General theoretical considerations on nanowire solar cell designs

We propose two novel solar cell designs, tapping the advantages of semiconductor nanowires. A silicon (Si) tandem cell can be achieved by growing sub-5 nm p-n junction in the radial direction of the Si nanowires on a planar crystalline Si cell. For series connected sub-cells, based on a size-band gap dependence obtained from Quantum Monte Carlo calculations, we obtained the maximum detailed balance limit of efficiency to be 48% and 37% for concentrated and unconcentrated sunlight, respectively, for a nanowire diameter of 3.6 nm. The same efficiency would be obtained if the top tandem cell would be composed of AlGaAs nanowires, with an Al content of 19%. Ultra-high efficiencies could be achieved by a radial p-n junction nanowire tandem cell, since it could enable growth of lattice mismatched sub-cells whose band gap energies can be optimized and secondly, due to decoupling of carrier collection and light absorption. However, for a parallel connection of the sub cells, we show that the detailed balance limit of efficiency for the tandem cell will always be lower than an optimized single junction. Finally, we discuss future challenges and directions in realizing the designs. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.