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news digest ♦ Novel Devices placing new demands on microoptics for these applications.


These needs include a broad NIR - MWIR transmission, high numerical aperture (NA), and small form factor for minimally invasive applications.


Jenoptik´s Grayscale lithographically fabricated GaP microlenses and microlens arrays meet these demands with:


1) A broad wavelength range from 600 nm beyond 5 µm


2) A high refractive index of 3.1 allowing a single element lens or lens array with NA’s up to 0.85.


3) Complex surface shapes providing for beam collimation and circularization of high divergence diode lasers with a single element.


Scanning confocal microscopy and minimally invasive optical coherence tomography are two examples where a single GaP microlens or microlens array can be used to extend the wavelength range over traditional GRIN and silicon lenses.


Jenoptik’s says it fabricates complex aspheric elements with high numerical aperture also provide for better imaging performance when compared with GRIN lens based systems.


What’s more, standard manufacturing processes are available for a range of different optical materials such as SiO2, GaAs, CaF2, Al2O3, ZnS, ZnSe, Ge, and chalcogenide glass.


Record-breaking antireflection coating on


solar cells A new development could significantly improve the performance of solar cells, LEDs and photodetectors


A team of researchers from Rensselaer Polytechnic Institute (RPI), Magnolia Solar, Inc. and Pohang University of Science and Technology have demonstrated a novel antireflection (AR) coating.


It beats the widely employed double-layer AR (DLAR) coating on state-of-the-art triple-junction solar cells.


The scientists demonstrated that the solar cells investigated gain over 4 percent in efficiency when replacing the industry- standard DLAR with an optimised four-layer AR coating.


efficiency.


What’s more, omnidirectional AR characteristics have become important for the rapidly expanding terrestrial application of solar cells. This is because solar irradiance in terrestrial applications usually has a large range of incident angles for non-tracking solar cells.


Both broadband and omnidirectional AR characteristics are attainable by four-layer AR coatings, as demonstrated by the RPI-led team.


RPI says that the excellent broadband and omnidirectional AR characteristics of the four-layer AR coating are achieved through solving the problem of refractive index matching at multiple layer interfaces.


By using tailored and low-refractive index nanoporous silica layers, the team has greatly reduced the refractive index contrast at the semiconductor / AR coating / air interfaces.


Through a multilayer design methodology powered by a genetic algorithm optimisation, favourable antireflective properties over a specified wavelength range and angle-of-incidence range were found.


Two porous layers of the four-layer AR coating were fabricated by oblique-angle deposition of silica thereby resulting in films with refractive indices of 1.32 and 1.11. This is less than the refractive index of silica. The other two layers are dense and were fabricated by co-deposition of silica / titania using sputtering.


According to the photocurrent measurements performed by the team, the angle-of-incidence (0°- 80°) averaged photocurrent enhancement (over an uncoated triple-junction solar cell) of the four-layer AR coating amounts to 34.4 percent. The enhancement of a DLAR coating is only 25.3 percent.


In the future, the team members Jaehee Cho and E. Fred Schubert, will integrate this novel AR coating technology on surface-textured devices.


They will also investigate innovative fabrication methods for depositing low-refractive index AR coatings on curved surfaces, such as a hemispherical lens.


Further details of this research are described in the paper, “Enhanced Omnidirectional Photovoltaic Performance of Solar Cells Using Multiple-Discrete-Layer Tailored - and Low-Refractive Index Anti-Reflection Coatings,” by X. Yan et al in Advanced Functional Materials, 23, 583 (2013).


Separating the core and shell


Considering that the solar spectrum is an intrinsically broadband spectrum, such broadband characteristics of the AR coating are undoubtedly beneficial for high power conversion


142 www.compoundsemiconductor.net March 2013


of CdSe quantum dots LEDs, solar cells and sensors could benefit from a new technology to study the interface between the core and shell of quantum dots composed of CdTe based materials


The J. William Fulbright College of Arts & Sciences at the


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