Light Enhancement in Intermediate Band Solar Cells with Near‐Field Structures Manuel J. Mendes, Elisa Antolín, Ignacio Tobías, Antonio Martí, Antonio Luque Instituto de Energía Solar, Universidad Politécnica de Madrid, E28040 Madrid, Spain
Background • The Intermediate Band Solar Cell (IBSC)1 can convert a greater portion of the solar spectrum into electrical energy than single‐gap cells due to the existence of an electronic band — the intermediate band (IB) — within the semiconductor bandgap. • This concept has a detailed balance efficiency limit of 63.2% compared to 40.7% for single‐gap cells1. • The Intermediate Band can be formed by the confined levels of a quantum dot (QD) array2; however the associated absorption coefficient is weak3. • To increase absorption we propose two procedures which employ near‐field structures designed to amplify light in the QDs array at the particular photon energies of the IBSC absorption bands.
InAs QDs in GaAs host
(a)
30 nm
1 mm side
InAs dots
(b)
(a)
Left – (a) TEM image of a 10 QD layer stack, with 10nm thick GaAs spacer layers. (b) AFM image of InAs dots grown on the surface of GaAs.
30 nm GaAs matrix
Surface Plasmons in Metallic Nano‐Spheroids
Wavelength‐sized Dielectric Spheroids
This alternative exploits the high near‐field in the vicinity of metal nanoparticles (MNPs) sustaining surface plasmons. The inclusion of these particles close to the QDs can amplify their absorption. The scattered field is computed using the electrostatic approach (EA) for particles which are small compared to the light’s wavelength4.
Dielectric particles perform better than metallic ones when their size is close to or above the incident wavelength (l). In the cases analyzed here6, high field enhancements are observed with spheroidal scatterers having a refractive index of nr=1.33 relative to the surrounding medium. This could be achieved, for instance, with Ge particles embedded in GaAs medium.
Right ‐ Preferential position for inclusion of MNPs in the plane of the QDs (pyramids). MNP sizes in the range of 10‐100nm should behave adequately under EA. The absorption enhancement produced by an MNP is significant up to a distance of about its size.
Left – Distributions of the total electric field intensity in the incidence plane, for an oblate (left) and prolate (right) spheroid with size parameter C=7 and aspect ratio 2. The length unit is l.
Left ‐ Spheroid aspect ratios that maximize scattering for coated and uncoated Ag MNPs in GaAs medium. Metallic particles act as recombination centers causing current degradation, so they should be enclosed in a passivating coating.
Right ‐ Maximum absorption enhancement that can be felt by the QDs5 at the MNP surface, for spheroids shaped for 0.3 and 1eV ‐ the sub‐gap bands in InAs/GaAs QD‐IBSC. The curves for the 0.3eV case are multiplied by 10 for better visualization. The inset is a polar‐plot of the scattered potential at the uncoated MNP surface.
uncoated with coating
References [1] A. Luque and A. Martí. Phys. Rev. Lett. 78, No. 26, 5014–5017 (1997) [2] A. Martí, L. Cuadra and A. Luque. 28th IEEE Photovoltaics Specialists Conference (2000) [3] E. Antolín et al. Thin Solid Films 516, 6919‐23 (2008) [4] M. J. Mendes, A. Luque, I. Tobías and A. Martí. App. Phys. Lett. 95, 1 (2009) [5] A. Luque, A. Marti, M. J. Mendes and I. Tobias. J. App. Phys. 104, 113118 (2008) [6] M. J. Mendes, I. Tobías, A. Martí and A. Luque. J. Opt. Soc. Am. B 27, 1221‐1231 (2010)
Left ‐ Angle (in degrees) between the incident E‐field direction and the position of maximum field enhancement. The schematic ES and KS vectors represent the polarization of the scattered field at the spot of maximum E‐field. Three regimes can be distin‐ guished: electrostatic, transition and forward scattering.
Right – Electric field intensity profiles along the radial direction of maximum field, for spheroids with sizes approximately equal to l. r is the distance to the particle center, in units of l. The grey crossed circles are at the particle surface. The inset shows the profiles of spheres with R=l and distinct refractive indices nr.
Acknowledgements This work has been supported by the IBPOWER Grant No. 211640 of European Commission, by the GENESIS FV grant No. CSD2006‐0004 of the Spanish program CONSOLIDER, and by the NUMANCIA grant No. S‐ 0505/ENE/0310 of Comunidad de Madrid. M. J. Mendes acknowledges the Portuguese Government fellowship SFRH/BD/21669/2005 of FCT‐MCTES.
For further information Please contact manuel.mendes@ies‐def.upm.es or elisa@ies‐def.upm.es. More information on these and related projects can be obtained at www.ies.upm.es.
