Continued development of high-efficiency multi-junction solar cells requires growth of latticemismatched materials. Today, the need for lattice matching both restricts the bandgap combinations available for multi-junctions solar cells and prohibits monolithic integration of high-efficiency III-V materials with low-cost silicon solar cells. The use of III-V nanowires is the only known method for circumventing this lattice-matching constraint, and therefore it is necessary to develop growth of nanowires with bandgaps 41.4 eV. Here we present the first gold-free gallium arsenide phosphide nanowires grown on silicon by means of direct epitaxial growth. We demonstrate that their bandgap can be controlled during growth and fabricate core-shell nanowire solar cells. We further demonstrate that surface passivation is of crucial importance to reach high efficiencies, and present a record efficiency of 10.2% for a core-shell single-nanowire solar cell. I mproving the cost/efficiency ratio of III-V-based multijunction cells can be done through efficiency enhancements, by adding additional junctions to the cell stack 1 or through the lowering of cost by replacing the expensive germanium substrate with silicon. Integrating III-V semiconductors and silicon requires overcoming their differences in lattice parameters and thermal expansion coefficient, as well as their polar/non-polar interfaces 2,3 . When constrained to a silicon-bottom cell, the optimum dualjunction solar cell, the simplest multi-junction solar cell, has a theoretical 1-sun peak efficiency between 33 and 43% (refs 4,5) when combined with a 1.7 eV bandgap top cell; this can be achieved with a III-V semiconductor consisting of GaAs 0.8 P 0.2.The small contact interface between the nanowire and the silicon substrate ensures that strain from lattice mismatch is relaxed within the first few monolayers 6 . Using the method of gallium (Ga)-assisted growth, gold-free perfect single crystal, gallium arsenide (GaAs) nanowires have been grown directly on silicon 7 . Higher bandgap gallium phosphide (GaP) 8 and gallium arsenide phosphide (GaAsP) 9,10 nanowires have also been grown, but only using gold as growth catalyst. Gold is incompatible with silicon 11 , and has been shown to incorporate into the III-V crystal 12 and degrade the optoelectronic properties of the nanowires 13 . In addition, a sparse array of nanowires can absorb almost all incoming light 14 , meaning that only minute amounts of the expensive III-V material is needed for making a nanowire top cell. Devices consisting of a contacted ensemble of free-standing nanowires have been exposed to temperature changes of up to 200 K, demonstrating that their strain-relieving ability is able to overcome the difference in thermal expansion 15 . To fully utilize their advantage with regards to non-lattice-matched growth, the nanowires themselves must also be able to function well as solar cells.
ResultsGa-assisted GaAs 0.8 P 0.2 nanowire growth. High-quality GaAsP nanowires were grown without the use of a buffer la...