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 °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.