Front Side Metallization Of Solar Cells Research Articles

Low Recombination Firing-Through Al Paste for N-Type Solar Cell with Boron Emitter

A kind of low recombination firing-through screen-printing aluminum (Al) paste is proposed in this work to be used for a boron-diffused N-type solar cell front side metallization. A front side fire-through contact (FTC) approach has been carried out for the formation of local contacts for a front surface passivated solar cell. With a low contact resistivity (ρc) of 1.0 mΩ·cm2, good ohmic contact between the boron-doped front surface of the silicon sample and the Al paste was realized. To obtain a good energy conversion efficiency, a balance can be achieved between the open circuit voltage (Voc) and contact resistivity (ρc) of the cell by combining suitable Al powders and appropriate additives. The detailed micro-contact difference in Si/metallization between the firing-through Al paste and silver-aluminum (Ag-Al) paste was analyzed. The dark saturation current density beneath the metal contact (J0, metal) of the Si/metallization region using our firing-through Al paste was discussed, which was proven to be 61% lower than using Ag-Al paste. The pseudo energy conversion efficiency of the cell using Al paste measured by Suns-VOC was also higher than using Ag-Al paste. The role of Al paste in low surface metal recombination is discussed. The utilization of this new kind of Al paste was much cheaper and more convenient, compared to the traditional process using Ag or Ag-Al paste.

Single-step printing of high-resolution, high-aspect ratio silver lines through laser-induced forward transfer

Laser-induced forward transfer (LIFT) has the advantage of the absence of contact and masking, which is beneficial for the front-side metallization of solar cells. However, due to the influence of the interaction between adjacent pulses, it is difficult to obtain continuous lines with a high resolution. In this paper, by optimizing relevant parameters (laser fluence, gap distance, viscosity, spot size, and spot pitch), we demonstrate that silver lines with a high resolution and high aspect ratio (thickness over width) can be printed stably. The printing of single-voxel aims to initially determine the range of parameters necessary to generate high-resolution and high-aspect ratio voxels. Subsequently, based on this parameter range, lines are printed at different spot pitches to define the optimal parameters required to produce continuous lines. A comparison of two different viscosities (250 and 50 Pa·s) reveals that high-resolution continuous lines can only be realized at a viscosity of 50 Pa·s. In addition, the continuous lines printed at a large spot size (150 μm) are more uniform and narrower than those printed at a small spot size (60 μm). Finally, according to the optimal parameters (50 Pa·s viscosity, 150 μm spot size, 30 μm gap distance, 70 μm spot pitch, and 1.3 J/cm2 laser fluence), continuous lines with a 44 μm width and 0.54 aspect ratio are achieved via the single-step printing process.

Front side metallization of silicon solar cells – A high-speed video imaging analysis of the screen printing process

Aiming at higher cell efficiency and lower Ag consumption, research focuses on achieving smaller finger lines for silicon solar cell front side metallization. In-depth understanding of the printing process is mandatory to achieve uniform, defect free lines with high aspect ratio. An innovative high-speed video imaging setup allows for tracking of the mechanisms happening within ≈10–50 ms during the screen printing process. Two pastes differing significantly with respect to their rheological as well as wetting properties are used in this study. A paste spreading mechanism consisting of different consecutive process steps determines the final line shape. During nip contact the paste is squeezed between screen emulsion and substrate. Paste spreading further proceeds as long as the screen clings to the substrate, it occurs in a regular pattern and is most pronounced in the regions were mesh wires approach the emulsion surface. Then sudden retraction takes place during snap-off and the final line width is essentially fixed. Spreading is more pronounced for the paste exhibiting low viscosity at high shear rates and strong wetting of screen emulsion and substrate. The overall paste transfer is promoted by poor screen wetting and strong paste cohesion. These insights will stimulate further improvement of Ag paste formulation for solar cell metallization.

Optimizing the Solar Cell Front Side Metallization and the Cell Interconnection for High Module Power Output

Improving the light trapping in a module results in an increase in the generated current. Consequently, an optimization of the front grid metallization of the cell is required for the best trade-off between series resistance, shading, and recombination losses. For this purpose, we combine ray tracing and electrical solar cell and module calculations that explicitly account for cell and module interactions. Our model bases on experimentally verified input parameters: We determine the electrical and optical properties of the front metal fingers of passivated emitter and rear cells (PERC). We show that the effective optical width of the front metal fingers in the module is significantly reduced by 54%. The optimized simulated module has 120 half-size PERC with 20.2% cell efficiency and has an output power of 295.2 W. This is achieved with an increased number of 120 front metal fingers per cell, four white-colored cell interconnection ribbons (CIR), and an increased cell spacing. Applying these optimized design changes to an experimental module we measure a module power output of 294.8 W and a cell-to-module (CTM) factor of 1.02. Measured and simulated power agree and the deviations in Voc, Isc and FF are less than 0.91%rel. We perform a module power gain analysis for the fabricated module and simulate a potential maximum module power of 374.1 W when including further improvements.

Long Term Stability Analysis of Copper Front Side Metallization for Silicon Solar Cells

In this work the development and evaluation of a copper solar cell front side metallization based on a screen printed silver seed layer, a plated nickel diffusion barrier, a plated copper conductive layer and a plated silver capping is presented. Due to a slight adjustment of the standard screen printing process, and subsequent plating of Ni, Cu and Ag, silver consumption was reduced to < 16 mg per cell front side on standard BSF 15.6 cm x 15.6cm mono crystalline wafers, produced only with inline capable techniques. Contact adhesion forces of 1.5 N/mm and, efficiencies of 18.4 % were achieved. For the long term stability analysis on cell and module level, cells with different nickel diffusion barrier masses (10-40mg/cell) were treated on hotplates at 200, 225 and 250°C while detecting degradation due to copper diffusion by measuring the pFF. Sufficient cell life times are predicted for 10 mg Ni by the resulting Arrhenius plot. After 750h damp heat test of one cell modules no visible degradation was determined. This means that accelerated degradation on hotplates show the degradation due to copper diffusion more strongly than 750hours damp heat test. In an additional SEM analysis the plated nickel diffusion barrier showed a closed layer on the fingers and an incomplete layer on the busbars, indicating busbars as possible weak points for copper diffusion due to inhomogeneous Ni plating

Aluminum Evaporation and Etching for the Front-Side Metallization of Solar Cells

In this paper, we report and discuss several strategies to produce solar cell front contacts by full-area metallization and etching (FAME). Our chemically structured contacts consume less expensive silver than screen-printed contacts. As a proof of principle for the FAME approach, we present a 148.6-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> -sized silicon solar cell that has about 100-μm-wide front-side fingers. These fingers consist of a 15-μm-thick evaporated aluminum layer, supplying the electrical conductance, and a sputtered capping stack (200 nm Ni:V plus 20 nm Ag), providing solderability. The entire metal stack is first deposited on the full area of the solar cells, then locally protected by a wax pattern, and subsequently etched with commercial Ni:V etch and NaOH. The efficiency of the best solar cell is 19.3%, the fill factor is 78%, the open-circuit voltage is 666 mV, and the short-circuit current density is 37.1 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Quick Determination of Copper-Metallization Long-Term Impact on Silicon Solar Cells

In this paper, we present a method to quickly evaluate the long-term effects of copper-containing metal stack systems for silicon solar cell front–side metallization. Copper diffusion, which is detrimental for the solar cell performance, is accelerated by exposing the cell to thermal stress. In this paper, we suggest to quantify the degree of copper diffusion into the cell by the very fast Suns- technique, measuring the pseudo fill factor (pFF). Using three or more different temperatures, and assuming a certain loss in pFF corresponds to a certain depth of diffusion, the effective activation energy for copper diffusion for a given system can be extracted from an Arrhenius plot of the measured data. An extrapolation into temperature regions typical for solar cell modules under outdoor conditions allows an estimation of the fill factor loss for any operation time and temperature. Compared to time- and cost-intensive methods such as transmission electron microscopy or secondary-ion mass spectrometry, this kind of investigation requires only sparse equipment and can typically be done in 1 week per stack system.