We explore the mechanisms for an efficient light trapping structure for thin-film silicon solar cells. The design combines a distributed Bragg reflector (DBR) and periodic gratings. Using photonic band theories and numerical simulations, we discover that light can be scattered into the DBR by gratings, with an unusual way of light trapping different from metal reflectors and photonic crystals. We further investigate the influence of DBR on generated photocurrent in different device configurations. These results would provide new design rules for photonic structures in thin-film solar cells. V
We developed a high-index-contrast photonic structure for improving the light extraction efficiency of light-emitting diodes (LEDs) by a self-assembly approach. In this approach, a two-dimensional grating can be non-lithographically integrated on the top of virtually any types of LEDs with controlled structural parameters and material indices. As a proof of concept, our designed photonic structure was implemented on a GaAs double heterojunction LED. Using numerical electromagnetic simulations, we explored the effects of the structural parameters (the grating period, layer thickness and material indices) on the device performances, followed by fabrication through a self-assembled porous alumina as a template. Device simulation and experimental results indicate that an optimized high-index-contrast (a-Si / air) grating obtains a much larger efficiency increase than using a low-index SiO2 grating. In addition, the devices maintain a Lambertian radiation pattern with the self-assembled grating. This technique provides an effective and low-cost method for improving LED efficiency.
A compact, single element concentrator comprising a near linear array of prisms has been designed to simultaneously split and concentrate the solar spectrum. Laterally aligned solar cells with different bandgaps are devised to be fabricated on a common Si substrate, with each cell absorbing a different spectral band optimized for highest overall power conversion efficiency. Epitaxial Ge on Si is used as a low cost virtual substrate for III-V materials growth. Assuming no optical loss for the prism concentrator, no shadowing and perfect carrier collection for the solar cells, simulations show that 39% efficiency can be achieved for a parallel four-junction (4PJ) InGaP-GaAs-Si-Ge cell under 200X concentration, and higher efficiency is possible with more junctions.
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