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 atomi....

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 ....

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