On the design and applicability of nanowire solar cells using low-grade semiconductors

Nanowire based radial junction structure has been widely proposed to achieve high efficiency solar cells with low grade semiconductors, in particular silicon. We present a concise model based on the collection probability of photogenerated carriers to investigate the effect of size and geometry on the photovoltaic behavior in radial junction devices. The model provides a simple tool to obtain the optimal range of radius sizes depending on material and device parameters. We use the model to derive guideline for the design of cells made of low grade silicon. The analysis of the radial structure in upgraded metallurgical grade silicon reveals how nanowire devices lose the improved efficiency advantage quickly at radii below the minority carrier diffusion length.