F-doped SnO2 thin films grown on flexible substrates at low temperatures by pulsed laser deposition

doi:10.1016/j.tsf.2011.07.025

Thin Solid Films

Volume 520, Issue 1, 31 October 2011, Pages 497-500

https://doi.org/10.1016/j.tsf.2011.07.025 Get rights and content

Abstract

Fluorine-doped tin oxide (SnO₂:F) films were deposited on polyethersulfone plastic substrates by pulsed laser deposition. The electrical and optical properties of the SnO₂:F films were investigated as a function of deposition conditions such as substrate temperature and oxygen partial pressure during deposition. High quality SnO₂:F films were achieved under an optimum oxygen pressure range (7.4–8 Pa) at relatively low growth temperatures (25–150 °C). As-deposited films exhibited low electrical resistivities of 1–7 mΩ-cm, high optical transmittance of 80–90% in the visible range, and optical band-gap energies of 3.87–3.96 eV. Atomic force microscopy measurements revealed a reduced root mean square surface roughness of the SnO₂:F films compared to that of the bare substrates indicating planarization of the underlying substrate.

Introduction

There is a growing need for transparent conducting oxides (TCOs) in the field of organic optoelectronics — specifically in applications such as substrates for flat panel organic light-emitting devices (OLEDs) and organic photovoltaic (OPV) devices. In general, glass substrates are widely used for most optoelectronic and electro-optic devices. However, glass substrates are unsuitable for many applications such as ultra-thin displays, electronic maps, smart cards and portable computers where flexibility, weight and/or safety issues are critical, and especially true for large area applications due to the inherent rigidity, brittleness and weight of the glass. These disadvantages can be overcome by using flexible substrates, which are robust, light-weight, and cost effective. For these reasons, optically transparent plastics, such as polyethersulfone (PES; T_g = 223 °C), polycarbonate (T_g = 150 °C), and polyethylene terephthalate (T_g = 70 °C), where T_g is the glass transition temperature, have been explored as substrate materials for the growth of transparent conducting oxide thin films in passive and active matrix display applications [1], [2], [3], [4], [5]. Among these substrates, PES is an excellent candidate because it exhibits ~ 90% of optical transmission in the visible and near-infrared spectrum with a high glass transition temperature (T_g = 223 °C), which allows for the growth of metal oxide films at relatively high temperatures (up to 150 °C) without compromising the mechanical and optical stability of the substrate.

Thin films of F-doped SnO₂ (SnO₂:F, or abbreviated as FTO) are commonly used as transparent conducting electrodes for many devices such as solar cells and flat panel displays because SnO₂ films are inexpensive as well as chemically and thermally stable [6], [7], [8]. There are numerous deposition techniques used to grow FTO films on glass substrates including chemical vapor deposition [9], [10], spray pyrolysis [11], [12], thermal evaporation [13], sol–gel [14] and sputtering [15], [16]. However, there have been very few reports of deposition of FTO films on flexible polymer substrates. Previously, high quality FTO films have been grown on glass substrates using pulsed laser deposition (PLD) and the deposition conditions were optimized for highly conductive films [17]. In general, the films grown by PLD crystallize at relatively lower substrate temperatures compared to those grown by other physical vapor deposition techniques due to the high kinetic energies (> 1 eV) of the ejected species in the laser-produced plasma [18]. Thus, TCO films grown on plastic substrates by PLD can achieve relatively lower resistivity compared to those grown by other techniques at low temperatures. In this paper, we report our study on the electrical and optical properties of FTO films deposited on flexible PES substrates by PLD without a postdeposition anneal. Film properties were measured as a function of the substrate temperature and oxygen pressure during deposition.

Section snippets

Experimental details

FTO thin films were deposited on PES substrates (200 μm, Sumilite®, courtesy of Sumitomo Bakelite) using a KrF excimer laser (Lambda Physik LPX 305, 248 nm, 30 ns full width half maximum). Details of the film deposition conditions are presented elsewhere [17], [19]. Briefly, the laser beam, operating at 10 Hz, was focused by a 50-cm focal length lens through a quartz window into a vacuum chamber onto a rotating target at a 45° angle of incidence. The energy density of the laser beam at the target

Results and discussion

Fig. 1 shows a plot of the electrical resistivity as a function of oxygen deposition pressure (Po₂) for FTO films grown on PES substrates at three different substrate temperatures, (T_s = 25, 100 and 150 °C). For the films grown at 25 °C, the resistivity is highly responsive to the oxygen deposition pressure. Low resistivity values can be obtained only in a small range of oxygen deposition pressures between 7.4 and 8 Pa. As the T_s increases, the resistivity of the FTO films becomes less dependent on

Conclusion

In summary, SnO₂:F thin films have been deposited on PES plastic substrates by pulsed laser deposition. FTO films (300 nm thick) grown at T_s = 25 °C and 8 Pa of oxygen exhibited a resistivity of 6.9 × 10^− 3 Ω-cm and an average transmittance of 80% in the visible range with an optical band-gap of 3.88 eV. For a 320 nm thick FTO film deposited at T_s = 150 °C and 8 Pa of oxygen, an electrical resistivity of 1.3 × 10^− 3 Ω-cm, an average transmittance of 89% in the visible range, and an optical band-gap of 3.96 eV were

Acknowledgments

This work was supported by the Office of Naval Research (ONR).

References (23)

B. O'Regan et al.
Nature
(1991)
B. Thangaraju
Thin Solid Films
(2002)
D. Das et al.
Thin Solid Films
(1987)
O. Varghese et al.
J. Appl. Phys.
(2000)
R. Eason
Pulsed Laser Deposition of Thin Films, 239–260
(2006)
G.P. Kushto et al.
Appl. Phys. Lett.
(2005)
H.-K. Kim et al.
Appl. Phys. Lett.
(2007)
H. Kim et al.
Appl. Phys. Lett.
(2001)
G. Gu et al.
Optics Lett.
(1997)
G. Gustafsson et al.
Nature
(1992)
Synth. Metals
(1993)

T. Fukano et al.

J. Appl. Phys.

(2005)

Cited by (20)

Highly transparent conductive F-doped SnO<inf>2</inf> films prepared on polymer substrate by radio frequency reactive magnetron sputtering
2022, Thin Solid Films
Citation Excerpt :
It has been widely used in solar cells [1], organic light emitting diodes [2], low emission glass [3] and other fields [4,5]. In general, FTO films can be prepared on the surface of rigid glass substrates by methods such as sol-gel [6], chemical vapor deposition (CVD) [7–9], spray pyrolysis [10–13], magnetron sputtering [14] and pulsed laser deposition (PLD) [15,16]. However, with the development of wearable devices and curved display screens, it is necessary to prepare FTO films on surface of flexible substrates.
F-doped SnO₂ (FTO) films have excellent optical-electrical properties and have been widely used, but the preparation of FTO film on the surface of flexible polymer substrate by magnetron sputtering has not been reported. In this study, Sn-SnF₂ mixture was used as the target, and then the FTO film was prepared by reactive magnetron sputtering on the surface of the polyethylene terephthalate substrate at substrate temperature of room temperature, 60 and 100 °C. The results show that highly transparent conductive FTO films can be obtained by optimizing the O₂ flow rate at each substrate temperature. In general, as the substrate temperature increases, the optimized range of O₂ flow rate shifts to the direction of lower O₂ flow rate and the corresponding transparent conductive properties are slightly enhanced. Currently, the films show sheet resistance of 209–423 Ω/sq., transmittance in visible-light range of 84.4–90.3% and figure of merit of 5.09–12.9 × 10^–4 Ω^–1, which is comparable with those of previously reported SnO₂-based films. At the same time, the films have better bending performance at the bending radius of 20 mm, but they have poor damp-heat stability. In addition, the influence of substrate temperature and O₂ flow rate on structure (thickness, crystallinity and surface roughness) and energy gap of the films is also investigated.
Effect of Target Morphology on Morphological, Optical and Electrical Properties of FTO Thin Film Deposited by Pulsed Laser Deposition for MAPbBr<inf>3</inf> Perovskite Solar Cell
2021, Surfaces and Interfaces
Three targets for pulsed laser deposition (PLD), having different morphology were used to deposit FTO thin film on three glass substrates to fabricate three FTO/SnO₂/perovskite/spiro-OMeTAD/Silver perovskite solar cells. 5000 pulses of KrF excimer laser (348 nm, 50 Hz, 10 ns), having 1.2J energy were used to ablate FTO targets. Absorption, transmittance, resistivity, charge mobility and bandgap of deposited FTO thin film were observed to be dependent on target morphology. SEM micrographs of FTO layer reviled significant changes in smoothness and compactness of the thin films due to change in target morphology. Random three dimensional structures of FTO are observed on FTO layer by AFM. These random structures played very important role in improvement of solar cell performance. Maximum efficiency of 12.5% was achieved for proposed FTO/SnO₂/perovskite/spiro-OMeTAD/Silver solar cell.
Effect of oxygen vacancies on photoluminescence and electrical properties of (2 0 0) oriented fluorine-doped SnO<inf>2</inf> films
2019, Materials Science and Engineering: B
Polycrystalline fluorine-doped SnO₂ (FTO) films have been deposited on glass substrate by ultrasonic spray pyrolysis at 500 °C. The effects of F/Sn atom ratio in precursor solution on the structural, optical and electrical properties of FTO films have been investigated. The XRD results indicate that all films consist of polycrystalline cassiterite tetragonal particles with obviously (2 0 0) preferred orientations. SEM images present that all the films consist of uniform, clear and sharp edge shaped pyramid particles. The film prepared with F/Sn = 1 in precursor solution possess the UV–vis transmittance about 70%, the lowest resistivity of 3.50 × 10⁻⁴ Ω·cm and the minimum value of [O_I]/[O_II] (oxygen atom at different condition determined by XPS) of 0.99 which can be contributed to the improved conductivity. In the room temperature photoluminescence (PL) spectra, there are four intense emission bands corresponding to defect of different oxygen vacancy states in FTO films.
Physical properties of rare earth metal (Gd<sup>3+</sup>) doped SnO<inf>2</inf> thin films prepared by simplified spray pyrolysis technique using nebulizer
2019, Optik
Citation Excerpt :
As the ionic radius of Gd3+ (0.94 Å) is slightly larger than the ionic radius of Sn4+ (0.69 Ǻ), the Sn4+ ions could be easily substituted by Gd ions without provoking any significant variation in lattice parameters and crystalline nature. Undoped and doped SnO2 thin films can be synthesized by numerous methods such as dip coating [16], pulsed laser deposition [17], DC reactive sputtering [18], Spray pyrolysis [19], and chemical vapour deposition [20], etc. Amongst, nebulizer used spray pyrolysis (NSP) is one of the unique technique possess similar advantages of spray pyrolysis technique in addition to this, it is a low-cost and non-vacuum technique for wider range of applications and also yield good quality film with minimum precursor volume, nevertheless the rate of deposition and film thickness could be easily controlled for large area deposition that are desirable for industrial applications.
Pristine and rare earth (i.e) gadolinium doped SnO₂ thin films have been coated on micro-glass substrates with different Gd doping concentration at constant temperature 450 °C by simplified spray pyrolysis technique using nebulizer unit. The variation of doping concentration from 0 to 6 wt.% in the steps of 2 wt.%. Structural, optical, electrical, morphological and photoluminescence properties had been examined as a function of gadolinium doping level. The X-ray diffraction study exposed that all the prepared pristine and Gd doped SnO₂ thin films are (110) preferred orientation with tetragonal crystal structure. The observed transmittance of Gd:SnO₂ thin film varies between 91–80% in the visible regions. The estimated optical band gap value was initially decreased from 3.79 to 3.74 eV and then it was slightly increased as 3.77 eV with respect to the increase of Gd doping concentrations. Homogeneous surface morphology with polyhedrons like grains without cracks for all the prepared samples was illustrated by SEM studies. EDS spectra confirms that the existence of Sn, O and Gd elements in the 4% Gd doped SnO₂ thin film surface. PL results indicates that three emission bands such as ultra violet, blue and green emission peaks at the wavelength of 360, 493 and 519 nm respectively, for all the prepared Gd:SnO₂ films. Minimum resistivity (ρ) 7.14 × 10⁻⁴ Ω-cm with activation energy (E_a) 0.05 eV, maximum carrier concentration (n) 2.36 × 10²⁰  cm⁻³ and figure of merit (ϕ) 52.96 × 10⁻³ (Ω/sq)⁻¹ were obtained for 4% Gd doped SnO₂ thin film using Hall effect measurements.
The optical and electrical properties of ZnO:Al thin films deposited at low temperatures by RF magnetron sputtering
2018, Ceramics International
Several ZnO:Al thin films have been successfully deposited on glass substrates at different substrate temperatures by RF (radio frequency) magnetron sputtering method. Effects of the substrate temperatures on the optical and electrical properties of these ZnO:Al thin films were investigated. The UV–VIS–NIR spectra of the ZnO:Al thin films revealed that the average optical transmittances in the visible range are very high, up to 88%. X-ray diffraction results showed that crystallization of these films was improved at higher substrate temperature. The band gaps of ZnO:Al thin films deposited at 25 ℃, 150 ℃, 200 ℃, and 250 ℃ are 3.59 eV, 3.55 eV, 3.53 eV, and 3.48 eV, respectively. The Hall-effect measurement demonstrated that the electrical resistivity of the films decreased with the increase of the substrate temperature and the electrical resistivity reached 1.990×10⁻³ Ω cm at 250 ℃.
Structural, electrical, and optical properties of F-doped SnO or SnO<inf>2</inf> films prepared by RF reactive magnetron sputtering at different substrate temperatures and O<inf>2</inf> fluxes
2017, Journal of Alloys and Compounds
The effects of substrate temperature and O₂ flux on structural, electrical, and optical properties of the films prepared by RF reactive magnetron sputtering with SnF₂-Sn target have been investigated by X-ray diffraction (XRD), Hall effect measurements, optical transmission spectra, and photoluminescence (PL) spectra. It is found that F-doped SnO films are produced and exhibit amorphous structure at lower O₂ fluxes for substrate temperature of RT-300 °C, and window of O₂ flux for depositing the films is reduced as substrate temperature increases. F-doped SnO₂ (FTO) films are formed at higher O₂ fluxes, and they exhibit amorphous state at substrate temperature of RT but crystalline state at substrate temperature of 150 and 300 °C. Furthermore, increasing O₂ flux or substrate temperature can promote crystallinity and affect growth orientation for crystalline FTO films. Highly conductive FTO films can be obtained only at suitable O₂ fluxes and higher substrate temperatures (150 and 300 °C), at which the films keep crystalline SnO₂ structure along with high amount of oxygen vacancy (V_O). Increasing substrate temperature from 150 to 300 °C, the resistivity of FTO films cannot be further decreased due to the out-diffusion of F although the film crystallinity improves. Compared with F-doped SnO films, the average transmittance in visible light range and band gap (E_g) of FTO films obviously increase. The FTO films show similar shaped PL spectra that can be attributed to substitutional fluorine (F_O) and V_O defects in the films, and PL emission intensity increases with increasing O₂ flux or substrate temperature due to the improvement of film crystallinity. It is observed that the PL emission characteristics (shape and intensity) of F-doped SnO film are obviously different from FTO films and they have great changes as substrate temperature from RT to 150 °C, but corresponding mechanisms need to further clarify.

View all citing articles on Scopus

View full text

Published by Elsevier B.V.

F-doped SnO2 thin films grown on flexible substrates at low temperatures by pulsed laser deposition

Abstract

Introduction

Section snippets

Experimental details

Results and discussion

Conclusion

Acknowledgments

Nature

Thin Solid Films

Thin Solid Films

J. Appl. Phys.

Appl. Phys. Lett.

Appl. Phys. Lett.

Appl. Phys. Lett.

Optics Lett.

Nature

Synth. Metals

J. Appl. Phys.

F-doped SnO₂ thin films grown on flexible substrates at low temperatures by pulsed laser deposition