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

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.