Crystalline silicon solar cells 10–15 times thinner than traditional commercial c-Si cells with 14.9% efficiency are presented with modeling, fabrication, and testing details. These cells are 14mm thick, 250mm wide, and have achieved 14.9% solar conversion efficiency under AM 1.5 spectrum. First, modeling results illustrate the importance of high-quality passivation to achieve high efficiency in thin silicon, back contacted solar cells. Then, the methodology used to fabricate these ultra thin devices by means of established microsystems processing technologies is presented. Finally, the optimization procedure to achieve high efficiency as well as the results of the experiments carried out with alumina and nitride layers as passivation coatings are discussed.