Browsing by Author "Chen, Po Yu"

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Arbitrarily-Wide-Band Dielectric Mirrors and Their Applications to SiGe Solar Cells
(2015-08-01)
Zhong, Yan Kai
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Fu, Sze Ming
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Yan, Sheng Lun
;
Chen, Po Yu
;
Lin, Albert
The dielectric mirror is an important optical component for optoelectronic devices, passive photonic devices, and solar cells. Unfortunately, the reflection bandwidth of distributed Bragg reflectors (DBRs) and high-index contrast mirrors (HCGs) are limited by the index contrast of the material system used. Here, an aperiodic design for dielectric mirrors is proposed, and it is shown that for a fixed index contrast, the bandwidth of the reflection band can be arbitrarily widened by simply incorporating more dielectric layers. This is pronouncedly different from the fixed bandwidth of HCGs and DBRs. The physics behind the broadband reflection for the aperiodic stacking is identified as the photonic bandgap widening due to the annihilation of the quasi-guided modes in nonperiodic structures. This observation applies very well to aperiodic auto-cloned 3-D photonic crystal reflectors, to aperiodic DBRs, and even to diffuse dielectric mirrors that have recently emerged to be very promising for solar cells due to their zero plasmonic absorption nature. Experimentally, the white paint diffuse medium reflectors are applied to SiGe solar cells to confirm their high reflectance and the feasibility of enhancing solar cell efficiency.
Experimentally-implemented genetic algorithm (Exp-GA): toward fully optimal photovoltaics
(2015-09-21)
Zhong, Yan Kai
;
Fu, Sze Ming
;
Ju, Nyan Ping
;
Chen, Po Yu
;
Lin, Albert
The geometry and dimension design is the most critical part for the success in nano-photonic devices. The choices of the geometrical parameters dramatically affect the device performance. Most of the time, simulation is conducted to locate the suitable geometry, but in many cases simulation can be ineffective. The most pronounced examples are large-area randomized patterns for solar cells, light emitting diode (LED), and thermophtovoltaics (TPV). The large random pattern is nearly impossible to calculate and optimize due to the extended CPU runtime and the memory limitation. Other scenarios that numerical simulations become ineffective include three-dimensional complex structures with anisotropic dielectric response. This leads to extended simulation time especially for the repeated runs during its geometry optimization. In this paper, we show that by incorporating genetic algorithm (GA) into real-world experiments, shortened trial-and-error time can be achieved. More importantly, this scheme can be used for many photonic design problems that are unsuitable for simulation-based optimizations. Moreover, the experimentally implemented genetic algorithm (Exp-GA) has the additional advantage that the resultant objective value is a real one rather than a theoretical one. This prevents the gaps between the modeling and the fabrication due to the process variation or inaccurate numerical models. Using TPV emitters as an example, 22% enhancement in the mean objective value is achieved. (C) 2015 Optical Society of America
Non-reciprocal meta-surfaces for nanophtonic light trapping: optical isolators versus solar cells
(2015)
Fu, Sze Ming
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Zhong, Yan Kai
;
Yan, Sheng Lun
;
Ju, Nyan Ping
;
Chen, Po Yu
Nano-photonic concept has been shown to be very effective for boosting solar cell conversion efficiency. In this work, the novel concepts of non-reciprocality and chiral meta-surface are demonstrated to be very useful for photovoltaics. The asymmetric chiral patterns are designed and optimized using genetic algorithm. The enhanced optical absorption is observed and it is attributed to the non-reciprocal nature of the solar cell front meta-surface. The asymmetric solar cell front surface enhances the anti-reflection and, at the same time, forbids the photon escape once it enters the devices. This concept is very similar to optical isolators where one-way transmission is intentionally designed for enhanced optical diode effect. Preliminary experimental results are supplied to confirm the superiority of asymmetric chiral design. The ongoing experimental effort is adjusting the PECVD deposition conditions to boost the efficiency on patterned substrates.
Photonic Density of State (PDOS) Enhancement by Brillouin Zone Engineering
(2015)
Fu, Sze Ming
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Zhong, Yan Kai
;
Yan, Sheng Lun
;
Ju, Nyan Ping
;
Chen, Po Yu
The material absorption can be enhanced by increased photonic density of state(PDOS). Here we demonstrate that through the design in the Brillouin zone of nanophotonic light trapping structures, the PDOS can be effectively increased. The theoretical work using rigorously coupled wave calculation shows the increased spectral absorption and the optimal quasi-guided mode excitations in terms of their quality factors and the number of excited modes. The experimental work is currently ongoing and the preliminary result confirms enhanced short circuit current if the PDOS is properly adjusted. Unlike random structures, the proposed Brillouin zone engineered structures are more feasible from modeling and device process viewpoints.
The rigorous wave optics design of diffuse medium reflectors for photovoltaics
(2014-04-21)
Lin, Albert
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Fu, Sze Ming
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Zhong, Yan Kai
;
Tseng, Chi Wei
;
Chen, Po Yu
Recently, diffuse reflectors are being incorporated into solar cells, due to the advantage of no metallic absorption loss, higher reflectance, decent light scattering property by embedded TiO2 scatterers, and the ease of fabrication. Different methods have been employed to analyze diffuse reflectors, including Monte Carlo method, N-flux method, and a one-dimensional approximation based on semi-coherent optics, and the calculated reflectance is around 80% by these methods. In this work, rigorous wave optics solution is used, and it is shown that the reflectance for diffuse medium mirrors can actually be as high as >99% over a broad spectral range, provided the TiO2 scatterer geometry is properly optimized. The bandwidth of diffuse reflectors is un-achievable by other dielectric mirrors such as distributed Bragg reflectors or high index contrast grating mirror, using the same index contrast. Finally, it is promisingly found that even if the distribution of TiO2 is random, the wide-band reflection can still be achieved for the optimized TiO2 geometry. Initial experimental result is included in the supplementary material which shows the high feasibility of diffuse medium mirrors for solar cells. (C) 2014 AIP Publishing LLC.