Study On The Lead-free Silver Thick-film Contacts Of Crystalline Silicon Solar Cells

Silver front contact paste is the key material for mass production of crystalline silicon solar cells using screen printing sintering process technology. To meet requirements of environmental protection, lead-free silver front contact paste is becoming a hot research topic recently.In this paper, the principle of the front silver metallization for crystalline silicon solar cells was elaborated and an new experimental scheme of lead-free silver front contact pastes was proposed. The effects of Te-Bi, Te-Bi-Si, Bi-Si, Bi-Ge-based glass on the performance of Ag thick-film contacts were studied and results show that the optimum lead-free glass formulation was（ by mass fraction）: TeO₂ of 30%, Bi₂O₃ of 50%, MO of 14%, R₂ O of 5%. Using the optimized glass frit, a new environmental friendly silver front contact paste was obtained, which adapt to polycrystalline silicon solar cells with Conversion efficiency of 16.48%, only slightly lower than that of commercial lead front-side silver paste in the same technological conditions. The conversion efficiency of solar cells used Te-Bi-Si/Bi-Ge glass frits were low, Because of their excellent adhesion to silver thick film and silicon surface, especially Bi-Ge glass frits（the adhesive Strength from 8N to 9N）, they still have the potential superiority.Thermogravimetric analysis（ TGA） and differential scanning calorimetry（ DSC） in flowing air were used to analyze thermal properties of glass frits and corresponding pastes. the glass transition temperature Tg（at the temperature range 325⁴25℃） of Te-Bi glass frits was decreasing as the increasing of the TeO₂ composition. the Bi-Te silver pastes would loss weight in an exothermic reaction at 610℃ were found for the first time, which would directly affect the formation of Ag crystallites at the Ag-Si interface, so the series resistance and conversion efficiency of solar cells were influenced. The paper also studied the reaction mechanism between lead-free silver paste and SiNx antireflection coating, which showed good etching result.The effect of TeO₂, Bi₂O₃ and SiO₂ content on the size, number and distribution of Ag-crystallites in Ag-Si interface was analyzed by scanning electron microscope.The effects of different sintering temperatures and glass content in pastes on the electrical performance of silver thick-film contacts of Si solar cells were also studied. The silver front contact paste used lead glass form Ag-Si ohmic contact in wide temperature range. However, silver front contact paste used lead-free bismuth glass are sensitive to sintering temperature. The sintering temperature range was as narrow as 50℃. Glass content in pastes was affected by the Tg of the glass. The high Tg should be appropriately increased glass content, which led to more Ag crystallites. The low Tg should be appropriately decreased glass content to Avoiding excess Ag crystallites, which may destroy the emitter junction.