20% Efficient screen-printed n-type solar cells using a spin-on source and thermal oxide/silicon nitride passivation

Due partly to the fact that n-type solar cells do not suffer from light-induced degradation, there has been a recent surge in research and industrial interest in n-type silicon solar cells. However, the most common dielectric materials that are used to passivate the surface of p-type cells - thermal oxide and silicon nitride - are well known to have problems with providing adequate passivation of heavily boron doped surfaces, especially on textured surfaces. As a result, the highest efficiency n-type cells have thus far relied on Al₂O₃ whose high negative charge density results in good passivation on both planar and textured boron diffused surfaces. We show here that though thermal SiO₂ and SiN_x alone provide poor passivation on p+ surfaces, a thin (15 nm) thermal-SiO₂ layer, capped with SiN_x can, after firing, provide high-quality passivation on a textured, boron emitter. Firing at ~ 750°C was found to result in a threefold improvement in the surface passivation quality with a final J₀ of ~ 150 fA/cm² being achieved. Additionally, a spin-on boric acid source was used to form a uniform, textured emitter as verified on completed solar cells which did not demonstrate junction shunting. Using a symmetric SiO₂/SiN_x stack passivation and screen-printed contacts, a verified n-type cell efficiency of 20.3% was achieved with V_OC up to 650mV and J_SC over 40 mA/cm². The stability of this structure was also tested, with no degradation being observed over 7 months of dark storage in air.