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 cutting step.</li>
	<li>Use a die saw to cut the wafer into 1&#8221; &#215; 1&#8221; (25.4 x 25.4 mm2) square substrates.</li>
</ol>
2. Substrate Cleaning
<ol>
	<li>Remove particulates and other residue that result from cutting step using a compressed nitrogen gun, followed by an ultrasonic bath in de-ionized (DI) water for 5 min at 45 &#8211; 60 &#176;C.</li>
	<li>Remove the photoresist layer with an ultrasonic bath in acetone for 5 min at 45 &#8211; 60 &#176;C.</li>
	<li>Clean the exposed substrate with 3 subsequent ultrasonic baths, all for 5 min at 45 &#8211; 60 &#176;C: acetone, ethanol, and isopropyl alcohol. Finish by drying with a compressed nitrogen gun, while substrates remain in the quartz carrier.</li>
</ol>
3. Mo Sputtering
<ol>
	<li>Load the clean Si / SiO2 substrates into a high vacuum sputtering system. Ensure that the substrate plate is unheated and substrate rotation is enabled. For the devices reported here, process in a commercial system with tilted magnetron guns with 2&#8221; targets and a throw distance of approximately 4&#8221;.</li>
	<li>Deposit the first layer (the adhesion layer) at relatively high background pressure such as 10 mTorr of Ar. For the devices reported here, process with a sputtering power of 180 W (DC), which gives a growth rate of 2.6 &#197;/sec, and a first Mo layer that is 360 nm thick.</li>
	<li>Deposit the second layer (the conductive layer) at a relatively low background pressure such as 2 mTorr of Ar. Use the same sputtering power as the first layer (180 W) and deposit the same thickness. 
	Note: The devices reported here had a second Mo layer that was 360 nm thick, same as the first layer.</li>
	<li>After Mo deposition, store the substrates under vacuum until the SnS deposition step.</li>
</ol>
4. SnS Deposition
Note: The ALD deposition technique is described in sub-section 4.1, and the TE deposition is described in sub-section 4.2. The ALD deposition system is shown in Figure 2, and TE deposition system is shown in Figure 3.
<ol>
	<li>Deposit SnS by ALD
	<ol>
		<li>Before loading into the reactor, put Mo substrates in a UV ozone cleaner for 5 min to remove organic particles. Then place the substrates on the substrate holder and insert into the deposition zone.</li>
		<li>Stabilize the furnace temperature at 200 &#176;C before starting deposition.</li>
		<li>Grow SnS thin films from the reaction of bis(N,N&#8217;-diisopropylacetamidinato)-tin(II) [Sn(MeC(N-iPr)2)2, referred to here as Sn(amd)2] and hydrogen sulfide (H2S)4.
		<ol>
			<li>Keep the Sn(amd)2 precursor at a constant temperature of 95 &#176;C. Use pure N2 gas to assist the delivery of Sn(amd)2 vapor from the container in the oven to the deposition zone. During each ALD cycle, supply three doses of Sn(amd)2 precursor for total exposure of 1.1 Torr second.</li>
			<li>Use a gas mixture of 4% H2S in N2 as the sulfur source. Ensure that the exposure to the hydrogen sulfide vapor is 1.5 Torr second per dose. Ensure that the partial pressure of H2S and the total pressure of H2S in N2 are 0.76 Torr and 19 Torr, respectively.</li>
		</ol>
		</li>
		<li>Set the pumping time between Sn precursor dose and H2S dose to be only 1 sec (short compared to most other conventional ALD procedures) in order to speed up the deposition. 
		Note: Because the Sn precursor is not completely removed by this short pumping time, some residual Sn precursor remains when the H2S arrives. Thus the process could be described as a pulsed CVD process. The growth rate of SnS film is 0.33 &#197;/cycle, or 0.04 &#197;/sec.</li>
	</ol>
	</li>
	<li>Deposit SnS by TE
	<ol>
		<li>Ensure that the process chamber pressure is 2 x 10-7 Torr or lower. Load substrates into the chamber through the load lock. Hold the substrates to the plate either with a single clip, or with a custom substrate holder with appropriately sized pockets that is screwed down to the substrate plate.</li>
		<li>Ramp the source and substrate heaters to their setpoints. For the device reported here the substrate temperature is 240 &#176;C and the growth rate is 1 &#197;/sec; to achieve this growth rate set the source temperature in the range 550 &#8211; 610 &#176;C (the required source temperature increases with time for a single load of source powder). The target film thickness is 1,000 nm.</li>
		<li>Measure deposition rate using the quartz crystal monitor (QCM) before and after the SnS film deposition by moving the QCM arm into the process chamber. For this measurement the substrate is raised so that the QCM can be moved into the substrate growth position. 
		Note: The deposition rate remains fairly constant throughout a deposition time of 3 hr (&#177;0.05 &#197;/sec deviation).</li>
		<li>After deposition, transfer the samples back into the load lock before venting to air. Quickly transport the samples through air into storage either in vacuum or in an inert atmosphere glovebox before the next processing step. 
		Note: The typical unintentional air exposure time is approximately 3 min. The typical storage time is between a day and a week.</li>
	</ol>
	</li>
</ol>
5. SnS Annealing
Note: This step is performed slightly differently for ALD and TE solar cells. The annealing procedure for ALD solar cells is described in sub-section 5.1, and the procedure for TE solar cells is described in sub-section 5.2. The purpose of annealing is discussed in the Discussion section.
<ol>
	<li>Anneal the ALD-grown SnS films in H2S gas. 
	Note: This step is performed in the same system used for ALD growth.
	<ol>
		<li>Use pure H2S gas (99.5% pure) at a flow rate of 40 sccm and pressure of 10 Torr.</li>
		<li>Heat the SnS film to a temperature of 400 &#176;C and hold for 1 hr in the H2S gas environment. Ensure that the gas is flowing throughout the whole process, including temperature ramping up and down.</li>
	</ol>
	</li>
	<li>Anneal the TE-grown SnS films in H2S gas. Perform this step in a dedicated tube furnace.
	<ol>
		<li>Load the samples onto a clean quartz plate and slide into the hot-zone region of the furnace.</li>
		<li>After the furnace is sealed, purge three times with pure N2 and allow to pump down to base pressure.</li>
		<li>Establish gas flow at 100 sccm of 4% H2S at 28 Torr.</li>
		<li>Ramp the temperature to 400 &#176;C over 10 min. Hold at 400 &#176;C for 1 hr, then allow the samples to cool unassisted in the hot-zone. Maintain constant H2S gas flow and pressure until samples cool below 60 &#176;C. Remove the samples and either proceed immediately to the next step, or place them into storage in an inert gas glove box.</li>
	</ol>
	</li>
</ol>
6. SnS Surface Passivation with a Native Oxide
Note: This step is performed slightly differently for ALD and TE solar cells. In sub-section 6.1 the surface passivation procedure for ALD solar cells is described, and the procedure for TE solar cells is described in sub-section 6.2. The function of this step is further discussed in the Discussion section.
<ol>
	<li>For the ALD-grown samples, grow a thin layer of SnO2 by ALD. 
	Note: We use a different reactor than that used for SnS growth.
	<ol>
		<li>Grow SnO2 by the reaction of cyclic amide of tin [(1,3-bis(1,1-dimethylethyl)-4,5-dimethyl-(4R,5R)-1,3,2-diazastannolidin-2-ylidene)Sn(II)] and hydrogen peroxide (H2O2). Store the cyclic amide tin precursor in an oven at 43 &#176;C, and the H2O2 in a bubbler at RT.</li>
		<li>Maintain substrate temperature at 120 &#176;C for deposition.</li>
		<li>Expose the tin precursor and H2O2 using 0.33 and 1.5 Torr second per cycle, respectively, for a total of 5 cycles. Check that the thickness of the resulting SnO2 is 0.6 0.7 nm, as measured by X-ray photoelectron spectroscopy (XPS) analysis10.</li>
	</ol>
	</li>
	<li>For the TE-grown samples, form a thin layer of SnO2 by air exposure.
	<ol>
		<li>Expose the samples to lab ambient air for 24 hr. Check that the thickness of the resulting SnO2 is approximately 0.5 nm, as measured by XPS analysis. 
		Note: The typical RT is 24 &#177; 1 &#176;C, and the typical humidity is 45% &#177; 13% (higher in the summer); for the devices reported here, the values were 24.6 &#730;C and &#60; 30%, respectively.</li>
	</ol>
	</li>
</ol>
7. Deposition of the Zn(O,S) / ZnO Buffer Layer
Note: This step is performed in the same ALD chamber that is used for SnS growth by ALD.
<ol>
	<li>Grow a Zn(O,S):N layer by ALD.
	<ol>
		<li>Maintain the substrate temperature at 120 &#176;C.</li>
		<li>Grow Zn(O,S):N by ALD from the reaction of diethylzinc (Zn(C2H5)2, DEZ), deionized water (H2O), 4% H2S in N2, and ammonia (NH3)11. Store the bubbler containing DEZ at RT. Use a cycle sequence of [DEZ-H2O-DEZ-NH3]14-[DEZ-H2S]1, and repeat this super cycle 12 times. Ensure that the exposure of ammonia is 11 Torr second.</li>
		<li>Check that the S/Zn ratio in the resulting film is 0.14, as measured by Rutherford backscattering spectroscopy12, and that the thickness of the film is approximately 36 nm.</li>
	</ol>
	</li>
	<li>Grow a ZnO layer by ALD.
	<ol>
		<li>Maintain the substrate temperature at 120 &#176;C.</li>
		<li>Grow ZnO with 50 ALD cycles of DEZ-H2O. 
		Note: The thickness of the resulting ZnO film is approximately 18 nm.</li>
	</ol>
	</li>
</ol>
8. Deposition of the Transparent Conducting Oxide (TCO), Indium Tin Oxide (ITO)
<ol>
	<li>Cut ITO shadow masks from a 0.024&#8221; (610 &#956;m) aluminum 6061 sheet using a laboratory laser cutter. 
	Note: The masks define 11 rectangular devices that are 0.25 cm2 in size plus a larger pad in one corner that is used for optical reflectivity measurements, see Figure 4.</li>
	<li>Mount the devices and masks in a mask aligner. 
	Note: This is an aluminum plate with nested pockets for the substrate and masks and clips to secure the masks in place.</li>
	<li>Deposit ITO by reactive magnetron sputtering.
	<ol>
		<li>Heat the substrate to approximately 80 &#8211; 90 &#730;C and enable substrate rotation.</li>
		<li>Use a 2 inch diameter ITO target (In2O3/SnO2 90/10 wt.%, 99.99% pure) at 65 W RF sputtering power with 40 / 0.1 sccm Ar / O2 gas flow at 4 mTorr total pressure.</li>
		<li>Grow a 240 nm thick ITO film. 
		Note: With these parameters, growth rates of 0.5 &#197;/sec and sheet resistances in the range 40 &#8211; 60 &#937;/sq are achieved.</li>
	</ol>
	</li>
</ol>
9. Metallization
<ol>
	<li>Cut metallization shadow masks from a 127 &#956;m thick austenitic stainless steel sheet. 
	Note: These masks are cut with +10/-5 &#181;m tolerance by a commercial company. The metal pattern consists of 2 fingers separated by 1.5 mm, each 7 mm long, and a 1 x 1 mm2 contact pad, see Figure 4.</li>
	<li>Mount the devices and masks in a mask aligner, as in Step 8.2.</li>
	<li>Deposit Ag (for TE devices) or Ni/Al (for ALD devices) by electron-beam evaporation.
	<ol>
		<li>Mount mask aligner onto the substrate plate of an electron beam metals evaporation system. Pump down to a base pressure below 1 x 10-6 Torr.</li>
		<li>Evaporate metal at a rate of 2 &#197;/sec. Deposit 500 nm total metal thickness.</li>
	</ol>
	</li>
</ol>
10. Device Characterization
<ol>
	<li>Perform current-voltage (&#8220;J-V&#8221;) measurements on all devices in the dark and in AM1.5 simulated solar light.
	<ol>
		<li>Calibrate the solar simulator by collecting J-V data from a calibrated silicon solar cell and adjusting the solar simulator lamp power and height until reaching the calibrated current value for AM1.5 insolation.</li>
		<li>Contact the devices in four-wire mode by using copper beryllium double probe tips to contact to both the top (anode, Ag or Al) and bottom (cathode, Mo) layers. Contact the bottom layer by scratching away the buffer and SnS layers with a scalpel blade.</li>
		<li>Measure light and dark J-V data using a source-meter by sourcing voltage and measuring current. 
		Note: Devices are typically measured within the range &#177;0.5 V. The devices are not responsive to the direction or rate of the voltage sweeps. For routine testing an area-defining light aperture is not used, see Discussion section for further details.</li>
	</ol>
	</li>
	<li>Perform external quantum efficiency (EQE) measurements on all devices, with variable light and voltage bias.
	<ol>
		<li>Calibrate the EQE system by measuring the response of a Si calibration photodiode. 
		Note: The software compares this data to measurements performed with a NIST-backed standard to adjust the light level accordingly.</li>
		<li>Contact the devices using the four-wire method, as in step 10.1.2.</li>
		<li>Measure EQE using a commercial system which illuminates the sample with monochromatic light chopped at 100 Hz over a wavelength range of 270 1,100 nm and measures the resulting current. Perform this measurement according to the manufacturer&#8217;s standard operating procedure.</li>
		<li>Repeat the EQE measurement with variable voltage and white light bias. Use a sourcemeter to supply the voltage bias, and a halogen lamp to supply the light bias. Measure devices in both forward and reverse voltage bias, and under variable white light intensity up to ~1 Suns.</li>
		<li>Measure optical reflectance (%R) of the ITO top surface using a spectrophotometer with an integrating sphere in order to convert external to internal quantum efficiency (IQE). Perform this measurement according to the manufacturer&#8217;s standard operating procedure.</li>
	</ol>
	</li>
</ol>

<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. 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The authors have nothing to disclose.

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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42.6K Views.  Massachusetts Institute of Technology. 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%.

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

Harvesting Solar Energy by Means of Charge-Separating Nanocrystals and Their Solids

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials offering a superior control over the spatial distribution of charge carriers across material interfaces. As this study demonstrates, a combination of donor-acceptor nanocrystal (NC) domains in a single nanoparticle can lead to the realization of efficient photocatalytic1-5 materials, while a layered assembly of donor- and acceptor-like nanocrystals films gives rise to photovoltaic materials.
Initially the paper focuses on the synthesis of composite inorganic nanocrystals, comprising linearly stacked ZnSe, CdS, and Pt domains, which jointly promote photoinduced charge separation. These structures are used in aqueous solutions for the photocatalysis of water under solar radiation, resulting in the production of H2 gas. To enhance the photoinduced separation of charges, a nanorod morphology with a linear gradient originating from an intrinsic electric field is used5. The inter-domain energetics are then optimized to drive photogenerated electrons toward the Pt catalytic site while expelling the holes to the surface of ZnSe domains for sacrificial regeneration (via methanol). Here we show that the only efficient way to produce hydrogen is to use electron-donating ligands to passivate the surface states by tuning the energy level alignment at the semiconductor-ligand interface. Stable and efficient reduction of water is allowed by these ligands due to the fact that they fill vacancies in the valence band of the semiconductor domain, preventing energetic holes from degrading it. Specifically, we show that the energy of the hole is transferred to the ligand moiety, leaving the semiconductor domain functional. This enables us to return the entire nanocrystal-ligand system to a functional state, when the ligands are degraded, by simply adding fresh ligands to the system4.
To promote a photovoltaic charge separation, we use a composite two-layer solid of PbS and TiO2 films. In this configuration, photoinduced electrons are injected into TiO2 and are subsequently picked up by an FTO electrode, while holes are channeled to a Au electrode via PbS layer6. To develop the latter we introduce a Semiconductor Matrix Encapsulated Nanocrystal Arrays (SMENA) strategy, which allows bonding PbS NCs into the surrounding matrix of CdS semiconductor. As a result, fabricated solids exhibit excellent thermal stability, attributed to the heteroepitaxial structure of nanocrystal-matrix interfaces, and show compelling light-harvesting performance in prototype solar cells7.

A general strategy for the development of charge-separating semiconductor nanocrystal composites deployable for solar energy production is presented. We show that assembly of donor-acceptor nanocrystal domains in a single nanoparticle geometry gives rise to a photocatalytic function, while bulk-heterojunctions of donor-acceptor nanocrystal films can be used for photovoltaic energy conversion.

Conjoining different semiconductor materials in a single nano-composite provides synthetic means for the development of novel optoelectronic materials ...

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O2 pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (&gt;80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO2, Al2O3, WO3 and Ag4O4.

We describe the experimental method to deposit nanostructured oxide thin films by nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas. By using this method Al-doped ZnO (AZO) films, from compact to hierarchically structured as nano-tree forests, can be deposited.

Nanosecond Pulsed  Laser Deposition (PLD) in the presence of a background gas allows the deposition  of metal oxides with tunable morphology, structure, ...

Integration of Light Trapping Silver Nanostructures in Hydrogenated Microcrystalline Silicon Solar Cells by Transfer Printing

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly known as plasmonic effects, play an important role. Research in this field has, however, been impeded owing to the difficulty of fabricating devices containing the desired functional metal nanostructures. In order to provide a viable strategy to this issue, we herein show a transfer printing-based approach that allows the quick and low-cost integration of designed metal nanostructures with a variety of device architectures, including solar cells. Nanopillar poly(dimethylsiloxane) (PDMS) stamps were fabricated from a commercially available nanohole plastic film as a master mold. On this nanopatterned PDMS stamps, Ag films were deposited, which were then transfer-printed onto block copolymer (binding layer)-coated hydrogenated microcrystalline Si (&#181;c-Si:H) surface to afford ordered Ag nanodisk structures. It was confirmed that the resulting Ag nanodisk-incorporated &#181;c-Si:H solar cells show higher performances compared to a cell without the transfer-printed Ag nanodisks, thanks to plasmonic light trapping effect derived from the Ag nanodisks. Because of the simplicity and versatility, further device application would also be feasible thorough this approach.

A viable transfer printing-based methodology to introduce plasmonic metal nanostructures in solar cells is described. Using nanopillar poly(dimethylsiloxane) stamps, an Ag-based ordered nanodisk array was integrated with standard hydrogenated microcrystalline Si solar cells, which led to improved device performances due to plasmonic light trapping.

One of the potential applications of metal nanostructures is light trapping in solar cells, where unique optical properties of nanosized metals, commonly ...

Seedless Growth of Bismuth Nanowire Array via Vacuum Thermal Evaporation

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. Conventionally reserved for the fabrication of metal thin films, thermal evaporation deposits bismuth into an array of vertical single crystalline nanowires over a flat thin film of vanadium held at RT, which is freshly deposited by magnetron sputtering or thermal evaporation. By controlling the temperature of the growth substrate the length and width of the nanowires can be tuned over a wide range. Responsible for this novel technique is a previously unknown nanowire growth mechanism that roots in the mild porosity of the vanadium thin film. Infiltrated into the vanadium pores, the bismuth domains (~ 1 nm) carry excessive surface energy that suppresses their melting point and continuously expels them out of the vanadium matrix to form nanowires. This discovery demonstrates the feasibility of scalable vapor phase synthesis of high purity nanomaterials without using any catalysts.

A protocol for seedless and high yield growth of bismuth nanowire arrays through vacuum thermal evaporation technique is presented.

Here a seedless and template-free technique is demonstrated to scalably grow bismuth nanowires, through thermal evaporation in high vacuum at RT. ...

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices have used devices prepared by spin coating, a process that is not amenable to large-scale fabrication. This mismatch in device fabrication makes it difficult to translate quantitative results obtained in the laboratory to the commercial level, making optimization difficult. Using a mini-slot die coater, this mismatch can be resolved by translating the commercial process to the laboratory and characterizing the structure formation in the active layer of the device in real time and in situ as films are coated onto a substrate. The evolution of the morphology was characterized under different conditions, allowing us to propose a mechanism by which the structures form and grow. This mini-slot die coater offers a simple, convenient, material efficient route by which the morphology in the active layer can be optimized under industrially relevant conditions. The goal of this protocol is to show experimental details of how a solar cell device is fabricated using a mini-slot die coater and technical details of running in situ structure characterization using the mini-slot die coater.

Printing Fabrication of Bulk Heterojunction Solar Cells and In Situ Morphology Characterization

Here, we present a protocol to fabricate organic thin film solar cells using a mini-slot die coater and related in-line structure characterizations using synchrotron scattering techniques.

Polymer-based materials hold promise as low-cost, flexible efficient photovoltaic devices. Most laboratory efforts to achieve high performance devices ...

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical properties. The possibility of doping was investigated by adding monovalent cation halides with similar ionic radii to Pb2+, including Cu+, Na+, and Ag+. A shift in the Fermi level and a remarkable decrease of sub-bandgap optical absorption, along with a lower energetic disorder in the perovskite, was achieved. An order-of-magnitude enhancement in the bulk hole mobility and a significant reduction of transport activation energy within an additive-based perovskite device was attained. The confluence of the aforementioned improved properties in the presence of these cations led to an enhancement in the photovoltaic parameters of the perovskite solar cell. An increase of 70 mV in open circuit voltage for AgI and a 2 mA/cm2 improvement in photocurrent density for NaI- and CuBr-based solar cells were achieved compared to the pristine device. Our work paves the way for further improvements in the optoelectronic quality of CH3NH3PbI3 perovskite and subsequent devices. It highlights a new avenue for investigations on the role of dopant impurities in crystallization and controls the electronic defect density in perovskite structures.

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Here, we present a protocol to adjust the properties of solution-processed CH3NH3PbI3 through the incorporation of monovalent cation additives in order to achieve highly efficient perovskite solar cells.

Here, we demonstrate the incorporation of monovalent cation additives into CH3NH3PbI3 perovskite in order to adjust the optical, excitonic, and electrical ...

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for Cu(In,Ga)Se2 Solar Cells

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity. 
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two 'Combined Stress test with in situ measurement' setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.

In Situ Monitoring of the Accelerated Performance Degradation of Solar Cells and Modules: A Case Study for CuIn,GaSe2 Solar Cells

Two &#39;Combined Stress test with in situ measurement&#39; setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.

The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction ...

Well-aligned Vertically Oriented ZnO Nanorod Arrays and their Application in Inverted Small Molecule Solar Cells

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule (SMPV1) and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM), by utilizing ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The inverted SMPV1:PC71BM solar cells with ZnO NRs that grew on both a sputtered and sol-gel processed AZO seed layer are fabricated. Compared with the AZO thin film prepared by the sol-gel method, the sputtered AZO thin film exhibits better crystallization and lower surface roughness, according to X-ray diffraction (XRD) and atomic force microscope (AFM) measurements. The orientation of the ZnO NRs grown on a sputtered AZO seed layer shows better vertical alignment, which is beneficial for the deposition of the subsequent active layer, forming better surface morphologies. Generally, the surface morphology of the active layer mainly dominates the fill factor (FF) of the devices. Consequently, the well-aligned ZnO NRs can be used to improve the carrier collection of the active layer and to increase the FF of the solar cells. Moreover, as an anti-reflection structure, it can also be utilized to enhance the light harvesting of the absorption layer, with the power conversion efficiency (PCE) of solar cells reaching 6.01%, higher than the sol-gel based solar cells with an efficiency of 4.74%.

This manuscript describes how to design and fabricate efficient inverted SMPV1:PC71BM solar cells with ZnO nanorods (NRs) grown on a high quality Al-doped ZnO (AZO) seed layer. The well-aligned vertically oriented ZnO NRs exhibit high crystalline properties. The power conversion efficiency of solar cells can reach 6.01%.

This manuscript describes how to design and fabricate efficient inverted solar cells, which are based on a two-dimensional conjugated small molecule ...

Production of Single Tracks of Ti-6Al-4V by Directed Energy Deposition to Determine the Layer Thickness for Multilayer Deposition

Directed Energy Deposition (DED), which is an additive manufacturing technique, involves the creation of a molten pool with a laser beam where metal powder is injected as particles. In general, this technique is employed to either fabricate or repair different components. In this technique, the final characteristics are affected by many factors. Indeed, one of the main tasks in building components by DED is the optimization of process parameters (such as laser power, laser speed, focus, etc.) which is usually carried out through an extensive experimental investigation. However, this sort of experiment is extremely lengthy and costly. Thus, in order to accelerate the optimization process, an investigation was conducted to develop a method based on the melt pool characterizations. In fact, in these experiments, single tracks of Ti-6Al-4V were deposited by a DED process with multiple combinations of laser power and laser speed. Surface morphology and dimensions of single tracks were analyzed, and geometrical characteristics of melt pools were evaluated after polishing and etching the cross-sections. Helpful information regarding the selection of optimal process parameters can be achieved by examining the melt pool features. These experiments are being extended to characterize the larger blocks with multiple layers. Indeed, this manuscript describes how it would be possible to quickly determine the layer thickness for the massive deposition, and avoid over or under-deposition according to the calculated energy density of the optimum parameters. Apart from the over or under-deposition, time and materials saving are the other great advantages of this approach in which the deposition of multilayer components can be started without any parameter optimization in terms of layer thickness.

In this research, a rapid method based on melt pool characterization is developed to estimate the layer thickness of Ti-6Al-4V components produced by directed energy deposition.

Directed Energy Deposition (DED), which is an additive manufacturing technique, involves the creation of a molten pool with a laser beam where metal ...

Atomic Layer Deposition of Vanadium Dioxide and a Temperature-dependent Optical Model

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with wafer-scale uniformity and angstrom level control of thickness, the method of atomic-layer deposition was chosen. This ALD process enables high-quality, low-temperature (&#8804;150 &#176;C) growth of ultrathin films (100-1000 &#197;) of VO2. For this demonstration, the VO2 films were grown on sapphire substrates. This low temperature growth technique produces mostly amorphous VO2 films. A subsequent anneal in an ultra-high vacuum chamber with a pressure of 7x10-4 Pa of ultra-high purity (99.999%) oxygen produced oriented, polycrystalline VO2 films. The crystallinity, phase, and strain of the VO2 were determined by Raman spectroscopy and X-ray diffraction, while the stoichiometry and impurity levels were determined by X-ray photoelectron spectroscopy, and finally the morphology was determined by atomic force microscopy. These data demonstrate the high-quality of the films grown by this technique. A model was created to fit to the data for VO2 in its metallic and insulating phases in the near infrared spectral region. The permittivity and refractive index of the ALD VO2 agreed well with the other fabrication methods in its insulating phase, but showed a difference in its metallic state. Finally, the analysis of the films&#39; optical properties enabled the creation of a wavelength- and temperature-dependent model of the complex optical refractive index for developing VO2 as a tunable refractive index material.

Thin films (100-1000 &#197;) of vanadium dioxide (VO2) were created by atomic-layer deposition (ALD) on sapphire substrates. Following this, the optical properties were characterized through the metal-insulator transition of VO2. From the measured optical properties, a model was created to describe the tunable refractive index of VO2.

Vanadium dioxide is a material that has a reversible metal-insulator phase change near 68 &#176;C. To grow VO2 on a wide variety of substrates, with ...