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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)

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Measuring open-circuit voltage (Voc) vs. light intensity (Suns) in solar cells permits the access of cell performance without the series resistance effect. This work shows the implementation of a Suns-Voc measurement system which consists of an off-the-shelf photograph flash-lamp, a digital oscilloscope and a specifically designed photodetector board with three silicon photodiodes that covers a light intensity measurement range from 10-5 to 102 suns. The whole system is controlled via USB ports from a personal computer allowing the measurement of cell characteristics (pseudo-dark and pseudo-light current density-voltage curves, as well as the pseudo fill factor and pseudo efficiency) automatically in a very short time., This work was supported in part by the Spanish Government under a FPU grant (FPU13/04381), and projects ENE2016-78933-C4-1-R, ENE2017-87671-C3-2-R and TEC2017-82305-R. The work was also supported in part by the project REFER COMRDI15-1-0036 funded by ACCIÓ and the European Regional Development Fund (FEDER). The authors wish to thank Mr. Miguel Garcia for its help in the mechanical and electrical photodetector interface development., Peer Reviewed, Postprint (author's final draft)

This book presents a set of solved examples on semiconductor device physics. Semiconductordevices is a core subject in electrical engineering and physics curricula. The level of the proposedexamples corresponds to a semester course at senior undergraduate or junior graduate level.Readers are expected to have a basic background on quantum and solid state physics, moreovera reasonable mathematical knowledge reaching differential equations is also assumed., 2019/2020

This work studies a novel electron-selective contact for n-type silicon solar cells based on the modification of the cathode with a polymeric interlayer. Specifically, a thin layer of the conjugated polyelectrolyte PFN is intercalated before the Al contact. During solution-processing, the amine groups in the PFN polymer form intense dipoles by the aggregation of acetate radicals. These dipoles are partially oriented in spin-coated layers because of the different interactions with the substrate and the solvent. As a result, an internal electrostatic potential appears that causes an apparent reduction of the cathode work-function. In this way, electron-selectivity at the rear contact is enhanced by inducing a favorable band-bending near the surface. A significant improvement is obtained in conventional electron-selective contacts with n-doped a-Si:H layers. Furthermore, good-performing solar cells can be also obtained even without intentionally doped layers. Compared to other solutions like the evaporation of metal-fluoride salts, the PFN cathode-modification can be readily done at ambient conditions. Besides, the soft solution-processing kindly preserves the properties of the underlying passivating layers., Postprint (author's final draft)

© <2019> Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, Very thin, thermally evaporated MoOx (x¿<¿3) layer has been used as transparent hole-selective contact on an n-type Germanium substrate to effectively demonstrate PV conversion capability. The fabricated MoOx/Ge heterojunction PV cell shows a photocurrent density of 44.8¿mA/cm2 under AM1.5G illumination, which is comparable to that of conventional Ge PV cells. However, a low open-circuit voltage of 138¿mV is obtained, which might be explained by the presence of tunnelling mechanisms through the MoOx/Ge interface. To our knowledge, this is the first demonstration of a hole-selective contact made of transition metal oxide on an n-type semiconductor different from c-Si. Thus, this work may have important implications toward the development of new device architectures, such as novel low-cost Ge PV cells with possible applications in multijunction solar cells and thermophotovoltaics., Peer Reviewed, Postprint (author's final draft)

In this project I will learn how to fabricate twisted van der Waals heterostructures using mechanical exfoliation and pick up & transfer methods. I will also learn how to perform device nanofabrication using modern electron beam lithography techniques. The devices will consist of heterostructures built by assembling two van der Waals materials on top of each other with a small angle of rotation between the two crystalline lattices. These exhibit a range of interesting quantum phases and electronic behaviors, such as correlated insulating states, superconductivity, nematicity, etc. I will investigate this behavior via quantum electronic transport experiments at low temperature and variable magnetic field., Outgoing

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Front surface texturization is a standard procedure used to improve optical properties of photovoltaic devices. In some particular cases, such as when dealing with ultrathin substrates, common texturization techniques can become unpractical or even unfeasible. Texturized polymer films applied on top of such devices may be used as an alternative. In this article, we report on the development of textured polydimethylsiloxane (PDMS) films to be placed on top of planar crystalline silicon solar cells based on thin substrates (=40 µ m). The PDMS polymer is deposited onto a rough surface (conventional random pyramid textured silicon), cured and detached from it. By scanning electron microscope images, we demonstrate that the dilution of PDMS into toluene helps in a better replica of the master surface. Next, we apply the optimized PDMS films on top of dummy samples based on 10, 20, and 40 µm thick crystalline silicon (c-Si) substrates whose reflectance is significantly reduced after placing the PDMS films. Accurate optical simulations indicate that the optical improvement comes from three mechanisms: higher light transmission into the device, lower reflectance at the c-Si surface, and better light trapping properties at the thin c-Si absorber. Experimental verification of the optical improvement with texturized PDMS films is reported based on 40 µ m thick solar cell, where a short-circuit current density gain of 1.7 mA/cm 2 is observed., This work was funded by MINECO from Spanish government under projects TEC2017-82305-R, ENE2016-78933-C4-1-R, ENE2017-87671-C3-2-R. The work was also supported in part by project REFER COMRDI15-1-0036 funded by ACCIÓ and the European Regional Development Fund (FEDER)., Peer Reviewed, Postprint (published version)

This is the peer reviewed version of the following article: Ortega, P, Garín, M, von Gastrow, G, et al. Black silicon back‐contact module with wide light acceptance angle. Prog Photovolt Res Appl. 2019; 1– 7, which has been published in final form at https://doi.org/10.1002/pip.3231. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving, In this work, a photovoltaic mini-module combining interdigitated back-contacted solar cells with black silicon in the front was implemented as a proof of concept. The module consists of nine solar cells connected in series with an active area of 86.5 cm2. Both the assembly and panel encapsulation were made using industrial back-contact module technology. Noticeable photovoltaic efficiencies of 18.1% and 19.9% of the whole module and the best individual cell of the module, respectively, demonstrate that fragile nanostructures can withstand standard module fabrication stages. Open-circuit voltage and fill factor values of 5.76 V and 81.6%, respectively, reveal that series interconnection between cells works as expected, confirming a good ohmic contact between cell busbars and the conductive backsheet. Additionally, the excellent quasi-omnidirectional antireflection properties of black silicon surfaces prevail at module level, as it is corroborated by light incidence angle dependence measurements of the short-circuit current parameter., Peer Reviewed, Postprint (author's final draft)

In the last years, our research group has focused on the development of Interdigitated Back Contacted (IBC) solar cells based on transition metal oxides. In particular, a very thin Al2O3/TiO2 stack deposited by ALD has demonstrated excellent results. The point of this work is to try to develop a transparent conductive oxide that could be inserted in between the passivating layers and the metal contact in order to improve rear reflector properties. As a good candidate, we focus on Zinc Oxide (ZnO) deposited from DEZ (Ditehylzinc) and water as gas precursors. A deposition rate of 1.76 A/cycle is found for layers deposited at 125 ºC indicating a self-limited reaction, i.e. atomic layer deposition process. This film has an optical band gap of 3.26 eV demonstrating good optical properties to work as a back reflector. Unfortunately, contact quality with these layers is very poor. In order to investigate the origin of such a poor performance, we measure ZnO workfunction by Scanning Kelvin Probe resulting in 4.85 eV. This value suggests that the developed ZnO films could work better in a hole selective contact. To demonstrate this point, contacts on MoOx layers were fabricated leading to reasonable contact resistance values of 43 mohmscm. Passivation properties of these contacts will be addressed in the future in order to consider their introduction into the final IBC solar cell., Peer Reviewed, Postprint (published version)

© 2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Dielectric/metal/dielectric structures based on vanadium pentoxide with a thin silver interlayer have been optimized to replace traditional transparent electrodes. As would be expected, there is a tradeoff in the metal thickness to achieve high transparency and low sheet resistance simultaneously. It has been demonstrated that an ultra-thin gold seed prevents the tendency of silver to form clusters. This wetting effect reduces the metal thickness needed to form a continuous film, which leads to a higher averaged transmittance and very low sheet resistance. On the other hand, vanadium pentoxide on silicon forms a high-quality hole-selective contact. Thus, these structures can be used as an all-in-one transparent electrode and selective contact for a new kind of heterojunction solar cells. This concept has been proved in a 13.3% efficient solar cell fabricated on n-type silicon wafers. Besides being dopant-free, the complete fabrication route did not require any sputtered transparent electrode., Peer Reviewed, Postprint (author's final draft)

III-IV semiconductors offer great potential due to their carrier mobility and direct bandgap, but their integration in Si wafers can be difficult due to lattice mismatch. Nanowire structures can solve this problem thanks to strain relaxation at the interface, as well as enhacing the functional properties of the III-IV semiconductor with respect to the bulk structure. Furthermore, MBE grown nanowires can be tuned in order to achieve quantum confinement, such as quantum wires and quantum dots, inside the nanowire structure, obtainig its consequently optical and electronical p, In the aim to grow vertical self-catalyzed GaAs nanowires on [100] Si, fancy structures with a tapered shape resembling a tree were observed. They were called nanotrees. This is the first time that vertical growth has been achieved without catalyzer in this conditions. Different techniques were used to characterize these structures, such as TEM, Raman spectroscopy, photoluminescence and cathodoluminescence. Additionally, simulations were made to analyze light confinement inside the nanotree. Overall, nanotrees are promising structures for devices, from solar cells to photodiodes., Con el propósito de crecer nanohilos de arseniuro de galio en silicio [100] sin catalizador se descubrieron unas estructuras con forma triangular que se llamaron nanoárboles. Esta ha sido la primera vez que se ha obtenido un crecimiento vertical en estas condiciones. Diferentes técnicas se han utilizado para caracterizar estas estructuras, como TEM, espectroscopia de Raman, fotoluminiscencia y catodoluminiscencia. Adicionalmente, se hicieron simulaciones para analizar el confinamiento de la luz dentro del nanoárbol. En perspectiva, los nanoárboles son unas estructuras prometedoras para futuros dispositivos, como células solares o fotodiodos., Amb el propòsit de créixer nanofils d'arsenur de gal·li sobre silici [100] sense catalitzador es van descobrir unes estructures de forma triangular que es van anomenar nanoarbres. Aquest és el primer cop que s'assoleix un creixement vertical en aquestes condicions. Diferents tècniques s'han fet servir per caracteritzar aquestes estructures, com ara TEM, espectroscòpia de Raman, fotoluminescència i catodoluminescència. Addicionalment, es van fer simulacions per analitzar el confinament de la llum als nanoarbres. En perspectiva, els nanoarbres són unes estructures prometedores per a futurs dispositius, com ara cèl·lules solars o fotodíodes.

Postprint (author's final draft)

© <2018>. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, Heterojunction solar cells based on molybdenum sub-oxide (MoOx) deposited on n-type crystalline silicon have been fabricated. The hole selective character of MoOx is explained by its high workfunction, which causes a strong band bending in the Si substrate. This bending pushes the surface into inversion. In addition, the sub-stoichiometry of the evaporated MoOx layers leads to a high density of states within the bandgap. This is crucial for charge transport. The J-V electrical characteristics at several temperatures were analysed to elucidate the dominant charge transport mechanisms of this heterojunction structure. We have identified two different transport mechanisms. At low bias voltage, transport is dominated by hole tunnelling through the MoOx gap states. At higher voltage the behaviour is similar to a Schottky junction with a high barrier value, due to the high MoOx work function. These results provide a better understanding of the hole selective character of MoOx/n-type silicon heterocontacts, which is key to further improve this new kind of solar cells., Peer Reviewed, Postprint (author's final draft)

© <2018>. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, Interdigitated back contact technology is a promising candidate to be applied to thin crystalline silicon solar cells because of its simpler one-side interconnection while allowing a more flexible front surface treatment. This work explores the efficiency potential of IBC c-Si solar cells applied to thin c-Si substrates through 3D device simulations. In particular, we explore the impact of substrate thickness and front surface recombination velocity on cell performance with special attention to the different behavior in carrier collection of two different rear-surface doping structures. Firstly, the model is validated by comparing simulation results to a fabricated device on 280¿µm-thick substrates with stripe-like p+ and n+ diffusions. It is revealed that efficiencies of 16–17% are reachable for substrates on the 10–15¿µm range without changing the technology developed for thick ones. Next, the rear doping structure is modified leading to doped regions just under the metal contacts. This type of structure is expected in solar cells where high-temperature diffusions are replaced by point-like laser doped contacts, which is a feasible alternative to be applied to thin substrates. Simulation results show that diffusion length requirements for those locally-doped structures are more demanding due to the reduction of emitter regions. As a result, very well passivated front and rear surfaces are required to maintain short-circuit current densities to reasonable values. Finally, for both structures open-circuit voltage is kept almost constant with reduced thickness, despite the strong reduction in short-circuit current. Simulations show a reduction of dark saturation current density with substrate thinning due to the redistribution of dark current densities that flow parallel to the device surface., Peer Reviewed, Postprint (author's final draft)

Explore different materials as selective contacts in Germanium. The final objective is to fabricate a Germanium based solar cells usin selective contacts and low temperature processe., New materials are currently being studied in the field of photovoltaics to improve the efficiency and reduce fabrication costs. Among them, transition metal oxides have recently achieved competitive efficiencies as selective contacts on crystalline silicon solar cells, so the idea to expand them to other semiconductors came to mind. The goal of this project is to study the viability of these transition metal oxides based contacts on germanium substrates. Besides, an extensive lifetime study has been performed so as to adapt the equipment Sinton WCT-120 to germanium samples., Nuevos materiales están siendo estudiados en el campo fotovoltaico para mejorar la eficiencia y reducir costes de fabricación. Entre ellos, los óxidos de metales de transición han obtenido eficiencias competitivas como contactos selectivos en células de silicio cristalino, así que se planteó la idea de probar en otros semiconductores. El objetivo de este trabajo es estudiar la viabilidad de estos contactos basados en óxidos de metales de transición en sustratos de germanio. Aparte, se ha llevado a cabo un extenso estudio del tiempo de vida para adaptar el equipo Sinton WCT-120 a muestras de germanio., Nous materials estan sent estudiats en el camp fotovoltaic per tal de millorar l'eficiència i reduir costos de fabricació. Entre ells, els òxids de metalls de transició han obtingut eficiències competitives com a contactes selectius en cèl·lules de silici cristal·lí, així que es va plantejar la idea de provar en altres semiconductors. L'objectiu d'aquest treball és estudiar la viabilitat d'aquests contactes basats en òxids de metalls de transició en substrats de germani. A part, s'ha dut a terme un extens estudi del temps de vida per tal d'adaptar l'equip Sinton WCT-120 a mostres de germani.

Over the last few years, transition metal oxide layers have been proposed as selective contacts both for electrons and holes and successfully applied to silicon solar cells. However, better published results need the use of both a thin and high quality intrinsic amorphous Si layer and TCO (Transparent Conductive Oxide) films. In this work, we explore the use of vanadium suboxide (V2Ox) capped with a thin Ni layer as a hole transport layer trying to avoid both the intrinsic amorphous silicon layer and the TCO contact layer. Obtained figures of merit for Ni/V2Ox/c-Si(n) test samples are saturation current densities of 175 fA cm-2 and specific contact resistance below 115 mO cm2 on 40 nm thick V2Ox layers. Finally, the Ni/V2Ox stack is used with an interdigitated back-contacted c-Si(n) solar cell architecture fully fabricated at low temperatures. An open circuit voltage, a short circuit current and a fill factor of 656 mV, 40.7 mA cm-2 and 74.0% are achieved, respectively, leading to a power conversion efficiency of 19.7%. These results confirm the high potential of Ni/V2Ox stacks as hole-selective contacts on crystalline silicon photovoltaics., Postprint (author's final draft)

In this work, we report on a novel structure of Interdigitated Back-Contacted (IBC) solar cells on c-Si p-type substrates that combines laser processed homojunction base contacts and silicon heterojunction (SHJ) emitters. These hybrid devices which can lead to potential benefits in device processing and/or conversion efficiency. In the proposed fabrication process special attention has been paid to the compatibility of both involved technologies: silicon heterojunction and laser doping from dielectric films. In particular, we focus on the surface passivation obtained by the heterojunction emitter after removing the aluminum oxide/silicon carbide (Al2O3/SiCx) layer stack needed for the laser doping process and previously deposited on the c-Si surface. A severe passivation degradation after plasma et ching process to remove the top SiCx film is observed, despite leaving the Al2O3 film on the c-Si surface. Based on high-frequency capacitance-voltage characterization, an increase in the interface state density and a strong impact on the fixed charge density is deduced. Next, in order to choose an optimized metallization technology that could simultaneously contact both the ITO film and the p+ laser processed regions, we evaluate the contact quality of Titanium and Aluminum on ITO. Results show that Titanium is a better option with a specific contact resistance of 1.1 mohm cm2. Finally, finished hybrid IBC solar cells with conversion efficiencies in the 18-19% range are reported., Peer Reviewed, Postprint (published version)

© <2017>. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, This work shows a novel fabrication process of interdigitated back-contacted c-Si(n) solar cells based on laser-doped point contacts. In this approach, all the highly-doped regions have been entirely fabricated through UV laser processing of dielectric layers, avoiding the high temperature steps typically involved in conventional diffusion processes. Additionally, the number of patterning steps has been significantly reduced. Aluminum oxide films deposited by thermal ALD on the front surface passivate and reduce reflection losses, while at the rear surface the same films are also used as aluminum source for p+ emitter contacts. n+ regions are created by laser processing a phosphorus-doped amorphous silicon carbide stack deposited by PECVD. As a proof of concept, solar cells (3×3 cm2) have been fabricated with efficiencies beyond 20% with short-circuit current densities and open-circuit voltages up to 40.7 mA/cm2 and 654 mV respectively., Peer Reviewed, Postprint (author's final draft)

La solución de Mie a las ecuaciones de Maxwell describe la interacción de una onda luminosa con una esfera. Esta solución - para dimensiones de esfera comparables a la longitud de onda de la luz - tiene un resultado muy interesante. Para algunas frecuencias de resonancia y radios de la esfera, la sección de scattering puede ser más de diez veces mayor que la sección geométrica real. Además, en algunas condiciones, la sección de absorción también puede ser mayor que la real. Esto significaría que la microesfera absorbería más luz que la que incidía en su superficie. Esto tendría una consecuencia muy impresionante en el aprovechamento de la luz en placas solares. Para probar esta capacidad y aplicando la ley de Kirchhoff de radiación térmica - que establece que la emisividad es igual a la absorbancia - hemos de medir la radiación térmica de una sola microesfera. En este proyecto diseñamos un set-up óptico que formará parte de la configuración final utilizada para medir la radiación térmica de una sola microesfera. Para realizar la medición se utilizaron tecnicas de espectroscopia FT-IR combinada con técnicas de lock-in. Hemos podido medir un área de 10?m de radio en contraste con los pocos milímetros que el FT-IR normal mide., Mie’s solution to Maxwell’s equations describes the interaction of a light wave with a sphere. This solution - for sphere dimensions comparable to the light wavelength- has a very a interesting result. For some resonant frequencies and sphere radii, scattering section can be more than ten times greater than actual geometric section. Furthermore, under some conditions, the absorption cross section can also be larger than the real one. This means that the microsphere would absorb more light than the one impinging on its surface. This would have a very impressive consequence on light harvesting. In order to test this ability and applying Kirchho↵’s law of thermal radiation - which states that the emissivity is equal to the absorptivity - we would measure thermal radiation of a single microshpere. In this project we design a optical set-up that will be part of the final set-up used to measure the thermal radiation of a single microsphere. In order to perform the measurement Step-Scan FT-IR spectroscopy combined with lock-in techniques were used. We where able to measure an area of 10µm radius’ circle in contrast to the few millimetres that normal FT-IR measures.

In this work, we report on the development of silicon nitride (SiNx) layers to be applied to crystalline silicon high-efficiency solar cells. In particular, our research group has developed the concept of Doped by Laser (DopLa) solar cells where the highly doped regions are based on laser processed dielectric films. This concept has been recently applied to Interdigitated Back-Contacted (IBC) solar cells where some limitations associated to silicon carbide films arisen. The objective is to replace these films by SiNx layers on both the rear and front surface. Focusing on the rear surface, we have determined a SiNx layer that could be etched by conventional wet processes avoiding the plasma etching used in our current devices that degrades their surface passivation. On the other hand, for the front surface we found deposition conditions of SiNx films more transparent than their SiCx counterparts and that could stand the subsequent cleaning procedures included in the solar cell fabrication process. A gain in short-circuit current of 0.1 mA/cm2 is expected., Peer Reviewed, Postprint (published version)

Under mesoscopic conditions, strong electron-electron interactions and weak electron-phonon coupling in graphene lead to an hydrodynamic behaviour of electron dynamics, resulting in unusual transport phenomena. During the last years, there have been several theoretical studies of macroscopic signatures of this collective cooperation of electrons. Here, we want to distinguish this hydrodynamic regime studying the conductance through a narrow geometrical constriction in graphene. To do so, we have fabricated high-quality, low-disorder graphene nano-constriction devices encapsulated by hexagonal boron nitride, where electron-electron scattering dominates impurity scattering. We carried out systematic four-probe conductance measurements on devices with different constriction widths as a function of carrier density and temperature. The observation of quantum transport phenomena that are inconsistent with the non-interacting ballistic free-fermion model suggests a macroscopic transport signature of electron viscosity., n condiciones mesoscópicas, las fuertes interacciones electrón-electrón, junto con el débil acoplamiento electrón-fonón en grafeno, dan lugar a un comportamiento hidrodinámico de la dinámica electrónica que resulta en fenómenos de transporte inusuales. Durante los últimos años ha habido varios estudios teóricos acerca de indicios macroscópicos de esta cooperación colectiva de los electrones. Aquí, intentamos distinguir este régimen hidrodinámico estudiando la conductividad a través de una estrecha constricción geométrica en grafeno. Para hacerlo, hemos fabricado dispositivos con nano-constricciones en grafeno, de alta calidad y bajo nivel de desorden, encapsulados en nitruro de boro hexagonal, en los que la dispersión electrón-electrón domina la dispersión por impurezas. Se llevaron a cabo medidas sistemáticas de la conductividad con cuatro sondas, en dispositivos con distintas anchuras de constricción, en función de la densidad de portadores y de la temperatura. La observación de fenómenos cuánticos de transporte que no son consistentes con el modelo balístico de fermiones libres sin interacción sugiere un indicio de transporte macroscópico de la viscosidad electrónica., Les fortes interaccions electró-electró conjuntament amb el dèbil acoblament electró-fonó al grafè en condicions mesoscòpiques provoquen un comportament hidrodinàmic de la dinàmica electrònica que resulta en fenòmens de transport inusuals. Durant els últims anys hi ha hagut diversos estudis teòrics que buscaven indicis macroscòpics d'aquesta cooperació col·lectiva dels electrons. En aquest treball intentem distingir aquest règim hidrodinàmic estudiant la conductivitat a través d'una constricció geomètrica estreta de grafè. Per fer-ho, hem fabricat dispositius amb nano-constriccions de grafè d'alta qualitat i baix nivell de desordre, encapsulats en dispositius de nitrur de bor hexagonal, en els quals la dispersió electró-electró domina sobre la dispersió per impureses. Es van prendre mesures sistemàtiques de la conductivitat emprant quatre sondes, en dispositius amb diferent amplada de constricció, i en funció de la densitat de portadors i de la temperatura. L'observació de fenòmens quàntics de transport que no són consistents amb el model balístic de fermions lliures sense interacció suggereix un indici de transport macroscòpic de la viscositat electrònica.

Confocal micro-photoluminescence (PL) spectroscopy has become a powerful characterization technique for studying novel photovoltaic (PV) materials and structures at the micrometer level. In this work, we present a comprehensive study about the effects and implications of photon reabsorption phenomena on confocal micro-PL measurements in crystalline silicon (c-Si), the workhorse material of the PV industry. First, supported by theoretical calculations, we show that the level of reabsorption is intrinsically linked to the selected experimental parameters, i.e., focusing lens, pinhole aperture, and excitation wavelength, as they define the spatial extension of the confocal detection volume, and therefore, the effective photon traveling distance before collection. Second, we also show that certain sample properties such as the reflectance and/or the surface recombination velocity can also have a relevant impact on reabsorption. Due to the direct relationship between the reabsorption level and the spectral line shape of the resulting PL emission signal, reabsorption phenomena play a paramount role in certain types of micro-PL measurements. This is demonstrated by means of two practical and current examples studied using confocal PL, namely, the estimation of doping densities in c-Si and the study of back-surface and/or back-contacted Si devices such as interdigitated back contact solar cells, where reabsorption processes should be taken into account for the proper interpretation and quantification of the obtained PL data., Peer Reviewed, Postprint (published version)

Transition metal oxides (TMOs) have recently been proved to efficiently serve as hole-selective contacts in crystalline silicon (c-Si) heterojunction solar cells. In the present work, two TMO/c-Si heterojunctions are explored using MoO3 (reference) and V2O5 as an alternative candidate. It has been found that V2O5 devices present larger (16% improvement) power conversion efficiency mainly due to their higher open-circuit voltage. While V2O5/c-Si devices with textured front surfaces exhibit larger short-circuit currents, it is also observed that flat solar cell architectures allow for passivation of the V2O5/n-Si interface, giving significant carrier lifetimes of 200 µs (equivalent to a surface recombination velocity of Seff ~140 cm s-1) as derived from impedance analysis. As a consequence, a significant open-circuit voltage of 662 mV is achieved. It is found that, at the TMO/c-Si contact, a TMO work function enhancement ¿FTMO occurs during the heterojunction formation with the consequent dipole layer enlargement ¿’=¿+¿FTMO. Our results provide new insights into the TMO/c-Si contact energetics, carrier transport across the interface and surface recombination allowing for further understanding of the nature of TMO/c-Si heterojunctions., Peer Reviewed, Postprint (published version)

Transition metal oxides (TMOs) have recently demonstrated to be a good alternative to boron/phosphorous doped layers in crystalline silicon heterojunction solar cells. In this work, the interface between n-type c-Si (n-Si) and three thermally evaporated TMOs (MoO3, WO3, and V2O5) was investigated by transmission electron microscopy, secondary ion-mass, and x-ray photoelectron spectroscopy. For the oxides studied, surface passivation of n-Si was attributed to an ultra-thin (1.9–2.8 nm) SiOx~1.5 interlayer formed by chemical reaction, leaving oxygen-deficient species (MoO, WO2, and VO2) as by-products. Carrier selectivity was also inferred from the inversion layer induced on the n-Si surface, a result of Fermi level alignment between two materials with dissimilar electrochemical potentials (work function difference ¿¿ = 1 eV). Therefore, the hole-selective and passivating functionality of these TMOs, in addition to their ambient temperature processing, could prove an effective means to lower the cost and simplify solar cell processing., Postprint (author's final draft)

The reorganization through high-temperature annealing of closely-packed pore arrays can be exploited to create ultra-thin (<20 µm) monocrystalline silicon layers that can work as cheap and flexible substrates for both the electronic and the photovoltaic industries. By introducing a periodic diameter modulation along deep etched pores, many thin layers can be produced from a single substrate and in a single technological process. Besides the periodicity, the exact shape of the modulation also has a profound impact on the process and subtle profile changes can lead to important differences on the process outcome. In this paper we study both theoretically and experimentally the effect of the initial profile on the pore reorganization dynamics and the morphology of the thin layers obtained through annealing. We show that process reliability, annealing time and final layer characteristics, all can be engineered and optimized by precisely controlling the initial pore profile., Postprint (published version)

In this work we show a baseline fabrication process of interdigitated back contacted IBC c-Si(p) solar cells, which combines conventional diffusion oven stages to define base p+ and emitter n+/n++ regions at the back side, with outstanding front surface passivation using atomic layer deposited Al2O3 films over random pyramids surfaces. Cells include a selective phosphorous n++ emitter in order to improve contact resistance and simultaneously reduce recombination current density. Fabricated devices reach efficiencies up to 22.2% (AM1.5G 1 kW/m2, T=25°C). This value is so far the highest efficiency reported by any Spanish institution using silicon substrates. 3D simulations envisage efficiencies beyond 24% introducing little changes in the fabrication process., Peer Reviewed, Postprint (published version)

In this paper, we study the recombination mechanisms in ion-implanted black silicon (bSi) emitters and discuss their advantages over diffused emitters. In the case of diffusion, the large bSi surface area increases emitter doping and consequently Auger recombination compared to a planar surface. The total doping dose is on the contrary independent of the surface area in implanted emitters, and as a result, we show that ion implantation allows control of emitter doping without compromise in the surface aspect ratio. The possibility to control surface doping via implantation anneal becomes highly advantageous in bSi emitters, where surface passivation becomes critical due to the increased surface area. We extract fundamental surface recombination velocities Sn through numerical simulations and obtain the lowest values at the highest anneal temperatures. With these conditions, an excellent emitter saturation current (J0e) is obtained in implanted bSi emitters, reaching 20 fA/cm2 ± 5 fA/cm2 at a sheet resistance of 170 O/sq. Finally, we identify the different regimes of recombination in planar and bSi emitters as a function of implantation anneal temperature. Based on experimental data and numerical simulations, we show that surface recombination can be reduced to a negligible contribution in implanted bSi emitters, which explains the low J0e obtained., Postprint (published version)

Cu(In,Ga)Se2 (CIGS) technology is one of the best absorber materials with record efficiencies among photovoltaic thin-film technologies (22.3% at lab scale and 16% at large commercial module). Although research on this material was originally motivated by low-cost, glass-glass applications focusing to fixed photovoltaic structures, the high efficiency values make CIGS an interesting alternative for low concentration systems. In this paper a 2D model for Cu(In,Ga)Se2 (CIGS) solar cells under low solar concentration is described and contrasted with experimental data. Using simulation, the effect of front electric contact design parameters: finger width, finger separation, and number of buses are analyzed for solar concentrations from 1 up to 10 suns. Efficiency maps allowing front contact grid optimization are shown and analyzed for each concentration factor (Cx), assessing the viability of CIGS solar cells for low concentration applications, where commercial CIGS solar cells may exhibit 35% of electrical power increases with proper front grid optimization under low concentration respect to conventional grid design., Preprint

Black silicon (b-Si) reduces drastically light reflectance in the front side of c-Si solar cells to values near zero for the whole absorbed solar spectrum. In this work, we apply 2D and 3D simulations to explore the efficiency limits of interdigitated back-contacted c-Si(n) solar cells with line or point contacts respectively, using ALD Al2O3 passivated b-Si in the front surface. Realistic physical and technological parameters involved in a conventional oven-based fabrication process are considered in the simulations, especially those related to surface recombination on the b-Si as well as high doped p+/n+ strip regions. One important issue is the temporal stability of surface passivation on b-Si surfaces. In this work experimental long-term b-Si surface passivation data after two years and its impact on cell performance are studied. Simulations demonstrate initial and final photovoltaic efficiencies over 24.6% and 23.2% respectively for an emitter coverage of 80% independently of the cell contact strategy. A photocurrent loss about 1.3 mA/cm2 occurs when surface recombination velocity at the b-Si surfaces degrades from 6 cm/s to a final value of 28 cm/s., Postprint (author's final draft)

Postprint (published version)

In this work, we report on improving efficiency of DopLa cells fabricated on p-type substrates. This type of solar cells has all the highly-doped regions based on laser doping from dielectric films resulting in a very simple fabrication process. Depending on the dopant type, emitter regions or high/low doping junctions related to base contacts can be created. The emitter regions are located at the rear surface in order to be contacted by a continuous metal film without penalizing in shadowing losses, while the front surface shows a typical finger grid configuration with the base contacts under the metal. In a previous work, the reported efficiency was limited by optical losses at the front planar surface. As a consequence, we focus on the introduction of a texturized front surface to the device. Firstly, we characterize the contact formation by laser processing on texturized surfaces by SEM image showing that the size of the contacted region is difficult to determine. By measuring contact resistance and surface recombination velocity, we deduce that the laser process of such surfaces leads to a contacted region which is smaller than the one where passivation is lost. The obtained information is included in 3D simulations to get the optimum size of the contacts, i.e. optimum laser power. Additionally, a new front grid metallization is introduced in order to reduce shadowing. Due to the rear emitter configuration of these devices, front surface recombination is crucial for collecting photogenerated carriers. Thus, optimum laser power is very close to the minimum to obtain a reliable contact. Finally, 2x2 cm 2 solar cells are fabricated with a best efficiency of 17.0 %., Peer Reviewed, Postprint (published version)

Organic-inorganic hybrid solar cells were fabricated by spin coating of conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as hole selective contact upon textured n-type crystalline silicon wafers. Two different PEDOT:PSS commercial products (Clevios HTL Solar and PH1000) were compared, enhancing their conductivity and wettability by addition of dimethyl sulfoxide cosolvent and Capstone FS-31 surfactant. Transmission line measurements (TLM) revealed that the sheet resistance of the PEDOT:PSS layer was comparable with that of conventional Indium-Tin oxide (ITO) electrodes, allowing its use as a conductive transparent layer in an ITO-free solar cell. Quasi-steady-state photoconductance measurements revealed an implied open-circuit voltage of 644 mV for HTL Solar, which is equivalent to a saturation current density of 255 fA/cm2. In general, the performance of HTL Solar was better than PH1000, exhibiting power conversion efficiency (1 × 1 cm2 cell area) of 11.6%, as compared with 8.5%., Peer Reviewed, Preprint

© 2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works., Cu2 ZnSn(S1-ySey )4 (CZTS) thin-film solar cells have been qualified as potential competitors of the more established CIGS ones. One of the more important handicaps of CZTS solar cells is the open-circuit voltage deficit. The rear-contact/absorber interface is known to be very sensitive to the formation of secondary phases, which are detrimental for the electrical behavior of photovoltaic devices. The addition of intermediate layers to favor the formation of an adequate interface has been repeatedly tested. In this work, an amorphous silicon carbide (a-SiC) layer is added to explore its influence on the material properties and electrical performance of CZTSe solar cells. According to scanning electron microscopy (SEM) analysis, when the a-SiC layer thickness is increased, bigger grains along the absorber are obtained. Additionally, a lower [VCu + ZnCu ] defect cluster density is also deduced from the analysis of Raman measurements. Both results indicate a favorable impact of a-SiC films on the material quality of the absorber. Fabricated solar cells show an enhancement of 0.9% abs. of efficiency compared to identical solar cells without a-SiC layers used as a reference. This increase is mainly related to an improvement of open-circuit voltage and fill factor (FF) when the proposed intermediate layer is included., Peer Reviewed, Postprint (author's final draft)

Reducing the wafer thickness for c-Si solar cell fabrication is an effective approach for cost-savings. Interdigitated Back Contacts (IBC) technology is a promising candidate to be applied to thin c-Si substrates due to its potential to facilitate thin device processing: thin c-Si solar cell could be processed attached to a glass with its rear surface, where all the contacts are to be defined, still accessible. The understanding of thin IBC c-Si solar cell performance has great significance in guiding the design of such devices. In this work, we explore the performance of thin IBC c-Si solar cells by both 3D TCAD device simulations and experimental fabrication based on conventional technology developed in our research group. On one hand, an optical model is proposed using 2D ray tracing method whose results are applied to 3D ATLAS TCAD simulator which is used to simulate the electrical performance of the devices. As a result, efficiencies in the range of 20 % are expected for substrates of 10-20 µm without changing our technology. On the other hand, a ~30 µm IBC solar cell is fabricated by thinning down a previously developed IBC c-Si solar cell on conventional thick high quality substrates demonstrating a 12.1% of efficiency. The front surface passivation provided by Al2O3 is deduced by comparing with the simulation results revealing a front surface recombination velocity of about 1500 cm/s., Peer Reviewed, Postprint (published version)

In order to understand the mechanical behaviour of Al2O3 inverse opals, nanoindentation techniques have been implemented in material layers with three different microstructures, in terms of hollow or polystyrene spheres, with Al2O3 shells of distinct wall thickness. Different indenter tip geometries as well as contact loading conditions have been used, in order to induce different stress field and fracture events to the layers. Field emission scanning electron microscopy and focused ion beam have been employed to understand accommodation of plastic deformation induced during the indentation process. Results show that materials with polystyrene spheres exhibit higher hardness and modulus under sharp indentation, and cracking resistance under spherical indentation. Furthermore, deformation is discerned to be mainly governed by the rotation of the microspheres. In the case of the inverse opals made of hollow spheres, the main deformation mechanisms activated under indentation are the rearrangement and densification of them, Postprint (author's final draft)

Hybrid organic/inorganic heterojunction solar cells were fabricated based on polished n-type crystalline silicon (c-Si) and p-type transparent conducting polymer poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS). The conductivity and wettability of PEDOT:PSS has been significantly improved by the addition of 5% dimethyl sulfoxide (DMSO) and 0.1% Capstone FS-31 surfactant. Transmission line measurements revealed that the sheet resistance of PEDOT:PSS with 5% DMSO is 122 O/sq, comparable to Indium-Tin Oxide (ITO). Quasi-steady-state photo-conductance measurements showed an implicit open-circuit voltage of 0.61 V with a minority carrier lifetime of 82 µs. The power conversion efficiency of the PEDOT:PSS/n-Si hybrid solar cells is 8.5% with an open-circuit voltage of 0.56 V, a short-circuit current density of 29.7 mA/cm2 and a fill factor of 49.7%., Peer Reviewed, Postprint (author's final draft)

Laser firing processes have emerged as a technologically feasible approach for the fabrication of local point contacts or local doped regions in advanced high-efficiency crystalline-Si (c-Si) solar cells. In this work, we analyze the local impact induced by the laser pulse on the passivation layers, which are commonly present in advanced c-Si solar cell architectures to reduce surface recombination. We use microphotoluminescence (PL) measurements with a spatial resolution of 7 mu m to evaluate the passivation performance at the surroundings of laser-processed regions (LPRs). In particular, we have studied LPRs performed on SiCx/Al2O3- and Al2O3 passivated c-Si wafers by an infrared (1064 nm) laser. Micro-PL results show that passivation quality of c-Si surface is affected up to about 100 mu m away from the LPR border and that the extension of this damaged zone is correlated with the laser power and to the presence of capping layers. In the final part of the work, the observed decrease in passivation quality is included in an improved 3-D simulation model that gives accurate information about the recombination velocities associated with the studied LPRs., Peer Reviewed, Postprint (author's final draft)

This work reports on a comparative study comprising three transition metal oxides, MoO3, WO3 and V2O5, acting as front p-type contacts for n-type crystalline silicon heterojunction solar cells. Owing to their high work functions (>5 eV) and wide energy band gaps, these oxides act as transparent hole-selective contacts with semiconductive properties that are determined by oxygen-vacancy defects (MoO3-x), as confirmed by X-ray photoelectron spectroscopy. In the fabricated hybrid structures, 15 nm thick transition metal oxide layers were deposited by vacuum thermal evaporation. Of all three devices, the V2O5/n-silicon heterojunction performed the best with a conversion efficiency of 15.7% and an open-circuit voltage of 606 mV, followed by MoO3 (13.6%) and WO3 (12.5%). These results bring into view a new silicon heterojunction solar cell concept with advantages such as the absence of toxic dopant gases and a simplified low-temperature fabrication process., Preprint

Postprint (published version)

Postprint (published version)

In this work we report on the long-term stability of black silicon surfaces passivated with atomic layer deposited (ALD) 20 nm thick Al2O3 films on p- and n-type FZ c-Si substrates. The results are directly compared with random pyramid textured counterparts. The effective surface recombination velocity Seff has been measured within a time frame of one year after activation of surface passivation. The results demonstrate that after an initial slight degradation during the first month Seff values stabilize around 45 and 25 cm/s on p- and n-type black silicon samples, respectively. These values are enough to guarantee stable high efficiency in interdigitated back-contacted (IBC) c-Si(n) solar cells (> 24.5%) using black silicon nanostructures on the front side. Similar, although weaker, losses are also observed in surface passivation on textured samples covered by Al2O3 with equal thickness, indicating that the origin of the instability might be independent of surface morphology., Peer Reviewed, Postprint (published version)

© 2016 Elsevier B.V. All rights reserved. This paper introduces the possibility to produce 1-2 cm2 and tens of layers colloidal crystals on different non-flat surfaces by means of electrospray deposition. Two different types of crystalline silicon surfaces were tested: nanostructured, Postprint (author's final draft)

In this work, we report on the fabrication and characterization of n-type c-Si solar cells whose p+ emitters are based on laser processed aluminum oxide/silicon carbide (Al2O3/SiCx) films. The p+ emitter is defined at the rear side of the cell and it consists of point-like laser-diffused p+ regions with a surface charge induced emitter in between based on the high negative charge located at the Al2O3/c-Si interface. These emitters are fabricated at low temperature (< 400 °C) and could be directly compared to silicon heterojunction emitters with the advantage that the deposition of a Transparent Conductive Oxide (TCO) film can be avoided, since they are based on p+/n c-Si homojunctions. Additionally, the involved films are transparent to the IR photons ( >1000 nm) that reach the rear surface of the cell resulting in an excellent back reflector. We fabricated solar cells with distance between p+ regions or pitch ranging from 200 to 350 µm with a front surface based on silicon heterojunction technology. Best efficiency (18.1%) is obtained for a pitch of 250 µm as a consequence of the trade-off between Voc and FF values., Peer Reviewed, Postprint (published version)

Selective thermal emitters concentrate most of their spontaneous emission in a spectral band much narrower than a blackbody. When used in a thermophovoltaic energy conversion system, they become key elements defining both its overall system efficiency and output power. Selective emitters' radiation spectra must be designed to match their accompanying photocell's band gap and simultaneously, withstand high temperatures (above 1000 K) for long operation times. The advent of nanophotonics has allowed the engineering of very selective emitters and absorbers; however, thermal stability remains a challenge since nanostructures become unstable at temperatures much below the melting point of the used materials. In this paper we explore a hybrid 3D dielectric-metallic structure that combines the higher thermal stability of a monocrystalline 3D silicon scaffold with the optical properties of a thin platinum film conformally deposited on top. We show experimentally that these structures exhibit a selective emission spectrum suitable for TPV applications and that they are thermally stable at temperatures up to 1100 K. These structures are ideal in combination with HI-V semiconductors in the range E-g=0.4-0.55 eV such as InGaAsSb (E-g=0.5-0.6 eV) and InAsSbP (E-g=0.3-0.55 eV). (C) 2014 Elsevier B.V. All rights reserved., Peer Reviewed, Postprint (author’s final draft)

In this work laser doping technique is used to create highly-doped regions defined in a point-like structure to form n+/p and p+/n junctions applying a pulsed Nd-YAG 1064 nm laser in the nanosecond regime. In particular, phosphorous-doped silicon carbide stacks (a-SiC x /a-Si:H (n-type)) deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and aluminum oxide (Al 2 O 3 ) layers deposited by atomic layer deposition (ALD) on 2 ± 0.5 cm p- and n-type FZ c-Si substrates respectively are used as dopant sources. Laser power and number of pulses per spot are explored to obtain the optimal electrical behavior of the formed junctions. To assess the quality of the p+ and n+ regions, the junctions are electrically contacted and characterized by means of dark J–V measurements. Additionally, a diluted HF treatment previous to front metallization has been explored in order to know its impact on the junction quality. The results show that fine tuning of the energy pulse is critical while the number of pulses has minor effect. In general the different HF treatments have no impact in the diode electrical behavior except for an increase of the leakage current in n+/p junctions. The high electrical quality of the junctions makes laser doping, using dielectric layers as dopant source, suitable for solar cell applications. Particularly, a potential open circuit voltage of 0.64 V (1 sun) is expected for a finished solar cell., Peer Reviewed, Postprint (author's final draft)