Name: making record-efficiency sns solar cells by thermal evaporation and atomic layer deposition
Uploaded: 2015-05-22T14:00:00
Description: Watch this Scientific Journal Video about Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition at JoVE.com

The authors would like to thank Paul Ciszek and Keith Emery from the National Renewable Energy Laboratory (NREL) for certified&#160;J-V&#160;measurements, Riley Brandt (MIT) for photoelectron spectroscopy measurements, and Jeff Cotter (ASU) for inspiration for the hypothesis testing section. This work is supported by the U.S. Department of Energy through the SunShot Initiative under contract DE-EE0005329, and by Robert Bosch LLC through the Bosch Energy Research Network under grant 02.20.MC11. V. Steinmann, R. Jaramillo, and K. Hartman acknowledge the support of, the Alexander von Humboldt foundation, a DOE EERE Postdoctoral Research Award, and Intel PhD Fellowship, respectively. This work made use of the Center for Nanoscale Systems at Harvard University which is supported by the National Science Foundation under award ECS-0335765.

1. Substrate Selection and Cutting
<ol>
	<li>Purchase polished Si wafers with a thick thermal oxide. For the devices reported here, use 500 &#956;m thick wafers with a 300 nm or thicker thermal oxide. The substrate selection criteria are discussed in the Discussion section.</li>
	<li>Spin coat the polished side of wafer with a typical positive photoresist (SPR 700 or PMMA A. 495) and soft bake (30 sec at 100 &#176;C). 
	Note: This is a protective layer to prevent damage or contamination during the subsequent cutti...

<ol>
	<li>Woodhouse, M., Goodrich, 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=Perspectives+on+the+pathways+for+cadmium+telluride+photovoltaic+module+manufacturers+to+address+expected+increases+in+the+price+for+tellurium.">Perspectives on the pathways for cadmium telluride photovoltaic module manufacturers to address expected increases in the price for tellurium.</a> Solar Energy Materials and Solar Cells. 115, 199-212 (2013).</li><li>Ramakrishna Reddy, K. T., Koteswara Reddy, N., Miles, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Photovoltaic+properties+of+SnS+based+solar+cells.">Photovoltaic properties of SnS based solar cells.</a> Solar Energy Materials and Solar Cells. 90 (18-19), 3041-3046 (2006).</li><li>Sinsermsuksakul, P., Heo, J., Noh, W., Hock, A. S., Gordon, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+Layer+Deposition+of+Tin+Monosulfide+Thin+Films.">Atomic Layer Deposition of Tin Monosulfide Thin Films.</a> Advanced Energy Materials. 1 (6), 1116-1125 (2011).</li><li>Noguchi, H., Setiyadi, A., Tanamura, H., Nagatomo, T., Omoto, O. <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+vacuum-evaporated+tin+sulfide+film+for+solar+cell+materials.">Characterization of vacuum-evaporated tin sulfide film for solar cell materials.</a> Solar Energy Materials and Solar Cells. 35, 325-331 (1994).</li><li>Hartman, K., Johnson, J. L., 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=SnS+thin-films+by+RF+sputtering+at+room+temperature.">SnS thin-films by RF sputtering at room temperature.</a> Thin Solid Films. 519 (21), 7421-7424 (2011).</li><li>Tanusevski, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optical+and+photoelectric+properties+of+SnS+thin+films+prepared+by+chemical+bath+deposition.">Optical and photoelectric properties of SnS thin films prepared by chemical bath deposition.</a> Semiconductor Science and Technology. 18 (6), 501 (2003).</li><li>Sharma, R. C., Chang, Y. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+S&#8722;Sn+(Sulfur-Tin)+system.">The S&#8722;Sn (Sulfur-Tin) system.</a> Bulletin of Alloy Phase Diagrams. 7 (3), 269-273 (1986).</li><li>Steinmann, V., Jaramillo, R., 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=3.88%+Efficient+Tin+Sulfide+Solar+Cells+using+Congruent+Thermal+Evaporation.">3.88% Efficient Tin Sulfide Solar Cells using Congruent Thermal Evaporation.</a> Advanced Materials. 26 (44), 7488-7492 (2014).</li><li>Sinsermsuksakul, P., Sun, L., 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+Efficiency+Limitations+of+SnS-Based+Solar+Cells.">Overcoming Efficiency Limitations of SnS-Based Solar Cells.</a> Advanced Energy Materials. 4 (15), 1400496 (2014).</li><li>Hejin Park, H., Heasley, R., Gordon, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+layer+deposition+of+Zn(O,S)+thin+films+with+tunable+electrical+properties+by+oxygen+annealing.">Atomic layer deposition of Zn(O,S) thin films with tunable electrical properties by oxygen annealing.</a> Applied Physics Letters. 102 (13), 132110 (2013).</li><li>Scofield, J. H., Duda, A., Albin, D., Ballard, B. L., Predecki, P. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sputtered+molybdenum+bilayer+back+contact+for+copper+indium+diselenide-based+polycrystalline+thin-film+solar+cells.">Sputtered molybdenum bilayer back contact for copper indium diselenide-based polycrystalline thin-film solar cells.</a> Thin Solid Films. 260 (1), 26-31 (1995).</li><li>Malone, B. D., Gali, A., Kaxiras, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=First+principles+study+of+point+defects+in+SnS.">First principles study of point defects in SnS.</a> Physical Chemistry Chemical Physics. 16, 26176-26183 (2014).</li><li>Vaux, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Research+methods:+Know+when+your+numbers+are+significant.">Research methods: Know when your numbers are significant.</a> Nature. 492 (7428), 180-181 (2012).</li></ol>

Substrate selection cleaning
Oxidized Si wafers are used as substrates. The substrates are the mechanical support for the resulting solar cells, and their electrical properties are not important. Si wafers are preferred to glass because commercially purchased Si wafers are typically cleaner than commercially purchased glass wafers, and this saves time in substrate cleaning. Si substrates also have higher thermal conductivity than glass, which leads to more even heating during ...

Thin film photovoltaics (PV) continue to attract interest and significant research activity. However, the economics of the PV market are shifting rapidly and developing commercially successful thin film PV has become a more challenging prospect. Manufacturing cost advantages over wafer-based technologies can no longer be taken for granted, and improvements in both efficiency and cost must be sought on an equal footing.1,2 In light of this reality we have chosen to develop SnS as an absorber material for thin film PV. SnS has intrinsic practical advantages that could translate into low manufacturing cost. If high efficiencies can be demonstrated, it could be considered as a drop-in replacement for CdTe in commercial thin film PV. Here, the fabrication procedure for the recently reported record SnS solar cells is demonstrated. We focus on practical aspects such as substrate selection, deposition conditions, device layout, and measurement protocols.
SnS is composed of non-toxic, Earth-abundant and inexpensive elements (tin and sulfur). SnS is an inert and insoluble semiconducting solid (mineral name Herzenbergite) with an indirect bandgap of 1.1 eV, strong light absorption for photons with energy above 1.4 eV (&#945; &#62; 104 cm-1), and intrinsic p-type conductivity with carrier concentration in the range 1015&#160;&#8211; 1017 cm-3.3&#8211;7 Importantly, SnS evaporates congruently and is phase-stable up to 600 &#176;C.8,9 This means that SnS can be deposited by thermal evaporation (TE) and its high-speed cousin, closed space sublimation (CSS), as is employed in the manufacture of CdTe solar cells. It also means that SnS phase control is far simpler than for most thin film PV materials, notably including Cu(In,Ga)(S,Se)2 (CIGS) and Cu2ZnSnS4 (CZTS). Therefore, cell efficiency stands as the primary barrier to commercialization of SnS PV, and SnS could be considered a drop-in replacement for CdTe once high efficiencies are demonstrated at the laboratory scale. However this efficiency barrier cannot be overstated. We estimate that the record efficiency must increase by a factor of four, from ~4% to ~15%, in order to stimulate commercial development. Developing SnS as a drop-in replacement for CdTe will also require growth of high quality SnS thin films by CSS, and the development of an n-type partner material on which SnS can be grown directly.
Below is described the step-by-step procedure for fabricating record SnS solar cells using two different deposition techniques, atomic layer deposition (ALD) and TE. ALD is a slow growth method but to-date has yielded the highest efficiency devices. TE is faster and industrially scalable, but lags ALD in efficiency. In addition to the different SnS deposition methods, the TE and ALD solar cells differ slightly in the annealing, surface passivation, and metallization steps. The device fabrication steps are enumerated in Figure 1.
After describing the procedure, test results for the certified record devices and related samples are presented. The record results have been reported previously. Here the focus is on the distribution of results for a typical processing run.

<table border="0" cellpadding="0" cellspacing="0" >
	
	<tbody>
		<tr >
			<td >Quartz wafer carrier</td>
			<td >AM Quartz, Gainesville, TX</td>
			<td ></td>
			<td >bespoke design</td>
		</tr>
		<tr >
			<td >Sputtering system</td>
			<td >PVD Products</td>
			<td ></td>
			<td>High vacuum sputtering system with load lock</td>
		</tr>
		<tr >
			<td >4% H2S in N2</td>
			<td >Airgas Inc.</td>
			<td>X02NI96C33A5626</td>
			<td></td>
		</tr>
		<tr >
			<td >99.5% H2S</td>
			<td >Matheson Trigas</td>
			<td>G1540250</td>
			<td></td>
		</tr>
		<tr >
			<td >SnS powder</td>
			<td >Sigma Aldrich</td>
			<td>741000-5G</td>
			<td></td>
		</tr>
		<tr >
			<td >Effusion cell</td>
			<td >Veeco</td>
			<td>35-LT</td>
			<td>Low temperature, single filament effusion cell</td>
		</tr>
		<tr >
			<td >diethylzinc (Zn(C2H5)2)</td>
			<td>Strem Chemicals</td>
			<td>93-3030</td>
			<td></td>
		</tr>
		<tr >
			<td >Laser cutter</td>
			<td>Electrox</td>
			<td >Scorpian G2</td>
			<td>Used for ITO shadow masks</td>
		</tr>
		<tr >
			<td >ITO sputtering target (In2O3/SnO2 90/10 wt.%, 99.99% pure)</td>
			<td >Kurt J. Lesker</td>
			<td>EJTITOX402A4</td>
			<td></td>
		</tr>
		<tr >
			<td >Metallization shadow masks</td>
			<td>MicroConnex</td>
			<td ></td>
			<td>bespoke design</td>
		</tr>
		<tr >
			<td >Electron Beam Evaporator</td>
			<td>Denton</td>
			<td ></td>
			<td>High vacuum metals evaporator with load-lock</td>
		</tr>
		<tr >
			<td >AM1.5 solar simulator</td>
			<td >Newport Oriel</td>
			<td align="right">91194</td>
			<td>1300 W Xe-lamp using an AM1.5G filter</td>
		</tr>
		<tr >
			<td >Spectrophotometer</td>
			<td >Perkin Elmer</td>
			<td>Lambda 950 UV-Vis-NIR</td>
			<td>150mm Spectralon-coated integrating sphere</td>
		</tr>
		<tr >
			<td >Calibrated Si solar cell</td>
			<td >PV Measurements</td>
			<td ></td>
			<td>BK-7 window glass</td>
		</tr>
		<tr >
			<td >Double probe tips</td>
			<td >Accuprobe</td>
			<td >K1C8C1F</td>
			<td></td>
		</tr>
		<tr >
			<td >Souce-meter</td>
			<td >Keithley</td>
			<td align="right" >2400</td>
			<td></td>
		</tr>
		<tr >
			<td >Quantum efficiency measurement system</td>
			<td >PV Measurements</td>
			<td>QEX7</td>
			<td></td>
		</tr>
		<tr >
			<td >Calibrated Si photodiode</td>
			<td >PV Measurements</td>
			<td ></td>
			<td></td>
		</tr>
		<tr >
			<td >High-throughput solar cell test station</td>
			<td >PV Measurements</td>
			<td ></td>
			<td>bespoke design</td>
		</tr>
		<tr >
			<td >Inert pump oil</td>
			<td >DuPont</td>
			<td >Krytox</td>
			<td>PFPE oil, grade 1514; vendor: Eastern Scientific</td>
		</tr>
		<tr >
			<td >H2S resistant elastomer o-rings</td>
			<td >DuPont</td>
			<td>Kalrez</td>
			<td>compound 7075; vendor: Marco Rubber</td>
		</tr>
		<tr >
			<td >H2S resistant elastomer o-rings</td>
			<td >Marco Rubber</td>
			<td >Markez</td>
			<td>compound Z1028</td>
		</tr>
		<tr >
			<td >H2S resistant elastomer o-rings</td>
			<td>Seals Eastern, Inc.</td>
			<td >Aflas</td>
			<td>vendor: Marco Rubber</td>
		</tr>
	</tbody>
</table>

making record-efficiency sns solar cells by thermal evaporation and atomic layer deposition

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

In Figures 6-8 results are shown for two representative &#8220;baseline&#8221; TE-grown samples as described above. Illuminated J-V data for these two samples is plotted in Figure 6. The first sample (&#8220;SnS140203F&#8221;) yielded the device with certified efficiency of 3.88% that was reported previously.9 Representative J-V distributions are also shown for each sample. For a given bias voltage, these distributions are calculated as <img alt="Equ...

Watch this Scientific Journal Video about Making Record-efficiency SnS Solar Cells by Thermal Evaporation and Atomic Layer Deposition at JoVE.com

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

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

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

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