Metal Induced Recombination Losses Research Articles

Microstructure beneath screen-printed silver contacts and its correlation to metallization-induced recombination parameters

In this work, we present an approach to model metallization-induced recombination losses j 0,met of screen-printed fire-through metallization pastes, which are used e.g. for the contacts on the front side of passivated emitter and rear cells (PERC). Modelling is based on the local phosphorus emitter doping profile and the local microstructure of the interface between metal contact and silicon surface, which is characterized during a quantitative microstructural analysis using the scanning electron microscope (SEM). By comparing the modelled j 0,met to measured j 0,met results determined at the same position of both, the microstructural analysis and emitter doping profile, we find that in most cases recombination is dominated by the etch back of the emitter by the glass frit rather than by penetration of crystallites into the doped region. However, for crystallites protruding deep into the emitter or rather up to the junction, recombination at the crystallites becomes significant as well. Our modelled results yield indications that the etch depth into the emitter doping profile by the glass frit contained in the metallization paste, is sensitive to the emitter doping profile itself. • Metallization-induced recombination is dominated by the etch back of the emitter doping profile • Etch back of the emitter doping profile is sensitive to doping profile itself • Plating useful to enhance contrast for microstructure analysis

Spatially Resolved Determination of Metallization-Induced Recombination Losses Using Photoluminescence Imaging

Metallization induced recombination losses are one dominant loss mechanism for current industrial solar cells. A precise determination of these losses is important for contacting technology optimization, as well as precise solar cell modeling. Usually, for state-of-the-art approaches to determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> , it is assumed that the samples itself exhibit spatially uniform properties (e.g., carrier lifetime or sheet resistance) or that the used reference samples are identical to the metallized samples. Finally, in most cases, only one global j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> -value for the entire sample is given, neglecting possible spatial inhomogeneities. In this article, we mostly eliminate the necessity for the assumptions of perfect sample homogeneity by means of an interpolation scheme of the photoluminescence (PL) signal. Thereby, we can predict the PL signal of a virtually nonmetallized test field with a relative standard deviation of about σ ≈ 0.7%. Additionally, we determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> for specific test fields at different positions on the sample and correlate the results to the local emitter sheet resistance R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sh</sub> , the local peak firing temperature of the sample during the fast firing process T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">peak</sub> , and the test field finger spacing d. For our samples, a reduction of d from d = 1000 μm to d = 200 μm leads to a reduction of j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> by up to 18%. This strong effect is physically unexpected and so far not considered by the state-of-the-art approach, frequently performed in the photovoltaic community. Further, we perform a sensitivity and error analysis which reveals that we are able to determine j <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0,met</sub> within an estimated accuracy between 15% and 18%.

Optimization of front metal contact design of Industrial Si solar cells using photoluminescence imaging technique

Abstract Silver (Ag) based front contact metallisation is a crucial process step in industrial Si solar cell fabrication. Lesser front metal coverage leads to high resistive losses, whereas higher coverage results in shadowing loss as well as increased metal-induced recombination losses. Both scenarios impede the power conversion efficiency of solar cells. Therefore, it is essential to monitor the shadow, resistive, and metal-induced recombination losses associated with the design of front Ag contact. In this work, we demonstrate the application of photoluminescence (PL) imaging technique in association with the other conventional characterization techniques used in the optimization of front contact metallisation.

A Comprehensive Evaluation of Contact Recombination and Contact Resistivity Losses in Industrial Silicon Solar Cells

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 ( ${J}_{\rm 0c}$ ) is often assumed to be constant for different finger width. Similarly, impact of finger width on contact resistivity ( ${\rho }_{\rm c}$ ) is rarely reported. Therefore, we performed a comprehensive evaluation of ${J}_{\rm 0c}$ and ${\rho }_{\rm c}$ as a function of finger width, spacing as well as firing temperature. We found out that ${J}_{\rm 0c}$ increases from $\approx$ 2000 to $\approx$ 8100 $\text{fA/cm}^{2}$ , when the finger width increases from 60 to 400 $\mu$ m; and ${\rho }_{\rm c}$ decrease from 7.2 to 2.2 $\text{m}{\Omega }\cdot \text{cm}^{2}$ when using a wide -TLM rather than a narrow -TLM structure, for samples fired at 840 $^{\circ }$ 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.

Mechanism of Metallization-Induced Losses in the Rear-Side of Fully Screen-Printed p-type PERC Solar Cells

This article investigates metallization-induced recombination losses associated with the presence of silicon (Si) in the 1) rear passivation layers and 2) aluminum (Al) paste in fully screen-printed p -type passivated emitter and rear cell (PERC) solar cells. For 1), the rear sides of PERCs passivated with aluminum oxide films stacked with silicon nitride (SiN y ) films with various refractive indexes (n), as defined by the composition of the SiN y films, were fabricated. Si-rich SiN y films strongly degraded the implied open-circuit voltages of symmetrically passivated samples, that include cells. Elemental mapping analysis confirmed further metallization-induced losses at the cell level because of the formation of an Al-Si alloy within the passivation stacks, with the density of the alloy dependent on the refractive index. For 2), the rear sides of PERCs were metallized with Si-free Al paste or Al pastes that contain Si. High-Si paste had both good and bad effects: it produced thicker aluminum back surface fields but left partially-formed Al-Si alloy within the passivation films.

Determination of Metal-Induced Recombination of n-Type Bifacial Si Solar Cells Using Special Print Patterns

A systematic routine to determine the front and rear metal induced recombination losses of bifacial silicon solar cells using an automated Griddler finite-element method fit procedure is presented in this paper. The method prescribes the preparation of easy-to-fabricate test patterns printed on the front and rear side of separate cells, nonmetallized cells, and simple photoluminescence imaging. Eight different samples with similar front boron emitter and different phosphorus back surface fields (BSFs) are analyzed. Choosing an optimized BSF is shown to lead to a cell efficiency gain of ∼0.4% absolute.

Determination of Metallization-Induced Recombination Losses of Screen-Printed Silicon Solar Cell Contacts and Their Dependence on the Doping Profile

This paper presents an easy-to-implement, but rigorous method to determine the dark saturation current densities $J_{01m}$ and $J_{02m}$ under the metallized region of a silicon solar cell contact. The method is based on an autofitting procedure by the Griddler artificial intelligence finite-element simulation program to measured photoluminescence images of samples with especially designed test structures. It is found that a deeper p-n junction strongly reduces the recombination losses at the metal–silicon interface. By carefully tailoring the boron diffusion profile, the $J_{01m}$ is reduced from 839.2 to 274.2 fA/cm2.

Understanding Metal Induced Recombination Losses in Silicon Solar Cells with Screen Printed Silver Contacts

There have been great improvements in silicon solar cell voltage across the last 5 years. A great deal of the gains can be attributed to application of lighter phosphorus emitter diffusions which deliver higher cell voltages, combined with the complementary silver metallization pastes which can contact those emitters. As the voltages continue to increase over time, metal recombination losses become more important to understand. The surface doping, junction depth, and metal contact used are all factors in determining the metal recombination rate which covers a range of approximately 400-1500 fA/cm2 for standard cases. Silver contact pastes modulate recombination by changing the way they etch the emitter surface, and semiconductor models indicate that the recombination rate can decrease to < 200 fA/cm2 as metallization paste technology continues to improve.

Microstructural characterization and current conduction mechanisms of front-side contact of n-type crystalline Si solar cells with Ag/Al pastes

Recently, high efficiency n-type crystalline Si cells made with the screen printed Ag/Al metallization have received considerable attention. We report here our microstructural investigations of the critical interfacial region between the front-side contact and the Si wafer of n-type cells fired under progressively higher temperatures. Our study revealed that the key characteristic microstructures of the interfacial region changed from one with a large fraction of residual SiNx, to one consisting of a thin glass layer with nano-Ag colloids, and finally to one decorated with Ag and Ag/Al crystallites attached to the emitter surface for cells with under-, optimally-, and over-fired conditions, respectively. We did not find any Al-Si eutectic layer on the emitter surface that would support a silicon dissolution and re-growth mechanism, which is operative in the back surface field formation process for the Al back contact of p-type industrial solar cells. The presence of the SiNx antireflection coating has likely altered the chemistry between Si and Al significantly. The observed microstructures lead us to conclude that the main current conduction mechanism in optimally-fired n-type cells is tunneling through those areas of thin interfacial glass containing nano-Ag colloids. This mechanism is similar to the current conduction model we have proposed previously for optimally-fired p-type crystalline Si solar cells. We believe that the intrusion of Ag/Al (and/or Ag) crystallites into the p+-Si emitter in over-fired cells is one of the major sources of metallization-induced recombination losses, which degrades cell performance.

Two-Dimensional Modeling of the Metallization-Induced Recombination Losses of Screen-Printed Solar Cells

We present an approach of implementing the experimental microscopic observations at metal-Si emitter interface into a 2-D simulation model to investigate the metallization-induced recombination losses of crystalline Si solar cells with firing through metallization. This metal-Si interface is typically characterized by few Ag crystallites grown into Si, whereas the main part of the interface area is covered by a glass layer. By using this simulation model, we were able to disentangle the effect that each of these microscopic features has on solar cell performance. The model assumes the metal-Si interface to be either Ohmic or Schottky, with current extraction occurring only through the direct Ag crystallites-Si interface. The simulation results show negligible degradation in open-circuit voltage (V OC ) and pseudo-fill factor (pF F) if the metal-Si interface comprises the sole presence of Ag crystallites, as long as these crystallites grow only superficially and are not penetrating the p-n junction. A more detrimental effect is observed if the emitter area beneath the glass layer is affected by etching. For more aggressive metallization pastes or for overfiring scenarios where larger in-grown crystallites are observed that are protruding through the p-n junction, the model assuming a Schottky contact qualitatively explains the shunting behavior observed experimentally.

Metallization‒induced recombination losses of bifacial silicon solar cells

AbstractIn this study, we investigate the metallization‒induced recombination losses of high efficiency bifacial n‒type and p‒type crystalline Si solar cells. From the experimental data, we found that the most efficiency limiting parameter by the screen‒printed metallization is the open‒circuit voltage (VOC) of the cells. We investigated the mechanism responsible for this loss by varying the metallization fraction on either side of the cell and determined the local enhancement in the dark saturation current density beneath the metal contacts (J0(met)). Under optimum fabrication conditions, the J0(met) at metal‒p+ (boron) emitter interfaces was found to be significantly higher compared with the values obtained for metal‒n+ emitters. A two‒dimensional simulation model was used to get further insight into the recombination mechanism leading to these VOC losses. The model assumes that metal contacts penetrate (or etch) into the diffused region following the firing process and depassivate the interface. Applying this model to our n‒type solar cells with a boron p+ emitter, we demonstrated that the simple loss of passivated area beneath the metal contact cannot explain the degradation observed in the VOC of the cell without considering a significant etching or metal penetration into the emitter region. Copyright © 2014 John Wiley &amp; Sons, Ltd.