Plated copper (Cu) contacts for silicon (Si) solar cells are an attractive alternative material to conventional screenprinted silver, but there are unresolved questions on the long-term integrity of plated contact structures. In this work, we perform characterization on plated Cu contacts from encapsulated cells that were degraded during extended exposure to damp heat (DH) stress. First, using energy-dispersive X-ray spectroscopy, we find evidence of Cu outdiffusion upward through capping layers made of both tin and silver applied with light-induced plating, resulting in a layer of Cu on the outer contact surface. We hypothesize that if Cu is mobile in the module, it may eventually find some route by which to enter the Si cells where it can degrade performance. Subsequently, in several types of Cu-plated, DH-degraded cells, secondary ion mass spectrometry detects elevated levels of Cu at the Si surface and in the Si cell bulk, which suggests that Cu can indeed migrate from contacts into Si over the course of DH stress.
Passivated, carrier-selective contacts have enabled a recent surge in the efficiency of crystalline silicon solar cells by reducing the Shockley-Read-Hall recombination at the electrical contacts for the cell. They operate by allowing the extraction of only the majority carriers from the absorber, i.e., the c-Si wafer. The molybdenum oxide and nickel stack is known to form an effective hole-selective contact. However, the parasitic optical losses they introduced limit the quantum efficiency and therefore efficiency of solar cells featuring these materials. In this work, we introduce photonic nanostructures with reduced parasitic losses in the contacts and enhancement of light trapping in the active area of the cell. The optimized structure shows its potential for increasing photogenerated current density and efficiency as a result.
To assess the reliability of PERC cells compared to Al-BSF in a commercial setting minimodules with cell and encapsulant combinations are compared in accelerated exposure. In both modified damp heat and modified damp heat with full spectrum light exposures, white EVA samples showed a higher susceptibility for metallization corrosion degradation than all other encapsulants. Al-BSF cells in particular showed higher power loss than PERC cells with white EVA. It was observed that the degree of degradation had a strong significance on the manufacturer of the white EVA encapsulant. In both exposures the encapsulant was a much stronger predictor of degradation than cell type. For modules with the same encapsulant, PERC cells showed the higher performance or were comparable to Al-BSF cells for all but one case.
Degradation due to acetic acid in photovoltaic modules has been a commonly observed phenomenon for both damp-heat exposure and outdoor operations. Acetic acid is formed as a decomposition product of ethylene-vinyl acetate (EVA), a common module encapsulation material. It interacts with the front metallization, causing degradation in the glass layer/metal-silicon interface. This results in the loss of cell performance, contact adhesion, increased resistance, dark areas seen in electroluminescence (EL) imaging, etc. It has been reported that lead oxide (PbO) contained in the glass layer is dissolved by acetic acid causing significant performance losses. To address this issue, robust metallic pastes and new metallization techniques are being developed. However, it is important to assess how these technologies perform in acetic acid environment and withstand degradation before they are implemented in the solar market. In this work, we investigate the impact of acetic acid exposure on novel screen-printable pastes, including a lead-tellurite (Pb-Te-O) based paste and a lead-free tellurite (Te-O) based paste. Solar cells fabricated using these pastes were exposed to acetic acid for different amounts of time. A subset of the cells were also exposed to water to monitor and compare the impact. We measure the change in contact resistivity for the samples by a non-destructive circular transmission line method (cTLM). The contact recombination characteristics are extracted by intensity-dependent photoluminescence imaging and multivariate regression analysis with Griddler AI. Additionally, Suns-V OC measurement was performed to further characterize the cells. Finally, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) were performed to understand the change in materials properties. Our study suggests that both the contact recombination and contact resistivity is affected by exposure to acetic acid. The lead-based paste performs better overall compared to lead-free paste even after different stages of exposure.
Silicon heterojunction (SHJ) solar cells have been studied extensively due to their potential to reach high energy conversion efficiencies. However, one of the limiting factors for this technology is the metallization, which uses low‐curing‐temperature silver pastes that result in fingers with higher bulk resistivity. Herein, a simple approach to fabricate the SHJ solar cell contacts is demonstrated with commercially available low‐cost silver (Ag) electrically conductive adhesive (ECA) pastes and aluminium (Al) foils. This technique can result in a transparent conductive oxide (TCO)‐free cell structure and has the potential to combine cell metallization and interconnection. Recombination and resistive loss analyses are performed on the fabricated test samples. A dark saturation current density at the contact ( J 0c ) of 3.05 fA cm −2 for test samples before annealing is reported. The analysis of the experimental and simulated photoluminescence (PL) images shows negligible added recombination. The contact resistivity ( ρ c ) value is 49.3 mΩ cm 2 after annealing which can be optimized by specialized ECA pastes.
This dataset contains contact resistivity, contact recombination and reflectance data associated with the following publication. In this work, a novel low-cost approach to fabricate metal contacts for silicon heterojunction solar cells, called the transferred foil contact, is presented using aluminium foils with silver-based electrically conductive adhesive (ECA) pastes. The extended transfer length method (TLM) measurement technique is used to calculate the contact resistivity values. The contact recombination values were obtained by applying intensity-dependent photoluminescence (PL) imaging and subsequent multivariate regression analysis with finite element method (FEM) simulation called Griddler AI. Reflectance was measured using a quantum efficiency (QE) measurement system. Iqbal, N., Li, M., Gregory, G., Dahal, S., Bowden, S. & Davis, K.O. Recombination and Resistive Losses of Transferred Foil Contacts for Silicon Heterojunction Solar Cells. physica status solidi (RRL) – Rapid Research Letters (2020). doi:10.1002/pssr.202000368
We have studied the degradation of both full-sized modules and minimodules with PERC and Al-BSF cell variations in fields while considering packaging strategies. We demonstrate the implementations of data-driven tools to analyze large numbers of modules and volumes of timeseries data to obtain the performance loss and degradation pathways. This data analysis pipeline enables quantitative comparison and ranking of module variations, as well as mapping and deeper understanding of degradation mechanisms. The best performing module is a half-cell PERC, which shows a performance loss rate ( PLR ) of −0.27 ± 0.12% per annum (%/ a ) after initial losses have stabilized. Minimodule studies showed inconsistent performance rankings due to significant power loss contributions via series resistance, however, recombination losses remained stable. Overall, PERC cell variations outperform or are not distinguishable from Al-BSF cell variations.
Reliable characterization techniques to accurately quantify the metallization-induced recombination losses as well as contact resistivity losses of screen-printed cells are crucial for successful optimization of the contact grid design. Previously, the dark saturation current density at the contact (J 0c ) is often assumed to be constant for different finger width. Similarly, impact of finger width on contact resistivity (ρc) is rarely reported. Therefore, we performed a comprehensive evaluation of J 0c and ρ c as a function of finger width, spacing as well as firing temperature. We found out that J0c increases from ≈2000 to ≈8100 fA/cm2, when the finger width increases from 60 to 400 μm; and ρ c decrease from 7.2 to 2.2 mΩ · cm 2 when using a wide-TLM rather than a narrow-TLM structure, for samples fired at 840 °C. Based on our cross-sectional and top-down scanning electron microscopy images, we believe that the physical root cause can be explained by the difference in the microstructure formed at the metal-silicon interface during the firing process for the screen-printed contacts.
A multicrystalline silicon solar cell was analyzed using Raman microspectroscopy. We measured the prominent Raman modes of silicon, nanocrystalline silicon and silver oxide in various regions of the solar cell to generate insights into the process and material quality of the finished device. First, by comparing the distribution of the transverse optical (TO) phonon peak position and full-width-at-half-maximum (FWHM) of the solar cell with a single crystal silicon wafer, the quality of the multicrystalline silicon surface was ascertained. Second, a similar analysis of the remnant saw marks on the device surface showed a discernably higher and wider distribution of TO phonon peak position and FWHM compared to a multicrystalline silicon surface. This indicated the presence of residual compressive stresses in the saw mark region. Third, by observing the silver fingers and bus bars, a residual silver oxide layer was identified, up to 25 μm away from the line edges. This was attributed to the screen printing of the silver paste and the subsequent firing process. Finally, Raman mapping on an embedded inclusion showed the presence of nanocrystalline silicon phase. The multicrystalline silicon region surrounding the inclusion was under tensile stress. A nondestructive, confocal Raman analysis of the inclusion provided a 3-D visualization of the defect, both inside and above the surface of the multicrystalline silicon wafer.
Screen printing is the dominant technique for contact formation of industrial high-efficiency crystalline silicon solar cells. Accurately quantify the metallization induced recombination losses of the metal contacts (J 0c ) remain a hot topic. We present a comprehensive analysis of the J 0c at the metal-Si interface as a function of the finger width and firing temperature. The dimension-dependent J 0c properties are investigated and possible explanations are discussed. By extracting J 0c directly from finished solar cells, it opens up the opportunity to quantify the contact recombination property in a fast and efficient manner, since no specially prepared midstream structures will be required for the analysis.
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