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621.383 --- Photoelectric tubes and cells --- Theses --- 621.383 Photoelectric tubes and cells

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In this work, thin-film polysilicon solar cells on foreign substrates are investigated. Three substrates are chosen that are able to withstand the deposition temperatures of 1130 °C, oxidized silicon, alumina and transparent glass-ceramic substrates. Fine-grained polysilicon is obtained by direct deposition of the silicon onto an oxidized silicon substrate, resulting in very small grains, i.e. an average grain size of only 0.2 µm. This material can be obtained relatively fast and is used as reference material to optimize certain processes on polysilicon layers. Coarse-grained polysilicon is obtained using an epitaxial thickening of a thin but large-grained AIC seed layer, which result in an average grain size of around 5 µm. The main improvements are repeated here. The progress of the efficiency of coarse-grained polysilicon layers on top of an alumina substrate as a function of the time shows a monotonously increasing curve with an average improvement in absolute efficiency of 1.5% per year. The total increase of the efficiency is due to four major improvements in the cell design. The first breakthrough is realized in the beginning of 2004 by inserting a thin FOx-layer between the alumina and the AIC seed layer. This layer reduces the substrate roughness and increases the material quality of the polysilicon layers. More details on the improvement of the material quality will be given in another Ph.D. from the solar cell technology group of IMEC. The second major improvement is obtained at the end of 2004 by the development and application of a thin (~ 17 nm thick) amorphous silicon - crystalline silicon heterojunction emitter instead of the conventional P-diffused homojunction emitter. This leads to a boost in Voc of the devices by nearly 100 mV. The main reason is found to be the absence of preferential P-diffusion spikes along grain boundaries, resulting in less space-charge-region recombination when a heterojunction emitter is used. Processing of the heterojunction emitter also leads to a more efficient hydrogen passivation, which can be done before emitter formation in case of a heterojunction. After plasma hydrogenation, the defect density of the fine-grained polysilicon layers decreases by a factor of three. This reduced defect concentration due to hydrogenation typically increases Voc by another 100 mV and nearly doubles Jsc. Different hydrogenation techniques are investigated showing that plasma hydrogenation techniques outperforms solid source hydrogenation based on a dense silicon-nitride layer. The process sequence using a short plasma hydrogenation followed by the deposition of the heterojunction emitter yields Voc-values that are independent of the grain size of the polysilicon, with the thinner fine-grained polysilicon layers showing as high Voc-values as the coarse-grained (~ 540 mV). Without hydrogenation this trend is also observed and a maximum Voc of 420 mV is reached for the unhydrogenated fine-grained polysilicon layer. In 2005, a third boost in the efficiency is realized using thinner p+- and p-type layers in combination with an interdigitated contact scheme. The use of thin p+- and p-type layers leads to higher Voc and Jsc because of a reduced volume recombination but lowers FF when the side-contacted process was used due to a higher Rseries. The development of interdigitated top contacts where the base contact fingers are closer to each other, results in a lower Rseries and a record FF-value of 74 % for polysilicon layers with an epitaxial p+ layer thickness of only 200 nm. These effects lead to a best efficiency of 5.9 % at the end of 2005. In 2006, the final improvement in this work is obtained through a better optical confinement of the light in the polysilicon layers. The light trapping structure that consists of a plasma textured front (acting like a Lambertian surface) and a highly reflective alumina substrate, shows a path length enhancement factor of 42. The alumina proves to be an excellent substrate with a back reflectance of 97 % while the plasma texturing is a useful technique to roughen the polysilicon surface without removing too much silicon. The combination of this light trapping structure with a thin p+ layer increases the Jsc of the devices from 17 to 21.2 mA/cm2. This results in the best efficiency so far of 8 % for a coarse-grained polysilicon layer on top of an alumina substrate. Except for the improvement of the material quality using a FOx layer between substrate and absorber, the same trends are observed for fine-grained layers on top of an oxidized silicon substrate resulting in a highest efficiency of 5.0 % and for coarse-grained polysilicon layers on top of glass substrates yielding an efficiency of 6.4 %. Both types of polysilicon solar cells show lower efficiencies than coarse-grained polysilicon on alumina mainly because of a lower current density. For both materials, the lower current density results from a worse back reflectance compared to coarse-grained polysilicon on alumina.   The efficiency of 8 % brings the performance of large-grained polysilicon solar cells close to the efficiencies reached in the field of microcrystalline silicon. Their main advantage over microcrystalline silicon solar cells is that these cells are fully stable. To surpass the 10 % efficiency barrier, some further research and development is clearly needed. Dunne-film polykristallijn silicium zonnecellen. Fotovoltaïsche zonnecellen op basis van dunne-film polykristallijn silicium zijn een goedkoop alternatief voor de huidige "wafer-gebaseerde" silicium zonnecellen. Door hun kleine korrelgrootte, de grote recombinatie aan de korrelgrenzen en de lage absorbantie van het licht in de dunne polysilicium laag, is het verkrijgen van een goede celefficiëntie een grote uitdaging. Deze thesis tracht de recombinatie aan de defecten te verminderen en de absorbantie in het materiaal te verhogen. Een heterojunktie emitter van amorf silicium en polykristallijn silicium in plaats van een homojunktie emitter resulteert in een sterke daling van de korrelgrens recombinatie. De concentratie aan bengelende bindingen in het polykristallijn materiaal vermindert met een factor drie door het toepassen van een extra waterstof passivatie. Ten slotte stijgt de nuttige absorbantie in de laag door het textureren van het oppervlak en door het gebruik van een sterk reflecterend alumina substraat. Een combinatie van de heterojunktie emitter, de waterstof passivatie en de texturisatie resulteerde in de verhoging van de efficiëntie van de dunne-film polykristallijne zonnecellen van 1.6 % naar 8 %.

Academic collection --- 621.383 <043> --- 621.383 <043> Photoelectric tubes and cells--Dissertaties --- Photoelectric tubes and cells--Dissertaties --- Theses

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Om photovoltaïsche energie concurrentieel te maken met andere vormen van energieopwekking staat men nog altijd voor de uitdaging om de productiekosten van zonnecellen te verlagen, terwijl het energetisch rendement voor de omzetting van zonnestraling in elektrische energie verhoogd moet worden. Voor de productie van kristallijn silicium zonnecellen betekent dit het gebruik van dunnere en goedkopere silicium substraten. Bij gevolg moet ook de productietechnologie voor de zonnecel zelf aangepast worden. In de eerste plaats moet het kleiner absorberend volume van een dunner substraat gecompenseerd worden met een verbeterde opvang en opsluiting van het zonlicht. Ten tweede moet daarbij het verlies van opgewekte ladingsdragers door recombinatie verhinderd worden. Droge procestechnologie biedt een goede mogelijkheid om dit te verwezenlijken. Een droge etstechniek is ontwikkeld die het mogelijk maakt om gladde silicium oppervlakken te verruwen, zodat het zonlicht minder reflecteert zonder dat de recombinatie stijgt. In deze techniek wordt er gebruik gemaakt van een plasma, waarin het etsgas gedissocieerd wordt in radicalen die het siliciumsubstraat langs een enkele zijde etsen. Bij geoptimaliseerde etscondities ontstaat er spontaan een piramideachtige textuur. Deze manier van textuuretsen kan eenvoudig geïntegreerd worden in het productieproces voor zeer dunne zonnecellen die een verbeterde passivatie aan de achterzijde nodig hebben om oppervlakterecombinatie tegen te gaan. Bovendien is het mogelijk om zeer dunne siliciumfilms te textuuretsen zonder veel verlies van materiaal, wat niet goed mogelijk is in een natte etstechniek. Het hedendaagse productieproces maakt al op grote schaal gebruik van plasmatechnologie die de recombinatie van ladingsdragers in de bulk van goedkope substraten tegen gaat. Een waterstofrijk siliciumnitride antireflectie coating wordt gedeponeerd met een plasma. Op hoge temperatuur komt dit waterstof vrij uit deze coating en passiveert recombinatiecentra, in het bijzonder in de bulk van multikristallijn silicium. Hoewel dit effect goed bekend is, is er tot nu toe weinig kennis aanwezig over de exacte omstandigheden hoe waterstof vrij komt en welke defecten gepassiveerd worden. In dit werk wordt het passiverende effect van waterstof in multikristallijn silicium zonnecellen gedurende het productieproces nader onderzocht. Ook wordt het depositieproces zelf verder bestudeerd en geoptimaliseerd om de parameters te kennen die het passivatieproces beïnvloeden. Aan de hand van die parameters en de eigenschappen van het siliciumnitride is er een model voorgesteld om het vrijkomen van atomair waterstof te beschrijven, wat essentieel is voor de snelle diffusie en defectpassivatie in multikristallijn silicium. To give photovoltaic energy a competitive position compared to other energy resources, there still is a major challenge to decrease the manufacturing costs of solar cells, while increasing the efficiency of converting solar radiation into electric energy. The production of crystalline silicon solar cells using thinner and cheaper silicon substrates is one route to reach this target. For these substrates the cell production technology has to be adapted. In the first place, the decreased absorbing volume of a thinner substrate has to be compensated with a better coupling and confinement of solar radiation into the solar cell. Secondly, losses by recombination of generated charge carriers have to be limited. Dry process technology is a suitable method to fulfill these requirements. A dry etching process has been developed to roughen smooth silicon surfaces to reduce the reflection of solar radiation without inducing additional recombination. This process uses a plasma in which gas is dissociated into radicals that etch silicon substrates on one side only. Under optimized conditions, a pyramid-like texture can be obtained spontaneously. This way of texturing is easy to integrate in the production process of very thin solar cells, requiring an improved passivation on the rear surface to reduce surface recombination losses. Additionally, with this method it is possible to texture very thin silicon films without much material loss, hard to obtain by a wet etch technology. Current cell manufacturing uses plasma technology on a large scale to reduce the recombination of charge carriers in the bulk of cheap substrates. A hydrogen rich silicon nitride anti-reflection coating is deposited with the aid of a plasma. Hydrogen is released from this coating during thermal annealing and passivates recombination centers in the silicon. This passivation is a known effect. However, until now a good understanding about the exact mechanisms of hydrogen release and the passivation of defects is not available. In this work the hydrogenation of multicrystalline silicon solar cells during the manufacturing process is investigated in detail. Also the deposition process is studied and optimized to know the parameters, influencing the passivation. According to these parameters and the characteristics of the silicon nitride layers a model is proposed to describe the release of atomic hydrogen, essential for the fast diffusion and defect passivation in multicrystalline silicon.

Academic collection --- 669 <043> --- 621.383 <043> --- 621.383 <043> Photoelectric tubes and cells--Dissertaties --- Photoelectric tubes and cells--Dissertaties --- 669 <043> Metallurgy--Dissertaties --- Metallurgy--Dissertaties --- Theses

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zonne-energie --- Relation between energy and economics --- Photovoltaic power generation --- Solar energy --- Conversion photovoltaïque --- Energie solaire --- Research --- Recherche --- 621.383 --- Photoelectric tubes and cells --- 621.383 Photoelectric tubes and cells --- Conversion photovoltaïque

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Photovoltaic power generation --- Solar energy industries --- 621.383 --- -Solar energy industries --- -Energy industries --- Photovoltaic energy conversion --- Photovoltaics --- Direct energy conversion --- Solar energy --- Solar batteries --- Photoelectric tubes and cells --- -Photoelectric tubes and cells --- 621.383 Photoelectric tubes and cells --- -621.383 Photoelectric tubes and cells --- Energy industries --- Report --- E-books --- Photovoltaic power generation - Europe --- Solar energy industries - Europe