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

Summary

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.

Abstract

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.

Introduction

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 sputtering^3,4. 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 applications^{8,9,10,11,12,13,14,15,16,17,18,19}. 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 (V_oc) 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.

Protocol

1. Etching and cleaning the substrate

1. Using a glass cutting system, form 2.5 x 2.5 cm substrates of fluoride thin oxide (FTO).
2. Protect part of the substrate surface with a thermal tape leaving 0.5 cm of one side exposed.
3. 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 with deionized (DI) water.
   CAUTION: Hydrogen gas in abundance is generated from zinc and HCl reaction.
4. Remove the tape and wash with DI water and soap using a small brush.
   NOTE: The brush helps to remove some residual glue from the tape.
5. Leave the etched substrate in a soap solution (50% in water) and keep it for 15 min in an ultrasonicate bath. Then, sonicate for 15 min in DI water (2 times), followed by 10 more min in acetone and finally 10 min in isopropyl alcohol. Dry the substrate with nitrogen gas.

2. Deposition of niobium oxide films

1. Fix the substrate through a shadow mask protecting 0.5 cm of both sides.
   NOTE: On the side where the FTO was etched, it is important to certify that the FTO is covered in order to prevent short circuits when building the cell.
2. Introduce the substrate into the sputtering chamber and seal the chamber.
3. Start the mechanic pump. In the first 10 min, change the 3-way valve into roughing position to heat its oil and release water to improve the pumping. The primary pump works alone until the pressure is 6 x 10^-2 Torr.
4. Change the 3-way valve to backing position, and turn the turbo molecular pump on. Once the molecular pump is started, open the gate valve at the vacuum pump entry. Deposition starts when the pressure reaches 3 x 10^-6 torr.
   NOTE: Before starting the molecular pump, the primary vacuum should be better than 6 x 10^-2 torr, however, not higher than 5 x 10^-2 torr in order to prevent contaminating the chamber with pump oil.
5. When the vacuum reaches 5 x 10^-5 torr, open the water cooler system and turn on the substrate heating system. Set the temperature at 500 °C. Increase the temperature slowly, 100 °C every 5 min until it reaches the desired value.
6. Set the gases parameters to be used in the deposition: argon of 40 sccm and oxygen of 3 to 10 sccm.
   NOTE: The oxygen flow rate was varied in each deposition: 3, 3.5, 4 and 10 sccm. Oxygen reacts with niobium forming niobium oxide.
7. Introduce argon onto the chamber, and set the pressure to 5 x 10^-3 torr and the radio frequency (RF) to 120 W. Turn the RF on and tune using the impedance matching box. In case the plasma does not start, increase the pressure slowly until it reaches 2 x 10^-2 Torr. In this pressure, the plasma should start. Set the pressure using a gate valve that can be opened or closed to change the pumping rate.
8. Keep the plasma at 120 W for 10 min to clean the niobium target removing any oxide layer present in its surface.
   NOTE: While cleaning the target, the substrate shutter is kept closed to protect the substrate from any material deposition.
9. Introduce oxygen into the chamber, after stabilization, set the radio frequency power to 240 W and open the substrate shutter. Deposition starts. Set the deposition time to have a final thickness of 100 nm based on previous studies that determined the deposition rate. For each deposition condition a different deposition rate is expected, so the deposition time does also differ.
10. Once the deposition time is completed, close the shutter immediately, turn the RF off, the close the gases and decrease the substrate temperature to room temperature.
11. As the substrate temperature reaches room temperature, introduce air to reestablish the ambient pressure and open the chamber..
    NOTE: Generally, the system takes 4 h to reach a temperature of 40 °C.

3. Constructing the solar cells

1. Preparing the solutions used to construct the devices
   1. TiO₂ paste solution: Mix 150 mg of TiO₂ paste in 1 mL of DI water. Stir it for 1 day before use.
      NOTE: Keep the suspension stirring even when you are not using it to be sure that the suspension is always homogeneous.
   2. Prepare the lead iodide solution (PbI₂) by mixing 420 mg of PbI₂ in 1 mL of anhydrous dimethylformamide. Use only anhydrous solvents.
   3. Prepare the methylammonium iodide (CH₃NH₃I) solution by adding 8 mg of CH₃NH₃I in 1 mL of isopropyl alcohol (IPA).
      NOTE: The water content in IPA must be less than 0.0005%.
2. Deposit TiO₂ mesoporous layer on top of the niobium oxide layer using a spin coater at 4,000 rpm for 30 s.
3. Put the substrate on the oven following the steps: 270 °C for 30 min; 370 °C for 30 min and 500 °C for 1 h. Wait until the oven reaches room temperature and remove the substrate.
   NOTE: The heat treatment decomposes the organic part of the paste leaving a porous layer over the film.
4. Deposit two layers of PbI₂ on top of the TiO₂ mesoporous using a spin coater at 6,000 rpm for 90 s and after each deposition put the substrate in a hot plate at 70 °C for 10 min.
   NOTE: The PbI₂ deposition must be inside a glove box filled with pure nitrogen or argon and with controlled atmosphere (water and oxygen < 0.1 ppm).
5. Deposit the CH₃NH₃I solution. Drop 0.3 mL of CH₃NH₃I solution onto PbI₂, wait 20 s and then spin at 4,000 rpm for 30 s. Put the substrate on a hot plate at 100 °C for 10 min.
   NOTE: The CH₃NH₃I deposition must be inside a glove box. The total amount of CH₃NH₃I solution must be dropped quickly in only one step.
6. Deposit Spiro-OMeTAD solution on top of the perovskite layer by spin coating at 4,000 rpm for 30 s. Leave the substrate in an oxygen atmosphere overnight.
   NOTE: The Spiro-OMeTAD deposition must be inside of a glove box. After the deposition, it is important to leave the substrate overnight in an oxygen atmosphere in order to oxidize the Spiro- OMeTAD increasing its conductivity.
7. Evaporate 70 nm of gold contact using a shadow mask at a rate of 0.2 A/s until 5 nm is reached and then increase the rate to 1 A/s.
   NOTE: It is important to use a slow rate at the beginning to prevent gold diffusion through the cell.

Results

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 <...

Discussion

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...

Materials

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