J Mater Sci: Mater Electron DOI 10.1007/s10854-016-4801-1

Synthesis and characterization of zinc stannate thin films prepared by spray pyrolysis technique Mahendra A. Patil1 • Sarfraj H. Mujawar1 • Vinayak V. Ganbavle2 Kesu Y. Rajpure2 • Harish P. Deshmukh3

•

Received: 22 December 2015 / Accepted: 8 April 2016 Ó Springer Science+Business Media New York 2016

Abstract The zinc stannate thin films were synthesized by simple and inexpensive spray pyrolysis technique on the glass and fluorine doped tin oxide coated conducting glass substrates. The as deposited films were further annealed at 500 °C temperature for 12 h. The structural optical and morphological characterization of as prepared and annealed films was carried out by XRD, UV–Vis spectroscopy, SEM and AFM techniques respectively. The structural analysis shows that films are polycrystalline and crystallized in cubic inverse spinel crystal structure. SEM studies show that grain size increases after annealing and exhibits spherical morphology. AFM study shows that roughness is higher for the post annealed film. Further the samples were tested for testing their applicability for dye sensitized solar cells. The as prepared, annealed and CNT doped samples exhibits photoconversion efficiencies 2.7, 2.8 and 3.1 % respectively.

1 Introduction Within last two decades dye sensitized solar cells (DSSCs) are researched extensively as an alternative to high cost traditional silicon solar cells. A typical DSSC module comprises a dye loaded photoanode of nanocrystalline & Harish P. Deshmukh [email protected] 1

Rayat Shikshan Sanstha’s Mahatma Phule Mahavidyalaya, Pimpri, Pune 411017, India

2

Electrochemical Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India

3

Department of Physics, Y. M. College, Bharati Vidyapeeth University, Erandwane, Pune, India

metal oxide, a liquid electrolyte containing I-/I3- redox couple, and a platinum counter electrode [1]. Among these the photoanode has a to play dual role by providing high surface area for dye loading and rapid transport of photogenerated electrons towards conducting electrode avoiding its recombination and hence governing the device performance. Several binary metal oxides are being extensively studied for their plausible use as photoanode in DSSC. Among these binary oxides nanocrystalline TiO2 proved its potential candidature with consistently improved photoconversion efficiency (PCE) from 7.1 % in 1991 to 12.3 % in 2012 [1–3]. Beside these binary oxides nowadays ternary oxides are being acknowledged as suitable candidates for photovoltaics [4–6]. Synthesis of ternary metal oxides is more flexible than those of binary oxides because the physical and chemical properties of these materials can be tuned by changing the composition of different components [7]. Zinc stannate (Zn2SnO4) is a ternary metal oxide of class Aii2 Biv O4 . Zn2SnO4 with a wide band gap of 3.6 eV, high electron mobility (10–15 cm2 V-1 s-1) [8] and owing to its complex crystal structure, provides improved stability in adverse conditions [9] as compared to binary oxides [7]. From last 10 years consistent efforts are being taken by scientific community for the synthesis of Zn2SnO4 nanostructures to improve PEC of Zn2SnO4based DSSCs. Wu et al. prepared Zn2SnO4 based DSSCs and obtained an efficiency of 3.8 % [10, 11]. Subsequently, using uniform Zn2SnO4 nanoparticles (8 nm), Hong and his co-workers improved PCE from 4.7 % for Zn2SnO4 to 6.0 % for the DSSC device architecture containing Zn2SnO4 compact layer and Zn2SnO4/ZnO core shell nanoparticles [12, 13]. Kim et al. [14] noted that Zn2SnO4– DSSCs based on organic donor-conjugate-acceptor (D-pA) structured orange organic dye displayed significantly

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20

30

40

50

60

(622)

(531)

X-ray diffraction (XRD) studies of the Zn2SnO4 were carried out using Bruker D2 phaser X-ray diffractometer. The patterns were recorded in the 2h range of 20–80° with step width 0.2° using CuKa radiation of wavelength ˚ . The patterns were analysed by matching with 1.54056 A the standard JCPDS card and indexed accordingly. Morphological study of the films was carried out using

As Prepared Annealed

(331)

2.2 Material characterization

XRD studies were carried out for the identification of crystal structure of the as-prepared and annealed films. Figure 1 shows XRD pattern of as prepared and post annealed film at 500 °C. The existence of well defined reflection along (311) plane at angle 34.3° and well matching of 2h and ‘d’ values with JCPDS card No. 73-1725 depicts the formation of phase pure Zn2SnO4 thin film with cubic face cantered crystal structure. It indicates that Sn4? atoms are in octahedral coordination, while half of the Zn2? atoms are distributed in tetrahedral coordination and the other half in octahedral coordination [18]. Along with (311) peak no more peaks with considerable intensity are observed which indicates preferential growth of the films along (311) plane. Slight shift in (311) peaks is observed upon annealing (2h = 34.5), (Fig. 2) which may be due to the removal of residual stresses in the films causing decrease in interplanar spacing [19]. After annealing, the marginal enhancement observed in the peaks intensity depicts improvement in the crystallinity of Zn2SnO4 thin film. The estimated lattice parameter is ˚ , which is in good agreement with the reported 8.610 A

(311)

For the synthesis of Zn2SnO4 thin films, zinc chloride (ZnCl2) and stannic chloride (SnCl45H2O) are used as cationic precursors of zinc and tin, respectively and deionized (DI) water is used as solvent. The chemicals were procured from S. D. Fine Chemicals Limited, Mumbai, and used as received without any further treatment. To obtain the Zn2SnO4 thin films, ZnCl2, SnCl4 of required quantity were dissolved in 200 ml of DI water to form 0.1 M solution keeping Zn/Sn ratio 2:1. The precursor solution formed is then sprayed onto the preheated glass substrates at 400 °C temperature. The spray solution quantity (200 ml), spray rate (10 ml/min) and the nozzle to substrate distance (28 cm) were optimized by observing uniformity and adherence of the films. The compressed air is used as carrier gas for spray pyrolysis process. The as prepared films were further annealed at 500 °C in air atmosphere for 12 h. to enhance the crystallinity of the films.

3.1 X-ray diffraction

(222)

2.1 Material synthesis

3 Results and discussion

(220)

2 Experimental

scanning electron microscopy (FESEM Model: JEOL JSM 6360) operating at 20 kV. Atomic force microscope (Bruker, USA-INNOVA 1B3BE) was used to study the topography and to measure surface roughness of the films. For the optical studies of the films the transmission spectra were obtained using UV–Vis Spectrophotometer-1800, Shimadzu spectrophotometer. Thickness of the films was measured by weight difference method.

Intensity (A.U.)

improved performance compared to the ruthenium complex sensitized DSSCs. Multi-functional, hierarchical Zn2SnO4 spheres comprising nanoparticles are effective photoanodes for DSSCs, with PCE of 5.36 % [15]. Ma et al. [16] improved the power conversion efficiency of Zn2SnO4–DSSCs from 3.47 % for bare Zn2SnO4 to 5.72 % surface modified Zn2SnO4 using TiO2. Recently Zou et al. [17] reported overall photoconversion efficiency up to 3.43 % for nanosheet assembling hierarchical Zn2SnO4 microspheres (NHMSs) based anode of 19 lm thickness. Very few reports are available on the synthesis of zinc stannate thin films by spray pyrolysis technique. In the present work we report the applicability of simple and inexpensive spray pyrolysis technique for the direct synthesis of phase pure, uniform and well adherent Zn2SnO4 thin films. The films were further characterized for structural optical and morphological characterizations. The applicability of these films for dye sensitized solar cell is also tested.

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2θ (Degree) Fig. 1 XRD patterns of as deposited and annealed (500 °C) Zn2SnO4 thin films

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recorded over the wavelength range of 350–850 nm at room temperature. The optical absorption data were analyzed using the following classical Eq. (2) of optical absorption in semiconductor near band edge [22].

As Prepared Annealed

Intensity (A. U.)

(311)

a¼

35.0

35.5

36.0

Fig. 2 Shift in (311) peak of Zn2SnO4 thin films after annealing at 500 °C

˚ for Zn2SnO4 (JCPDS card No. 73-1725). value of 8.610 A In order to determine the crystallite size, a slow scan of XRD pattern between 34° and 37° was carried out within the step of 0.02 min-1 for both the samples. The size of the crystallites oriented along (311) plane can be deduced using Scherer’s formula shown Eq. (1). D¼

0:9k bCosh

ð1Þ

where D is the size of crystallite, b the broadening of diffraction line measured at half of its maximum intensity ˚ ). It in radius and k is the wavelength of X-rays (1.5405 A was presumed that values of instrumental error are common for both the samples. The calculated values of crystallite size are 13 and 41 nm for as prepared and annealed sample respectively. During annealing, smaller crystallites agglomerate and larger crystallites are formed thus increasing the crystallite size after annealing. Intensity of the less intense peaks increases after annealing films. The existence of single peak in the XRD reveals preferential growth of nanocrystallites along (311) plane. Morinaga and Suwanboon et al. [20, 21] proposed a qualitative idea for the growth mechanism of the preferential orientated thin films of Zinc oxide. The observed oriented growth may be due to the minimization of the surface free energy of each crystal plane and usually films grow so as to minimize the surface energy. Due to the minimization of the surface energy heterogeneous nucleation readily happens at the interface of thin film and substrate. 3.2 Optical absorption studies The optical energy band gap of Zn2SnO4 thin film was estimated from optical absorption measurement. The optical absorption spectrum for the Zn2SnO4 thin film is

3.3 Scanning electron microscopy Figure 4 shows the scanning electron micrograph (SEM) of as deposited and annealed Zn2SnO4 thin film. From the figure it is clear that no well defined grains were observed in case of as deposited sample whereas the annealed

As prepared o Annealed at 500 C -1 2

34.5

2θ (degree)

-10

34.0

where Eg is the separation between top of the valence band and bottom of the conduction band, hm the photon energy and n is a constant. The value of n depends on the probability of transition; it takes values as 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transition respectively. Thus, if plot of (ahm)2 versus (hm) is linear the transition is direct allowed. Extrapolation, of the straight-line portion to zero absorption coefficient (a = 0), leads to estimation of band gap energy (Eg) values. Figure 3 shows variation of (ahm)2 as a function of photon energy (hm). From figure it is observed that the optical band gap energy decreases from 3.9 to 3.7 eV after annealing. These two values of Zn2SnO4 nanocrystals are apparently greater than the value for bulk Zn2SnO4 (3.6 eV), the observed increase in the band gap of the Zn2SnO4 nanostructures is indicative of quantum confinement effects arising from the small size regime [23] because the size of nanostructures is smaller in comparison with the bulk materials, as indicated by FESEM observations. The observed decrease in band gap after annealing is analogous to increase in crystallite size. Reported values of the band gap of Zn2SnO4 based semiconductors are ranging from 3.35 to 4.1 eV [24, 25].

2

33.5

ð2Þ

(α hν ) ,10 (eV.cm )

33.0

a0 ðhm  EgÞn hm

3.0

3.2

3.4

3.6

3.8

4.0

h ν (eV) Fig. 3 Tauc plot of (ahm)2 versus hm of as deposited and annealed Zn2SnO4 thin films

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Fig. 4 SEM images of Zn2SnO4 thin films a as deposited and b after annealing at 500 °C

sample shows crystallites of an average diameter 700–800 nm were developed in the film. 3.4 Atomic force microscopy The three dimensional (3D) morphology of films can be observed by AFM. A quantitative method to examine the surface morphology and structure is analyzing the surface roughness using AFM histogram. Figure 5 shows 3D and 2D images of as prepared and annealed samples, respectively. The most important measurement of surface roughness can be given with a statistical parameter root mean square (rms) that is the standard deviation of the height (Z) values within a given area. It is calculated using relation (3) and roughness values are shown in Table 1. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 I¼1 ðZi  Zave Þ rms ¼ ð3Þ N where Zave is the average of z values within given area, Zi the current Z value and N is the number of points within given area. It has been observed that after annealing crystallinity of the Zn2SnO4 film increases and leads to decrease in the surface roughness from 166 (as prepared) to 71 (annealed) nm. Decrease in the surface roughness makes films smooth and uniform. 3.5 Fabrication of Zn2SnO4 photoanodes and dye sensitized solar cell properties measurements Spray pyrolysis technique is used to fabricate Zn2SnO4 films on fluorine-doped tin oxide conductive glass (Asahi

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Glass Co., Ltd.; 10 ohm/square). Film thickness was 7–8 lm. For the preparation of CNT–Zn2SnO4 thin films appropriate amount of functionalized canrbon nano tubes (CNTs) was added in the spray solution and sonicated for 10 h for their uniform dispersion. Subsequently films were dipped in a dye solution of 5 9 10-4 M N719 dissolved in acetonitrile and 4-tert-butyl alcohol (volume ratio = 1:1), and kept at room temperature for 20 h. Finally, the solar cells were fabricated using a dye-sensitized Zn2SnO4 photoanode, a sputtered Pt counter electrode, and a redox (I-/I3-) electrolyte solution. For the photovoltaic measurements of DSSCs, the current density–voltage (J–V) curves of DSSCs were measured under simulated AM 1.5 illumination (100 mW cm-2, Newport USA) using a Keithley digital source meter (Keithley 2601, USA). The performance of Zn2SnO4 DSSCs for as prepared and annealed film was measured. The plot of current density–voltage (J–V) characteristics of as prepared and annealed Zn2SnO4 based solar cells are shown in Fig. 6. J–V measurements of both the devices were carried out by simulated sun light of power 100 mW/cm2 irradiation. The illuminated area of Zn2SnO4 working electrode was 0.25 cm2 (0.5 cm 9 0.5 cm). The obtained photo-electrochemical parameters of DSSCs such as short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) were summarized in Table 2. The as prepared Zn2SnO4 device produced short circuit photocurrent density 6.4 mA cm-2 whereas for the postannealed Zn2SnO4 device it is 8.4 mA cm-2. The observed improvement in the Isc may be due to the improved crystallinity and decreased surface roughness of the sample which can promote easy charge transport within

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Fig. 5 2D and 3D AFM images of Zn2SnO4 thin films a, c as deposited and b, d after annealing at 500 °C

Table 1 Roughness and particle size measurement of as deposited and annealed Zn2SnO4 thin films Sample ID

Roughness (nm)

Particle size (nm)

As prepared

166

51.8

71

278

Annealed

the film due to less grain boundary scattering. Open circuit voltage in DSSC is governed by the difference in conduction band (CB) edge of photoanode and LUMO level of organic dye. After annealing, shrinkage in band gap

causing the lowering of conduction band edge is observed and hence Voc decreased from 0.73 to 0.68. For CNT doped films Jsc found to be increased up to 7.5 mA/cm2. It is observed that the photo-conversion efficiency of CNT doped sample increases by 0.25 % than that of annealed sample. The observed increase in the efficiency may be the collective effect of provision of conducive channels by CNTs for the transport of photo-generated electrons and from Fig. 7 it is also observed that the reflectance of CNT doped films decreased drastically as compared to undoped films, which may trap more photons and enhance photogenerated charge careers [26]. The overall photoconversion

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References

Current density (mA/cm2)

2.4 As Prepared Annealed With CNT

2.1 1.8 1.5 1.2 0.9 0.6 0.3 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

V (volts) Fig. 6 Current density–voltage characteristics of as prepared and annealed Zn2SnO4 based solar cells

Table 2 Photo-electrochemical parameters of DSSCs of as deposited and annealed Zn2SnO4 thin films Isc (mA/cm2)

Sample ID

Voc (V)

Fill factor

Efficiency (%)

As prepared

6.4

0.73

57.72

2.7

Annealed

8.4

0.68

48.86

2.85

CNT doped

7.5

0.73

57.3

3.1

100

Zn2SnO4 with CNT

90

Zn2SnO4 without CNT

Reflectance (%)

80 70 60 50 40 30 20 10 0

200

400

600

800

1000

1200

1400

Wavelength (nm)

Fig. 7 DRS Spectra of undoped and CNT doped zinc stannate (Zn2SnO4) thin film

efficiencies for the as-prepared, post-annealed and CNT doped samples were 2.7, 2.85 and 3.1 % respectively. Acknowledgments The authors would like to acknowledge Dr. S. B. Ogale (NCL, Pune) for availing facilities for DSSC measurement and Director BCUD Savitribai Phule University of Pune, Pune for financial support through minor research project.

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