UNDERSTANDING AND DEVELOPING FIRE-THROUGH DIELECTRIC CONTACTS ON HIGHLY EFFICIENT SILICON SOLAR CELLS
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Abstract
Photovoltaic is a quick and efficient way of responding to the scarcity of fossil fuels and increasing environmental pollution. For the last two decades, photovoltaic technology has been the fastest growing industry among the renewable energy sources. However, the cost of per kWh of solar electricity is still the main challenge to be tackled. Cost and efficiency are the two opposing challenges that must be overcome for cost-effective solar electricity. Metallization is one of the key fabrication steps, especially for crystalline silicon solar cells which dominates the market, that can be tailored to reduce cost by using less silver along with fine gridlines while increasing the efficiency. Thus, the comprehensive investigation of the front grid metallization designs to reduce the amount of Ag used and the alternative such as Ni/Cu to reduce the cost of metallization even further have been carried out.Firstly, a comprehensive empirical grid model was first established to investigate the front grid designs with 3-, 4- and 5 busbars. The results are compared to numerical analysis using Griddler 2-D modeling program. A combination of segmented tapered metal grids (SG) and uneven busbars (UEB) led to increased short circuit current density (JSC) and open circuit voltage (VOC) without sacrificing the fill factor (FF). The 5-busbar SG-UEB combination resulted in ≥ 20 % efficient Al-BSF and ≥21 % PERC solar cell. The results demonstrated that, in addition to high efficiency, the cost of front Ag metallization can be reduced by 1.4¢ per cell through SG-UEB combination as opposed to non-segmented approach.Secondly, the front designs were implemented taking into account the impact of Ag paste composition and sintering on contact and series resistance, which tend to impact the FF. The Ag paste used in the experiments had the right composition to result in (i) narrow gridlines according to the screen design, after printing (ii) low contact and gridline resistances after sintering. This resulted in PERC structure efficiency of ~21%. Noted in the used paste, according to the SEM study, is the Ag crystallites, which tend to be more when nano Ag particle is used than the micro Ag particle counterpart. Also, gridline porosity is reduced by use of nano Ag particle in the paste as well as the contact resistance from high density and uniform Ag crystallites with very thin (~ 0.1 nm) glass layer.Thirdly, having established the baseline process to achieve ~21% PERC cell with a belt speed of 230 IPM in the infrared belt rapid thermal annealing furnace, the impact of belt speed on the cell performance was investigated. Since rapid thermal processing (RTP) is a key technology in the screen-printed Ag paste contacts to silicon solar cells; the ramp up and ramp down rates, which depends on belt speed, were investigated. It is noted that, the faster the belt speed the shorter the dwell time, which enhances the front contact quality. By doubling the belt speed from 180 to 375 ipm, the VOC was observed to increase by ~5 mV along with ~1-2 % absolute increase in fill factor with zero cost to production. Thus, by doubling the belt speed, higher efficiency is obtained and cost is conserved. This is one of the ways to further decrease cost of production with increased output performance.Finally, in quest to reduce cost further, starting with metallization cost, alternative to Ag, Ni/Cu is a contender. However, aerosol jet printing, which is a non-contact printing method, may be explored instead of screen-printing to maintain control over gridline width and height. Aerosol printing is a high throughput process that can easily be integrated into commercial silicon solar manufacturing. Gridlines of silver frit, nickel frit, silver/nickel stacks, and silver/nickel/copper stacks were investigated as a first approach. The Ag Frit/Ni/Cu stack displayed ~19% pseudo efficiency with ~85% pFF and gridlines of 100 µm wide, and 3.25 µm height. However, the Ni frit ink showed better adhesion over the Ag counterpart. This is quite fascinating preliminary results, which suggests that the Ni/Cu is a good candidate as alternative to Ag. Ni is lower cost than Ag and can provide up to 30% cost reduction in manufacturing.