Environmentally-friendly, Pb-free Cu front electrode for Si-based solar cell applications

In this study, a double glass system with PbO base and ZnO base of Cu paste is designed, prepared and printed with narrow line screen printing process on polycrystalline Si solar cell which has already finished the back Al printing and deposition of double anti-reflection coatings (DARCs). And then, a two-step firing process is applied to sinter the front electrode and obtain the ohmic contact between front electrode and solar cell. The first step of firing is conducted in air atmosphere. In this process, PbO-based glass frit etched the DARCs and Ag recrystallized at the surface of Si, constructing the preliminary contact. The second step of firing is conducted in a reducing atmosphere (H₂/N₂ = 5/95). In this process, the CuO is reduced into Cu which is sintered with ZnO based-glass. Ag nanoparticles recrystallized in the glass layer at interface due to the interactions between H₂, Ag and PbO-based glass frit and the volatility of Pb, improve the Ohmic contact between electrode and solar cell. According to our investigation, a model of the reaction mechanisms in the double glass system of Cu paste in each sintering stage is proposed. And we have shown that Ohmic contact and good sheet resistance for the Cu front electrode with Pb-free could both be obtained by applying the newly-invented Cu paste and two steps of firing process. The lowest sheet resistance and contact resistance for such a Cu front electrode measured are 0.090 Ω/□ and 0.0040 Ω/□, respectively.