Effects of Annealing on Firing Stability of a Al2O3/SiNx Stack Passivation Layer for Crystalline Silicon Solar Cells

Abstract

Because of its high passivation quality, Al2O3 is used as the front passivation layer in commercial n-type silicon solar cells. The front passivation layer of a solar cell should have passivation properties and antireflection properties. For process efficiency and protection, SiNx is used as a capping layer on Al2O3. However, the Al2O3/SiNx stack layer has an issue with firing stability during screen printing of the cell, similar to the Al2O3 layer. This is because the Al2O3/SiNx stack layer needs to be fired to form the metal contact. In this study, the firing stability of the Al2O3/SiNx stack layer is investigated and the relation between blister formation and passivation is elucidated. Annealing improves the passivation quality of layers after firing. The order of annealing and stack formation is also one of parameters for firing stability. We used thermal atomic layer deposition to form Al2O3 and plasma enhanced chemical vapor deposition to form SiNx. The refractive index of each layer is 1.6 and 2.0, respectively, and the thickness is 10 nm and 70 nm, respectively. Rapid thermal processing was used for the annealing and firing. Passivation qualities were determined by quasi steady-state photo conductance, and blistering was observed by optical microscopy. Although there is some blistering on the sample annealed at 600 °C, good passivation and good firing stability was observed. We identified that a low density of blisters formed during the annealing step improves the firing stability of the passivation layer by preventing abrupt blister formation during the firing step, which is the cause of thermal degradation.