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To overcome transmission and thermalization losses from single-junction solar cells, the two-terminal tandem cell configuration is one of the most widely followed approaches. Unfortunately, the need to match the current from both cells, in addition to the fabrication of an interlayer with excellent optical and electronic performances, makes such two-terminal configurations a challenging approach. Herein, the fabrication of a four-terminal tandem incorporating a front transparent cell made using a 7¿nm Ag back electrode and a PM6:L8-BO blend which exhibits a bandgap close to the optimal according to the detailed balance theory is reported. For the opaque back cell in such tandem, a PTB7-Th:O6T-4F blend is used. A 16.94% power conversion efficiency is achieved when such two cells are configured in a four-terminal device separated by a WO3/LiF/WO3 multilayer providing an optimal light absorption distribution among the two cells., The authors acknowledge Dr. Mariia Kramarenko for her help with the detailed balance calculations. F.B.T. and J. M. acknowledge the financial support by the European Commission through the LICROX project (grant no. 951843) and the SOREC2 project (101084326). In addition, the work was partially funded by Ministerio de Ciencia e Innovación (grants nos. CEX2019-000910-S and PID2020-112650RB-I00), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya through Centres de Recerca de Catalunya. F.B.T acknowledges financial support from the Agencia Estatal de Investigación (grant no. PRE2018-084881)., Peer Reviewed, Postprint (published version)

Resolved, 2022/2023, 1r quadrimestre

(English) To mitigate the present environmental crisis, caused by the excessive use of fossil fuels and associated release of carbon dioxide into the atmosphere, it is necessary to significantly reduce worldwide energy consumption, to rely more strongly on clean and renewable sources of energy, but also to maximize energy efficiency in currently existent technologies that make use of energy. To reach such maximal energy efficiency, it is necessary to optimize light propagation, harvesting, and utilization in the different existent optoelectronic technologies. Given that a considerable portion of the global energy consumption is dedicated to illumination or devices incorporating illumination sources in them, a clear path to maximize energy-efficiency would imply minimizing the light losses in such kind of systems. In addition, for maximal energy conversion efficiency it is essential to optimize light absorption in systems that perform an unassisted sunlight transformation into other forms of energy, such as electrical or chemical. To reach the double goal of optimizing light utilization and transformation, in this thesis we consider the study of optical ergodic configurations, where light rays are randomized after a few bounces at the interfaces, losing any correlation with the external incident state and giving rise to an isotropic radiation inside the material. In Chapter 2 of the thesis, we demonstrate that an ergodic geometry can be used to obtain homogeneously distributed polarized light emission. In the same ergodic system, we also demonstrate that the light with the unwanted polarization can be trapped and transformed back into electricity by using a couple of perovskite solar cells. Such features are potentially useful to increase energy efficiency in optoelectronic devices incorporating illumination sources in them, as is the case of liquid crystal displays. A similar ergodic light propagation is also considered in Chapter 3 to determine what the maximal lig, (Català) Per a mitigar l'actual crisi mediambiental, causada per l'ús excessiu de combustibles fòssils i l'alliberament associat de diòxid de carboni a l'atmosfera, és necessari reduir significativament el consum d'energia a escala mundial, confiar més en fonts d'energia netes i renovables, però també maximitzar l'eficiència energètica en les tecnologies actualment existents que utilitzen l'energia. Per assolir aquesta eficiència energètica màxima, és necessari optimitzar la propagació de la llum, la recol·lecció i la utilització en les diferents tecnologies optoelectròniques existents. Atès que una part considerable del consum global d'energia es dedica a la il·luminació o dispositius que incorporen fonts d'il·luminació en ells, un camí clar per maximitzar l'eficiència energètica implicaria minimitzar les pèrdues de llum en aquest tipus de sistemes. A més, per a una eficiència màxima de conversió d'energia, és essencial optimitzar l'absorció de llum en sistemes que realitzen una transformació de la llum solar no assistida en altres formes d'energia, com ara l'electricitat o la química. Per assolir el doble objectiu d'optimitzar la utilització i transformació de la llum, en aquesta tesi considerem l'estudi de configuracions ergòdiques òptiques, on els raigs de llum són aleatoris després d'uns quants rebots a les interfícies, perdent qualsevol correlació amb l'estat d'incidència extern i donant lloc a una radiació isotròpica dins del material. En el Capítol 2 de la tesi, es demostra que una geometria ergòdica pot ser utilitzada per obtenir una emissió de llum polaritzada distribuïda homogèniament. En el mateix sistema ergòdic, també demostrem que la llum amb la polarització no desitjada pot ser atrapada i transformada de nou en electricitat utilitzant dues cèl·lules solars de perovskita. Aquestes característiques són potencialment útils per augmentar l'eficiència energètica en dispositius optoelectrònics que incorporen fonts d'il·luminació en ells, com és el cas de l, (Español) Para mitigar la actual crisis ambiental, provocada por el uso excesivo de combustibles fósiles y la consiguiente liberación de dióxido de carbono a la atmósfera, es necesario reducir significativamente el consumo mundial de energía, confiar más en fuentes de energía limpias y renovables, pero también maximizar la eficiencia energética en las tecnologías actualmente existentes que hacen uso de la energía. Para alcanzar tal eficiencia energética máxima, es necesario optimizar la propagación, recolección y utilización de la luz en las diferentes tecnologías optoelectrónicas existentes. Dado que una parte considerable del consumo energético mundial se dedica a la iluminación o a dispositivos que incorporan fuentes de iluminación, un camino claro para maximizar la eficiencia energética pasaría por minimizar las pérdidas de luz en este tipo de sistemas. Además, para obtener la máxima eficiencia de conversión de energía, es esencial optimizar la absorción de luz en los sistemas que realizan una transformación de la luz solar, sin aplicar un voltaje externo, en otras formas de energía, como eléctrica o química. Para alcanzar el doble objetivo de optimizar la utilización y transformación de la luz, en esta tesis consideramos el estudio de configuraciones ópticas ergódicas, donde los rayos de luz se aleatorizan tras unos pocos rebotes en las interfaces, perdiendo cualquier correlación con el estado incidente externo y dando lugar a un radiación isotrópica en el interior del material. En el Capítulo 2 de la tesis, demostramos que se puede utilizar una geometría ergódica para obtener una emisión de luz polarizada homogéneamente distribuida. En el mismo sistema ergódico, también demostramos que la luz con la polarización no deseada puede atraparse y transformarse de nuevo en electricidad mediante el uso de un par de células solares de perovskita. Tales características son potencialmente útiles para aumentar la eficiencia energética en dispositivos optoelectrónicos que, Postprint (published version)

(English) To mitigate the energy and climate crisis we are already facing and address the increasing energy consumption, it is essential to foster energy transition strategies. The energy transition success largely depends on being able to transform the global energy sector from fossil fuel-based to neutral or zero carbon dioxide emission sources. A decades-long study of photovoltaics has produced several generations of solar energy conversion technologies that do not emit greenhouse gases or air pollution when they operate. However, considering the sun is our planet's most abundant energy source, when compared to fossil fuels, the use of photovoltaics is still very limited. In such a transition from fossil fuels to photovoltaics, it is essential to enhance affordability combined with high efficiency. In recent years, perovskite solar cells have emerged as the most prominent thin-film PV technology to provide high efficiency simultaneously at a low cost. In this thesis, we study perovskite-based solar cells and optical routes to maximise their power conversion efficiency. The theoretical limit for converting sunlight into electricity by the photovoltaic effect was first established by William Shockley and Hans-Joachim Queisser in 1961 based on the principle of detailed balance. According to their work, a solar cell can reach a limiting efficiency if all loss mechanisms are eliminated and the fluorescence quantum yield is equal to unity. The conversion efficiency for all PV technologies is still considerably lower than the Shockley – Queisser efficiency limit, but perovskite-based ones have demonstrated outstanding optoelectronic properties and high fluorescence quantum yields, which may help get their maximum PV efficiency closer to such limit. In this thesis, we provide several optical routes to optimise the photovoltaic parameters of perovskite solar cells, placing a special emphasis on the open circuit voltage by studying its relation to the absorption and emissi, (Català) Per mitigar la crisi energètica i climàtica que ja estem enfrontant i abordar l'augment del consum d'energia, és fonamental fomentar estratègies de transició energètica. L'èxit de la transició energètica depèn en gran menera de poder transformar el sector energètic mundial de fonts basades en combustibles fòssils a fonts d'emissió de diòxid de carboni neutres o zero. Un estudi de dècades sobre la fotovoltaica ha produït diverses generacions de tecnologies de conversió d'energia solar que no emeten gasos d'efecte hivernacle ni contaminació de l'aire quan funcionen. Tanmateix, tenint en compte que el sol és la Font d'energia més abundant del nostre planeta, en comparació amb els combustibles fòssils, l'ús de la fotovoltaica és encara molt limitat. En aquesta transició dels combustibles fòssils a la fotovoltaica, és essencial millorar l'assequibilitat combinada amb una alta eficiència. En els darrers anys, les cèl·lules solars de perovskita s'han convertit en la tecnologia fotovoltaica de pel·lícula prima més destacada per proporcionar una alta eficiència simultàniament a un baix cost. En aquesta tesi, estudiem cèl·lules solars basades en perovskites i rutes òptiques per maximitzar la seva eficiència de conversió d'energia. El límit teòric per convertir la llum solar en electricitat mitjançant l'efecte fotovoltaic va ser establert per primera vegada per William Shockley i Hans-Joachim Queisser el 1961 basant-se en el principi de l'equilibri detallat. Segons el seu treball, una cèl·lula solar pot assolir una eficiència limitadora si s'eliminen tots els mecanismes de pèrdua i el rendiment quàntic de fluorescència és igual a la unitat. L'eficiència de conversió de totes les tecnologies fotovoltaiques encara és considerablement inferior al límit d'eficiència de Shockley - Queisser, però les basades en perovskita han demostrat propietats optoelectròniques excepcionals i rendiments quàntics de fluorescència elevats, que poden ajudar a apropar la seva màxima eficiènc, Postprint (published version)

Bismuth vanadate has attracted much attention as one of the most promising materials for photoelectrochemical (PEC) water splitting. This Master Thesis is aimed at understanding BiVO4 films growth to use it as photoanodes. We fabricate samples by spin- coating method modifying BiVO4 thickness up to ∼ 40 nm and adding a thin (∼ 5 nm) SnO2 hole blocking layer. Morphological characterization with scanning electron microscopy have confirmed the formation of BiVO4 layers on FTO and have shown an increase on the grain sizes from 64 nm to 98 nm for increasing film thickness. Electrochemical experiments are performed on thin films to characterize its electrical parameters, showing an increase of the photocurrent density with increasing number of layers up to 0.72 mA · cm−2 at 1.23 VRHE for the thicker sample. Moreover, it is demonstrated an optimization of the fill factor for the samples that include a intermediate SnO2 layer, confirmed by Mott-Schottky analysis, obtaining a shift of 80 mV of the flat band potential, improving photogenerated voltage and thus, performance of the photoanode. We have proposed and demonstrated a method to obtain sample thicknesses by a linear relation with the incident photon to current conversion efficiency value at 400 nm.

Singlet fission is a photophysical process which converts an excited singlet state into two independent triplet states. This has attracted significant attention for photovoltaic devices due to the fact that more than one electron can be generated per each absorbed photon, which can boost the quantum efficiency over 100 %. In this project, research on the understanding of singlet fission dynamics of organic thin films has been conducted by using transient absorption spectroscopy as the main characterisation method of the proposed polymer-based films, which are a mixture of TIPS-pentacene and Y6. Results have shown that charge generation is due to the triplet states generated thanks to the excitation at 800 nm of TIPS-pentacene in blends at certain ratios. Moreover, as an extra work, solar cells have been fabricated obtaining a 10 % of quantum efficiency due only to the triplet states generation.

Examen resolt de mig curs Fonaments de Física., Resolved, 2022/2023, 1r quadrimestre

The performance of nonfullerene-acceptor-(NFA)-based organic solar cells is rapidly approaching the efficiency of inorganic cells. The chemical versatility of NFAs extends the light-harvesting range to the infrared, while preserving a considerably high open-circuit- voltage, crucial to achieve power-conversion efficiencies >17%. Such low voltage losses in the charge separation process have been attributed to a low-driving-force and efficient exciton dissociation. Here, we address the nature of the subpicosecond dynamics of electron/hole transfer in PM6/Y6 solar cells. While previous reports focused on active layers only, we developed a photocurrent-detected two-dimensional spectroscopy to follow the charge transfer in fully operating devices. Our measurements reveal an efficient hole-transfer from the Y6- acceptor to the PM6-donor on the subpicosecond time scale. On the contrary, at the same time scale, no electron-transfer is seen from the donor to the acceptor. These findings, putting ultrafast spectroscopy in action on operating optoelectronic devices, provide insight for further enhancing NFA solar cell performance., This research received funding from the Clean Planet Program supported by Fundació Joan Ribas Araquistain (FJRA) and the Ignite program (Q-SPET) supported by the Barcelona Institute of Science and Technology. N.F.v.H. acknowledges the financial support by the European Commission (ERC Advanced Grant 670949-LightNet and Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agree- ment No 713729), the Ministry of Science & Innovations (“Severo Ochoa” program for Centers of Excellence in R&D CEX2019-000910-S and Plan Nacional PGC2018-096875−B- I00), the Catalan AGAUR (2017SGR1369), Fundació Privada Cellex, Fundació Privada Mir-Puig and the Generalitat de Catalunya through the CERCA program. J.M. and F.B. acknowledge the financial support by the European Commis- sion (grant 951843), Spanish Ministry MINECO and FEDER (grant MAT2017-89522-R), the Severo Ochoa program (Grant SEV-2015-0522) and “Agencia Estatal de Investiga- ción” (Grant PRE2018-084881). E.C. acknowledges the financial support of the H2020 FET Project COPAC (766563)., Peer Reviewed, Postprint (published version)

The phenomena related to charge trapping are among the most relevant open issues that affect the long-term stability of perovskite-based devices. According to this, the objective of this paper is to report experimental results in which a charge control strategy is used for the first time in a solar cell structure that has a high trap density perovskite absorber. This device has also noticeable J-V hysteresis, produced by non-capacitive effects. The control strategy proposed, based on sigma-delta modulation, applies to the device an appropriate sequence of voltage waveforms determined after periodical current measurements made at a constant voltage. The experimental results obtained and the fittings made with a phenomenological model indicate that this approach allows controlling several charge-related effects. As a consequence, the J-V characteristic of the device is successfully shifted and stabilized to predetermined positions., This research was funded by the Spanish Ministry MINECO (under grant RTI2018-098728- B-C33), the Severo Ochoa program (under grant SEV-2015-0522), MINECO and FEDER (under grant MAT2017-89522-R), the Fundacio Privada Cellex and the CERCA program from the Generalitat de Catalunya., Peer Reviewed, Postprint (published version)

The environmental issues associated with the use of conventional fuels necessitates the utilisation of renewable energy sources, as well as the implementation of energy efficient designs, in order to decrease electricity consumption. Photovoltaic (PV) technology can be employed for both approaches by converting not only natural but, also, artificial light into electricity. Among the different emerging PVs, perovskites achieve the highest power conversion efficiency, providing a widely tuneable bandgap with minimum open circuit losses. Moreover, their fabrication uses readily available materials, and does not necessarily require either the use of high temperature processes or vacuum deposition techniques. In this thesis, we enhance light harvesting in perovskite solar cells, and approach the energy efficiency concept through their optimised fabrication and integration in light selective structures. This is accomplished by the implementation of optical and material strategies applied to specific perovskite solar cell designs. The results prove that such strategies provide enhanced light absorption and optimal PV performance in low temperature devices, and enable the recycling of light into electricity for alternative photonic applications. The approaches presented could be utilised in future procedures to decrease the amount of Pb employed in perovskite solar cells, and to reduce the energy consumption during fabrication and the operation of other optoelectronic devices. The thesis is organised into four chapters. Chapter 1 serves as an introduction, where the current energy situation and PV technology are analysed, together with an insight into light harvesting and energy efficiency in perovskite solar cells. In Chapter 2, we demonstrate the employment of a periodic structure to propagate ergodic light in order to increase light absorption in perovskite solar cells, as would happen by employing randomly textured surfaces. This structure serves as a tool to decrease t, Los problemas medioambientales asociados al uso de combustibles convencionales requieren del uso de fuentes de energía renovables, así como de la implementación de diseños eficientemente energéticos para reducir el consumo de energía. La tecnología fotovoltaica puede emplearse para cubrir ambas estrategias convirtiendo no sólo la luz natural, sino también la artificial, en electricidad. De entre las diferentes tecnologías fotovoltaicas emergentes, las perovskitas alcanzan la más alta eficiencia en conversión de potencia, al mismo tiempo que proporcionan una banda de energía prohibida ampliamente ajustable con pérdidas mínimas de tensión de circuito abierto. Además, su fabricación usa materiales abundantemente disponibles, y no requiere necesariamente de procesos a alta temperatura ni de técnicas de deposición en vacío. En esta tesis, mejoramos la colección de luz en celdas de perovskitas, a la vez que abordamos el concepto de eficiencia energética a través de una fabricación optimizada y su integración en estructuras selectivas de luz. Esto es conseguido gracias a la implementación de estrategias ópticas y materiales aplicadas a diseños específicos de celdas solares de perovskita. Los resultados demuestran que tales estrategias proporcionan una colección de luz y un rendimiento fotovoltaico mayor aplicable a dispositivos fabricados a baja temperatura, y permiten el reciclaje de luz en electricidad para aplicaciones fotónicas alternativas. Las técnicas presentadas podrían ser utilizadas en procedimientos futuros para disminuir la cantidad de Pb empleado en celdas solares de perovskita, y para reducir el consumo de energía durante su fabricación y el funcionamiento de otros dispositivos optoelectrónicos. La tesis está organizada en cuatro capítulos. El Capítulo 1 sirve como una introducción, donde la actual situación energética y la tecnología fotovoltaica son analizadas junto a una descripción de la recolección de luz y la eficiencia energética en celdas solares de p, Postprint (published version)

Bringing the Voc of a perovskite solar cell toward its radiative value, corresponding to a 100% external fluorescence quantum yield (QY) of the cell, has been pursued to reach the highest performance photovoltaic devices. Therefore, much research has been focused on maximizing the QY of the active layer isolated from the rest of the cell layers. However, such quantity does not often correlate with the Voc following the ideal diode relation. Herein, the QYs of complete FA0.8MA0.2PbI3-yBry solar cells are reported, ranging from 0.1% to 3%, and compared with their Vocs, ranging from 1 to 1.13¿V. By combining these measurements with electromagnetic simulations based on a full-wavevector detailed balance and a fluorescence power-loss model, it is demonstrated that a nonoptimal Voc in mixed-cation lead halide perovskite cells is not only due to nonradiative photocarrier recombination at traps. In addition to the expected parasitic absorption of the emitted photons in the electrode layers, discrepancies appear between Voc and QY. These discrepancies are attributed to the rise of energy barriers, a side effect of trap removal. Indeed, although surface passivation may enhance the QY, its beneficial effect may be counterbalanced by the emergence of such barriers between active and charge-transporting layers. Bringing the Voc of a perovskite solar cell toward its radiative value, corresponding to a 100% external fluorescence quantum yield (QY) of the cell, has been pursued to reach the highest performance photovoltaic devices. Therefore, much research has been focused on maximizing the QY of the active layer isolated from the rest of the cell layers. However, such quantity does not often correlate with the Voc following the ideal diode relation. Herein, the QYs of complete FA0.8MA0.2PbI3-yBry solar cells are reported, ranging from 0.1% to 3%, and compared with their Vocs, ranging from 1 to 1.13¿V. By combining these measurements with electromagnetic simulations based on a full, This work was supported by the Spanish Ministry MINECO (grant RTI2018- 098728-B-C33), the Spanish Ministry of Economy and Competitiveness (OrgEnergyExcelence Network CTQ2016-81911-REDT), the Severo Ochoa program (grant SEV-2015-0522), the MINECO and the FEDER (grant MAT2017-89522-R), the Fundació Mir Puig, the Fundacio Privada Cellex, and the CERCA program from the Generalitat de Catalunya. C.G.F. acknowledges European Union’s Horizon 2020 Research and Innovation Program under the Marie Skłodowska-Curie grant agreement no. 665884. J.M. acknowledges support from Generalitat de Catalunya (Program ICREA Academia)., Peer Reviewed, Postprint (author's final draft)

For many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two ¿/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day., Q.L., L.G., F.B., J.T., and J.M. acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) through the “Severo Ochoa” program for Centres of Excellence in R&D (SEV2015-0522), from Fundació Privada Cellex, from Generalitat de Catalunya through the Impulsa-Llavor (Llav-00073) and CERCA programs, and also acknowledge financial support from MINECO through project MAT2017-89522-R. T.L. and X.Z. thank the NSFC (No. 21734001). F.B. also thanks the CONACyT international grants program., Postprint (published version)

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of largecations to obtain a perovskite with larger lattice parameter together with a large crystalsize to minimize interface nonradiative recombination. When applying the two-stepmethod for a better crystal control, it is rather challenging to fabricate perovskites withFA+cations, given the small penetration depth of such large ions into a compact PbI2film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl-anions achieving,via a solvent annealing-controlled dissolution-recrystallization, larger than 1µmperovskite crystals in a solar cell. This solar cell, with a largely increasedfluorescencequantum yield, exhibited an open-circuit voltage equivalent to 93% of thecorresponding radiative limit one. This, together with the low band gap achieved(1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum., Peer Reviewed, Postprint (author's final draft)

Graphane offers a safe and high capacity hydrogen storage. Unfortunately, production of graphane directly from its parent material graphene requires breaking the extended p bond, implying either harsh chemical environments or highly energetic plasmas. In here, we propose to use graphene oxide (GO) to initially have the lattice irregularities and local curvature conferring to some of the carbons a favorable partial negative charge or a sp3 hybridization suitable for C–H bond formation. When GO covers the cathode of a water splitting cell powered at 1.7¿V, we demonstrate an effective hydrogen chemisorption exhibiting a logarithmic growth with time. Such GO undergoes a dynamic evolution, combining a continuous change in the local corrugation and partial charge distribution with deoxygenation, opening additional sites for hydrogen chemisorption. Using density functional theory combined with the experimental parameters we can monitor the H atom gravimetric density increase as the water splitting experiment takes place., Peer Reviewed, Postprint (author's final draft)

In an optical medium slab, a randomization of the interfaces will lead to an ergodic light propagation, implying a kind of light trapping that sets an upper limit for a broadband absorption enhancement factor. However, in thin film solar cells where such light trapping is most needed, it is not straightforward how the implementation of a surface randomization should be done. In addition, such randomization may also severely limit the electronic device performance. Here, a cylindrical periodic corrugation, which is named halfcylinder photonic plate, is introduced at the light entering interface which leads to an ergodic light propagation. Advantage of such ergodicity is taken to enhance light absorption in a perovskite cell, physically separated by a planar glass substrate from such half-cylinder photonic plate. Theoretically and experimentally it is demonstrated that the enhancement light absorption factor measured is close to the maximum possible for light trapping schemes using one interface periodic corrugations. This upper limit results from the ergodic light propagation that is achieved with a half-cylinder photonic plate made from an optical transparent material with a low refractive index that minimizes light rejection by reflectivity., Peer Reviewed, Postprint (author's final draft)

In the subwavelength regime, several nanophotonic configurations have been proposed to overcome the conventional light trapping or light absorption enhancement limit in solar cells also known as the Yablonovitch limit. It has been recently suggested that establishing such limit should rely on computational inverse electromagnetic design instead of the traditional approach combining intuition and a priori known physical effect. In the present work, by applying an inverse full wave vector electromagnetic computational approach, a 1D nanostructured optical cavity with a new resonance configuration is designed that provides an ultrabroadband (˜450 nm) light absorption enhancement when applied to a 107 nm thick active layer organic solar cell based on a low-bandgap (1.32 eV) nonfullerene acceptor. It is demonstrated computationally and experimentally that the absorption enhancement provided by such a cavity surpasses the conventional limit resulting from an ergodic optical geometry by a 7% average over a 450 nm band and by more than 20% in the NIR. In such a cavity configuration the solar cells exhibit a maximum power conversion efficiency above 14%, corresponding to the highest ever measured for devices based on the specific nonfullerene acceptor used., Peer Reviewed, Postprint (author's final draft)

Solution-processed metal halide perovskite materials haverevealed outstanding optoelectronic features that make them uniquely suitedfor photovoltaic applications. Although a rapidprogresshasledtoperformances similar to inorganic thinfilm technologies, the fabricationmethod of some of the most widely used electron selective layers, based oneither mesoporous architectures or high annealing temperatures, may limit yeta future large-scale production. In that regard, planar perovskite solar cellconfigurations that can be processed at low temperatures are more desirable.Herein, we demonstrate that a few tens of nanometers thick bilayer, made oftwo types of inorganic oxide nanoparticles, can perform as a robust and low-temperature-processed electron-selective contact for planar perovskite solarcells. Aside from boosting the average efficiency of planar opaque devices, theproposed method allowed us to preserve the main photovoltaic characteristicswhen thinner active layers, usually exhibiting a noncontinuous morphology, were integrated for semitransparent cells. Byproviding excellent electronic and coverage features against the bottom electrode, this novel configuration may hence offer analternative route to approach future inexpensive printable methodologies for the fabrication of efficient low-temperatureperovskite solar cells., Peer Reviewed, Postprint (author's final draft)

Cotutela Universitat Politècnica de Catalunya i Karlsruher Institut für Technologie, Solution-processed thin film solar cells emerged as very promising photovoltaic technologies suitable for a low cost roll-to-roll upscale production. Such thin film character also ensures lightweight and flexibility for the solar cell modules, making them ideal for a wide variety of applications where silicon panels cannot be used. In addition to the above-mentioned advantages, common in all solution-processed thin film technologies, polymer solar cells (PSCs) have a unique semitransparency, which makes them very useful for solar window applications and very competitive in building integrated photovoltaics. In recent years, a remarkable progress has been achieved in the field of PSCs. The power conversion efficiency of PSCs has already surpassed the 11% barrier. However, to be able to eventually compete with other solution-processed thin film technologies, such device efficiency must be further improved. Given the low charge carrier mobility in commonly used organic p-conjugated semiconductors, the tradeoff between optical absorption and charge collection, limits the thickness of the majority of photoactive layers currently being used to approximately 100 nm. To overcome the limited light absorption in such thin active layers, an adequate optical management becomes very important. Ideally, a light absorption or short-circuit current enhancement should be achieved without affecting the other photovoltaic parameters, such as the photovoltaic device open circuit voltage and fill factor. In this thesis, we implement a one-dimensional new optical planar cavity that exhibits a resonant character at two different nonharmonic frequencies of each other, which we named two-resonance tapping cavity (TRTC). With such TRTC we demonstrate that one may reach an optimal broadband light trapping in thin film cells, largely improving the photocurrent of the solar without sacrificing the device electrical properties. A limited stability is another obstacle that may prevent any industr, Las celdas solares de capa fina procesadas por métodos en solución surgieron como una interesante tecnología fotovoltaica para una producción roll-to-roll a bajo coste. El carácter de película delgada también asegura el bajo peso y la flexibilidad de los módulos solares, haciéndolos ideales para una gran variedad de aplicaciones donde los paneles de silicio no pueden ser usados. Además de las ventajas mencionadas, compartidas por todas las tecnologías de capa fina procesadas en solución, las celdas solares de polímero (PSCs) presentan propiedades únicas de semi-transparencia, lo que las hace útiles para aplicaciones en ventanas solares y muy competitivas como elementos integrados en edificios. En los últimos años, se ha logrado un progreso notable en el campo de las PSCs. La eficiencia de conversión de energía de las PSCs ha superado ya la barrera del 11%. Sin embargo, para poder competir finalmente con otras tecnologías de capa delgada procesadas en solución, la eficiencia del dispositivo debe mejorarse aún más. Debido a la baja movilidad de los portadores de carga para los típicos semiconductores orgánicos p-conjugados, el compromiso entre absorción óptica y recolección de carga limita el espesor de la mayoría de las capas fotoactivas que se utilizan actualmente a aproximadamente 100 nm. Para mejorar la limitada absorción de luz en dichas capas activas, el uso de estrategias ópticas adecuadas es muy relevante. Idealmente, se debería lograr un aumento de la absorción de la luz o de la corriente a cortocircuito sin afectar a los demás parámetros fotovoltaicos, tales como el voltaje a circuito abierto y el factor de llenado del dispositivo fotovoltaico. En esta tesis, incorporamos una nueva cavidad óptica planar unidimensional que presenta un carácter resonante en dos frecuencias no armónicas diferentes, y que llamamos two-resonance tapping cavity (TRTC). Con la TRTC demostramos que puede alcanzarse una captura óptima de la luz en celdas de capa fina sobre un amplio, Postprint (published version)

We present a novel monolithic architecture for optimal light harvesting in multijunction thin film solar cells. In the configuration we consider, formed by a perovskite (PVK) cell overlying a CIGS cell, the current extracted from the two different junctions is decoupled by the insertion of a dielectric nonperiodic photonic multilayer structure. This photonic multilayer is designed by an inverse integration approach to confine the incident sunlight above the PVK band gap in the PVK absorber layer, while increasing the transparency for sunlight below the PVK band gap for an efficient coupling into the CIGS bottom cell. To match the maximum power point voltages in a parallel connection of the PVK and CIGS cells, the latter is divided into two subcells by means of a standard three-laser scribing connection. Using realistic parameters for all the layers in the multijunction architecture we predict power conversion efficiencies of 28%. This represents an improvement of 24% and 26% over the best CIGS and PVK single-junction cells, respectively, while at the same time outperforms the corresponding current-matched standard tandem configuration by more than two percentage points., Peer Reviewed, Postprint (author's final draft)

Aquest article pot ser utilitzat per a fins no comercials, d'acord amb els termes i condicions d’auto-arxiu de Wiley., Large area flexible electronics rely on organic or hybrid materials prone to degradation limiting the device lifetime. For many years, photo-oxidation has been thought to be one of the major degradation pathways. However, intense illumination may lead to a burn-in or a rapid decrease in performance for devices completely isolated from corrosive elements as oxygen or moisture. The experimental studies which are presented in here indicate that a plausible triggering for the burn-in is a spin flip after a UV photon absorption leading to the accumulation of electrostatic potential energy that initiates a rapid destruction of the nanomorpholgy in the fullerene phase of a polymer cell. To circumvent this and achieve highly stable and efficient devices, a robust nanocrystalline ordering is induced in the PCBM phase prior to UV illumination. In that event, PTB7-Th:PC71BM cells are shown to exhibit T80 lifetimes larger than 1.6 years under a continuous UV-filtered 1-sun illumination, equivalent to 7 years for sunlight harvesting at optimal orientation and 10 years for vertical applications., Peer Reviewed, Postprint (author's final draft)

This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., An optimal photon absorption in thin film photovoltaic technologies can only be reached by effectively trapping the light in the absorber layer provided a considerable portion of the photons is rejected or scattered out of such layer. Here, a new optical cavity is proposed that can be made to have a resonant character at two different nonharmonic frequencies when adjusting the materials or geometry configurations of the partially transmitting cavity layers. Specific configurations are found where a reminiscence of such two fundamental resonances coexists leading to a broadband light trapping. When a PTB7-Th:PC71BM organic cell is integrated within such cavity, a power conversion efficiency of 11.1% is measured. This study also demonstrates that when materials alternative to organics are used in the photoactive cell layer, a similar cavity can be implemented to also obtain the largest light absorption possible. Indeed, when it is applied to perovskite cells, an external quantum efficiency is predicted that closely matches its corresponding internal one for a broad wavelength range., Peer Reviewed, Postprint (author's final draft)

Thin film photovoltaics encompass a group of technologies able to harvest light within a few microns thickness. The reduced thickness allows a low cost of manufacture while making the films flexible and adaptable to different surfaces. This, combined with their low weight, positioned thin film solar cells as ideal candidates for building integrated photovoltaics. For the latter, organic solar cells (OSC) can provide a high quality semi-transparency that closely mimics the aesthetics of standard windows. Indeed, some unique features of organic solar cells make them the optimal solution for applications where standard Si technology cannot be used. However, for large-scale electricity production where efficiency is, perhaps, the most determining factor, newer thin film technologies like perovskites solar cells may be a more adequate option. At the moment of writing this thesis, state of the art efficiencies of single junction perovskites nearly double that of the best single junction organic solar cell. A limitation found in both technologies, especially in organics and to a lesser degree in perovskites, is the low mobility of the carriers. This, together with other processing shortcomings in the organic absorbers and perovskites limit their thickness to 100-130 nm, and 500-600 nm, respectively. In summary, light management must be an essential ingredient when designing device architectures to achieve the optimal performance in the specific application being considered. In this thesis, in order to achieve an optimal light harvesting and therefore increase the performance of thin film solar cells, we take two approaches. On one hand, we increase the total thickness of the absorber material used in the device without increasing the thickness of the single active material layer and, on the other hand, we combine complementary absorbers to cover a wider portion of the solar spectra. These approaches pose the double challenge of finding the optimal electromagnetic field dis, La fotovoltaica de capa delgada engloba un grupo de tecnologías capaces de capturar la luz en tan sólo unos pocos nanómetros de espesor. Su bajo costo de manufactura, flexibilidad y bajo peso, hace a las capas delgadas candidatas ideales para la integración en edificios. En particular, las celdas orgánicas pueden proveer una transparencia de alta calidad similar a las ventanas convencionales irrealizable con tecnologías basadas en Silicio. Sin embargo, para la producción de electricidad a gran escala en donde la eficiencia es, tal vez, el factor determinante, existen nuevas tecnologías como las celdas solares de perovskita que pueden resultar más adecuadas. Al momento de escribir esta tesis, las eficiencias de celdas de perovskita de simple unión casi duplican la de las mejores celdas orgánicas de simple unión. Una limitante de ambas tecnologías, en especial de las celdas orgánicas y en menor medida de las perovskitas, es la baja movilidad de las cargas. Esta, junto a otras desventajas de los absorbentes orgánicos y perovskitas limita su espesor al rango de los 100 a los 130 nm, y entre los 500 a 600 nm, respectivamente. En resumen, el manejo de la luz debe constituir un ingrediente esencial para el diseño de los dispositivos, tal que se consiga un desempeño óptimo en la aplicación para la cual sean considerados. En esta tesis, con el fin de alcanzar un aprovechamiento óptimo de la luz y por ende aumentar el desempeño de las celdas solares de capa delgada, utilizamos dos enfoques. Por un lado, aumentamos el espesor total de material absorbente dentro del dispositivo sin incrementar el espesor de las capas actives individuales y por otro lado, combinamos absorbentes complementarios para cubrir una porción más amplia del espectro solar. Estos enfoques conllevan al doble reto de encontrar la distribución de campo electromagnético óptima dentro de una estructura compleja de multicapas con dos o más capas activas, junto a la implementación de una recolección o recombinac, Postprint (published version)

Organic electronics has the potential to be incorporated in any kind of surface morphology for wearable or fully portable applications. Unfortunately when organic devices, such as solar cells, are fabricated on flexible substrates, the device performance is severely limited unless the physical properties of such substrates are carefully chosen. Here, it is demonstrated that layers of nanoparticles with a size gradient distribution can be used to obtain high performance solar cell devices that can be effectively delaminated from an original flat and rigid glass substrate. Such sacrificial nanoparticles layers are incorporated in between the glass substrate and the semitransparent electrode of a polymer:fullerene (PTB7:PC71BM) cell. After the cell delamination, freestanding flexible devices with power conversion efficiencies as high as 7.12% are obtained, which corresponds to 90% of the performance of the same cell fabricated on a standard glass smooth surface., Peer Reviewed, Postprint (author's final draft)

This document is the Accepted Manuscript version of a Published Work that appeared in final form in CS catalysis, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acscatal.6b01036, We report a photovoltaics-electrochemical (PV-EC) assembly based on a compact and easily processable triple homojunction polymer cell with high fill factor (76%), optimized conversion efficiencies up to 8.7%, and enough potential for the energetically demanding water splitting reaction (V-oc = 2.1 V). A platinum-free cathode made of abundant materials is coupled to a ruthenium oxide on glassy carbon anode (GC-RuO2) to perform the reaction at optimum potential (Delta E = 1.70-1.78 V, overpotential = 470-550 mV). The GC-RuO2 anode contains a single monolayer of catalyst corresponding to a superficial concentration (Gamma) of 0.15 nmol cm(-2) and is highly active at pH 7. The PV-EC cell achieves solar to hydrogen conversion efficiencies (STH) ranging from 5.6 to 6.0%. As a result of the solar cell's high fill factor, the optimal photovoltaic response is found at 1.70 V, the minimum potential at which the electrodes used perform the water splitting reaction. This allows generating hydrogen at efficiencies that would be very similar (96%) to those obtained as if the system were to be operating at 1.23 V, the thermodynamic potential threshold for the water splitting reaction., Peer Reviewed, Postprint (author's final draft)

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS applied materials and interfaces, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acsami.6b07985, Solution-processed ZnO sol–gel or nanoparticles are widely used as the electron-transporting layer (ETL) in optoelectronic devices. However, chemisorbed oxygen on the ZnO layer surface has been shown to be detrimental for the device performance as well as stability. Herein, we demonstrate that chemisorbed oxygen removal based on UV illumination of the ZnO surface layer under a nitrogen atmosphere can, simultaneously, improve the power conversion efficiency and photostability of PTB7-Th:PC71BM-based inverted polymer solar cells. By a systematic study of such a UV illumination procedure, we obtained optimal conditions where both the cell efficiency and stability were improved. We fabricated cells with a power conversion efficiency higher than 9.8% and with a T80 lifetime longer than 500 h, corresponding to about a 2.5-fold enhancement relative to non-UV-treated ZnO reference devices., Peer Reviewed, Postprint (author's final draft)

Organic polymeric solar cells require thin absorber layers due to intrinsic low exciton diffusion distances, thus showing low absorption of light. They are also characterized by having a low absorption peak, whose position lies in the near-infrared, slightly affecting the visible spectrum. By depositing thin silver layers of 10 nm for the back electrode, a semi-transparent organic solar cell can be created. Attractive outcomes could result from these devices, like solar cell windows that would remain visually transparent, while still collecting energy from the sun, or tandem polymeric solar cells. The formation of a non-homogeneous and discontinuous silver layer when depositing an ultra-thin film turns into a high resistivity electrode, therefore limiting the area that such devices can reach and the maximum power that the cell can offer. The goal of this bachelor thesis was to up-scale semi-transparent organic solar cells by characterizing the thin electrode conductivity and enhancing its charge collection using thick grid-like layers of silver. The cause for such high resistivity was confirmed by finding silver agglomerations of 30 nm in size, and a lower limit for its value was found by studying the silver bulk resistivity. The upscaling losses were determined in terms of the increase in area, and charge-collecting grids were consequently tested. The building of organic solar cells by spin-coating and vapour depositions was proved functional on top of non-flat substrates with roughness of the order of 100 nm. Fill Factor enhancements were found by designing optimal grid structures compatible with highly transparent cells.

At the moment the use our society makes of the available energy sources is far from optimal. Only a small 5% fraction of the energy used for the electricity production comes from new renewable energy sources. For over several decades, in an attempt to maximize sunlight energy harvesting, researchers in thin film devices have been searching for the optimal materials. Out of the many thin film cell options available one of the most promising is the one based on perovskites. According to the NREL chart the efficiency of such cells almost doubled in just two years. These attrac

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS applied materials and interfaces, copyright © American Chemical Society, after peer review and technical editing by the publisher., Future optoelectronic devices and their low-cost roll-to-roll production require mechanically flexible transparent electrodes (TEs) and substrate materials. Indium tin oxide (ITO) is the most widely used TE because of its high optical transmission and low electrical sheet resistance. However, ITO, besides being expensive, has very poor performance under mechanical stress because of its fragile oxide nature. Alternative TE materials have thus been sought. Here we report the development of a multilayer TiO2/Ag/Al-doped ZnO TE structure and an ITO-free polymer solar cell (PSC) incorporating it. Electro-optical performances close to those of ITO can be achieved for the proposed TE and corresponding PSC with an additional advantage in their mechanical flexibility, as demonstrated by the fact that the cell efficiency maintains 94% of its initial value (6.6%) after 400 cycles of bending, with 6 and 3 cm maximum and minimum radii, respectively. Instead of common plastic materials, our work uses a very thin (0.14 mm) flexible glass substrate with several benefits, such as the possibility of high-temperature processes, superior antipermeation properties against oxygen and moisture, and improved film adhesion., Peer Reviewed, Postprint (author's final draft)

We report a thin and robust interconnecting layer (ICL) for polymer tandem solar cells. This ICL shows low absorption, good electrical contacts, large work function contrast and robustness. Its use yields tandem cells with a very high fill factor of 76%, making this ICL a promising component of future highly efficient multijunction organic solar cells., Peer Reviewed, Postprint (author's final draft)

We report a thin and robust interconnecting layer (ICL) for polymer tandem solar cells. This ICL shows low absorption, good electrical contacts, large work function contrast and robustness. Its use yields tandem cells with a very high fill factor of 76%, making this ICL a promising component of future highly efficient multijunction organic solar cells., Peer Reviewed, Postprint (published version)

© 2015. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, We studied the performance over time of opaque and semi-transparent PTB7:PC71BM bulk heterojunction solar cells. For unsealed inverted configuration cells we observe that when the isolation from the environment is improved, the degradation observed is dominated by one single exponential decay. We demonstrate that a dielectric multilayer stack of approximately 550 nm provides an isolation that increases the lifetime of the cell close to ten times. In that event the fill factor appears to be the PV parameter dominating cell degradation resulting from a decrease in the shunt resistance. An Impedance analysis we performed indicates that a Warburg element, attributed to the presence of slowly moving charges such as heavy ions, must be included in the description of the experimental data. The contribution from such element increases as the cell degrades in good agreement with a degradation dominated by the corrosive effects from external agents reaching the active layer of the device., Peer Reviewed, Postprint (author's final draft)

Aquest article pot ser utilitzat per a fins no comercials, d'acord amb els termes i condicions d’auto-arxiu de Wiley. https://authorservices.wiley.com/author-resources/Journal-Authors/licensing-open-access/open-access/self-archiving.html, An optical metal cavity configuration is used to trap near infrared light in a semitransparent organic solar cell. The efficiency for such cell, which has a higher than 20% transparency in the visible region, is equal to 90% of the efficiency of the corresponding opaque cell., Peer Reviewed, Postprint (author's final draft)

This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving. https://authorservices.wiley.com/author-resources/Journal-Authors/licensing-open-access/open-access/self-archiving.html, Transparent electrodes (TEs) having electrooptical trade-offs better than state-of-the-art indium tin oxide (ITO) are continuously sought as they are essential to enable flexible electronic and optoelectronic devices. In this work, a TiO2-Ag-ITO (TAI)-based TE is introduced and its use is demonstrated in an inverted polymer solar cell (I-PSCs). Thanks to the favorable nucleation and wetting conditions provided by the TiO2, the ultrathin silver film percolates and becomes continuous with high smoothness at very low thicknesses (3-4 nm), much lower than those required when it is directly deposited on a plastic or glass substrate. Compared to conventional ITO-TE, the proposed TAI-TE exhibits exceptionally lower electrical sheet resistance (6.2 sq(-1)), higher optical transmittance, a figure-of-merit two times larger, and mechanical flexibility, the latter confirmed by the fact that the resistance increases only 6.6% after 10(3) tensile bending cycles. The I-PSCs incorporating the TAI-TE show record power conversion efficiency (8.34%), maintained at 96% even after 400 bending cycles., Peer Reviewed, Postprint (author's final draft)

Copyright 2015 Society of Photo Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this publication for a fee or for commercial purposes, or modification of the contents of the publication are prohibited., Over the last three decades, progress in the organic photovoltaic field has resulted in some device features which make organic cells applicable in electricity generation configurations where the standard silicon-based technology is not suitable, for instance, when a semi-transparent photovoltaic panel is needed. When the thin film solar cell performance is evaluated in terms of the device's visible transparency and power conversion efficiency, organic solar cells offer the most promising solution. During the last three years, research in the field has consolidated several approaches for the fabrication of high performance semi-transparent organic solar cells. We have grouped these approaches under three categories: devices where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity light trapping configuration. We herein review these approaches., Peer Reviewed, Postprint (author's final draft)

© 2016. This version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, We have used impedance spectroscopy technique to monitor degradation mechanisms in organic solar cells based on a blend of PTB7:PC71BM. We have measured the impedance of the cell on a periodical basis for almost four months, and experimental data have been modeled using three different circuits. The evolution of the circuital parameters gives information about the device dynamical mechanisms. We have observed at high voltages a low frequency feature that is more pronounced along days of measurement. This low frequency arc has been associated to charge accumulation that is related to a worsening of charge extraction through the contacts. The simultaneous increase of recombination and low frequency resistances at high voltages (around V-oc) results in a decrease of the fill factor and therefore of the efficiency., Peer Reviewed, Postprint (author's final draft)

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS applied materials and interfaces, copyright © American Chemical Society, after peer review and technical editing by the publisher and may be found at http://dx.doi.org/10.1021/acsami.5b04537, A 4-terminal architecture is proposed in which two thin active layers (<100 nm) of PTB7:PC71BM are deposited on a two-sided ITO covered glass substrate. By modeling the electric field distribution inside the multilayer structure and applying an inverse solving problem procedure, we designed an optimal device architecture tailored to extract the highest photocurrent possible. By adopting such a 4-terminal configuration, we numerically demonstrated that even when the two subcells use identical absorber materials, the performance of the 4-terminal device may overcome the performance of the best equivalent single-junction device. In an experimental implementation of such a 4-terminal device, we demonstrate the viability of the approach and find a very good match with the trend of the numerical predictions., Peer Reviewed, Postprint (author's final draft)

Over the last three decades, progress in the organic photovoltaic field has resulted in some device features which make organic cells applicable in electricity generation configurations where the standard silicon-based technology is not suitable, for instance, when a semi-transparent photovoltaic panel is needed. When the thin film solar cell performance is evaluated in terms of the device’s visible transparency and power conversion efficiency, organic solar cells offer the most promising solution. During the last three years, research in the field has consolidated several approaches for the fabrication of high performance semi-transparent organic solar cells. We have grouped these approaches under three categories: devices where the absorber layer includes near-infrared absorption polymers, devices incorporating one-dimensional photonic crystals, and devices with a metal cavity light trapping configuration. We herein review these approaches., Postprint (published version)

Considering that the most abundant renewable energy source is the Sun, photovoltaic technology possesses one of the highest potentials to provide environmental benign and sustainable energy worldwide. Currently, most of commercial available modules are fabricated from crystalline silicon because of its high efficiency. To lower fabrication costs and increase the functionality of the solar modules, several thin film technologies are under development. Among them, organic photovoltaics has created large expectations provided it possesses some intrinsic advantages, such as light weight, flexibility or semi-transparency. However, the low charge mobility in the majority of the organic semiconductor materials prevents the use of active layers thicker than a few hundred nanometers. This leads to a limited light harvesting capacity and, consequently, a limited conversion efficiency. Different optical approaches have been considered to enhance the absorption of organic solar cells and increase their efficiency. In this thesis, we propose a novel configuration based on the use of fiber arrays to effectively trap light and efficiently couple it into the active layer to enhance absorption. The thesis work is presented in five chapters. After an introductory chapter, in chapter 2 light absorption of an organic solar cell deposited on the backside of a fiber array is studied theoretically. A strong enhancement in light harvesting is predicted using such configuration. For small diameter fibers the enhancements originated from light coupling to some low quality whispering gallery modes, while for large diameter fibers light seemed to be effectively trapped inside the fiber structure. In chapter 3 and 4, we consider the dip-coating procedure, a fabrication technique that can be applied to deposit from a precursor solution, layers on a substrate irrespective of its shape. Its viability is demonstrated by applying it to different device architectures. The deposition on such non-flat, Si considerem que la font d'energia renovable més a abundant és el Sol, la tecnologia fotovoltaica posseeix un dels potencials més alts per poder produir l'energia mundial de forma sostenible i benigne amb el medi ambient. Actualment la majoria dels mòduls comercials estan fabricats de silici cristal.lí ja que aquest material té una gran eficiència. Per tal de rebaixar els costos de producció i incrementar la funcionalitat d'aquest panells solars, diverses tecnologies de capa prima s'estan desenvolupant. Entre elles, la tecnologia fotovoltaica amb materials orgànics ha creat grans expectatives gràcies a les seves propietats intrínseques, com per exemple la seva lleugeresa, flexibilitat o bé semi transparència. Per altra banda, la baixa mobilitat de les càrregues en la majoria dels semiconductors orgànics impedeix l'ús de capes actives no molt més gruixudes que uns pocs nanòmetres. Això provoca que tinguin una capacitat de col.lecció lumínica limitada i com a conseqüència, la eficiència de conversió energètica també ho és. S'han considerat diferents estratègies òptiques per tal de millorar l'absorció en les cel.les solars orgàniques i incrementar la seva eficiència. En aquesta tesi proposem una configuració innovadora basada en l'ús d'una matriu de fibres, les quals atrapen i acoblen la llum en la capa activa per millorar l'absorció d'aquesta. La present tesi consta de cinc capítols. Després d'un capítol introductori, en el capítol 2 s'estudia teòricament l'absorció de llum d'una cel·la solar orgànica dipositada en la part posterior de la matriu de fibres. Per diàmetres de fibra petits, les millores són degudes a l'acoblament d'uns modes recirculants de llum de baixa qualitat. Mentre que per diàmetres grans, la llum sembla estar atrapada de forma efectiva dins de l'estructura formada per les fibres. En els capítols 3 i 4 hem considerat el recobriment per immersió, aquesta tècnica de fabricació pot ser aplicada per dipositar capes des d'una solució precursora a un sub, Postprint (published version)

In recent years, it has been proposed that circular microresonators may become an important element in the core of many photonic devices. The high Q-factors seen in fused silica micro-spheres and micro-toroids for light coupled in the whispering gallery modes (WGMs) inside the micro-resonator led to many new developments in a diversity of fields. Indeed, WGM micro-resonators have found applications in laser oscillation, optical filtering, bio and chemical sensing, frequency stabilization, quantum electrodynamics experiments, nonlinear parametric conversion and in many other light-matter interaction processes where light recirculation is an essential ingredient. For second and third order nonlinear optical phenomena a high-Q micro circular cavity is an ideal framework to lower the light intensity or material density and still obtain a measurable interaction. This may become particularly useful when the nonlinear interaction is considered on the sphere surface because at an interface centro-symmetry is always broken. In this thesis, we approach the problem of obtaining SHG with the smallest amount of material possible. Our goal is to demonstrate that WGMs in micro-sphere resonators are an optimal option to consider such type of non-linear interaction. SHG from a small amount of material may found interesting applications in high sensitivity unmarked detection of low numbers of very small objects such as molecules, viruses or other types of nano-particles. The different experimental and theoretical developments we implemented to achieve such goal are reported in the four chapters of the current thesis. In chapter 1 we introduce basic concepts of spherical micro-resonators an their interest. Theoretical aspects of light propagation and nonlinear light generation in the whispering gallery modes in such micro-resonators are discussed in Chapter 2. A new method to obtain patterns of non-linear material is presented in Chapter 3. In Chapter 4, the developments presented in, En años recientes los microresonadores circulares han sido propuestos como un elemento central para formar parte de muchos dispositivos fotónicos. El alto factor de calidad observado en microesferas o microtoroides de sílice cuando la luz se propaga en modos "whispering gallery" (WG) ha dado lugar a un gran número de nuevos desarrollos en campos muy diversos. En efecto, los micro resonadores con modos WG han encontrado aplicación en la oscilación laser, en el filtrado óptico, en sensores bioquímicos, como estabilizadores de frecuencia, en experimentos de electrodinámica cuántica, en la conversión paramétrica no lineal y en muchas otros procesos donde la recirculación de luz es un ingrediente esencial para su interacción con la materia. En fenómenos ópticos no lineales de segundo y tercer orden, la micro cavidad circular con un alto factor de calidad constituye una estructura ideal para poder obtener una interacción medible incluso cuando se consideran pequeñas intensidades de luz o bajas densidades de materia. Esto puede resultar particularmente útil en la superficie de la microesfera ya que en la interface entre dos materiales se rompe la simetría de inversión incluso cuando los materiales son centro simétricos. En esta tesis abordamos la generación de segundo armónico con una cantidad mínima de material. Nuestra meta es demostrar que los modos WG en resonadores de microesfera son una opción óptima para poder considerar este tipo de interacción no lineal. La generación de segundo armónico con una cantidad muy pequeña de material puede encontrar aplicaciones interesantes en la detección de muy pocos objetos pequeños tales como moléculas, viruses o cualquier otro tipo de nanopartículas. Los diferentes desarrollos experimentales y teóricos que implementamos para alcanzar nuestro objetivo están explicados en los cuatro capítulos de esta tesis. En el Capítulo 1 introducimos conceptos básicos de microresonadores esféricos y su interés. Aspectos teóricos de la propagación, Postprint (published version)

Disordered media with refractive index variations can be found in the atmosphere, the ocean, and in many materials or biological tissues. Several technologies that make use of such random media, as image formation, satellite communication, astronomy or microscopy, must deal with an unavoidable light scattering or diffusion. This is why for many years light propagation through random media has been a subject of intensive study. Interesting phenomena such as speckle, coherent backscattering or random lasing have been discovered and studied. More recently, researchers are beginning to investigate mechanisms to control light propagation through such media to enhance light transmission and sharpen the focus. On the other hand, it has been known for several years that nonlinear random structures are able to generate light in an ultra-broad frequency range, without the need of angle or temperature tuning. Particularly interesting is the nonlinear light diffusion observed from materials with no change in the refractive index and which appear to be fully diffusion less to linear light propagation. However, a comprehensive understanding of the scattering when a nonlinear interaction takes place has not yet been given. The core of the thesis focuses on the study of the nonlinear light generation and propagation from crystalline structures with disordered nonlinear domains but with a homogenous refractive index. A random distribution of non-linear domains is found naturally in the Strontium Barium Niobate (SBN) ferroelectric crystal. As opposed to other mono-domain nonlinear optical crystals commonly used for frequency up-conversion, such as Potassium Titanyl Phosphate (KTP) or Lithium Niobate (LiNbO3), in SBN the nonlinear domain size is, typically, on the order of the coherence length or many times smaller than the size of the whole crystal. Such domains are usually several times longer in the c-axis direction relative to the plane perpendicular to that axis. Adjacent domains, Postprint (published version)

We propose the use of whispering gallery mode coupling in a novel configuration based on implementing a thin film cell on the backside of an array of parallel fibers. We performed numerical calculations using the parameters of a thin film organic cell which demonstrate that light coupling becomes more effective as the angle for the incident light relative to the fiber array normal increases up to an optimal angle close to 55 deg. At this angle the power conversion efficiency of the fiber array solar cell we propose becomes 30% times larger than the one from an equivalent planar cell configuration. We demonstrate that the micro fiber array solar cell we propose may perform an effective partial tracking of the sun movement for over 100 degrees without any mechanical help. In addition, in the event that such fiber array cell would be installed with the adequate orientation on a vertical façade, an optimal photon-to-charge conversion would be reached for sunlight incident at 55 deg with respect to the horizon line, very close to the yearly average position for the sun at Latitude of 40 deg., Peer Reviewed, Postprint (published version)

Organic solar cells have unique properties that make them very attractive as a renewable energy source. Of particular interest are semi-transparent cells, which have the potential to be integrated into building fac¸ades yet not completely block light. However, making organic cells transparent limits the metal electrode thickness to a few nanometres, drastically reducing its reflectivity and the device photon-harvesting capacity. Here, we propose and implement an ad hoc path for light-harvesting recovery to bring the photon-to-charge conversion up to almost 80% that of its opaque counterpart. We report semi-transparent PTB7:PC71BM cells that exhibit 30% visible light transmission and 5.6% power conversion efficiency. Non-periodic photonic crystals are used to trap near-infrared and near-ultraviolet photons. By modifying the layer structure it is possible to tune the device colour without significantly altering cell performance., Peer Reviewed, Postprint (published version)

Peer Reviewed, Postprint (published version)

Organic photovoltaic (OPV) technology has emerged as a potential cost-effective solution to produce electrical energy. The foreseen low manufacturing costs combined with features as semi-transparency or mechanical flexibility give to OPV devices a strong potential for industrial applicability. However, the commercial implementation of this technology faces the challenge of increasing the relatively low power conversion efficiency of the current state-of-the-art OPV devices. This thesis presents an optical based approach to enhance the performance of OPV devices by effectively controlling sunlight photons. Such control is possible because of the coherent interaction between light and the multilayered structure constituting the OPV device. Accordingly, we studied the dependence of the optical field distribution inside the solar cell relative to the optical properties of the different layers including their refractive index , extinction coefficient , and thickness. This optical study led to the prediction of optimal OPV device structures. The first implementation of a photon control was done by changing the relative thicknesses of the different layers in the device. An optimal combination of thicknesses was found and confirmed experimentally. A significant reduction of the energy lost in the device was demonstrated. As a consequence, the photon harvesting improved, which led to a close matching between the external and internal quantum efficiencies in a broad wavelength range. A second photon control strategy to enhance the performance of OPV cells was implemented by modifying the complex refractive index of the nonactive device layers. Both and were changed in specific layers by considering new materials. Three different cases were considered: in the first example a BCP layer was used to replace calcium as electron transporting layer. The parasitic absorption induced by the highly absorptive calcium layer was diminished almost to zero after replacing this layer with B, La tecnología fotovoltaica orgánica (OPV) ha surgido como una solución potencial rentable para producir energía eléctrica. Los bajos costos de manufactura previstos combinados con propiedades como semi-transparencia o flexibilidad mecánica le dan a los dispositivos OPV un gran potencial de ser aplicados industrialmente. Sin embargo, la implementación comercial de esta tecnología se enfrenta al reto de incrementar la relativamente baja eficiencia de los dispositivos OPV del estado del arte. Esta tesis presenta una aproximación óptica para aumentar la eficiencia de los dispositivos OPV mediante un control efectivo de los fotones de la radiación solar. Tal control es posible debido a la interacción coherente entre la luz y la estructura de multi-capas que constituye el dispositivo OPV. Consecuentemente, en esta tesis se estudia la dependencia de la distribución del campo óptico dentro de la celda solar con las propiedades ópticas de las diferentes capas. Entre esas propiedades se incluyen el índice de refracción , el coeficiente de extinción y espesor de cada una de las capas. Este estudio óptico ha permitido predecir estructuras óptimas para los dispositivos OPV. La primera implementación del control de fotones fue hecha al cambiar los espesores relativos de las diferentes capas en el dispositivo. Una combinación óptima fue encontrada y confirmada experimentalmente. Una reducción significativa de la energía perdida por reflexión especular fue demostrada y como consecuencia, la recolección de fotones fue mejorada lo cual condujo a la concordancia entre las eficiencias cuánticas externa e internas en un amplio rango de longitudes de onda. Una segunda estrategia de control de fotones para mejorar el desempeño de los dispositivos OPV fue implementada tras modificar las propiedades ópticas de las capas en el dispositivo distintas a la capa activa. Tanto como fueron cambiados en capas específicas tras considerar nuevos materiales. Tres casos diferentes fueron considerados, Postprint (published version)

Cotutela ICFO-Universitat Politècnica de Catalunya i Institut Fresnel-Université Aix-Marseille, In this century some of our main issues are energy shortage and pollution. This work will briefly describe these problems, proposing a plan of action combining energy saving and different sustainable energy sources. Within different types of renewable energy sources, solar energy is the most abundant one. To make solar energy a more sustainable and cost effective technology we focus on enhancing the optical characteristics of thin film solar cells. In this category, organic solar cells are good options for their exiguous amount of material and the low energy needed for the fabrication process. This technology can be lightweight, transparent, flexible and conformal in order to be applied to and integrated in various architectural solutions and consumer electronics. After a study of the physics of such devices and on how to optically enhance their performances, we will show some examples where we theoretically and experimentally collect the solar radiation with optical antennas. We report, for the first time in literature, a nanogap antenna that efficiently couples the light in our active material thin film. Finally, we elaborate on the concept of building integrated photovoltaics introducing some examples of solar façades. Based on our research, we are able to design and fabricate an organic transparent solar cell with a visible transparency above 20% and an optically enhanced photon-electron conversion efficiency remarkably similar to its opaque equivalent., En el presente siglo, algunas de las prioridades son la escasez de la energía y la contaminación. Este trabajo describirá brevemente estos problemas y propondrá un plan de acción que combina el ahorro energético con diferentes fuentes sostenibles de energía. Dentro de estas fuentes de energía renovables, la energía solar es la más abundante. Con el objetivo de hacer la tecnología solar más sostenible y eficiente económicamente nos concentramos en aumentar las características ópticas en celdas solares de película delgada. Dentro de esta categoría, las celdas solares orgánicas son una buena opción porque su desarrollo requiere bajas cantidades de materiales y su fabricación es de baja energía embebida. Adicionalmente, esta tecnología puede ser liviana, transparente, flexible mecánicamente y modular para ser aplicada e integrada en varias soluciones arquitectónicas y de electrónica de consumo. Luego de estudiar los procesos físicos en tales dispositivos y de determinar las metodologías para aumentar ópticamente sus desempeños, mostraremos algunos ejemplos donde teórica y experimentalmente se colecta la radiación solar mediante antenas ópticas. Se reporta por primera vez, una antena de nanogap que acopla eficientemente la luz en la capa activa de la celda solar. Finalmente, se desarrolla el concepto de tecnología fotovoltaica integrada en edificaciones tras introducir algunos ejemplos de fachadas solares. Basados en nuestra investigación, fue posible diseñar y fabricar una celda solar orgánica transparente cuya transparencia en el rango visible estuvo por encima del 20% y una eficiencia de conversión foton-electron aumentada ópticamente que resulto notoriamente similar a la celda solar orgánica opaca equivalente., La rareté grandissante des ressources en énergie associée à une augmentation de la pollution font partie des enjeux plus importants de ce siècle. Cette thèse décrira brièvement ces deux problématiques et proposera un plan d’action combinant économie d’énergie et diversité des sources d’énergies renouvelables. Parmi les formes d’énergies renouvelables disponibles, l’énergie solaire est la plus abondante. Pour faire de l’énergie solaire une ressource plus durable et plus rentable économiquement, nous proposons d’amplifier les propriétés optiques de cellules solaires en couches minces. Dans cette catégorie, les cellules solaires organiques représentent un choix pertinent de part la faible quantité de matériau nécessaire ainsi que la faible énergie nécessaire au procédé de fabrication. Cette technologie peut être légère, transparente et flexible de sorte qu’elle peut être utilisée dans différentes solutions architecturales s’adaptant à des produits électroniques pour le grand publique. Suivra la théorie sous jacente à ces dispositifs et l’explication de la manière dont leurs performances sont améliorées. Nous présenterons quelques exemples où l’on collecte la radiation solaire avec une antenne optique. Ainsi, nous faisons la toute première démonstration d’une antenne auto-assemblée qui couple efficacement la lumière dans le matériau constituant la couche mince que nous utilisons. Finalement, nous développons le concept de cellules photovoltaïques intégrées en présentant différents cas de façades solaires. Ces travaux nous ont permis de concevoir et de fabriquer une cellule solaire organique transparente avec une transparence dans le visible de 20% et une efficacité de conversion photon-électron améliorée, similaire à une cellule équivalente opaque., La difficile reperibilità di risorse energetiche e l’inquinamento sono alcuni dei problemi più importanti di questo secolo. In questo lavoro saranno presentati brevemente questi temi proponendo un piano d’azione che abbini il risparmio energetico alle differenti fonti di energia rinnovabili. Nell’insieme delle fonti energetiche rinnovabili l’energia solare è senz’altro la più abbondante. Con l’obbiettivo di rendere lo sfruttamento di tale energia più sostenibile ed economicamente vantaggioso, ci premuriamo di migliorare le caratteristiche ottiche di celle fotovoltaiche a film sottile. In questa categoria utilizziamo, tra le diverse opzioni, le celle solari organiche in quanto la loro fabbricazione richiede una quantità di materiale minimo e un basso consumo energetico. Inoltre questi tipi di dispositivi possono essere leggeri, trasparenti, flessibili e conformabili alle superfici su cui sono applicati. Questa è una tecnologia che potrebbe essere implementata e integrata in varie soluzioni architettoniche o nell’ elettronica di consumo. Dopo aver presentato i principi fisici di tali dispositivi e determinato le metodologie ottiche per aumentarne le prestazioni, vengono illustrati alcuni esempi dove, teoricamente e sperimentalmente, riusciamo a intercettare la radiazione solare con antenne ottiche. Riportiamo, per la prima volta in letteratura, un’antenna ottica con nano-gap che accoppia efficacemente la luce solare nel nostro materiale attivo a film sottile. Nell’ultima parte sviluppiamo il concetto di tecnologia solare integrata negli edifici, introducendo alcuni esempi di facciate solari. Basando il design sulla nostra ricerca, è possibile realizzare una cella solare fotovoltaica organica trasparente, con una trasparenza superiore del 20% e un’ efficienza di conversione fotone-elettrone migliorata grazie all’ottica, che risulta molto vicina all’ equivalente cella fotovoltaica organica non trasparente., Postprint (published version)

We theoretically investigate how phase-only spatial light modulation can enable controlling and focusing the second-harmonic light generated in transparent nonlinear random structures. The studied structures are composed of domains with random sizes and antiparallel polarization, which accurately model widely used ferroelectric crystals such as strontium barium niobate. Using a first-principles Green-function formalism, we account for the effect that spatial light modulation of the fundamental beam introduces into the second-order nonlinear frequency conversion occurring in the considered class of structures. This approach provides a complete description of the physical origin of the second-harmonic light generation in the system, as well as the optimization of the light intensity in any arbitrary direction. Our numerical results show how the second-harmonic light is influenced by both the disorder in the structure and the boundaries of the crystal. Particularly, we find that the net result from the interplay between disorder and boundary effects is strongly dependent on the dimensions of the crystal and the observation direction. Remarkably, our calculations also show that although in general the maximum possible enhancement of the second-order light is the same as the one corresponding to linear light scattering in turbid media, in the Cerenkov phase matching direction the enhancement can exceed the linear limit. The theoretical analysis presented in this work expands the current understanding of light control in complex media and could contribute to the development of a new class of imaging and focusing techniques based on nonlinear frequency mixing in random optical materials., Peer Reviewed, Postprint (published version)

We propose a transparent electrode consisting of an aluminum doped zinc oxide (AZO) layer capped with an ultrathin oxidized Ti film for indium-free bulk-heterojunction polymer solar cells (PSCs). The oxidized Ti increases the chemical, environmental, stability and the surface smoothness of AZO while still maintaining its electrical and optical properties. The application potential of the proposed transparent electrode is demonstrated in an inverted PSC, which shows an efficiency of 6.3%, very close to the value (7%) obtained in a similar structure using indium tin oxide. This efficiency is the highest reported to date for PSCs incorporating AZO electrodes., Peer Reviewed, Postprint (published version)