Showing papers in "IEEE Journal of Photovoltaics in 2013"

Reassessment of the Limiting Efficiency for Crystalline Silicon Solar Cells

Abstract: Recently, several parameters relevant for modeling crystalline silicon solar cells were improved or revised, e.g., the international standard solar spectrum or properties of silicon such as the intrinsic recombination rate and the intrinsic carrier concentration. In this study, we analyzed the influence of these improved state-of-the-art parameters on the limiting efficiency for crystalline silicon solar cells under 1-sun illumination at 25°C, by following the narrow-base approximation to model ideal solar cells. We also considered bandgap narrowing, which was not addressed so far with respect to efficiency limitation. The new calculations that are presented in this study result in a maximum theoretical efficiency of 29.43% for a 110-μm-thick solar cell made of undoped silicon. A systematic calculation of the I-V parameters as a function of the doping concentration and the cell thickness together with an analysis of the loss current at maximum power point provides further insight into the intrinsic limitations of silicon solar cells.

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

Abstract: 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.

CdTe Solar Cells at the Threshold to 20% Efficiency

Abstract: After a long period of stagnancy for record cell efficiency and several years of growth of the industry, CdTe solar cell efficiency has been rapidly increasing recently. First Solar (FSLR) fabricated an 18.7% NREL certified cell at the end of 2012 and reports an increase to a certified 19.0% in this paper. Although the improvements were dominated by increases in short-circuit current and fill factor, there is now evidence that the open-circuit voltage of polycrystalline CdTe is not fundamentally limited to ~850 mV, and first devices exceeding 900 mV have been demonstrated. In-depth device characterization indicates that the responsible third-level metric is the increase of carrier lifetime, which can be characterized by transient photoluminescence. The progress at hand suggests that the near-term achievable target for CdTe solar cells should be raised from 19% to 22%. A detailed numerical model is used to translate cell results into predicted module efficiency. These simulations are in agreement with FSLR's recently certified 16.1% total-area module efficiency record. Forward-looking simulations show that with already demonstrated technology components, a module efficiency exceeding 17% is attainable. Disruptive changes and implementation of new device architectures can provide further room for improvement for cell efficiency beyond 22%.

>21% Efficient Silicon Heterojunction Solar Cells on n- and p-Type Wafers Compared

Abstract: The properties and high-efficiency potential of front- and rear-emitter silicon heterojunction solar cells on n- and p-type wafers were experimentally investigated. In the low-carrier-injection range, cells on p-type wafers suffer from reduced minority carrier lifetime, mainly due to the asymmetry in interface defect capture cross sections. This leads to slightly lower fill factors than for n-type cells. By using high-quality passivation layers, however, these losses can be minimized. High open-circuit voltages (Vocs) were obtained on both types of float zone (FZ) wafers: up to 735 mV on n-type and 726 mV on p-type. The best Voc measured on Czochralski (CZ) p-type wafers was only 692 mV, whereas it reached 732 mV on CZ n-type. The highest aperture-area certified efficiencies obtained on 4 cm2 cells were 22.14% (Voc = 727 mV , FF = 78.4%) and 21.38% (Voc = 722 mV, FF = 77.1%) on n- and p-type FZ wafers, respectively, proving that heterojunction schemes can perform almost as well on high-quality p-type as on n-type wafers. To our knowledge, this is the highest efficiency ever reported for a full silicon heterojunction solar cell on a p-type wafer, and the highest Voc on any p-type crystalline silicon device with reasonable FF.

Efficient Approaches for Modeling and Simulating Photovoltaic Power Systems

Abstract: Modeling and simulation of photovoltaic (PV) power systems have become increasingly important with wide acceptance and integration of solar energy in modern electric grids. The transcendental nonlinear equations describing the PV generator, which are coupled with the detailed switching models of the power electronic converters, generally result in slow and inefficient simulations, especially when long-term analyses are required. This paper focuses on simple and efficient modeling approaches that are suitable for long-term and large PV system analyses. This study provides a simplified PV-cell model and its parameterization, guaranteeing that the I-V characteristic curves pass through the typical points given in manufacturers' datasheets. Furthermore, several power interface models are provided for fast simulation purpose. A classical two-stage power processing system with intermediate dc link used as a string inverter, as well as a single-stage conversion unit used in distributed module-dedicated PV applications, are taken as application examples. The generalized modeling approach is thoroughly evaluated by comparing the simulation results with the experimental data of a practical 2.4-kW grid-tied PV solar unit. The proposed methodology is shown to have advantages over conventional modeling approaches to simulate long-term grid-tied operation.

Maximum efficiencies of indoor photovoltaic devices

Abstract: Photovoltaic (PV) converters on the centimeter scale are considered to be the most promising energy supplier for energy-autarkic microsystems in indoor applications, i.e., to power wireless sensor nodes in automated buildings. We provide the first systematic discussion of the optimum material and efficiency limits of indoor photovoltaics. The limiting efficiency η of an ideal photovoltaic converter for various indoor radiation sources and two calculation models ranges between 46% for a fluorescent tube and a bandgap Eg = 1.95 eV and 67% for a sodium discharge lamp and Eg = 2.10 eV. The optimal bandgap for typical narrow-band artificial light sources is 1.90-2.00 eV. For Eg = 2.25 eV, i.e., matched to the photon energy of a monochromatic source emitting at 555 nm, η calculated from the detailed balance model reaches 72.98%. The performance when irradiated by indoor Planckian radiators, such as incandescent bulbs and halogen tubes, is always less than under any solar irradiance. The experimental efficiencies of 11 sample groups with a maximum efficiency of 16%, which were measured with a radiometric indoor characterization setup especially developed for this study, support the theoretically predicted changes in efficiency when compared with efficiencies under the conditions of IEC 60904-3.

Inline Cu(In,Ga)Se $_{2}$ Co-evaporation for High-Efficiency Solar Cells and Modules

Abstract: In this paper, co-evaporation of Cu(In,Ga)Se2 (CIGS) in an inline single-stage process is used to fabricate solar cell devices with up to 18.6% conversion efficiency using a CdS buffer layer and 18.2% using a Zn1-xSnxOy Cd-free buffer layer. Furthermore, a 15.6-cm2 mini-module, with 16.8% conversion efficiency, has been made with the same layer structure as the CdS baseline cells, showing that the uniformity is excellent. The cell results have been externally verified. The CIGS process is described in detail, and material characterization methods show that the CIGS layer exhibits a linear grading in the [Ga]/([Ga]+[In]) ratio, with an average [Ga]/([Ga]+[In]) value of 0.45. Standard processes for CdS as well as Cd-free alternative buffer layers are evaluated, and descriptions of the baseline process for the preparation of all other steps in the Angstrom Solar Center standard solar cell are given.

Fabrication of GaInP/GaAs//Si Solar Cells by Surface Activated Direct Wafer Bonding

Abstract: GaInP/GaAs//Si solar cells with three active p-n junctions were fabricated by surface activated direct wafer bonding between GaAs and Si. The direct wafer bond is performed at room temperature and leads to a conductive and transparent interface. This allows the fabrication of high-efficiency monolithic tandem solar cells with active junctions in both Si and the III-V materials. This technology overcomes earlier challenges of III-V and Si integration caused by the large difference in lattice constant and thermal expansion. Transmission electron microscopy revealed a 5-nm thin amorphous interface layer formed by the argon fast atom beam treatment before bonding. No further defects or voids are detected in the photoactive layers. First triple-junction solar cell devices on Si reached an efficiency of 23.6% under concentrated illumination.

A Fill Factor Loss Analysis Method for Silicon Wafer Solar Cells

Abstract: The fill factor of silicon wafer solar cells is strongly influenced by recombination currents and ohmic resistances. A practical upper limit for the fill factor of crystalline silicon solar cells operating under low-level injection is set by recombination in the quasi-neutral bulk and at the two cell surfaces. Series resistance, shunt resistance, and additional recombination currents further lower the fill factor. For process optimization or loss analysis of solar cells, it is important to determine the influence of both ohmic and recombination loss mechanisms on the fill factor. In this paper, a method is described to quantify the loss in fill factor due to series resistance, shunt resistance, and additional recombination currents. Only the 1-Sun J-V curve, series resistance at the maximum power point, and shunt resistance need to be determined to apply the method. Application of the method is demonstrated on an 18.4% efficient inline-diffused p-type silicon wafer solar cell and a 21.1% efficient heterojunction n-type silicon wafer solar cell. Our analysis does not require J-V curve fitting to extract diode saturation current densities or ideality factor; however, the results are shown to be consistent with curve fitting results if the cell's two-diode model parameters can be unambiguously determined by curve fitting.

Design of GaAs Solar Cells Operating Close to the Shockley–Queisser Limit

Abstract: With recent advances in device design, single-junction GaAs solar cells are approaching their theoretical efficiency limits. Accurate numerical simulation may offer insights that can help close the remaining gap between the practical and theoretical limits. Significant care must be taken, however, to ensure that the simulation is self-consistent and properly comprehends thermodynamic limits. In this paper, we use rigorous photon recycling simulation coupled with carrier transport simulation to identify the dominant loss mechanisms that limit the performance of thin-film GaAs solar cells.

Effective Passivation of Black Silicon Surfaces by Atomic Layer Deposition

Abstract: The poor charge-carrier transport properties attributed to nanostructured surfaces have been so far more detrimental for final device operation than the gain obtained from the reduced reflectance. Here, we demonstrate results that simultaneously show a huge improvement in the light absorption and in the surface passivation by applying atomic layer coating on highly absorbing silicon nanostructures. The results advance the development of photovoltaic applications, including high-efficiency solar cells or any devices, that require high-sensitivity light response.

An Optimization Method for Designing Large PV Plants

Abstract: Large-scale photovoltaic (PV) plants enable the reduction of the PV plant cost per watt of nominal power that is installed. In this paper, a new method is presented for the calculation of the optimal configuration of large PV plants, such that the levelized cost of the generated electricity (LCOE) is minimized. The proposed design optimization process is performed by considering the impact of the number of components, as well as their type and arrangement within the installation field, on the tradeoff between the lifetime cost and the corresponding energy production of the PV plant. The high-accuracy feature of the energy production calculations that are performed by the proposed design tool has been validated using experimental operational data of an existing PV plant. The design results demonstrate that using the proposed optimization method allows a reduction of the cost of the energy that is generated by the large-scale PV plant, thus enabling the maximization of the economic benefit that is obtained during the operational lifetime period of the PV system.

Record Infrared Internal Quantum Efficiency in Silicon Heterojunction Solar Cells With Dielectric/Metal Rear Reflectors

Abstract: Inserting a dielectric between the absorber and rear metal electrode of a solar cell increases rear internal reflectance by both limiting the transmission cone and suppressing the plasmonic absorption of light arriving outside of the cone. We fabricate rear reflectors with low-refractive-index magnesium fluoride (MgF2) as the dielectric, and with local electrical contacts through the MgF2 layer. These MgF2/metal reflectors are introduced into amorphous silicon/crystalline silicon heterojunction solar cells in place of the usual transparent conductive oxide/metal reflector. An MgF2/Ag reflector yields an average rear internal reflectance of greater than 99.5% and an infrared internal quantum efficiency that exceeds that of the world-record UNSW PERL cell. An MgF2/Al reflector performs nearly as well, enabling an efficiency of 21.3% and a short-circuit current density of nearly 38 mA/cm2 in a silicon heterojunction solar cell without silver or indium tin oxide at the rear.

A Dynamic Photovoltaic Model Incorporating Capacitive and Reverse-Bias Characteristics

Abstract: Photovoltaics (PVs) are typically modeled only for their forward-biased dc characteristics, as in the commonly used single-diode model. While this approach accurately models the I-V curve under steady forward bias, it lacks dynamic and reverse-bias characteristics. The dynamic characteristics, primarily parallel capacitance and series inductance, affect operation when a PV cell or string interacts with switching converters or experiences sudden transients. Reverse-bias characteristics are often ignored because PV devices are not intended to operate in the reverse-biased region. However, when partial shading occurs on a string of PVs, the shaded cell can become reverse biased and develop into a hot spot that permanently degrades the cell. To fully examine PV behavior under hot spots and various other faults, reverse-bias characteristics must also be modeled. This study develops a comprehensive mathematical PV model based on circuit components that accounts for forward bias, reverse bias, and dynamic characteristics. Using a series of three experimental tests on an unilluminated PV cell, all required model parameters are determined. The model is implemented in MATLAB Simulink and accurately models the measured data.

Minority Carrier Lifetime Analysis in the Bulk of Thin-Film Absorbers Using Subbandgap (Two-Photon) Excitation

Abstract: We describe a new time-resolved photoluminescence (TRPL) analysis method for the determination of minority carrier lifetime τB. This analysis is based on subbandgap excitation (two-photon excitation, or 2PE) and allows selective lifetime determination at the surface or in the bulk of semiconductor absorbers. We show that for single-crystal CdTe, τB could be determined even if surface recombination velocity is >105 cm s-1. Two-photon excitation TRPL measurements indicate that radiative lifetime in undoped CdTe is >>66 ns. We also compare one-photon excitation (1PE) and 2PE TRPL data for polycrystalline CdS/CdTe thin films.

Comparing Capacity Value Estimation Techniques for Photovoltaic Solar Power

Abstract: In this paper, we estimate the capacity value of photovoltaic (PV) solar plants in the western U.S. Our results show that PV plants have capacity values that range between 52% and 93%, depending on location and sun-tracking capability. We further compare more robust but data- and computationally-intense reliability-based estimation techniques with simpler approximation methods. We show that if implemented properly, these techniques provide accurate approximations of reliability-based methods. Overall, methods that are based on the weighted capacity factor of the plant provide the most accurate estimate. We also examine the sensitivity of PV capacity value to the inclusion of sun-tracking systems.

An Analytical Solution for Tracking Photovoltaic Module MPP

Abstract: 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.

Effects of Internal Luminescence and Internal Optics on $V_{\bf oc}$ and $J_{\bf sc}$ of III--V Solar Cells

Abstract: For solar cells dominated by radiative recombination, the performance can be significantly enhanced by improving the internal optics. We demonstrate a detailed model for solar cells that calculates the external luminescent efficiency and discuss the relationship between the external and internal luminescence. The model accounts for wavelength-dependent optical properties in each layer, parasitic optical and electrical losses, multiple reflections within the cell, and assumes isotropic internal emission. For single-junction cells, the calculation leads to Voc, and for multijunction cells, the calculation leads to the Voc of each junction as well as the luminescent coupling constant. In both cases, the effects of the optics are most prominent in cells with high internal radiative efficiency. Exploiting good material quality and high luminescent coupling, we demonstrate a two-junction nonconcentrator cell with a conversion efficiency of (31.1 ± 0.9)% under the global spectrum.

Boron Emitter Passivation With Al $_{\bf 2}$ O $_{\bf 3}$ and Al $_{\bf 2}$ O $_{\bf 3}$ /SiN $_{\bm x}$ Stacks Using ALD Al $_{\bf 2}$ O $_{\bf 3}$

Abstract: Thin layers of Al2O3 are known to feature excellent passivation properties on highly boron-doped silicon surfaces. In this paper, we present a detailed study of the passivation quality of Al2O3 single layers and stacks of Al2O3 and antireflection SiNx on boron-doped emitters, where the Al2O3 was deposited by plasma-assisted atomic layer deposition and the SiNx by plasma-enhanced chemical vapor deposition. The passivation quality was studied for different atomic layer deposition temperatures, as a function of the Al2O3 layer thickness, as well as on samples with planar and random pyramids textured surfaces. These investigations were performed for different boron emitter diffusions, such as shallow, industrial emitters with high surface concentrations, as well as driven-in emitters with low surface concentrations. For all these variations, we compared systematically different thermal post-deposition treatments to activate the Al2O3 passivation, i.e., annealing processes at moderate temperatures and short high-temperature processes, as required for firing printed metal contacts. Therefore, symmetrically processed p+np + samples were fabricated, which were characterized with the photoconductance decay technique to determine emitter saturation current densities. Finally, the longtime stability of the Al2O3/SiNx stacks with planar and textured surfaces was investigated with an accelerated ultraviolet (UV) exposure experiment, miming about 34 month of outdoor performance.

Analysis of Multijunction Solar Cell Current–Voltage Characteristics in the Presence of Luminescent Coupling

Abstract: Luminescent coupling in multijunction solar cells is the phenomenon in which a junction in forward bias radiates photons that are absorbed in and converted to photocurrent by the junction beneath the radiating one. This effect can be significant in modern high-efficiency multijunction cells. We have previously developed a combined measurement and analytical approach to characterize the short-circuit current including the effects of nonlinear coupling in terms of measurable parameters η and φ that describe the coupling strength and linearity, respectively. Here, we develop an analytical model for the full current-voltage characteristic V (J) of a multijunction cell in the presence of luminescent coupling, in terms of the η and φ parameters. We compare the model with the measured V (J) parameters of GaInP/GaAs two-junction cells that exhibit differing degrees of luminescent coupling, and show that the model well describes the measurements. We then use the model to explore the consequences of luminescent coupling on the operating parameters of an idealized two-junction cell as a function of the top-junction thickness, focusing on the open-circuit voltage, fill factor, and efficiency. The results demonstrate that the strong luminescent coupling can significantly alter the dependence of cell efficiency on junction thickness, and that consequently the well-known optical-thinning design rules must be modified.

Misconceptions and Misnomers in Solar Cells

Abstract: Some of the terms that are currently used in solar cell technology, such as “emitter” and “back surface field,” perpetuate old misconceptions about the role of the highly doped n+ and p+ regions commonly implemented near their front and back surfaces. This paper reviews the physics of the p+ back surface region of silicon solar cells and concludes that, whereas electric fields are important to describe equilibrium conditions, the main force behind carrier transport under illumination is the gradient of the carrier concentration itself, i.e., of the chemical potential. The function of the back p+ region in a photovoltaic device is to facilitate the transfer of holes toward the metal contact, while suppressing the concentration of electrons. An appropriate name for it is hole collector. Similarly, the function of the n+ region is to collect and transfer electrons to the front metal contact and should be called the electron collector.

Impact of Impurities From Crucible and Coating on mc-Silicon Quality—the Example of Iron and Cobalt

Abstract: The aim of this paper is to analyze the limiting role of crucible and coating impurities on material quality of multicrystalline silicon. Both solid body diffusion and diffusion into the silicon melt are considered in this study. Two ingots of size G1 have been analyzed. One of them was crystallized in a standard crucible, whereas the other was crystallized in a quartz crucible of very high purity. Focus is put on iron and cobalt as examples of typical impurity species. Iron was found in large concentrations in standard crucibles, and cobalt was proven to be a suitable marker impurity that is mainly found in the coating. Inductively coupled plasma mass spectroscopy data are exploited for the determination of impurity concentrations in crucible, coating, and within the crystal. With higher sensitivity for low concentration, PL imaging is applied for carrier lifetime and interstitial iron concentration measurements. The different findings are compared with modeling results of iron and cobalt in-diffusion by Sentaurus Process. The analysis of silicon wafers before and after gettering steps enable a quantification of impurity-limiting cell efficiency potential. Conclusions about the role of impurities from coated crucibles in large-scale crystallization are deduced.

Synthesis of Group IV Clathrates for Photovoltaics

Abstract: Although Si dominates the photovoltaics market, only two forms of Si have been thoroughly considered: amorphous Si and Si in the diamond structure ( d-Si). Silicon can also form in other allotropes, including clathrate structures. Silicon clathrates are inclusion compounds, which consist of an Si framework surrounding templating guest atoms (e.g., Na). After formation of the type II Na 24Si136 clathrate, the guest atoms can be removed (Si136), and the material transitions from degenerate to semiconducting behavior with a 1.9 eV direct band gap. This band gap is tunable in the range of 1.9-0.6 eV by alloying the host framework with Ge, enabling a variety of photovoltaic applications that include thin-film single-junction devices, Si136 top cells on d-Si for all-Si tandem cells, and multijunction cells with varying Si/Ge ratios. In this study, we present electronic structure calculations that show the evolution of the direct transition as a function of Si/Ge ratio across the alloy range. We demonstrate the synthesis of type II Si/Ge clathrates spanning the whole alloy range. We also demonstrate a technique for forming Si clathrate films on d-Si wafers and sapphire substrates.

Luminescent Coupling in GaAs/GaInNAsSb Multijunction Solar Cells

Abstract: Multijunction subcell short-circuit currents are typically calculated by integrating the product of the external quantum efficiency and the input solar irradiance. The actual subcell current can be much greater than the result of this calculation when luminescence from a high bandgap subcell couples into a lower bandgap subcell. A model is developed, based on detailed balance, which quantifies luminescent coupling current under different spectral conditions. Steady-state laser and pulsed flash measurements on GaAs/GaInNAsSb dual-junction solar cells demonstrate a luminescent coupling factor of ~35%, compared with the theoretical maximum value of ~48%. The deviation from maximum is due to nonradiative current in the GaAs subcell.

Potential-Induced Degradation of CuIn $_{1-x} \hbox{Ga}_{x}$ Se $_{2}$ Thin Film Solar Cells

Abstract: The use of Na-free or low Na content glass substrates is observed to enhance the resiliency to potential-induced degradation, as compared with glass substrates with high Na content, such as soda lime glass (SLG). The results from stress tests in this study suggest that degradation caused by a combination of heat and bias across the SLG substrate is linked to increased Na concentration in the CdS and Cu(In,Ga)Se2 (CIGS) layers in CIGS-based solar cells. The degradation during the bias stress is dramatic. The efficiency drops to close to 0% after 50 h of stressing. On the other hand, cells on Na-free and low Na content substrates exhibited virtually no efficiency degradation. The degraded cells showed partial recovery by resting at room temperature without bias; thus, the degradation is nonpermanent and may be due to Na migration and accumulation rather than chemical reaction.

Simulation of Quantum Dot Solar Cells Including Carrier Intersubband Dynamics and Transport

Abstract: This paper presents a device-level model of quantum dot (QD) solar cells, coupling the classical drift-diffusion equations for transport of bulk carriers with a set of rate equations describing the QD carrier dynamics. The model is applied to carry out a detailed study of the impact of thermal-assisted processes on the electrical performance of InAs/GaAs QD solar cells (QDSCs), by exploiting experimentally determined parameters for QD photogeneration and carrier kinetics. Special emphasis is given on the analysis of the open circuit voltage degradation, as well as its dependence on QD size and carrier lifetime. The modeling approach is validated by comparing simulated trends against experimental data in the literature.

Cu(In,Ga)Se $_{\bf 2}$ Solar Cells With Varying Na Content Prepared on Nominally Alkali-Free Glass Substrates

Abstract: In this paper, Cu(In,Ga)Se2 (CIGS) thin-film solar cells are prepared on nominally alkali-free glass substrates using an in-line CIGS growth process As compared with, for example, borosilicate glass or quartz, the glass is engineered to have similar thermal expansion coefficient as soda-lime glass (SLG) but with alkali content close to zero Na is incorporated in the CIGS material using an ex-situ deposited NaF precursor layer evaporated onto the Mo back contact Several thicknesses of the NaF layer were tested The results show that there is a process window, between 15 and 225 nm NaF, where the solar cell conversion efficiency is comparable with or exceeding that of SLG references The effect of an NaF layer that is too thin on the solar cell parameters was mainly lowering the open-circuit voltage, which points to a lower effective dopant concentration in the CIGS layer and is also consistent with presented C - V measurements and modeling results For excessively thick NaF layers, delamination of the CIGS layer occurred Additional measurements, such as scanning electron microscopy (SEM), secondary ion mass spectrometry, capacitance-voltage analysis (C - V), time-resolved photoluminescence (TRPL), external quantum efficiency (EQE), current-voltage analysis (J-V), and modeling, are presented, and the results are discussed

Sizing and Parametric Analysis of a Stand-Alone Photovoltaic Power Plant

Abstract: A hybrid approach, combining analytical sizing equations with long-term performance, for an optimal design of a stand-alone photovoltaic (PV)-battery system is proposed in this paper. This study explicitly examines the interaction of hourly variation of the load on the energy supplied from a stand-alone PV-generating source keeping the system 100% reliable, and a modification is proposed in the sizing algorithm to include the effect of loading profile. The correctness of the methodology is validated using an experiment exercise and by comparing it with other existing models. An adaptive feedback iteration technique, for fast convergence, is presented to obtain the best optimum combination for PV-battery configuration. A parametric analysis is carried out to examine the effect of load duration and charge controller low-voltage disconnect (LVD) on system sizing. A significant reduction of system requirements, i.e., up to 14%, is observed when the load is operating between 6 a.m. and 12 p.m. For a given condition, the optimum LVD of the charge controller is reported to be 11.4 V. The results of this study will serve any PV design engineer to decide the optimum system requirements and charge controller settings without compromising system reliability.

The Influence of Absorber Thickness on Cu(In,Ga)Se $_{\bf 2}$ Solar Cells With Different Buffer Layers

Abstract: This study investigates the interplay between the absorber layer of Cu(In,Ga)Se2 solar cells and the other layers of these devices. Cu(In,Ga)Se2 devices with absorbers of different thicknesses and different buffer layers are fabricated. Absorber layers and finished devices are characterized. Good efficiencies are obtained, also for devices of substandard thickness down to 0.3 μm. Best open-circuit voltages and fill factors are found for cells with half the standard absorber thickness, but the highest efficiencies are found for cells with the standard thickness of 1.6 μm due to their higher short-circuit current density. Cu(In,Ga)Se2 cells with Zn(O,S) buffer layers are more efficient than CdS reference devices for the same absorber thickness due to a higher short-circuit current. For cells with thin absorber layers, a part of the higher current is caused by higher quantum efficiency at long wavelengths. Electrical simulations indicate that the loss in the open-circuit voltage for the thinnest devices is due to recombination in the back contact region. The difference in long-wavelength quantum efficiency between the buffer layers is attributed to a difference in the CIGS band bending. Acceptors at the Cu(In,Ga)Se2-CdS interface are proposed as an explanation for this difference. A low-quality back contact region enhances the effect.

Optical Properties of Sputtered SnS Thin Films for Photovoltaic Absorbers

Abstract: 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.