Name: close-space sublimation-deposited ultra-thin cdsete/cdte solar cells for enhanced short-circuit current density and photoluminescence
Uploaded: 2020-03-06T15:00:00
Description: Watch this Scientific Journal Video about Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence at JoVE.com

The authors would like to thank Professor W.S. Sampath for use of his deposition systems, Kevan Cameron for system support, Dr. Amit Munshi for his work with thicker bilayer cells and supplemental footage of the in-line automated CSS vacuum deposition system, and Dr. Darius Kuciauskas for assistance with TRPL measurements. This material is based upon work supported by the U.S. Department of Energy&#8217;s Office of Energy Efficiency and Renewable Energy (EERE) under Solar Energy Technologies Office (SETO) Agreement Number DE-EE0007543.

CAUTION: Gloves must be worn when handling substrates to prevent film contamination and material-to-skin contact. This fabrication process requires the handling of structures containing cadmium compounds; therefore, a lab coat and gloves should be worn in the lab at all times.
1. Substrate cleaning
<ol>
	<li>Place TCO-coated glass substrates (9.1 cm x 7.8 cm) in a stainless steel rack with ample spacing such that cleaning solution and compressed air can be applied to each glass face.</li>
	<...

<ol>
	<li>. Global energy demand rose by 2.3% in 2018, its fastest pace in the last decade Available from: <a target="_blank" href="https://www.iea.org/newsroom/news/2019/march/global-energy-demand-rose-by-23-in-2018-its-fastest-pace-in-the-last-decade.html">https://www.iea.org/newsroom/news/2019/march/global-energy-demand-rose-by-23-in-2018-its-fastest-pace-in-the-last-decade.html</a> (2019)</li><li>Morton, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Solar+energy:+A+new+day+dawning?:+Silicon+valley+sunrise.">Solar energy: A new day dawning?: Silicon valley sunrise.</a> Nature. 443 (7107), 19-22 (2006).</li><li>. Best research-cell efficiency chart Available from: <a target="_blank" href="https://www.nrel.gov/pv/cell-efficiency.html">https://www.nrel.gov/pv/cell-efficiency.html</a> (2019)</li><li>Munshi, A., 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=Polycrystalline+CdSeTe/CdTe+absorber+cells+with+28+mA/cm2+short-circuit+current.">Polycrystalline CdSeTe/CdTe absorber cells with 28 mA/cm2 short-circuit current.</a> IEEE Journal of Photovoltaics. 8 (1), 310-314 (2018).</li><li>Metzger, W. K., 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=Exceeding+20%+efficiency+with+in+situ+group+V+doping+in+polycrystalline+CdTe+solar+cells.">Exceeding 20% efficiency with in situ group V doping in polycrystalline CdTe solar cells.</a> Nature Energy. 4, 837-845 (2019).</li><li>Hsiao, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Electroplated+CdTe+solar+technology+at+Reel+Solar.">Electroplated CdTe solar technology at Reel Solar.</a> Proceedings of 46thIEEE PVSC. , (2019).</li><li>Bothwell, A. M., Drayton, J. A., Jundt, P. M., Sites, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Characterization+of+thin+CdTe+solar+cells+with+a+CdSeTe+front+layer.">Characterization of thin CdTe solar cells with a CdSeTe front layer.</a> MRS Advances. 4 (37), 2053-2062 (2019).</li><li>Fiducia, T. A. M., 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=Understanding+the+role+of+selenium+in+defect+passivation+for+highly+efficient+selenium-alloyed+cadmium+telluride+solar+cells.">Understanding the role of selenium in defect passivation for highly efficient selenium-alloyed cadmium telluride solar cells.</a> Nature Energy. 4, 504-511 (2019).</li><li>Swanson, D. E., 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=Single+vacuum+chamber+with+multiple+close+space+sublimation+sources+to+fabricate+CdTe+solar+cells.">Single vacuum chamber with multiple close space sublimation sources to fabricate CdTe solar cells.</a> Journal of Vacuum Science and Technology A. 34, 021202 (2016).</li><li>McCandless, B. E., Sites, J. R., Luque, A., Hegedus, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cadmium+telluride+solar+cells.">Cadmium telluride solar cells.</a> Handbook of Photovoltaic Science and Engineering. , 617-662 (2003).</li><li>Abbas, A., 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=Cadmium+chloride+assisted+re-crystallization+of+CdTe:+the+effect+of+annealing+over-treatment.">Cadmium chloride assisted re-crystallization of CdTe: the effect of annealing over-treatment.</a> Proceedings of 40th IEEE PVSC. , (2014).</li><li>Munshi, A., 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=Effect+of+varying+process+parameters+on+CdTe+thin+film+device+performance+and+its+relationship+to+film+microstructure.">Effect of varying process parameters on CdTe thin film device performance and its relationship to film microstructure.</a> Proceedings of 40th IEEE PVSC. , (2014).</li><li>Metzger, W. K., 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=Time-resolved+photoluminescence+studies+of+CdTe+solar+cells.">Time-resolved photoluminescence studies of CdTe solar cells.</a> Journal of Applied Physics. 94 (5), 3549-3555 (2003).</li><li>Moseley, 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=Luminescence+methodology+to+determine+grain-boundary,+grain-interior,+and+surface+recombination+in+thin+film+solar+cells.">Luminescence methodology to determine grain-boundary, grain-interior, and surface recombination in thin film solar cells.</a> Journal of Applied Physics. 124, 113104 (2018).</li><li>Amarasinghe, M., 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=Obtaining+large+columnar+CdTe+grains+and+long+lifetime+on+nanocrystalline+CdSe,+MgZnO,+or+CdS+layers.">Obtaining large columnar CdTe grains and long lifetime on nanocrystalline CdSe, MgZnO, or CdS layers.</a> Advanced Energy Materials. 8, 1702666 (2018).</li><li>Tencor Instruments. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Alpha-Step 100 User's Manual. , (1984).</li><li>Wojtowicz, A., Huss, A. M., Drayton, J. A., Sites, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+CdCl2+passivation+on+thin+CdTe+absorbers+fabricated+by+close-space+sublimation.">Effects of CdCl2 passivation on thin CdTe absorbers fabricated by close-space sublimation.</a> Proceedings of 44th IEEE PVSC. , (2017).</li><li>Kirchartz, T., Ding, K., Rau, U., Abou-Ras, D., Kirchartz, T., Rau, U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fundamental+electrical+characterization+of+thin+film+solar+cells.">Fundamental electrical characterization of thin film solar cells.</a> Advanced Characterization Techniques for Thin Film Solar Cells. , 47 (2011).</li><li>Swanson, D. E., Sites, J. R., Sampath, W. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Co-sublimation+of+CdSexTe1-x+layers+for+CdTe+solar+cells.">Co-sublimation of CdSexTe1-x layers for CdTe solar cells.</a> Solar Energy Materials &amp; Solar Cells. 159, 389-394 (2017).</li><li>McCandless, B. E., 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=Overcoming+Carrier+Concentration+Limits+in+Polycrystalline+CdTe+Thin+Films+with+In+Situ+Doping.">Overcoming Carrier Concentration Limits in Polycrystalline CdTe Thin Films with In Situ Doping.</a> Scientific Reports. 8, 14519 (2018).</li><li>Romeo, N., Bossio, A., Rosa, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+back+contact+in+CdTe/CdS+thin+film+solar+cells.">The back contact in CdTe/CdS thin film solar cells.</a> Proceedings ISES Solar World Congress 2017. , (2017).</li><li>Munshi, A. 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=Doping+CdSexTe1-x/CdTe+graded+absorber+films+with+arsenic+for+thin+film+photovoltaics.">Doping CdSexTe1-x/CdTe graded absorber films with arsenic for thin film photovoltaics.</a> Proceedings of 46th IEEE PVSC. , (2019).</li><li>Danielson, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Doping+CdTe+Absorber+Cells+using+Group+V+Elements.">Doping CdTe Absorber Cells using Group V Elements.</a> Proceedings of WCPEC-7. , (2018).</li><li>Hsiao, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Electron Reflector Strategy for CdTe Thin film Solar Cells. , (2010).</li><li>Swanson, D. E., 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=Incorporation+of+Cd1-xMgxTe+as+an+electron+reflector+for+cadmium+telluride+photovoltaic+cells.">Incorporation of Cd1-xMgxTe as an electron reflector for cadmium telluride photovoltaic cells.</a> Proceedings of MRS Spring Meeting and Exhibit. , (2015).</li><li>Wendt, R., Fischer, A., Grecu, D., Compaan, A. 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Thin bilayer CdSeTe/CdTe photovoltaic devices demonstrate improvements in efficiency compared to their CdTe counterparts because of better material quality and increased current collection. Such enhanced efficiencies have been demonstrated in bilayer absorbers greater than 3 &#956;m5,7, and now with optimized fabrication conditions, it has been demonstrated that increased efficiencies are also achievable for thinner, 1.5-&#956;m bilayer absorbers.
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Global energy demand is quickly accelerating, and the year 2018 demonstrated the fastest( 2.3%) growth rate in the last decade1. Paired with increasing awareness of the effects of climate change and the burning of fossil fuels, the need for cost-competitive, clean, and renewable energy has become abundantly clear. Of the many renewable energy sources, solar energy is distinctive for its total potential, as the amount of solar energy that reaches earth far exceeds global energy consumption2.
Photovoltaic (PV) devices directly convert solar energy to electrical power and are versatile in scalability (e.g., personal use mini-modules and grid-integrated solar arrays) and material technologies. Technologies such as multi- and single-junction, single-crystal gallium arsenide (GaAs) solar cells have efficiencies reaching 39.2% and 35.5%, respectively3. However, fabrication of these high efficiency solar cells is costly and time-consuming. Polycrystalline cadmium telluride (CdTe) as a material for thin film PVs is advantageous for its low cost, high-throughput fabrication, variety of deposition techniques, and favorable absorption coefficient. These attributes make CdTe propitious for large- scale manufacturing, and improvements in efficiency have made CdTe cost-competitive with PV-market-dominant silicon and fossil fuels4.
One recent advancement that has driven the increase in CdTe device efficiency is the incorporation of cadmium selenium telluride (CdSeTe) alloy material into the absorber layer. Integrating the lower ~1.4 eV band gap CdSeTe material into a 1.5 eV CdTe absorber reduces the front band gap of the bilayer absorber. This increases the photon fraction above the band gap and thus improves current collection. Successful incorporation of CdSeTe into absorbers that are 3 &#956;m or thicker for increased current density has been demonstrated with various fabrication techniques (i.e., close-space sublimation, vapor transport deposition, and electroplating)5,6,7. Increased room temperature photoluminescence emission spectroscopy (PL), time-resolved photoluminescence (TRPL), and electroluminescence signals from bilayer absorber devices5,8 indicate that in addition to increased current collection, the CdSeTe appears to have better radiative efficiency and minority carrier lifetime, and a CdSeTe/CdTe device has a larger voltage relative to the ideal than with CdTe only. This has largely been attributed to selenium passivation of bulk defects9.
Little research has been reported on the incorporation of CdSeTe into thinner (&#8804;1.5 &#956;m) CdTe absorbers. We have therefore investigated the characteristics of thin 0.5 &#956;m CdSeTe/1.0 &#956;m CdTe bilayer-absorber devices fabricated by close-space sublimation (CSS) to determine whether the benefits seen in thick bilayer absorbers are also attainable with thin bilayer absorbers. Such CdSeTe/CdTe absorbers, more than twice as thin as their thicker counterparts, offer a notable decrease in deposition time and material and lower manufacturing costs. Finally, they hold potential for future device architecture developments which require absorber thicknesses of less than 2 &#956;m.
CSS deposition of absorbers in a single automated in-line vacuum system offers many advantages over other fabrication methods10,11. Faster deposition rates with CSS fabrication boosts device throughput and promotes larger experimental datasets. Additionally, the single vacuum environment of the CSS system in this work limits potential challenges with absorber interfaces. Thin-film PV devices have many interfaces, each of which can act as a recombination center for electrons and holes, thus reducing the overall device efficiency. The use of a single vacuum system for the CdSeTe, CdTe, and cadmium chloride (CdCl2) depositions (necessary for good absorber quality12,13,14,15,16) can produce a better interface and reduce interfacial defects.
The in-line automated vacuum system developed at Colorado State University10 is also advantageous in its scalability and repeatability. For example, deposition parameters are user-set, and the deposition process is automated such that the user does not need to make adjustments during absorber fabrication. Although small area research devices are fabricated in this system, the system design can be scaled up for larger area depositions, enabling a link between research-scale experimentation and module-scale implementation.
This protocol presents the fabrication methods used to manufacture 0.5-&#956;m CdSeTe/1.0-&#956;m CdTe thin-film PV devices. For comparison, a set of 1.5 &#956;m CdTe devices are fabricated. Single and bilayer absorber structures have nominally identical deposition conditions in all process steps, excluding the CdSeTe deposition. To characterize whether thin CdSeTe/CdTe absorbers retain the same benefits demonstrated by their thicker counterparts, current density-voltage (J-V), quantum efficiency (QE), and PL measurements are performed on the thin single and bilayer absorber devices. An increase in short-circuit current density (JSC) as measured by J-V and QE, in addition to an increase in PL signal for the CdSeTe/CdTe vs. CdTe device, indicate that thin CdSeTe/CdTe devices fabricated by CSS show notable improvement in current collection, material quality, and device efficiency.
Although this work focuses on the benefits associated with the incorporation of a CdSeTe alloy into a CdTe PV device structure, the complete fabrication process for CdTe and CdSeTe/CdTe devices is described subsequently in full. Figure 1A,B shows completed device structures for CdTe and CdSeTe/CdTe devices respectively, comprised of a transparent conducting oxide (TCO)-coated glass substrate, n-type magnesium zinc oxide (MgZnO) emitter layer, p-type CdTe or CdSeTe/CdTe absorber with CdCl2 treatment and copper doping treatment, thin Te layer, and nickel back contact. Excluding the CSS absorber deposition, the fabrication conditions are identical between the single and bilayer structure. Thus, unless otherwise noted, each step is performed on both CdTe and CdSeTe/CdTe structures.

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	<tbody>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Alpha Step Surface Profilometer</td>
			<td style="width:71px;">Tencor Instruments</td>
			<td style="width:119px;">10-00020</td>
			<td style="width:167px;">Instrument for measuring film thickness</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">CdCl2 Material</td>
			<td style="width:71px;">5N Plus</td>
			<td style="width:119px;">N/A</td>
			<td>Material for absorber passivation treatment</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">CdSeTe Semiconductor Material</td>
			<td style="width:71px;">5N Plus</td>
			<td style="width:119px;">N/A</td>
			<td>P-type semiconductor material for absorber layer</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">CdTe Semiconductor Material</td>
			<td style="width:71px;">5N Plus</td>
			<td style="width:119px;">N/A</td>
			<td>P-type semiconductor material for absorber layer</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">CESAR RF Power Generator</td>
			<td style="width:71px;">Advanced Energy</td>
			<td style="width:119px;">61300050</td>
			<td>Power generator for MgZnO sputter deposition</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">CuCl Material</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:119px;">N/A</td>
			<td>Material for absorber doping</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Delineation Material</td>
			<td style="width:71px;">Kramer Industries Inc.</td>
			<td style="width:119px;">Melamine Type 3 60-80 mesh</td>
			<td>Plastic beading material for film delineation</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Glovebox Enclosure</td>
			<td style="width:71px;">Vaniman Manufacturing Co.</td>
			<td style="width:119px;">Problast 3</td>
			<td>Glovebox enclosure for film delineation</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Gold Crystal</td>
			<td style="width:71px;">Kurt J. Lesker Company</td>
			<td>KJLCRYSTAL6-G10</td>
			<td>Crystal for Te evaporation thickness monitor</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">HVLP and Standard Gravity Feed Spray Gun Kit</td>
			<td style="width:71px;">Husky</td>
			<td style="width:119px;">HDK00600SG</td>
			<td>Applicator spray gun for Ni paint back contact application</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">MgZnO Sputter Target</td>
			<td style="width:71px;">Plasmaterials, Inc.</td>
			<td style="width:119px;">PLA285287489</td>
			<td>N-type emitter layer material</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Micro 90 Glass Cleaning Solution</td>
			<td style="width:71px;">Cole-Parmer</td>
			<td>EW-18100-05</td>
			<td>Solution for initial glass cleaning</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">NSG Tec10 Substrates</td>
			<td style="width:71px;">Pilkington</td>
			<td style="width:119px;">N/A</td>
			<td>Transparent-conducting oxide glass for front electrical contact</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Super Shield Ni Conductive Coating</td>
			<td style="width:71px;">MG Chemicals</td>
			<td style="width:119px;">841AR-3.78L</td>
			<td>Conductive paint for back contact layer</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Te Material</td>
			<td style="width:71px;">Sigma Aldrich</td>
			<td style="width:119px;">MKBZ5843V</td>
			<td>Material for back contact layer</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Thickness Monitor</td>
			<td style="width:71px;">R.D. Mathis Company</td>
			<td style="width:119px;">TM-100</td>
			<td>Instrument for programming and monitoring Te evaporation conditions</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Thinner 1</td>
			<td style="width:71px;">MG Chemicals</td>
			<td style="width:119px;">4351-1L</td>
			<td>Paint thinner to mix with Ni for back contact layer</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Ultrasonic Cleaner 1</td>
			<td style="width:71px;">L &#38; R Electronics</td>
			<td style="width:119px;">Q28OH</td>
			<td>Ultrasonic cleaner 1 for glass cleaning</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Ultrasonic Cleaner 2</td>
			<td style="width:71px;">Ultrasonic Clean</td>
			<td style="width:119px;">100S</td>
			<td>Ultrasonic cleaner 2 for glass cleaning</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">UV/VIS Lambda 2 Spectrometer</td>
			<td style="width:71px;">PerkinElmer</td>
			<td style="width:119px;">166351</td>
			<td>Spectrometer used for transmission measurements on CdSeTe films</td>
		</tr>
	</tbody>
</table>

close-space sublimation-deposited ultra-thin cdsete/cdte solar cells for enhanced short-circuit current density and photoluminescence

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst growing global energy demands and climate change. Thin film CdTe technology has demonstrated cost-competitiveness and increasing efficiencies due partially to rapid fabrication times, minimal material usage, and introduction of a CdSeTe alloy into a ~3 &#956;m absorber layer. This work presents the close-space sublimation fabrication of thin, 1.5 &#181;m CdSeTe/CdTe bilayer devices using an automated in-line vacuum deposition system. The thin bilayer structure and fabrication technique minimize deposition time, increase device efficiency, and facilitate future thin absorber-based device architecture development. Three fabrication parameters appear to be the most impactful for optimizing thin CdSeTe/CdTe absorber devices: substrate preheat temperature, CdSeTe:CdTe thickness ratio, and CdCl2 passivation. For proper sublimation of the CdSeTe, the substrate temperature prior to deposition must be ~540 &#176;C (higher than that for CdTe) as controlled by dwell time in a preheat source. Variation in the CdSeTe:CdTe thickness ratio reveals a strong dependence of device performance on this ratio. The optimal absorber thicknesses are 0.5 &#956;m CdSeTe/1.0 &#956;m CdTe, and non-optimized thickness ratios reduce efficiency through back-barrier effects. Thin absorbers are sensitive to CdCl2 passivation variation; a much less aggressive CdCl2 treatment (compared to thicker absorbers) regarding both temperature and time yields optimal device performance. With optimized fabrication conditions, CdSeTe/CdTe increases device short-circuit current density and photoluminescence intensity compared to single-absorber CdTe. Additionally, an in-line close-space sublimation vacuum deposition system offers material and time reduction, scalability, and attainability of future ultra-thin absorber architectures.

The addition of CdSeTe to a thin CdTe absorber improves device efficiency through superior absorber material quality and higher short-circuit current density (JSC). Figure 3A and Figure 3B, (adapted from Bothwell et al.8) show PL and TRPL, respectively, for the single CdTe absorber and CdSeTe/CdTe bilayer absorber devices. Both PL and TRPL measurements clearly show improved photoluminescence with the CdSeTe/CdTe bilayer absorbe...

Watch this Scientific Journal Video about Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence at JoVE.com

Close-Space Sublimation-Deposited Ultra-Thin CdSeTe/CdTe Solar Cells for Enhanced Short-Circuit Current Density and Photoluminescence

This work describes the complete fabrication process of thin absorber cadmium selenium telluride/cadmium telluride photovoltaic devices for enhanced efficiency. The process utilizes an automated in-line vacuum system for close-space sublimation deposition that is scalable, from fabrication of small area research devices as well as large-scale modules.

Developments in photovoltaic device architectures are necessary to make solar energy a cost-effective and reliable source of renewable energy amidst ...

The addition of cadmium selenium telluride to cadmium telluride absorbers is significant to improvements for photovoltaic efficiencies. Higher efficiencies and material savings with ultra thin layers advance photovoltaic and renewable energy development. The main advantages of automated in-line close-space sublimation deposition or CSS is that the close-space sublimation is fast and the automated deposition ensures reproducibility across devices.Close-space sublimation addresses one of the challenges in the deposition of film absorbers for solar cells. The challenge is that it can be pretty slow to deposit thin films so close-space sublimation allows us to do that at a faster rate and therefore manufacture more solar cells in one day. This deposition method is also very useful for other thin film depositions in that many times atmospheric exposure is not desirable.As with any fabrication system, initial oversight from an experienced user is highly advisable. Since the fabrication of these bilayer absorber photovoltaic devices requires many steps, visualization of the sample after each stage is critical to understanding which visual film qualities are required. Begin by using a handheld air blower to gently remove any dust particles from a clean transparent conducting oxide coated substrate marked with permanent marker and use a pair of rubber tipped tweezers to load the clean substrate onto the sample holder transparent conducting oxide side down.With the load lock door closed, pump the load lock down until the load lock pressure gauge reads below five times 10 to the negative two torr and switch off the load lock pump. Switch open the load lock gate and wait for the pressure to level before manually inserting the transfer arm so that the sample sits above the shuttered cathode. Set the desired deposition time on a timer and begin timing as the shutter is manually opened.Immediately close the shutter as the timer goes off and completely retract the transfer arm before closing the load lock gate and unloading the sample. To obtain the magnesium zinc oxide deposition rate, use a cotton tipped applicator dipped in methanol to remove the marker from the witness sample and measure the thickness with a profilometer. For close-space sublimation of the absorber layers, set the top and bottom sources in the deposition system with a temperature differential for proper material sublimation.Use a handheld air blower to gently remove any dust particles from the clean magnesium zinc oxide and transparent conducting oxide coated substrate and load the clean substrate onto the sample holder magnesium zinc oxide side down. After closing the load lock door, turn on the load lock roughing switch to pump down the load lock. While pumping down, set the deposition recipe for the cadmium telluride witness sample to 110 seconds dwell time in the preheat source to raise the glass to approximately 480 degrees Celsius, 110 seconds in the cadmium telluride source for cadmium telluride deposition, 180 seconds in the cadmium chloride source for passivization of the polycrystalline cadmium telluride, 240 seconds in the anneal source to drive the cadmium chloride into the absorber, and 300 seconds in the cooling source.When the load lock is pumped down below 40 millitorr, open the gate valve and start the deposition recipe in the software. The transfer arm will automatically move into the preset positions returning to the home position upon completion. When the deposition is complete, vent the load lock to atmosphere and open the load lock door.When the sample is cool enough to handle, remove it with a lint-free cloth. Use deionized water to rinse the visible cadmium chloride residue from the film surface into a graduated beaker and dry the film with compressed nitrogen. Then use a razor blade to scrape a small area of cadmium telluride material off of the substrate and use a surface profilometer to measure the cadmium telluride film thickness to determine the deposition rate.When the load lock pressure is below 40 millitorr, run a bakeoff recipe in the software. When the bakeoff is complete, set the deposition recipe for the cadmium selenium telluride witness sample to establish thickness. Set 140 seconds dwell time in the preheat source to raise the glass to approximately 540 degrees Celsius, 300 seconds in the cadmium selenium telluride source for cadmium selenium telluride deposition, and 300 seconds in the cooling source.When the load lock pressure reaches below 40 millitorr, execute the deposition recipe. When the cadmium selenium telluride film deposition is complete, unload the cooled sample with a lint-free cloth and scratch off a small section of material to determine the cadmium selenium telluride deposition rate with a profilometer as previously demonstrated. To fabricate a 1.5 micron single cadmium telluride absorber, set the deposition recipe based on the cadmium telluride deposition rate and a cadmium chloride treatment previously optimized for ultra thin absorbers.Use a preheat dwell time of 110 seconds, a cadmium telluride dwell time of 60 seconds, a cadmium chloride dwell time of 150 seconds, and anneal time of 240 seconds, and a cooling time of 300 seconds. When the load lock pressure reaches below 40 millitorr, open the load lock gate valve and select start. The program will automatically run the selected deposition recipe returning to the home position upon completion of the cooling step.To fabricate a 0.5 micron cadmium selenium telluride and 1.0 micron cadmium telluride bilayer absorber, set the deposition recipe based on the absorber deposition rate with a preheat dwell time of 140 seconds, a cadmium selenium telluride dwell time of 231 seconds, a cadmium telluride dwell time of 50 seconds, a cadmium chloride dwell time of 150 seconds, an anneal time of 240 seconds, and a cooling time of 300 seconds. When the load lock pressure reaches below 40 millitorr, execute deposition recipe. Then unload the sample upon recipe completion and sample cooling as demonstrated.When the top and bottom sources have reached operating temperature, load the sample onto the sample holder and sequentially move the transfer arm into the preheat copper chloride and anneal positions in accordance with a timer set for the deposition time of each position. The copper recipe in this protocol has been optimized for 1.5 micron devices. When the final timer goes off, manually return the transfer arm to the home position and close the load lock gate valve.For evaporation deposition of thin tellurium, load the sample film side down onto the sample holder and close the chamber top. Manually move the lever into the roughing position. When the pressure drops below 10 millitorr, turn the lever back to the foreline position and wait about 30 seconds for any momentary spike in pressure to be resolved before opening the high vacuum valve.When the chamber pressure reader has based out, the proper deposition pressure of one times 10 to the negative fifth torr has been reached. Turn on the power switch, open the shutter, and turn up the current control to begin deposition. When the quartz crystal monitor display shows the desired tellurium thickness, quickly and simultaneously turn the current to zero, turn off the power switch and close the shutter.Before applying the paint back contact, shake the back contact solution to ensure complete mixing. Spray the solution across the sample in a slow lateral motion to apply uniform nickel back contact onto the sample. After allowing the back contact to dry slightly, repeat the application as many times as necessary for complete coverage.To finish the thin film structure into electrically contactable devices, place the sample loaded into a metal mask into a glove box and use a siphon hose to apply glass beaded medium to the unmasked portions of the sample. Repeat the application with a second mask such that upon completing the delineation, 25 small-area square devices appear in a five by five pattern on the sample. Then clean the film side of the samples with a cotton tipped applicator dipped in deionized water.To minimize the lateral resistance in electrical measurements of the finished devices, solder a grid pattern between the devices with an indium solder. The addition of cadmium selenium telluride to a thin cadmium telluride absorber improves the device efficiency through superior absorber material quality demonstrated by higher photoluminescence and longer time resolved photoluminescence decay lifetimes. Greater efficiencies are also achieved with higher short circuit current density.The downward shift in the light current density voltage curve along the current density axis corresponds to an increase in short circuit current density for the best performing bilayer absorber device compared to the best performing single cadmium telluride absorber device. Quantum efficiency measurements of the cadmium telluride and cadmium selenium telluride/cadmium telluride devices show the additional photon conversion in the long wavelength range of the bilayer device and corroborate the increase in the short circuit current density for that device. The importance of optimizing the cadmium selenium telluride to cadmium telluride thickness ratio is demonstrated by a comparison of current density voltage results.Data for a 0.5 to 1.0 micron ratio and a 1.25 to 0.25 micron ratio show a significant kink in the latter non-optimal device and a resultant decrease in photovoltaic efficiency. The most important thing to remember is that the thickness ratio between the cadmium selenium telluride and the cadmium telluride is imperative for respectable device performance and should be optimized for each bilayer absorber thickness. Following this procedure, an additional material layer can be deposited after the bilayer to act as an electron reflector.Introducing this layer can minimize the voltage deficit obstacle in cadmium telluride-based devices. The incorporation of a cadmium selenium telluride alloy into the bilayer solar cells has been useful not only for the solar cells, but for developing the properties of that alloy into other photovoltaic applications. Cadmium compounds can be hazardous.When we use such compounds and when we rinse the residue from the films, it's important to wear gloves, a lab coat, and then to use appropriate procedures to dispose of the hazardous waste.

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