Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

High-performance photovoltaic cells use semiconductors to convert sunlight into clean electrical power, and transparent dielectrics or conductive oxides as antireflection coatings. A common feature of these materials is their high refractive index. Whereas high-index materials in a planar form tend to produce a strong, undesired reflection of sunlight, high-index nanostructures afford new ways to manipulate light at a subwavelength scale. For example, nanoscale wires, particles and voids support strong optical resonances that can enhance and effectively control light absorption and scattering processes. As such, they provide ideal building blocks for novel, broadband antireflection coatings, light-trapping layers and super-absorbing films. This Review discusses some of the recent developments in the design and implementation of such photonic elements in thin-film photovoltaic cells.

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Light management for photovoltaics using high-index nanostructures.

Recent biomedical applications of gold nanoparticles: A review.

Most photovoltaic (solar) cells are made from crystalline silicon (c-Si), which has an indirect band gap. This gives rise to weak absorption of one-third of usable solar photons. Therefore, improved light trapping schemes are needed, particularly for c-Si thin film solar cells. Here, a photonic crystal-based light-trapping approach is analyzed and compared to previous approaches. For a solar cell made of a 2 µm thin film of c-Si and a 6 bilayer distributed Bragg reflector (DBR) in the back, power generation can be enhanced by a relative amount of 24.0% by adding a 1D grating, 26.3% by replacing the DBR with a six-period triangular photonic crystal made of air holes in silicon, 31.3% by a DBR plus 2D grating, and 26.5% by replacing it with an eight-period inverse opal photonic crystal.

Improving thin-film crystalline silicon solar cell efficiencies with photonic crystals

We use a rigorous electromagnetic approach to develop a light-trapping theory, which reveals that the standard limit developed by Yablonovitch can be substantially surpassed in nanophotonic regimes, opening new avenues for highly efficient solar cells

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Fundamental limit of nanophotonic light-trapping in solar cells

An efficient light-trapping scheme is developed for solar cells that can enhance the optical path length by several orders of magnitude using a textured photonic crystal as a backside reflector. It comprises a reflection grating etched on the backside of the substrate and a one-dimensional photonic crystal deposited on the grating. Top-contacted crystalline Si solar cells integrated with the textured photonic crystal back reflector were designed and fabricated. External quantum efficiency was significantly improved between the wavelengths of 1000 and 1200nm (enhancement up to 135 times), and the overall power conversion efficiency was considerably increased.

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

Herein the authors report the experimental application of a powerful light trapping scheme, the textured photonic crystal (TPC) backside reflector, to thin film Si solar cells. TPC combines a one-dimensional photonic crystal as a distributed Bragg reflector with a diffraction grating. Light absorption is strongly enhanced by high reflectivity and large angle diffraction, as designed with scattering matrix analysis. 5 μm thick monocrystalline thin film Si solar cells integrated with TPC were fabricated through an active layer transfer technique. Measured short circuit current density Jsc was increased by 19%, compared to a theoretical prediction of 28%.

Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector

Aperiodic one-dimensional Si∕SiO2 Thue–Morse (T–M) multilayer structures have been fabricated in order to investigate both the band gap properties with respect to the system size (band gap scaling) and the omnidirectional reflectance at the fundamental optical band gap. Variable angle reflectance data have experimentally demonstrated a large reflectance band gap in the optical spectrum of a T–M quasicrystal, in agreement with transfer matrix simulations. We explain the physical origin of the T–M omnidirectional band gap as a result of periodic spatial correlations in the complex T–M structure. The unprecedented degree of structural flexibility of T–M systems can provide an attractive alternative to photonic crystals for the fabrication of photonic devices.

Photon band gap properties and omnidirectional reflectance in Si∕SiO2 Thue–Morse quasicrystals

Light-emitting silicon-rich, SiNx∕SiO2 Thue-Morse (T-M) multilayer structures have been fabricated in order to investigate the generation and transmission of light in strongly aperiodic deterministic dielectrics. Photoluminescence and optical transmission data experimentally demonstrate the presence of emission enhancement effects occurring at wavelengths corresponding to multiple T-M resonance states. Emission enhancement effects by a factor of almost 6 with respect to homogeneous SiNx dielectrics have been experimentally measured, in good agreement with transfer matrix simulations. The unprecedented degree of structural flexibility of T-M systems can provide alternative routes towards the fabrication of optically active multiwavelength photonic devices.

Spectrally enhanced light emission from aperiodic photonic structures

Both theoretically and experimentally, we demonstrate that the photonic band gaps in Thue–Morse aperiodic systems can be separated into two flavors, the fractal gaps and the traditional gaps, distinguished by the presence or absence of fractal structure, respectively. The origin of two kind gaps is explained by the different interface correlations. This explanation is confirmed by the gap width behaviors. In addition, the eigenstates near the fractal gaps have a cluster-periodic form, while those near the traditional gaps have the Bloch wavelike form. Our detailed study of these differences is essential for understanding the spectra and light localization in aperiodic systems.

Yasha Yi

Papers

Efficiency enhancement in Si solar cells by textured photonic crystal back reflector

Demonstration of enhanced absorption in thin film Si solar cells with textured photonic crystal back reflector

Photon band gap properties and omnidirectional reflectance in Si∕SiO2 Thue–Morse quasicrystals

Spectrally enhanced light emission from aperiodic photonic structures

Photonic band gaps and localization in the Thue–Morse structures