This research was supported by the In-House Research and Development Program of the Korea Institute of Energy Research (KIER) (B9-2411) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant NRF-2016R1D1A1B03934840).

1. Preparation of Mo-coated glass by DC magnetron sputtering
<ol>
	<li>Load cleaned glass substrates into a DC magnetron and pump down to below 4 x 10-6 Torr.</li>
	<li>Flow Ar gas and set the working pressure to 20 mTorr.</li>
	<li>Turn on plasma and increase the DC output power to 3 kW.</li>
	<li>After pre-sputtering of 3 min for target cleaning, begin the Mo deposition until the Mo film thickness reaches approximately 350 nm.</li>
	<li>Set the working pressure to 15 mTorr while maintaining the same output power (i.e., 3 kW).</li>
	<li>Resume the Mo deposition until the total thickness of Mo reaches approximately 750 nm.</li>
</ol>
2. CIGS absorber layer deposition by means of a three-stage coevaporation
<ol>
	<li>Load Mo-coated glass into a preheated co-evaporator under a vacuum lower than 5 x 10-6 Torr.</li>
	<li>Set the temperatures of In, Ga, and Se effusion cells yielding deposition rates of 2.5 &#197;/s, 1.3 &#197;/s, and 15 &#197;/s, respectively.
	<ol>
		<li>Check the deposition rates using the quartz crystal microbalance (QCM) technique. The deposition rates are dependent on the set temperature of effusion cells and the amount of materials in the effusion cells.</li>
	</ol>
	</li>
	<li>Begin to supply In, Ga and Se onto the Mo-coated glass to form a 1 &#956;m-thick (In,Ga)xSey precursor layer at the substrate temperature of 450 &#176;C. The deposition time is 15 min (namely, 1st stage).</li>
	<li>Stop the In and Ga supplies and increase the substrate temperature to 550 &#176;C.</li>
	<li>Begin to supply Cu (deposition rate: 1.5 &#197;/s) onto the (In,Ga)xSey precursor and continue until the Cu/(In + Ga) compositional ratio of the film reaches 1.15. Note that the Se deposition rate is maintained at 15 &#197;/s through the 2nd stage (namely, 2nd stage).</li>
	<li>Stop supplying Cu and evaporate In and Ga again with the same deposition rates as the 1st stage to finally form an approximately 2 &#956;m-thick CIGS film with Cu/(In+Ga) compositional ratio of 0.9. Maintain the Se deposition rate and substrate temperature at 15 &#197;/s and 550 &#176;C, respectively. The deposition time of this stage is 4 min (namely 3rd stage).</li>
	<li>In order to ensure a complete reaction, anneal the deposited CIGS film under ambient Se (15 &#197;/s) for 5 min at the substrate temperature of 550 &#176;C.</li>
	<li>Cool down the substrate temperature to 450 &#176;C under ambient Se (15 &#197;/s) and then unload the CIGS-deposited substrate when the substrate temperature is below 250 &#176;C.</li>
</ol>
3. Growth of the CdS buffer layer on the CIGS absorber layer using a chemical bath deposition (CBD) method
<ol>
	<li>Prepare the CdS reaction bath solution in a 250 mL beaker by adding 97 mL of DI water, 0.079 g of Cd(CH3COO)2&#183;2H2O, 0.041 g of NH2CSNH2, and 0.155 g of CH3COONH4. Stir the solution for several minutes to mix. Make sure that all added solutes are completely dissolved.</li>
	<li>Add 3 mL of NH4OH (28% NH3) into the bath solution and stir the solution for 2 min. Figure 1 shows the experimental setup of CBD for CdS.</li>
	<li>Put the CIGS sample into the reaction bath solution using a Teflon sample holder.</li>
	<li>Put the reaction bath into the water-heat bath maintained at 65 &#176;C and stir the reaction bath solution at 200 rpm using a magnetic bar during the deposition process.</li>
	<li>React for 20 min to generate an approximately 70 to 80 nm CdS buffer layer on the CIGS.</li>
	<li>After the reaction, remove the sample from the reaction bath, wash with a flow of DI water, and dry with N2 gas.</li>
	<li>Anneal the sample at 120 &#176;C for 30 min on a hot plate.</li>
</ol>
4. Fabrication of the AgNW TCE network
<ol>
	<li>Prepare a diluted AgNW dispersion (1 mg/mL) by mixing 19 mL of ethanol with 1 mL of a purchased ethanol-based AgNW dispersion (20 mg/mL).</li>
	<li>Pour 0.2 mL of the diluted AgNW dispersion onto a CdS/CIGS sample (2.5 cm x 2.5 cm) to cover the whole surface of the sample and rotate the sample with 1,000 rpm for 30 s.</li>
	<li>Repeat step 4.2 as needed to achieve the desired optical and electrical properties. Spin-coat the AgNWs 3x. A scanning electron microscopy (SEM) image of spin-coated AgNW TCE is shown in Figure 2.</li>
	<li>After spin-coating, anneal the sample at 120 &#176;C for 5 min on a hot plate.</li>
</ol>
5. Deposition of the 2nd CdS layer
<ol>
	<li>Prepare a new CdS reaction bath solution as described in step 3.1.</li>
	<li>Deposit CdS as in section 3, except change the reaction time as necessary. 
	NOTE: We optimized the reaction time, and 10 min resulted in the CIGS device with the best performance. The effect of 2nd CdS deposition time on a CIGS thin film solar cell device performance can be found in our previous work14.</li>
</ol>
6. Characterization techniques
<ol>
	<li>Characterize the surface and cross-sectional morphology of AgNWs and CdS-coated AgNWs by field emission SEM and transmission electron microscopy (TEM).</li>
	<li>Measure solar cell performance using a current-voltage source equipped with a solar simulator (1,000 W/m2, AM1.5G).</li>
</ol>

<ol>
	<li>Lee, S., 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=Determination+of+the+lateral+collection+length+of+charge+carriers+for+silver-nanowire-electrode-based+Cu(In,Ga)Se2+thin-film+solar+cells.">Determination of the lateral collection length of charge carriers for silver-nanowire-electrode-based Cu(In,Ga)Se2 thin-film solar cells.</a> Solar Energy. 180, 519-523 (2019).</li><li>Langley, D., 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=Flexible+transparent+conductive+materials+based+on+silver+nanowire+networks:+a+review.">Flexible transparent conductive materials based on silver nanowire networks: a review.</a> Nanotechnology. 24 (45), 452001 (2013).</li><li>Chung, C. -. 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=Silver+nanowire+composite+window+layers+for+fully+solution-deposited+thin-film+photovoltaic+devices.">Silver nanowire composite window layers for fully solution-deposited thin-film photovoltaic devices.</a> Advanced Materials. 24 (40), 5499-5504 (2012).</li><li>Liu, C. -. H., Yu, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Silver+nanowire-based+transparent,+flexible,+and+conductive+thin+film.">Silver nanowire-based transparent, flexible, and conductive thin film.</a> Nanoscale Research Letters. 6 (1), (2011).</li><li>Yu, 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=Highly+flexible+silver+nanowire+electrodes+for+shape-memory+polymer+light-emitting+diodes.">Highly flexible silver nanowire electrodes for shape-memory polymer light-emitting diodes.</a> Advanced Materials. 23 (5), 664-668 (2011).</li><li>Chung, C. -. H., Hong, K. -. H., Lee, D. -. K., Yun, J. H., Yang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ordered+vacancy+compound+formation+by+controlling+element+redistribution+in+molecular-level+precursor+solution+processed+CuInSe2+thin+films.">Ordered vacancy compound formation by controlling element redistribution in molecular-level precursor solution processed CuInSe2 thin films.</a> Chemistry of Materials. 27 (21), 7244-7247 (2015).</li><li>Kim, A., Won, Y., Woo, K., Kim, C. -. H., Moon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+transparent+low+resistance+ZnO/Ag+Nanowire/ZnO+composite+electrode+for+thin+film+solar+cells.">Highly transparent low resistance ZnO/Ag Nanowire/ZnO composite electrode for thin film solar cells.</a> ACS Nano. 7 (2), 1081-1091 (2013).</li><li>Singh, M., Jiu, J., Sugahara, T., Suganuma, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin-film+copper+indium+gallium+selenide+solar+cell+based+on+low-temperature+all-printing+process.">Thin-film copper indium gallium selenide solar cell based on low-temperature all-printing process.</a> ACS Applied Materials and Interfaces. 6 (18), 16297-16303 (2014).</li><li>Kim, A., Won, Y., Woo, K., Jeong, S., Moon, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-solution-processed+indium-free+transparent+composite+electrodes+based+on+Ag+Nanowire+and+Metal+Oxide+for+thin-film+solar+cells.">All-solution-processed indium-free transparent composite electrodes based on Ag Nanowire and Metal Oxide for thin-film solar cells.</a> Advanced Functional Materials. 24 (17), 2462-2471 (2014).</li><li>Shin, D., Kim, T., Ahn, B. T., Han, S. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solution-processed+Ag+Nanowires+++PEDOT:PSS+hybrid+electrode+for+Cu(In,Ga)Se2+thin-film+solar+cells.">Solution-processed Ag Nanowires + PEDOT:PSS hybrid electrode for Cu(In,Ga)Se2 thin-film solar cells.</a> ACS Applied Materials and Interfaces. 7 (24), 13557-13563 (2015).</li><li>Wang, M., Choy, K. -. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=All-nonvacuum-processed+CIGS+solar+cells+using+scalable+Ag+NWs/AZO-based+transparent+electrodes.">All-nonvacuum-processed CIGS solar cells using scalable Ag NWs/AZO-based transparent electrodes.</a> ACS Applied Materials and Interfaces. 8 (26), 16640-16648 (2016).</li><li>Jang, J., 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=Cu(In,Ga)Se2+thin+film+solar+cells+with+solution+processed+silver+nanowire+composite+window+layers:+buffer/window+junctions+and+their+effects.">Cu(In,Ga)Se2 thin film solar cells with solution processed silver nanowire composite window layers: buffer/window junctions and their effects.</a> Solar Energy Materials and Solar Cells. 170, 60-67 (2017).</li><li>Chung, C. -. H., Bob, B., Song, T. -. B., Yang, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current-voltage+characteristics+of+fully+solution+processed+high+performance+CuIn(S,Se)2+solar+cells:+crossover+and+red+kink.">Current-voltage characteristics of fully solution processed high performance CuIn(S,Se)2 solar cells: crossover and red kink.</a> Solar Energy Materials and Solar Cells. 120, 642-646 (2014).</li><li>Lee, S., 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=Robust+nanoscale+contact+of+silver+nanowire+electrodes+to+semiconductors+to+achieve+high+performance+chalcogenide+thin+film+solar+cells.">Robust nanoscale contact of silver nanowire electrodes to semiconductors to achieve high performance chalcogenide thin film solar cells.</a> Nano Energy. 53, 675-682 (2018).</li></ol>

The authors declare that they have no competing financial interests.

Note that the deposition time of the 2nd CdS layer must be optimized to achieve the optimal cell performance. As the deposition time increases, the thickness of the 2nd CdS layer increases, and consequently, the electrical contact will improve. However, further deposition of the 2nd CdS layer will result in a thicker layer that reduces light absorption, and the device efficiency will decrease. We achieved the best cell performance with 10 min of deposition time for the 2nd CdS ...

Silver nanowire (AgNW) networks have been extensively studied as an alternative to indium tin oxide (ITO) transparent conducting thin films due to their advantages over conventional transparent conducting oxides (TCOs) in terms of lower processing cost and better mechanical flexibility. Solution-processed AgNW network transparent conducting electrodes (TCEs) have thus been employed in Cu(In,Ga)Se2 (CIGS) thin-film solar cells1,2,3,4,5,6. Solution-processed AgNW TCEs are normally fabricated in the form of embedded-AgNW or sandwich-AgNW structures in a conductive matrix such as PEDOT:PSS, ITO, ZnO, etc.7,8,9,10,11 The matrix layers can enhance that the collection of the charge carriers present in the empty spaces of the AgNW network.
However, the matrix layers can generate interfacial defects between the matrix layer and underlying CdS buffer layer in CIGS thin-film solar cells12,13. The interfacial defects often cause a kink in the current density-voltage (J-V) curve, resulting in a low fill factor (FF) in the cell, which is detrimental to solar cell performance. We previously reported a method to resolve this issue by selectively depositing an additional thin CdS layer (2nd CdS layer) between the AgNWs and the CdS buffer layer14. The incorporation of an additional CdS layer enhanced the contact properties in the junction between the AgNW and CdS layers. Consequently, the carrier collection in the AgNW network was greatly improved, and the cell performance was enhanced. In this protocol, we describe the experimental procedure to fabricate robust electrical contact between the AgNW network and the CdS buffer layer using a 2nd CdS layer in a CIGS thin-film solar cell.

<table border="0" cellpadding="0" cellspacing="0" style="width:616px;" width="617">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Mo</td>
			<td style="width:115px;">Materion</td>
			<td style="width:119px;">Purity: 3N5</td>
			<td style="width:167px;">Mo sputtering</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Cu</td>
			<td style="width:115px;">5N Plus</td>
			<td style="width:119px;">Purity: 4N7</td>
			<td>CIGS deposition</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">In</td>
			<td style="width:115px;">5N Plus</td>
			<td style="width:119px;">Purity: 5N</td>
			<td>CIGS deposition</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ga</td>
			<td style="width:115px;">5N Plus</td>
			<td style="width:119px;">Purity: 5N</td>
			<td>CIGS deposition</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Se</td>
			<td style="width:115px;">5N Plus</td>
			<td style="width:119px;">Purity: 5N</td>
			<td>CIGS deposition</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ammonium acetate</td>
			<td style="width:115px;">Alfa Aesar</td>
			<td style="width:119px;">11599</td>
			<td>CdS reaction solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ammonium hydroxide</td>
			<td style="width:115px;">Alfa Aesar</td>
			<td style="width:119px;">L13168</td>
			<td>CdS reaction solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Cadmium acetate dihydrate</td>
			<td style="width:115px;">Sigma-Aldrich</td>
			<td style="width:119px;">289159</td>
			<td>CdS reaction solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Thiourea</td>
			<td style="width:115px;">Sigma-Aldrich</td>
			<td style="width:119px;">T8656</td>
			<td>CdS reaction solution</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Silver Nanowire</td>
			<td style="width:115px;">ACSMaterial</td>
			<td style="width:119px;">AgNW-L30</td>
			<td>AgNW dispersion</td>
		</tr>
	</tbody>
</table>

fabrication of robust nanoscale contact between a silver nanowire electrode and cds buffer layer in cu(in,ga)se2 thin-film solar cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes normally result in very poor cell performance. Embedding or sandwiching silver nanowires using moderately conductive transparent materials, such as indium tin oxide or zinc oxide, can improve cell performance. However, the solution-processed matrix layers can cause a significant number of interfacial defects between transparent electrodes and the CdS buffer, which can eventually result in low cell performance. This manuscript describes how to fabricate robust electrical contact between a silver nanowire electrode and the underlying CdS buffer layer in a Cu(In,Ga)Se2 solar cell, enabling high cell performance using matrix-free silver nanowire transparent electrodes. The matrix-free silver nanowire electrode fabricated by our method proves that the charge-carrier collection capability of silver nanowire electrode-based cells is as good as that of standard cells with sputtered ZnO:Al/i-ZnO as long as the silver nanowires and CdS have high-quality electrical contact. The high-quality electrical contact was achieved by depositing an additional CdS layer as thin as 10 nm onto the silver nanowire surface.

The layer structures of the CIGS solar cells with (a) standard ZnO:Al/i-ZnO and (b) AgNW TCE are shown in Figure 3. The surface morphology of CIGS is rough, and a nanoscale gap can form between the AgNW layer and the underlying CdS buffer layer. As highlighted in Figure 3A, the 2nd CdS layer can be selectively deposited onto the nanoscale gap to create a stable electrical contact. The detailed explanation on the format...

Watch this Scientific Journal Video about Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells at JoVE.com

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in Cu(In,Ga)Se2 Thin-film Solar Cells

Silver nanowire transparent electrodes have been employed as window layers for Cu(In,Ga)Se2 thin-film solar cells. Bare silver nanowire electrodes ...

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6.2K Views.  Hanbat National University. In this protocol, we describe the detailed experimental procedure for the fabrication of a robust nanoscale contact between a silver nanowire network and CdS buffer layer in a CIGS thin-film solar cell.

Video: Fabrication of Robust Nanoscale Contact between a Silver Nanowire Electrode and CdS Buffer Layer in CuIn,GaSe2 Thin-film Solar Cells

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 ...

Probing and Mapping Electrode Surfaces in Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen 1-7. The performance of SOFCs and the rates of many chemical and energy transformation processes in energy storage and conversion devices in general are limited primarily by charge and mass transfer along electrode surfaces and across interfaces. Unfortunately, the mechanistic understanding of these processes is still lacking, due largely to the difficulty of characterizing these processes under in situ conditions. This knowledge gap is a chief obstacle to SOFC commercialization. The development of tools for probing and mapping surface chemistries relevant to electrode reactions is vital to unraveling the mechanisms of surface processes and to achieving rational design of new electrode materials for more efficient energy storage and conversion2. Among the relatively few in situ surface analysis methods, Raman spectroscopy can be performed even with high temperatures and harsh atmospheres, making it ideal for characterizing chemical processes relevant to SOFC anode performance and degradation8-12. It can also be used alongside electrochemical measurements, potentially allowing direct correlation of electrochemistry to surface chemistry in an operating cell. Proper in situ Raman mapping measurements would be useful for pin-pointing important anode reaction mechanisms because of its sensitivity to the relevant species, including anode performance degradation through carbon deposition8, 10, 13, 14 ("coking") and sulfur poisoning11, 15 and the manner in which surface modifications stave off this degradation16. The current work demonstrates significant progress towards this capability. In addition, the family of scanning probe microscopy (SPM) techniques provides a special approach to interrogate the electrode surface with nanoscale resolution. Besides the surface topography that is routinely collected by AFM and STM, other properties such as local electronic states, ion diffusion coefficient and surface potential can also be investigated17-22. In this work, electrochemical measurements, Raman spectroscopy, and SPM were used in conjunction with a novel test electrode platform that consists of a Ni mesh electrode embedded in an yttria-stabilized zirconia (YSZ) electrolyte. Cell performance testing and impedance spectroscopy under fuel containing H2S was characterized, and Raman mapping was used to further elucidate the nature of sulfur poisoning. In situ Raman monitoring was used to investigate coking behavior. Finally, atomic force microscopy (AFM) and electrostatic force microscopy (EFM) were used to further visualize carbon deposition on the nanoscale. From this research, we desire to produce a more complete picture of the SOFC anode.

We present a unique platform for characterizing electrode surfaces in solid oxide fuel cells (SOFCs) that allows simultaneous performance of multiple characterization techniques (e.g. in situ Raman spectroscopy and scanning probe microscopy alongside electrochemical measurements). Complementary information from these analyses may help to advance toward a more profound understanding of electrode reaction and degradation mechanisms, providing insights into rational design of better materials for SOFCs.

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution to utilization of a wide variety of fuels beyond hydrogen ...

Nanomoulding of Functional Materials, a Versatile Complementary Pattern Replication Method to Nanoimprinting

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding combined with layer transfer enables the replication of arbitrary surface patterns from a master structure onto the functional material. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes. In particular we demonstrate the fabrication of patterned transparent zinc oxide electrodes for light trapping applications in solar cells.

We describe a nanomoulding technique which allows low-cost nanoscale patterning of functional materials, materials stacks and full devices. Nanomoulding can be performed on any nanoimprinting setup and can be applied to a wide range of materials and deposition processes.

We describe a nanomoulding technique which  allows low-cost nanoscale patterning of functional materials, materials stacks  and full devices. Nanomoulding ...

Atom Probe Tomography Studies on the Cu(In,Ga)Se2 Grain Boundaries

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the nanoscale and in three dimensions. Indeed, APT possesses high sensitivity (in the order of ppm) and high spatial resolution (sub nm).
Considerable efforts were done here to prepare an APT tip which contains the desired grain boundary with a known structure. Indeed, site-specific sample preparation using combined focused-ion-beam, electron backscatter diffraction, and transmission electron microscopy is presented in this work. This method allows selected grain boundaries with a known structure and location in Cu(In,Ga)Se2 thin-films to be studied by atom probe tomography.
Finally, we discuss the advantages and drawbacks of using the atom probe tomography technique to study the grain boundaries in Cu(In,Ga)Se2 thin-film solar cells.

Atom Probe Tomography Studies on the CuIn,GaSe2 Grain Boundaries

In this work, we describe the use of the atom-probe tomography technique for studying the grain boundaries of the absorber layer in a CIGS solar cell. A novel approach to prepare the atom probe tips containing the desired grain boundary with a known structure is also presented here.

Compared with the existent techniques, atom probe tomography is a unique technique able to chemically characterize the internal interfaces at the ...

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO2 interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO2 surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO2 substrate.

A technique was developed that removes Ni/Au contact metal films from their substrate to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN nanowire devices.

Single GaN nanowire (NW) devices fabricated on SiO2 can exhibit  a strong degradation after annealing due to the  occurrence of void formation at the ...

Fabrication of Uniform Nanoscale Cavities via Silicon Direct Wafer Bonding

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned and bonded silicon wafers. Unlike confinements in porous materials often used for these types of experiments3, bonded wafers provide predesigned uniform spaces for confinement. The geometry of each cell is well known, which removes a large source of ambiguity in the interpretation of data.
Exceptionally flat, 5 cm diameter, 375 &#181;m thick Si wafers with about 1 &#181;m variation over the entire wafer can be obtained commercially (from Semiconductor Processing Company, for example). Thermal oxide is grown on the wafers to define the confinement dimension in the z-direction. A pattern is then etched in the oxide using lithographic techniques so as to create a desired enclosure upon bonding. A hole is drilled in one of the wafers (the top) to allow for the introduction of the liquid to be measured. The wafers are cleaned2 in RCA solutions and then put in a microclean chamber where they are rinsed with deionized water4. The wafers are bonded at RT and then annealed at ~1,100 &#176;C. This forms a strong and permanent bond. This process can be used to make uniform enclosures for measuring thermal and hydrodynamic properties of confined liquids from the nanometer to the micrometer scale.

A method for permanently bonding two silicon wafers so as to realize a uniform enclosure is described. This includes wafer preparation, cleaning, RT bonding, and annealing processes. The resulting bonded wafers (cells) have uniformity of enclosure ~1%1,2. The resulting geometry allows for measurements of confined liquids and gasses.

Measurements of the heat capacity and superfluid fraction of confined 4He have been performed near the lambda transition using lithographically patterned ...

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 ...

Ohmic Contact Fabrication Using a Focused-ion Beam Technique and Electrical Characterization for Layer Semiconductor Nanostructures

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor performance, which suggest a new direction for the development of next-generation ultrathin and flexible photonic and electronic devices. Enhanced luminescence quantum efficiency has been widely observed in these atomically thin 2D crystals. However, dimension effects beyond quantum confinement thicknesses or even at the micrometer scale are not expected and have rarely been observed. In this study, molybdenum diselenide (MoSe2) layer crystals with a thickness range of 6-2,700 nm were fabricated as two- or four-terminal devices. Ohmic contact formation was successfully achieved by the focused-ion beam (FIB) deposition method using platinum (Pt) as a contact metal. Layer crystals with various thicknesses were prepared through simple mechanical exfoliation by using dicing tape. Current-voltage curve measurements were performed to determine the conductivity value of the layer nanocrystals. In addition, high-resolution transmission electron microscopy, selected-area electron diffractometry, and energy-dispersive X-ray spectroscopy were used to characterize the interface of the metal&#8211;semiconductor contact of the FIB-fabricated MoSe2 devices. After applying the approaches, the substantial thickness-dependent electrical conductivity in a wide thickness range for the MoSe2-layer semiconductor was observed. The conductivity increased by over two orders of magnitude from 4.6 to 1,500 &#937;&#8722;1 cm&#8722;1, with a decrease in the thickness from 2,700 to 6 nm. In addition, the temperature-dependent conductivity indicated that the thin MoSe2 multilayers exhibited considerably weak semiconducting behavior with activation energies of 3.5-8.5 meV, which are considerably smaller than those (36-38 meV) of the bulk. Probable surface-dominant transport properties and the presence of a high surface electron concentration in MoSe2 are proposed. Similar results can be obtained for other layer semiconductor materials such as MoS2 and WS2.

We describe the approaches for the device fabrication and electrical characterization of molybdenum diselenide (MoSe2) layer semiconductor nanostructures with different thicknesses. In addition, the fabrication of ohmic contacts for MoSe2-layer nanocrystals by the focused-ion beam deposition method using platinum (Pt) as a contact metal is described.

Layer semiconductors with easily processed two-dimensional (2D) structures exhibit indirect-to-direct bandgap transitions and superior transistor ...

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 ...

A Fabrication Method for Highly Stretchable Conductors with Silver Nanowires

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of stretchable electronic devices is the preparation of stretchable conductors with great mechanical stability. In this study, we developed a simple fabrication method to chemically solder the contact points between silver nanowire (AgNW) networks. AgNW nanomesh was first deposited on a glass slide via spray coating method. A reactive ink composed of silver nanoparticle (AgNPs) precursors was applied over the spray coated AgNW thin films. After heating for 40 min, AgNPs were preferentially generated over the nanowire junctions to solder the AgNW nanomesh, and reinforced the conducting network. The chemically modified AgNW thin film was then transferred to polyurethane (PU) substrates by casting method. The soldered AgNW thin films on PU exhibited no obvious change in electrical conductivity under stretching or rolling process with elongation strains up to 120%.

A simple synthesis method is used to chemically solder silver nanowire thin film to fabricate highly stretchable and conductive metal conductors.

Stretchable electronics are identified as a key technology for electronic applications in the next generation. One of the challenges in fabrication of ...