Nickel-plated front contacts for front and rear emitter silicon solar cells

We investigate the application of nickel-plated front contacts to front and rear emitter silicon solar cells. We compare identically processed p-and n-type Si solar cells featuring (i) a homogeneous phosphorus-diffused n(+) front, acting as emitter or front surface field and (ii) a full-area aluminum-alloyed p(+) rear, acting as back surface field or rear emitter. This results in an n(+) pp(+) front emitter and an n(+) np(+) rear emitter solar cell structure, respectively. We show that the contact annealing temperature for the thermal formation of the nickel silicide (NiSix) after the electroless Ni plating has a significant influence on the NiSix layer thickness. Increasing the temperature from 300 to 450 degrees C shifts the average thickness to higher values and augments deep NiSix spikes. The intensified penetration of the n(+) front by the NiSix spikes leads to a strong degradation of the front emitter p-type Si solar cell performances due to (i) decreased shunt resistances and (ii) the contamination of the emitter space charge region. We demonstrate that the rear emitter n-type Si solar cells, in contrast, are explicitly more stable. Therefore, by applying n-instead of p-type Si as base material, an improved stability against the thermal Ni contact formation can be easily realized without significant changes in the solar cell fabrication process. (C) 2013 The Authors. Published by Elsevier Ltd.