Abstract
Long diffusion length is essential to reach high efficiencies (>15%) on rear-junction solar cells. As a rule of thumb the minority carrier diffusion length should be at least three times as long as the wafer thickness. Thanks to the smaller capture cross-section of impurities measured in n-type material, n-type silicon can exhibit a diffusion length exceeding 600 pm even in its multi-crystalline form. High-efficiency rear-junction cells become feasible at reasonable cost with wafers as thick as 200 mu m. The development of the p+ emitter on the rear is essential for this type of cell, and Al alloying is one of the techniques of choice to realize it. After optimizing the emitter, we obtained very good values for Voc (similar to 615 mV) and FF (>80%). Afterwards, focus was laid on improving the short-circuit current. This was achieved by an optimization of the phosphorus-diffused front-surface field, and by thinning down the wafers. These two actions had the consequence of increasing independently of each other the Jsc by 2.3 mA.cm(-2) for the optimization of the front-surface field, and by 3 mA.cm(-2) when wafer thickness decreased from 300 pm to 150 mu m. The characteristics obtained are observed to be extremely dependent on the wafer quality, which varies a lot along the ingot. An efficiency of 15.0% (30.7 mA. CM 2) was reached on rear-junction n-type 150-mu m-thick 5x5-cm(2) 2.5-ohms.cm multi-crystalline Si.