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Subcell Characterization in Multijunction Solar Cells Using Pulsed Light
Abstract
The photocurrent versus open-circuit voltage characteristic of individual subcells in a multijunction solar cell can be measured by illuminating the subcell with a pulse sequence of spatially homogeneous laser light. In this paper, we demonstrate the applicability of this method for four junction solar cells. Furthermore, the degradation of triple junction solar cells after electron irradiation is analyzed on the subcell level. Subcell performance parameters are derived, and results are compared with electroluminescence-based results. The influence of luminescent coupling and semitransparency effects to lower subcells are quantified by simulations.
... Based on the terminal measurement of electrical output characteristics, the PV system model is identified using parameter extraction [9], which can further be used to predict the system output under varying ambient conditions [10], along with cell degradation-based aging studies [11] and fault diagnosis [12]. Characterization for low-power solar cells is found extensively both in literature [13][14][15][16] with several low-power single cell/module characterization devices available in industry. The focus of the above-mentioned cell-level characterization is typically in the early design and performance validation stages of the PV device development [17,18]. ...
Photovoltaic energy generation potential can be tapped with maximum efficacy by characterizing the source behaviour. Characterization refers to the systematic terminal measurement-based PV modeling which can further facilitate output prediction and fault detection. Most of the existing PV characterization methods fail for high-power PV array due to increased thermal losses in electronic components. Here, we propose a switched-mode power converter-based PV characterization setup which is designed with input filter to limit switching ripple entering into PV array under test, thereby enhancing system life and efficiency. The high resonant frequency input filter ensures its compactness with high-speed characterization capability. To further enhance the system performance, a closed-loop current control of the system is designed for high bandwidth and stable phase margins. Variation of the controller parameters under varying ambient conditions of 200–1000 W/m2 irradiation and 25–70 °C temperature is documented and an adaptive PI controller is proposed. Experimental and simulation results validate the high performance of the closed loop operation of the PV characterization at 1.2 kW range power level in real-time field conditions. Compared to the open loop operation, the closed-loop operation eliminates the waveform ringing by 100% during characterization.
A thin, lightweight, flexible solar cell is developed that maximizes the power‐to‐mass ratio under AM0 illumination and has a competitive efficiency after typical high energy electron irradiation. The inverted metamorphic triple junction (IMM3J) solar cells with Ga0.51In0.49P/GaAs/Ga0.73In0.27As subcells are grown on GaAs substrates and have a total epitaxy thickness of about 10 μm. After epitaxial growth, the inverted layer stack is metallized, with the metal serving as back‐contact, back reflector and support layer for the ultra‐thin solar cells before the GaAs substrate is separated by an epitaxial lift‐off (ELO) process. The nondestructive nature of the ELO process makes multiple reuses of the GaAs substrate possible. The solar cell structure is optimized for maximum EOL efficiency, that is, after 1‐MeV electron irradiation with a fluence of 1 × 10¹⁵ cm⁻², by means of simulations that include the irradiation induced defects in the various semiconductor alloys. Assuming realistic charge carrier lifetime in the materials, we predict a near‐term efficiency potential for the IMM3J ELO of 30.9% under AM0 illumination before and 26.7% after irradiation. Several IMM3J ELO solar cells with an area of approximately 20 cm² from different development stages were tested under AM0 illumination. The newest solar cells (generation III) with a mass density of only 13.2 mg/cm² reach conversion efficiencies of 30.2% at 25°C. The resulting power‐to‐mass ratio of 3.0 W/g for the bare solar cell is one of the highest published ratios. After irradiation, a conversion efficiency of 25.4% was measured for “generation II” devices under AM0 illumination, which corresponds to a power‐to‐mass ratio of 2.6 W/g. IMM3J ELO solar cells from “generation I” were also tested for mechanical stability as “de‐risking” test of this new cell technology. No degradation of the cell performance was found after dipping the cell in liquid N2 and then heating up to 25°C for five times, despite of strong deformation of the flexible cell during the temperature cycle.
We investigate the luminescent coupling (LC) effects in a four-junction GaInP/GaAs//GaInAsP/GaInAs concentrator solar cell based on transient open-circuit voltage (Voc) measurements under monochromatic illumination. Photocurrent generation in the non-absorbing GaInAs bottom subcell due to LC from upper subcells shows superlinear behavior with increasing light intensity. Along with this, a Voc enhancement is observed and quantified for illumination intensities that span almost six orders of magnitude. The Voc increase is explained and studied using a series-connected diode model including subcell shunt resistances, capacitances, and LC effects. The impact of unilluminated subcells on the subcell Voc determination is discussed for multi-junction solar cells. Finally, in the analysis of the LC generated photocurrent, namely, the coupling factor from the GaInAsP to the non-absorbing GaInAs subcell, a characteristic dependency on bias voltage is shown and explained
by a result of competing photo- and electroluminescence mechanisms.
The response of triple-junction solar cells to proton and electron irradiation is analyzed using electroluminescence (EL) measurements. This analysis allows the dark current of each individual subjunction to be determined providing insight into the radiation response mechanisms.
The I-V characteristics of the individual subcells of a monolithic Ga0.50In0.50P/Ga0.99In0.01As/Ge triple-junction solar cell have been extracted from measurements of the electroluminescence peak intensity as a function of the electroluminescence injection current. By using the spectral reciprocity relation between the electroluminescence and the quantum efficiency, the individual subcell I-V characteristics were derived. It is shown that the subcell dark I-V characteristics and the subcell illuminated I-V characteristics are accessible under variable spectral illumination conditions.
Mit der vorliegenden Arbeit werden wichtige Beiträge auf dem Gebiet der Charakterisierung und hinsichtlich des Degradationsverhaltens von III-V Mehrfachsolarzellen für die Anwendung im Weltraum geleistet. Im Rahmen einer umfassenden Degradationsstudie an der aktuellen Weltraumsolarzelle des europäischen Herstellers RWE SSP (RWE3G28) unter Einbeziehung von Komponentenzellen wurde die Anwendbarkeit der bestehenden Methoden zur Vorhersage von Solarzellendegradation auf Mehrfachsolarzellen überprüft. Dabei wurde demonstriert, dass eine direkte Übertragung dieser zunächst nur für Einfachsolarzellen entwickelten Methoden auf Mehrfachsolarzellen streng genommen nicht möglich ist. Eine im mathematischen und physikalischen Sinne korrekte Erweiterung sowohl der JPL (Jet Propulsion Laboratory) aber vor allem auch der NRL (Naval Research Laboratory) Methode bedarf der Kenntnis der Degradationscharakteristiken der Komponentenzellen der zu untersuchenden 3J- oder Mehrfachsolarzelle. Die bestehenden Modelle wurden daher entsprechend erweitert, um mit diesen auf die Komplexität der 3J- oder Mehrfachzelle angepassten Modellen, eine konsistente Beschreibung des Degradationsverhaltens unter Teilchenbestrahlung zu gewährleisten. Mit der Analyse von Dunkelkennlinienmessungen an Komponentenzellen vor und nach Bestrahlung wurde eine neue Methode entwickelt, bei der die Degradationscharakteristiken der Hellparameter dieser Zellen - aber auch die der zugehörigen Mehrfachsolarzelle - aus dem Zweidiodenmodell abgeleitet werden können. Diese Methode basiert auf Grundgleichungen der Physik und lässt sich auf jeden beliebigen Solarzellentyp anwenden. Ein weiterer Schwerpunkt dieser Arbeit lag in der Durchführung einer detaillierten Messunsicherheitsanalyse für die Kalibrierung von 3J-Zellen, wie sie im Messlabor des Fraunhofer ISE (ISE CalLab) durchgeführt wird. Diese zeichnet sich dabei durch eine in hohem Maße systematische und präzise Vorgehensweise aus, wodurch sie ein sehr genaues Verständnis für die komplexen Zusammenhänge innerhalb einer Mehrfachsolarzelle liefert. Die unterschiedlichen Einflussgrößen wurden dabei soweit wie möglich in Unsicherheiten der Photoströme der einzelnen Teilzellen umgerechnet. Die Schwierigkeit bestand dann letztlich darin, die Auswirkungen dieser Unsicherheiten auf die Zellparameter der kompletten 3J-Zelle zu bestimmen. Mittels Anwendung der spektrometrischen Charakterisierung wurde der Einfluss dieser Unsicherheiten auf die Zellparameter experimentell bestimmt. Das Ergebnis der Messunsicherheitsanalyse zeigte, dass die Messunsicherheiten bei der Kalibrierung von 3J-Zellen nur wenig verschieden sind von denen, die sich typischerweise für Einfachsolarzellen ergeben. Neben den beiden Schwerpunkten Messunsicherheitsanalyse und Degradationsstudie wurde im Rahmen dieser Arbeit bei der Entwicklung der aktuell standardmäßig eingesetzten 3J-Zelle von RWE SSP (RWE3G28) maßgeblich mitgewirkt. Eine präzise Charakterisierung und eine genaue Analyse bzw. Modellierung der Messergebnisse ist hierbei von Bedeutung, um die Richtung für den nächsten Optimierungsschritt festzulegen. Eine mögliche neue Charakterisierungsmethode zur Bestimmung oder Abschätzung von Lebensdauern von Minoritäten wurde in der Auswertung des durch optische Kopplung entstehenden Signals bei der Messung der spektralen Empfindlichkeit von Ge Komponentenzellen identifiziert. Eine quantitative Ableitung der Lebensdauern aus diesem Modell bedarf allerdings zunächst einer genauen Kenntnis der Materialstruktur der Komponentenzelle. Darüber hinaus müssen die Abhängigkeiten des Photonrecyclings von verschiedenen Einflussgrößen (in Abhängigkeit der Materialstruktur) bekannt sein. Am Beispiel der Einflussgröße Bestrahlung (spektrale Verteilung und Bestrahlungsintensität) an zwei verschiedenen Schichtstrukturen zeigten sich gegensätzliche Abhängigkeiten von dieser Einflussgröße auf das durch optische Kopplung entstehende Signal. Erste Erklärungsmöglichkeiten für die verschiedenen Abhängigkeiten wurden beschrieben. Schließlich wurden aktuelle Forschungsschwerpunkte beschrieben, die das Potential haben, zu einer Verbesserung von Wirkungsgrad und/oder Strahlungsstabilität von III-V Mehrfachsolarzellen zu führen. Wichtig ist dabei vor allem die Entwicklung des 1 eV Materials aus der Materialkombination (GaIn)(NAs). Deutliche Verbesserungen der Materialqualität von (GaIn)(NAs) durch Anpassung der Annealingbedingungen führten auf Solarzellenstrukturen, die zumindest in einer 6J-Zelle schon gewinnbringend eingesetzt werden könnten. Langfristiges Ziel ist die Entwicklung einer 4J-Zelle, für die die Qualität der (GaIn)(NAs)-Zelle jedoch noch deutlich verbessert werden muss. With this work an important contribution in the field of characterisation of multi-junction solar cells and in the understanding of the degradation behaviour of III-V multi-junction cells for space applications has been made. An extensive degradation study has been performed on the current state-of-the-art space solar cell of the European manufacturer RWE SSP (RWE3G28). Thereby, the applicability of available methods for the prediction of solar cell degradation in space due to particle irradiation has been investigated on multi-junction solar cells. It was demonstrated that a direct transfer of those methods, which were previously developed only for single-junction solar cells, to multi-junction solar cells is not possible in a strict sense. A correct extension both, from a mathematical and physical point of view, of the so called JPL (Jet Propulsion Laboratory) and especially of the NRL (Naval Research Laboratory) method for the prediction of the degradation behaviour of solar cells in space requires the knowledge of the degradation characteristics of all the subcells of the multi-junction stack under investigation. This was proven by the parallel investigation of the degradation behaviour of component cells. The knowledge of the degradation behaviour of component cells and therefore the subcells in a multi-junction stack allows the extension of the existing models to arrive at a consistent description of the degradation behaviour of the more complex 3J or multi-junction solar cells. By analysing dark I-V characteristics of component cells before and after irradiation a new method was developed which allows the derivation of the light I-V parameters and their degradation behaviours from applying the two-diode model which represents a mathematical description of the I-V characteristic of solar cells. This method is based on fundamental physics and is easy to apply to any kind of multi-junction solar cell. Another main focus of this work was put on the conduction of a systematic uncertainty analysis for the calibration of multi-junction solar cells as it is performed at the calibration lab of the Fraunhofer ISE (ISE CalLab). Thereby, the complex interactions between the different subcells of a multi-junction solar cell were taken into account. This was done by first converting all influencing variables into uncertainties of the photocurrents of the different subcells. The impact of those uncertainties on the uncertainties of the relevant parameters of the multi-junction solar cells was then derived experimentally by carrying out spectrometric characterisations. Thereby, the changes of the solar cell parameters were monitored when changing the photocurrents of one of the subcells. The results showed that the uncertainties of the solar cell parameters when measuring multi-junction solar cells are close to the ones typically obtained when measuring single-junction solar cells. Apart from the main topics of this work uncertainty analysis and degradation study significant contributions were made for the development of the current state-of-the-art 3J solar cell from RWE SSP (RWE3G28). It was demonstrated that precise characterisations and correct interpretation of the measurement results are of great importance in defining the direction for the next development steps. A new possibility for deriving or assessing the lifetimes of the minority carriers in solar cells was identified by analysing the signals obtained within spectral response measurements of Ge component cells due to optical coupling. However, to quantify the result a very detailed knowledge of the structure of the component cell is required. Moreover, the dependencies of the optical coupling or photon recycling on various influencing parameters (in dependence of the structure) have to be known. Taking the insolation (spectral distribution and intensity) as an example for an influencing parameter, opposing dependencies of the signal in the spectral response measurement due to optical coupling were obtained for two different structures. First possibilities that explain these differences are described. Finally, the main fields of interest of current research programs were described which have the potential of improving efficiency and/or the radiation stability of III-V multi-junction solar cells. As an important point hereby, the development of a 1 eV material based on the material combination (GaIn)(NAs) has been identified. Significant improvements of the material quality of (GaIn)(NAs) have been achieved by applying appropriate annealing steps after the growth. These improvements would allow already a profitable introduction of a (GaIn)(NAs) subcell into a 6J solar cell. Nevertheless, the aim in the long-term is the development of a 4J solar cell for which the quality of the (GaIn)(NAs) subcell still has to be improved.
The open circuit voltages Voc of individual subcells in a multijunction solar cell are measured by illuminating a given subcell with a pulse of spatially homogeneous, nearly monochromatic light with a rising edge in the μs regime. The influence of luminescent coupling and semi-transparency on Voc is eliminated by over-illuminating all subcells below this subcell with a preceding light pulse. By using a suns-Voc approach, the two-diode model dark saturation currents of each subcell are extracted. The proposed method is verified experimentally as well as through simulations on three and four-junction solar cells.
We demonstrate a photoluminescence (PL)-based contactless method to determine the current–voltage (I-V) characteristics of the individual subcells in a multijunction solar cell. The method relies upon the reciprocity relation between the absorption and emission properties on a solar cell. Laser light with a suitable energy is used to excite carriers selectively in one junction (1J), and the internal voltages are deduced from the intensity of the resulting luminescence. The I–V curves obtained this way on 1J, 2J, and 6J devices are compared with those obtained using electroluminescence (EL). Good agreement is obtained at high-injection conditions, while discrepancies at low injection are attributed to in-plane carrier transport.
A method for the determination of the subcell I-V characteristics of multijunction solar cells in the presence of optical coupling is presented and applied to a Ga0.50In0.50P/Ga0.99In0.01As/Ge triple-junction solar cell. Each of the subcells is described by a two-diode model and can be illuminated by a narrowband light source externally. Optical coupling is then used explicitly to generate current in one subcell, which is not illuminated externally. This approach yields the magnitude of optical coupling and a relationship between the two diode parameters of each subcell. The remaining cell parameters are determined with the help of pulsed illumination. In this fashion, the open circuit voltage of individual subcells is accessible, despite the fact that not all junctions are illuminated.
Consolidated tables showing an extensive listing of the highest independently confirmed efficiencies for solar cells and modules are presented. Guidelines for inclusion of results into these tables are outlined, and new entries since July 2015 are reviewed.
AZUR's latest lattice-matched triple junction space solar cell product has been designed for a high EOL efficiency by focusing the development on radiation hardness right from the beginning. These solar cells named as 3G30-Advanced show an efficiency of 29.5% at BOL and of 28.1% for EOL at 5E14 (1MeV) e-/cm2 (AM0, 1367 W/m2, 28°C). Meanwhile more than 125,000 cells of this cell type have been delivered to customers worldwide. Cell thicknesses of 150 μm, 80μm or even as thin as 20μm are available. Besides the standard size of 8×4 cm2 (with cropped corners) larger sizes such as 8×8cm2 and 12×6 cm2 are also available and provide equal performance. AZUR's next generation product will comprise a metamorphic 4-junction device targeting at 30% efficiency at EOL to be qualified in 2015/16. Furthermore, for arriving at more than 30% EOL efficiency with 4 - 6 junction solar cells in a subsequent step, appropriate lattice-matched 1eV dilute nitride materials, inversely grown solar cells and semiconductor bonded cells are studied in cooperation with external partners.
The open circuit voltage of a single subcell in a multijunction cell stack can be measured with the help of pulsed, millisecond illumination. This concept makes use of the fact that the charging of the non-illuminated cell capacitances takes place on a much longer timescale than of the illuminated one. Optical coupling introduces a photocurrent in the subcell underneath. Its efficiency can be quantified in parallel under short circuit conditions. A suns-Voc approach, applied to this subcell pair, yields all relevant diode parameters. Applied to all subcells of a Ga0.50In0.50P/Ga0.99In0.01As/Ge triple junction cell, a very good match to the dark I-V curve is obtained.
The assumption of superposition or linearity of photocurrent with solar flux is widespread for calculations and measurements of solar cells. The well-known effect of luminescent coupling in multijunction solar cells has also been assumed to be linear with excess current. Here we show significant non-linearities in luminescent coupling in III-V multijunction solar cells and propose a simple model based on competition between radiative and nonradiative processes in the luminescent junction to explain these non-linearities. We demonstrate a technique for accurately measuring the junction photocurrents under a specified reference spectrum, that accounts for and quantifies luminescent coupling effects.
Micro-lenses and micro-lens arrays are widely used for various applications. Monolithic arrays of cylindrical lenslets made of glass, semiconductors or crystals provide great advantages to laser applications, e.g. high efficiency, intensity stability and very low absorption. However, up to now, mainly symmetrical micro-lens surfaces are utilized in most applications due to design and manufacturing restrictions. The manufacture and application benefits of asymmetrical cylindrical-like micro-lens surfaces are enabled by LIMO's unique production technology. The asymmetrical shape is defined by uneven-polynomial terms and/or an asymmetrical cut-off from an even polynomial surface. Advantages of asymmetrical micro-lenses are off-axis light propagation, the correction of aberration effects or intensity profile deformations when the illuminated surfaces are not orthogonal to the optical axis. First application results of such microlens arrays are presented for beam shaping of high power diode lasers. The generation of a homogeneous light field by a 100 W laser with tilted illumination under an angle of 30-50° is shown. A homogeneity of better than 90% was achieved for a field size of 270 mm x 270 mm. In laser direct write processes a top hat profile has several advantages compared to a Gaussian beam profile, especially the throughput of the system and quality of the structures can be improved. Novel patterning results with TopHat-converted single mode lasers and a special Gaussian-to-TopHat galvo scan system are demonstrated for solar cell technology.
A new circuit simulation program, SPICE, is described. The simulation capabilities of nonlinear dc analysis, small signal analysis, and nonlinear transient analysis are combined in a nodal analysis program to yield a reasonably general purpose electronic circuit simulation program. Particular emphasis is placed upon the circuit models for the BJT and the FET which are implemented in SPICE.
A rigorous proof for a reciprocity theorem that relates the spectral and
angular dependences of the electroluminescence of solar cells and light
emitting diodes to the spectral and angular quantum efficiency of
photocarrier collection is given. An additional relation is derived that
connects the open circuit voltage of a solar cell and its
electroluminescence quantum efficiency.
Written by scientists from leading institutes in Germany, USA and Spain who use these techniques as the core of their scientific work and who have a precise idea of what is relevant for photovoltaic devices, this text contains concise and comprehensive lecture-like chapters on specific research methods. They focus on emerging, specialized techniques that are new to the field of photovoltaics yet have a proven relevance. However, since new methods need to be judged according to their implications for photovoltaic devices, a clear introductory chapter describes the basic physics of thin-film solar cells and modules, providing a guide to the specific advantages that are offered by each individual method. The choice of subjects is a representative cross-section of those methods enjoying a high degree of visibility in recent scientific literature. Furthermore, they deal with specific device-related topics and include a selection of material and surface/interface analysis methods that have recently proven their relevance. Finally, simulation techniques are presented that are used for ab-initio calculations of relevant semiconductors and for device simulations in 1D and 2D. For students in physics, solid state physicists, materials scientists, PhD students in material sciences, materials institutes, semiconductor physicists, and those working in the semiconductor industry, as well as being suitable as supplementary reading in related courses.
A simple method for implementing the steady-state photoconductance technique for determining the minority-carrier lifetime of semiconductor materials is presented. Using a contactless instrument, the photoconductance is measured in a quasi-steady-state mode during a long, slow varying light pulse. This permits the use of simple electronics and light sources, Despite its simplicity, the technique is capable of determining very low minority carrier lifetimes and is applicable to a wide range of semiconductor materials. In addition, by analyzing this quasi-steady-state photoconductance as a function of incident light intensity, implicit current-voltage characteristic curves can be obtained for noncontacted silicon wafers and solar cell precursors in an expedient manner. (C) 1996 American Institute of Physics.
We analyze electroluminescence spectra of a Ga In P / Ga In As / Ge triple-junction solar cell at different injection currents. Using the reciprocity theorem between electroluminescent emission and external quantum efficiency of solar cells allows us to derive the current/voltage curves and the diode quality factors of all individual subcells.
The letter describes a method for determining the photovoltage, i.e., the quasifermi level splitting at open-circuit conditions, associated with each subcell in a series connected multijunction solar cell structure by voltage-dependent capacitance analysis. Experimental verification and accuracy analysis of subcell photovoltage determination is provided for a 6-tuple AlGaInP/GaInP/AlGaInAs/GaInAs/GaInNAs/Ge solar cell device.
The quasi-static capacitance-voltage ( C-V) technique measures the dependence of junction capacitance on the bias voltage by applying a slow, reverse-bias voltage ramp to the solar cell in the dark, using simple circuitry. The resulting C-V curves contain information on the junction area and base dopant concentration, as well as their built-in potential. However, in the case of solar cells made on low to medium resistivity substrates and having thick emitters, the emitter dopant profile has to be taken into account. A simple method can then be used to model the complete C-V curves, which, if the base doping is known, permits one to estimate the emitter doping profile. To illustrate the method experimentally, several silicon solar cells with different base resistivities have been measured. They comprise a wide range of areas, surface faceting conditions and emitter doping profiles. The analysis of the quasi-static capacitance characteristics of the flat surface cells resulted in good agreement with independent data for the wafer resistivity and the emitter doping profile. The capacitance in the case of textured surfaces is a function of the effective junction area, which is otherwise difficult to measure, and is essential to understand the emitter and space charge region recombination currents. The results indicate that the effective area of the junction is not as large as the area of the textured surface.