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Determination of subcell open circuit voltages and Iph–Voc curves in multijunction solar cells by sequentially pulsed, monochromatic illumination
Applied Physics Letters
Abstract
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.
... This fact does notvallow directly obtaining subcell dark IV-curve. Simultaneously, several methods are already developed to solve this problem using additional measurements [1][2][3][4][5][6]. The first method is based on analyzing the total I -V characteristic of multi-junction solar cells under various conditions [1]. ...
... Comparison searches for such a wavelength range in which the Rau's [7] condition is satisfied. The third method [5,6] is based on exposure of individual subcell to a light pulse and recording and analyzing the time dependences of the photogenerated current and open-circuit voltage. In contrast to the previous two, this method considers the effect of luminescent coupling, which can lead to a significant error in the measurement of the characteristics of subcells [8][9][10]. ...
... So, direct use of subcell EL intensities particularly allows taking into account the luminescent coupling effect. Unlike the third type of existing methods [5,6], this does not require additional measurements A significant drawback of the method's current implementation is the complexity of measuring the electroluminescence intensity with the accuracy required for the method. An experimental study of the method showed that the error in measuring the IV characteristic could be on the order of 0.2 volts. ...
An electroluminescent method for determining the IV characteristics of subcells in multi-junction solar cells has been experimentally investigated. The method makes it possible to determine the coefficients L0 for sub-elements, which determine the relationship between the voltage at the p-n junction and the intensity of electroluminescence. The method is based on determining the voltage and subcells’ electroluminescence intensity under conditions when one of the subcells is in the short-circuit current mode. Applying the method is substantiated theoretically and studied experimentally on a prototype double-junction solar cell.
... Several techniques have been developed to overcome this limitation and to obtain information at a subcell level, e.g., the external quantum efficiency 2 or the subcell current voltage characteristics. 3,4 The subcell capacitance C i is also not directly accessible, as the externally measured cell capacitance C is expressed as 1=C ¼ P 1=C i . In addition to several destructive capacitance measurement methods, 5,6 non-destructive standard methods of measuring the total cell capacitance C are small signal AC capacitance measurements using an impedance analyzer 7 and time domain techniques, in which the illuminated cell is released from the short circuit to open circuit operating conditions while the gradient dV/dt 8,9 is measured. ...
... The measurement sequence is similar to Ref. 4. To measure subcell J4, the 1450 nm LED is turned on, generating a very low photocurrent density in J4. ...
... This is illustrated by the green, red, and black curves in Fig. 2. As documented in Table I, the photocurrent created by the laser illumination is at least two orders of magnitude higher than the ones created by the LEDs. Using the identical rationale as presented in Ref. 4, the impact of optical coupling on the measured voltage is less than 0.5 mV in this case and thus negligible. The LED-induced voltage signal therefore only originates from the subcell of interest. ...
A method for measuring subcell capacitance voltage (C–V) in a multijunction solar cell is introduced. The subcell of interest is illuminated by a monochromatic light pulse with a ns rise time. The subcell capacitance is calculated from the measured rise time of the solar cell voltage. The effect of optical coupling is eliminated by introducing a high intensity bias illumination to all subcells below the one measured. The method is verified by comparing the subcell capacitance obtained from four junction solar cells with the results from corresponding component cells, which can be measured using well-established methods. From the C–V curves, the built-in voltage and the base layer doping density for each subcell are calculated.
... We recently proposed a completely different approach to gain access to subcell characteristics: the pulsed photocurrent versus open-circuit voltage (I ph − V oc ) method [7]. The method is based on measuring the subcell V oc by using pulsed, monochromatic, and spatially homogeneous illumination. ...
... The open-circuit voltage V n oc of subcell n is measured with sequentially pulsed illumination [7]. If either one or multiple light sources are switched on, the initial voltage response of the cell is the sum of the directly and indirectly (if either LC or ST are present) illuminated subcell voltages, i.e., the voltage of the primary absorbing subcells and all subcells below. ...
... This is done for the same laser powers at which the different I bot ph were determined. To gain access to V mid oc , sequential pulsing is used to suppress the impact of LC and ST [7]. First, the 1470 nm laser is switched on with maximum laser power as bias light, resulting in a voltage plateau. ...
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.
... Due the logarithmic dependence of open circuit voltage on photocurrent this introduces a considerable additional voltage contribution and completely distorts the voltage measurement. To circumvent this problem, a sequence of light pulses is used [5]. First, the subcell i-1 is flooded with a high intensity light pulse. ...
... 5. J4 is shown in black, J3 in red,J2 in green and J1 in blue. The intersection of the linearly extrapolated curve with the voltage axis indicates the built in voltage ( -50 mV) according to(5). ...
... The experimental procedure to measure the subcell capacitance is based on a measurement of the subcell voltage as it has been presented in Ref. [14]. There, a MJSC is illuminated using a sequence of monochromatic light pulses. ...
... To eliminate the LC contribution, the reabsorbing subcell is illuminated by a high intensity laser such that the additional LC photocurrent contribution becomes negligible. This is the same concept, which was also used for the extraction of subcell current voltage curves in Ref [14]. ...
The evolution of the subcell capacitance of triple junction solar cells upon 3 MeV electron irradiation is studied. Photocurrent is induced subcell specific using a sequence of spatially homogeneous, monochromatic light pulses. The used method is based on the externally measured transient voltage, which allows to calculate the depletion layer capacitance. Capacitance-voltage curves for each subcell are obtained. From these curves, the base layer effective doping density and thus the carrier removal rate are extracted for the top and middle subcell. Additionally, the built-in voltage and its degradation is measured.
A new experimental method for obtaining resistive-less dark current-voltage (IV) characteristic of multi-junction solar cells has been developed. The method realizes by measuring dark voltage, dark current, and additional measurements of the electroluminescence (EL) intensity of subcells. It has been shown that in the resistive-less mode (low currents), the dark voltage depends logarithmically on the product of EL intensities of subcells. Experimental definition of this dependence has made it possible to determine the relationship between the intensities and the voltage. The method has been applied to a GaInP/GaAs solar cell, in which three effects have an influence on IV characteristic: the mismatch of photocurrents generated by subcells, the nonlinear series resistance, and counter-electromotive force. It has been experimentally found that the result of the method does not depend on these effects and allows obtaining a fundamental dark resistive-less IV characteristic of multi-junction solar cells.
The IV characteristics of previously found structure fragment with not optimized isotype hetero-barriers in the bottom connecting part of triple-junction GaInP/GaAs/Ge solar cell has been investigated. It has been shown that there are various ways to obtain the IV-curves of such fragment, including by creating structures containing only an isotype substrate and layers forming the barriers. It has been found that if hetero-barrier fragment contacts with the p-n junction layers, the shape of its reverse IV characteristic dramatically changes and depends on the incident light intensity.
Laser power converters (LPCs) based on InxGa1-xAs/GaAs metamorphic heterostructure grown by MOVPE have been investigated. Both direct study of transmission electron microscopy images and analysis of the dark I–V curves at a low photogenerated current have shown the absence of the effect of threading dislocations from the metamorphic buffer on the LPC parameters. It has been shown, according to the “voltage” and “current” invariants that voltage loss in all investigated LPCs evidences the good quality of p-n junctions for the operating mode of high power laser radiation conversion. It has been also demonstrated that the method of saturation current approximation with current invariant allows obtaining the limit voltage losses irrespective of photogenerated current. Band-gap and thickness of the InGaAs LPCs have been adjusted using spectral characteristics. Fundamental losses (thermal and resistive) and ways to minimize them have been discussed. As a result, LPCs based on In0.23Ga0.77As have demonstrated the laser conversion efficiency (λ = 1064 nm) of more than 50% with maintaining the efficiency of more than 48% up to 13 W/cm².
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.
Further development and optimization of modern optoelectronic devices requires fast and reliable procedures which may evaluate the quality of interfaces. For thick multilayer devices mixing effect significantly may prevent proper interpretation of secondary ion mass spectrometry depth profiles especially if a region of interest is located far from the sample surface. In this work we present how to overcome this problem with a so called a-crater-within-a-crater approach. In this notion a high energetic primary ion beam is used to rapidly remove most of the material forming a large crater. Then the energy is significantly reduced and a new smaller crater is formed at the bottom of the previous one. Close to the region of interest the impact energy is decreased to 150eV and thus an interface can be analyzed with minimal mixing effect and thus its quality can be adequately assessed. Usefulness of this approach is tested on an epitaxial structure of a triple-junction solar cell and reliable information about the structure imperfection has been obtained: p and n dopants in the tunnel junction overlapped deteriorating the operation of the device.
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.
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.
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.
A combined electroluminescence and photoluminescence setup for a fast, nondestructive, and high-resolution characterization of large-area lattice matched GaInP
2
/Ga(In)As/Ge triple-junction space solar cells was developed. In contrast with electroluminescence, where coupling effects between the subcells appear, all subcells can be analyzed independently in photoluminescence imaging, using three monochromatic light sources. This will be demonstrated by way of example for a partly irradiated cell. Alternatively, electroluminescence combined with simulation program with integrated circuit emphasis network simulations constitutes a powerful tool to extract quantitative information despite the coupling effects. Sheet resistances and shunts in individual subcells can be accurately modeled with this method.
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.
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.
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