Name: fabrication of high contrast gratings for the spectrum splitting dispersive element in a concentrated photovoltaic system
Uploaded: 2015-07-18T14:00:00.000+00:00
Duration: 12 min 8 s
Description: Watch this Scientific Journal Video about Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System at JoVE.com

This research was supported as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award Number DE-SC0001013. We also want to thank Dr. Max Zhang and Dr. Jianhua Yang of HP Labs for their help on TiO2 film sputtering and refractive indices measurement.

1. Prepare the Blank Polydimethylsiloxane (PDMS) Substrate for Nanoimprint Mold
<ol>
	<li>Silicon Wafer Treatment Process
	<ol>
		<li>Clean a 4 inch silicon wafer by rinsing with acetone, methanol and isopropanol.</li>
		<li>Blow it dry using the nitrogen gun.</li>
		<li>Clean it using piranha solution (3:1 mixture of sulfuric acid with 30% hydrogen peroxide) by soaking inside for 15 min.</li>
		<li>Rinse it with DI water. Blow dry using the nitrogen gun.</li>
		<li>Place the wafer in a glass desiccator. Add a drop (20 drops = 1 ml) of releasing agent (trichlorosilane) into the desiccator.</li>
		<li>Pump down the desiccator until the gauge reads -762 Torr and wait for 5 hr.</li>
		<li>Take the wafer out, which has been treated with releasing agent.</li>
	</ol>
	</li>
	<li>Preparation of PDMS Film (Used as Mold in Nanoimprint)
	<ol>
		<li>Weigh 10 g of silicone elastomer base and 1 g of curing agent.</li>
		<li>Add them in the same glass beaker.</li>
		<li>Stir and mix with a glass rod for 5 min.</li>
		<li>Put the mixture into a vacuum desiccator until the gauge reads -762 Torr to pump out all the trapped air bubbles.</li>
		<li>Spread them evenly onto the treated 4-inch silicon wafer.</li>
		<li>Bake the wafer with PDMS on top in the vacuum oven for 7 hr at 80 &#176;C to cure the PDMS film.</li>
	</ol>
	</li>
</ol>
2. Prepare the Nanoimprint Mold (Duplication from the Master Mold)
<ol>
	<li>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.</li>
	<li>Carefully peel a piece of PDMS film off the treated silicon wafer.</li>
	<li>Put the PDMS film onto the UV curable resist and let it absorb the UV resist for 5 min then peel it off.</li>
	<li>Repeat 2.1-2.3 on the same PDMS film for two times. Absorb the UV resist for 3 min and 1 min respectively.</li>
	<li>Place the PDMS film (after three-time UV resist absorption) onto a silicon master mold.</li>
	<li>Put it into a chamber with nitrogen environment.</li>
	<li>Turn on UV lamp to cure the sample for 5 min.</li>
	<li>Peel off the PDMS film. The cured UV resist on the PDMS will keep the negative pattern of the master mold.</li>
	<li>Use RF O2 plasma to treat the PDMS mold. (RF power: 30 W, pressure: 260 mTorr, time: 1 min)</li>
	<li>Place the PDMS mold in a vacuum chamber with one drop (20 drops = 1 ml) of releasing agent for 2 hr.</li>
</ol>
3. Nanoimprint Pattern Transfer
<ol>
	<li>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.</li>
	<li>Bake it on a hotplate for 5 min at 120 &#176;C.</li>
	<li>Wait for the sample to cool down.</li>
	<li>Spin eight drops (20 drops = 1 ml) of UV curable resist (3.9%) on the same substrate.</li>
	<li>Place the PDMS mold (prepared in step 2) onto the sample (with both UV resist and PMMA).</li>
	<li>Put it into a chamber with nitrogen environment.</li>
	<li>Turn on the UV lamp to cure for 5 min.</li>
	<li>Peel the PDMS mold off the sample and the pattern on the PDMS mold gets transferred to the sample.</li>
</ol>
4. Cr Lift-off Process
<ol>
	<li>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&#160;
	<ol>
		<li>Log in RIE ICP machine.</li>
		<li>Load a blank 4 inch silicon wafer. Run the clean recipe for 10 min.</li>
		<li>Take the blank silicon wafer out.</li>
		<li>Mount the sample on another clean silicon wafer and load it into the machine.</li>
		<li>Run the UV resist etching recipe for 2 min (the recipe can be found in Table 1).</li>
		<li>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.</li>
		<li>Mount the sample on a clean silicon wafer and load it into the machine.</li>
		<li>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.</li>
	</ol>
	</li>
	<li>Cr E-beam Evaporation
	<ol>
		<li>Log into e-beam evaporator.</li>
		<li>Load the Cr metal source and sample into the chamber.</li>
		<li>Set the thickness (20 nm) and deposition rate (0.03 nm/sec).</li>
		<li>Pump the chamber until required vacuum (10-7 Torr) is reached.</li>
		<li>Start the deposition process.</li>
		<li>Take the sample out after the deposition finishes.</li>
	</ol>
	</li>
	<li>Cr Lift-off Procedure
	<ol>
		<li>Immerse the sample in acetone with ultrasonic agitation for 5 min.</li>
		<li>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.</li>
	</ol>
	</li>
</ol>
5. TiO2 Deposition
<ol>
	<li>Load sample.</li>
	<li>Set the parameters for the direct current magnetron sputtering machine
	<ol>
		<li>Use a chamber pressure of 1.5 mTorr, Ar flow of 100 sccm and a sputtering power of 130 W.</li>
		<li>Use a temperature of 27 &#176;C and a stage rotation speed of 20 rpm.</li>
	</ol>
	</li>
	<li>Start the sputter process and stop at desired thickness.</li>
	<li>Take the sample out and anneal the TiO2 film in oxygen environment at 300 &#176;C for 3 hr.</li>
</ol>
6. High Contrast Grating Etching
<ol>
	<li>Log in the inductively coupled plasma (ICP) reactive ion etching (RIE) machine.</li>
	<li>TiO2 etching
	<ol>
		<li>Load a blank 4-inch silicon wafer.</li>
		<li>Start and run the clean recipe (can be found in Table 1) for 10 min.</li>
		<li>Unload load the blank wafer and load the sample with Cr mask.</li>
		<li>Set etching time. Start TiO2 etching recipe. The etching process will automatically stop.</li>
		<li>Unload the sample.</li>
	</ol>
	</li>
	<li>SiO2 Etching
	<ol>
		<li>Repeat step 5.2 except use the SiO2 etching recipe.</li>
	</ol>
	</li>
</ol>
7. Reflectance Measurement
<ol>
	<li>Log in and turn on the measurement system.</li>
	<li>Place the reflectance standard mirror on the sample holder and align the optical path.</li>
	<li>Calibrate the system for the 100% reflectance.</li>
	<li>Take off the reflectance standard mirror and place the HCG.</li>
	<li>Measure the reflectance of the HCG.</li>
	<li>Save the data and log out of the measurement system.</li>
</ol>

<ol>
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Express. 20, 10888-10895 (2012).</li><li>Chou, S. Y., Krauss, P. R., Renstrom, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoimprint+lithography.">Nanoimprint lithography.</a> Journal of Vacuum Science &amp; Technology B: Microelectronics and Nanometer Structures. 14, 4129-4133 (1996).</li><li>Namiki, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+FDTD+algorithm+based+on+alternating-direction+implicit+method.+Microwave+Theory+and+Techniques.">A new FDTD algorithm based on alternating-direction implicit method. Microwave Theory and Techniques.</a> IEEE Transactions on. 47, 2003-2007 (1999).</li><li>Moharam, M. G., Gaylord, T. 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H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sub-50+nm+period+patterns+with+EUV+interference+lithography.">Sub-50 nm period patterns with EUV interference lithography.</a> Microelectronic Engineering. 67, 56-62 (2003).</li><li>Li, Z., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hybrid+nanoimprint&#8722;+soft+lithography+with+sub-15+nm+resolution.">Hybrid nanoimprint&#8722; soft lithography with sub-15 nm resolution.</a> Nano letters. 9, 2306-2310 (2009).</li><li>Yu, Z., Chen, L., Wu, W., Ge, H., Chou, S. 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The authors have nothing to disclose.

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 should be avoided. The roughness introduced in fabrication will cause more light to be scattered, so the reflectance will become lower. The deviation of parameters in HCG fabrication including line width, height and pitch will not allow the device to work optimally as in simulation. Moreover, the reflectance of HCG strongly depends on the etching profile, i.e. the angle of sidewall. In Figure 11, the effect of sidewall angles on the reflectance of HCG is numerically calculated. As the sidewall angles decrease from 90&#176; to 84&#176;, the average reflectance drops from over 90% to less than 50%, because the HCG behaves more like a cone-shaped anti-reflection coating when the sidewall angle is small.
The optical efficiency of the dispersive element is important for the overall efficiency of the CPV system, so the reflectance of each layer of HCG should be as high as possible. Based on the discussion above, while the optical efficiency for the fabricated layer is about 60%, there are several possible improvements for a better HCG reflectance. The TiO2 sputtering condition can be further optimized to generate the film with a higher index, less surface roughness and lower optical loss. The dry etching recipes should be further adjusted for a better etching profile, making the grating straighter, which can be achieved by adjusting the combination of gases (C4F8, SF6 and O2) to balance the etching and re-deposition process. The nanoimprint and lift-off process should be improved to avoid roughness and fabrication errors so that the unnecessary scattering can be reduced to increase the overall optical efficiency.
By stacking multiple layers of two-dimensional HCGs with different pitches, the dispersive mirror can operate in much broader spectrum. The mirror can reflectively direct light into different angles according to wavelengths, in a way of packaging all HCG layers subsequently in different tilting angles. Moreover, the dispersive mirror can be fabricated using nanoimprint lithography (NIL) in a large area and at a low cost. Moreover, the proposed system features an easy integration with existing concentrator photovoltaic (CPV) setup so it has the potential to be accepted widely by the industry to improve solar energy conversion efficiency.

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.

<table><tbody><tr><td>184 Silcone elastomer kit</td><td>Sylgard</td><td></td><td>Polydimethylsiloxane (PDMS)</td></tr><tr><td>4 inch silicon wafer</td><td>Universitywafer</td><td></td><td></td></tr><tr><td>4 inch fused silica wafer</td><td>Universitywafer</td><td></td><td></td></tr><tr><td>Poly(methyl methacrylate)</td><td>Sigma-Aldrich</td><td>182265</td><td></td></tr><tr><td>UV-curable resist</td><td></td><td></td><td>Nor available on market</td></tr><tr><td>PlasmaLab System 100</td><td>Oxford Instruments</td><td></td><td>ICP IRE machine</td></tr><tr><td>UV curing system for nanoimprint fabrication</td><td></td><td></td><td>Not available on market</td></tr><tr><td>Ocean Optics HR-4000&amp;nbsp;</td><td>Ocean Optics</td><td>HR-4000</td><td>Spectrometer with normal detector</td></tr><tr><td>Lambda 950 UV / VIS</td><td>PerkinElmer</td><td></td><td>spectrometer with hemisphere intergration detector</td></tr><tr><td>JSM-7001F-LV</td><td>JEOL</td><td></td><td>Field emission SEM</td></tr><tr><td>DC magnetron sputtering machine</td><td></td><td></td><td>Equipment is in HP labs, who helped us to sputter the TiO&lt;sub&gt;2&lt;/sub&gt;</td></tr><tr><td>Metal e-beam evaporator</td><td>Temescal</td><td>BJD-1800</td><td></td></tr></tbody></table>

fabrication of high contrast gratings for the spectrum splitting dispersive element in a concentrated photovoltaic system

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.

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 implementation of the dispersive element, which is composed of multiple layers of HCG.
Figure 2 shows the numerical optimization result for each layer in the dispersive element. The results was calculated by the finite-difference time-domain (FDTD)7 based commercial simulation software &#8220;Lumerical&#8221; and further validated by rigorous coupled-wave analysis (RCWA)8. The refractive index of TiO2 was from the SOPRA9 online database. The optimized six-layer dispersive element can provide a total reflection of more than 90% over the entire solar spectrum10,11.
To demonstrate the broadband reflectance of HCG experimentally, one of the six layers in the dispersive element HCG structure is fabricated using nanoimprint fabrication. As shown in Figure 3, each grating block consists of two parts. The material of the top grating is TiO2 and the material of the sub grating is fused silica. The pitch of the 2D HCG is 453 nm. The line width of each grating is 220 nm. The height of both top and sub grating is 340 nm. The material of the substrate is the same as the sub grating.
TiO2 was deposited on fused silica at HP Labs using a direct current magnetron sputter machine. The chamber pressure was 1.5 mTorr with an Ar flow about 100 sccm. The sputter power was 130 W and the rate was 4 nm/min. Two batches of TiO2 film were sputtered at different temperatures, 27 &#176;C and 270 &#176;C respectively. To ensure an even film deposition, substrate stage rotation was turned on (20 rpm) during sputtering. Both batches of TiO2 films were annealed at 300 &#176;C for 3 hr after sputtering to improve film quality. After deposition, both batches of TiO2 films were examined using a scanning electron microscope (SEM) (Figure 4). The refractive indices of TiO2 films were also measured (Figure 5). The measured refractive indices were 10% lower than standard database, because the film was porous which can also be observed in Figure 4. A higher sputtering temperature could increase the refractive index, however the roughness of the film was much higher. To reach a good balance between refractive indices and film roughness, the TiO2 film which was sputtered at 27 &#176;C was chosen as the grating material.
The major steps for nanoimprint fabrication are schematically shown in Figure 6. First, a mold with certain patterns is pressed onto the UV-curable resist on the substrate. Then UV light is applied to cure the resist. After curing, the mold can be separated from the substrate and the shape of resist is exactly the opposite of the mold. The imprinted pattern can be used as the mask to etch the residual resist, deposit metal, lift off and finally etch into the substrate. In this way, the shape of the mold gets transferred into the substrate.
To fabricate 2D HCG, a mold is duplicated from a 1D periodic grating silicon master which was fabricated by interference lithography12. Then the same mold is used to imprint twice in orthogonal directions on the same silicon substrate to pattern a 2D hole array (Figure 7). The hybrid nanoimprint13 process can make large-area samples with high-resolution and little defects. The imprinted results (2D hole array silicon array) is shown in Figure 8. The roughness of edges can be further reduced with the help of edge smoothing technologies14.
After nanoimprint patterning and Cr mask array is completed, an ICP RIE machine is used to etch the sample. Two different etching recipes were developed for TiO2 and fused silica respectively, which is shown in Table 1. The fabricated structure is shown in Figure 9.
The reflectance (from the normal incidence) of 2D HCG was measured using two different spectrometers with different types of detectors, the normal detector and the sphere integration detector. In contrast to sphere integration detector, the normal detector has a relatively small angle of acceptance and therefore will not receive the scattered light. As shown in Figure 10, the difference in reflectance curves measured by both detectors indicates that the light is scattered by the HCG due to the structure roughness. The difference between integration sphere measurement and simulation data is mainly due to the loss of material and fabrication errors. The reflectance curves can demonstrate that the fabricated device can work as a band reflector as one layer in the dispersive element. Due to the high contrast of index between the grating and the substrate, HCG has good angle independence. The reflectance curve will not change much when the incidence angle is less than 15&#176;.
<img alt="Figure 1" src="/files/ftp_upload/52913/52913fig1highres.jpg" width="700" /> 
Figure 1: The implementation of the dispersive element (multiplayer HCG) in a concentrated photovoltaic (CPV) system.
<img alt="Figure 2" src="/files/ftp_upload/52913/52913fig2highres.jpg" width="550" /> 
Figure 2: Numerically optimized reflectance curves for the dispersive element design (six-layer stacked HCG) that can cover most of the solar spectrum.
<img alt="Figure 3" src="/files/ftp_upload/52913/52913fig3highres.jpg" width="700" /> 
Figure 3: The optimized structure of a HCG for demonstration of nanoimprint fabrication.
<img alt="Figure 4" src="/files/ftp_upload/52913/52913fig4highres.jpg" width="700" /> 
Figure 4: The SEM images (cross-sectional view) of sputtered TiO2 films at (a) 27 &#176;C and (b) 270 &#176;C. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/52913/52913fig4highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/52913/52913fig5highres.jpg" width="550" /> 
Figure 5: Measured and standard refractive (SOPRA database) indices of sputtered TiO2 films.
<img alt="Figure 6" src="/files/ftp_upload/52913/52913fig6highres.jpg" width="700" /> 
Figure 6: Nanoimprint fabrication process. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/52913/52913fig6highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/52913/52913fig7highres.jpg" width="600" /> 
Figure 7: The SEM image of 2D hole array silicon master (top-down view).
<img alt="Figure 8" src="/files/ftp_upload/52913/52913fig8highres.jpg" width="600" /> 
Figure 8: The photo of 2D hole array silicon master fabricated by PDMS-based nanoimprint.
<img alt="Figure 9" src="/files/ftp_upload/52913/52913fig9highres.jpg" width="600" /> 
Figure 9: The SEM image (cross-sectional view) of the fabricated 2D HCG.
<img alt="Figure 10" src="/files/ftp_upload/52913/52913fig10highres.jpg" width="550" /> 
Figure 10: One simulated reflectance curve and two measured reflectance curves using sphere integration detector and normal detector respectively.
<img alt="Figure 11" src="/files/ftp_upload/52913/52913fig11highres.jpg" width="700" /> 
Figure 11: (a) Effect of refractive index on HCG reflectance; (b) Effect of sidewall angle on HCG reflectance. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/52913/52913fig11highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td class="xl63"></td>
			<td class="xl64">ICP Power</td>
			<td class="xl64">Forward Power</td>
			<td class="xl64">SF6 Flow</td>
			<td class="xl64">C4F8 Flow</td>
			<td class="xl64">O2 Flow</td>
			<td class="xl64">Pressure</td>
			<td class="xl64">Etching Rate</td>
		</tr>
		<tr>
			<td class="xl65">TiO2</td>
			<td class="xl66">0 W</td>
			<td class="xl66">25 W</td>
			<td class="xl66">25 sccm</td>
			<td class="xl66">10 sccm</td>
			<td class="xl66">10 sccm</td>
			<td class="xl66">10 mTorr</td>
			<td class="xl66">43 nm/min</td>
		</tr>
		<tr>
			<td class="xl65">Fused silica</td>
			<td class="xl66">0 W</td>
			<td class="xl66">100 W</td>
			<td class="xl66">0 sccm</td>
			<td class="xl66">15 sccm</td>
			<td class="xl66">15 sccm</td>
			<td class="xl66">10 mTorr</td>
			<td class="xl66">20 nm/min</td>
		</tr>
		<tr>
			<td class="xl65">Resist</td>
			<td class="xl66">0 W</td>
			<td class="xl66">25 W</td>
			<td class="xl66">25 sccm</td>
			<td class="xl66">15 sccm</td>
			<td class="xl66">0</td>
			<td class="xl66">10 mTorr</td>
			<td class="xl66">22 nm/min</td>
		</tr>
		<tr>
			<td class="xl65">PMMA</td>
			<td class="xl66">0 W</td>
			<td class="xl66">30 W</td>
			<td class="xl66">0</td>
			<td class="xl66">0</td>
			<td class="xl66">30 sccm</td>
			<td class="xl66">2 mTorr</td>
			<td class="xl66">55 nm/min</td>
		</tr>
		<tr>
			<td class="xl65">Clean</td>
			<td class="xl66">1,000 W</td>
			<td class="xl66">200 W</td>
			<td class="xl66">0</td>
			<td class="xl66">0</td>
			<td class="xl66">50 sccm</td>
			<td class="xl66">50 mTorr</td>
			<td class="xl66">NA</td>
		</tr>
	</tbody>
</table>
Table 1: The etching recipes for TiO2, fused silica, UV resist, PMMA and clean.

Watch this Scientific Journal Video about Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System at JoVE.com

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

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.

High contrast gratings are designed and fabricated and its application is proposed in a parallel spectrum splitting dispersive element that can improve ...

fabrication-high-contrast-gratings-for-spectrum-splitting-dispersive

Nanyang Technological University

Research

JoVE Journal

Engineering

10.7K Views.  University of Sothern California. 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. 

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

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To compensate for lower light absorption such physically thin devices have to incorporate light-trapping which increases their optical thickness. Light scattering by textured surfaces is a common technique but it cannot be universally applied to all solar cell technologies. Some cells, for example those made of evaporated silicon, are planar as produced and they require an alternative light-trapping means suitable for planar devices. Metal nanoparticles formed on planar silicon cell surface and capable of light scattering due to surface plasmon resonance is an effective approach.
The paper presents a fabrication procedure of evaporated polycrystalline silicon solar cells with plasmonic light-trapping and demonstrates how the cell quantum efficiency improves due to presence of metal nanoparticles.
To fabricate the cells a film consisting of alternative boron and phosphorous doped silicon layers is deposited on glass substrate by electron beam evaporation. An Initially amorphous film is crystallised and electronic defects are mitigated by annealing and hydrogen passivation. Metal grid contacts are applied to the layers of opposite polarity to extract electricity generated by the cell. Typically, such a ~2 &mu;m thick cell has a short-circuit current density (Jsc) of 14-16 mA/cm2, which can be increased up to 17-18 mA/cm2 (~25% higher) after application of a simple diffuse back reflector made of a white paint.
To implement plasmonic light-trapping a silver nanoparticle array is formed on the metallised cell silicon surface. A precursor silver film is deposited on the cell by thermal evaporation and annealed at 23&deg;C to form silver nanoparticles. Nanoparticle size and coverage, which affect plasmonic light-scattering, can be tuned for enhanced cell performance by varying the precursor film thickness and its annealing conditions. An optimised nanoparticle array alone results in cell Jsc enhancement of about 28%, similar to the effect of the diffuse reflector. The photocurrent can be further increased by coating the nanoparticles by a low refractive index dielectric, like MgF2, and applying the diffused reflector. The complete plasmonic cell structure comprises the polycrystalline silicon film, a silver nanoparticle array, a layer of MgF2, and a diffuse reflector. The Jsc for such cell is 21-23 mA/cm2, up to 45% higher than Jsc of the original cell without light-trapping or ~25% higher than Jsc for the cell with the diffuse reflector only.
Introduction
Light-trapping in silicon solar cells is commonly achieved via light scattering at textured interfaces. Scattered light travels through a cell at oblique angles for a longer distance and when such angles exceed the critical angle at the cell interfaces the light is permanently trapped in the cell by total internal reflection (Animation 1: Light-trapping). Although this scheme works well for most solar cells, there are developing technologies where ultra-thin Si layers are produced planar (e.g. layer-transfer technologies and epitaxial c-Si layers) 1 and or when such layers are not compatible with textures substrates (e.g. evaporated silicon) 2. For such originally planar Si layer alternative light trapping approaches, such as diffuse white paint reflector 3, silicon plasma texturing 4 or high refractive index nanoparticle reflector 5 have been suggested.
Metal nanoparticles can effectively scatter incident light into a higher refractive index material, like silicon, due to the surface plasmon resonance effect 6. They also can be easily formed on the planar silicon cell surface thus offering a light-trapping approach alternative to texturing. For a nanoparticle located at the air-silicon interface the scattered light fraction coupled into silicon exceeds 95% and a large faction of that light is scattered at angles above critical providing nearly ideal light-trapping condition (Animation 2: Plasmons on NP). The resonance can be tuned to the wavelength region, which is most important for a particular cell material and design, by varying the nanoparticle average size, surface coverage and local dielectric environment 6,7. Theoretical design principles of plasmonic nanoparticle solar cells have been suggested 8. In practice, Ag nanoparticle array is an ideal light-trapping partner for poly-Si thin-film solar cells because most of these design principle are naturally met. The simplest way of forming nanoparticles by thermal annealing of a thin precursor Ag film results in a random array with a relatively wide size and shape distribution, which is particularly suitable for light-trapping because such an array has a wide resonance peak, covering the wavelength range of 700-900 nm, important for poly-Si solar cell performance. The nanoparticle array can only be located on the rear poly-Si cell surface thus avoiding destructive interference between incident and scattered light which occurs for front-located nanoparticles 9. Moreover, poly-Si thin-film cells do not requires a passivating layer and the flat base-shaped nanoparticles (that naturally result from thermal annealing of a metal film) can be directly placed on silicon further increases plasmonic scattering efficiency due to surface plasmon-polariton resonance 10.
The cell with the plasmonic nanoparticle array as described above can have a photocurrent about 28% higher than the original cell. However, the array still transmits a significant amount of light which escapes through the rear of the cell and does not contribute into the current. This loss can be mitigated by adding a rear reflector to allow catching transmitted light and re-directing it back to the cell. Providing sufficient distance between the reflector and the nanoparticles (a few hundred nanometers) the reflected light will then experience one more plasmonic scattering event while passing through the nanoparticle array on re-entering the cell and the reflector itself can be made diffuse - both effects further facilitating light scattering and hence light-trapping. Importantly, the Ag nanoparticles have to be encapsulated with an inert and low refractive index dielectric, like MgF2 or SiO2, from the rear reflector to avoid mechanical and chemical damage 7. Low refractive index for this cladding layer is required to maintain a high coupling fraction into silicon and larger scattering angles, which are ensured by the high optical contrast between the media on both sides of the nanoparticle, silicon and dielectric 6. The photocurrent of the plasmonic cell with the diffuse rear reflector can be up to 45% higher than the current of the original cell or up to 25% higher than the current of an equivalent cell with the diffuse reflector only.

Polycrystalline silicon thin-film solar cells on glass are fabricated by deposition of boron and phosphorous doped silicon layers followed by crystallisation, defect passivation and metallisation. Plasmonic light-trapping is introduced by forming Ag nanoparticles on the silicon cell surface capped with a diffused reflector resulting in ~45% photocurrent enhancement.

One of major approaches to cheaper solar cells is reducing the amount of semiconductor material used for their fabrication and making cells thinner. To ...

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped crystallites. Neutrons are passed through two well defined regions of magnetic field; one before and one after the sample. The two magnetic field regions have opposite polarity and are tuned such that neutrons travelling through both regions, without being perturbed, will undergo the same number of precessions in opposing directions. In this case the neutron precession in the second arm is said to &#34;echo&#34; the first, and the original polarization of the beam is preserved. If the neutron interacts with a sample and scatters elastically the path through the second arm is not the same as the first and the original polarization is not recovered. Depolarization of the neutron beam is a highly sensitive probe at very small angles (&#60;50&#160;&#956;rad) but still allows a high intensity, divergent beam to be used. The decrease in polarization of the beam reflected from the sample as compared to that from the reference sample can be directly related to structure within the sample.
In comparison to scattering observed in neutron reflection measurements the SERGIS signals are often weak and are unlikely to be observed if the in-plane structures within the sample under investigation are dilute, disordered, small in size and polydisperse or the neutron scattering contrast is low. Therefore, good results will most likely be obtained using the SERGIS technique if the sample being measured consist of thin films on a flat substrate and contain scattering features that contains a high density of moderately sized features (30 nm to 5 &#181;m) which scatter neutrons strongly or the features are arranged on a lattice. An advantage of the SERGIS technique is that it can probe structures in the plane of the sample.

Progress has been made in utilizing spin echo resolved grazing incidence scattering (SERGIS) as a neutron scattering technique to probe the length-scales in irregular samples. Crystallites of [6,6]-phenyl-C61-butyric acid methyl ester have been probed using the SERGIS technique and the results confirmed by optical and atomic force microscopy.

The spin echo resolved grazing incidence scattering (SERGIS) technique has been used to probe the length-scales associated with irregularly shaped ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal inks. For the photoactive absorber layer, colloidal CdTe and CdSe nanocrystals (3-5 nm) are synthesized using an inert hot injection technique and cleaned with precipitations to remove excess starting reagents. Similarly, gold nanocrystals (3-5 nm) are synthesized under ambient conditions and dissolved in organic solvents. In addition, precursor solutions for transparent conductive indium tin oxide (ITO) films are prepared from solutions of indium and tin salts paired with a reactive oxidizer. Layer-by-layer, these solutions are deposited onto a glass substrate following annealing (200-400 &#176;C) to build the nanocrystal solar cell (glass/ITO/CdSe/CdTe/Au). Pre-annealing ligand exchange is required for CdSe and CdTe nanocrystals where films are dipped in NH4Cl:methanol to replace long-chain native ligands with small inorganic Cl- anions. NH4Cl(s) was found to act as a catalyst for the sintering reaction (as a non-toxic alternative to the conventional CdCl2(s) treatment) leading to grain growth (136&#177;39 nm) during heating. The thickness and roughness of the prepared films are characterized with SEM and optical profilometry. FTIR is used to determine the degree of ligand exchange prior to sintering, and XRD is used to verify the crystallinity and phase of each material. UV/Vis spectra show high visible light transmission through the ITO layer and a red shift in the absorbance of the cadmium chalcogenide nanocrystals after thermal annealing. Current-voltage curves of completed devices are measured under simulated one sun illumination. Small differences in deposition techniques and reagents employed during ligand exchange have been shown to have a profound influence on the device properties. Here, we examine the effects of chemical (sintering and ligand exchange agents) and physical treatments (solution concentration, spray-pressure, annealing time and annealing temperature) on photovoltaic device performance.

This protocol describes the synthesis and solution deposition of inorganic nanocrystals layer by layer to produce thin film electronics on non-conductive surfaces. Solvent-stabilized inks can produce complete photovoltaic devices on glass substrates via spin and spray coating following post-deposition ligand exchange and sintering.

We demonstrate a method for the preparation of fully solution processed inorganic solar cells from a spin and spray coating deposition of nanocrystal ...

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to achieve device efficiencies exceeding other thin film device technologies. Yet, large variations in device efficiency and basic physical properties are reported. This is due to unintentional variations during film processing, which have not been sufficiently investigated so far. We therefore conducted an extensive study of the morphology and electronic structure of a large number of CH3NH3PbI3 perovskite where we show how the preparation method as well as the mixing ratio of educts methylammonium iodide and lead(II) iodide impact properties like film formation, crystal structure, density of states, energy levels, and ultimately the solar cell performance.

We present an extensive study on the effects of different fabrication methods for organic/inorganic perovskite thin films by comparing crystal structures, density of states, energy levels, and ultimately the solar cell performance.

Hybrid organic/inorganic halide perovskites have lately been a topic of great interest in the field of solar cell applications, with the potential to ...

Chemistry

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is composed of two materials, a plastic and an elastomer, whose dispersion characteristics (refractive index variation with wavelength) are different. We first designed the lens geometry and then used ray-tracing simulation, based on the Monte Carlo method, to analyze its performance from the point of view of both optical efficiency and the maximum attainable concentration. Afterwards, ADG Fresnel lens prototypes were manufactured using a simple and reliable method. It consists of a prior injection of plastic parts and a consecutive lamination, together with the elastomer and a glass substrate to fabricate the parquet of ADG Fresnel lenses. The accuracy of the manufactured lens profile is examined using an optical microscope while its optical performance is evaluated using a solar simulator for concentrator photovoltaic systems. The simulator is composed of a xenon flash lamp whose emitted light is reflected by a parabolic mirror. The collimated light has a spectral distribution and an angular aperture similar to the real Sun. We were able to assess the optical performance of the ADG Fresnel lenses by taking photographs of the irradiance spot cast by the lens using a charge-coupled device (CCD) camera and measuring the photocurrent generated by several types of multi junction (MJ) solar cells, which have been previously characterized at a solar simulator for concentrator solar cells. These measurements have demonstrated the achromatic behavior of ADG Fresnel lenses and, as a consequence, the suitability of the modelling and manufacturing methods.

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass ADG Fresnel Lens for Concentrating Photovoltaics

The achromatic doublet on glass (ADG) Fresnel lens makes use of two materials with differing dispersion to reduce chromatic aberration and increase attainable concentration. In this paper, a protocol for the complete characterization of the ADG Fresnel lens is presented.

We present a method to characterize achromatic Fresnel lenses for photovoltaic applications. The achromatic doublet on glass (ADG) Fresnel lens is ...

Developing High Performance GaP/Si Heterojunction Solar Cells

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar cells. In this work, we present high performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime and high crystal quality of epitaxial GaP layers. It is shown that by applying phosphorus (P)-diffusion layers into the Si substrate and a SiNx layer, the Si minority-carrier lifetime can be well-maintained during the GaP growth in the molecular beam epitaxy (MBE). By controlling the growth conditions, the high crystal quality of GaP was grown on the P-rich Si surface. The film quality is characterized by atomic force microscopy and high-resolution x-ray diffraction. In addition, MoOx was implemented as a hole-selective contact that led to a significant increase in the short-circuit current density. The achieved high device performance of the GaP/Si heterojunction solar cells establishes a path for further enhancement of the performance of Si-based photovoltaic devices.

Here, we present a protocol to develop high-performance GaP/Si heterojunction solar cells with a high Si minority-carrier lifetime.

To improve the efficiency of Si-based solar cells beyond their Shockley-Queisser limit, the optimal path is to integrate them with III-V-based solar ...

In situ Grazing Incidence Small Angle X-ray Scattering on Roll-To-Roll Coating of Organic Solar Cells with Laboratory X-ray Instrumentation

We present an in-house, in situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) experiment, developed to probe the drying kinetics of roll-to-roll slot-die coating of the active layer in organic photovoltaics (OPVs), during deposition. For this demonstration, the focus is on the combination of P3HT:O-IDTBR and P3HT:EH-IDTBR, which have different drying kinetics and device performance, despite their chemical structure only varying slightly by the sidechain of the small molecule acceptor. This article provides a step-by-step guide to perform an in situ GISAXS experiment and demonstrates how to analyze and interpret the results. Usually, performing this type of in situ X-ray experiments to investigate the drying kinetics of the active layer in OPVs relies on access to synchrotrons. However, by using and further developing the method described in this paper, it is possible to perform experiments with a coarse temporal and spatial resolution, on a day-to-day basis to gain fundamental insight in the morphology of drying inks.

This paper is a demonstration and a guideline to perform and analyze in-house (with a laboratory X-ray instrument) in situ GISAXS experiments of drying inks on roll-to-roll slot-die coated, non-fullerene organic photovoltaics.

We present an in-house, in situ Grazing Incidence Small Angle X-ray Scattering (GISAXS) experiment, developed to probe the drying kinetics of roll-to-roll ...

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

Summary

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Chapters in this video

Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Using Neutron Spin Echo Resolved Grazing Incidence Scattering to Investigate Organic Solar Cell Materials

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Fabrication of Fully Solution Processed Inorganic Nanocrystal Photovoltaic Devices

Influence of Hybrid Perovskite Fabrication Methods on Film Formation, Electronic Structure, and Solar Cell Performance

Indoor Experimental Assessment of the Efficiency and Irradiance Spot of the Achromatic Doublet on Glass (ADG) Fresnel Lens for Concentrating Photovoltaics

Developing High Performance GaP/Si Heterojunction Solar Cells

In situ Grazing Incidence Small Angle X-ray Scattering on Roll-To-Roll Coating of Organic Solar Cells with Laboratory X-ray Instrumentation