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In this study we investigate metal spike formation of screen-printed Ag/Al pastes during contact firing in an infrared belt furnace and its influence on the characteristics of n-type bi-facial silicon solar cells. The boron emitters are formed in a co-diffusion step using boron doped PECVD layers. It is demonstrated that the formation of Ag/Al spik...
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... This prevents the reduction in pores after silver particle sintering, which could lead to glass residue in the silver layer [50][51][52][53][54]. This not only increases the resistivity of the silver electrode but also results in an insufficient glass interlayer at the silver-silicon interface [55][56][57][58][59]. Hence, it is generally considered that silver powder suitable for front-side solar silver paste should possess an average particle size of 1-1.5 µm, a narrow size distribution, and high sphericity. ...
Silver powder, as the primary component of solar silver paste, significantly influences various aspects of the paste’s performance, including printing, sintering, and conductivity. This study reveals that, beyond the shape and size of the silver powders, their microstructure is a critical factor influencing the performance of both silver powders and silver pastes in solar cell applications. The growth process leads to the formation of either polycrystalline aggregated silver powder or crystal growth silver powder. Analyzing the performance characteristics of these different microstructures provides guidance for selecting silver powders for silver pastes at different sintering temperatures. Polycrystalline aggregated silver powder exhibits higher sintering activity, with a sintering initiation temperature around 450 °C. The resulting silver paste, sintered at 750 °C, demonstrates a low sheet resistance of 2.92 mΩ/sq and high adhesion of 2.13 N. This silver powder is suitable for formulating silver pastes with lower sintering temperatures. The solar cell electrode grid lines have a high aspect ratio of 0.37, showing poor uniformity. However, due to the high sintering activity of the silver powder, the glass layer dissolves and deposits more silver, resulting in excellent conductivity, a low contact resistance of the silver electrode, a low series resistance of the solar cell of 1.23 mΩ, and a high photoelectric conversion efficiency of 23.16%. Crystal growth silver powder exhibits the highest tap density of 5.52 g/cm3. The corresponding silver paste shows improved densification upon sintering, especially at 840 °C, yielding a sheet resistance of 2.56 mΩ/sq and adhesion of 3.05 N. This silver powder is suitable for formulating silver pastes with higher sintering temperatures. The solar cell electrode grid lines are uniform with the highest aspect ratio of 0.40, resulting in a smaller shading area, a high fill factor of 81.59%, and a slightly higher photoelectric conversion efficiency of 23.17% compared to the polycrystalline aggregated silver powder.
... (2.5.1), while the differences are discussed in the following subsections. The contact formation strongly depends on the firing conditions [49]. Long firing generates a thick glass layer while a certain temperature input is required to build the contacts at all [50]. ...
... In Fritz et al. [60] contact resistivities between 4.5 and 0.86 mΩcm 2 are achieved with AgAl pastes on p+ emitters with SiN x :H coating. Frey et al. [49] shows that the depth of crystallites depends strongly on the peak firing temperature. Concluding, the contact of AgAl pastes on B emitters is characterised by the temperature dependent formation of deep Ag crystals. ...
... The high throughput makes belt driven systems suitable for industrial purpose, while for screenings in research a rapid thermal processing (RTP) furnace is appropriate due to short preheating times. According to [49,60,64] there is a significant impact of the firing step on the contact formation. Therefore, every available furnace was tested. ...
... However, usually Al containing Ag pastes are used for contact formation on boron emitters, since adding Al into Ag pastes reduces ρc significantly, as shown in [134,135,138]. Although the addition of Al lowers ρc, it suffers from several disadvantages such as the formation of deep metal spikes, which can penetrate the SCR and thereby induce essential power loss because of enhanced recombination, as reported in [61,135,[138][139][140]. Another drawback is the slightly higher line resistivity rl of Ag/Al metal fingers since Al has a lower conductivity than Ag, which causes slightly increased resistance related power losses. ...
... In this section, the much more challenging issue of contact formation on boron-doped emitters is discussed. Contact formation on boron emitters by means of screen-printing of metal pastes has been intensely investigated in recent years by several research groups [61,134,135,138,139,157,[160][161][162][163]. ...
... However, the use of Ag/Al pastes suffers from several disadvantages such as the formation of metal spikes which can potentially penetrate the SCR, as reported in [61,135,[138][139][140], resulting in essential power loss due to recombination at the metal contacts and in the SCR. This power loss can potentially compensate the benefits of n-type Si substrates. ...
... Besides the diffusion processes and surface passivation, contact formation is an important process step. Investigations from Frey et al. [22] showed the challenges occurring when contacting B emitters by screen-printed Ag/Al paste. The detrimental effect of Al spiking should be overcome. ...
This review presents the current state of the art and interesting questions with regard to CVD BSG layers. The advantages of CVD doping sources over the conventional POCl3 and BBr3 or BCl3 gaseous sources are the simple way to deposit a diffusion source on only a single side of the wafer and structuring the diffusion source to achieve dopant concentration profiles next to each other on the same side of the wafer. In addition, these CVD glasses are multifunctional. The same CVD BSG can serve e. g. as doping source, passivation layer, antireflective coating and as electrical insulator.
Laser‐assisted current‐injection treatment, also known as Laser Enhanced Contact Optimization (LECO), has great potential to reduce the front contact resistance and metal‐induced recombination of n‐type tunnel oxide passivated contact (n‐TOPCon) solar cells, thereby improving the cell efficiency. Herein, we investigate the interfacial Ag–Si contact characteristics on boron‐doped p ⁺ emitters and the electrical properties of industrial n‐TOPCon solar cells that feature a LECO treatment and a specific Ag paste and compare them with those of n‐TOPCon solar cells with a standard Ag/Al paste process. LECO causes some Current Fired Contacts (CFCs) that, when removed by sequential selective etching, leave bowl‐shaped imprints on the emitter, indicating that isotropic alloying behavior occurs between Ag and Si at these local positions during LECO. Unlike the standard Ag/Al metallization process, the LECO process does not significantly damage the passivation layer or emitter. More interestingly, the n‐TOPCon solar cells prepared with the specific Ag paste did not initially form an effective metal‐semiconductor contact, with an average efficiency of only 0.14%, which increased to 25.65% after LECO treatment, even 0.2% abs higher than that of the reference counterparts with standard Ag/Al electrodes. Ultimately, a physical model of LECO‐induced Ag–Si contact formation on boron emitters is proposed.
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