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.

<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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The authors have nothing to disclose.

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.
<p class="j...

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.

<table border="0" cellpadding="0" cellspacing="0" style="width:572px;" width="573">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<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 ...

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8.0K Views.  Colorado State University. 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.

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

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O substrates outside vacuum at low temperature. The performance of photovoltaic devices featuring atmospherically fabricated ZnO/Cu2O heterojunction was dependent on the conditions of AP-SALD film deposition, namely, the substrate temperature and deposition time, as well as on the Cu2O substrate exposure to oxidizing agents prior to and during the ZnO deposition. Superficial Cu2O to CuO oxidation was identified as a limiting factor to heterojunction quality due to recombination at the ZnO/Cu2O interface. Optimization of AP-SALD conditions as well as keeping Cu2O away from air and moisture in order to minimize Cu2O surface oxidation led to improved device performance. A three-fold increase in the open-circuit voltage (up to 0.65 V) and a two-fold increase in the short-circuit current density produced solar cells with a record 2.2% power conversion efficiency (PCE). This PCE is the highest reported for a Zn1-xMgxO/Cu2O heterojunction formed outside vacuum, which highlights atmospheric pressure spatial ALD as a promising technique for inexpensive and scalable fabrication of Cu2O-based photovoltaics.

Improved Heterojunction Quality in Cu2O-based Solar Cells Through the Optimization of Atmospheric Pressure Spatial Atomic Layer Deposited Zn1-xMgxO

Here we present a protocol for synthesizing Zn1-xMgxO/Cu2O heterojunctions in open-air at low temperature via atmospheric pressure spatial atomic layer deposition (AP-SALD) of Zn1-xMgxO on cuprous oxide. Such high quality conformal metal oxides can be grown on a variety of substrates including plastics by this cheap and scalable method.

Atmospheric pressure spatial atomic layer deposition (AP-SALD) was used to deposit n-type ZnO and Zn1-xMgxO thin films onto p-type thermally oxidized Cu2O ...

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

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

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

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

Nanosponge Tunability in Size and Crosslinking Density

We describe a protocol for the synthesis of linear polyesters containing pendant epoxide functionality and their incorporation into a nanosponge with controlled dimensions. This approach begins with synthesis of a functionalized lactone which is key to the pendant functionalization of the resulting polymer. Valerolactone (VL) and allyl-valerolactone (AVL) are then copolymerized using ring-opening polymerization. Post-polymerization modification is then used to install an epoxide moiety on some or all of the pendant allyl groups. Epoxy-amine chemistry is employed to form nanoparticles in a dilute solution of both polymer and small molecule diamine crosslinker based on the desired nanosponge size and crosslinking density. Nanosponge sizes can be characterized by transmission electron microscopy (TEM) imaging to determine the dimension and distribution. This method provides a pathway by which highly tunable polyesters can create tunable nanoparticles, which can be used for small molecule drug encapsulation. Due to the nature of the backbone, these particles are hydrolytically and enzymatically degradable for a controlled release of a wide range of hydrophobic small molecules.

This article describes a process for tuning the size and crosslinking density of covalently crosslinked nanoparticles from linear polyesters containing pendant functionality. By tailoring synthesis parameters (polymer molecular weight, pendant functionality incorporation, and crosslinker equivalents), a desired nanoparticle size and crosslinking density can be achieved for drug delivery applications.

We describe a protocol for the synthesis of linear polyesters containing pendant epoxide functionality and their incorporation into a nanosponge with ...

Observation and Analysis of Blinking Surface-enhanced Raman Scattering

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on how to prepare the SERS-active silver nanoaggregate, record a video of certain blinking spots in the microscopic image, and analyze the blinking statistics. In this analysis, a power law reproduces the probability distributions for bright events relative to their duration. The probability distributions for dark events are fitted by a power law with an exponential function. The parameters of the power law represent molecular behavior in both bright and dark states. The random walk model and the speed of the molecule across the entire silver surface can be estimated. It is difficult to estimate even when using averages, autocorrelation functions, and super-resolution SERS imaging. In the future, power law analyses should be combined with spectral imaging, because the origins of blinking cannot be confirmed by this analysis method alone.

This protocol describes the analysis of blinking surface-enhanced Raman scattering due to the random walk of a single molecule on a silver surface using power laws.

From a single molecule at a silver nanoaggregate junction, blinking surface-enhanced Raman scattering (SERS) is observed. Here, a protocol is presented on ...

Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

Sb2S3 is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical properties. Recently, we demonstrated its potential as next-generation solar cells by achieving a high photovoltaic efficiency of &#62; 6% in Sb2S3-sensitized solar cells using a simple thiourea (TU)-based complex solution method. Here, we describe the key experimental procedures for the deposition of Sb2S3 on a mesoporous TiO2 (mp-TiO2) layer using a SbCl3-TU complex solution in the fabrication of solar cells. First, the SbCl3-TU solution is synthesized by dissolving SbCl3 and TU in N,N-dimethylformamide at different molar ratios of SbCl3:TU. Then, the solution is deposited on as-prepared substrates consisting of mp-TiO2/TiO2-blocking layer/F-doped SnO2 glass by spin coating. Finally, to form crystalline Sb2S3, the samples are annealed in an N2-filled glove box at 300 &#176;C. The effects of the experimental parameters on the photovoltaic device performance are also discussed.

Key Factors Affecting the Performance of Sb2S3-sensitized Solar Cells During an Sb2S3 Deposition via SbCl3-thiourea Complex Solution-processing

This work provides a detailed experimental procedure for the deposition of Sb2S3 on a mesoporous TiO2 layer using a SbCl3-thiourea complex solution for applications in Sb2S3-sensitized solar cells. This article also determines key factors governing the deposition process.

Sb2S3 is considered as one of the emerging light absorbers that can be applied to next-generation solar cells because of its unique optical and electrical ...

Solution-Processed "Silver-Bismuth-Iodine" Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. However, poor surface morphologies and relatively high bandgap energies have limited their potential. Silver-bismuth-iodine (Ag-Bi-I) is a promising semiconductor for optoelectronic devices. Therefore, we demonstrate the fabrication of Ag-Bi-I ternary thin films using material solution processing. The resulting thin films exhibit controlled surface morphologies and optical bandgaps according to their thermal annealing temperatures. In addition, it has been reported that Ag-Bi-I ternary systems crystallize to AgBi2I7, Ag2BiI5, etc. according to the ratio of the precursor chemicals. The solution-processed AgBi2I7 thin films exhibit a cubic-phase crystal structure, dense, pinhole-free surface morphologies with grains ranging in size from 200 to 800 nm, and an indirect bandgap of 1.87 eV. The resultant AgBi2I7 thin films show good air stability and energy band diagrams, as well as surface morphologies and optical bandgaps suitable for lead-free and air-stable single-junction solar cells. Very recently, a solar cell with 4.3% power conversion efficiency was obtained by optimizing the Ag-Bi-I crystal compositions and solar cell device architectures.

Solution-Processed &quot;Silver-Bismuth-Iodine&quot; Ternary Thin Films for Lead-Free Photovoltaic Absorbers

Herein, we present detailed protocols for solution-processed silver-bismuth-iodine (Ag-Bi-I) ternary semiconductor thin films fabricated on TiO2-coated transparent electrodes and their potential application as air-stable and lead-free optoelectronic devices.

Bismuth-based hybrid perovskites are regarded as promising photo-active semiconductors for environment-friendly and air-stable solar cell applications. ...

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part of the Properties of Actinides under Extreme Conditions laboratory (PAMEC), allows the preparation of films and studies of sample surfaces using surface analytical techniques such as X-ray and ultra-violet photoemission spectroscopy (XPS and UPS, respectively). All studies are made in situ, and the films, transferred under ultra-high vacuum from their preparation to an analyses chamber, are never in contact with the atmosphere. Initially, a film of UO2 is prepared by direct current (DC) sputter deposition on a gold (Au) foil then oxidized by atomic oxygen to produce a UO3 film. This latter is then reduced with atomic hydrogen to U2O5. Analyses are performed after each step involving oxidation and reduction, using high-resolution photoelectron spectroscopy to examine the oxidation state of uranium. Indeed, the oxidation and reduction times and corresponding temperature of the substrate during this process have severe effects on the resulting oxidation state of the uranium. Stopping the reduction of UO3 to U2O5 with single U(V) is quite challenging; first, uranium-oxygen systems exist in numerous intermediate phases. Second, differentiation of the oxidation states of uranium is mainly based on satellite peaks, whose intensity peaks are weak. Also, experimenters should be aware that X-ray spectroscopy (XPS) is a technique with an atomic sensitivity of 1% to 5%. Thus, it is important to obtain a complete picture of the uranium oxidation state with the entire spectra obtained on U4f, O1s, and the valence band (VB). Programs used in the Labstation include a linear transfer program developed by an outside company (see Table of Materials) as well as data acquisition and sputter source programs, both developed in-house.

U2O5 Film Preparation via UO2 Deposition by Direct Current Sputtering and Successive Oxidation and Reduction with Atomic Oxygen and Atomic Hydrogen

This protocol presents the preparation of U2O5 thin films obtained in situ under ultra-high vacuum. The process involves oxidation and reduction of UO2 films with atomic oxygen and atomic hydrogen, respectively.

We describe a method to produce U2O5 films in situ using the Labstation, a modular machine developed at JRC Karlsruhe. The Labstation, an essential part ...

Experimental Methods for Efficient Solar Hydrogen Production in Microgravity Environment

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the Earth's atmosphere. So-called 'solar fuel' devices, currently developed for terrestrial applications in the quest for realizing a sustainable energy economy on Earth, provide promising alternative systems to existing air-revitalization units employed on the International Space Station (ISS) through photoelectrochemical water-splitting and hydrogen production. One obstacle for water (photo-) electrolysis in reduced gravity environments is the absence of buoyancy and the consequential, hindered gas bubble release from the electrode surface. This causes the formation of gas bubble froth layers in proximity to the electrode surface, leading to an increase in ohmic resistance and cell-efficiency loss due to reduced mass transfer of substrates and products to and from the electrode. Recently, we have demonstrated efficient solar hydrogen production in microgravity environment, using an integrated semiconductor-electrocatalyst system with p-type indium phosphide as the light-absorber and a rhodium electrocatalyst. By nanostructuring the electrocatalyst using shadow nanosphere lithography and thereby creating catalytic 'hot spots' on the photoelectrode surface, we could overcome gas bubble coalescence and mass transfer limitations and demonstrated efficient hydrogen production at high current densities in reduced gravitation. Here, the experimental details are described for the preparations of these nanostructured devices and further on, the procedure for their testing in microgravity environment, realized at the Bremen Drop Tower during 9.3 s of free fall.

Efficient solar-hydrogen production has recently been realized on functionalized semiconductor-electrocatalyst systems in a photoelectrochemical half-cell in microgravity environment at the Bremen Drop Tower. Here, we report the experimental procedures for manufacturing the semiconductor-electrocatalyst device, details of the experimental set-up in the drop capsule and the experimental sequence during free fall.

Long-term space flights and cis-lunar research platforms require a sustainable and light life-support hardware which can be reliably employed outside the ...

Gold Nanoparticle Modified Carbon Fiber Microelectrodes for Enhanced Neurochemical Detection

For over 30 years, carbon-fiber microelectrodes (CFMEs) have been the standard for neurotransmitter detection. Generally, carbon fibers are aspirated into glass capillaries, pulled to a fine taper, and then sealed using an epoxy to create electrode materials that are used for fast scan cyclic voltammetry testing. The use of bare CFMEs has several limitations, though. First and foremost, the carbon fiber contains mostly basal plane carbon, which has a relatively low surface area and yields lower sensitivities than other nanomaterials. Furthermore, the graphitic carbon is limited by its temporal resolution, and its relatively low conductivity. Lastly, neurochemicals and macromolecules have been known to foul at the surface of carbon electrodes where they form non-conductive polymers that block further neurotransmitter adsorption. For this study, we modify CFMEs with gold nanoparticles to enhance neurochemical testing with fast scan cyclic voltammetry. Au3+ was electrodeposited or dipcoated from a colloidal solution onto the surface of CFMEs. Since gold is a stable and relatively inert metal, it is an ideal electrode material for analytical measurements of neurochemicals. Gold nanoparticle modified (AuNP-CFMEs) had a stability to dopamine response for over 4 h. Moreover, AuNP-CFMEs exhibit an increased sensitivity (higher peak oxidative current of the cyclic voltammograms) and faster electron transfer kinetics (lower &#916;EP or peak separation) than bare unmodified CFMEs. The development of AuNP-CFMEs provides the creation of novel electrochemical sensors for detecting fast changes in dopamine concentration and other neurochemicals at lower limits of detection. This work has vast applications for the enhancement of neurochemical measurements. The generation of gold nanoparticle modified CFMEs will be vitally important for the development of novel electrode sensors to detect neurotransmitters in vivo in rodent and other models to study neurochemical effects of drug abuse, depression, stroke, ischemia, and other behavioral and disease states.

In this study, we modify carbon-fiber microelectrodes with gold nanoparticles to enhance the sensitivity of neurotransmitter detection.

For over 30 years, carbon-fiber microelectrodes (CFMEs) have been the standard for neurotransmitter detection. Generally, carbon fibers are aspirated into ...

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number of photoactive assemblies have emerged where a photosensitizer and cobaloxime-based H2 production catalysts work in tandem to convert light energy into the H-H chemical bonds. However, the long-term instability of these assemblies and the need for hazardous proton sources have limited their usage. Here, in this work, we have integrated a stilbene-based organic dye into the periphery of a cobaloxime core via a distinct axial pyridine linkage. This strategy allowed us to develop a photosensitizer-catalyst hybrid structure with the same molecular framework. In this article, we have explained the detailed procedure of the synthesis of this hybrid molecule in addition to its comprehensive chemical characterization. The structural and optical studies have exhibited an intense electronic interaction between the cobaloxime core and the organic photosensitizer. The cobaloxime was active for H2 production even in the presence of water as the proton source. Here, we have developed a simple airtight system connected with an online H2 detector for the investigation of the photocatalytic activity by this hybrid complex. This photosensitizer-catalyst dyad present in the experimental setup continuously produced H2 once it was exposed in the natural sunlight. This photocatalytic H2 production by the hybrid complex was observed in aqueous/organic mixture media in the presence of a sacrificial electron donor under complete aerobic conditions. Thus, this photocatalysis measurement system along with the photosensitizer-catalyst dyad provide valuable insight for the development of next generation photocatalytic H2 production devices.

Developing Photosensitizer-Cobaloxime Hybrids for Solar-Driven H2 Production in Aqueous Aerobic Conditions

We have directly incorporated a stilbene-based organic dye into a cobaloxime core to generate a photosensitizer-catalyst dyad for photocatalytic H2 production. We have also developed a simple experimental setup for evaluating the light-driven H2 production by photocatalytic assemblies.

Developing photocatalytic H2 production devices is the one of the key steps for constructing a global H2-based renewable energy infrastructure. A number ...