Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 162-174).

This diploma thesis analyzes the current state of company´s purchasing strategy. In the introduction is described the company and its production program, followed by theoretical part of the thesis. The suggestion part deals with the application of the issue to the practical example as well as the calculation of savings as a result of the purchasing strategy change over.

The objective of this thesis is to understand and improve optical and electrical confinement to achieve cost-effective high-efficiency thin p-type Si solar cells. Optical confinement is achieved by front surface texturing in conjunction with an internal reflective layer on the back surface. Electrical confinement is obtained through the use of a high-lifetime material coupled with high-quality passivation on both surfaces. This research is divided into five tasks. In the first task, Ga-doped Cz Si was investigated to achieve a high and stable lifetime. It was found that for 1 ohm-cm nominal-resistivity screen-printed Al-back surface filed (BSF) cells, the Ga-doped ingot gave ~1.5% higher absolute efficiency after light-soaking relative to the B-doped counterpart. The benefit of using Ga is therefore quite explicit. In the second task, the screen-printed Al-BSF was investigated to explore its potential and limitations for achieving high-efficiency cells. It was found that there exists a critical alloying temperature for a given Al-thickness, above which the Al-BSF becomes non-uniform and cell performance starts to degrade. This puts a limit on the quality of the Al-BSF that can be achieved. An alternative way of back passivation involving dielectric/metal layers was therefore explored. In Task three, two key requirements for achieving high-efficiency dielectric back-passivated cells were established through device modeling. These are (1) a formation of a high-quality BSF underneath the local back contact through vias in the dielectric and (2) a high-quality dielectric passivation with either a moderate positive charge density or a high negative charge density. Task four involved the development of a metallization technique through vias in the dielectric to achieve a high-quality contact and an efficient internal reflector in conjunction with a high-quality local BSF. Further, a novel dielectric system composed of a spin-on SiO<SUB>2</SUB> layer capped with SiN<SUB>x</SUB> was developed that exhibited excellent passivation and a moderate positive charge density. The final task involved fabrication and analysis of dielectric back-passivated cells. The new dielectric and process sequence developed in this thesis resulted in screen-printed solar cells with efficiency as high as 19% with the potential for 20% efficient cells on 100-µm thick Si substrates.

NiSi and Ni(Pt)Si, and of the effects of dissociated ammonia on oxide reduction was carried out under controlled ultrahigh vacuum (UHV) conditions. X-ray photoelectron spectroscopy (XPS) has been used to characterize the evolution of surface composition. Vicinal surfaces on NiSi and Ni(Pt)Si were formed in UHV by a combination of Ar+ sputtering and thermal annealing. Oxidation of these surfaces in the presence of either O+O2 or pure O2 at room temperature results in the initial formation of a SiO2 layer ~ 7 Å thick. Subsequent exposure to O2 yields no further oxidation. Continued exposure to O+O2, however, results in rapid silicon consumption and, at higher exposures, the kinetically-driven oxidation of the transition metal(s), with oxides >35&#506; thick formed on all samples, without passivation. The addition of Pt retards but does not eliminate oxide growth or Ni oxidation. At higher exposures, in Ni(Pt)Si surface the kinetically-limited oxidation of Pt results in Pt silicate formation. Substrate dopant type has almost no effect on oxidation rate. Reduction of the silicon oxide/metal silicate is carried out by reacting with dissociated NH3 at room temperature. The reduction from dissociated ammonia (NHx+H) on silicon oxide/ metal silicate layer shows selective reduction of the metal oxide/silicate layer, but does not react with SiO2 at ambient temperature.

Cette thèse est axée sur la fabrication et l’optimisation d’une nouvelle structure permettant théoriquement d’améliorer les performances des cellules à base de silicium cristallin. Cette nouvelle architecture de cellule utilise la technologie des hétérojonctions de silicium a-Si:H/ c-Si (Si-HJ) appliquée sur des structures à contacts interdigités en face arrière (IBC). Le potentiel de rendement des cellules IBC Si-HJ est supérieur à 25%, mais leur fabrication nécessite une localisation des couches de a-Si:H de dopage différent et de leurs métallisations. L’intégration de ces étapes dans un procédé simplifié utilisant des techniques industrielles (PECVD, pulvérisation, sérigraphie et laser) a été étudiée. De plus, une structure obtenue sans séparation entre le BSF et l’émetteur est présentée, permettant de réduire le nombre d’étapes de fabrication. Les avantages ainsi que les limites liés à cette architecture simplifiée ont été illustrés du point de vue expérimental et par simulation. Dans le cadre de ces travaux, le rendement maximum atteint sur les dispositifs IBC Si-HJ simplifiés de 25cm² est de 19% (substrats de type n), ce qui constitue le 3e meilleur résultat au niveau mondial. Les performances des cellules restent encore limitées par l’absorption des couches de a-Si:H utilisées pour la passivation de la face avant, et par la conductivité des couches dopées en face arrière. De nombreuses pistes d’amélioration sont explorées dans cette étude. Un procédé de métallisation innovant a également été élaboré pour le passage sur des substrats de grande taille (150cm²). Il permet de limiter les pertes résistives tout en offrant de la flexibilité au niveau de la géométrie des contacts. La mise en module de cellules ayant ce design de métallisation a ensuite été étudiée, et un module de 4 cellules IBC Si-HJ a pu être fabriqué. / This thesis studies the fabrication and the optimization of a new structure to enhance the efficiency of crystalline silicon based solar cells. This new cell design uses a-Si:H/c-Si heterojunction (Si-HJ) technology applied on interdigitated back contact structures (IBC). With IBC Si-HJ solar cells, the efficiency potential is theoretically higher than 25%. Their fabrication requires to pattern doped a-Si:H and the associated metallization on the same side. The implementation of those process steps has been carefully studied. All processes used in this study are potentially industrial (PECVD, sputtering, screen-printing, and laser) and the obtained structure without buffer layer between the BSF and the emitter allows to reduce fabrication steps. Issues linked to this design have been investigated. Within the frame of this work, the maximum efficiency reached on reduced size devices (25cm²) with n-type substrate and is 19% which is the 3rd best result worldwide. The cell performances are still limited by the absorption of front surface passivating layer (a-Si:H) and by the low doped layer conductivity. Several optimization ways are explored in this study. An innovative metallization process is then elaborated to allow large area solar cell fabrication while limiting resistive losses and offering more flexibility on metallized pattern. The interconnection and the encapsulation of cells with this metallization design have been illustrated and a module with 4 cells has been fabricated.

Aluminum was deposited onto both Teflon AF and Parylene AF surfaces by chemical vapor deposition of trimethylaluminum. This work shows that similar thin film (100 Angstroms) aluminum oxide adlayers form on both polymers at the low temperature dosing conditions used in the studies. Upon anneal to room temperature and above, defluorination of the polymer surfaces increased and resulted in fluorinated aluminum oxide adlayers; the adlayers were thermally stable to the highest temperatures tested (600 K). Angle-resolved spectra showed higher levels of fluorination toward the polymer/adlayer interface region. Copper films were also deposited at low temperature onto Teflon AF using a copper hexafluoroacetylacetonate-cyclooctadiene precursor. Annealing up to 600 K resulted in the loss of precursor ligands and a shift to metallic copper. As with aluminum adlayers, some polymer defluorination and resulting metal (copper) fluoride was detected. Parylene AF and polystyrene films surfaces were modified by directly dosing with water vapor passed across a hot tungsten filament. Oxygen incorporation into polystyrene occurred exclusively at aromatic carbon sites, whereas oxygen incorporation into parylene occurred in both aromatic and aliphatic sites. Oxygen x-ray photoelectron spectra of the modified polymers were comparable, indicating that similar reactions occurred. The surface oxygenation of parylene allowed enhanced reactivity toward aluminum chemical vapor deposition. Silicon-carbon (Si-Cx) films were formed by electron beam bombardment of trimethylvinylsilane films which were adsorbed onto metal substrates at low temperatures in ultra-high vacuum. Oxygen was also added to the films by coadsorbing water before electron beam bombardment; the films were stable to more than 700 K, with increasing silicon-oxygen bond formation at elevated temperatures. Copper metal was sputter deposited in small increments onto non-oxygenated films. X-ray photoelectric spectra show three-dimensional copper growth (rather than layer-by-layer growth), indicating only weak interaction between the copper and underlying films. Annealing at elevated temperatures caused coalescence or growth of the copper islands, with spectra indicating metallic copper rather than copper oxide.

This thesis focuses on improving the adhesion of electroless metal layers plated to pit contacts in interdigitated, backside buried contact (IBBC) solar cells. In an electrolessly plated, pit contact IBBC cell, the contact grooves are replaced with lines of pits which are interconnected by the plated metal. It is shown, however, that electroless metal layers, plated by the standard IBBC plating sequence, are not adherent on pit contact IBBC solar cells. The cause of this adhesion problem is investigated by examining the adhesive properties of each of the metal layers in the electroless metallization sequence on planar test structures. This investigation reveals that Pd activation of heavily P diffused Si impedes Ni silicide growth and that, in the absence of a silicide at the Ni/Si interface, an electrolessly plated Cu layer will cause the underlying Ni layer to peel away from the substrate. It is also found that the Ni silicidation process itself intermittently causes the unreacted Ni to spontaneously peel away from the substrate. An electroless metallization sequence that results in thick, adhesive Cu deposits on planar &lt 100&gt surfaces is developed in this thesis. It is shown that this process leads to the formation of a Ni silicide on both n- and p- type, heavily diffused surfaces. Fully plated, pit contact IBBC solar cells were not able to be fabricated during the course of this work but it is reasonable to expect that the modified plating sequence developed in this work will result in the metal layers being adhesive on these cells.