Fundamental limits in the external quantum efficiency of single nanowire solar cells

Heiß, Martin; Morral, Anna Fontcuberta i

doi:10.1063/1.3672168

Cited by 47 publications

(45 citation statements)

References 24 publications

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“…20). For a device geometry similar to ours, it is predicted that roughly half of the projected sunlight is absorbed 21 , which accounts well for the discrepancy between measured and Shockley-Queisser I SC . The measured V OC is also about half of the Shockley-Queisser limit, but this cannot be accounted for by the low absorption in this solar-cell geometry.…”

Section: Resultssupporting

confidence: 54%

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

Holm¹,

Jørgensen²,

et al. 2013

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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...

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Section: Resultssupporting

confidence: 54%

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

Holm¹,

Jørgensen²,

et al. 2013

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“…To investigate the possible differences in temperature, we simulated light absorption for the two geometries using the finite-difference time-domain package MEEP [34] treating the incident light as a plane wave polarized along the NW axis. For the absorption efficiency, we calculated the flux inside a box around the NW given by a top plane wave incoming onto it [35] with reference to a simulation performed without the NW. The flux through the projected area of the NW on the bottom face of the box was calculated.…”

Section: B Amentioning

confidence: 99%

Morphology and composition of oxidized InAs nanowires studied by combined Raman spectroscopy and transmission electron microscopy

et al. 2016

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Any device exposed to ambient conditions will be prone to oxidation. This may be of particular importance for semiconductor nanowires because of the high surface-to-volume ratio and only little is known about the consequences of oxidation for these systems. Here, we study the properties of indium arsenide nanowires which were locally oxidized using a focused laser beam. Polarization dependent micro-Raman measurements confirmed the presence of crystalline arsenic, and transmission electron microscopy diffraction showed the presence of indium oxide. The surface dependence of the oxidation was investigated in branched nanowires grown along the [ ] 0001 and [¯] 0110 wurtzite crystal directions exhibiting different surface facets. The oxidation did not occur at the [¯] 0110 direction. The origin of this selectivity is discussed in terms transition state kinetics of the free surfaces of the different crystal families of the facets and numerical simulations of the laser induced heating.

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“…For low-absorbing materials (for example, indirect bandgap materials such as silicon), waveguiding effects plays a key role 23,24 , whereas highly absorbing semiconductors (such as direct-bandgap GaAs) exhibit resonances that increase the total absorption several times. Nanowires lying on a substrate also exhibit such resonances, often described by Mie theory 25,26 , although the total absorption rate is significantly lower 27,28 . Even though the optical absorption of nanowires arranged in an array has been shown to be far more complex than in thin films, nanowire vertical arrays currently seem to be the most reasonable device proposal.…”

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confidence: 99%

Single-nanowire solar cells beyond the Shockley–Queisser limit

Jørgensen²,

et al. 2013

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Light management is of great importance in photovoltaic cells, as it determines the fraction of incident light entering the device. An optimal p-n junction combined with optimal light absorption can lead to a solar cell efficiency above the Shockley-Queisser limit. Here, we show how this is possible by studying photocurrent generation for a single core-shell p-i-n junction GaAs nanowire solar cell grown on a silicon substrate. At 1 sun illumination, a short-circuit current of 180 mA cm -2 is obtained, which is more than one order of magnitude higher than that predicted from the Lambert-Beer law. The enhanced light absorption is shown to be due to a light-concentrating property of the standing nanowire, as shown by photocurrent maps of the device. The results imply new limits for the maximum efficiency obtainable with III-V based nanowire solar cells under 1 sun illumination.N anowire-based solar cells hold great promise for third-generation photovoltaics and for powering nanoscale devices 1,2 . With the advent of third-generation photovoltaics, solar cells will become cheaper and more efficient than current devices. In particular, a cost reduction may be achieved by reducing material use through the fabrication of nanowire arrays and radial p-n junctions [3][4][5] . The geometry of nanowire crystals is expected to favour elastic strain relaxation, providing great freedom in the design of new compositional multijunction solar cells 6 grown on mismatched materials 7,8 . The efficiencies of nanostructured solar cells have increased over time and have now reached up to 13.8%, due to improvements in materials and new device concepts [9][10][11][12][13][14] .Light absorption in standing nanowires is a complex phenomenon, with a strong dependence on nanowire dimensions and the absorption coefficient of the raw materials [15][16][17][18] . In low-absorbing microwire arrays, such as those composed of silicon, light absorption is understood via ray optics or by calculation of the integrated local density of optical states of the nanowire film 19,20 . Interestingly, when these arrays stand on a Lambertian back-reflector, an asymptotic increase in light trapping for low filling factors (FFs) is predicted 19 . This is advantageous for improvement of the efficiency-to-cost ratio of solar cells and has led to the demonstration of microwire arrays exhibiting higher absorption than in the equivalent thickness of textured film 19,21,22 . The case for nanowires is quite different. Nanowire diameters are smaller than or comparable to the radiation wavelength. In this case, optical interference and guiding effects play a dominant role in relation to reflectivity and absorption spectra. For low-absorbing materials (for example, indirect bandgap materials such as silicon), waveguiding effects plays a key role 23,24 , whereas highly absorbing semiconductors (such as direct-bandgap GaAs) exhibit resonances that increase the total absorption several times. Nanowires lying on a substrate also exhibit such resonances, often described by Mi...

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Fundamental limits in the external quantum efficiency of single nanowire solar cells

Cited by 47 publications

References 24 publications

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

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

Morphology and composition of oxidized InAs nanowires studied by combined Raman spectroscopy and transmission electron microscopy

Single-nanowire solar cells beyond the Shockley–Queisser limit

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