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Measurement of Subcell Capacitance in Triple Junction Solar Cells with Pulsed Illumination
Abstract
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.
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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.
World-wide studies on multi-junction (tandem) solar cells have led to record-breaking improvements in conversion efficiencies year after year. To obtain detailed and proper feedback for solar-cell design and fabrication, it is necessary to establish standard methods for diagnosing subcells in fabricated tandem devices. Here, we propose a potential standard method to quantify the detailed subcell properties of multi-junction solar cells based on absolute measurements of electroluminescence (EL) external quantum efficiency in addition to the conventional solar-cell external-quantum-efficiency measurements. We demonstrate that the absolute-EL-quantum-efficiency measurements provide I-V relations of individual subcells without the need for referencing measured I-V data, which is in stark contrast to previous works. Moreover, our measurements quantify the absolute rates of junction loss, non-radiative loss, radiative loss, and luminescence coupling in the subcells, which constitute the "balance sheets" of tandem solar cells.
Precisely how the short circuit current (JSC) is produced in a proton irradiated n+p InP/Si solar cell at very high fluence levels has been determined from combined measurements of the cell structure using electrochemical capacitance-voltage profiling and detailed analysis of the spectral quantum efficiency. Type conversion in the base region of the cell is shown to occur before an anomalous peak in the degradation curve for JSC is reached at high damage levels. The short circuit current, and hence the cell efficiency, ultimately collapse because the high absorption coefficient of InP eventually prevents the generation of electron-hole pairs close enough to the effective cell junction from being collected.
The effect of 1 MeV electron and 3 MeV proton irradiation on the performance of n+p InP solar cells grown heteroepitaxially on Si (InP/Si) substrates is presented. The radiation response of the cells was characterized by a comprehensive series of measurements of current versus voltage (I–V), capacitance versus voltage (C–V), quantum efficiency (QE), and deep level transient spectroscopy (DLTS). The degradation of the photovoltaic response of the cells, measured under simulated 1 sun, AM0 solar illumination, is analyzed in terms of displacement damage dose (Dd) which enables a characteristic degradation curve to be determined. This curve is used to accurately predict measured cell degradation under proton irradiation with energies from 4.5 down to 1 MeV. From the QE measurements, the base minority carrier diffusion length is determined as a function of particle fluence, and a diffusion length damage coefficient is calculated. From the C–V measurements, the radiation-induced carrier removal rate in the base region of the cells is determined. The DLTS data show the electron and proton irradiations to produce essentially the same defect spectra, and the spectra are essentially the same as observed in irradiated homoepitaxial n+p InP. From the DLTS data, the introduction rate of each defect level is determined. From the dark I–V curves, the effect of irradiation on the various contributions to the dark current are determined. The data are analyzed, and a detailed description of the physical mechanisms for the radiation response of these cells is given. The results enable a model to be developed for the radiation response of the cells. © 1997 American Institute of Physics.
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.
The emission of light from each junction in a series-connected multijunction solar cell both complicates and elucidates the understanding of its performance under arbitrary conditions. Bringing together many recent advances in this understanding, we present a general 1-D model to describe luminescent coupling that arises from both voltage-driven electroluminescence and voltage-independent photoluminescence in nonideal junctions that include effects such as Sah-Noyce-Shockley (SNS) recombination with n ≠ 2, Auger recombination, shunt resistance, reverse-bias breakdown, series resistance, and significant dark area losses. The individual junction voltages and currents are experimentally determined from measured optical and electrical inputs and outputs of the device within the context of the model to fit parameters that describe the devices performance under arbitrary input conditions. Techniques to experimentally fit the model are demonstrated for a four-junction inverted metamorphic solar cell, and the predictions of the model are compared with concentrator flash measurements.
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.
Electroluminescence measured at different injection current densities is used to study the effect of particle irradiation on GaInP/GaInAs/Ge triple-junction solar cells. By employing the optoelectronic reciprocity relation, the irradiation-induced degradation in the J–V characteristics of all individual subcells and their underlying diode saturation parameters is derived. Also, the dependence of the solar cell irradiation response on the position of the irradiation-induced non-radiative recombination centers within the cell active region, i.e., the quasi-neutral regions and the space charge region, is discussed.
We monitor the short circuit current and the open circuit voltage of Si, GaAs and GaInP solar cells versus the fluence of 1 MeV electrons. From these data, we extract the fluence dependences of the lifetime and of the concentration of compensating centers. This allows to compare in detail the radiation hardness of cells made with these materials and to modelize their degradation under a given irradiation.
The study presents detailed isothermal and isochronal annealing recovery of photovoltaic parameters in n+/p InGaP solar cells after 1 MeV electron irradiation. Correlation of the solar cells characteristics with changes in the deep level transient spectroscopy data observed in irradiated and annealed n+/p InGaP diodes and solar cells shows that the H2 (Ev+0.50 eV) and H3 (Ev+0.76 eV) defects have a dominant role in governing the minority-carrier lifetime as well as carrier removal. However, capacitance–voltage measurements indicate that other defects must also play a role in the carrier removal process. In addition, the concentration of the H2 defect is found to decay significantly as a result of room temperature storage for 40 days, suggesting that InGaP-based solar cells will display superior radiation tolerance in space. Finally, the deep donor-like-defect H2 is tentatively identified as a phosphorus Frenkel pair.
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.
Degradation modeling of InGaP/GaAs/Ge triple-junction (3J) solar cells subjected to proton irradiation is performed with the use of a one-dimensional optical device simulator, PC1D. By fitting the external quantum efficiencies of 3J solar cells degraded by 30 keV, 150 keV, 3 MeV, or 10 MeV protons, the short-circuit currents (ISC) and open-circuit voltages (VOC) are simulated. The damage coefficients of minority carrier diffusion length (KL) and the carrier removal rate of base carrier concentration (RC) of each sub-cell are also estimated. The values of ISC and VOC obtained from the calculations show good agreement with experimental values at an accuracy of 5%. These results confirm that the degradation modeling method developed in this study is effective for the lifetime prediction of 3J solar cells.
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