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Photovoltaics, IEEE Journal of

Issue 3 • Date July 2013

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Displaying Results 1 - 25 of 38
  • Table of Contents

    Page(s): C1 - C4
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    Freely Available from IEEE
  • IEEE Journal of Photovoltaics publication information

    Page(s): C2
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  • Passivation of n ^{+} -Type Si Surfaces by Low Temperature Processed SiO _{2} /Al _{2} O _{3} Stacks

    Page(s): 925 - 929
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    The surface passivation of SiO2/Al 2O3 stacks prepared at low process temperatures was investigated on phosphorous diffused n+-type Si surfaces with a broad range of sheet resistances. Two kinds of SiO2 films were prepared, the first with plasma-enhanced chemical vapor deposition (PECVD) and the second in a wet chemical process. After atomic layer deposition of the Al2O3 capping layer, the resulting SiO2/Al2O3 stacks differ in the polarity of their fixed charge density, i.e., the PECVD SiO2 stacks had a positive and the wet chemically grown SiO2 stacks a negative fixed charge density. The PECVD SiO2/Al2O3 stacks resulted in a high surface passivation over a broad range of sheet resistances whereas the wet chemically grown SiO2 stacks were only feasible for diffused surfaces with low sheet resistances (<; 100 Ω/□). By corona charging experiments, it was established that the field effect based on a negative fixed charge density was the reason for the loss in surface passivation in the specific range of diffused surfaces. View full abstract»

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  • Aluminum Oxide Deposited by Pulsed-DC Reactive Sputtering for Crystalline Silicon Surface Passivation

    Page(s): 930 - 935
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    In this paper, we report on the surface passivation of crystalline silicon (c-Si) by pulsed-dc (p-dc) reactive-sputtered aluminum oxide (AlOx) films. For the activation of surface passivation, the films were subjected to post deposition annealing (PDA) in different ambients namely N2, N2 + O2, and forming gas (FG) in the temperature range of 420-520°C. The surface passivation was quantified by surface recombination velocity, which was correlated to the interface states at the silicon-dielectric interface and fixed charges in the dielectric. A good quality surface passivation with effective surface recombination velocity Seff of 41 cm · s-1 is obtained for PDA in N2 or N2 + O2 gas ambient. PDA in FG ambient at high temperature is found to degrade the passivation. The AlOx film annealed in FG ambient shows poorer thermal stability as compared with films annealed in the other two ambients. A clear path for further improvements in surface passivation quality of p-dc reactive sputter-deposited AlOx is suggested based on cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy analysis and electrical data. View full abstract»

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  • Modeling Recombination at the Si–Al _{2} O _{3} Interface

    Page(s): 936 - 943
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    In this paper, we present a complete set of data on the silicon surface passivation parameters of Al2O3 deposited by atmospheric pressure chemical vapor deposition with triethyldialuminum-tri-( sec-butoxide) and H2O precursors at temperatures between 325 and 520°C. Using measured values of the total interface charge Qtot and of the interface defect density Dit(E), apparent electron capture cross section σn (E), and apparent hole capture cross section σp(E) as a function of the energy within the bandgap E, we calculate surface recombination velocities using the Shockley-Read-Hall (SRH) model and compare these with measured values, finding excellent agreement when Qtot is large and reasonable agreement otherwise. The resulting model is valid for both n- and p-type substrates, under the condition that holes are the majority carrier at the surface, as is generally the case for typical (negative) values of Qtot. It is shown that, under these conditions, recombination is dominated by a single donor-like defect species located just below midgap. These results support the direct correspondence between Qtot, Dit (E), σn, and σp determined by capacitance and conductance measurements of metal-insulator-semiconductor structures and the carrier lifetimes measured by photoconductance. View full abstract»

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  • Understanding and Development of Screen-Printed Front Metallization for High-Efficiency Low-to-Medium Concentrator Silicon Solar Cells

    Page(s): 944 - 951
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    This paper reports on a methodology to achieve low-cost high-efficiency screen-printed low-to-medium concentrator Si cells and validates it by fabricating some of the highest efficiency metal paste printed cells with a simple cell design. The model shows that the highest achievable efficiency at any given concentration is a function of metal paste, contact parameters, number of fingers, and finger length due to the tradeoff between resistive and shadow losses. Consistent with the model calculations, first, 52 mm × 27 mm ~19% cells were fabricated at ~5X (suns) with a Dupont 16 A paste and ~110 μm wide 32 lines. Then, with improved 50 μm wide direct extrusion printed fingers, >20% efficient cells were achieved in the concentration range of 3-16X, along with a roadmap to ~21% efficient cells. This approach provides excellent guidelines to design grid pattern and selecting cell dimensions to achieve maximum efficiency at the desired concentration from a given printing technology and cell structure. View full abstract»

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  • High-Resolution Lock-in Thermography Investigation on Industrial Multicrystalline Silicon Solar Cells

    Page(s): 952 - 956
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    Industrial multicrystalline silicon (mc-Si) solar cells with different types of shunts have been analyzed in detail by dark lock-in thermography (DLIT). Several types of nonlinear shunts were found in our samples and most of them could only be detected in low forward-bias images of DLIT. However, we also observed nonlinear shunts that are only visible or have much stronger signal under the low reverse-bias than equivalent forward-bias condition, which is a new finding compared with the common observations. The edge of an mc-Si solar cell was found vulnerable for shunting. A weak leakage current around edges was frequently observed under 0.5 V forward bias on both shunted and normal cells. It reveals that the edge is one of the major recombination paths under the cell operation condition, which is due to the imperfect edge passivation and material quality limitation of mc-Si. Light-beam-induced current (LBIC) was also applied on one material-induced shunt. LBIC mapping with long wavelength revealed the degraded current response due to poor wafer quality. However, an LBIC image of short wavelength did not show the defect structure because the current was dominated by the Auger recombination, while not influenced by the bulk lifetime. Some pre-breakdown sites were found in the material-induced shunt sample and were only visible under the reverse-bias condition of DLIT. View full abstract»

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  • Impact of Iron Surface Contamination on the Lifetime Degradation of Samples Passivated by Fired Al _{\bf 2} O _{\bf 3} /SiN _{bm x} Stacks

    Page(s): 957 - 961
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    We present results on the impact of iron surface contamination on the firing stability of the bulk lifetime of silicon samples passivated by Al2O3/SiNx stacks. The effect of blistering, hydrogen passivation, and formation and transformation of the interfacial silicon oxide layer, respectively, has been widely discussed. This paper focuses on an up-to-now largely ignored effect that may dominate the observed degradation in frequent cases: bulk degradation by in-diffusion of impurities from the interface between silicon and atomic layer deposition layer during firing. In order to enhance the apparent passivation quality of thin Al2O3 layers, an appropriate predeposition cleaning plays an important role. By experimental data and theoretical calculation, we could prove that the short firing step is sufficient to promote the diffusion of iron from the surface into the bulk. Even low iron surface contamination levels of less than 1.4 × 1011 at/cm2 can have a detrimental impact on bulk recombination. The detected enhancement of the global Shockley-Read-Hall recombination was attributed essentially to interstitial iron contamination of the bulk. This indicates that a close control of iron surface contamination is a key factor to guarantee for firing stable Al2O3/SiNx stack systems. View full abstract»

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  • Full Spectrum Photoluminescence Lifetime Analyses on Silicon Bricks

    Page(s): 962 - 969
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    Bulk lifetime and doping images on silicon bricks can be obtained by spectral luminescence intensity ratio analysis as established recently. Here, we report on calibrated full spectrum band-to-band luminescence measurements taken on the flat side faces of mono- and multicrystalline silicon bricks at room temperature. Our results verify the physical modeling used for the spectral intensity ratio imaging. We further investigate three fitting methods employing spectrally resolved photoluminescence data to obtain bulk lifetime information. View full abstract»

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  • Surface Passivation and Simulated Performance of Solar Cells With Al _{\bf 2} O _{\bf 3} /SiN _{bm x} Rear Dielectric Stacks

    Page(s): 970 - 975
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    Al2O3/SiNx stacks that are prepared at low temperatures in chemical vapor deposition processes excel in terms of surface passivation applicable in industrial p-type Si solar cells. The conversion efficiencies that are feasible for solar cells with Al 2O3/SiNx rear dielectric stacks, have been investigated by numerical simulations, including the optical performance of the stacks, which was considered for various Al 2O3 and SiNx film thicknesses. The optically optimized film thicknesses were found to be 15-30 nm for Al2O3 and 100-120 nm for the SiNx films. Experimentally, the surface passivation was found to be similar for annealed Al2O3/SiNx stacks and single-layer Al2O3 films with an almost equal level of field-effect and chemical passivation, as determined by optical second harmonic generation and corona charging experiments. View full abstract»

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  • Increased Front Surface Recombination by Rear-Side Laser Processing on Thin Silicon Solar Cells

    Page(s): 976 - 984
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    We show the degradation of the front surface passivation by rear-side laser processing of thin silicon solar cells when using a laser with a pulse length of 8 ps. 45-μm-thick back-contact back-junction monocrystalline silicon solar cells show an energy conversion efficiency of 18.8% without rear-side laser processing, whereas they show only 7.5% with an additional rear-side laser process step for contact separation. This low efficiency is due to the degradation of the front surface passivation, which is confirmed by quantum efficiency measurements. The internal quantum efficiency at short wavelength is 0.88 without laser processing, whereas it is only 0.33 with the rear-side laser process step. View full abstract»

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  • Projected Efficiency of Polarization-Matched p-In _{bm x} Ga _{bm {1-x}} N/i-In _{bm y} Ga _{bm{1-y}} N/n-GaN Double Heterojunction Solar Cells

    Page(s): 985 - 990
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    Traditional p-GaN/i-InGaN/n-GaN double heterojunction solar cells have limited power conversion efficiency due to large polarization charges that accumulate at the heterojunction interfaces, leading to severe band bending that, in turn, hinders the carrier transport. In this study, we proposed the use of a p-type InGaN layer to reduce the polarization field and projected the power conversion efficiencies of p-In xGa1-xN/i-In yGa1-y N/n-GaN double heterojunction solar cells that are grown on a c-facet sapphire substrate with various indium components. Numerical simulations predict that a maximal power conversion efficiency that is close to 7% with a short-circuit current density of 4.05 mA/cm2 and an open-circuit voltage of 1.94 V can be achieved with a p-In0.2 Ga0.8N/i-In0.2Ga0.8N/n-GaN structure due to a polarization-matched p-i interface. Further efficiency enhancement with a higher indium composition over 20% is also possible via the redistribution of the built-in potential with n-GaN doping. View full abstract»

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  • Triple-Junction GaInP/GaAs/Ge Solar Cells With an AZO Transparent Electrode and ZnO Nanowires

    Page(s): 991 - 996
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    In this paper, the fabrication of GaInP/GaAs/Ge triple-junction (TJ) solar cells with an Al-doped ZnO (AZO) transparent electrode and a ZnO nanowire (NW) antireflection (AR) layer is reported. It was found that ZnO NWs/AZO could provide a smaller reflectance, as compared with AZO and MgF2/Ta2O5. By inserting a 4-nm-thick AuGeNi between AZO and n+ -AlInP, it was found that the rectifying contact could be transformed into an ohmic contact with a specific contact resistance of 1.02 × 10-5 Ω·cm2. Furthermore, it was found that the ZnO NWs/ZnO used in this study could enhance the conversion efficiency of TJ solar cells from 21.91% to 28.16%, which corresponds to a 25.4% relative enhancement in the conversion efficiency. View full abstract»

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  • Evaluation of a Selection of Intermediate Band Materials Based on Their Absorption Coefficients

    Page(s): 997 - 1003
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    The intermediate band materials BSSi214, Cu4CrGa3S8, Cu4TiGa3S8, Mg2In3VS8, S32Zn31Cr, and Te32Zn31Cr, as well as a certain configuration of InAs quantum dots in GaAs, are evaluated as candidates to implement highly efficient intermediate band solar cells. The evaluation implies calculating theoretical efficiencies by combining an existing mathematical model and the absorption coefficients for the investigated materials. The model takes into account the energy dependence and spectral overlaps of the absorption coefficients related to transitions between various pairs of electronic bands. The presented results represent theoretical efficiencies for flat-plate solar cells, without light-trapping schemes, based on absorption coefficients publicly available in scientific journals. Only BSSi214 and InAs quantum dots in GaAs turn out to have theoretical efficiencies close to or above the detailed balance efficiency limit for single-bandgap cells. It appears unlikely that cells made of the other materials will be able to show efficiencies higher than single-bandgap cells either due to unfortunate absorption coefficients or due to bandgap combinations that are too far from the optimal. The results highlight the fact that materials have to be selected with great care when attempting to make IBSC prototypes with higher efficiency than conventional solar cells. View full abstract»

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  • Fabrication of Spacer and Catalytic Layers in Monolithic Dye-Sensitized Solar Cells

    Page(s): 1004 - 1011
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    Over the past few years, dye-sensitized solar cell (DSC) research has been focused on the material and process cost reduction, and on the electronics integration of the devices. Monolithic design is one of the most promising DSC architectures for mass production, because it allows the elimination of one conductive substrate and offers the possibility of printing layer-by-layer the materials that compose the structure. In this study, the formulation, the realization, and the processing of the spacer and the catalyst layers are proposed, and the relative performance in terms of J-V characteristics, incident photon to current conversion efficiency, and impedance analysis of the device with the optimized material thickness is reported. The optimized profile of the overall structure permits us to obtain masked cells with a conversion efficiency of about 5% with no chemical treatment. View full abstract»

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  • Carrier Lifetime from Dynamic Electroluminescence

    Page(s): 1012 - 1015
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    In this paper, the injection-level-dependent measurement of the effective minority carrier lifetime of solar cells from dynamic time-modulated electroluminescence is introduced. The analogy between optical and electrical injection of excess carriers is elaborated, the relevant specifications of the experimental setup are addressed, and an experimental proof of concept is given. Lifetime measurements over a very broad injection range are presented and found to agree well with dynamic photoluminescence measurements. View full abstract»

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  • Uncertainty in PV Module Measurement—Part I: Calibration of Crystalline and Thin-Film Modules

    Page(s): 1016 - 1026
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    This paper presents recent progress in reducing the measurement uncertainty for crystalline silicon (c-Si) and thin-film PV modules. It describes the measurement procedure and the uncertainty analysis, as applied at the CalLab PV Modules, Fraunhofer ISE's laboratory for module measurements. The uncertainty analysis covers the complete calibration process in detail, including measurements, correction to standard testing conditions, and determination of electrical module parameters (ISC, PMPP, VOC, etc.) from the I-V curve. Differences between c-Si and thin-film modules are addressed, most importantly in terms of spectral mismatch factor and short timescale stability problems. The paper outlines the importance of a comprehensive quality assurance system in a calibration laboratory as a prerequisite for accurate measurements on a daily basis. Particular attention is paid to results from a series of measurements taken every three weeks over a three-year period and conducted as part of the quality assurance system. In conclusion, this paper introduces a best-case uncertainty for c-Si module calibration of 1.6% for PMPP and 1.3% for ISC. This represents the lowest reported uncertainty for full size module calibration in a laboratory so far. The presented uncertainty in PMPP of cadmium telluride and single-junction amorphous silicon modules is 2.9% and 1.8%, respectively. All mentioned uncertainties are expanded uncertainties (k = 2). View full abstract»

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  • Thermoelectrical Modeling of Wavelength Effects on Photovoltaic Module Performance—Part I: Model

    Page(s): 1027 - 1033
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    A better understanding of the factors that affect the performance of photovoltaic (PV) modules will improve the design, optimization, and development of them. Researchers have introduced thermal models to predict module temperatures, and thermoelectrical models to study the interaction of electrical and thermal module characteristics. However, these models have relied on the assumption that the short-circuit current is proportional to the total incident irradiance, neglecting wavelength-specific effects. While it is true that the main input power to the PV module is the incident solar irradiance, various optical properties of module materials vary with wavelength. At each wavelength some energy is reflected, some is absorbed in the PV cells (contributing to electricity production), some is absorbed in other module materials, and some is transmitted through the module. A model capable of predicting this wavelength-specific behavior will generally allow a better assessment of the module performance especially when it is combined with subsystems such as optical filters. In this study, a wavelength-based thermoelectrical model is proposed to predict the PV module temperature and output power. A lab-built PV module is used to validate both the temperature and output power results. The predicted results are shown to agree with the experimental measurements. View full abstract»

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  • Thermoelectrical Modeling of Wavelength Effects on Photovoltaic Module Performance—Part II: Parameterization

    Page(s): 1034 - 1037
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    Intervals of the incident solar spectrum associated with long wavelengths do not contribute to electricity production due to their low energy levels. However, light with such wavelengths partially passes through the module layers, while the remainder is either reflected or absorbed as heat into them. Absorbed heat increases the solar module temperature and reduces its efficiency. In Part I, a thermoelectrical model is proposed in order to study the effects of the individual wavelengths on the photovoltaic (PV) module performance. In this paper, a method to characterize the model is proposed. A case study that shows the extraction and the derivation of all required parameters for the model is presented. The experiments are conducted on a lab-built monocrystalline silicon PV module. An optical model is presented to predict the optical properties of the module layers. It is found that characterizing the model using the proposed parameterization methodology leads to predicted results that are consistent with the experimental measurements. View full abstract»

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  • Modeling the Spectral Luminescence Emission of Silicon Solar Cells and Wafers

    Page(s): 1038 - 1052
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    A generalized expression for the theoretical description of luminescence spectra of silicon solar cells and wafers is outlined. Models of the spectral luminescence emission, which can be found in the literature, are reviewed and compared with spectrally resolved photoluminescence measurements carried out on specially prepared samples. These models describe the spectrum of samples with either two planar or two rough (lambertian reflecting) surfaces. Good qualitative agreement is shown. Moreover, we introduce a new model which is valid for samples with any configuration of planar and textured surfaces and also takes free carrier absorption into account. The accuracy of the model is experimentally confirmed by a comparison to an electroluminescence spectrum of an industrial silicon solar cell. View full abstract»

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  • An Analytical Solution for Tracking Photovoltaic Module MPP

    Page(s): 1053 - 1061
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    In this paper, a new method for estimating the maximum power point (MPP) of a solar array based on its characteristic equation is introduced. The proposed method is simple fast and returns the MPP accurately. Moreover, the sensitivity of the introduced scheme to the model parameters is analyzed. Finally, the practicability of the proposed method is demonstrated by both simulation and experimental results on REC-AE220 solar modules. View full abstract»

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  • Statistic and Parallel Testing Procedure for Evaluating Maximum Power Point Tracking Algorithms of Photovoltaic Power Systems

    Page(s): 1062 - 1069
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    Maximum power point tracking (MPPT) methods are essential for photovoltaic (PV) systems to take full advantage of the available solar energy. Over the past few years, an increasing number of new MPPT methods have been proposed in the literature, and they show better capability of capturing the maximum power point. The testing and evaluation procedure of any new MPPT method is a crucial step for assessing its robustness and performance. PV panel manufacturers always specify power output tolerances, ranging from ±2% to ±5%. Thus, nonideal factors might dominate the subsystem output and defeat any performance comparison attempt and might lead to inaccurate results. This paper highlights the main shortcomings of previous MPPT testing procedures and proposes a comprehensive testing approach using paired difference tests to evaluate the performance of MPPT. The dual channel bench system demonstrates a systematic framework to deal with the nonideal factors associated with PV manufacturing, quantify experimental data, and correctly illustrate the performance improvement of MPPT. The case study shows that the 1% improvement of MPPT could be easily overshadowed by the nonideal factors shown previously. The proposed test setup and analysis method provide the effective solution for evaluating the MPPT performance. View full abstract»

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  • On the Perturb-and-Observe and Incremental Conductance MPPT Methods for PV Systems

    Page(s): 1070 - 1078
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    This paper presents a detailed analysis of the two most well-known hill-climbing maximum power point tracking (MPPT) algorithms: the perturb-and-observe (P&O) and incremental conductance (INC). The purpose of the analysis is to clarify some common misconceptions in the literature regarding these two trackers, therefore helping the selection process for a suitable MPPT for both researchers and industry. The two methods are thoroughly analyzed both from a mathematical and practical implementation point of view. Their mathematical analysis reveals that there is no difference between the two. This has been confirmed by experimental tests according to the EN 50530 standard, resulting in a deviation between their efficiencies of 0.13% in dynamic and as low as 0.02% under static conditions. The results show that despite the common opinion in the literature, the P&O and INC are equivalent. View full abstract»

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  • Comparison of Cell Performance of ZnS(O,OH)/CIGS Solar Cells With UV-Assisted MOCVD-ZnO:B and Sputter-Deposited ZnO:Al Window Layers

    Page(s): 1079 - 1083
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    ZnO:B films deposited by ultraviolet light-assisted metal-organic chemical vapor deposition (UM-ZnO:B) were applied to CBD-ZnS(O,OH)/CIGS solar cells in order to eliminate plasma damages during the subsequent ZnO sputtering. It was verified that the conversion efficiency of CIGS solar cells with a UM-ZnO:B window layer was higher than that of the device with a sputter-deposited(Sp-) ZnO:Al window layer; in both cases, thick (120 nm) and thin (10 nm) ZnS(O,OH) buffer layers were used. The conversion efficiency of a CIGS solar cell was improved from 16.3% to 17.5% upon replacement of the Sp-ZnO:Al by a UM-ZnO:B window layer when the thick ZnS(O,OH) (120 nm) buffer layer was used. Notably, the conversion efficiency was remarkably improved from 0.2% to 15.6% by the replacement of the window layer even when the ultrathin ZnS(O,OH) (10 nm) buffer layer was used. The temperature dependence of open-circuit voltage revealed that interface recombination decreased owing to the use of a UM-ZnO:B window layer. View full abstract»

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  • Optical Properties of Sputtered SnS Thin Films for Photovoltaic Absorbers

    Page(s): 1084 - 1089
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    Tin monusulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high-efficiency solar cells. The optical properties of SnS thin films are investigated to assess their compatibility with the solar spectrum. SnS thin films were RF magnetron sputter-deposited at target powers of 105-155 W and total pressures of 5 to 60 mtorr in argon at room temperature. X-ray diffraction patterns confirmed a dominant tin monosulfide herzenbergite phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 105 -106 cm-1. The direct gap, indirect gap, and forbidden direct gap for the films were found to be in the range of 1.2-1.6 eV, indicating a strong match with the solar irradiance spectrum. 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