We report back-contact solar cells with short-circuit currents as high as 18.4 mA cm⁻², giving 70% external charge-collection efficiency. We then explore the behaviour of charge carriers in continuously illuminated metal-halide perovskite devices. We show that continuous illumination of perovskite devices gives rise to a segregated charge carrier population, and we find that the distance photo-induced charges travel increases significantly under these conditions. Finally, we examine intermittancy in the photoluminescence intensity of metal-halide perovskite films.

Neste trabalho desenvolvemos rotinas computacionais em Python para o cálculo de propriedades dinâmicas (espectros de fotocorrente e absorção) de heteroestruturas semicondutoras baseadas em Dinâmica Quântica. Em uma primeira etapa do desenvolvimento do projeto, a formulação baseada na evolução temporal das soluções da equação de Schrödinger dependente do tempo foi aplicada a sistemas com soluções analíticas conhecidas ou com resultados já reportados na literatura. Devido à excelente concordância entre nossos dados e aqueles já conhecidos, em uma etapa seguinte, foram calculadas as energias de transição observadas em espectros de fotoluminescência para poços quânticos de InGaAs/GaAs, crescidos por MBE, levando-se em conta os efeitos de tensão e segregação de átomos de índio. Na continuidade do projeto, especial atenção foi dada ao desenvolvimento de estratégias para calcular os espectros de absorção e fotocorrente para dispositivos do Estado Sólido. O conjunto de resultados apresentados neste trabalho demonstra que a metodologia desenvolvida é precisa e pode ser utilizada com baixo custo computacional para o modelamento de heteroestruturas semicondutoras mais complexas, que servem de base para o desenvolvimento de dispositivos optoeletrônicos. / In this work we developed computational routines in Python for the calculation of the dynamic properties (spectrum of photocurrent and absorption) of semiconductor heterostructures based on Quantum Dynamics Theory. In a first stage of the development of the project the formulation based on the time evolution of the solutions of the time dependent Schrödinger equation was applied to systems with known analytical solutions or results already reported in the literature. Due to the excellent agreement between our data and those already known, in the next stage the transition energies observed in photoluminescence spectra for InGaAs/GaAs quantum wells, grown by MBE, were calculated taking into account the effects of stress and segregation of indium atoms. In the continuity of the project, special attention was given to the development of strategies to calculate absorption and photocurrent spectra for solid state devices. The set of results presented in this work demonstrates that the methodology developed is accurate and can be used with low computational cost for the modeling of more complex semiconductor heterostructures, which are used for the development of optoelectronic devices.

Στην διατριβή αυτή μελετήθηκαν και εξακριβώθηκαν θεωρητικά όλες οι φυσικές διεργασίες που υπεισέρχονται στις μετρήσεις του διαμορφωμένου φωτορεύματος. Με βάση αυτές δείξαμε ότι μπορούμε να εξάγουμε όλες τις πιθανές διεργασίες παγίδευσης και επανασύνδεσης σε διάφορες κατηγορίες ανόργανων ημιαγωγών. Σαν μελλοντική μελέτη μπορεί να γίνει η επέκταση της θεωρητικής ανάλυσης για υλικά όπου έχουμε ατέλειες που μπορούν να βρεθούν σε περισσότερες από δύο καταστάσεις φορτίου όπως είναι οι αιωρούμενοι δεσμοί του άμορφου . Επίσης η ανάλυση που προτείνουμε μπορεί να εφαρμοστεί και στην κατηγορία των οργανικών ημιαγωγών. / In this thesis we studied and verified theoretically, all the physical processes involved in modulated photocurrent measurements. Based on these measurements, we show that we can extract all possible trapping and recombination processes in different classes of inorganic semiconductors. As future study, we can extend the theoretical analysis for materials which have defects that may be found in more than two charge states such as the dangling bonds in amorphous silicon. Also, the analysis we propose can be applied to the class of organic semiconductors.

Ce travail présente la caractérisation des propriétés d'absorption de lumière par des nanofils uniques (NF) de silicium en utilisant la spectroscopie de photocourant, ainsi qu'une étude préliminaire des processus d'incorporation des dopants et de réalisation de jonction dans les NFs. Tout d'abord, nous commençons par décrire les méthodes de croissance utilisées pour synthétiser des NFs actifs pour la génération de photocourant, avec l'utilisation du chlorure d'hydrogène dans les procédés classiques de croissance CVD catalysée or de fils dopés. Cette méthode offre des structures très faiblement coniques, élargit les températures de procédé, permettant en particulier d'incorporer très efficacement le bore, avec des densités d'accepteurs ionisés allant jusqu'à 1.8E19 cm-3, tout en inhibant la diffusion d'or depuis le catalyseur. L'attention est ensuite portée à la fabrication de jonctions, l'étude de ses caractéristiques électriques, ainsi que sur l'influence de paramètres morphologiques (rayon, position axiale) du fil sur sa résistivité apparente. Dans une seconde partie, nous étudions la réponse en photocourant d'un jeu de NFs actifs de différents diamètres et corrélons nos résultats à un traitement analytique de l'absorption des photons à l'échelle du nanoobjet dans le cadre de la théorie de Mie adaptée au cas cylindrique. L'accord expérience-théorie est très bon pour les deux polarisations (TE-TM). Des résonances dans le spectre d'absorption sont mises en évidence, correspondant à l'excitation de modes propres du fil, et associées à des sections efficaces d'absorption pouvant être supérieures à l'unité. Dans une dernière partie, nous adaptons la stratégie de dépôt antireflet utilisée dans les cellules solaires pour améliorer le couplage de la lumière incidente aux NFs. Pour cela, des dépôts de SiO2 et Si3N4 sont réalisés sur des NFs via la technique de PECVD, nous fournissant par là-même un jeu de structures pourvues d'un dépôt de diélectrique à haute conformité. Se basant sur les spectres d'absorption ainsi acquis, nous obtenons les gains relatifs d'absorption induits par le dépôt de diélectrique et les comparons aux calculs analytiques développés spécifiquement pour obtenir l'absorption dans le cœur seulement du cylindre coaxial, ceci nous permettant également d'estimer la partie du rayonnement incident absorbé dans la coquille diélectrique. / In this work we present the characterization of the light absorption properties of single silicon NWs (NW) using photocurrent spectroscopy along with the preliminary work done at the wire scale to characterize the dopant incorporation and the fabrication of junctions. First, we start with a description of the growth methods used to synthesize active NW's for photocurrent generation, with results obtained on the use of hydrogen chloride in the CVD VLS growth of doped NWs. This method offers highly straight structures, widened process temperatures allowing in particular very efficient boron incorporation—ionized acceptors densities up to 1.8E19 cm-3—and inhibited gold diffusion, thereby greatly reducing elemental contamination from the catalyst. Focus is made on the junction formation, the study of its electrical characteristics and the influence of morphological parameters—radius, axial position—to obtain the desired doping properties. In a second part, we present the photocurrent response of a set of different diameter active Si NWs and correlate our results with an analytical treatment of the photon absorption at the nanoscale using the Lorentz Mie theory adapted to the cylindrical geometry under study. Very good agreement is found between experiment and theory for both polarization spectra (TE-TM). Absorption resonances are resolved, corresponding to leak resonant modes, and can display absorption efficiencies higher than one, making downscaling an efficient tool to increase energy harvesting capabilities. In a last part, we adapt the antireflective coating strategy used in solar cells to improve the coupling of the incoming light to Si NWs. For this, SiO2 and Si3N4 films are deposited on NWs using PECVD, providing a set of structures coated with a high level of conformity. Based on the new set of spectra we obtain the relative gain curves and compare them with analytical calculations specifically derived for getting the absorption in the core of coaxial nanocylinders only, which allows estimating the magnitude of the absorbed energy in the dielectric shell.