The authors thank New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade, and Industry (METI), Japan, for the financial support.

1. Preparation of PDMS Stamps
<ol>
	<li>Set a nanohole mold (nanoimprinted cyclo olefin polymer plastic film, size: 50 mm &#215; 50 mm) in a polytetrafluoroethylene (PTFE) container.</li>
	<li>Weigh vinylmethylsiloxane-dimethylsiloxane copolymer (0.76 g for the 50 mm &#215; 50 mm mold) in a disposable glass bottle and mix it with Pt-divinyltetramethyldisiloxane complex (6 &#181;l, using a digital micro pipette with a disposable polypropylene tip) and 2,4,6,8-tetramethyltetra-vinylcyclotetrasiloxane (24 &#181;l, using a digital micro pipette with a disposable polypropylene tip).</li>
	<li>Add methylhydrosiloxane-dimethylsiloxane copolymer (0.24 ml, using a digital micro pipette with a disposable polypropylene tip) in the glass bottle and mix it quickly using a disposable glass pipette. After the surface of the mold set in the PTFE container is blown by N2, pour the resulting mixture (&#8220;hard&#8221; PDMS prepolymer) on the mold and start spin-coating at 1,000 rpm for 40 sec to achieve the layer thickness of ~40 &#181;m.</li>
	<li>Place the spin-coated sample in a hot chamber preheated at 65 &#176;C for 30 min to briefly cross-link the hard PDMS.</li>
	<li>During the heating, weigh silicone (6 g) and mix it with catalyst (0.6 g) in a disposable glass bottle. Place the glass bottle in a vacuum desiccator and apply vacuum (~133 Pa) for 15 min to remove the air trapped in the silicone mixture (&#8220;soft&#8221; PDMS prepolymer).</li>
	<li>Take out the mold and soft PDMS prepolymer from the heating chamber and vacuum desiccator, respectively, and quickly pour the soft PDMS prepolymer onto the heated mold. The thickness of soft PDMS layer is ~3 mm.</li>
	<li>Place the resulting sample in the vacuum desiccator again for further degassing at ~133 Pa at least for 1 hr.</li>
	<li>Transfer the degassed sample to the heating chamber and start heating gradually up to 80 &#176;C (heating rate ~3 &#176;C/min). Keep this temperature for 5 hr to cross-link the hard and soft PDMS completely.</li>
	<li>After cooling the sample down to RT, peel off the PDMS stamp carefully from the mold. Reuse the mold to prepare the second (or more) stamps, if necessary. Note: The same mold can be used at least five times without degradation of the stamp quality.</li>
	<li>Cut the resulting nanopillar stamp (double-layered hard/soft PDMS composite)27 into pieces of desired sizes (typically 7 mm &#215; 7 mm for our solar cells) using a knife and store them under air until use.</li>
</ol>
2. Preparation of Block Copolymer Solutions
<ol>
	<li>Weigh the powder of polystyrene-block-poly-2-vinylpyridine (PS-b-P2VP) in a glass bottle and mix it with o-xylene in the ratio of 3 mg/ml (PS-b-P2VP/o-xylene).</li>
	<li>Stir the mixture using a PTFE-coated magnetic stirring bar at 70 &#176;C for 1 hr.</li>
	<li>Keep the resulting solution for more than 24 h at RT without stirring to complete the formation of self-assembled micelles. Tightly seal the solution and store it under ambient conditions. Note: The quality of the solution is unchanged even one year after preparation.</li>
</ol>
3. Preparation of &#181;c-Si:H Substrates
<ol>
	<li>Wash glasses covered with SnO2:F (denote glass/SnO2:F, hereafter) with H2O (500 ml), detergent (500 ml), and H2O (500 ml) at RT using an ultrasonic bath (15 min for each). Dry them by blowing N2.</li>
	<li>Load the cleaned glass/SnO2:F substrates in a substrate holder and deposit ZnO:Ga (20 nm) using a direct current (DC) sputtering system with the conditions specified in Table 2.</li>
	<li>Load the glass/SnO2:F/ZnO:Ga substrates in another substrate holder and deposit &#181;c-Si:H p (10 nm), i (500 nm), and n (40 nm) layers using a plasma-enhanced chemical vapor deposition (PECVD) system with the conditions specified in Table 2.</li>
	<li>Store the resulting &#181;c-Si:H (glass/SnO2:F/ZnO:Ga/&#181;c-Si:H p-i-n) substrates under vacuum or N2 until the transfer-printing step.</li>
</ol>
4. Ag-coating of PDMS Stamps
<ol>
	<li>Wash the PDMS stamps (prepared in step 1) with EtOH (30 ml) using an ultrasonic bath for 15 min and dry by blowing N2.</li>
	<li>Load the cleaned PDMS stamps on a sample holder using double-sided adhesive tape and deposit an Ag film (10-80 nm) using an electron beam (EB) evaporation system with following conditions: deposition rate = 5-10 &#197;/sec, pressure = ~1.5 &#215; 10-4 Pa.</li>
	<li>Take out the Ag-coated stamps from the EB evaporation system and use them immediately in the following transfer printing step.</li>
</ol>
5. Transfer Printing of Ag Nanodisks on Thin-film Si Surfaces
<ol>
	<li>Take out the thin-film Si substrates stored under vacuum or N2 and spin-coat with the PS-b-P2VP solution (0.3 ml for 50 mm &#215; 50 mm sample, using a digital micro pipette with a disposable polypropylene tip) at 5,000 rpm for 40 sec.</li>
	<li>Wet the PS-b-P2VP-coated surface with EtOH using a digital micro pipette (5 &#181;m/cell area) and apply the Ag-coated PDMS stamp softly to the EtOH-wet surface. Do not press the stamp.</li>
	<li>Place the thin-film Si substrate with the stamp in a vacuum chamber and apply vacuum (~133 Pa).</li>
	<li>After 5 min, fill the vacuum chamber with air and take out the thin-film Si substrate.</li>
	<li>Remove the stamp from the thin-film Si substrate by holding the both side of the stamp with tweezers to transfer-print Ag nanodisks. Note: If successful, the trace of stamping is visible as a greenish spot.</li>
	<li>Rinse the transfer-printed thin-film Si substrate with a continuous flow of EtOH for 15 sec (~30 ml) and dry by blowing N2.</li>
	<li>Remove the PS-b-P2VP coating using an Ar plasma system.
	<ol>
		<li>Place the transfer-printed thin-film Si substrate in the process chamber of the Ar plasma system.</li>
		<li>Pump out the air in the process chamber for ~5 min (pressure ~20 Pa).</li>
		<li>Open the valve of an Ar gas line and manually adjust the flow rate to 4 sccm. Wait for ~5 min to stabilize the pressure to 40 Pa.</li>
		<li>Generate Ar plasma for 108 sec.</li>
		<li>Close the valve of the Ar gas line, stop pumping, and fill air into the process chamber to take out the plasma-cleaned, transfer-printed thin-film Si substrates.</li>
	</ol>
	</li>
</ol>
6. Completion of Thin-film Si Solar Cell Fabrication
<ol>
	<li>Attach metal masks to the transfer-printed thin-film Si substrates after the Ar plasma treatment using polyimide tapes.</li>
	<li>Load the masked substrates in a substrate holder of a DC sputtering system and deposit ZnO:Ga (100 nm), Ag (250 nm), and ZnO:Ga (40 nm) sequentially with the conditions specified in Table 2.</li>
	<li>Detach the metal masks from the substrates and remove the masked thin-film Si layers (i.e., the area where ZnO:Ga and Ag were not deposited) using a reactive ion etching (RIE) system.
	<ol>
		<li>Place the samples in the process chamber of the RIE system.</li>
		<li>Pump out the air in the process chamber following manufacturer&#8217;s instructions.</li>
		<li>Set the process conditions as follows following the manufacturer&#8217;s instruction: SF6/O2 flow rate =100/20 sccm, pressure = 20 Pa, power 100 W, time = 1 min 20 sec.</li>
		<li>Open the SF6 and O2 gas lines, stabilize the pressure, and generate plasma.</li>
		<li>Close the valve of the SF6 and O2 gas line, stop pumping, and fill N2 into the process chamber to take out the samples.</li>
	</ol>
	</li>
	<li>Put the samples in a vacuum-annealing chamber and start heating gradually up to 175 &#176;C under vacuum (~133 Pa). Keep this temperature for 2 hr, and then allow to cool to RT. Fill air in the chamber and take out samples, which now can be called cells.</li>
	<li>Solder Sn-Zn-based alloy on the front transparent electrode (glass/SnO2:F/ZnO:Ga, the exposed part by the RIE treatment) using an ultrasonic soldering device.</li>
</ol>
7. Measurement of External Quantum Efficiency (EQE)
<ol>
	<li>Attach a light-shielding mask to a fabricated cell using polyimide tape and set the masked cell in a cell holder. Connect probes to the front soldered electrode (+) and back Ag/ZnO:Ga electrode (-).</li>
	<li>Measure EQE spectra using an EQE measurement system following manufacturer&#8217;s instructions with a wavelength range and step of 300-1,100 nm and 5 nm, respectively.</li>
</ol>
8. Measurement of Photovoltaic Current-Voltage (J-V) Characteristics
<ol>
	<li>Calibrate the light intensity of a J-V characteristics measurement system using an amorphous Si reference cell.
	<ol>
		<li>Set the amorphous Si reference cell to a cell holder of the J-V characteristics measurement system, and illuminate the light.</li>
		<li>Read the photogenerated current using a digital multi-meter equipped in the J-V characteristics measurement system. Adjust the light intensity until the photogenerated current shows the right value for the reference cell (8.34 mA/cm2).</li>
	</ol>
	</li>
	<li>Attach a light-shielding mask to a cell and set the masked cell in a cell holder. Connect probes to the front soldered electrode (+) and back Ag/ZnO:Ga electrode (-).</li>
	<li>Illuminate the calibrated light (100 mW/cm2, 1 sun) on the cell and measure photogenerated currents using the J-V characteristics measurement system following manufacturer&#8217;s instructions with a voltage step of 0.02 V.</li>
</ol>

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The authors have nothing to disclose.

In this article, a double-layered hard/soft PDMS composite was employed as stamp materials.27 This combination was found to be essential to precisely replicate the parent nanostructure in the mold, which was a hexagonally close-packed round-hole array whose diameter of 230 nm, depth of 500 nm, and hole center-to-center spacing of 460 nm. When only soft PDMS was used, the stamp always resulted in a poorly nanostructured surface (for example, no sharp edge in the inverted pillar structure) due to the low Young&#...

There has been a long-standing demand for the application of functional nanostructures in a broad range of technological field. One of the expectations for this trend is to open new design of device architectures leading to improved or innovative performances. In the field of solar cells, for example, the use of metal nanostructures has been actively explored because of their intriguing optical (i.e., plasmonic) properties,1 potentially beneficial to construct effective light trapping systems.2,3 Indeed, some theoretical studies4-6 have suggested that such plasmonic light trapping could achieve effects exceeding the conventional ray optics (texturing)-based light trapping limit.7 As a result, developing strategies to integrate desired metal nanostructures with solar cells has become increasingly important in order to realize these theoretical predictions.
A number of strategies have been proposed to meet this challenge.8-24 These include, for instance, simple (low-cost) thermal annealing of metal films8,9 or dispersion of pre-synthesized metal nanoparticles,10,11 both of which resulted in successful demonstrations of plasmonic light trapping. However, it should be pointed out that the metal nanostructures fabricated by these approaches are usually challenging to match to the theoretical models. In contrast, the traditional nanofabrication techniques in semiconductor industries, such as photolithography and electron beam lithography,12,13 can control structures well below the sub-100 nm level, but they are often too expensive and time-consuming to apply to solar cells, where large-area capability with low cost is essential. In order to fulfill the low-cost, high-throughput, and large-area requirements with nanoscale controllability, methods such as nanoimprint lithography,14-16 soft lithography,17,18 nanosphere lithography,19-21 and hole-mask colloidal lithography22-24 would be promising. Among these choices, we have developed a soft lithographic, advanced transfer printing technique.25 Using a nanostructured poly(dimethylsiloxane) (PDMS) stamps and block copolymer-based adhesive layers, patterning of ordered metal nanostructures could be readily achieved on a number of technologically relevant materials, including the ones for solar cells.
The focus of this article is to describe the detailed procedure of our transfer printing approach to incorporate effective light trapping plasmonic nanostructures in existing solar cell structures. As a demonstrative case, Ag nanodisks and thin-film hydrogenated microcrystalline Si (&#181;c-Si:H) solar cells were selected in this study (Figure 1),26 although other types of metals and solar cells are compatible with this approach. Together with its process simplicity, the approach would be of interest to diverse researchers as a handy tool to integrate functional metal nanostructures with devices.

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		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Nanohole mold</td>
			<td style="width:200px;">Scivax 
			http://www.scivax.com</td>
			<td style="width:129px;">FLH230/500-120</td>
			<td style="width:319px;"></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">PTFE container</td>
			<td style="width:200px;">Eishin 
			http://www.colbyeishin.com</td>
			<td style="width:129px;">n/a</td>
			<td style="width:319px;">Custom made</td>
		</tr>
		<tr height="21">
			<td height="63" rowspan="3" style="height:63px;width:216px;">Hard-PDMS materials</td>
			<td rowspan="3" style="width:200px;">Gelest 
			http://www.gelest.com/gelest/forms/Home/home.aspx</td>
			<td style="width:129px;">VDT-731</td>
			<td>Vinylmethylsiloxane-dimethylsiloxane copolymer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:129px;">SIP6831.1</td>
			<td>Pt-divinyltetramethyldisiloxane complex</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:129px;">HMS-301</td>
			<td>Methylhydrosiloxane-dimethylsiloxane copolymer</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">2,4,6,8-tetramethyltetra-vinylcyclotetrasiloxane</td>
			<td style="width:200px;">Sigma-Aldrich 
			http://www.sigmaaldrich.com</td>
			<td style="width:129px;">396281</td>
			<td>Additive for hard-PDMS</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Soft-PDMS materials</td>
			<td style="width:200px;">Dow Corning 
			http://www.dowcorning.com</td>
			<td style="width:129px;">Sylgard-184</td>
			<td>Silicone precursor</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;">PS-b-P2VP</td>
			<td style="width:200px;">Polymer Source 
			http://polymersource.com</td>
			<td style="width:129px;">P5742-S2VP</td>
			<td>Mn &#215; 103 = 133-b-132</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Glass/SnO2:F substrates</td>
			<td style="width:200px;">Asahi Glass Co. Ltd. 
			http://www.agc.com/english/company</td>
			<td style="width:129px;">Type VU</td>
			<td style="width:319px;">Chemical mechanical polished by D-process Inc. (http://d-process.jp/index.html) to flatten the surfaces</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Detergent</td>
			<td style="width:200px;">Fruuchi Chemical Co. 
			http://www.furuchi.co.jp/eng/main.htm</td>
			<td style="width:129px;">Semico-clean 56</td>
			<td style="width:319px;">Used for the cleaning of Glass/SnO2:F substrates</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">ZnO:Ga supputtering target</td>
			<td style="width:200px;">AGC Ceramics Co. Ltd. 
			http://www.agcc.jp/2005/en/index.html</td>
			<td style="width:129px;">5.7GZO</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Ag supputtering target</td>
			<td style="width:200px;">Mitsubishi Materials Co. 
			http://www.mitsubishicarbide.com/mmc/en/index.html</td>
			<td style="width:129px;">4NAg</td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Double-sided adhesive tape</td>
			<td style="width:200px;">Nisshin EM Co. 
			http://nisshin-em.co.jp/information/carbontape.html</td>
			<td style="width:129px;">732</td>
			<td></td>
		</tr>
		<tr height="120">
			<td height="120" style="height:120px;width:216px;">Polyimide tape</td>
			<td style="width:200px;">Dupont 
			http://www.dupont.com/products-and-services/membranes-films/polyimide-films/brands/kapton-polyimide-film.html</td>
			<td style="width:129px;">Kapton 650S#25</td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Sn-Zn-based Solder</td>
			<td style="width:200px;">Kuroda Techno Co., Ltd. 
			http://www.kuroda-techno.com/english/index.html</td>
			<td style="width:129px;">Cerasolzer AL-200</td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="84" style="height:84px;width:216px;">Digital micro pipette</td>
			<td style="width:200px;">Nichiryo 
			http://www.nichiryo.co.jp/en/product/pipette/ex/index.html</td>
			<td style="width:129px;">00-NPX2-20 
			00-NPX2-200 
			00-NPX2-1000</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Heating chamber</td>
			<td style="width:200px;">Tokyo Rikakikai Co., Ltd. 
			http://www.eyelaworld.com/product_view.php?id=120</td>
			<td style="width:129px;">VOS-201SD</td>
			<td></td>
		</tr>
		<tr height="84">
			<td height="126" rowspan="2" style="height:126px;width:216px;">Electron beam evaporator 
			(two types)</td>
			<td style="width:200px;">Canon-Anelva 
			https://www.canon-anelva.co.jp/english/index.html</td>
			<td style="width:129px;">n/a</td>
			<td style="width:319px;">Custom made</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:200px;">Arios 
			http://arios.com/</td>
			<td style="width:129px;">n/a</td>
			<td>Custom made</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Sputtering system</td>
			<td style="width:200px;">Ulvac 
			http://www.ulvac.co.jp/en</td>
			<td style="width:129px;">SBR-2306</td>
			<td style="width:319px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">PECVD system&#160;</td>
			<td style="width:200px;">Shimadzu Emit Co. Ltd. 
			http://www.shimadzu.co.jp/emit/en/</td>
			<td style="width:129px;">SLCM-13</td>
			<td style="width:319px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Ar plasma system&#160;</td>
			<td style="width:200px;">Diner Electric Gmbh 
			http://www.plasma.de/index.html</td>
			<td style="width:129px;">Femto&#160;</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">RIE system</td>
			<td style="width:200px;">Samco Inc. 
			http://www.samcointl.com</td>
			<td style="width:129px;">RIE-10NR</td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Ultrasonic soldering device</td>
			<td style="width:200px;">Colby-Eishin Enterprises, Inc. 
			http://www.colbyeishin.com/sub_sunbonder.htm</td>
			<td style="width:129px;">SUNBONDER</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">EQE measurement system</td>
			<td rowspan="3" style="width:200px;">Bunkoukeiki Co. Ltd. 
			http://www.bunkoukeiki.co.jp/</td>
			<td style="width:129px;">CEP-25BXS</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">J-V characteristics measurement system</td>
			<td style="width:129px;">OTENTOSUN-5S-I/V</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Amorphous Si reference cell</td>
			<td style="width:129px;">WPVS-NPB-S1</td>
			<td>For light intensity calibration</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Digital multi-meter</td>
			<td style="width:200px;">Keithley Instruments Inc. 
			http://www.keithley.com/</td>
			<td style="width:129px;">2400</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

Figure 2 outlines the general process for the transfer printing of Ag nanodisks on the surface of &#181;c-Si:H (n layer). Briefly, an Ag film (thickness: 10-80 nm) is first deposited on the surface of a nanopillar PDMS stamp by electron beam evaporation. In parallel, a PS-b-P2VP solution is spin-coated on the surface of a freshly prepared &#181;c-Si:H n layer. Subsequently, a droplet of EtOH is placed on the PS-b-P2VP-coated surface, and the Ag-deposited PDMS stamp is ...

Watch this Scientific Journal Video about Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing at JoVE.com

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

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 ...

integration-light-trapping-silver-nanostructures-hydrogenated

Research

JoVE Journal

Engineering

7.7K Views.  National Institute of Advanced Industrial Science and Technology, Koriyama, Fukushima, Japan. 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.

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

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 ...

A Method to Fabricate Disconnected Silver Nanostructures in 3D

The standard nanofabrication toolkit includes techniques primarily aimed at creating 2D patterns in dielectric media. Creating metal patterns on a submicron scale requires a combination of nanofabrication tools and several material processing steps. For example, steps to create planar metal structures using ultraviolet photolithography and electron-beam lithography can include sample exposure, sample development, metal deposition, and metal liftoff. To create 3D metal structures, the sequence is repeated multiple times. The complexity and difficulty of stacking and aligning multiple layers limits practical implementations of 3D metal structuring using standard nanofabrication tools. Femtosecond-laser direct-writing has emerged as a pre-eminent technique for 3D nanofabrication.1,2 Femtosecond lasers are frequently used to create 3D patterns in polymers and glasses.3-7 However, 3D metal direct-writing remains a challenge. Here, we describe a method to fabricate silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm. The method enables the fabrication of patterns not feasible using other techniques, such as 3D arrays of disconnected silver voxels.8 Disconnected 3D metal patterns are useful for metamaterials where unit cells are not in contact with each other,9 such as coupled metal dot10,11or coupled metal rod12,13 resonators. Potential applications include negative index metamaterials, invisibility cloaks, and perfect lenses.
In femtosecond-laser direct-writing, the laser wavelength is chosen such that photons are not linearly absorbed in the target medium. When the laser pulse duration is compressed to the femtosecond time scale and the radiation is tightly focused inside the target, the extremely high intensity induces nonlinear absorption. Multiple photons are absorbed simultaneously to cause electronic transitions that lead to material modification within the focused region. Using this approach, one can form structures in the bulk of a material rather than on its surface.
Most work on 3D direct metal writing has focused on creating self-supported metal structures.14-16 The method described here yields sub-micrometer silver structures that do not need to be self-supported because they are embedded inside a matrix. A doped polymer matrix is prepared using a mixture of silver nitrate (AgNO3), polyvinylpyrrolidone (PVP) and water (H2O). Samples are then patterned by irradiation with an 11-MHz femtosecond laser producing 50-fs pulses. During irradiation, photoreduction of silver ions is induced through nonlinear absorption, creating an aggregate of silver nanoparticles in the focal region. Using this approach we create silver patterns embedded in a doped PVP matrix. Adding 3D translation of the sample extends the patterning to three dimensions.

Femtosecond-laser direct-writing is frequently used to create three-dimensional (3D) patterns in polymers and glasses. However, patterning metals in 3D remains a challenge. We describe a method for fabricating silver nanostructures embedded inside a polymer matrix using a femtosecond laser centered at 800 nm.

The standard nanofabrication toolkit includes  techniques primarily aimed at creating 2D patterns in dielectric media. Creating  metal patterns on a ...

Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent thermal evaporation, and it absorbs visible light strongly. However, for a long time the record power conversion efficiency of SnS solar cells remained below 2%. Recently we demonstrated new certified record efficiencies of 4.36% using SnS deposited by atomic layer deposition, and 3.88% using thermal evaporation. Here the fabrication procedure for these record solar cells is described, and the statistical distribution of the fabrication process is reported. The standard deviation of efficiency measured on a single substrate is typically over 0.5%. All steps including substrate selection and cleaning, Mo sputtering for the rear contact (cathode), SnS deposition, annealing, surface passivation, Zn(O,S) buffer layer selection and deposition, transparent conductor (anode) deposition, and metallization are described. On each substrate we fabricate 11 individual devices, each with active area 0.25 cm2. Further, a system for high throughput measurements of current-voltage curves under simulated solar light, and external quantum efficiency measurement with variable light bias is described. With this system we are able to measure full data sets on all 11 devices in an automated manner and in minimal time. These results illustrate the value of studying large sample sets, rather than focusing narrowly on the highest performing devices. Large data sets help us to distinguish and remedy individual loss mechanisms affecting our devices.

Tin sulfide (SnS) is a candidate material for Earth-abundant, non-toxic solar cells. Here, we demonstrate the fabrication procedure of the SnS solar cells employing atomic layer deposition, which yields 4.36% certified power conversion efficiency, and thermal evaporation which yields 3.88%.

Tin sulfide (SnS) is a candidate absorber material for Earth-abundant, non-toxic solar cells. SnS offers easy phase control and rapid growth by congruent ...

Selective Area Modification of Silicon Surface Wettability by Pulsed UV Laser Irradiation in Liquid Environment

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of biosensing devices. We report on a protocol of using KrF and ArF lasers irradiating Si (001) samples immersed in a liquid environment with low number of pulses and operating at moderately low pulse fluences to induce Si wettability modification. Wafers immersed for up to 4 hr in a 0.01% H2O2/H2O solution did not show measurable change in their initial contact angle (CA) ~75&#176;. However, the 500-pulse KrF and ArF lasers irradiation of such wafers in a microchamber filled with 0.01% H2O2/H2O solution at 250 and 65 mJ/cm2, respectively, has decreased the CA to near 15&#176;, indicating the formation of a superhydrophilic surface. The formation of OH-terminated Si (001), with no measurable change of the wafer&#8217;s surface morphology, has been confirmed by X-ray photoelectron spectroscopy and atomic force microscopy measurements. The selective area irradiated samples were then immersed in a biotin-conjugated fluorescein-stained nanospheres solution for 2 hr, resulting in a successful immobilization of the nanospheres in the non-irradiated area. This illustrates the potential of&#160;the method for selective area biofunctionalization and fabrication of advanced Si-based biosensing architectures. We also describe a similar protocol of irradiation of wafers immersed in methanol (CH3OH) using ArF laser operating at pulse fluence of 65 mJ/cm2 and in situ formation of a strongly hydrophobic surface of Si (001) with the CA of 103&#176;. The XPS results indicate ArF laser induced formation of Si&#8211;(OCH3)x compounds responsible for the observed hydrophobicity. However, no such compounds were found by XPS on the Si surface irradiated by KrF laser in methanol, demonstrating the inability of the KrF laser to photodissociate methanol and create -OCH3 radicals.

We report on a process of in situ alteration of HF treated Si (001) surface into a hydrophilic or hydrophobic state by irradiating samples in microfluidic chambers filled with&#160;H2O2/H2O solution (0.01%-0.5%) or methanol solutions using pulsed UV laser of a relative low pulse fluence.

The wettability of silicon (Si) is one of the important parameters in the technology of surface functionalization of this material and fabrication of ...

Light Enhanced Hydrofluoric Acid Passivation: A Sensitive Technique for Detecting Bulk Silicon Defects

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) of silicon wafers by temporarily attaining a very high level of surface passivation when immersing the wafers in hydrofluoric acid (HF) is presented. By this procedure three critical steps are required to attain the bulk lifetime. Firstly, prior to immersing silicon wafers into HF, they are chemically cleaned and subsequently etched in 25% tetramethylammonium hydroxide. Secondly, the chemically treated wafers are then placed into a large plastic container filled with a mixture of HF and hydrochloric acid, and then centered over an inductive coil for photoconductance (PC) measurements. Thirdly, to inhibit surface recombination and measure the bulk lifetime, the wafers are illuminated at 0.2 suns for 1 min&#160;using a halogen lamp, the illumination is switched off, and a PC measurement is immediately taken. By this procedure, the characteristics of bulk silicon defects can be accurately determined. Furthermore, it is anticipated that a sensitive RT surface passivation technique will be imperative for examining bulk silicon defects when their concentration is low (&#60;1012 cm-3).

A RT liquid surface passivation technique to investigate the recombination activity of bulk silicon defects is described. For the technique to be successful, three critical steps are required: (i) chemical cleaning and etching of silicon, (ii) immersion of silicon in 15% hydrofluoric acid and (iii) illumination for 1 min.

A procedure to measure the bulk lifetime (&#62;100 &#181;sec) of silicon wafers by temporarily attaining a very high level of surface passivation when ...

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 ...

Digital Printing of Titanium Dioxide for Dye Sensitized Solar Cells

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less environmentally damaging with lower processing temperatures presenting a viable and low cost alternative to conventional production. This paper further enhances these environmental credentials by evaluating the digital printing and therefore additive production route for these cells. This is achieved here by investigating the formation and performance of a metal oxide photoelectrode using nanoparticle sized titanium dioxide. An ink-jettable material was formulated, characterized and printed with a piezoelectric inkjet head to produce a 2.6 &#181;m thick layer. The resultant printed layer was fabricated into a functioning cell with an active area of 0.25 cm2 and a power conversion efficiency of 3.5%. The binder-free formulation resulted in a reduced processing temperature of 250 &#176;C, compatible with flexible polyamide substrates which are stable up to temperatures of 350 &#730;C. The authors are continuing to develop this process route by investigating inkjet printing of other layers within dye sensitized solar cells.

This paper investigates the suitability of inkjet printing for the manufacturing of dye-sensitized solar cells. A binder-free TiO2 nanoparticle ink was formulated and printed onto a FTO glass substrate. The printed layer was fabricated into a cell with an active area of 0.25 cm2 and an efficiency of 3.5%.

Silicon solar cell manufacturing is an expensive and high energy consuming process. In contrast, dye sensitized solar cell production is less ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

Challenges in Rheological Characterization of Highly Concentrated Suspensions &#8212; A Case Study for Screen-printing Silver Pastes

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the manifold requirements during processing and application of such complex fluids. In this investigation, measuring protocols for a conclusive assessment of different process relevant rheological parameters have been evaluated. This includes the determination of yield stress, viscosity, wall slip velocity, structural recovery after large deformation and elongation at break as well as tensile force during filament stretching.
The importance of concomitant video recordings during parallel-plate rotational rheometry for a significant determination of rheological quantities is demonstrated. The deformation profile and flow field at the sample edge can be determined using appropriate markers. Thus, measurement parameter settings and plate roughness values can be identified for which yield stress and viscosity measurements are possible. Slip velocity can be measured directly and measuring conditions at which plug flow, shear banding or sample spillover occur can be identified clearly. 
Video recordings further confirm that the change in shear moduli observed during three stage oscillatory shear tests with small deformation amplitude in stage I and III but large oscillation amplitude in stage II can be directly attributed to structural break down and recovery. For the pastes investigated here, the degree of irreversible, shear-induced structural change increases with increasing deformation amplitude in stage II until a saturation is reached at deformations corresponding to the crossover of G' and G'', but the irreversible damage is independent of the duration of large amplitude shear. 
A capillary breakup elongational rheometer and a tensile tester have been used to characterize deformation and breakup behavior of highly filled pastes in uniaxial elongation. Significant differences were observed in all experiments described above for two commercial screen-printing silver pastes used for front side metallization of Si-solar cells.

Challenges in Rheological Characterization of Highly Concentrated Suspensions &amp;#8212; A Case Study for Screen-printing Silver Pastes

A protocol for a robust and application relevant rheological characterization of highly concentrated suspensions is presented. Silver pastes used for screen-printing application in solar cell production are employed as model systems.

A comprehensive rheological characterization of highly concentrated suspensions or pastes is mandatory for a targeted product development meeting the ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...