This research project is supported by Australian Research Council through the linkage grant with CSG Solar Pty. Ltd. Jing Rao acknowledges her University of NSW Vice-Chancellor Postdoctoral Fellowship. SEM images were taken by Jongsung Park using the equipment provided by the Electron Microscopy Unit of the University of NSW.

1. Fabrication of polycrystalline silicon solar cells (Animation 3)
<ol>
 <li>Silicon film deposition
 <ol class="lalpha">
 <li>Prepare the e-beam evaporation tool by baking it out at ~100 &deg;C overnight to reach the base pressure of &lt;3E-8 Torr. Preset the sample heater to the 150 &deg;C standby temperature.</li>
 <li>Use a substrate made of 5x5 cm2 (or 10 x10 cm2) substrate borosilicate glass (Schott Borofloat33), 1.1 or 3.3 mm thick, coated with ~80 nm of silicon nitride (prepared by PECV...

<ol>
	<li>Henley, F. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Kerf-free+wafering.">Kerf-free wafering.</a> , 1184-1192 (2010).</li><li>Kunz, O., Wong, J., Janssens, J., Bauer, J., Breitenstein, O., Aberle, A. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Shunting+problems+due+to+sub-micron+pinholes+in+evaporated+solid-phase+crystallised+poly-Si+thin-film+solar+cells+on+glass.">Shunting problems due to sub-micron pinholes in evaporated solid-phase crystallised poly-Si thin-film solar cells on glass.</a> Progress Photovoilt.: Res. Appl. 17, 35-46 (2009).</li><li>Kunz, O., Ouyang, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=5%+Efficient+evaporated+solid-phase+crystallised+polycrystalline+silicon+solar+cells.">5% Efficient evaporated solid-phase crystallised polycrystalline silicon solar cells.</a> Progress Photovolt.: Res. Appl. 17, 567-573 (2009).</li><li>Van Nieuwenhuysen, K., Payo, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epitaxially+grown+emitters+for+thin+film+silicon+solar+cells+result+in+16%+efficiency.">Epitaxially grown emitters for thin film silicon solar cells result in 16% efficiency.</a> Thin Solid Films. 518, S80-S82 (2008).</li><li>Lee, B. G., Stradin, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light-trapping+by+a+dielectric+nanoparticle+back+reflector+in+film+silicon+solar+cells.">Light-trapping by a dielectric nanoparticle back reflector in film silicon solar cells.</a> Appl. Phys. Lett. 99, 064101 (2011).</li><li>Catchpole, K. R., Polman, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Plasmonic+solar+cells.">Plasmonic solar cells.</a> Optics Express. 16, 21793-21800 (2008).</li><li>Ouyang, Z., Zhao, X. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nanoparticle+enhanced+light-trapping+in+thin-film+silicon+solar+cells.">Nanoparticle enhanced light-trapping in thin-film silicon solar cells.</a> Progress Photovolt.: Res. Appl. 19, 917-926 (2011).</li><li>Catchpole, K. R., Polman, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+principle+for+particle+plasmon+enhanced+solar+cells.">Design principle for particle plasmon enhanced solar cells.</a> Appl. Phys. Lett. 93, 191113 (2008).</li><li>Beck, F. J., Mokkapati, S., Polman, A., Catchpole, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Asymmetry+in+photocurrent+enhancement+by+plasmonic+nanoparticle+arrays+located+on+the+front+or+on+the+rear+of+solar+cells.">Asymmetry in photocurrent enhancement by plasmonic nanoparticle arrays located on the front or on the rear of solar cells.</a> Appl. Phys. Lett. 96, 033113 (2008).</li><li>Beck, F. J., Verhagen, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resonant+SPP+modes+supported+bt+discrete+metal+nanoparticles+on+high+index+substrates.">Resonant SPP modes supported bt discrete metal nanoparticles on high index substrates.</a> Optics Express. 19, 146-156 (2010).</li><li>Kunz, O., Ouyang, Z., al, a. t. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=5%+Efficient+evaporated+solid-phase+crystallised+polycrystalline+silicon+thin-film+solar+cells.">5% Efficient evaporated solid-phase crystallised polycrystalline silicon thin-film solar cells.</a> Progress Photovolt. 17, 567-573 (2009).</li><li>Keevers, M. J., Young, T. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=10%+Efficient+CSG+minimodules.">10% Efficient CSG minimodules.</a> , 1783-1790 (2007).</li></ol>

No conflicts of interest declared.

Evaporated polycrystalline silicon solar cells and the light-scattering plasmonic nanoparticles are ideal partners for light-trapping. Such cells are planar, hence they cannot rely on light-scattering from textured surfaces, nor can plasmonic nanoparticles be easily formed on textured surfaces. The cells have only one, rear surface with directly exposed silicon, which also happens to be the best nanoparticle location for most effective plasmonic light-scattering. Moreover, the easiest method for nanoparticle formation by...

<table border="1">
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 <td>Name of the reagent</td>
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 <td>Silver granular</td>
 <td>Sigma-Aldrich</td>
 <td>303372</td>
 <td>99.99%</td>
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 <td>MgF2, random crystals, optical grade</td>
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 <td>Dulux one-coat ceiling paint</td>
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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.

Watch this Scientific Journal Video about Polycrystalline Silicon Thin-film Solar cells with Plasmonic-enhanced Light-trapping at JoVE.com

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

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 ...

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18.7K Views.  University of New South Wales. 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.