Tailoring the properties of nebulizer spray pyrolysis coated FTO thin films through rare earth element terbium for optoelectronic applications

Journal Pre-proof Tailoring the properties of nebulizer spray pyrolysis coated FTO thin films through rare earth element terbium for optoelectronic applications R. Thomas, T. Mathavan, M.A. Jothirajan, V. Ganesh, Mohd Shkir, I.S. Yahia, H.Y. Zahran, S. AlFaify PII:

S0921-4526(19)30796-3

DOI:

https://doi.org/10.1016/j.physb.2019.411916

Reference:

PHYSB 411916

To appear in:

Physica B: Physics of Condensed Matter

Received Date: 15 September 2019 Revised Date:

27 November 2019

Accepted Date: 28 November 2019

Please cite this article as: R. Thomas, T. Mathavan, M.A. Jothirajan, V. Ganesh, M. Shkir, I.S. Yahia, H.Y. Zahran, S. AlFaify, Tailoring the properties of nebulizer spray pyrolysis coated FTO thin films through rare earth element terbium for optoelectronic applications, Physica B: Physics of Condensed Matter (2019), doi: https://doi.org/10.1016/j.physb.2019.411916. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Tailoring the properties of nebulizer spray pyrolysis coated FTO thin films through rare

earth element Terbium for optoelectronic applications R. Thomas1*, T. Mathavan2, M.A. Jothirajan1, V. Ganesh3, Mohd. Shkir3, I. S. Yahia3,4, H.Y. Zahran3,4, S. AlFaify3 1

PG and Research Department of Physics, Arul Anandar College, Karumathur, Madurai, India 2 PG and Research Department of Physics, N.M.S.S.V.N College, Madurai, India 3 Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia 4 Metallurgical Lab., Nanoscience Laboratory for Environmental and Bio-medical Applications (NLEBA), Semiconductor Lab., Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt.

Corresponding Author R. Thomas PG and Research Department of Physics Arul Anandar College, Karumathur Madurai, India Email: [email protected]

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Abstract

In optoelectronic device, transparent conducting oxide (TCO) acts as an electrode. In this work the rare earth element terbium (Tb) was doped with FTO by simple and inexpensive nebulizer spray pyrolysis (NSP) technique. Structural, optical end electrical properties were investigated for all the synthesized films. X-Ray diffraction (XRD) analysis confirmed that all the prepared films exhibited polycrystalline nature with tetragonal crystal structure and size of the crystalline reduced with increasing Tb concentration. Raman active doubly degenerate mode (Eg), IR active mode (Eu) and vibration mode (B2g) were found from Raman analysis. Atomic force microscope (AFM) images visualises the granular sized particle and the roughness of the Tb doped films. Elememental analysis spectrum exhibited Sn, O, F and Tb elements for 1.5 wt.% Tb doped thin film. Photoluminacence (PL) analysis revealed that UV, blue and green (visible) emission and UV emission intensity was reduced systamatically for the doped films. From UVVis analysis, highest optical transmittace were deduced for 1.5 wt.% Tb doped film. Reflectance, absorbance and band gap also been observed for the prepared films. Refractive index, extinction coefficient and dielectric constant values were decreased with increasing Tb doping concentration. High electrical conductivity and carrier concentration were measured using four probe hall effect system. Figure of merit value for the 1.5 wt.% Tb doped film is 1.5x10-3 Ω-1 and therefore the prepared film suits to be an electrode in optoelectronic devices. Keywords: Optoelectronic device, Nebulizer spray, Raman analysis, Figure of merit, photoluminacence, carrier concentration.

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1. INTRODUCTION In recent years, transparent conducting oxide (TCO) matetrials attracted the attention of the researchers since it has good electrical conductivity and high optical transparency. The TCO can also been articulated as wide band gap semiconductor metal oxide. The TCO film has good charge transport capacities and wider transparancy window so it has been utilized as an electrode in optoelectronic devices such as heat reflectors, energy efficient windows, photovoltaics, solar cells, thin film transistor liquid crystal displays, quantum dot-sensitized solar cells, low-e windows, touch screens, automobile window deicing and defogging and flat pannel displays [1,2]. The current research is mainly concentrated to enhance the transparency more than 75% and conductivityofabout 103 Ω cm-1. Various TCO films like Indium doped tin oxide (ITO), Fluorine doped tin oxide (FTO), Antimony doped tin oxide (ATO) and Aluminium doped zinc oxide (AZO) were used as an electrode. In these materials ITO and FTO are the predominant material used as an electrode in optoelectronic devices and photovoltaic cells [3]. In solar cells, ITO has been used as an anode material since it has high conductivity and transparency but the main drawbacks are less stable in hydrogen plasma, high cost, less resource and toxic in nature [4,5]. FTO can be the capable TCO material that can full fill all the limitations such stability, nontoxic, wide band gap, higher carrier density and high transmittance in visible region [6]. FTO film is obtained, where the oxygen ions replaced by the fluorine ions while doping fluorine ions into the SnO2 lattice and it can act as free electron. FTO is a fine electrode for solar cells and optoelectronic devices because it promotes the flow of electrons [7] and this work is mainly focusing on optoelectronic application.

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In order to reduce the resistivity (i.e. increasing the conductivity) we could add some external elements (impurities) like manganese, iron, copper and rare earth metals to the host material (FTO) [8]. However, rare earth element can still enhance the electrical conductivity and optical transmittance when doped with the host material (FTO). There are only limited researches existing for rare earth doping with different semiconducting oxide materials. Recently, Habib Elhouich et al. [9] reported the Tb and Sb doped SnO2 thin films for increasing electrical performance through spin coating method. They observed that the reduction of film series resistance by the inclusion of Tb element with the SnO2:Sb structure due to interfacial effects and crystalline quality. Fang et al. [10] also reported the drastic reduction of electrical resistivity for ZnO thin film when doping with Tb concentration and keeping a good optical transparency due to the Tb element act as donor by supplying a single free electron to the ZnO. It is known that the Tb is a 4f electronic states considering as promising luminescence behavior and to attain a high electrical conductivity in the semiconducting films. Based on the above reports, for the first time Tb element has been chosen to dope with FTO films for increasing optical and electrical properties. Different physical and chemical methods have been subjected to prepare TCO thin films like atmospheric pressure chemical vapor deposition, sol-gel, sputtering, ultrasonic spray pyrolysis, pulsed laser deposition, spray pyrolysis, and nebulizer spray pyrolysis [11-12]. The novel nebulizer spray pyrolysis technique is used to fabricate FTO and Tb doped FTO thin films since it has some special features like efficient, simple and inexpensive and the particle size can be controlled over large area deposition. The nebulizer spray deposited FTO:Tb films using various Tb doping concentrations (0, 0.5, 1.0 and 1.5 wt.%) and their key properties are reported in this investigation. This would be the first literature on rare earth Tb doped FTO thin film by

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nebulized spray pyrolysis method. The structural, optical, morphological and electrical characteristics have been scientifically investigated for the pure and Tb doped FTO films and the results are compared with previous reports. 2. MATERIALS AND METHODS 2.1 Chemicals The precursors used to prepare pure and Tb doped FTO films were of analytical grade pure chemicals which is used without further purification. Stannous chloride anhydride (SnCl2) was received from alfa aesar, Terbium (III) nitrate hydrate [Tb(NO3).xH2O] and Ammonium fluoride [NH4F] obtained from Sigma-Aldrich, USA. Propan-2-ol and hydrogen chloride (HCl) were purchased from SRL, India. The silica glass substrates of 75mm×25mm×1.2 mm was supplied by Blue Star, Mumbai and doubly deionized water used for entire thin film preparation process. 2.2 FTO and Tb doped FTO thin films Preparation Simple and novel spray pyrolysis technique was used to fabricate pure FTO and different doping concentration of Tb doped FTO thin films. First and foremost, 0.07 M of stannous chloride anhydrate and 0.03 M of ammonium fluoride was dissolved in 10 ml of propan-2-ol and deionized water mixed solution (ie.7.5 ml +2.5 ml) respectively. 0.5 to 1.5 wt.% of terbium nitrate hydrate is mixed with the above prepared solution and 3 drops of HCl has been added to get a clear solution which ease to spray. Before starting the deposition process, the glass substrates were well cleaned using soap solution, chromic acid and acetone and the cleaned glass substrates placed on hot plate which was maintained at 450 ºC using PID controller. The solution pressure (normal compressed air) was maintained at 1.5 Pascal which convert droplets into

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fumes and the fumes were easily struck on the glass plate. Spray gun was moved gently and sprayed the solution over the hot substrate to get very smooth and transparent films. 2.3 Characterization Tools Pristine and terbium doped FTO films were investigated by X-ray diffractometer (Japan Shimadzu X-600) over a range of 20° to 70° using CuKα (λ = 1.5406 Å) radiation with 0.02 steps. The phase transformation and vibrational mode of the pure and doped films were analyzed by Laser Raman Spectrometer DXR THERMO SCIENTIFIC using λexc= 532 nm at 5 mW laser power. Surface roughness of the films were visualized by atomic force microscope (AFM) NTMDT, NEXT spectrum instrument, Russia. EDAX instrument of JSM 7600 F, JEOL, Japan, was used to show the existing elements of the prepared film. Photoluminescence spectrum was observed by Thermo Fisher Scientific with 325 nm laser as a light source for excitation. optical properties of the films were measured by UV–Vis–NIR spectrophotometer (UV-3600 Shimadzu instrument). Stylus profilo meter was used to measure the thickness of the prepared thin films. Electrical properties of the prepared films were analyzed by Hall Effect (four-point probe) instrument with van der Pauw configuration. 3. RESULTS AND DISCUSSION 3.1 Structural analysis X-ray diffraction (XRD) analysis was intended to examine the structural properties of pristine and Tb doped FTO films. Fig.1 shows the high intense (predominant) peak along (110) plane and other (101), (200), (211), (220), (310) and (301) peaks with less intensity for pure at different 2θ values. XRD analysis clearly mentioned that all the prepared films exhibit polycrystalline nature with tetragonal crystal structure which corresponds to SnO2 crystal structure and was confirmed with JCPDS file number 88-0287. No other peaks corresponding to

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Tb, F- and its oxides were observed which implies that Tb ions are placed into SnO2 sites [13]. High intense preferentially oriented peak (110) was obtained at (2θ) 26.8º. Intensity of the predominant peak (110) was reduced systematically and could observe the change of orientation from (110) plane to (211) plane with increasing doping concentration of Tb from 0 wt.% to 1.5 wt.% which is shown in fig.1. XRD pattern also confirmed that the change of predominant orientation from (110) plane to (211) by the inclusion of Tb element within the host material. The growth orientation change may occur due to the grain reorientation effect on increasing Tb doping. This reorientation can be simplified that the SnO2 grains possess minimum interfacial energy [14]; when grown the FTO:Tb system. Same tendency was previously observed by Turgut et.al [15] for Pr doped SnO2 thin films. The orientation change towards higher 2θ values may occur due to impact of strain level on the crystal lattice [16]. Crystallite size of the pure and Tb doped FTO films were calculated using the following equation [17].

D=

0.9λ β cosθ

(1)

where, D, and λ are the average grain size and wave length of Cu Kα radiation (λ=1.5418 Å). β and θ are the (110) peak FWHM value and Bragg diffraction angle. The size of the crystallite was decreased gradually with increasing wt.% of Tb for the reason that the Tb ions are introduced into host lattice and this can disturb the grain orientation of host lattice owing to lattice imperfection. When Tb3+ ion is added with SnO2 host lattice, some oxygen atoms are lost causing a small adjustment in position of Sn atoms, which leads to the formation of SnO phase. As the 3+ charged Tb is doped with 4+ charged Sn, a charge compensation is taken place in the formation of SnO2 by reducing Sn4+ to Sn2+, as 3Sn4+= 3Sn2+ + 2Tb

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3+

[18]. Another reason is

that the Tb ions placed into the host lattice, which may create some oxygen defects. These defects can produce the small Tb-O bond; as a result, the reduced crystallite size was observed [19]. The range of crystalline defects of the pure and Tb doped FTO films was calculated from dislocation density (δ) and micro strain (ε) values using the following relations [17].

ε= δ =

β Cot θ

( 2)

4

1 D2

(3)

where, D and β are the average crystallite size and FWHM. The δ and ε are dislocation density and micro strain. δ and ε values have been calculated for all the prepared films, and the calculated values are given in table 1. As per the calculated results, the dislocation density and micro strain values were increased and this may due to decrease in crystallite size. Lattice constant ‘a’, ‘c’ and cell volume ‘V’ was calculated for all the prepared films by using following relations [20].

1 h2 + k 2 l 2 = + 2 d2 a2 c

(4)

V = a 2c

(5)

here, d is the inter planer distance, h, k, and l are the miller indices of the given plane. Table 1 shows the values of ‘a’, ‘c’ and ‘V’ for the pure and different (0.5, 1, 1.5 wt.%) doping concentration of Tb. Lattice constant and cell volume values were increased with the increase of Tb doping concentration, this may arise due to the ionic radius of Tb and tin ions.

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3.2 Raman investigation Raman scattering is a versatile technique to investigate the phase transformation, vibrational modes and the structural defects. Raman spectrum of pristine and Tb doped films in the wave number range from 200 to 900 cm-1 was shown in fig.2. Since SnO2 being a tetragonal crystal structure, it possesses the space group D4h. The tetragonal crystal structured SnO2 has 18 types of lattice mode of vibrations and they have been articulated as A1g + A2g + A2u + B1g + B2g + 2Bu + Eg + 3Eu. From these A1g, B1g, B2g and Eg are mentioned as Raman active modes; A2u represents transverse-optical vibration (TO) mode and Eu represents longitudinal-optical vibration (LO) mode [21]. A high intense mode and two broad modes are observed at 565 cm-1 ,469 cm-1 and 788 cm-1 respectively. The first peak at 469 cm-1 corresponds to raman active Eg doubly degenerate mode arising from the vibration mode of oxide ions. This mode was called as classical mode and it confirms SnO2 as a tetragonal structure [22]. IR active Eu mode is observed at 565 cm-1 and this peak may arise from oxygen vacancy. Third peak at 788 cm-1 which endorsed to B2g vibrational mode and it was associated with asymmetric stretching on Sn-O bonds [20]. Intensity of the peaks were reduced with increasing doping concentration of Tb which may arises from the Tb ions are placed into host lattice sites. 3.3 AFM Analysis Atomic Force microscope (AFM) is an important tool to visualize the grain shape, growth and the quality of the films especially the surface roughness. Fig. 3 (a,b) shows 2D & 3D AFM images of 0, 0.5, 1.0, 1.5 wt.% Tb doped FTO thin films. Granular sized and tightly packed particles without any annulled can be visualised from all the prepared films. When rare earth element Tb at different concetration doped with FTO, the size of the particles gradually

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decreases, which may arises from the Tb ions are placed inside the SnO2 lattice. Roughness of the films gradually decreases, which denotes that the smoothness of the film increased gradually and this can enhance the transparency of the film [23]. The surface roughness and grain size values are given in table 2. Energy dispersive X-ray spectroscopy is an important tool to identify the existing chemical element of the prepared thin film. The EDAX spectrum of 1.5 wt.% Tb doped FTO thin film is shown in Fig.4. From the figure we can recognize the existing elements like terbium, oxygen, fluorine, and tin. The trace of unidentified peaks of Si and Ca at 1.8 KeV and 3.8 KeV are possibly instigated from the glass substrate. 3.4 Photoluminescence Ananlysis The optical charecteristics can be investigated throughly for all the semiconductors and metal oxide thin films using a versatile tool of room temperature photoluminacence (PL). PL spectra of pristine and Tb doped FTO thin film at excitation of 325 nm is shown in fig.5. In Ultra violet region (368 nm), the strong intense emission peak was observed and the corresponding energy of 3.37 eV was noted. With high intense peak two more broad emission peaks were observed in visible region at 467 nm and 499 nm. In general, the high intense peak was noticed in UV region due to radioactive recombination of free excitons or band to band transition or donar or acceptor bound excitons. Due to the defects formed by premediated doping the broad peaks were noticed in visible region. The emission peak was observed at 368 nm corresponds to the recombination of electrons and holes in SnO2 structure [24]. The strong intensity peak at UV region has been reduced as well the slight shift were noticed with increasing doping concentration of Tb which ascribed as decreasing crystalline size of tin oxide paricles. Blue emission peak was observed due to the transition between Sn ions and the valance band at 467

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nm. Green emission peak at 499 nm was observed due to recombination of delocalized electron in conduction band with the ionized oxygen vacancies [25]. The emissions related to terbium are not observed for all the films. 3.5 Optical Properties 3.5.1 Thickness measurement Thickness is one of the key factor for TCO thin films since it decides the film properties. Film thickness was measuredby stylus-profilometer and the values were given in table 3. From the observed values, we can noticed that the thickness of the films were systamatically decreases from 630 nm to 500 nm for pure FTO to 1.5 wt.% of Tb doped FTO thin film, 3.5.2. Optical Analysis Optical parameter such as transmittance, absorbance, reflectance and the band gap were investigated in the range from 300 nm to 1200 nm for pure and Tb doped thin films is shown in fig 6 (a-d). The transmittance for pristine FTO shows 61% and it could be increased systamatically with increasing doping concentration of Tb at different wt. % (0.5, 1, & 1.5 wt.%.) is shown in fig.6 (a). The maximum transparency was observed for 1.5 wt.% doped FTO film which is about 79%. The crystalline defect occurred during Tb ions doped with the host material and this defect can allow more light to pass [26]. By increasing the doping concentration of Tb, the thickness of the films gradually decreases and the smoothness of the surface increases, which improve the transmittance.Since the prepared film has higher tansparency, it can be employed as an electrode in optoelectronic devices. Fig 6 (b) shows the absorbance spectrum of prepared thin films. From the spectrum, particularly in visible region the absorbance decreases with increasing doping concentration and in high frequency range the absorption remains invariable, which implies that the prepared films consistently allow the visible light for

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controlling the phonon scattering mechanism. Fig 6 (c) shows the minimum reflectance occurred in order of increasing doping concentration which attributes the behavior of surface uniformity. In toting up, the doped films exhibit low reflectance in both visible and IR region with an average of 11% and this confirms the minimum transparency loss. TCO material must possess minimum reflectance and the prepared film (1.5 wt.% Tb) satisfies the minimum reflectance standard which can be utilized for optoelectronic applications. The optical band gap (Eg) for the Tb doped FTO thin films were measured by Tauc model and parabolic bands

αhν = B(hν − E g )n

(6)

where ‘α’, ‘hν’ are the absorption coefficient and incident photon energy respectively. Fig.6 (d) shows the ሺߙℎߥ ሻ2 vs hν plots of different Tb doped FTO films. The direct energy band gap values, 3.88 eV, 3.94 eV, 3.97 eV and 3.99 eV are measured for 0, 0.5, 1.0 and 1.5 wt.% Tb doped FTO thin films. These observed band gap values are compared with the standard value of 3.67 eV. It is in good agreement with the direct allowed band gap values reported by sputtering method [27]. The band gap values gradually increase with gradual increasing of doping concentration and this may arise from the blue shift phenomena in the absorption or transmittance spectrum [28]. The present work, the broad band gap was observed with respect to the doping concentration and this may suit for TCO applications. 3.5.3 Optical constants In optoelectronic device fabrication process, the most considering factor was the optical constants (refractive index (n) and extinction coefficient (k)). n and k are linked with intrinsic material properties of electronic polarization and the local field inside the optical material. The following equations are used to find ‘n’ and ‘k’ of the material [29].

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1+ R  n= − 1− R  k=

4R

(1 − R )2

−k2

aλ 4π

(7)

(8)

R, λ represent the reflectance and the wavelength respectively. Fig.7 (a) shows the refractive index of pure and Tb doped FTO thin films. The average optical parameter values in the visible region (400 nm to 800 nm) is calculated and shown in table 3. The refractive index values gradually decrease with respect to the increasing doping concentration of Tb. The reduced value of refractive index is associated with the fundamental band gap absorption and at the same time the refractive index varies inversely to the transmittance [30]. The low refractive index is obtained for 1.5 wt.% Tb doped FTO film, since it has high transmittance than the other films. The extinction coefficient values gradually decrease as increasing the doping concentration. This may be due to the decreasing the film thickness and surface roughness of the films [31] which is shown in fig.7 (b) and the values are given in table 3. The porosity of the material can be calculated from the refractive index value using the expression [29].

  n 2 − 1  Porosity = 1 −  2  ×100%   nd − 1 

(9)

nd and n are the refractive index of SnO2 tetragonal crystal and the prepared FTO thin films. The fig.8 (a) is drawn between the change of porosity and wavelength. From the figure we can observe that the porosity of the film increases with respect to the doping concentration and this may be from the gradual decrement of packing density of the films. Using Clausius- Mossati local field polarizability model [29] the electronic polarizability is calculated for the prepared films. The relation is 13

 n 2 − 1  Lρ  2  = αp  n + 2  3M

(10)

here, M is the molecular weight, L is the Avogadro’s number and ρ is the density of the material. The photon energy depends on the ratio of (n2-1) and (n2+2). From the fig.8 (b) the electronic polarizability decreases with increasing doping concentration and the calculated values are also acknowledging the same (i.e., (1.64×10-22 to 1.26×10-22). Electronic polarizability values decreases may be due to the decreasing the electron volume and the volume occupied by the electrons. 3.5.4 Dielectric Constants In general, the dielectric constant is represented as ε * = ε 1 + i ε 2 where ε1 and ε2 are the real and imaginary part respectively. ε1 value reveals that dispersion of electromagnetic waves through the dielectric medium and ε2 shows that the energy was absorbed from electric field due to dipole motion. The ε1 and ε2 values can be calculated from the below relations [29].

ε1 = n2 − k 2

(11)

ε 2 = 2 nk

(12)

Fig. 9 (a & b) represents the real and imaginary part of the dielectric constant as a function of incident photon wavelength for various doping concentration of Tb. From the figure, the real and imaginary part of the dielectric constants are gradually decreases with increasing Tb doping concentration. This decrement is strongly based the refractive index and the extinction coefficient values. Since the value of ‘n’ and ‘k’ being decreass, the real and imaginary part of the dielectric constants also decreased. The observed values ε1 and ε2 (shown in table 3) are small then the electromagnetic waves can move more easier and faster within the Tb doped films [29].

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The conductance of charge carriers in the material due to optical excitation is meant for optical conductivity. The optical conductivity ( σ op ) of the material can be calculated using the below relation [29].

σ op =

α nc 4π

(13)

where, n and α are the refractive index and absorption coefficient; c is the velocity of light in vacuum. Fig. 10 shows the optical conductivity by means of photon energy. σ op gradually decreases from pristine to 1.5 wt.% Tb doped FTO film. The 1.5 wt.% Tb-doped FTO film has less optical conductivity than the other films, which shows that the photon energy absorption was very well limited by the electrons [32]. The optical conductivity values are given in table3. Energy loss function plays an inevitable role in optoelectronic devices. Analyzing the energy loss function of the prepared material is most important, since the material used as an electrode in optoelectronic devices. The fast-moving electrons were subjected into the prepared material; it loses its energy while moving into the material. According to the loss it is discriminated surface energy loss function as well as volume energy loss function. Surface energy loss function (SELF) which express the transition of electron only on the surface of the material whereas the volume energy loss function (VELF) which express the transition of electron in the bulk materials. Both SELF and VELF can be calculated from the real and imaginary part of the dielectric constant values [29].

VELF =

SELF =

ε2

(14)

ε + ε 22 2 1

[(ε

ε2

2 1 + 1) + ε 2 2

15

]

(15)

The plot was drawn between energy loss function and the photon energy which is shown in fig. 11 (a & b). From the observed values, energy losses of the electrons are merely same when moving through the bulk material and through the surface of the material. And also, we can notice the VELF is greater than the SELF. The VELF and SELF are found decreased with increasing doping concentration of Tb, which may be due to decreasing of electron energy loss. 3.6 Electrical Analysis by Hall Effect The resistivity (ρ), mobility (µ) and carrier concentration (n) are the important parameters of TCO materials. These electrical parameters were measured by room temperature. Carrier concentration and Hall mobility charge carriers can be calculated from n e = 1 R H and e





1  where e, RH and ρ are the electron charge, Hall coefficient and resistivity of the µ e =   n e eρ 

prepared films [33]. Resistivity decreases; and the carrier concentration increases systematically from pure to 1.5 wt.% Tb doped films. Especially for 1.5 wt.% Tb doped FTO thin film it attained very stumpy electrical resistivity and higher carrier concentration since the Tb has various oxidation states Tb2+, Tb3+ Tb4+ [34] like Pr and Sb elements. For increasing Tb doping level, the Tb ions can be substituted with SnO2 lattice which creates oxygen vacancies and thus these vacancies can enhance the electrical conductivity. 1.5 wt.% Tb doped FTO film shows highest carrier mobility since it has very less grain boundary scattering [35]. The electrical parameters graph is shown in fig.12. 3.7 Optoelectronic study by the figure of merit Figure of merit (Φ) is an essential parameter to analyze optoelectronic property of the prepared TCO material. In Haack’s relation, sheet resistance and optical transmittance values are used to calculate the figure of merit (FOM) [36]. 16

(φ ) = T

10

(16)

R sh

T and Rsh are the average transmittance in the visible range (400-800 nm) and sheet resistance value respectively. Sheet resistance and transmittance are the most important factors, which decide the prepared film to be an electrode in an optoelectronic device. In present study, the figure of merit values increases gradually from 1.1x10-4 Ω-1 to 1.5×10-3 Ω-1 by increasing the doping concentration of Tb from 0 wt.% to 1.5 wt.%. As a result, 1.5 wt.% Tb doped FTO film can be exploited as an anode material in optoelectronic device. Generally, high conducting, high transmittance and high FOM thin film is acting as an electrode in optoelectronic device. In this case, 1.5 wt.% Tb doped FTO film accomplished all the conditions which pointed out above; it is perfectly opt for being an electrode in optoelectronic device. Low sheet resistance, high transmittance and conducting films are prepared using effective nebulized spray pyrolysis technique for the first time with the doping element Tb. The obtained results are compared with the earlier reports and outlined in table 4. The prepared FTO:Tb films via nebulized spray deposited shows better optical and electrical properties, and it can fulfill optoelectronics based energy demand. Conclusion Pristine and terbium doped FTO thin films were synthesized using nebulizer spray pyrolysis technique and the important results were summarized below (1) The structural analysis showed that the pure and Tb doped FTO films were polycrystalline nature with tetragonal crystal structure and the crystalline size was reduced systematically with increasing doping concentration. Raman active mode (Eg), IR active mode (Eu), asymmetric stretching Sn-O bonds were found from Raman analysis. 17

(2) Granular sized tightly packed particles and smoothness of the films are visualized from AFM. PL study reveals the UV, blue and green (visible) emission. Highest optical transmittance is noticed for 1.5 wt.% Tb doped FTO thin film and the observed band gap is 3.99 eV. Optical and dielectric constants were decreased with increasing doping concentration. (3) Low resistivity, high carrier concentration and high figure of merit are obtained for 1.5 wt. % Tb doped film. (4) From this novel work, we could conclude that the rare earth element Tb doped FTO thin film has good electrical conductivity, high transmittance and high figure of merit which perfectly suit for optoelectronic applications. Acknowledgment: The authors express their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P.2/9/40.

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21

Fig.1 X-ray diffraction pattern of FTO:Tb thin films with different doping concentration

22

Fig.2 Raman characteristic spectrum of FTO:Tb thin films of different doping concentration

23

(b)

(a)

(c)

(d)

Fig.3a. 2D AFM images of FTO:Tb thin films with different doping concentration

24

(b)

(a)

(d)

(c)

Fig.3b. 3D AFM images of FTO:Tb thin films with different doping concentration

25

cps/eV

5

4

3 Tb

O Sn F

Tb

Sn

Tb

2

1

0 1

2

3

4

5 keV

6

7

Fig.4 EDAX spectrum of 1.5wt.% Tb doped FTO thin film

26

8

9

10

Fig.5 Photoluminacence spectrum of FTO:Tb thin films with different doping concentration

27

Fig.6 (a) Transmittance, (b) Absorbance (c) Reflectance and (d) Band gap spectrum of FTO:Tb thin films with different doping concentration

28

Fig 7 (a) Refractive index and (b) Extinction coefficient of FTO:Tb thin films with different doping concentration

29

Fig.8 (a) Porosity (b) electronic polarisability of FTO:Tb thin films with different doping concentration

30

Fig.9 (a) Real and (b) Imaginary part of dielectric constant for FTO:Tb thin films with different doping concentration

31

Fig. 10 Optical conductivity of FTO:Tb thin films with different doping concentration

32

Fig.11(a) VELF and (b) SELF of FTO:Tb thin films with different doping concentration

33

Fig.12 Resistivity, Carrier concentration and carrier mobility of FTO:Tb thin films with different doping concentration

Table 1 Structural parameters and Lattice constants of prepared FTO:Tb thin films Tb doping level (wt.%) 0 0.5 1 1.5

Crystallite size (nm)

Dislocation density (x1015) lines.m-2

Strain (x10-3 )

22 20 18 17

1.989 2.423 3.002 3.291

6.669 7.360 8.201 8.563

Lattice constants (Å) a

c

Cell volume (Å3)

4.722 4.727 4.731 4.735

3.172 3.165 3.161 3.159

70.72 70.74 70.75 70.84

Table 2 Surface roughness parameter of FTO:Tb thin films Tb doping level (wt.%) 0 0.5 1 1.5

Surface roughness (nm) 18 17 16 14

34

Grain size (nm) 60 57 41 30

Table 3 Optical and Dielectric parameters of prepared FTO:Tb thin films Tb doping level (wt.%) 0 0.5 1 1.5

Thickness (nm)

Band gap (eV)

Refractive index (n)

Extinction coefficient (k)

ε1

ε2

Optical conductivity (x1013)

630 570 540 500

3.88 3.94 3.97 3.99

1.74 1.70 1.64 1.62

0.038 0.036 0.034 0.033

3.035 2.878 2.618 2.607

0.127 0.118 0.112 0.104

3.46 3.22 3.05 2.79

Table 4 Comparison of various parameter values for FTO thin films prepared by various chemical methods S.No

Technique

Transmittance, (%)

Resistivity, (Ω-cm)

Carrier mobility, (cm2 V−1 s−1)

Figure of merit (ɸ) (Ω)-1

1

Pulsed spray pyrolysis Chemical vapor deposition Sol-gelevaporation method Spray pyrolysis technique Chemical vapor deposition NSP

60

2.19× 10-5

39.2

4.1 × 10-3

7.28× 1021

[35]

70

1.09 × 10-3

26.6

2.01 × 10-3

2.64 × 1020

[37]

59.84

-

-

2.58× 10-3

-

[38]

80

2.2×10−4

21.6

35.7× 10-4

1.7 × 1020

[39]

70.59

5× 10-4

6

3.12 × 10-3

7.98 × 1020

[40]

79

1.67× 10-3

30

1.5× 10-3

1.26× 1020

Present work

2

3

4

5

6

35

Carrier References concentration (n) (cm−3)

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The author declares that 1) The manuscript has not been previously published 2) It is not currently submitted for review to any other journal, and will not be submitted elsewhere before a decision is made by this journal