科研之友
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摘要
A lateral multijunction photovoltaic (PV) concept is introduced that explores the unique ability of plasmonic nanoantennas to locally concentrate optical energy and spectrally filter incoming light at the subwavelength level. This electromagnetic field enhancement near the localized plasmon resonance modes of the metallic nanoantennas can be used to selectively increase light absorption in semiconductor nanowires at specific spectral and spatial regions. In our geometry, we take advantage of the ring antenna's ability to excite two distinct plasmon modes in order to carry out spectral splitting and concentration of the electromagnetic field. A localized dipolar surface plasmon mode near the material resonance of the silver nanoantenna results from the ring behaving as an effective disk in the visible region and focuses the field on the external surface of the ring while a dipolar bonding resonance mode dependent on the coupling of modes excited on the inner and outer surface of the ring geometry in the near infrared (NIR) region focuses energy in the cavity of the ring. Using finite difference time domain (FDTD) simulations, we describe the basic mechanisms at work and demonstrate that the subwavelength ring antennas can couple incident light into semiconductor nanowires placed both inside and outside the ring through the two modes with minimal loss in the metal. The modes are used to laterally split different spectral regions of broadband incident light optimized to the material bandgap of the nanowires located in the regions of field enhancement to produce the lateral multijunction effect. We demonstrate that, for example, a ring antenna with both an internal diameter and a thickness of 40 nm can enhance absorption by 6x in the visible region for a 100 nm tall AlAs nanowire placed just outside the ring and by 380x in the NIR region for a geometrically similar GaAs nanowire placed inside the ring. Both enhancements occur just above the material band gaps of the nanowires. These findings, hence, show promise for the design of multijunction nanowire-based PV cells since the enhanced absorption allows for shorter nanowires, reducing both surface recombination and material cost. The dipolar bonding mode is particularly dependent on the geometry and size of the antenna, allowing the incoupling cross section to be spectrally tuned and enhanced across a wide range of wavelengths. Enhancement suffers, however, if the resonance peak is redshifted too far into the NIR region. We also explored the effects of periodicity and incident angle on absorption and find that there is a trade-off between several different resonant phenomena that can change both the spectral features and coupling efficiency of the geometry.
- 出版日期2011-6-1
