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 usually used in most concentrated photovoltaic (CPV) systems¹ to achieve a high concentration of solar power incidence on the small-area solar cells, so it is economically viable to exploit expensive tandem multi-junction solar cells² in CPV systems, and to maintain a reasonable cost at the same time. However, for most non-concentrated photovoltaic systems, which usually require a large-area installment of solar cells, the high-cost tandem solar cells cannot be incorporated, although they usually have a broader solar spectrum response and a higher overall conversion efficiency than the single junction solar cells³.

Recently, with the help of the parallel spectrum splitting optics (i.e. dispersive element), the parallel spectrum splitting photovoltaic technology⁴ has made it possible that a similar or better spectrum coverage and conversion efficiency can be achieved without using the expensive tandem solar cells. The solar spectrum can be split into different bands and each band can be absorbed and converted to electricity by the specialized single-junction solar cells. In this way, the expensive tandem solar cells in CPV systems can be replaced by a parallel distribution of single-junction solar cells without any compromise on the performance.

The dispersive element that was designed in this report can be applied in a reflective CPV system (which is based on dish reflectors) to realize parallel spectrum splitting for the improved solar-electricity conversion efficiency and reduced cost. Multilayer high contrast gratings (HCG)⁵ is used as the dispersive element by designing each layer of HCG to work as an optical band reflector. The structures and parameters of the dispersive element are numerically optimized. Moreover, the fabrication of high contrast gratings for the dispersive element by using dielectric (TiO₂) sputtering, nanoimprint lithography⁶ and reactive ion etching is studied and demonstrated.

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 for 5 hr.
   7. Take the wafer out, which has been treated with releasing agent.
2. Preparation of PDMS Film (Used as Mold in Nanoimprint)
   1. Weigh 10 g of silicone elastomer base and 1 g of curing agent.
   2. Add them in the same glass beaker.
   3. Stir and mix with a glass rod for 5 min.
   4. Put the mixture into a vacuum desiccator until the gauge reads -762 Torr to pump out all the trapped air bubbles.
   5. Spread them evenly onto the treated 4-inch silicon wafer.
   6. Bake the wafer with PDMS on top in the vacuum oven for 7 hr at 80 °C to cure the PDMS film.

2. Prepare the Nanoimprint Mold (Duplication from the Master Mold)

1. Spin twelve drops (20 drops = 1 ml) of UV curable resist (15.2%) on a clean blank silicon wafer for 30 sec at 1,500 rpm.
2. Carefully peel a piece of PDMS film off the treated silicon wafer.
3. Put the PDMS film onto the UV curable resist and let it absorb the UV resist for 5 min then peel it off.
4. Repeat 2.1-2.3 on the same PDMS film for two times. Absorb the UV resist for 3 min and 1 min respectively.
5. Place the PDMS film (after three-time UV resist absorption) onto a silicon master mold.
6. Put it into a chamber with nitrogen environment.
7. Turn on UV lamp to cure the sample for 5 min.
8. Peel off the PDMS film. The cured UV resist on the PDMS will keep the negative pattern of the master mold.
9. Use RF O₂ plasma to treat the PDMS mold. (RF power: 30 W, pressure: 260 mTorr, time: 1 min)
10. Place the PDMS mold in a vacuum chamber with one drop (20 drops = 1 ml) of releasing agent for 2 hr.

3. Nanoimprint Pattern Transfer

1. Spin eight drops (20 drops = 1 ml) of PMMA (996k, 3.1%) on the substrate to be imprinted for 50 sec at 3,500 rpm.
2. Bake it on a hotplate for 5 min at 120 °C.
3. Wait for the sample to cool down.
4. Spin eight drops (20 drops = 1 ml) of UV curable resist (3.9%) on the same substrate.
5. Place the PDMS mold (prepared in step 2) onto the sample (with both UV resist and PMMA).
6. Put it into a chamber with nitrogen environment.
7. Turn on the UV lamp to cure for 5 min.
8. Peel the PDMS mold off the sample and the pattern on the PDMS mold gets transferred to the sample.

4. Cr Lift-off Process

1. Reactive ion etching residual layer of UV resist and PMMA
   Note: The SOP for ICP machine can be found at https://www.nanocenter.umd.edu/equipment/fablab/sops/etch-07/Oxford%20Chlorine%20Etcher%20SOP.pdf 
   1. Log in RIE ICP machine.
   2. Load a blank 4 inch silicon wafer. Run the clean recipe for 10 min.
   3. Take the blank silicon wafer out.
   4. Mount the sample on another clean silicon wafer and load it into the machine.
   5. Run the UV resist etching recipe for 2 min (the recipe can be found in Table 1).
   6. Take the sample out. Load a blank 4 inch silicon wafer. Re-run the clean recipe (can be found in Table 1) for 10 min.
   7. Mount the sample on a clean silicon wafer and load it into the machine.
   8. Run the PMMA etching recipe (can be found in Table 1) for 2 min.
      Note: Now the residual resist has been etched and substrate is exposed.
2. Cr E-beam Evaporation
   1. Log into e-beam evaporator.
   2. Load the Cr metal source and sample into the chamber.
   3. Set the thickness (20 nm) and deposition rate (0.03 nm/sec).
   4. Pump the chamber until required vacuum (10^-7 Torr) is reached.
   5. Start the deposition process.
   6. Take the sample out after the deposition finishes.
3. Cr Lift-off Procedure
   1. Immerse the sample in acetone with ultrasonic agitation for 5 min.
   2. Clean the sample by rinsing with acetone, methanol and isopropanol.
      Note: The Cr evaporated on the resist will be lifted off and a Cr mask for substrate etching is formed.

5. TiO₂ Deposition

1. Load sample.
2. Set the parameters for the direct current magnetron sputtering machine
   1. Use a chamber pressure of 1.5 mTorr, Ar flow of 100 sccm and a sputtering power of 130 W.
   2. Use a temperature of 27 °C and a stage rotation speed of 20 rpm.
3. Start the sputter process and stop at desired thickness.
4. Take the sample out and anneal the TiO₂ film in oxygen environment at 300 °C for 3 hr.

6. High Contrast Grating Etching

1. Log in the inductively coupled plasma (ICP) reactive ion etching (RIE) machine.
2. TiO₂ etching
   1. Load a blank 4-inch silicon wafer.
   2. Start and run the clean recipe (can be found in Table 1) for 10 min.
   3. Unload load the blank wafer and load the sample with Cr mask.
   4. Set etching time. Start TiO₂ etching recipe. The etching process will automatically stop.
   5. Unload the sample.
3. SiO₂ Etching
   1. Repeat step 5.2 except use the SiO₂ etching recipe.

7. Reflectance Measurement

1. Log in and turn on the measurement system.
2. Place the reflectance standard mirror on the sample holder and align the optical path.
3. Calibrate the system for the 100% reflectance.
4. Take off the reflectance standard mirror and place the HCG.
5. Measure the reflectance of the HCG.
6. Save the data and log out of the measurement system.

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

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