Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

Summary

The fabrication of high contrast gratings as the parallel spectrum splitting dispersive element in a concentrated photovoltaic system is demonstrated. Fabrication processes including nanoimprint lithography, TiO₂ sputtering and reactive ion etching are described. Reflectance measurement results are used to characterize the optical performance.

Abstract

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve the solar conversion efficiency of a concentrated photovoltaic system. The proposed system will also lower the solar cell cost in the concentrated photovoltaic system by replacing the expensive tandem solar cells with the cost-effective single junction solar cells. The structures and the parameters of high contrast gratings for the dispersive elements were numerically optimized. The large-area fabrication of high contrast gratings was experimentally demonstrated using nanoimprint lithography and dry etching. The quality of grating material and the performance of the fabricated device were both experimentally characterized. By analyzing the measurement results, the possible side effects from the fabrication processes are discussed and several methods that have the potential to improve the fabrication processes are proposed, which can help to increase the optical efficiency of the fabricated devices.

Introduction

Our modern society will not survive without moving a significant portion of energy consumption to renewable energy sources. To make this happen, we have to find a way to harvest renewable energy at a cost lower than petroleum-based energy sources in the near future. Solar energy is the most abundant renewable energy on earth. Despite that a lot of progresses have been made in solar energy harvesting, it is still very challenging to compete with petroleum-based energy sources. Improving the efficiency of solar cells is one of the most efficient ways to lower the system cost of solar energy harvesting.

Optical lenses and dish reflectors are u....

Protocol

1. Prepare the Blank Polydimethylsiloxane (PDMS) Substrate for Nanoimprint Mold

1. Silicon Wafer Treatment Process
   1. Clean a 4 inch silicon wafer by rinsing with acetone, methanol and isopropanol.
   2. Blow it dry using the nitrogen gun.
   3. Clean it using piranha solution (3:1 mixture of sulfuric acid with 30% hydrogen peroxide) by soaking inside for 15 min.
   4. Rinse it with DI water. Blow dry using the nitrogen gun.
   5. Place the wafer in a glass desiccator. Add a drop (20 drops = 1 ml) of releasing agent (trichlorosilane) into the desiccator.
   6. Pump down the desiccator until the gauge reads -762 Torr and wait ....

Results

Figure 1 shows the implementation of the dispersive element (multilayer high contrast grating (HCG)) in a concentrated photovoltaic system. The sun light is first reflected by the primary mirror and impinges on the reflective dispersive element, where the beam is reflected and split into different bands of different wavelengths. Each band will impinge on a certain location on the solar cell array for the best absorption and conversion to electricity. The key to this system is the design and implementatio.......

Discussion

First, the quality of the TiO₂ film is very crucial for the HCG performance. The reflectance peak will be higher if the TiO₂ film has less loss and surface roughness. The TiO₂ film with a higher refractive index is also favorable because the optical mode confinement will be enhanced by a higher contrast in index, which can give rise to a flatter and broader reflectance band in HCG.

Second, the fabrication errors will have significant effects on the HCG and shou.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
184 Silcone elastomer kit	Sylgard		Polydimethylsiloxane (PDMS)
4 inch silicon wafer	Universitywafer
4 inch fused silica wafer	Universitywafer
Poly(methyl methacrylate)	Sigma-Aldrich	182265
UV-curable resist			Nor available on market
PlasmaLab System 100	Oxford Instruments		ICP IRE machine
UV curing system for nanoimprint fabrication			Not available on market
Ocean Optics HR-4000	Ocean Optics	HR-4000	Spectrometer with normal detector
Lambda 950 UV / VIS	PerkinElmer		spectrometer with hemisphere intergration detector
JSM-7001F-LV	JEOL		Field emission SEM
DC magnetron sputtering machine			Equipment is in HP labs, who helped us to sputter the TiO2
Metal e-beam evaporator	Temescal	BJD-1800