Nanophotonic light trapping with patterned transparent conductive oxides

Geballe Laboratory for Advanced Materials, Stanford University, Stanford, CA, USA.
Optics Express (Impact Factor: 3.49). 05/2012; 20(10):A385-94. DOI: 10.1364/OE.20.00A385
Source: PubMed


Transparent conductive oxides (TCOs) play a crucial role in solar cells by efficiently transmitting sunlight and extracting photo-generated charge. Here, we show how nanophotonics concepts can be used to transform TCO films into effective photon management layers for solar cells. This is accomplished by patterning the TCO layer present on virtually every thin-film solar cell into an array of subwavelength beams that support optical (Mie) resonances. These resonances can be exploited to concentrate randomly polarized sunlight or to effectively couple it to guided and diffracted modes. We first demonstrate these concepts with a model system consisting of a patterned TCO layer on a thin silicon (Si) film and outline a design methodology for high-performance, TCO-based light trapping coatings. We then show that the short circuit current density from a 300 nm thick amorphous silicon (a-Si) cell with an optimized TCO anti-reflection coating can be enhanced from 19.9 mA/cm2 to 21.1 mA/cm2, out of a possible 26.0 mA/cm2, by using an optimized nanobeam array. The key differences and advantages over plasmonic light trapping layers will be discussed.

Download full-text


Available from: Alok Vasudev
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The coupling between free space radiation and optical media critically influences the performance of optical devices. We show that, for any given photonic structure, the sum of the external coupling rates for all its optical modes are subject to an upper bound dictated by the second law of thermodynamics. Such bound limits how efficient light can be coupled to any photonic structure. As one example of application, we use this upper bound to derive the limit of light absorption in broadband solar absorbers.
    Full-text · Article · Oct 2012 · Physical Review Letters
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: High-index dielectric or semiconductor nanoparticles support strong Mie-like geometrical resonances in the visible spectral range. We use 30 keV angle-resolved cathodoluminescence imaging spectroscopy to excite and detect these resonant modes in single silicon nanocylinders with diameters ranging from 60 - 350 nm. Resonances are observed with wavelengths in the range 400 - 700 nm, with quality factors in the range Q = 9 - 77, and show a strong redshift with increasing cylinder diameter. The photonic wavefunction of all modes is determined at deep-subwavelength resolution and shows good correspondence with numerical simulations. An analytical model is developed that describes the resonant Mie-like optical eigenmodes in the silicon cylinders using an effective index of a slab waveguide mode. It shows good overall agreement with the experimental results and enables qualification of all resonances with azimuthal (m = 0 - 4) and radial (q = 1 - 4) quantum numbers. The single resonant Si nanocylinders show characteristic angular radiation distributions in agreement with the modal symmetry.
    Full-text · Article · Jan 2013 · ACS Nano
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, a rigorous full-vectorial finite-element (FE)-based beam propagation method (BPM) has been implemented to study second-harmonic generation (SHG) in planar zinc oxide (ZnO) waveguides for the first time. It is shown here that the SHG output power is significantly improved when the waveguide structure is optimized. Furthermore, phase matching between the fundamental and second-harmonic modes, through the use of the quasi-phase matching technique, is discussed.
    Full-text · Article · Apr 2013 · IEEE Photonics Journal
Show more