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Photovoltaics, IEEE Journal of

Issue 2 • Date March 2014

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Displaying Results 1 - 25 of 38
  • Table of contents

    Page(s): C1 - C2
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  • IEEE Journal of Photovoltaics publication information

    Page(s): C2
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  • Numerical Current Density Loss Analysis of Industrially Relevant Crystalline Silicon Solar Cell Concepts

    Page(s): 533 - 539
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (399 KB) |  | HTML iconHTML  

    In order to improve the solar cell efficiency of the solar cell concept of interest most efficiently, it is essential to analyze its loss mechanisms. Numerical simulations are used to calculate the individual solar cell loss mechanisms. The calculated current losses can be used to identify the main losses and can give recommendation for the next improvement steps. We present a method for a current density loss analysis of crystalline silicon solar cells based on 2-D and 3-D numerical simulations. With the presented method, a collection of relevant crystalline silicon solar cell concepts are investigated. By determining the main loss mechanisms of each cell concept, the limitations and the potential of the different concepts are pointed out. View full abstract»

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  • Dynamic Infrared Lifetime Mapping for the Measurement of the Saturation Current Density of Highly Doped Regions in Silicon

    Page(s): 540 - 548
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (686 KB) |  | HTML iconHTML  

    Previously, the dynamic infrared lifetime mapping (ILM) approach was used for a spatially resolved determination of the reverse saturation current density J0 of local highly doped regions in silicon. However, possible restrictions of the method have not been considered yet. We show that 1) injection dependent lifetimes, 2) a nonlinearity between camera signal and excess charge-carrier density, as well as 3) an additional signal due to a modulated sample temperature may affect the lifetime measurement and thus the correct determination of J0. Moreover, we consider the impact of injection dependent lifetimes and the modulated sample temperature under high-level injection. We apply our approach to symmetrically phosphorous diffused and textured samples with sheet resistances between 23 and 150 Ω/sq. Using the adopted evaluation algorithm of the dynamic ILM technique, we obtain an agreement in J0 evaluated by dynamic ILM and photo-conductance decay measurements of 8%. View full abstract»

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  • Current Conduction Mechanism of Front-Side Contact of N-Type Crystalline Si Solar Cells With Ag/Al Pastes

    Page(s): 549 - 553
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    Recently, n-type crystalline Si (c-Si) cells with front-side (FS) metallization Ag/Al paste have attracted considerable attention. However, a clear understanding of current conduction mechanism is still lacking. We report here the results of our microstructural investigation of the interfacial contact region using electron microscopy techniques. In optimally fired cells, we did not find any Al-Si eutectic layer on the emitter surface that would support a regrowth mechanism as found during the back surface field formation process commonly practiced to create the full plane Al back contact of p-type industrial solar cells. The presence of SiN x antireflection coating has possibly altered significantly the chemistry between Si and Al. The observed microstructures suggest that the current conduction is predominantly tunneling through ultrathin interfacial glass, assisted by the presence of nano-Ag colloids. We believe this mechanism is similar to the current conduction model we have proposed previously for FS Ag-contact of p-type c-Si solar cells with Ag paste. View full abstract»

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  • Light Trapping in Thin Crystalline Si Solar Cells Using Surface Mie Scatterers

    Page(s): 554 - 559
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    Dielectric nanoparticles placed on top of a thin-film solar cell strongly enhance light absorption in the cell over a broad spectral range due to the preferential forward scattering of light from leaky Mie resonances in the particle. In this study, we systematically study with numerical simulations the absorption of light into thin (1-100 μm) crystalline Si solar cells patterned with Si nanocylinder arrays on top of the cell. We then use an analytical model to calculate the solar cell efficiency, based on the simulated absorption spectra. Using realistic values for bulk and surface recombination rates, we find that a 20-μm-thick Si solar cell with 21.5% efficiency can be made by using the Si nanocylinder Mie coating. View full abstract»

    Open Access
  • Measurement and Parameterization of Carrier Mobility Sum in Silicon as a Function of Doping, Temperature and Injection Level

    Page(s): 560 - 565
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    Based on contactless photoconductance measurements of silicon wafers, we have determined the sum of the electron and hole mobilities as a function of doping, excess carrier concentration, and temperature. By separately analyzing those three functional dependences, we then develop a simple mathematical expression to describe the mobility sum as a function of carrier injection wafer doping and temperature from 150 to 450 K. This new parameterization also provides experimental validation to Klaassen's and Dorkel-Leturcq's mobility models in a range of temperatures. View full abstract»

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  • Doped Layer Optimization for Silicon Heterojunctions by Injection-Level-Dependent Open-Circuit Voltage Measurements

    Page(s): 566 - 574
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    Besides passivation of the c-Si absorber, provided mainly by the undoped buffer layer, the net doping of the silicon thin films plays a major role in the performance of silicon-based heterojunction (SHJ) solar cells. However, junction engineering is complex as high net doping often interferes with the interface passivation and the optical properties of the silicon thin films. We show that injection-level-dependent open-circuit voltage (Suns- Voc) measurements are a simple and valuable method for the characterization and optimization of the doped amorphous silicon (a-Si:H) layers. It is shown by experiment and device simulations that at high illumination intensities the Suns- Voc characteristic exhibits a strong signature of defect recombination within the a-Si:H, which is determined by the a-Si:H doping and the interfacial transparent conducting oxide (TCO) properties. This fact is exploited for a qualitative interpretation of the interplay between a-Si:H and the interfacial TCO properties. As a clear correlation between the Suns- Voc characteristic and the maximum power point conditions of the devices exists, fill factor (FF) losses attributed to the doped a-Si:H and the interfacial TCO properties can 1) be easily predicted in the early stage of device optimization on simple test structures, or 2) these FF losses can be identified and distinguished from other FF losses in the final device. View full abstract»

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  • Study of Dielectric Layers for Bifacial n-type Silicon Solar Cells with Regard to Optical Properties, Surface Passivation Quality, and Contact Formation

    Page(s): 575 - 580
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    We have studied surface passivation layers for the application on n -type p+ nn+ bifacial silicon solar cells. Thereby, we have examined their optimal composition and thickness with regards to passivation quality, optical properties, and especially the contact formation during a co-firing step. These parameters were addressed in separate investigations: 1) simulation of the optical properties of a bifacial silicon solar cell, 2) measurement of the passivation quality on lifetime samples, 3) measurement of contact resistance (of aerosol printed fingers) to analyze the contact formation during the co-firing process, and 4) differential scanning calorimetry measurement were conducted to fundamentally understand reactions during contact formation in a fast firing furnace. The passivation layers tested were silicon nitride (SiNx), titanium oxide (TiO 2), and silicon oxide (SiO2) on lowly phosphorus-doped silicon n+-layers, whereas aluminum oxide (Al2O3) stacks, capped with SiNx and TiO2, were studied on lowly boron-doped silicon p+-layers. The results show that a dielectric stack, consisting of 10-nm-thick Al2O3 and 60-nm-thick SiNx layers on the boron-diffused silicon front side and a single 50-nm SiNx layer on the phosphorus-diffused silicon rear side, provides low emitter saturation current density (J0e), high optical absorption current density, and low contact resistance for printed and co-fired contacts. View full abstract»

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  • Evaluation of the Silicon Ingot With Addition of SiCl _{\bf 4} in Atmosphere During Unidirectional Solidification

    Page(s): 581 - 584
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    Adding SiCl4 to the atmosphere during the growth of a crystal suppressed crystalline defects and impurities from the edge of silicon ingots. Crystalline silicon ingots with seed crystal were grown by using the unidirectional solidification technique. Most of the grain boundaries were inactive Σ3 in the ingot with the SiCl4 injection. There were no small-angle grain boundaries at the top and edge of ingots. The carbon concentration with SiCl4 was decreased to less than half of that without the SiCl4 injection. Carbon precipitation did not occur even at the surface of ingots and the surface of grown ingots had a metallic luster. View full abstract»

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  • Detailed Current Loss Analysis for a PV Module Made With Textured Multicrystalline Silicon Wafer Solar Cells

    Page(s): 585 - 593
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (4323 KB) |  | HTML iconHTML  

    We present a top-down method to quantify optical losses due to encapsulation of textured multicrystalline silicon wafer solar cells in a photovoltaic module. The approach is based on a combination of measurements and mathematical procedures. Seven different loss mechanisms are considered: 1) reflection at the glass front surface, 2) reflection at the metal fingers, 3) reflection at the textured solar cell surface, 4) absorption in the antireflection coating, 5) absorption in the glass pane and the encapsulation layer, 6) front surface escape, and 7) losses due to a non-perfect solar cell internal quantum efficiency. Losses for each of these mechanisms are obtained as a function of wavelength, and the corresponding current loss for each loss mechanism is calculated. Comparing simulated and measured results, the method predicts the module quantum efficiency with an error of less than 2% and the collected current with an error of less than 1%. In the presented example, the biggest loss (7.4 mA/cm 2) is due to the nonperfect quantum efficiency, followed by reflection losses at the glass front (2.2 mA/cm 2) and absorption in the glass and encapsulation layer (1.1 mA/cm 2). View full abstract»

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  • The Impact of SiO _{2} /SiN _{\rm x} Stack Thickness on Laser Doping of Silicon Solar Cell

    Page(s): 594 - 600
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    Laser doping of semiconductors has been the subject of intense research over the past decades. Previous work indicates that the use of SiO2/SiNx stacks instead of a single dielectric film as the anti-reflection coating and passivation layer results in laser doped lines with superior properties. In this paper, the impact of the SiNx layer thickness in the SiO2/SiNx stacks on the properties of laser doped lines is investigated through resistance measurements of the laser doped line and the silicon-metal contact and the doping profile near the edge of the dielectric window, the latter being an important factor in determining the likelihood of high recombination or even shunting from the subsequent metallization process. Fundamentally, a problem of exposed and undoped silicon near the dielectric window is identified for most of the investigated parameter range. However, optimization of the laser parameters and dielectric film conditions is shown to be capable of preventing or at least minimizing this problem. The results indicate that for the used laser system, samples with thick dielectric stack processed using a low pulse energy and pulse distance yield the most favorable properties, such as low line resistance and low contact resistivity. Under these conditions, the laser doped regions laterally extend underneath the dielectric films, thus reducing the likelihood of high surface recombination. View full abstract»

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  • Simulations, Practical Limitations, and Novel Growth Technology for InGaN-Based Solar Cells

    Page(s): 601 - 606
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    Indium gallium nitride (InGaN) alloys exhibit substantial potential for high-efficiency photovoltaics. However, theoretical promise still needs to be experimentally realized. This paper presents a detailed theoretical study to provide guidelines to achieve high-efficiency InGaN solar cells. While the efficiency of heterojunction devices is limited to ~11%, homojunction devices can achieve suitable efficiencies, provided that highly p-type-doped InGaN layers and thick, single-phase InGaN films can be grown. Thus, we have developed a novel growth technology that facilitates growth of p-type nitride films with greatly improved hole concentration and growth of InGaN without phase separation, offering promise for future high-efficiency InGaN solar cells. View full abstract»

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  • Carrier Escape Time and Temperature-Dependent Carrier Collection Efficiency of Tunneling-Enhanced Multiple Quantum Well Solar Cells

    Page(s): 607 - 613
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    Tunneling enhancement of cell performance in InGaAs/GaAsP multiple quantum well (MQW) solar cells has been studied to investigate the potential in overcoming the carrier collection problem, which hinders the maximum performance of quantum structure solar cells. To accurately investigate the effects of the tunneling effect, the study was carried out in samples with different GaAsP barrier thickness, controlled absorption edge, and constant built-in field. The tunneling effect has been confirmed by evaluating carrier escape times using the time-resolved photoluminescence technique and measuring carrier collection efficiency at various temperatures. The collection efficiencies at low temperature are found to be remarkably improved when barrier thickness was below 3 nm, which can be regarded as the critical thickness for efficiently facilitating tunneling enhancement. It can also be concluded that the carrier transport model based on thermal and tunneling processes is practical enough to describe most of the carrier sweep-out dynamics in MQW solar cells. View full abstract»

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  • GaInP/GaAs Tandem Solar Cells With InGaAs/GaAsP Multiple Quantum Wells

    Page(s): 614 - 619
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    Lattice-matched multiple quantum wells (MQWs) consisting of InxGa1-xAs wells with very thin GaAs0.2P0.8 barriers have been incorporated into a GaInP/GaAs tandem solar cell. InGaAs/GaAsP MQWs increase the short-circuit current of the GaAs cell by extending the absorption range, with minimal impact on an open-circuit voltage, thus alleviating current matching restrictions placed by the GaAs cell on multijunction solar cells. MQWs with very thin, tensile strained, high phosphorus content GaAsP barriers allow tunneling to dominate carrier transport across the MQWs and balance the compressive strain of the InGaAs wells such that material quality remains high for subsequent top cell growth. We show that the addition of the QW layers enhances the GaAs cell, does not degrade the performance of the GaInP top cell, and leads to potential efficiency enhancements. View full abstract»

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  • Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon

    Page(s): 620 - 625
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    Two different process technologies were investigated for the fabrication of high-efficiency GaInP/GaAs dual-junction solar cells on silicon: direct epitaxial growth and layer transfer combined with semiconductor wafer bonding. The intention of this research is to combine the advantages of high efficiencies in III-V tandem solar cells with the low cost of silicon. Direct epitaxial growth of a GaInP/GaAs dual-junction solar cell on a GaAsyP1-y buffer on silicon yielded a 1-sun efficiency of 16.4% (AM1.5g). Threading dislocations that result from the 4% lattice grading are still the main limitation to the device performance. In contrast, similar devices fabricated by semiconductor wafer bonding on n-type inactive Si reached efficiencies of 26.0% (AM1.5g) for a 4-cm2 solar cell device. View full abstract»

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  • A Maximum Power Point Tracking Method Based on Perturb-and-Observe Combined With Particle Swarm Optimization

    Page(s): 626 - 633
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    Conventional maximum power point tracking (MPPT) methods such as perturb-and-observe (P&O) method can only track the first local maximum point and stop progressing to the next maximum point. MPPT methods based on particle swarm optimization (PSO) have been proposed to track the global maximum point (GMP). However, the problem with the PSO method is that the time required for convergence may be long if the range of the search space is large. This paper proposes a hybrid method, which combines P&O and PSO methods. Initially, the P&O method is employed to allocate the nearest local maximum. Then, starting from that point on, the PSO method is employed to search for the GMP. The advantage of using the proposed hybrid method is that the search space for the PSO is reduced, and hence, the time that is required for convergence can be greatly improved. The excellent performance of the proposed hybrid method is verified by comparing it against the PSO method using an experimental setup. View full abstract»

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  • Progress Toward Realizing an Intermediate Band Solar Cell—Sequential Absorption of Photons in a Quantum Well Solar Cell

    Page(s): 634 - 638
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    In order to realize an intermediate band solar cell, which promises high photovoltaic energy conversion efficiency, achieving higher photocurrent while maintaining the cell voltage is essential. We report on a transient photocurrent due to the sequential absorption of photons in a single quantum well by continuously pumping to stimulate interband transitions (from a valence band to an intermediate band) and showing an intersubband transition (from an intermediate band to a conduction band) with a pulsed infrared laser. We demonstrate the extent to which multiple-photon absorption can be achieved in quantum well devices and propose that a quantum well is a suitable candidate for an intermediate band solar cell. From the combination of this and other sequential absorption results, it is clear that enhancing the short lifetime of a carrier in the intermediate band is the next step toward achieving a working intermediate band solar cell. In light of this, we enhance our previous suggestion, the photon ratchet intermediate band solar cell, as a means of increasing the electron lifetime. View full abstract»

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  • Optimization of Microtextured Light-Management Films for Enhanced Light Trapping in Organic Solar Cells Under Perpendicular and Oblique Illumination Conditions

    Page(s): 639 - 646
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (681 KB) |  | HTML iconHTML  

    To improve light absorption in organic solar cells, microscale surface-textured light-management (LM) films are applied on top of the front glass substrate. In this study, numerical simulations are employed to determine the optimal texture of the LM films that would result in the highest short-circuit current density of the solar cells in perpendicular, as well as oblique, illumination conditions. Different types of 2-D periodic surface textures are analyzed (pyramidal, parabolic, sinusoidal), and the effects of the period and groove height sizes are investigated. Numerical simulations are based on a model that combines geometric optics and wave optics and, thus, enables simulation of light propagation through the thick microtextured LM film and glass, as well as thin layers of the device, respectively. Results show that parabolic textures are the most advantageous for the solar cells to achieve high performance operating in changing illumination conditions. When properly optimized, they enable over 14% boost of the short-circuit current density in a broad range of illumination incident angles, with the maximum of 22% for perpendicular incidence, with respect to that of the nontextured cell. View full abstract»

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  • Optimal Orientation and Tilt Angle for Maximizing in-Plane Solar Irradiation for PV Applications in Singapore

    Page(s): 647 - 653
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    The performance of photovoltaic (PV) modules and systems is affected by the orientation and tilt angle, as these parameters determine the amount of solar radiation received by the surface of a PV module in a specific region. In this study, three sky models (Liu and Jordan, Klucher, and Perez et al .) are used to estimate the tilted irradiance, which would be received by a PV module at different orientations and tilt angles from the measured global horizontal irradiance (GHI) and diffuse horizontal irradiance (DHI) in Singapore (1.37°N, 103.75°E). Modeled results are compared with measured values from irradiance sensors facing 60° NE, tilted at 10°, 20°, 30°, 40°, and vertically tilted irradiance sensors facing north, south, east, and west in Singapore. Using the Perez model, it is found that a module facing east gives the maximum annual tilted irradiation for Singapore's climatic conditions. These findings are further validated by one-year comprehensive monitoring of four PV systems (tilted at 10° facing north, south, east, and west) deployed in Singapore. The PV system tilted 10° facing east demonstrated the highest specific yield, with the performance ratio close to those of other orientations. View full abstract»

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  • Evolution of Leakage Current Paths in MC-Si PV Modules From Leading Manufacturers Undergoing High-Voltage Bias Testing

    Page(s): 654 - 658
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    The evolution of leakage currents in photovoltaic modules undergoing outdoor high-voltage bias testing is studied using data from high-voltage bias testing of multicrystalline silicon modules from leading manufacturers. An analysis of the module leakage currents as a function of environmental conditions including temperature, relative humidity, rain, and wetness is carried out. The behavior of the modules was found to be dependent on the module construction and the materials used. The Arrhenius model was used to fit the experimental data and activation energies were computed for various relative humidity values. The effect of dew and rain (wetness) on the front glass was investigated. Changes in the leakage current during dry conditions were studied using the temperature dependence of resistivity of bulk soda-lime glass. Because of the approximately tenfold increase in leakage currents during the wet conditions, it is suggested that the accelerated tests should not be limited exclusively to noncondensing environments but should also be complemented with tests that include wet conditions. View full abstract»

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  • Online Recording a PV Module Fingerprint

    Page(s): 659 - 668
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    In many applications, the current versus voltage curve of a photovoltaic cell, module, string, or field is acquired. A high number of samples are usually acquired, but the curve contains the main information in the open- and short-circuit points, as well as where it has a strong change in the slope. In this paper, these parts are called “the fingerprint” of the photovoltaic generator. The fingerprint allows us to recognize the working conditions of the photovoltaic generator, e.g., if it is affected by a partial shadowing or not. Saving the fingerprint and discarding the other points of the original curve allows us to minimize the memory needs for storing the curve without losing the main information content. In this paper, a numerical technique for selecting, from among the samples of the acquired current versus voltage curve of any photovoltaic generator, the ones to be included in the fingerprint is proposed. The processing steps and the memory needed to achieve the result are minimized in order to allow an implementation of the algorithm also in a low-cost processor for on-field real-time applications. The technique is validated through curves generated by using analytical models as well as by means of some curves acquired experimentally in outdoor conditions. View full abstract»

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  • Current Equalization in Photovoltaic Strings With Module Integrated Ground-Isolated Switched Capacitor DC–DC Converters

    Page(s): 669 - 678
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    A ground-isolated switched capacitor (SC) dc-dc converter-based current equalization scheme for partially shaded photovoltaic (PV) strings of a grid-connected system is presented. SC converters are compact, light, and have very high efficiency even for a wide variation in load under certain operating conditions. These features make them ideal for integration with the PV module for current equalization. The factors affecting the maximum output power that the SC converter can deliver, the limiting value of the maximum output power, and efficiency issues of the SC converter are studied. These studies are required to optimize the design of the equalizing SC converter and to maximize its efficiency. A novel algorithm that utilizes the results of the aforementioned analysis to maximize the net power available due to the SC converter-based current equalization scheme for grid-connected applications is proposed. Experimental results showing the advantages of current equalization with SC converters as compared to that with conventional dc-dc converters are presented. View full abstract»

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  • Maximum Power from PV Arrays Using a Fixed Configuration Under Different Shading Conditions

    Page(s): 679 - 686
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    A major challenge in photovoltaic (PV) systems is making them energy efficient. One of the major factors that contribute to the reduction of PV power is partial shading. The reduction in power depends on module interconnection scheme and shading pattern. Different interconnection schemes are used to reduce the losses caused by partial shading. This paper presents a fixed interconnection scheme for PV arrays that enhances the PV power under different shading conditions. The proposed scheme facilitates distribution of the effect of shading over the entire array thereby reducing the mismatch losses caused by partial shading. The performance of the system is investigated for different shading conditions and the MATLAB/SIMULINK results are presented to show that the power extracted from the PV arrays under partial shading conditions is improved. Experimental results are provided to validate the proposed approach using a laboratory experimental setup. A comparison is also made between the electrical array reconfiguration scheme and the proposed scheme for a 5 × 5 PV array. View full abstract»

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  • A New Mass Production Technology for High-Efficiency Thin-Film CIS-Absorber Formation

    Page(s): 687 - 692
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    A new mass production technology for CIS-absorber formation yielding high-average module efficiencies is introduced. A novel custom-designed oven very successfully exploits the principle of forced convection during heating, CIS formation reaction, and cooling. Cu(In,Ga)(Se,S) 2 absorbers are formed by metal precursor deposition on soda lime glass followed by reaction in selenium/sulfur atmosphere. Processing is performed in a multiple-chamber equipment which handles corrosive, flammable, and toxic process gases from atmospheric pressure to vacuum at high durability. The substrates (size: 50 cm × 120 cm) are processed in batches up to 102 substrates, applying forced convection for very homogenous heat transfer and high heating and cooling rates. Multiple-chamber design and batch size yield high throughput at cycle times above 1 h. This approach combines the specific advantages of batch type and inline processing. An excellent average efficiency of 14.3% with a narrow distribution (+/-0.31%) and a peak efficiency of 15.1% is shown with this technology. Module characteristic distributions during pilot production are presented. Detailed layer analytics is discussed. This straightforward reliable mass production technology is a key for highest module performance and for upscaling. Module efficiencies of 17% can be reached, enabling production costs below 0.38 US$/Wp in a projected GWp plant. 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).

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