Effects of Current-injection Firing with Ag Paste in a Boron Emitter

N-type silicon wafers are particularly advantageous for the design of high-efficiency solar cells because metal impurities are less detrimental to n-type silicon, which leads to a higher minority carrier lifetime1. Additionally, n-type silicon solar cells do not undergo light-induced degradation, which reduces cell performance2. However, contact problems arise for boron emitters that are applied to n-type silicon solar cells using the screen-printing technique. After firing with Ag paste, the contact resistance is higher than a phosphorus emitter; this problem can be resolved by adding aluminum to the Ag paste3,4,5,6. However, this produces other problems, such as shunting behavior and a high line resistance3. The Ag becomes less dense as the aluminum content increases6; thus, pastes with little or no aluminum are beneficial. In a recent study, Engelhardt et al. reported a contact resistivity of approximately 1 mΩ∙cm2 with a boron emitter using Ag paste7 when B-doped SiOx (SiOx:B) was used as the passivation layer. They reported a low contact resistivity at a peak temperature of 850 °C even above the Ag-Si eutectic point. However with a peak wafer temperature higher than the Ag-Si eutectic point, many Ag crystallites embed into the n-type base that causes a shunt path and recombination. When phosphorus emitters are fired with Ag paste, the Ag nucleates on the silicon emitter surface and grows as a crystallite. In a recent study by Hong, K. et al., Ag (Ag+) and oxygen (O2−) ions etched a phosphorus emitter, which was found to contribute to Ag crystallite formation during the firing process8. Also, Schubert reported that lead oxide (PbO) etches the silicon emitter9. Both of these reports confirm that the silicon etching reaction is critical for Ag crystallite formation. However this effect would be markedly reduced in boron emitters. In solution etching, silicon is etched in an alkaline solution such as a potassium hydroxide (KOH) solution. The silicon surface is etched with hydroxide ions (OH−) that are generated by the decomposition of water molecules (H2O) using the electrons at the silicon surface; thus, electrons are important in the etching of silicon. The etching reaction is particularly weak in emitters with high levels of boron doping due to the presence of fewer electrons. The etch rate is markedly reduced at boron concentrations above 1019 cm−3 in alkaline solutions10. Similar to the etching process in alkaline solutions, electrons play a significant role in the etching of silicon during the paste etching process. Ag ions that are created from the Ag bulk participate in the silicon etching reaction8, and electrons that are present at the surface of a boron emitter participate in the Ag ion reduction reaction. Positively charged Ag ions etch the silicon less strongly in boron emitters than in phosphorus emitters. This study investigated the use of conventional Ag paste in n-type silicon solar cells. We injected electrons into the boron emitter to increase the number of electrons at the boron emitter surface during the firing process. The nano- or micro-structure of Ag and its contact behavior were investigated using field emission scanning electron microscopy (FE-SEM) and the transfer length method.