Predicted efficiency of Si wire array solar cells

Solar cells based on arrays of CVD-grown Si nano- or micro-wires have attracted interest as potentially low-cost alternatives to conventional wafer-based Si photovoltaics [1-6], and single-wire solar cells have been reported with efficiencies of up to 3.4% [7]. We recently presented device physics simulations which predicted efficiencies exceeding 17%, based on experimentally observed diffusion lengths within our wires [8]. However, this model did not take into account the optical properties of a wire array device - in particular the inherently low packing fraction of wires within CVD-grown wire arrays, which might limit their ability to fully absorb incident sunlight. For this reason, we have combined a device physics model of Si wire solar cells with FDTD simulations of light absorption within wire arrays to investigate the potential photovoltaic efficiency of this cell geometry. We have found that even a sparsely packed array (14%) is expected to absorb moderate (66%) amounts of above-bandgap solar energy, yielding a simulated photovoltaic efficiency of 14.5%. Because the wire array comprises such a small volume of Si, the observed absorption represents an effective optical concentration, which enables greater operating voltages than previously predicted for Si wire array solar cells.