Academic literature on the topic 'Plasmonic silicon solar cells'

This research project is focused on the process optimisation and optical enhancement of the hydrogenated amorphous silicon solar cell design, achieved by the incorporation of light scattering plasmonic nano-particles. These treatments consist of a very thin layer of finely tuned silver metal-island films, which preferentially scatter light within a wavelength range tailored to the device absorption characteristic. This serves to increase the optical path length without the need for surface texturing of the semiconductor material. Within this study, the PECVD process is used to explore the parameter space and fabricate silicon thin films with excellent optical and electrical performance, and a P-I-N amorphous silicon device structure is fabricated with a high performance of 6.5% conversion efficiency, 14.04mA/cm2 current density and 0.82V open circuit voltage. The effects of metallic nano-particle arrays is demonstrated by numerical simulation, showing that variations in particle size, shape, position within the structure and surrounding material greatly influence the enhancement of the nano-particles on silicon absorber layers, and that particles positioned at the rear of the device structure adjacent to a back reflector avoid absorption losses which occur below the particle resonance frequency when such structures are positioned at the front surface. It is shown than an improvement in optical absorption of just over 1% is possible using this method. Silicon thin films are fabricated with self-organised nano-particle arrays via means of annealed metal films, positioned at the front or back adjacent to a metallic reflector, and measurements of optical transmittance, reflectance and absorption are taken. The optimum annealing temperature and duration is identified, and it is shown that these variables can greatly affect the absorption of the device stack. To conclude the study, an amorphous silicon P-I-N photovoltaic device is fabricated featuring self-organised nanoparticle arrays within the back reflector, and a modest improvement of energy conversion efficiency is observed with scope for further optimisation and enhancement.

The aim of this thesis is to explore the optical properties of localized surface plasmon resonance sustained by self-assembled metallic nanoparticles (NPs) for the light trapping application in thin lm photovoltaics (PV). Photovoltaics is able provide safe and clean electricity of the future, inparticular, thin lms solar cells have a potential to increase the competitiveness of PV through a substantial reduction of the manufacturing cost. However, an essential step it to develop an e cient, reliable and inexpensive light trapping scheme in order to maximize absorption of the near-infrared radiation in the cell and balance the reduced volume of semiconductor material. Recently there is a growing interest in the application of subwavelength metallic NPs for light trapping as they can scatter light e ciently over a broad wavelength range of the solar spectrum, due to the to the phenomena known as localized surface plasmon (LSP) resonance. A systematic study of the correlation between the structural and the optical properties of self-assembled silver nanostructures fabricated on soda-lime glass by a solid-state dewetting (SSD) process, which consist in thermallyinduced morphology transformation from a thin lm to an array of islands or nanoparticles is reported. It is shown that four distinct types of morphology tend to form in speci c ranges of fabrication parameters, which is quantitatively summarized by a proposed structural-phase diagram and allows to identify the fabrication conditions in which preferable, uniformly spaced and circular NPs are obtainable. The optical properties of the NPs stay in qualitative agreement with the trends predicted by Mie theory, and correlate with the surface coverage (SC) distributions and the mean SC size. As a step forward towards the implementation in thin lm photovoltaics, the NPs are incorporated on the rear side of thin silicon fillm in two distinct arrangements, namely superstrate and substrate. In superstrate configration,The coupling e ciency increases with NPs' average size, decreases with increasing distance between silicon, and is signi cantly smaller for spherical than for hemispherical NPs, which stay in qualitative agreement with theoretical predictions. A novel procedure, involving a combination of phothermal de ection spectroscopy and fourier transform photocurrent spectroscopy, employed for substrate con guration lms allowed for the quanti cation of useful and parasitic absorption. It is demonstrated that the optical losses in the NPs are insigni cant in the 500-730nm wavelength range, beyond which they increase rapidly with increasing illumination wavelength. Furthermore, a broadband enhancement of 89.9% of useful absorption has been achieved. Susequantly, a successful implementation of a plasmonic light trapping scheme implemented in a thin lm a-Si:H solar cell in plasmonic back re ector (PBR) con guration. The optical properties of the PBRs are systematically investigated according to the morphology of the self-assembled silver nanoparticles (NPs), which can be tuned by the fabrication parameters. By analyzing sets of solar cells built on distinct PBRs, it is shown that the photocurrent enhancement achieved in the a-Si:H light trapping window (600-800 nm) stays in linear relation with the PBRs di use re ection. The best-performing PBRs allow a pronounced broadband photocurrent enhancement in the cells which is attributed not only to the plasmon-assisted light scattering from the NPs but also to the front surface texture originated from the conformal growth of the cell material over the particles. As a result, remarkably high values of Jsc and Voc are achieved in comparison to those previously reported in the literature for the same type of devices. Furthermore an attempt on implementation of the plasmonic light trapping in the industrial a-Si/ c-Si double junction solar cells is reported.

Organic solar cells (OSCs) have recently attracted considerable research interest. However, there is a mismatch between their optical absorption length and charge transport scale. Attempts to optimize both the optical and electrical properties of the photoactive layer of OSCs have inevitably resulted in demands for rationally designed device architecture. Plasmonic nanostructures have recently been introduced into solar cells to achieve highly efficient light harvesting. The remaining challenge is to improve OSC performance using plasmonic nanotechnology, a challenge taken up by the research reported in this thesis. I systematically investigated two types of plasmonic effect: localized plasmonic resonances (LPRs) and surface plasmonic resonances (SPRs). Broadband plasmonic absorption is obviously highly desirable when the LPR effect is adopted in OSCs. Unfortunately, typical nanomaterials possess only a single resonant absorption peak, which inevitably limits the power conversion efficiency (PCE) enhancement to a narrow spectral range. To address this issue, I combined Ag nanomaterials of different shapes, including nanoparticles and nanoprisms. The incorporation of these mixed nanomaterials into the active layer resulted in wide band absorption improvement. My results suggest a new approach to achieving greater overall enhancement through an improvement in broadband absorption. I also explored the SPR effect induced by a metal patterned electrode with two parts. Most reports to date on back reflector realization involve complicated and costly techniques. In this research, however, I adopted a polydimethylsiloxane (PDMS)-nanoimprinted method to produce patterned back electrodes in OSCs directly, which is a very simple and efficient technique for realizing high-performance OSCs in industrial processes. Besides, a remaining challenge is that plasmonic effects are strongly sensitive to light polarization, which limits plasmonic applications in practice. To address this issue, I designed three-dimensional patterns as the back electrode of inverted OSCs, which simultaneously achieved highly efficient and polarization-independent plasmonic OSCs. In addition to investigating the two types of plasmonic effect individually, I also investigated their integrated function by introducing both LPRs and SPRs in one device structure. With the aim of achieving high-performance OSCs, I first demonstrated experimentally a dual metal nanostructure composed of Au nanoparticles (i.e. LPRs) embedded in the active layer and an Ag nanograting electrode (i.e. SPRs) as the back reflectors in inverted OSCs, which can generate a very strong electric field, in a single junction to improve the light absorption of solar cells. As a result, the PCE of the OSC reached 9.1%, making it one of the best-performing OSCs reported to date. In addition, as an important extension, I subsequently achieved tremendous near-field enhancement owing to multiple couplings, including nanoparticle-nanoparticle (LPR-LPR) couplings and nanoparticle-film (LPR-SPR) couplings, by designing a novel nanoparticle-film coupling system through the introduction of ultrathin monolayer graphene as a well-defined sub-nanogap between the Ag nanoparticles and Ag film. The graphene sub-nanogap is the thinnest nanogap (in atomic scale terms) to date, and thus constitutes a promising light-trapping strategy for improving future OSC performance.<br>published_or_final_version<br>Electrical and Electronic Engineering<br>Doctoral<br>Doctor of Philosophy

This thesis explores the use of plasmonic nanovoids for enhancing the efficiency of thin-film solar cells. Devices are fabricated inside plasmonically resonant nanostructures, demonstrating a new class of plasmonic photovoltaics. Novel cell geometries are developed for both organic and amorphous silicon solar cell materials. An external-quantum efficency rig was set up to allow simultaneous microscope access and micrometer-precision probe-tip control for optoelectronic characterisation of photovoltaic devices. An experimental setup for angle-resolved re ectance was extended to allow broadband illumination from 380 - 1500nm across incident angles 0 - 70 degrees giving detailed access to the energy-momentum dispersion of optical modes within nanostructured materials. A four-fold enhancement of overall power conversion efficiency is observed in organic nanovoid solar cells compared to at solar cells. The efficiency enhancement is shown to be primarily due to strong localised plasmon resonances of the nanovoid geometry, with close agreement observed between experiment and theoretical simulations. Ultrathin amorphous silicon solar cells are fabricated on both nanovoids and randomly textured silver substrates. Angle-resolved re ectance and computational simulations highlight the importance of the spacer layer separating the absorbing and plasmonic materials. A 20% enhancement of cell efficiency is observed for nanovoid solar cells compared to at, but with careful optimisation of the spacer layer, randomly textured silver allows for an even greater enhancement of up to 50% by controlling the coupling to optical modes within the device. The differences between plasmonic enhancement for organic and amorphous silicon solar cells are discussed and the balance of surface plasmon absorption between a semiconductor and a metal is analytically derived for a broad range of solar cell materials, yielding clear design principles for plasmonic enhancement. These principles are used to outline future directions of research for plasmonic photovoltaics.

On constate une forte demande mondiale d' énergie propre et renouvelable en raison de la consommation rapide des combustibles fossiles non renouvelables et l'effet de serre qui en résulte. Une solution prometteuse pour produire une énergie propre et renouvelable est d'utiliser des cellules solaires pour convertir l' énergie solaire directement en électricité. Comparativement à leurs homologues inorganiques, les cellules solaires organiques (OSCs) sont maintenant intensivement étudiées en raison des avantages tels que le poids léger, la flexibilité, la compatibilité avec les procédés de fabrication à faibles coûts. Malgré ces avantages, l'efficacité de conversion (PCE) des OSCs doit encore être améliorée pour la commercialisation à grande échelle. Les cellules solaires organiques sont réalisées en pile de couches minces comprenant des électrodes, la couche de transport d' électrons, la couche de polymère actif et la couche de transport de trous. Dans cette étude, nous sommes concernés par la couche de PEDOT:PSS qui est couramment utilisée comme une couche tampon entre l'électrode anodique et la couche de polymère actif de cellules solaires organiques. Cette étude vise à intégrer différentes concentrations de nanoprismes (NPSMs) d'argent de taille sub-longueur d'onde dans du PEDOT: PSS afin de profiter de leurs propriétés optiques uniques nées de résonances de plasmons de surface localisées (LSPR) pour améliorer la collecte lumineuse et l'efficacité de génération de charge en optimisant l' absorption et la diffusion de la lumière. Nous avons constaté que les facteurs clés qui contrôlent les performances des cellules solaires plasmoniques comprennent non seulement les propriétés optiques, mais également les propriétés structurelles et électriques des couches hybrides de PEDOT:PSS comprenant des NPSMs d' Ag. D'une part, l'ajout de NPSMs d' Ag conduit ¨¤ (1) une augmentation de l'absorption optique; (2) de la diffusion de la lumière ¨¤ de grands angles ce qui pourrait conduire ¨¤ un meilleur piégeage de la lumière dans les OSCs. D'autre part, (1) la rugosité de surface est augment¨¦e en raison de la formation d'agglomérats de NPSMs d' Ag, ce qui conduit ¨¤ une meilleure efficacité de collecte de charge; (2) la résistance globale des films hybrides est également augment¨¦e en raison de l'excès de PSS introduit par les NPSMs d' Ag incomplètement purifiées, inférieur courant de court-circuit (Jsc) qui en résulte; (3) les Ag NPSMs et leurs agglomérats ¨¤ l'interface PEDOT:PSS/couche photo-active pourraient agir comme des centres de recombinaison, conduisant ¨¤ une réduction de la résistance de shunt, du Jsc et de la tension en circuit ouvert (Voc). Afin de résoudre partiellement l'inconvénient (2) et (3), en intégrant des NPSMs d¡¯Ag davantage purifiés et une petite quantité de glycérol dans le PEDOT:PSS, la résistance des couches hybrides de PEDOT:PSS-Ag-NPSMs peut ¨être réduite à une valeur comparable ou inférieure ¨¤ celles couches vierges. Les futurs progrès en chimie de surface colloïdale et l'optimisation sur le processus d'incorporation des nanoparticules seront nécessaires pour produire des cellules solaires organiques plasmoniques de meilleures performances<br>Nowadays there has been a strong global demand for renewable and clean energy due to the rapid consumption of non-renewable fossil fuels and the resulting greenhouse effect. One promising solution to harvest clean and renewable energy is to utilize solar cells to convert the energy of sunlight directly into electricity. Compared to their inorganic counterparts, organic solar cells (OSCs) are now of intensive research interest due to advantages such as light weight, flexibility, the compatibility to low-cost manufacturing processes. Despite these advantages, the power conversion efficiency (PCE) of OSCs still has to be improved for large-scale commercialization. OSCs are made of thin film stacks comprising electrodes, electron transporting layer, active polymer layer and hole transporting layer. In this study, we are concerned with PEDOT:PSS layer which is commonly used as a buffer layer between the anodic electrode and the organic photoactive layer of the OSC thin film stack. We incorporated different concentrations of silver nanoprisms (NPSMs) of sub-wavelength dimension into PEDOT:PSS. The purpose is to take advantage of the unique optical properties of Ag MPSMs arisen from localized surface plasmon resonance (LSPR) to enhance the light harvest and the charge generation efficiency by optimizing absorption and scattering of light in OSCs. We found that the key factors controlling the device performance of plasmonic solar cells include not only the optical properties but also the structural and electrical properties of the resulting hybrid PEDOT:PSS-Ag-NPSM-films. On one hand, the addition of Ag NPSMs led to (1) an increased optical absorption; (2) light scattering at high angles which could possibly lead to more efficient light harvest in OSCs. On the other hand, the following results have been found in the hybrid films: (1) the surface roughness was found to be increased due to the formation of Ag agglomerates, leading to increased charge collection efficiency; (2) the global sheet resistance of the hybrid films also increases due to the excess poly(sodium styrenesulphonate) introduced by incompletely purified Ag NPSMs, resulting in lower short circuit current (Jsc); (3) the Ag nanoprisms and their agglomerates at the PEDOT:PSS/photoactive layer interface could act as recombination centers, leading to reductions in shunt resistance, Jsc and open circuit voltage (Voc). In order to partially counteract the disadvantage (2) and (3), by incorporating further purified Ag NPSMs and/or a small amount of glycerol into PEDOT:PSS, the sheet resistance of hybrid PEDOT:PSS-Ag-NPSM-films was reduced to a resistance value comparable to or lower than that of pristine film

<p>This thesis work consists of two parts, each with a different motivation. Part II is the main part and was partly conducted in industry, at ScanWafer ASA’s plant no.2 in Glomfjord.</p><p>The large growth in the Photo Voltaic industry necessitates a dedicated feedstock for this industry, a socalled Solar Grade (SoG) feedstock, since the currently used feedstock rejects from the electronic industry can not cover the demand. Part I of this work was motivated by this urge for a SoG- feedstock. It was a cooperation with the Sintef Materials and Chemistry group, where the aim was to study the kinetics of the removal reactions for dissolved carbon and boron in a silicon melt by oxidative gas treatment. The main focus was on carbon, since boron may be removed by other means. A plasma arc was employed in combination with inductive heating. The project was, however, closed after only two experiments. The main observations from these two experiments were a significant boron removal, and the formation of a silica layer on the melt surface when the oxygen content in the gas was increased from 2 to 4 vol%. This silica layer inhibited further reactions.</p><p>Multi-crystalline (mc) silicon produced by directional solidification constitutes a large part of the solar cell market today. Other techniques are emerging/developing and to keep its position in the market it is important to stay competitive. Therefore increasing the knowledge on the material produced is necessary. Gaining knowledge also on phenomenas occurring during the crystallisation process can give a better process control.</p><p>Part II of this work was motivated by the industry reporting high inclusion contents in certain areas of the material. The aim of the work was to increase the knowledge of inclusion formation in this system. The experimental work was divided into three different parts;</p><p>1) Inclusion study</p><p>2) Extraction of melt samples during crystallisation, these were to be analysed for carbon- and nitrogen. Giving thus information of the contents in the liquid phase during soldification.</p><p>3) Fourier Transform Infrared Spectroscopy (FTIR)-measurements of the substitutional carbon contents in wafers taken from similar height positions as the melt samples. Giving thus information of the dissolved carbon content in the solid phase.</p><p>The inclusion study showed that the large inclusions found in this material are β-SiC and β-Si3N4. They appear in particularly high quantities in the top-cuts. The nitrides grow into larger networks, while the carbide particles tend to grow on the nitrides. The latter seem to act as nucleating centers for carbide precipitation. The main part of inclusions in the topcuts lie in the size range from 100- 1000 µm in diameter when measured by the Coulter laser diffraction method.</p><p>A method for sampling of the melt during crystallisation under reduced pressure was developed, giving thus the possibility of indicating the bulk concentration in the melt of carbon and nitrogen. The initial carbon concentration was measured to ~30 and 40 ppm mass when recycled material was employed in the charge and ~ 20 ppm mass when no recycled material was added. Since the melt temperature at this initial stage is ~1500 °C these carbon levels are below the solubility limit. The carbon profiles increase with increasing fraction solidified. For two profiles there is a tendency of decreasing contents at high fraction solidified.</p><p>For nitrogen the initial contents were 10, 12 and 44 ppm mass. The nitrogen contents tend to decrease with increasing fraction solidified. The surface temperature also decreases with increasing fraction solidified. Indicating that the melt is saturated with nitrogen already at the initial stage. The proposed mechanism of formation is by dissolution of coating particles, giving a saturated melt, where β-Si3N4 precipitates when cooling. Supporting this mechanism are the findings of smaller nitride particles at low fraction solidified, that the precipitated phase are β-particles, and the decreasing nitrogen contents with increasing fraction solidified.</p><p>The carbon profile for the solid phase goes through a maximum value appearing at a fraction solidified from 0.4 to 0.7. The profiles flatten out after the peak and attains a value of ~ 8 ppma. This drop in carbon content is associated with a precipitation of silicon carbide. It is suggested that the precipitation of silicon carbide occurs after a build-up of carbon in the solute boundary layer.</p><p>FTIR-measurements for substitutional carbon and interstitial oxygen were initiated at the institute as a part of the work. A round robin test was conducted, with the Energy Research Centre of the Netherlands (ECN) and the University of Milano-Bicocci (UniMiB) as the participants. The measurements were controlled against Secondary Ion Mass Spectrometer analyses. For oxygen the results showed a good correspondence between the FTIR-measurements and the SIMS. For carbon the SIMS-measurements were significantly lower than the FTIR-measurements. This is probably due to the low resistivity of the samples (~1 Ω cm), giving free carrier absorption and an overestimation of the carbon content.</p>

Master of Science<br>Department of Chemistry<br>Jun Li<br>With the worldly consumption of energy continually increasing and the main source of this energy, fossil fuels, slowly being depleted, the need for alternate sources of energy is becoming more and more pertinent. One promising approach for an alternate method of producing energy is using solar cells to convert sunlight into electrical energy through photovoltaic processes. Currently, the most widely commercialized solar cell is based on a single p-n junction with silicon. Silicon solar cells are able to obtain high efficiencies but the downfall is, in order to achieve this performance, expensive fabrication techniques and high purity materials must be employed. An encouraging cheaper alternative to silicon solar cells is the dye-sensitized solar cell (DSSC) which is based on a wide band gap semiconductor sensitized with a visible light absorbing species. While DSSCs are less expensive, their efficiencies are still quite low compared to silicon. In this thesis, Grätzel cells (DSSCs based on TiO2 NPs) were fabricated and optimized to establish a reliable standard for further improvement. Optimized single layer GSCs and double layer GSCs showing efficiencies >4% and efficiencies of ~6%, respectively, were obtained. Recently, the incorporation of metallic nanoparticles into silicon solar cells has shown improved efficiency and lowered material cost. By utilizing their plasmonic properties, incident light can be scattered, concentrated, or trapped thereby increasing the effective path length of the cell and allowing the physical thickness of the cell to be reduced. This concept can also be applied to DSSCs, which are cheaper and easier to fabricate than Si based solar cells but are limited by lower efficiency. By incorporating 20 nm diameter Au nanoparticles (Au NPs) into DSSCs at the FTO/TiO2 interface as sub wavelength antennae, average photocurrent enhancements of 14% (maximum up to ~32%) and average efficiency enhancements of 13% (maximum up to ~23% ) were achieved with well dispersed, low surface coverages of nanoparticles. However the Au nanoparticle solar cell (AuNPSC) performance is very sensitive to the surface coverage, the extent of nanoparticle aggregation, and the electrolyte employed, all of which can lead to detrimental effects (decreased performances) on the devices.