The work was supported by Funda&#231;&#227;o de Amparo &#224; Pesquisa do Estado de S&#227;o Paulo (FAPESP), Centro de&#160;Desenvolvimento de Materiais Cer&#226;micos (CDMF- FAPESP&#160;N&#186; 2013/07296-2, 2017/11072-3, 2013/09963-6 and 2017/18916-2). Special thanks to Professor M&#225;ximo Siu Li for PL measurements.

1. Etching and cleaning the substrate
<ol>
	<li>Using a glass cutting system, form 2.5 x 2.5 cm substrates of fluoride thin oxide (FTO).</li>
	<li>Protect part of the substrate surface with a thermal tape leaving 0.5 cm of one side exposed.</li>
	<li>Deposit a small amount of zinc powder (enough to cover the area to be etched) on the top of the exposed FTO and drop concentrated hydrochloric acid (HCl) on the zinc powder slowly until all zinc powder is consumed by the reaction. Immediately after, rinse the substrate wit...

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A., Zoolfakar, A. S., O'Mullane, A. P., Austin, M. W., Kalantar-Zadeh, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thin+films+and+nanostructures+of+niobium+pentoxide:+fundamental+properties,+synthesis+methods+and+applications.">Thin films and nanostructures of niobium pentoxide: fundamental properties, synthesis methods and applications.</a> Journal Materials Chemistry A. 2, 15683-15703 (2014).</li><li>Foroughi-Abari, A., Cadien, K. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth,+structure+and+properties+of+sputtered+niobium+oxide+thin+films.">Growth, structure and properties of sputtered niobium oxide thin films.</a> Thin Solid Films. 519, 3068-3073 (2011).</li><li>Numata, Y., 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=Nb-doped+amorphous+titanium+oxide+compact+layer+for+formamidinium-based+high+efficiency+perovskite+solar+cells+by+low-temperature+fabrication.">Nb-doped amorphous titanium oxide compact layer for formamidinium-based high efficiency perovskite solar cells by low-temperature fabrication.</a> Journal Materials Chemistry A. 6, 9583-9591 (2018).</li><li>Gra&#231;a, M. P. F., Meireles, A., Nico, C., Valente, M. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb2O5+nanosize+powders+prepared+by+sol-gel+-+Structure,+morphology+and+dielectric+properties.">Nb2O5 nanosize powders prepared by sol-gel - Structure, morphology and dielectric properties.</a> Journal of Alloys and Compounds. 553, 177-182 (2013).</li><li>Kogo, A., Numata, Y., Ikegami, M., Miyasaka, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Nb+2+O+5+Blocking+Layer+for+High+Open-circuit+Voltage+Perovskite+Solar+Cells.">Nb 2 O 5 Blocking Layer for High Open-circuit Voltage Perovskite Solar Cells.</a> Chemistry Letters. 44, 829-830 (2015).</li><li>Ueno, S., Fujihara, S. <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+an+Nb2O5+nanolayer+coating+on+ZnO+electrodes+in+dye-sensitized+solar+cells.">Effect of an Nb2O5 nanolayer coating on ZnO electrodes in dye-sensitized solar cells.</a> Electrochimica Acta. 56, 2906-2913 (2011).</li><li>Fernandes, S. 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=Exploring+the+Properties+of+Niobium+Oxide+Films+for+Electron+Transport+Layers+in+Perovskite+Solar+Cells.">Exploring the Properties of Niobium Oxide Films for Electron Transport Layers in Perovskite Solar Cells.</a> Frontiers in Chemistry. 7, 1-9 (2019).</li><li>Shirani, 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=Tribologically+enhanced+self-healing+of+niobium+oxide+surfaces.">Tribologically enhanced self-healing of niobium oxide surfaces.</a> Surface and Coatings Technology. 364, 273-278 (2014).</li><li>Yan, 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=Nb2O5/TiO2+heterojunctions:+Synthesis+strategy+and+photocatalytic+activity.">Nb2O5/TiO2 heterojunctions: Synthesis strategy and photocatalytic activity.</a> Applied Catalysis B: Environmental. 152 (1), 280-288 (2014).</li></ol>

The niobium oxide films prepared in this work was used as electron transport layer in perovskite solar cells. The most important characteristic required for an electron transport layer is to prevent recombination, blocking holes and transferring efficiently electrons.
In this respect the use of reactive sputtering technique is advantageous since it produces dense and compact films. Also, as already mentioned, compared to sol-gel, anodization, hydrothermal, and chemical vapor deposition synthes...

The sputtering technique is widely used to deposit high-quality films. Its main application is in the semiconductor industry, although it is also used in surface coating for improvement in mechanical properties, and reflective layers1. The main advantage of sputtering is the possibility to deposit different materials over different substrates; the good reproducibility and control over the deposition parameters. The sputtering technique allows deposition of homogeneous films, with good adhesion over large areas and at low-cost when compared with other deposition methods like chemical vapor deposition (CVD), molecular beam epitaxy (MBE) and atomic layer deposition (ALD)1,2. Commonly, semiconductor films deposited by sputtering are amorphous or polycrystalline, however, there are some reports on epitaxial growth by sputtering3,4. Nevertheless, the sputtering process is highly complex and the range of the parameter is wide5, so in order to achieve high-quality films, a good comprehension of the process and parameter optimization is necessary for each material.
There are several articles reporting on the deposition of niobium oxide films by sputtering, as well as niobium nitride6 and niobium carbide7. Among Nb-oxides, niobium pentoxide (Nb2O5) is a transparent, air-stable and water-insoluble material that exhibits extensive polymorphism. It is an n-type semiconductor with band gap values ranging from 3.1 to 5.3 eV, giving these oxides a wide range of applications8,9,10,11,12,13,14,15,16,17,18,19. Nb2O5 has attracted considerable attention as a promising material to be used in perovskite solar cells due to its comparable electron injection efficiency and better chemical stability compared to titanium dioxide (TiO2). In addition, the band gap of Nb2O5 could improve the open-circuit voltage (Voc) of the cells14.
In this work, Nb2O5 was deposited by reactive sputtering under different oxygen flow rates. At low oxygen flow rates, the conductivity of the films were increased without making use of doping, which introduces impurities on the system. These films were used as electron transport layer in perovskite solar cells improving the performance of these cells. It was found that decreasing the amount of oxygen induces the formation of oxygen vacancies, which increases the conductivity of the films leading to solar cells with better efficiency.

<table>
	<tbody>
		<tr>
			<td>2-propanol</td>
			<td>Merck</td>
			<td>67-63-0</td>
			<td>solvent with maximum of 0.005% H2O</td>
		</tr>
		<tr>
			<td>4-tert-butylpyridine</td>
			<td>Sigma Aldrich</td>
			<td>3978-81-2</td>
			<td>chemical with 96% purity</td>
		</tr>
		<tr>
			<td>acetonitrile</td>
			<td>Sigma Aldrich</td>
			<td>75-05-8</td>
			<td>anhydrous solvent , 99.8% purity</td>
		</tr>
		<tr>
			<td>bis(trifluoromethane)sulfonimide lithium salt</td>
			<td>Sigma Aldrich</td>
			<td>90076-65-6</td>
			<td>chemical with &#8805;99.95% purity</td>
		</tr>
		<tr>
			<td>chlorobenzene</td>
			<td>Sigma Aldrich</td>
			<td>108-90-7</td>
			<td>anhydrous solvent , 99.8% purity</td>
		</tr>
		<tr>
			<td>ethanol</td>
			<td>Sigma Aldrich</td>
			<td>200-578-6</td>
			<td>solvent</td>
		</tr>
		<tr>
			<td>Fluorine doped tin oxide (SnO2:F) glass substrate</td>
			<td>Solaronix</td>
			<td>TCO22-7/LI</td>
			<td>substrate to deposit films</td>
		</tr>
		<tr>
			<td>Kaptom tape</td>
			<td>Usinainfo</td>
			<td>04227</td>
			<td>thermal tape used to cover the substrates</td>
		</tr>
		<tr>
			<td>Kurt J Lesker magnetron sputtering system</td>
			<td>Kurt J Lesker</td>
			<td>------</td>
			<td>Sputtering equipment used to deposit compact films</td>
		</tr>
		<tr>
			<td>Lead (II) iodide</td>
			<td>Alfa Aesar</td>
			<td>10101-63-0</td>
			<td>PbI2 salt- 99.998% purity</td>
		</tr>
		<tr>
			<td>methylammonium iodide</td>
			<td>Dyesol</td>
			<td>14965-49-2</td>
			<td>CH3NH3I salt</td>
		</tr>
		<tr>
			<td>N2,N2,N2&#8242;,N2&#8242;,N7,N7,N7&#8242;,N7&#8242;-octakis (4-methoxyphenyl)-9,9&#8242;-spirobi [9H-fluorene]-2,2&#8242;,7,7&#8242;-tetramine</td>
			<td>Sigma Aldrich</td>
			<td>207739-72-8</td>
			<td>Spiro-OMeTAD salt, 99% purity</td>
		</tr>
		<tr>
			<td>Niobium target of 3&#8221;</td>
			<td>CBMM- Brazilian Metallurgy and Mining Company</td>
			<td>------</td>
			<td>niobium sputtering target used in the sputtering system</td>
		</tr>
		<tr>
			<td>N-N dimethylformamide</td>
			<td>Merck</td>
			<td>68-12-2</td>
			<td>solvent with maximum of 0.003% H2O</td>
		</tr>
		<tr>
			<td>TiO2 paste</td>
			<td>Dyesol</td>
			<td>DSL 30NR-D</td>
			<td>titanium dioxide paste</td>
		</tr>
		<tr>
			<td>tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide]</td>
			<td>Dyesol</td>
			<td>329768935</td>
			<td>FK 209 Co(III) TFSL salt</td>
		</tr>
	</tbody>
</table>

niobium oxide films deposited by reactive sputtering: effect of oxygen flow rate

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition parameters such as gas flow rate that results in changes on composition and thus in the film required properties. In this report, reactive sputtering is used to deposit niobium oxide films. A niobium target is used as metal source and different oxygen flow rates to deposit niobium oxide films. The oxygen flow rate was changed from 3 to 10 sccm. The films deposited under low oxygen flow rates show higher electrical conductivity and provide better perovskite solar cells when used as electron transport layer.

In the sputtering system, the deposition rate is strongly influenced by the oxygen flow rate. The deposition rate decreases when the oxygen flow is increased. Considering the present conditions of the target area used and plasma power, it is observed that from 3 to 4 sccm there is an expressive decrease on the deposition rate, however, when the oxygen is increased from 4 to 10 sccm it becomes less pronounced. In the regime of 3 sccm the deposition rate is 1.1 nm/s, decreasing abruptly to 0.1 nm/s for 10 sccm as seen in <...

Watch this Scientific Journal Video about Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate at JoVE.com

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Here, we present a protocol for niobium oxide films deposition by reactive sputtering with different oxygen flow rates for use as an electron transport layer in perovskite solar cells.

Reactive sputtering is a versatile technique used to form compact films with excellent homogeneity. In addition, it allows easy control over deposition ...

In the reactive sputter technique it's possible to have a fine control of the parameters which allows to deposit niobium oxide fume with different stochiometer and preference. The main advantage of this technique is the deposition of homogenous fumes with good adhesion over large areas and at low cost and low hinders of production. It's important to pay attention to each step and to not skip any.Realizing how to handle equipment and the final appearance of the fumes helps to achieve a good deposition. Begin by protecting the substrate surface with a thermal tape, leaving 0.5 cm of one side exposed. Deposit enough zinc powder to cover the area to be etched on the top of the exposed fluoride thin oxide.Then slowly drop concentrated hydrochloric acid until all of the zinc powder is consumed by the reaction. Immediately wash the substrate with the ionized water. Remove the tape.And sonicate the substrate with soap for 15 minutes, followed by two times in water, acetone, and isopropanol alcohol. After fixing the substrate through a metal shadow mask, place the substrate into the sputtering chamber. After sealing the chamber, start the mechanic pump, and turn on the turbo molecular pump.When the vacuum reaches five times 10 to the negative five torr, open the water cooler system and turn on the substrate heating system. Set the temperature to 500 degrees Celsius, increasing 100 degrees Celsius every five minutes until it reaches the desired value. Set the argon to 40 SCCM, and the oxygen to three standard cubic centimeters per minute.Introduce argon into the chamber. And set the pressure to five times 10 to the negative three torr, and the radio frequency to 120 watts. Turn on the radio frequency.Using the impedance matching box to tune the frequency as necessary. If the plasma does not start, increase the pressure slowly until it reaches two times 10 to the negative two torr. Using a gate valve that can be opened or closed to change the pumping rate to set the pressure.Keep the plasma at 120 watts for 10 minutes to clean the Niobium target and to remove any oxide layer present in its surface. After stabilization, introduce Oxygen in to the chamber, set the RF power to 240 watts, and open the substrate shutter. Start the deposition and set the deposition time, to achieve a final thickness of 100 nanometers.As soon as the deposition is complete, close the shutter, turn off the radio frequency, close the gases, and decrease the substrate temperature. As the substrate temperature reaches room temperature, introduce air to re-establish the ambient pressure before opening the chamber, and removing the substrate. For solar cell construction, protect both sides of the substrate with a piece of tape and use a spin coater at 4, 000 rotations per minute for 30 seconds to deposit a mesoporous titanium dioxide layer onto the niobium oxide layer.Then place the substrate in the oven according to the indicated warming sequence. When the oven reaches room temperature, use the spin coater to deposit two layers of the lead iodide into the titanium dioxide layer at 6, 000 rotations per minute for 90 seconds. Placing the substrate onto a hot plate or 70 degrees Celsius for 10 minutes after each deposition.After the heat treatment, drop 300 milliliters of methylammonium iodide solution onto the lead iodide layers and wait 20 seconds before spinning at 4, 000 rotations per minute for 30 seconds. At the end of the spin, place the substrate on a hot plate for 10 minutes at 100 degrees Celsius, before depositing Spiro OMetTAD solution on top of the perovskite layer in the spin coater at 4, 000 rotations per minute for 30 seconds. Then store the films in desiccator in air overnight to oxidize Spiro OMetTAD.The next morning, scratch the perovskite film to expose the FTO. Use a shadow mask to deposit gold contact in an evaporator machine at a rate of 0.2 angstroms per second until the thickness reaches five nanometers, before increasing the rate to one angstrom per second to obtain 17 nanometers of gold contact. Then the cell is ready to be tested.In the sputtering system, the deposition rate is strongly influenced by the oxygen flow rate, decreasing when the oxygen flow increases. For example, from three to four SCCM, there is an expressive decrease on the deposition rate. When the oxygen is increased, from four to 10 SCCM however the deposition rate becomes less pronounced.The niobium oxide phase formed is dependent on the oxygen flow rate, and for flows less than three SCCM niobium dioxide is the main phase formed. For flows equal or higher than 3.5 SCCM, the oxygen volume is too high to generate niobium dioxide. Instead niobium pentoxide is observed as the main phase.Electronmicroscopy images show the nano metric spherical particles of the films deposited at three point five, four, and 10 SCCM. In contrast the film deposited at three SCCM reveals sheet shaped particles. The films deposited by reactive sputtering in different oxygen flow rates demonstrate different electrical properties.The conductivity of the films increases when three SCCM of oxygen is used. When the Oxygen flow rate is increased to three point five, four or 10 SCCM, a decrease in the conductivity is observed. The performance of perovskite solar cells also depends on the niobium oxide used.As a cell made with electron transport layers deposited at three point five SCCM has the best performance with the highest short circuit current. Remember to check that all of parameters are correctly set before starting the deposition of niobium oxide films. The Niobium oxides films can also be the cause that putting off chemical solutions.However, the metal does not allow the deposition of different stoichiometry. Is the development of parts that analyzes of how the conductivity of Niobium oxide fumes influences the performance of the perovskite solar cells. Take care when using the chemicals for the perovskite deposition and be sure to follow all of the laboratory safety rules.

Niobium Oxide Films Deposited by Reactive Sputtering: Effect of Oxygen Flow Rate

Abstract