Surface-passivated GaAsP single-nanowire solar cells exceeding 10% efficiency grown on silicon

Abstract

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...

Tailoring the diameter and density of self-catalyzed GaAs nanowires on silicon

“…Furthermore, GaAs nanowire-based solar cells can be grown on Si, easily resulting in a dual-junction device [7]. Proof-of-concept radial p-i-n GaAs nanowire solar cells have been demonstrated in the past [8][9][10][11][12][13]. Recently, it has been shown that the achievement of high efficiencies can only be obtained after the accurate design of the individual junction, the nanowire diameter, and density so that light absorption and conversion are maximized [14,15].…”

Position-Controlled Uniform GaAs Nanowires on Silicon using Nanoimprint Lithography

“…Holm et al fabricated solar cells from broken GaAsP NWs that were grown on Si substrates [200]. By using an InGaP passivation layer on the GaAsP NWs, the efficiency of the best cell was reported to improve by 50%, from 6.8% in unpassivated NWs to 10.2% in passivated NWs.…”

“…-The efficiency of the solar cell has been reported to improve by using a passivation layer [32,200]. Holm et al fabricated solar cells from broken GaAsP NWs that were grown on Si substrates [200].…”

Nanophotonic integrated circuits from nanoresonators grown on silicon

“…While SiO 2 offers remarkable surface passivation properties for silicon 10 , a comparable solution for III-V semiconductors remains absent. On the other hand, low-defect single-crystal nanostructures can be grown with atomically flat surfaces and in situ surface passivation via higher band gap cladding layers 11,12 , which have historically proven to provide the best passivation for III-V materials. With tiny footprints, nanostructures incur minimal real estate cost while offering full photonic functionality.…”