Top-down Aluminum Induced Crystallization for Photovoltaics

Passivating silicon solar cell surfaces is critical to fabricating very high efficiency and low cost photovoltaic devices. The sun-facing surface of the solar cell, known as the emitter, is particularly important when designing a solar cell. This work focused first on an alternative method of forming the emitter of silicon solar cells, and secondly on a method for improving the surface passivation of both these non-traditional and standard n-type solar cells. Top-down aluminum induced crystallization (TAIC) was used for forming a polycrystalline silicon layer from amorphous silicon using aluminum to catalyze the crystallization at much lower temperatures than otherwise possible. Inherent to TAIC is the doping of the resultant crystalline silicon by the aluminum, an acceptor impurity. Thus, n-type solar cells with p-type polycrystalline emitters were fabricated. It was found that several variations of this crystallization process occurred and their effect on solar cell performance was analyzed. An inherent disadvantage to this method was the presence of defects at the junction of the highest efficiency solar cells fabricated. These defects were passivated by an atomic hydrogen treatment. Another method of improving solar cells was invented, theoretically modeled, and experimentally explored. The process improves silicon solar cells by hydrogen inactivation of acceptor impurities in the emitter (shown for both aluminum and boron in silicon). Low surface doping has been linked to lower measured surface recombination velocities for solar cell emitters with high quality dielectric passivation layers. By lowering emitter doping levels, n-type solar cell efficiencies were increased