Stark effect, polarizability and electroabsorption in silicon nanocrystals

Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects is extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling, $\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC diameter. Finally, based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Given the fact that bulk silicon lacks the linear electro-optic effect as being a centrosymmetric crystal, this may offer a viable alternative for electrical modulation using p-doped Si NCs.Comment: Published version, 7 pages, 7 figure

Exploration of the horizontally staggered light guides for high concentration CPV applications

The material and processing costs are still the major drawbacks of the c-Si based photovoltaic (PV) technology. The wafer cost comprises up to 35-40% of the total module cost. New approaches and system designs are needed in order to reduce the share of the wafer cost in photovoltaic energy systems. Here we explore the horizontally staggered light guide solar optics for use in Concentrated Photovoltaic (CPV) applications. This optical system comprises a lens array system coupled to a horizontal light guide which directs the incoming light beam to its edge. We have designed and simulated this system using a commercial ray tracing software (Zemax). The system is more compact, thinner and more robust compared to the conventional CPV systems. Concentration levels as high as 1000x can easily be reached when the system is properly designed. With such a high concentration level, a good acceptance angle of + -1 degree is still be conserved. The analysis of the system reveals that the total optical efficiency of the system could be as high as %94.4 without any antireflection (AR) coating. Optical losses can be reduced by just accommodating a single layer AR coating on the initial lens array leading to a %96.5 optical efficiency. Thermal behavior of high concentration linear concentrator is also discussed and compared with a conventional point focus CPV system. (c) 2012 Optical Society of Americ

A THEMATIC APPROACH TO BILDUNGSROMAN: DAVID COPPERFIELD & ÇALIKUŞU

Bu çalışmada, İngiliz edebiyatından Charles Dickens’ın David Copperfield (1850) ve Türk edebiyatından Reşat Nuri Güntekin’in Çalıkuşu (1922) romanları, Batı edebiyatlarında eğitim romanı, oluşum romanı veya sanatçı romanı olarak da adlandırılan Bildungsroman geleneği açısından incelenmektedir. Adı geçen romanların bu çalışmaya konu olmasının sebebi her iki romanın farklı ülke edebiyatlarında ve dönemlerde yazılmalarına rağmen, benzer tematik özelliklere sahip olmalarıdır. Yöntem olarak çalışmada Batı edebiyatına Bildungsroman olarak yer edinmiş, bir karakterin çocukluğundan yetişkinliğine kişisel gelişimini ele alan roman türünün tematik özelliklerine başvurulmuştur. Bunun sonucunda, çalışmada öncelikli olarak Batı edebiyatlarında görülen Bildungsroman geleneğinin tanımı yapılmış ve daha sonra Bildungsroman üzerine ortaya konmuş belli başlı çalışmalar çerçevesinde türün genel özellikleri üzerinde durulmuştur. Daha sonra sırasıyla David Copperfield ve Çalıkuşu romanlarının tematik özellikleri, olay örgüsü bağlamında ele alınmıştır. Son bölümde ise, her iki romanda tespit edilen bu tematik özellikler ve olay örgüsü yapısı Bildungsroman geleneği ışığında değerlendirilmiş ve iki romanda tespit edilen bu özellikler tartışılmıştır. Bunun sonucunda -bu çalışmada gerçekleştirilen tematik tartışmayla bağlantılı olarak- elde edilen sonuç, Türk edebiyatının önemli bir eseri olan Çalıkuşu romanın farklı bir kültüre ve döneme ait olmasına rağmen David Copperfield romanının da bir parçası olduğu Batı edebiyatlarında görülen Bildungsroman geleneği tematik özelliklerini büyük ölçüde taşıdığı tespit edilmiştir. Bu sonucun elde edilmesinde, Çalıkuşu romanında, David Copperfield’da olduğu gibi, kahramanın çocukluğundan yetişkinliğine göstermiş olduğu bütün gelişim sürecinin detaylı olarak ele alınması büyük rol oynamaktadır.In this paper, Charles Dickens’s David Copperfield (1850), from English literature, and Reşat Nuri Güntekin’s Çalıkuşu (1920), from Turkish literature, are analyzed through the Bildungsroman tradition which is also named as the novel of education, the novel of formation or the novel of artist. The reason why these two novels are subject to this study is that though they were written in different countries and periods they have similar thematic features. As a methodology, the thematic features of Bildungsroman in the Western literatures – which is a formative novel of a character from his/her youth to his/her maturity – is applied for the study. Therefore, initially the definition of Bildungsroman in Western literatures is given in the study and then according to the certain literary works on Bildungsroman, the general literary features of Bildungsroman are discussed in this paper. Later, the thematic features of David Copperfield and Çalıkuşu are elaborated respectively in the context of plot. In the last part of the paper, the thematic features and the plot structure observed in both novels are compared each other through the Bildungsroman tradition. As a result, it can be asserted that although the novel Çalıkuşu which is an important work of Turkish Literature, belongs to a different culture and period, it has the thematic features of Western Bildungsroman tradition in which David Copperfield is also included to a large extent, related to the theoretical discussion conducted in this study. The whole maturation process of the heroine observed in Çalıkuşu, as it is in David Copperfield, has an important role for achieving this result

Advanced plasmonic interfaces for optimized light trapping in photovoltaics

Plasmonic interfaces are integrated to photovoltaic devices to enhance light trapping and improve efficiency. The optimum thickness of the spacer layer used to passivate the absorber layer and adjust its distance from the metal nanoparticles remains unclear. We integrate plasmonic interfaces consisting of Ag nanoparticles and silicon nitride spacers of different thicknesses to the back of a-Si:H absorber to investigate the optimum thickness of the spacer layer and use the photocurrent in a-Si:H to indicate the enhancement in light-trapping. For integration to the back or front of the device, the localized surface plasmon resonance (LSPR) is shifted and broadened into the red with increased spacer layer thickness and the effect is more pronounced for integration to the back. An appreciable enhancement of photocurrent in a-Si:H is consistent with this broadening of LSPR and has a critical dependence on spacer layer thickness

Lazer Kristalizasyon Yöntemi ile Yüksek Verimli İnce Film Kristal Si Güneş Gözesi Geliştirilmesi

TÜBİTAK MAG Proje01.12.2018Gerçeklestirdigimiz projede silisyum tabanlı polikristal-amorf silisyum ince film günes gözesiüretimine özellesmis bir lazerle kristalizasyon platformunun sanayi-üniversite isbirligisayesinde gelistirilmesi saglanmıstır. Yola çıkıs hedeflerimiz bu teknoloji ve üniversitearastırma olanakları kullanılarak ince film silisyum günes gözesi üretilebileceginin gösterilmesive üretim yöntem basamaklarının ortaya konmasıdır. Günümüz fotovoltaik piyasasındaki enbüyük pazar payı yaklasık %90 ile kristal silisyum (c-Si) pul tabanlı gözelere aittir. Ancakavantajlarına ragmen pul tabanlı Si göze teknolojisinin dünya enerji pazarındaki yeri halensınırlıdır. Bu konudaki en büyük engellerden biri yüksek malzeme giderlerine baglı üretimmaliyetinin diger enerji sektörleri ile rekabet edecek seviyede olmamasıdır. Aktif maddekullanımını optimize ederek üretim masraflarını düsürmeyi amaçlayan ince-film teknolojilerimaliyet bariyerini asarak fotovoltaik enerji dönüsüm teknolojilerini diger teknolojilerle rekabetedebilir hale getirecek bir çözüm potansiyeli vadetmektedir. Ince film poli-Si yaklasımı ile tekkristal silisyuma yakın kalitede, mikrometre ölçeginde tane büyüklügüne sahip poli-Si incefilmlerin proje kapsamında büyütülmesi ve fotovoltaik hücre üretiminde kullanılmasıhedeflenmistir. Cam alttas üzerine elektron demet buharlastırma yöntemi ile katkılanmısamorf silisyum tabakalar biriktirilerek ve kristalizasyon için özel olarak tasarlanıp gelistirilmislazer sisteminin çizgisel odagı bu yüzey üzerinde taranarak amorf silisyumun polikristalsilisyuma kristalizasyonu saglanmıstır. Üretilen polikristal silisyum yine katkılanmıs ince biramorf silisyum ile kaplanarak heteroeklem elde edilmis, bu aygıtın üst ve alt kontakları Altermal buharlastırılmasıyla olusturularak aygıt üretimi tamamlanmıstır. Tüm üretimbasamakları arasında optik, mekanik ve elektriksel ölçümler gerçeklestirilerek islembasamakları iyilestirilmesi saglanmıs, gerektigi durumlarda lazer sisteminde de iyilestirmeleregidilmistir. Verimliliklerde artıs eldesi için kenar yalıtımı ve plazmonik yapıların entegrasyonuda gerçeklestirilmistir. Üretilen polikristal Si malzemenin dinamigi moleküler dinamik sayısalbenzetim yöntemiyle kuramsal olarak çalısılmıstır. Projemiz sonucunda çagrı programıylauyumlu olarak göze üretiminde yerli üretim altyapısı olusturmaya dogrudan katkı, genis alanlı,düsük üretim maliyetli, polikristal-amorf silisyum heteroeklem ince film günes gözesiüretilmesi, bu üretimi saglayabilmek için fiber lazer teknolojisine dayanan yenilikçi, çokfonksiyonlu ve özgün bir ince film kristalizasyon ve isleme platformu prototipi gelistirilmesibasarılmıstır.Anahtar Kelimeler: ince film günesThis project has achieved to develop a laser crystallization platform designated for theproduction of silicon based polycrystalline-amorphous silicon thin film solar cells throughcollaboration between industry and academia. The main goals of the project is to demonstratethat thin film crystalline silicon solar cells can be developed by utilizing the technological andacademic research capabilities, as well as establishing the methodological approach foraccomplishing this challenging task. The current photovoltaics market is dominated bycrystalline silicon (c-Si) wafer based cells, with a share of about 90%. Despite its advantagesSi-wafer based cells have yet to gain a considerable standing in the world energy market. Oneof the major obstacles for the Si-wafer cells is the raw material costs that keeps the productioncosts from reaching competitively low levels. Thin film technologies offer a solution to theovercome the cost problem by optimizing the active material usage. In this project, in-situdoped a-Si layers are deposited on glass. The a-Si layers are subsequently crystallized byscanning the linear focus of a dedicated laser system designed for this purpose on the filmsurface. Following heterojunction structure formation by growing a doped thin a-Si layer on thepoly-Si, top and bottom contact are produced by thermal evaporation of Al to finalize the devicestructure. During and in between each production step, optical, mechanical and electricalanalysis will be carried out in order to optimize process parameters, and where foundnecessary, modifications to improve the laser system will be undertaken. In order to furtherimprove device performace, edge isolation and plasmonic light trapping schemes have beenutilized. To understand the crystallization process on a theoretical level, molecular dynamicsnumerical simulation methods are used. The heterojunction properties will be studied via DFTsimulation method. As a result of our project, in line with the call program, direct contributionto national infrastructure for production of wide area, low cost polycrystalline-amorphous siliconhetero-junction thin-film solar cells; and an innovative, multifunctional and original fiber lasertechnology based thin film crystallization and processing platform prototype has beencommissioned

Angular Dependence of Solar Cell Parameters in Crystalline Silicon Solar Cells Textured with Periodic Array of Microholes

Surface texturing is an indispensable way of increasing absorption in solar cells. In order to properly characterize the effect of texturing, the angular dependence of the incidence light should be addressed. This is particularly important when the actual application where the incidence angle of the sunlight varies during the day is considered. This study presents the angular dependence of solar cell parameters in the case of periodically textured crystalline silicon (c-Si) solar cells with microholes. A standard solar cell with pyramid texturing is also studied for comparison. It is shown that the incidence angle for the highest efficiency depends on the surface structure. While a standard pyramid-textured surface performs best at the zero angle of incidence, it is needed to tilt the sample with microholes textures 15 degrees with respect to the surface normal. This is also confirmed by the simulation study performed for the structures presented in this study

Gözenekli si ve ge nanoyapılarının ileri teknoloji güneş gözelerine uygulanması

TÜBİTAK MFAG01.04.2014Uluslararası ikili işbirliği projesi olan bu çalışmada amaca uygun olarak gözenekli silisyum ve germanyum nanoyapılar silisyum oksit matris içerisinde büyütülmüş ve optoleektronik aygıt üretiminde kullanılmıştır. Söz konusu yapıları elde edebilmek için silisyum ve germanyum-zengin silisyum oksit ince filmler fiziksel yolla buhar depolama yöntemlerinden yararlanarak üretilmiş ve kristal oluşumunu sağlamak için de yüksek sıcaklık fırını, hızlı termal tavlama ve lazer tavlama yöntemleri gibi farklı teknikler kullanılarak tavlanmıştır. Bu sayede farklı boyutlarda nanokristaller oluşturulmuş ve elde edilen yapıların optik band aralığının değişken olması sağlanmıştır. Böylece bu proje ile güneş spektrumundan daha fazla ve daha verimli yararlanmak amaçlı optik band aralığı ayarlanabilir malzemeler üretmiş bulunmaktayız. Öte yandan ürtetilen filmler, aygıt üretiminde gerekli olan elektrik akım geçişini sağlamk açısından da istenen şartları sağlamıştır. Böylece proejnin en önemli hedefi olan kuantum etkileri koruyarak, iletken tabakaların elde edilmesi hedefine ulaşılmıştır. Daha sonra bu yapılar, heteroeklem türü p-n eklemlei üretilmiş ve bu eklemlerin elektro- optik özellikleri belirlenmiştir. Si nanoyapılı örnekler güneş gözesi özellikleri sergilerken, Ge nanoyapılı örnekler de daha çok fotoalgılama yeteneği öne çıkmıştır. Bu çalışma ile ilk defa üretilen ve umut verici sonuçlar veren bu tür aygıtların daha da geliştirilmesi için yeni çalışmaların yürütülmesi gerekmektedir. Proje önerisi 24 ay gibi nispeten kısa bir sürede çok sayıda iş yapmayı hedefleyen bir proje olarak sunulmuştur. Uluslararası bir organizasonu da içeren zorlu bir programı olanproje hedeflerine büyük ölçüde ulaşmıştır. Çok sayıda deney ve çalışma yürütülmüştür. Bu çalışmaların bir özeti bu sonuç raporunda sunulmuştur.In this international bilateral cooperation project, silicon and germanium nanostructures embedded in silicon oxide matrix have been produced, optimized and used in some optoelectronic devices. In order to obtained these new material types, Si and Ge materials were co-sputtered together with SiO2 on a substrate, and annealed using conventional and rapid thermal annealing systems as well as laser annealing systems. In this way, nanostructures embedded in a dielectric matrix have been obtained. Such nanostructures are expected to yield materials with tunable band gap which depends on this size of the nanoparticles. A device having different band gap is expected to provide a more efficient light harvesting from the solar radiation. On the other hand, when these nanostructure are somehow interconnected, electrical transport becomes more feasible, which is contrary to the case where isolated nanocrystals are used. We have reached this goal through a series of experiments during this project. We have shown that both electrical transport and quantum confinement can be obtained in the same material system. After having optimized the material system, we have applied them to heterojunction type p-n diodes for demenstration. Devices with Si nanostructure have exhibited solar cell properties with weak efficiency, while those with Ge nanostructures have shown photodiode features only. These type of device have been produced for he first time in this project. They have shown some promising features. However, they need to be studied and optimized for an actual appliction. This porject was submitted as an ambitious study to be performed in a relatively short time. In addition to its scientific and tecjhnical program, it also included international oragnization tasks, which sometimes slows down the prject execution In spite of laa these difficulties, and the heavy content, we have achived most of goals through extensive experiments and studies. Below, we present a summary of our project acitivities and the results obtained from these studies

Development of a Selective Wet-Chemical Etchant for 3D Structuring of Silicon via Nonlinear Laser Lithography

Recently-demonstrated high-quality three-dimensional (3D) subsurface laser processing inside crystalline silicon (c-Si) wafers opens a door to a wide range of novel applications in multidisciplinary research areas. Using this technique, a novel maskless micro-pillars with precise control on the surface reflection and coverage are successfully fabricated by etching the laser processed region of c-Si wafer. To achieve this, a particular selective wet chemical etching is developed to follow subsurface laser processing of c-Si to reveal the desired 3D structures with smooth surfaces. Here, we report the development of a novel chromium-free chemical etching recipe based on copper nitrate, which yields substantially smooth surfaces at high etch rate and selectivity on the both laser-processed Si surface and subsurface, i.e., without significant etching of the unmodified Si. Our results show that the etch rate and surface morphology are interrelated and strongly influenced by the composition of the adopted etching solution. After an extensive compositional study performed at room temperature, we identify an etchant with a selectivity of over 1600 times for laser-modified Si with respect to unmodified Si. We also support our findings using density functional theory calculations of HF and Cu adsorption energies, indicating significant diversity on the c-Si and laser-modified surfaces

Production of upgraded metallurgical-grade silicon for a low-cost, high-efficiency, and reliable PV technology

Upgraded metallurgical-grade silicon (UMG-Si) has the potential to reduce the cost of photovoltaic (PV) technology and improve its environmental profile. In this contribution, we summarize the extensive work made in the research and development of UMG technology for PV, which has led to the demonstration of UMG-Si as a competitive alternative to polysilicon for the production of high-efficiency multicrystalline solar cells and modules. The tailoring of the processing steps along the complete Ferrosolar’s UMG-Si manufacturing value chain is addressed, commencing with the purification stage that results in a moderately compensated material due to the presence of phosphorous and boron. Gallium is added as a dopant at the crystallization stage to obtain a uniform resistivity profile of ∼1 Ω cm along the ingot height. Defect engineering techniques based on phosphorus diffusion gettering are optimized to improve the bulk electronic quality of UMG-Si wafers. Black silicon texturing, compatible with subsequent gettering and surface passivation, is successfully implemented. Industrial-type aluminum back surface field (Al-BSF) and passivated emitter and rear cell (PERC) solar cells are fabricated, achieving cell efficiencies in the range of those obtained with conventional polysilicon substrates. TOPCon solar cell processing key steps are also tested to further evaluate the potential of the material in advanced device architectures beyond the PERC. Degradation mechanisms related to light exposure and operation temperature are shown to be insignificant in UMG PERC solar cells when a regeneration step is implemented, and PV modules with several years of outdoor operation demonstrated similar performance to reference ones based on poly-Si. Life cycle analysis (LCA) is carried out to evaluate the environmental impact of UMG-based PV technology when compared to poly-Si-based technology, considering different scenarios for both the manufacturing sites and the PV installations