FRONT SURFACE FIELD FORMATION AND DIFFUSION PROFILES FOR INDUSTRIAL INTERDIGITATED BACK CONTACT SOLAR CELLS

Optimization of the Front Surface Field (FSF) for IBC cells is important for passivation, lowering series resistance and reducing UV light degradation. This work presents results for optimizing the FSF diffusion from an industrial perspective, focusing on optimizing the process flow to achieve excellent FSF performance, whilst at the same time reducing the number of process steps. The ideal FSF profile is a compromise since a lightly doped deep diffusion reduces recombination losses close the cell surface where the light is captured, whilst increased doping reduces series resistance. This work investigates diffusing the FSF (1) at the beginning, (2) in the middle and (3) towards the end of the IBC process flow. The advantage of the first option is that the diffusion depth can be increased by subsequent thermal steps. However a diffusion barrier is required to protect the FSF throughout the subsequent processing, which increases the number of process steps and results in increased costs. By placing the FSF diffusion later in the process flow it is possible to simplify the process reducing the number of steps. Experimental results show excellent FSF diffusion passivation performance over 156mm, with lifetime values of over 500 µs. Simulations confirm that high current generation can be achieved with a short circuit current of over 40 mA cm-2.