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Photovoltaics, IEEE Journal of

Issue 1 • Date Jan. 2014

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Displaying Results 1 - 25 of 83
  • Table of contents

    Publication Year: 2014 , Page(s): C1 - 3
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  • IEEE Journal of Photovoltaics publication information

    Publication Year: 2014 , Page(s): C2
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  • Structural Order and Staebler–Wronski Effect in Hydrogenated Amorphous Silicon Films and Solar Cells

    Publication Year: 2014 , Page(s): 4 - 9
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    The structure of hydrogenated amorphous silicon films is investigated by Raman spectroscopy and X-ray diffraction. Raman spectroscopy probes the phonon density of states, whereas X-ray diffraction measures the distribution of the electron density. Yet, both methods can yield information on the microstructure of the material represented by certain parameters like, e.g., the position or the width of the transverse optical phonon or the width of the first scattering peak. Interdependences between these parameters are investigated and evaluated. A correlation was found between the structural disorder and the relative efficiency loss caused by the Staebler-Wronski effect for intrinsic films applied as absorbing layers in solar cells. This correlation could be used to estimate the solar cell degradation without time-consuming light-soaking experiments. View full abstract»

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  • Implications of TCO Topography on Intermediate Reflector Design for a-Si/μc-Si Tandem Solar Cells—Experiments and Rigorous Optical Simulations

    Publication Year: 2014 , Page(s): 10 - 15
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (4605 KB) |  | HTML iconHTML  

    The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay. View full abstract»

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  • Light-Trapping and Interface Morphologies of Amorphous Silicon Solar Cells on Multiscale Surface Textured Substrates

    Publication Year: 2014 , Page(s): 16 - 21
    Cited by:  Papers (1)
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    The short circuit current and quantum efficiency of silicon thin-film solar cells can be increased by using multiscale surface textures consisting of micro- and nanoscale textures. Adding microtextures to the already existing nanosurface textures leads to an increase of the short circuit current from 15.5 mA/cm2 to almost 17 mA/cm2 for thin amorphous silicon solar cells. To gain insights into the light-trapping properties, finite difference time domain simulations were carried out using realistic interface morphologies. The simulations reveal that the gain of the short circuit current is caused by an increased effective thickness of the solar cell and the scattering properties of the microtextured back reflector. The thickness of the solar cells is increased by the growth of the silicon p-i-n diode on the microtextured surface. The influence of the nanoscale and multiscale surface textures on the quantum efficiency and short circuit current will be discussed. View full abstract»

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  • Optical and Electrical Simulation of μc-Si:H Solar Cells: Effect of Substrate Morphology and Crystalline Fraction

    Publication Year: 2014 , Page(s): 22 - 27
    Cited by:  Papers (1)
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    Hydrogenated microcrystalline silicon (μc-Si:H) is an important material for high-efficiency multijunction solar cells. Due to its complex microstructural properties, it is difficult to describe the electronic behavior clearly. In this study, we measure opto-electronic properties including the mobility gap of μc-Si:H films in solar cells, as well as physical properties such as the crystalline fraction profile. The height distribution function of the ZnO substrates is obtained by AFM scans, which is used for optical simulation. All the parameters that we obtained from measurements were used as input parameters of a model in the ASA simulator. We obtained a good fit between measurements and simulations. View full abstract»

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  • Silicon Nanowire Solar Cells With Radial p-n Heterojunction on Crystalline Silicon Thin Films: Light Trapping Properties

    Publication Year: 2014 , Page(s): 28 - 32
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1939 KB) |  | HTML iconHTML  

    We present a concept for a core-shell silicon nanowire thin-film solar cell showing strong light trapping. Nanowires are wet chemically etched into a several micrometer-thick laser-crystallized silicon thin film on glass. The nanowires are equipped with an a-Si heteroemitter deposited as a shell around the nanowires by plasma-enhanced chemical vapor deposition to achieve a radial p-n heterojunction. The space between the nanowires is filled with ZnO:Al, acting as a transparent contact. Our core-shell nanowire solar cells reached an efficiency of 8.8%. The main emphasis of this study is on the optical properties of the nanowire solar cell system. View full abstract»

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  • Progress in Laser-Crystallized Thin-Film Polycrystalline Silicon Solar Cells: Intermediate Layers, Light Trapping, and Metallization

    Publication Year: 2014 , Page(s): 33 - 39
    Cited by:  Papers (5)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1141 KB) |  | HTML iconHTML  

    Diode laser crystallization of thin silicon films on the glass has been used to form polycrystalline silicon layers for solar cells. Properties of an intermediate layer stack of sputtered SiOx/SiNx/SiOx between the glass and the silicon have been improved by reactively sputtering the SiNx layer, which result in enhanced optical and electrical performance. Light trapping is further enhanced by texturing the rear surface of the silicon prior to metallization. An initial efficiency of 11.7% with VOC of 585 mV has been achieved using this technique, which are the highest values reported for poly-Si solar cells on glass substrates. Cells suffer a short term, recoverable degradation of VOC, and fill factor. The magnitude of the degradation is reduced via the repeated thermal treatment. A selective p+ metallization scheme has been developed which eliminates the degradation altogether. View full abstract»

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  • Local Shunting in Multicrystalline Silicon Solar Cells: Distributed Electrical Simulations and Experiments

    Publication Year: 2014 , Page(s): 40 - 47
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (876 KB) |  | HTML iconHTML  

    In this paper, we analyze the effect of local shunts in photovoltaic (PV) solar cells by experimental characterization and distributed electrical simulations. To this purpose, we developed a quasi-3-D distributed electrical network that is based on two-diode circuit elementary units. It allows accounting for resistive losses associated to the transport through the emitter, the fingers and the busbars, and to local defects in the semiconductor. The electrical parameters of the equivalent circuit units are calibrated according to experiments performed on multicrystalline (mc-Si) silicon solar cells, including samples that feature local shunts due to localized defects, which lead to nonuniform distribution of electrical and optical properties. The distributed electrical simulations account for the degradation of fill factor and power conversion efficiency in case of local shunting. Moreover, by combining the proposed tool with a RC thermal network it is possible to estimate the temperature distribution in a shunted solar cell. Our analysis shows how a shunted cell that operates under hot-spot conditions is subject to significant local overheating, which possibly lead to permanent PV cell damages. View full abstract»

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  • Fully Ion Implanted and Coactivated Industrial n-Type Cells With 20.5% Efficiency

    Publication Year: 2014 , Page(s): 48 - 51
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (490 KB) |  | HTML iconHTML  

    We present our progress in fully implanted n-type cell for industrial manufacturing. Screen-printed n-type cells with boron emitters are a viable contender to succeed p-type cell in industrial production. A major open topic is the availability of a robust manufacturing flow. The synergistic qualities of ion implantation allow for a very efficient and potentially robust process sequence. We review some of the challenges arising from this approach, such as activation of the implanted boron emitter and the reverse current characteristics. Or current process allows manufacturing of 156 × 156 mm 2 cells with efficiencies up to 20.5% with a single coactivation anneal. View full abstract»

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  • Application of ion Implantation Emitter in PERC Solar Cells

    Publication Year: 2014 , Page(s): 52 - 57
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    Ion-implantation offers numerous advantages (i.e., single-side precise control and reproducibility of the dopant, simultaneous SiO2 passivation during annealing, no phosphosilicate glass formation) for solar cell manufacturing. Canadian Solar Inc. has developed an average efficiency 19.23% blank emitter solar cell (156 mm Cz) process using a high-throughput Varian (Applied Materials) Solion ion-implant tool. In order to improve solar cell efficiency, focus is placed on the well-known advanced passivated emitter and rear cell solar cell architecture with optimized backside passivation. The approach is to combine the surface passivation provided by a thin atomic layer deposition aluminum oxide layer grown after the post implantation annealing process with a deposited capping silicon nitride layer. Laser ablation and proper aluminum paste is also used to locally remove the dielectric layers and to form local contact. Based on this development, implanted emitter and local Al-BSF with Al2O3/SiNx back passivation are integrated in solar cells, reaching an average efficiency of 19.96% and champion 20.12%. View full abstract»

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  • Fully Ion-Implanted and Screen-Printed 20.2% Efficient Front Junction Silicon Cells on 239 cm ^{\bf 2} n-Type CZ Substrate

    Publication Year: 2014 , Page(s): 58 - 63
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (716 KB) |  | HTML iconHTML  

    In this study, we present fully ion-implanted screen-printed high-efficiency 239 cm2 n-type silicon solar cells that are fabricated on pseudosquare Czochralski wafers. Implanted boron emitter and phosphorous back-surface field (BSF) were optimized to produce n-type front junction cells with front and back SiO2 /SiNx surface passivation and rear point contacts. Average efficiency of 19.8%, with the best efficiency of 20.2%, certified by Fraunhofer ISE, Freiburg, Germany, was achieved. In addition, the planarized rear side gave better surface passivation, in combination with optimized BSF profile, raised the average efficiency to ~20% for the fully implanted and screen-printed n-type passivated emitter, rear totally diffused cells. View full abstract»

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  • Effect of Boron Codoping and Phosphorus Concentration on Phosphorus Diffusion Gettering

    Publication Year: 2014 , Page(s): 64 - 69
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    Compared with phosphorus diffusions, conventional boron diffusions for n-type solar cells are not effective at impurity gettering without the presence of a boron-rich layer. In this paper, we investigate the gettering effectiveness of light phosphorus diffusions for removing Fe impurities, applied on an underlying boron diffusion, similar to the buried emitter concept, as an option for achieving effective gettering on boron diffused substrates. Our experimental results on monocrystalline silicon samples demonstrate that the underlying boron diffusion does not affect the gettering effectiveness of the phosphorus diffusion, even though much of the phosphorus diffused region is overdoped by the boron diffusion. Furthermore, we investigate the gettering effectiveness of low surface concentration phosphorus diffusions that can result in reduced recombination in the n+ region. Our results show that the gettering effectiveness decreases when the surface phosphorus concentration is reduced, either through manipulating the deposition gas flows or through subsequent driving in. Driving in the surface phosphorus concentration from 2 × 1020 to 3.5 × 1019 cm-3 decreased the gettering effectiveness by about one order of magnitude. View full abstract»

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  • Improving the Quality of Epitaxial Foils Produced Using a Porous Silicon-based Layer Transfer Process for High-Efficiency Thin-Film Crystalline Silicon Solar Cells

    Publication Year: 2014 , Page(s): 70 - 77
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (739 KB) |  | HTML iconHTML  

    A porous silicon-based layer transfer process to produce thin (30-50 μm) kerfless epitaxial foils (epifoils) is a promising approach toward high-efficiency solar cells. For high efficiencies, the epifoil must have high minority carrier lifetimes. The epifoil quality depends on the properties of the porous layer since it is the template for epitaxy. It is shown that by reducing the thickness of this layer and/or its porosity in the near-surface region, the near-surface void size is reduced to <;65 nm and in certain cases achieve a 100 nm-thick void-free zone below the surface. Together with better void alignment, this allows for a smoother growth surface with a roughness of <;35 Å and reduced stress in the porous silicon. These improvements translate into significantly diminished epifoil crystal defect densities as low as ~420 defects/cm 2. Although epifoils on very thin porous silicon were not detachable, a significant improvement in the lifetime (diffusion length) of safely detachable n-type epifoils from ~85 (~300 μm) to ~195 μs (~470 μm) at the injection level of 10 15/cm 3 is achieved by tuning the porous silicon template. Lifetimes exceeding ~350 μs have been achieved in the reference lithography-based epifoils, showing the potential for improvement in porous silicon-based epifoils. View full abstract»

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  • Leveraging Silicon Epitaxy to Fabricate Excellent Front Surface Regions for Thin Interdigitated Back Contact Solar Cells

    Publication Year: 2014 , Page(s): 78 - 83
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    Epitaxy can be used to fabricate doped front surface regions that enable high interdigitated back contact (IBC) silicon solar cell efficiency. One- and two-dimensional simulations show that an epitaxial layer with a constant phosphorus dopant concentration on the order of 1017 -1018 cm-3 can possess the properties of an excellent front surface region for an n-type IBC cell. With appropriate control of dopant concentration and thickness, the epitaxially grown region passivates a textured surface, and provides the lateral conductivity necessary to enable high fill factor. The combination of these two factors drives a simulated efficiency improvement above 0.3% absolute over an n-type IBC cell with a typical 200-Ω/sq phosphorus diffusion (e.g., from POCl3). Importantly, the epitaxial front surface region can occupy the entire volume of the pyramidal texture. We, therefore, propose an exemplary process sequence for device fabrication that places texture etching after epitaxial growth. View full abstract»

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  • Mono-Like Silicon Growth Using Functional Grain Boundaries to Limit Area of Multicrystalline Grains

    Publication Year: 2014 , Page(s): 84 - 87
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    We propose a new growth method for mono-like silicon (Si): the suppression of multicrystallization using functional grain boundaries artificially formed by multiseed crystals. In our previous study, we demonstrated such suppression in an ingot 30 mm in diameter. In this paper, we grew mono-like Si ingots of 100 and 400 mm on a side. Functional grain boundaries successfully suppressed the increase in the area of multicrystalline grains nucleated on crucible side walls, which indicates a large volume of quasi-monocrystalline Si up to the top of the ingots. This enables a large increase in the yield of quasi-monocrystalline wafers in an ingot and would lead to a reduction in the cost of the solar cells. View full abstract»

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  • Advanced Bulk Defect Passivation for Silicon Solar Cells

    Publication Year: 2014 , Page(s): 88 - 95
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (844 KB) |  | HTML iconHTML  

    Through an advanced hydrogenation process that involves controlling and manipulating the hydrogen charge state, substantial increases in the bulk minority carrier lifetime are observed for standard commercial grade boron-doped Czochralski grown silicon wafers from 250-500 μs to 1.3-1.4 ms and from 8 to 550 μs on p-type Czochralski wafers grown from upgraded metallurgical grade silicon. However, the passivation is reversible, whereby the passivated defects can be reactivated during subsequent processes. With appropriate processing that involves controlling the charge state of hydrogen, the passivation can be retained on finished devices yielding independently confirmed voltages on cells fabricated using standard commercial grade boron-doped Czochralski grown silicon over 680 mV. Hence, it appears that the charge state of hydrogen plays an important role in determining the reactivity of the atomic hydrogen and, therefore, ability to passivate defects. View full abstract»

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  • 24.7% Record Efficiency HIT Solar Cell on Thin Silicon Wafer

    Publication Year: 2014 , Page(s): 96 - 99
    Cited by:  Papers (24)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (322 KB) |  | HTML iconHTML  

    A new record conversion efficiency of 24.7% was attained at the research level by using a heterojunction with intrinsic thin-layer structure of practical size (101.8 cm2, total area) at a 98-μm thickness. This is a world height record for any crystalline silicon-based solar cell of practical size (100 cm2 and above). Since we announced our former record of 23.7%, we have continued to reduce recombination losses at the hetero interface between a-Si and c-Si along with cutting down resistive losses by improving the silver paste with lower resistivity and optimization of the thicknesses in a-Si layers. Using a new technology that enables the formation of a-Si layer of even higher quality on the c-Si substrate, while limiting damage to the surface of the substrate, the Voc has been improved from 0.745 to 0.750 V. We also succeeded in improving the fill factor from 0.809 to 0.832. View full abstract»

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  • Assessing the Performance of Surface Passivation Using Low-Intensity Photoluminescence Characterization Techniques

    Publication Year: 2014 , Page(s): 100 - 106
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    This paper applies quasi-steady-state photoluminescence (QSS-PL) and photoluminescence imaging to characterize the recombination properties of various surface passivation techniques. Particular interest is given to the performance at low excess carrier densities where many types of surface passivation show a strong increase in surface recombination velocity. These techniques are then used to further understand the ability of parasitic effects such as nonuniform illumination, edge recombination and areas of high recombination to affect these measurements. Furthermore, a new technique for edge isolation using laser doping is shown to be effective against the effect of edge recombination. This technique is useful to implement when using QSS-PL to analyze small samples as carriers conducted to the edge regions can dramatically alter the effective lifetime in low injection. View full abstract»

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  • Sensitivity Analysis of Industrial Multicrystalline PERC Silicon Solar Cells by Means of 3-D Device Simulation and Metamodeling

    Publication Year: 2014 , Page(s): 107 - 113
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    How successfully an improved solar cell concept is transferred to mass production depends not only on the realized cell efficiency, but crucially on the stability of the fabrication process, i.e., on the distribution of the current-voltage (I-V) parameters. To model such distributions, we use three-dimensional (3-D) full-size device simulations of passivated emitter and rear cells (PERCs). The number of these time-consuming simulations is drastically reduced by changing all input parameters concurrently in a design of experiment approach. A simple polynomial response surface methodology (RSM) model is obtained from these simulations by regression analysis. The RSM contains all the mutual nonlinear interactions between the device parameters, and is therefore called a metamodel. The metamodel is applied: 1) to find maximum efficiency; 2) to compute how sensitively each device parameter influences the I-V parameters; 3) to explain, predict, and manipulate the distribution of the I-V parameters in mass production; and 4) to find an optimum starting point for experiments. For example, we demonstrate how the choice of the distance between the rear local point contacts leads to either maximal median efficiency but with a broad distribution, or to a slightly reduced median cell efficiency but with a narrow distribution and a reduced number of bad cells. View full abstract»

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  • An Analytical Model for Interdigitated Back Contact Solar Cells

    Publication Year: 2014 , Page(s): 114 - 121
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    An analytical model for an interdigitated back contact solar cell structure is presented. This model is based on the concept of diffusion resistance and on the superposition principle. The analytical model is compared with finite-element simulations that are based on a conductive boundary model. For most practical cases, the model presented here is in very good agreement with the simulation, with less than 1% deviation on the Voc, jsc, fill factor (FF) and efficiency. View full abstract»

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  • Solar Cell Efficiency Losses Due to Impurities From the Crucible in Multicrystalline Silicon

    Publication Year: 2014 , Page(s): 122 - 129
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (717 KB) |  | HTML iconHTML  

    The electrical material quality of multicrystalline (mc) silicon for photovoltaic applications suffers from crystal defects as well as from impurities that originate from the feedstock, the quartz crucible, and its coating. In this study, we investigate the influence of impurities from the crucible on efficiency losses in mc silicon solar cells, focusing on the limitation due to iron. The applicability of p-type mc silicon, crystallized in G1 sized crucibles of industrial material quality and very pure electrically fused silica, for a high-efficiency solar cell process is examined by measuring lifetime and interstitial iron concentration in the wafers after different processing steps and by estimating the cell efficiency potential from injection-dependent bulk lifetime measurements. Interstitial iron concentrations extracted from 2-D simulations of iron precipitation at crystal defects and gettering during processing agree well with Fei measurements at different process stages and explain the observations. Efficiency losses are quantified to losses due to segregated impurities diffused into the silicon melt, losses due to decorated crystal defects and losses due to solid-state diffusion into the crystal. By using a high-purity crucible, losses are reduced significantly and an efficiency gain of 0.5% absolute is estimated to be attainable on wafers with edge region. View full abstract»

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  • High-Efficiency Full Back Contacted Cells Using Industrial Processes

    Publication Year: 2014 , Page(s): 130 - 133
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    Full-size (156 ×156 mm2) interdigitated back contact (IBC) solar cells have been developed with conventional industry processes. With PC1D simulation and short-flow experiment verification, we found that the tunnel junction shunting of rear n+/p+ could be mitigated significantly by controlling the boron surface concentration; therefore, it is not necessary to form a gap between rear emitter and back surface field. Made by a novel yet relative simple process, the IBC cells preliminarily achieved 19.65% best efficiency with Jsc and Voc as high as 40.5 mA/cm2 and 655 mV, respectively, while FF was only 73.9% due to the low pseudo fill factor (Pff) and high series resistance. Through the optimization of the rear pattern process, Pff was improved up to 82.5% and FF up to 77%. With further optimization of emitter, front surface field, passivation, and rear pattern design, the cells potentially can achieve up to 22.0% efficiency in the near future. View full abstract»

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  • Investigation of the Mechanism Resulting in low Resistance Ag Thick-Film Contact to Si Solar Cells in the Context of Emitter Doping Density and Contact Firing for Current-Generation Ag Paste

    Publication Year: 2014 , Page(s): 134 - 141
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    Screen-printed thick-film Ag metallization has become highly successful in crystalline Si (c-Si) photovoltaics. However, a complete understanding of the mechanism resulting in low resistance contact is still lacking. In order to shed light on this mechanism for current-generation Ag paste, Si solar cells were fabricated using a range of emitter doping densities and contact firing conditions. Low resistance contact was found to vary as a function of emitter surface P concentration ( [Psurface]) and peak firing temperature. Scanning electron microscope (SEM) analysis revealed thin interfacial glass films (IGF) under the bulk Ag gridline. SEM analysis also showed increasing Ag crystallite density as both emitter [Psurface] and peak firing temperature increased. Two mechanisms are proposed in forming low resistance contact to highly doped emitters: 1) formation of ultrathin IGF and/or nano-Ag colloids at low firing temperature, and 2) formation of Ag crystallites at high firing temperature. However, on lightly doped emitters, low resistance contact was achieved only at higher firing temperatures, concomitant with increasing Ag crystallite density, and suggests that thin IGF decorated with nano-Ag colloids may not be sufficient for low resistance contact to lightly doped emitters. View full abstract»

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  • Gettering of Iron in Silicon Solar Cells With Implanted Emitters

    Publication Year: 2014 , Page(s): 142 - 147
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (3989 KB) |  | HTML iconHTML  

    We present here experimental results on the gettering of iron in Czochralski-grown silicon by phosphorus implantation. The gettering efficiency and the gettering mechanisms in a high resistivity implanted emitter are determined as a function of both initial iron level and gettering anneal. The results show that gettering in implanted emitters can be efficient if precipitation at the emitter is activated. This requires low gettering temperatures and/or high initial contamination level. The fastest method to getter iron from the bulk is to rapidly nucleate iron precipitates before the gettering anneal. Here, this was achieved by a fast ramp to the room temperature in between the implantation anneal and the gettering anneal. View full abstract»

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Aims & Scope

The IEEE Journal of Photovoltaics is a peer-reviewed, archival publication reporting original and significant research results that advance the field of photovoltaics (PV).

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Timothy J. Anderson
Chemical Engineering Department
University of Florida