Influence of Defects on Solar Cell Characteristics
TL;DR: In this article, the authors reviewed the present knowledge of the origin of non-ideal I-V characteristics of silicon solar cells and introduced new results on recombination involving coupled defect levels.
Abstract: The current-voltage (I-V) characteristics of most industrial silicon solar cells deviate rather strongly from the exponential behavior expected from textbook knowledge. Thus, the recombination current may be orders of magnitude larger than expected for the given material quality and often shows an ideality factor larger than 2 in a wide bias-range, which cannot be explained by classical theory either. Sometimes, the cells contain ohmic shunts although the cell’s edges have been perfectly insolated. Even in the absence of such shunts, the characteristics are linear or super-linear under reverse bias, while a saturation would be classically expected. Especially in multicrystalline cells the breakdown does not tend to occur at -50 V reverse bias, as expected, but already at about -15 V or even below. These deviations are typically caused by extended defects in the cells. This paper reviews the present knowledge of the origin of such non-ideal I-V characteristics of silicon solar cells and introduces new results on recombination involving coupled defect levels.
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TL;DR: This review paper addresses nondestructive testing techniques that are used to detect microfacial and subfacial cracks in bulk solar cells and uses the multi-attribute decision-making method to evaluate the different inspection tools that are available on the market.
Abstract: Microcracks at the device level in bulk solar cells are the current subject of substantial research by the photovoltaic (PV) industry. This review paper addresses nondestructive testing techniques that are used to detect microfacial and subfacial cracks. In this paper, we mainly focused on mono- and polycrystalline silicon PV devices and the root causes of the cracks in solar cells are described. We have categorized these cracks based on size and location in the wafer. The impact of the microcracks on electrical and mechanical performance of silicon solar cells is reviewed. For the first time, we have used the multi-attribute decision-making method to evaluate the different inspection tools that are available on the market. The decision-making tool is based on the analytical hierarchy process and our approach enables the ranking of the inspection tools for PV production stages, which have conflicting objectives and multi-attribute constraints.
101 citations
Cites background from "Influence of Defects on Solar Cell ..."
...[11], [40] reported that such cracks could act as a linear or nonlinear edge shunt, and that cracks in processed solar cells led to a...
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TL;DR: By evaluating dark lock-in thermography images taken at one reverse and three forward biases, images of all two-diode-parameters J01, J02, n (ideality factor of J02), and Gp (the parallel Ohmic conductivity) of the dark current-voltage characteristic are obtained as discussed by the authors.
96 citations
Cites background from "Influence of Defects on Solar Cell ..."
...The analysis of many global and local I-V characteristics of solar cells leads to n2 > 2 [2-4, 7], which cannot be explained by standard point defect recombination theory....
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...It had been shown previously that ideality factors larger than 2 may be caused by recombination via extended defects [7]....
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01 Jan 2013
TL;DR: In this article, a lognormal analysis is applied to the accelerated lifetime test data, considering failure at 80% of the initial module power, and the probability of module failure at an arbitrary temperature is predicted.
Abstract: Acceleration factors are calculated for crystalline silicon photovoltaic modules under system voltage stress by comparing the module power during degradation outdoors with that in accelerated testing at three temperatures and 85% relative humidity. A lognormal analysis is applied to the accelerated lifetime test data, considering failure at 80% of the initial module power. Activation energy of 0.73 eV for the rate of failure is determined for the chamber testing at constant relative humidity, and the probability of module failure at an arbitrary temperature is predicted. To obtain statistical data for multiple modules over the course of degradation in situ of the test chamber, dark I–V measurements are obtained and transformed using superposition, which is found to be well suited for rapid and quantitative evaluation of potential-induced degradation. It is determined that shunt resistance measurements alone do not represent the extent of power degradation. This is explained with a two-diode model analysis that shows an increasing second diode recombination current and ideality factor as the degradation in module power progresses. Failure modes of the modules stressed outdoors are examined and compared with those stressed in accelerated tests.
80 citations
Cites background or methods from "Influence of Defects on Solar Cell ..."
...density of defect states in the p-n junction [21]....
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...Rsh and all the other parameters in the model, including pre-exponentials Jo1 and Jo2 and ideality factors n1 and n2 within the first and second diode terms referred to, respectively, as the diffusion and recombination current densities [21], and the area-specific series resistance Rs , were varied to achieve fitting of the dark I–V curve....
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TL;DR: In this article, a thermochemical treatment (TT) for CuSbS2 thin films was developed, which consists of annealing in Sb2S3 vapor followed by a selective KOH surface chemical etch.
Abstract: CuSbS2 is a promising nontoxic and earth-abundant photovoltaic absorber that is chemically simpler than the widely studied Cu2ZnSnS4. However, CuSbS2 photovoltaic (PV) devices currently have relatively low efficiency and poor reproducibility, often due to suboptimal material quality and insufficient optoelectronic properties. To address these issues, here we develop a thermochemical treatment (TT) for CuSbS2 thin films, which consists of annealing in Sb2S3 vapor followed by a selective KOH surface chemical etch. The annealed CuSbS2 films show improved structural quality and optoelectronic properties, such as stronger band-edge photoluminescence and longer photoexcited carrier lifetime. These improvements also lead to more reproducible CuSbS2 PV devices, with performance currently limited by a large cliff-type interface band offset with CdS contact. Overall, these results point to the potential avenues to further increase the performance of CuSbS2 thin film solar cell, and the findings can be transferred t...
62 citations
TL;DR: In this paper, a two-diode model with an analytically given variable series resistance is proposed, which may describe both the dark and the illuminated characteristic up to large current densities in good approximation with one and the same physically meaningful parameter set.
60 citations
Cites background from "Influence of Defects on Solar Cell ..."
...In contrast to this strong material-induced influence on J01, the recombination current density, which is described by J02 and n2 in the dark I–V curve, is essentially process-related, since it is mostly caused by extended defects crossing the p–n junction as cracks or badly passivated cell edges [21]....
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...As already mentioned before, the different parameters obtained from the I–V curves characterizes effects with different physical origins [21]....
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References
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TL;DR: In this article, nine different types of shunt have been found in state-of-the-art mono and multicrystalline solar cells by lock-in thermography and identified by SEM investigation.
Abstract: Nine different types of shunt have been found in state-of-the-art mono- and multicrystalline solar cells by lock-in thermography and identified by SEM investigation (including EBIC), TEM and EDX. These shunts differ by the type of their I–V characteristics (linear or nonlinear) and by their physical origin. Six shunt types are process-induced, and three are caused by grown-in defects of the material. The most important process-induced shunts are residues of the emitter at the edge of the cells, cracks, recombination sites at the cell edge, Schottky-type shunts below grid lines, scratches, and aluminum particles at the surface. The material-induced shunts are strong recombination sites at grown-in defects (e.g., metal-decorated small-angle grain boundaries), grown-in macroscopic Si3N4 inclusions, and inversion layers caused by microscopic SiC precipitates on grain boundaries crossing the wafer. Copyright © 2004 John Wiley & Sons, Ltd.
274 citations
"Influence of Defects on Solar Cell ..." refers background in this paper
...The different types of ohmic shunts appearing in silicon solar cells have been discussed already in [24]....
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TL;DR: In this paper, the effect of the radius of curvature of the metallurgical junction (or junction curvature) on avalanche breakdown voltage is computed numerically for spherical and cylindrical p-n junctions in Ge, Si, GaAs and GaP.
Abstract: The effect of the radius of curvature of the metallurgical junction (or junction curvature) on avalanche breakdown voltage is herein computed numerically for spherical and cylindrical p-n junctions in Ge, Si, GaAs and GaP. Three types of impurity distribution have been considered: abrupt, linearly graded and composite. For the composite distribution the space charge terminates in a graded region on one side of the junction and in a uniformly doped region on the other side.
The results, which are presented graphically, show that for abrupt junctions the breakdown voltage, VB, decreases with decreasing radius of curvature, rj, the effect being larger for spherical junction than for cylindrical junction. Typically, for Si abrupt junctions with a background doping of 1015 cm−3, VB is 330 V for a plane junction, 80 V for a cylindrical junction with rj = 1 μ, and 39 V for a spherical junction with the same junction radius. For linearly graded junctions however, VB is essentially independent of rj. For composite junctions, at small impurity gradient VB is nearly equal to that of linearly graded junctions; at large impurity gradient, however, VB is determined by rj and the background doping.
The results presented here can be applied to junctions formed by planar technology where the edge of the junction has cylindrical and/or spherical geometry with radius of curvature approximately equal to the junction depth. It is further suggested that the radius of curvature is an important design parameter in high power and higher frequency semiconductor devices as well as in integrated circuits.
250 citations
"Influence of Defects on Solar Cell ..." refers background in this paper
...There, due to field enhancement by the tip effect, the avalanche breakdown voltage reduces locally from -50 V to -13 V [23]....
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Book•
01 Jan 2003
TL;DR: In this article, physical and technical basics, experimental technique, theory, measurement strategies, and typical applications are presented for measurement strategies and their application in the field of computer vision..
Abstract: Introduction.- Physical and Technical Basics.- Experimental Technique.- Theory.- Measurement Strategies.- Typical Applications.- Summary and Outlook.- References.- Appendices.- Index.
176 citations
TL;DR: In this paper, the steady state recombination rate for two coupled defect levels and implemented the model into a device simulator, where the field effect was discussed in terms of tunnel assisted multiphonon capture or direct tunneling into the levels, respectively.
Abstract: We calculated the steady‐state recombination rate for two coupled defect levels and implemented the model into a device simulator. This model generalizes the familiar single‐level Shockley–Read–Hall (SRH) formula. If the intercenter transition probability vanishes, it reduces to the sum of two individual SRH rates, which are only linked via the band occupancies. The cases, where one of the levels or even both behave like traps in carrier capture, and the case of a rate‐limiting intercenter transition are derived from the general expression. The important feature of the model is a possible increased field effect which might lead to large excess currents. The field effect is discussed in terms of tunnel‐assisted multiphonon capture or direct tunneling into the levels, respectively. We show by means of numerical simulation that the large ideality factors found for liquid phase epitaxy grown diodes with weak intrinsic fields can be the result of a rapid direct charge transfer between donors and acceptors and ...
113 citations
"Influence of Defects on Solar Cell ..." refers background in this paper
...Since the spatial extension of the wave function of deep levels is very small (typically some nm), Schenk and Krumbein assumed that at least one shallow level participates, whose wave function is considerably more extended....
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...Schenk and Krumbein [16] have tried to solve this problem by assuming two-level (pair) a b recombination with participation of at least one shallow level....
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...However, since single shallow levels show a very weak coupling to the more distant band, Schenk and Krumbein had to assume infinitely strong coupling between the shallow level and a second deep level or a complimentary shallow level....
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...Schenk and Krumbein [16] already considered recombination via two coupled levels as a cause for the observed recombination current....
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...Schenk and Krumbein [16] have tried to solve this problem by assuming two-level (pair) a b Solid State Phenomena Vols....
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TL;DR: In this paper, a systematic study of these regions by various electron microscopy techniques has shown that the avalanche breakdown occurs at cone-shaped holes, located at dislocations and caused by acidic texture etch.
Abstract: Multicrystalline silicon solar cells typically show hard breakdown beginning from about –13 V bias, which leads to the well-known hot-spot problem. Using special lock-in thermography techniques, hard breakdown has been found to occur in regions of avalanche multiplication. A systematic study of these regions by various electron microscopy techniques has shown that the avalanche breakdown occurs at cone-shaped holes, located at dislocations and caused by acidic texture etch. At their bottom, these etch pits lead to a strongly curved p–n junction exhibiting an electrostatic tip effect which quantitatively explains the field enhancement needed for enabling avalanche breakdown. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
96 citations
"Influence of Defects on Solar Cell ..." refers background in this paper
...The mechanism of breakdown "III" has been identified as avalanche breakdown at the tips of dislocation-induced etch pits [20]....
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...In reality, the characteristics may become superlinear already at -5 V, and at about -13 V hard breakdown begins [20]....
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