Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 136 (2015) 1671–1678
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dye-sensitized solar cell using sprayed ZnO nanocrystalline thin films on ITO as photoanode P. Dhamodharan a, C. Manoharan a,⇑, S. Dhanapandian a, P. Venkatachalam b a b
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608 002, Tamil Nadu, India Department of Physics (DDE), Annamalai University, Annamalai Nagar, Chidambaram 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ZnO thin films are successfully
deposited onto ITO glass substrate with preferential orientation along (0 0 2) plane. Films with better crystallinity and maximum transmittance are obtained for the as deposited film (annealing is not required). The absorption of dye in mesoporous ZnO films was confirmed by UV–Vis spectra. The obtained band gap value of 3.2 eV expected to facilitate electron injection from photo-exited dye molecules in DSSC.
a r t i c l e
i n f o
Article history: Received 7 June 2014 Received in revised form 8 October 2014 Accepted 15 October 2014 Available online 24 October 2014 Keywords: Wurtzite ZnO Porous film c-axis orientation DSSCs Electrical properties
a b s t r a c t ZnO thin films had been successfully prepared by spray pyrolysis (SP) technique on ITO/Glass substrates at different substrate temperature in the range 250–400 °C using Zinc acetylacetonate as precursor. The X-ray diffraction studies confirmed the hexagonal wurtzite structure with preferred orientation along (0 0 2) plane at substrate temperature 350 °C and the crystallite size was found to vary from 18 to 47 nm. The morphology of the films revealed the porous nature with the roughness value of 8–13 nm. The transmittance value was found to vary from 60% to 85% in the visible region depending upon the substrate temperature and the band gap value for the film deposited at 350 °C was 3.2 eV. The obtained results revealed that the structures and properties of the films were greatly affected by substrate temperature. The near band edge emission observed at 398 nm in PL spectra showed better crystallinity. The measured electrical resistivity for ZnO film was 3.5 104 X cm at the optimized temperature 350 °C and was of n-type semiconductor. The obtained porous nature with increased surface roughness of the film and good light absorbing nature of the dye paved way for implementation of quality ZnO in DSSCs fabrication. DSSC were assembled using the prepared ZnO film on ITO coated glass substrate as photoanode and its photocurrent – voltage performance was investigated. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Zinc oxide (ZnO) thin films have appeared as an attractive option in the design of transparent electrodes in thin films solar ⇑ Corresponding author. Tel.: +91 9443787811. E-mail address: [email protected] (C. Manoharan). http://dx.doi.org/10.1016/j.saa.2014.10.063 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
cells due to the simultaneous occurrence of high transmittance in the visible (>90%) and a low resistivity (from 103 to 104 O cm) [1]. The material can be made transparent in the whole visible range in the form of thin film. ZnO also has high exciton binding energy of 60 meV, which is much higher than the values of other widely used wide band gap materials, such as ZnSe (20 meV) and GaN (21 meV). This large exciton binding energy provides excitonic
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emission more efficiently even at high temperature. Some of them are good candidates for transparent conductive films, if they are prepared off-stoichiometry [2]. Such films have applications in electronic and optoelectronic devices, photothermal and photovoltaic conversion [3]. In the last two decades, Zinc oxide thin films have emerged as a transparent conducting oxide for photovoltaic application and as a gas sensor device. ZnO is a kind of n-type semiconductor with a direct band gap of 3.37 eV and has been found to have the similar energy band structure and physical properties with TiO2, which has acted as the frequently used material for DSSC photoanodes, while its electron mobility is higher by 2– 3 order so magnitude which will help to improve the efficiency of photo-generated electron transfer and reduce the electron recombination probability. Therefore, ZnO is expected to exhibit faster electron transport with reduced loss of recombination and increases the photocurrent which is produced in the dye sensitized solar cell [4]. The natural dye i.e. the extract from the fruits, leaves, and flowers contain the natural pigments ascribed to anthocyanine is responsible for the several colours red–blue range of the flower. These natural pigments which are used as dye sensitized material in the solar cell produces photocurrent. ZnO can be deposited by numerous techniques such as thermal evaporation [5], sputtering [6], chemical vapour deposition [7], dip coating [8], chemical bath deposition [9], spray pyrolysis [10], spin coating [11] and SILAR [12]. Among the various deposition techniques, the spray pyrolysis is well suited for the preparation of ZnO thin films because of its simple and inexpensive experimental arrangement, reproducibility, high growth rate, and mass production capability for uniform large area coatings which are desirable for industrial solar cell applications [2]. The essential characteristic of the simple spray pyrolysis technique is economical and covers a large area film production [13]. In the present work, ZnO thin films have been prepared by the spray pyrolysis (SP) technique at different substrate temperatures (Ts = 250–400 °C) using Zn acac precursor. The aim of this work is to optimize the substrate temperature for the preparation of ZnO thin films on ITO coated glass plate in the preferred orientation (0 0 2) so as to apply as a photoanode for DSSCs. The obtained results have been compared and discussed with the specified results by several researchers.
Experimental details In the typical synthesis procedure, ZnO thin films were deposited onto the preheated, ultrasonically cleaned ITO glass substrates by SPT from an aqueous zinc acetylacetonate precursor solution (0.1 M) at different substrate temperatures. ITO glass substrates were previously cleaned with ethanol and acetone in an ultrasonic bath, followed by 10 min successively. Finally, the substrates were rinsed in deionised water and allowed to dry in air. The substrate holder was equipped with thermocouples and heating elements were equipped with a temperature controller. The compressed air was used to atomize the solution containing the precursor compounds through a specially designed spray nozzle over the preheated substrate. The spray head was allowed to move in the X–Y plane using the microcontroller stepper motor, in order to achieve a uniform coating on the substrate. The spray head could scan an area of 200 200 mm with X-movement at a speed of 20 mm/s and Y-movement insteps of 5 mm/s simultaneously which was connected to the electronic kit. The hazardous fumes evolving at the time of spray deposition were succeeded out using external exhaust system connected to the deposition chamber. In the spray unit, the microcontroller device was communicated with PC through the serial port; the data of each spray could be stored in the PC.
Fabrication of dye-sensitized solar cell About 10 g of fresh Hibiscus rosasinensis flowers was put into 10 ml of alcohol and it was heated by indirect hydronic heating method in boiling water for 30 min to extract anthocyanine and chlorophyll. The pure and natural dye solution was obtained by filtering out the solid dregs using filter paper. The mesoporous ZnO working electrode was immersed into the solution of Hibiscus rosasinensis dye for 24 h and it was dried at 80 °C in a hot air oven. A cobalt sulphide counter electrode was clipped on the top of the photoanode. The redox electrolyte was injected into the small holes and sealed with a small square of sealing sheet. The redox electrolyte consisted of 0.3 M LiI, 15 mM I2, 0.5 M PMII (1-propyl-3-methylimidiazoline iodide) and 0.2 M polyethelyene glycol in acetonitrile. Characterization techniques The structural characterization of the deposited films were carried out by X-ray diffraction technique on SHIMADZU-6000 0 (monochromatic CuKa radiation, k = 1.5406 Å A). The surface morphology was studied by using SEM (JEOL-JES-1600) with a magnification of 5000–10,000. Optical absorption spectrum was recorded in the range 350–1200 nm using JASCO V-670 spectrophotometer. The photoluminescence spectrum (PL) was studied at room temperature using plorolog 3-HORIBAJOBINYVON with excitation source wavelength of 375 nm. The surface topological studies were carried out using Atomic force Microscope (Nano surf Easy scan2) AGILENT-N9410A-5500. The Electrical resistivity, mobility, and carrier concentration were measured at room temperature using standard Hall Effect system equipment (ECOPIA HMS-3000) in van der pauw configuration. The photovoltaic properties of the DSSCs were characterized by recording the photocurrent voltage (I–V) curves under illumination of A.M.1.5 G (100 mW/cm2). Results and discussion Structural analysis The influence of different substrate temperature on the crystalline state of the ZnO thin films on ITO glass substrate has been studied using XRD (Fig. 1). XRD pattern of the film deposited with the substrate temperature 250 °C exhibits crystalline peaks attributed to indium tin oxide (ITO), whereas polycrystalline wurtzite phase of ZnO is evidenced at higher temperatures along with ITO peaks with less intensity. The absence of ZnO peaks at 250 °C show that temperature is not sufficient for the formation of crystalline ZnO. The XRD pattern shows polycrystalline nature with a pronounced (0 0 2) peak of ZnO at 2h = 34.55° (JCPDS Card No.361451) indicating a preferential c-axis orientation of the ZnO films at temperatures 300 and 350 °C, which is in good agreement with the reported results [14]. The occurrence of c-axis orientation is due to minimization of internal stress and surface energy, and it can also result from an easy growth because of high atomic density along (0 0 2) plane. The (1 0 0), (1 0 1) and (1 0 3) peaks are also observed at 2h = 31.84°, 36.46° and 62.91° respectively but these peaks have much lower intensity than the (0 0 2) peak [15,16]. The decrease in full width half maximum (FWHM) of X ray diffraction peaks as small as 0.172 nm at 350 °C show the highest degree of crystallinity and is the optimized temperature for further studies [17]. At Ts > 350 °C, the intensity of (0 0 2) peak decreases and this may be due to the higher rate of re-evaporation process. The mass transport to the substrate is reduced due to the vaporisation of solvent before reaching the substrate and is the reason for the decreased intensity of ZnO plane [13]. At this temperature the peak
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presented in Table 1. The increased crystallite size and the decreased strain and dislocation density show the improvement of the crystallite quality at the substrate temperature 350 °C. The decrease in strain indicates the decrease in concentration of lattice imperfections and the formation of high quality film [18]. Quantitative information concerning the preferential crystallite orientation is obtained from the texture coefficient TC (hkl) defined as
IðhklÞ =I0ðhklÞ TCðhklÞ ¼ 1 P IðhklÞ =I0ðhklÞ n
Fig. 1. XRD spectra of the ZnO films deposited on ITO glass at different substrate temperatures.
with weak intensity of (0 0 2) and (1 0 1) plane corresponds to the hexagonal crystal structure of wurtzite phase of ZnO are observed along with ITO peaks. The observed ‘d’ values and standard ‘d’ values taken from JCPDS standard data card are in good agreement with each other. X-ray diffraction studies of the films show that they are textured in the (0 0 2) direction at substrate temperature 350 °C. The crystallite size is calculated by Debye–Scherrer formula. The crystal defect parameters like strain and dislocation density are calculated by using the respective formulae [18,19] and are
ð1Þ
where I(hkl) is the measured relative intensity of a plane (hkl), I0(hkl) is the standard intensity of the plane (hkl) taken from the JCPDS data and n is the number of diffraction peaks. A sample with randomly oriented crystallite presents TC(hkl) = 1, while larger value represents the abundance of crystallites oriented along the particular (hkl) direction [20]. The observed larger values of texture co efficient for the plane (0 0 2) indicates the abundance of crystallites along that direction for the films deposited at 300 and 350 °C with improved crystallinity. Surface morphological studies Fig. 2 shows the SEM image of ZnO thin films deposited on ITO substrate with different temperatures. SEM images of the film with substrate temperature 250 °C and 300 °C show the formation of submicrometre crystallites distributed more or less uniformly over the entire surface of the ITO glass substrate. The petal shaped grains with uniform size and porous nature is revealed from the micrograph of the film deposited with substrate temperature 350 °C. The porous nature of the film is an important parameter
Table 1 Structural properties of ZnO thin film deposited at various Ts. Substrate temperature (°C)
hkl plane
Crystallite Size (D) (nm)
Strain (e) 103
Dislocation density (d) 1014 lines/m2
Texture coefficient
300 350 400
002 002 100
27.18 47.10 18.12
1.2752 0.7359 1.9126
13.5363 4.5077 30.4567
1.68 1.78 1.22
Fig. 2. SEM image of the ZnO films deposited on ITO glass at different substrate temperatures, (a) 250 °C, (b) 300 °C, (c) 350 °C, (d) 400 °C.
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for DSSC as it can absorb more dye molecules. The dye which can easily occupy the porous absorb the light radiation and inject electrons into the conduction band of ZnO semiconductors and hence enhances the performance of DSSC. Above 350 °C the pores have linear extent of around 300 nm and hence their sizes are similar to the dimension of the grain aggregates. The majority of these grains appear several times larger than the average crystallite sizes calculated from X-ray diffraction data (18–47 nm); thus, indicating that the most of the grains comprise multiple crystallites. Systematic variation in the microstructure of the ZnO films as a function of the different substrate temperatures is detected. To reveal the chemical composition of the film an energy dispersive X-ray (EDX) spectroscopy analysis is performed for representative samples and is shown in Fig. 3. EDX spectrum exhibits the presence of elements Zn, O, In, and Sn for the film at lower temperature (250 °C) and at higher temperature (350 °C). The presence Sn and In peak in this spectrum is due to the ITO coating on glass substrate. Even though the presence of Zn and O is evident from the EDX spectrum of the film deposited at 250 °C, the absence of peaks corresponding to ZnO in XRD pattern indicate that the deposited film is in amorphous nature. The surface topography of the films is also studied by AFM. 2D (two-dimensional) and 3D (three-dimensional) AFM images of ZnO films prepared at various substrate temperatures are shown Fig. 4. The sporadic distribution of grains with different sizes for the films at 250 °C is an indication of the amorphous nature of the film which is in agreement with XRD results. From the AFM images, it is observed that the grain size becomes larger and the crystallinity is improved with the increase in the substrate temperature. When the substrate temperature is increased from 300 to 400 °C, the distributions of grains are uniform with grain size of 60–70 nm. The root mean square (RMS) value of surface roughness is found to be 8.2–12.4 nm in the films. The higher roughness and the porous nature which are established from AFM is a useful parameter for dye absorption.
Optical studies The transmittance spectra of the grown films in the wavelength range of 350–1200 nm are shown in Fig. 5. The films are highly transparent in the visible region with an average transmittance reaching up to 85% and present a sharp ultraviolet cut-off at approximately 380 nm. The transmittance of the sample increases with the increase of substrate temperature. The best transparencies found for Ts = 350 °C are in good agreement with XRD results which describe the best arrangement with a preferred orientation along (0 0 2). An increase in the transmittance of ZnO films can also be attributed to the removal of organic species on the film at higher temperature. Scattering is less for c-axis oriented film, which is the reason for the increased transmittance value of the films with substrate temperature 300 and 350 °C. The decrease in transmittance value for further increase of temperature is due to the strong scattering and absorption [15]. The higher transparency with an optical transmittance value of 60–85% in the visible region is attributed to the higher crystalline nature of the film with preferential orientation along c-axis. This result shows that the transmittance of the c-axis oriented film is suitable for the fabrication of dye-sensitized solar cell [21]. Using the absorption data, the band gap energy is estimated using Tauc relationship [22] as follows
ahm ¼ Aðhm Eg Þn
ð2Þ
where hm is the photon energy, Eg is the optical band gap of the film material, A is a constant and n is 1/2, for direct band gap semiconductor. An extrapolation of the linear region of (ahm)2 on the y-axis versus photon energy (hm) on the x-axis give the value of the optical band gap Eg (Fig. 6). The calculated band gap (Eg) are found to be 2.8, 2.9, 3.2 and 2.8 eV, for the films deposited at 250, 300, 350, and 400 °C, respectively. It is evident from the figure that increase of band gap with substrate temperature up to 350 °C shows the improvement of crystallinity with lesser defects [2]. The extinction coefficients (k) of ZnO thin film are determined using the formula
k¼
ak 4p
ð3Þ
The variations of k with the wavelengths are shown in Fig. 7. It has been found that k values vary in the range of 0.15–0.09. The observed low extinction coefficient value is a qualitative indication of the homogeneity of the films. The refractive index of ZnO thin film is determined using the formula
n¼
Fig. 3. EDX spectrum of the ZnO films deposited on ITO glass at, (a) 250 °C, (b) 350 °C.
ð1 þ RÞ1=2 ð1 RÞ1=2
ð4Þ
The variation of refractive index with wavelength for the ZnO thin films deposited on ITO glass substrate are shown in Fig. 8. The increase in refractive index is attributed to improvement in film crystallinity. Fig. 9 shows the room temperature PL spectra of the ZnO thin films. In PL spectra of the film with substrate temperature 250 °C, the peak observed at 418 and 437 nm are ascribed to radiactive defects related to the interface traps exciting at grain boundaries and electron transition from zinc interstitial energy level to valence band respectively [23]. A strong UV emission is observed at around 398 nm corresponding to the near band edge emission of ZnO material for the film deposited with substrate temperature above 250 °C and this peak is an indicator of the good crystallinity [15]. The near band edge emission (NBE) has been attributed to free exciton annihilation from the position peak energy [14]. The absence of peak at 520 nm (which is due to
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Fig. 4. AFM image of the ZnO films deposited on ITO glass at different substrate temperatures. (a) 250 °C, (b) 300 °C, (c) 350 °C, (d) 400 °C.
oxygen vacancy in ZnO) indicates that it is free from such defects [15]. The intensity of the UV emission of ZnO films is dependent on the microcrystalline structure and stoichiometry [15]. Proper substrate temperature is favourable for the migration of atoms to favourable positions and the enhancement of crystallinity takes place, and this will decrease the concentration of intrinsic defects
and improves the NBE luminescence efficiency. Hence, the peak intensity of the UV emission increases with increase of substrate temperature up to 350 °C and then decreases for higher temperature at 400 °C. At high growth temperature (400 °C) zinc acetylacetonate decomposes and evaporation of Zn occurs before reaching the substrate, which degrade the stoichiometry of ZnO films. The
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Fig. 8. Refractive index of the ZnO films deposited on ITO glass at different substrate temperatures. Fig. 5. Transmission spectra of the ZnO films deposited on ITO glass at different substrate temperatures as a function of wavelength.
Fig. 9. PL spectra of the ZnO films deposited on ITO glass at different substrate temperatures. Fig. 6. Band gap of the ZnO films deposited on ITO glass at different substrate temperatures.
degradation of stoichiometry leads to the decrease in intensity of near band edge emission [15]. From PL spectra, it is concluded that the films prepared with substrate temperature 350 °C has good crystalline quality and less defect density. Such spray deposited ZnO thin films are one of the promising candidates for the UV optical devices. Electrical properties
Fig. 7. Extinction Coefficient of the ZnO films deposited on ITO glass at different substrate temperatures.
Hall – effect measurements are performed in order to investigate the electrical properties of the sprayed ZnO thin films on ITO glass substrate with different substrate temperatures. The measured values of resistivity (q), Hall mobility (l), carrier concentration (n) and conductivity (r) are shown in Table 2. The results show that the prepared film is an n-type semiconductor material with an resistivity of 3.52–6.22 104 X cm. It is observed that the resistivity decreases with increase in substrate temperature up to 350 °C whereas carrier concentration increases with it. The enhancement of carrier concentration leads to increase in conductivity up to 350 °C and then decreases. An increased resistivity and decreased carrier concentration are observed for the film deposited at 400 °C is due to structural changes taken place [23].
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Resistivity (q) X cm 4
300 350 400
4.38 10 3.52 104 6.22 104
Carrier concentration (n) cm3
Hall mobility (l) cm2/Vs
Conductivity (r) 1/X cm
5.01 6.59 2.68
29.22 36.99 26.89
1.98 103 2.83 103 1.58 103
Photovoltaic characterization Optical absorption peak of Hibiscus rosasinensis extract and Hibiscus rosasinensis absorbed ZnO deposited ITO coated glass substrate is shown in Fig. 10. The extract of Hibiscus rosasinensis exhibits peak at 551 nm. The appearance of 545 nm peak in the dye-sensitized ZnO confirms the incorporation of dye into the ZnO mesoporous film [24]. The photocurrent voltage (J–V) characteristics of the DSSCs with sprayed ZnO on ITO coated glass substrate as photoanode is shown in Fig. 11. It is observed that a maximum conversion efficiency of 0.42% with short circuit density 1.4 mA/cm2, open circuit voltage of 400 mV and a fill factor of 0.75. The observed low conversion
efficiency may be due to low light harvesting nature of the natural dye (Hibiscus rosasinensis) used, as well as smaller absorption of dye molecules by ZnO which inject only few photo-electrons at ZnO photoanode. The UV–visible spectra shows the low level absorption of dye molecules by ZnO photoanode. An efficiency of 0.3% is reported for the ZnO nanowires on ITO substrate and 0.2% for the fabricated hybrid polymer/ZnO photovoltaic devices with vertically oriented nanorods. The observed efficiency of 0.42% for the DSSCs prepared using Hibiscus rosasinensis as natural dye with ZnO coated on ITO glass substrate (spray pyrolysis technique) as photoanode is reasonably in agreement with the reported values [24–26] . Conclusions Wurtzite ZnO thin films with porous structure were obtained by spray pyrolysis using Zinc acetylacetonate as precursor. In this study, annealing of film was not required for crystallization of ZnO as better crystalline structure was obtained with preferential orientation along c-axis for the deposited film. The observed petal shaped grains with porous nature, increased surface roughness, higher transmittance value and n-type conductivity of the film made it as a promising candidate for DSSC. The NBE observed in PL showed better crystallinity with lesser defects. The absorbance of dye in ZnO mesoporous structure was confirmed from the UV analysis, which played a key factor in DSSC. An efficiency of 0.42% was observed for the prepared DSSC using the extract of Hibiscus rosasinensis as dye. To enhance the efficiency of DSSC, the growth of nanorods from the seed layer formed by spray pyrolysis is the future plan of work. Acknowledgement
Fig. 10. Optical absorption spectra of H.R extract and H.R extract absorbed ZnO thin film.
The authors thank the UGC Major Project, New Delhi, for the financial support through research Grant No. 42-860/2013(SR). References
Fig. 11. J–V characteristics of DSSC in tests ZnO thin film.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Dye-sensitized solar cell using sprayed ZnO nanocrystalline thin films on ITO as photoanode P. Dhamodharan a, C. Manoharan a,⇑, S. Dhanapandian a, P. Venkatachalam b a b
Department of Physics, Annamalai University, Annamalai Nagar, Chidambaram 608 002, Tamil Nadu, India Department of Physics (DDE), Annamalai University, Annamalai Nagar, Chidambaram 608 002, Tamil Nadu, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ZnO thin films are successfully
deposited onto ITO glass substrate with preferential orientation along (0 0 2) plane. Films with better crystallinity and maximum transmittance are obtained for the as deposited film (annealing is not required). The absorption of dye in mesoporous ZnO films was confirmed by UV–Vis spectra. The obtained band gap value of 3.2 eV expected to facilitate electron injection from photo-exited dye molecules in DSSC.
a r t i c l e
i n f o
Article history: Received 7 June 2014 Received in revised form 8 October 2014 Accepted 15 October 2014 Available online 24 October 2014 Keywords: Wurtzite ZnO Porous film c-axis orientation DSSCs Electrical properties
a b s t r a c t ZnO thin films had been successfully prepared by spray pyrolysis (SP) technique on ITO/Glass substrates at different substrate temperature in the range 250–400 °C using Zinc acetylacetonate as precursor. The X-ray diffraction studies confirmed the hexagonal wurtzite structure with preferred orientation along (0 0 2) plane at substrate temperature 350 °C and the crystallite size was found to vary from 18 to 47 nm. The morphology of the films revealed the porous nature with the roughness value of 8–13 nm. The transmittance value was found to vary from 60% to 85% in the visible region depending upon the substrate temperature and the band gap value for the film deposited at 350 °C was 3.2 eV. The obtained results revealed that the structures and properties of the films were greatly affected by substrate temperature. The near band edge emission observed at 398 nm in PL spectra showed better crystallinity. The measured electrical resistivity for ZnO film was 3.5 104 X cm at the optimized temperature 350 °C and was of n-type semiconductor. The obtained porous nature with increased surface roughness of the film and good light absorbing nature of the dye paved way for implementation of quality ZnO in DSSCs fabrication. DSSC were assembled using the prepared ZnO film on ITO coated glass substrate as photoanode and its photocurrent – voltage performance was investigated. Ó 2014 Elsevier B.V. All rights reserved.
Introduction Zinc oxide (ZnO) thin films have appeared as an attractive option in the design of transparent electrodes in thin films solar ⇑ Corresponding author. Tel.: +91 9443787811. E-mail address: [email protected] (C. Manoharan). http://dx.doi.org/10.1016/j.saa.2014.10.063 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
cells due to the simultaneous occurrence of high transmittance in the visible (>90%) and a low resistivity (from 103 to 104 O cm) [1]. The material can be made transparent in the whole visible range in the form of thin film. ZnO also has high exciton binding energy of 60 meV, which is much higher than the values of other widely used wide band gap materials, such as ZnSe (20 meV) and GaN (21 meV). This large exciton binding energy provides excitonic
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emission more efficiently even at high temperature. Some of them are good candidates for transparent conductive films, if they are prepared off-stoichiometry [2]. Such films have applications in electronic and optoelectronic devices, photothermal and photovoltaic conversion [3]. In the last two decades, Zinc oxide thin films have emerged as a transparent conducting oxide for photovoltaic application and as a gas sensor device. ZnO is a kind of n-type semiconductor with a direct band gap of 3.37 eV and has been found to have the similar energy band structure and physical properties with TiO2, which has acted as the frequently used material for DSSC photoanodes, while its electron mobility is higher by 2– 3 order so magnitude which will help to improve the efficiency of photo-generated electron transfer and reduce the electron recombination probability. Therefore, ZnO is expected to exhibit faster electron transport with reduced loss of recombination and increases the photocurrent which is produced in the dye sensitized solar cell [4]. The natural dye i.e. the extract from the fruits, leaves, and flowers contain the natural pigments ascribed to anthocyanine is responsible for the several colours red–blue range of the flower. These natural pigments which are used as dye sensitized material in the solar cell produces photocurrent. ZnO can be deposited by numerous techniques such as thermal evaporation [5], sputtering [6], chemical vapour deposition [7], dip coating [8], chemical bath deposition [9], spray pyrolysis [10], spin coating [11] and SILAR [12]. Among the various deposition techniques, the spray pyrolysis is well suited for the preparation of ZnO thin films because of its simple and inexpensive experimental arrangement, reproducibility, high growth rate, and mass production capability for uniform large area coatings which are desirable for industrial solar cell applications [2]. The essential characteristic of the simple spray pyrolysis technique is economical and covers a large area film production [13]. In the present work, ZnO thin films have been prepared by the spray pyrolysis (SP) technique at different substrate temperatures (Ts = 250–400 °C) using Zn acac precursor. The aim of this work is to optimize the substrate temperature for the preparation of ZnO thin films on ITO coated glass plate in the preferred orientation (0 0 2) so as to apply as a photoanode for DSSCs. The obtained results have been compared and discussed with the specified results by several researchers.
Experimental details In the typical synthesis procedure, ZnO thin films were deposited onto the preheated, ultrasonically cleaned ITO glass substrates by SPT from an aqueous zinc acetylacetonate precursor solution (0.1 M) at different substrate temperatures. ITO glass substrates were previously cleaned with ethanol and acetone in an ultrasonic bath, followed by 10 min successively. Finally, the substrates were rinsed in deionised water and allowed to dry in air. The substrate holder was equipped with thermocouples and heating elements were equipped with a temperature controller. The compressed air was used to atomize the solution containing the precursor compounds through a specially designed spray nozzle over the preheated substrate. The spray head was allowed to move in the X–Y plane using the microcontroller stepper motor, in order to achieve a uniform coating on the substrate. The spray head could scan an area of 200 200 mm with X-movement at a speed of 20 mm/s and Y-movement insteps of 5 mm/s simultaneously which was connected to the electronic kit. The hazardous fumes evolving at the time of spray deposition were succeeded out using external exhaust system connected to the deposition chamber. In the spray unit, the microcontroller device was communicated with PC through the serial port; the data of each spray could be stored in the PC.
Fabrication of dye-sensitized solar cell About 10 g of fresh Hibiscus rosasinensis flowers was put into 10 ml of alcohol and it was heated by indirect hydronic heating method in boiling water for 30 min to extract anthocyanine and chlorophyll. The pure and natural dye solution was obtained by filtering out the solid dregs using filter paper. The mesoporous ZnO working electrode was immersed into the solution of Hibiscus rosasinensis dye for 24 h and it was dried at 80 °C in a hot air oven. A cobalt sulphide counter electrode was clipped on the top of the photoanode. The redox electrolyte was injected into the small holes and sealed with a small square of sealing sheet. The redox electrolyte consisted of 0.3 M LiI, 15 mM I2, 0.5 M PMII (1-propyl-3-methylimidiazoline iodide) and 0.2 M polyethelyene glycol in acetonitrile. Characterization techniques The structural characterization of the deposited films were carried out by X-ray diffraction technique on SHIMADZU-6000 0 (monochromatic CuKa radiation, k = 1.5406 Å A). The surface morphology was studied by using SEM (JEOL-JES-1600) with a magnification of 5000–10,000. Optical absorption spectrum was recorded in the range 350–1200 nm using JASCO V-670 spectrophotometer. The photoluminescence spectrum (PL) was studied at room temperature using plorolog 3-HORIBAJOBINYVON with excitation source wavelength of 375 nm. The surface topological studies were carried out using Atomic force Microscope (Nano surf Easy scan2) AGILENT-N9410A-5500. The Electrical resistivity, mobility, and carrier concentration were measured at room temperature using standard Hall Effect system equipment (ECOPIA HMS-3000) in van der pauw configuration. The photovoltaic properties of the DSSCs were characterized by recording the photocurrent voltage (I–V) curves under illumination of A.M.1.5 G (100 mW/cm2). Results and discussion Structural analysis The influence of different substrate temperature on the crystalline state of the ZnO thin films on ITO glass substrate has been studied using XRD (Fig. 1). XRD pattern of the film deposited with the substrate temperature 250 °C exhibits crystalline peaks attributed to indium tin oxide (ITO), whereas polycrystalline wurtzite phase of ZnO is evidenced at higher temperatures along with ITO peaks with less intensity. The absence of ZnO peaks at 250 °C show that temperature is not sufficient for the formation of crystalline ZnO. The XRD pattern shows polycrystalline nature with a pronounced (0 0 2) peak of ZnO at 2h = 34.55° (JCPDS Card No.361451) indicating a preferential c-axis orientation of the ZnO films at temperatures 300 and 350 °C, which is in good agreement with the reported results [14]. The occurrence of c-axis orientation is due to minimization of internal stress and surface energy, and it can also result from an easy growth because of high atomic density along (0 0 2) plane. The (1 0 0), (1 0 1) and (1 0 3) peaks are also observed at 2h = 31.84°, 36.46° and 62.91° respectively but these peaks have much lower intensity than the (0 0 2) peak [15,16]. The decrease in full width half maximum (FWHM) of X ray diffraction peaks as small as 0.172 nm at 350 °C show the highest degree of crystallinity and is the optimized temperature for further studies [17]. At Ts > 350 °C, the intensity of (0 0 2) peak decreases and this may be due to the higher rate of re-evaporation process. The mass transport to the substrate is reduced due to the vaporisation of solvent before reaching the substrate and is the reason for the decreased intensity of ZnO plane [13]. At this temperature the peak
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presented in Table 1. The increased crystallite size and the decreased strain and dislocation density show the improvement of the crystallite quality at the substrate temperature 350 °C. The decrease in strain indicates the decrease in concentration of lattice imperfections and the formation of high quality film [18]. Quantitative information concerning the preferential crystallite orientation is obtained from the texture coefficient TC (hkl) defined as
IðhklÞ =I0ðhklÞ TCðhklÞ ¼ 1 P IðhklÞ =I0ðhklÞ n
Fig. 1. XRD spectra of the ZnO films deposited on ITO glass at different substrate temperatures.
with weak intensity of (0 0 2) and (1 0 1) plane corresponds to the hexagonal crystal structure of wurtzite phase of ZnO are observed along with ITO peaks. The observed ‘d’ values and standard ‘d’ values taken from JCPDS standard data card are in good agreement with each other. X-ray diffraction studies of the films show that they are textured in the (0 0 2) direction at substrate temperature 350 °C. The crystallite size is calculated by Debye–Scherrer formula. The crystal defect parameters like strain and dislocation density are calculated by using the respective formulae [18,19] and are
ð1Þ
where I(hkl) is the measured relative intensity of a plane (hkl), I0(hkl) is the standard intensity of the plane (hkl) taken from the JCPDS data and n is the number of diffraction peaks. A sample with randomly oriented crystallite presents TC(hkl) = 1, while larger value represents the abundance of crystallites oriented along the particular (hkl) direction [20]. The observed larger values of texture co efficient for the plane (0 0 2) indicates the abundance of crystallites along that direction for the films deposited at 300 and 350 °C with improved crystallinity. Surface morphological studies Fig. 2 shows the SEM image of ZnO thin films deposited on ITO substrate with different temperatures. SEM images of the film with substrate temperature 250 °C and 300 °C show the formation of submicrometre crystallites distributed more or less uniformly over the entire surface of the ITO glass substrate. The petal shaped grains with uniform size and porous nature is revealed from the micrograph of the film deposited with substrate temperature 350 °C. The porous nature of the film is an important parameter
Table 1 Structural properties of ZnO thin film deposited at various Ts. Substrate temperature (°C)
hkl plane
Crystallite Size (D) (nm)
Strain (e) 103
Dislocation density (d) 1014 lines/m2
Texture coefficient
300 350 400
002 002 100
27.18 47.10 18.12
1.2752 0.7359 1.9126
13.5363 4.5077 30.4567
1.68 1.78 1.22
Fig. 2. SEM image of the ZnO films deposited on ITO glass at different substrate temperatures, (a) 250 °C, (b) 300 °C, (c) 350 °C, (d) 400 °C.
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for DSSC as it can absorb more dye molecules. The dye which can easily occupy the porous absorb the light radiation and inject electrons into the conduction band of ZnO semiconductors and hence enhances the performance of DSSC. Above 350 °C the pores have linear extent of around 300 nm and hence their sizes are similar to the dimension of the grain aggregates. The majority of these grains appear several times larger than the average crystallite sizes calculated from X-ray diffraction data (18–47 nm); thus, indicating that the most of the grains comprise multiple crystallites. Systematic variation in the microstructure of the ZnO films as a function of the different substrate temperatures is detected. To reveal the chemical composition of the film an energy dispersive X-ray (EDX) spectroscopy analysis is performed for representative samples and is shown in Fig. 3. EDX spectrum exhibits the presence of elements Zn, O, In, and Sn for the film at lower temperature (250 °C) and at higher temperature (350 °C). The presence Sn and In peak in this spectrum is due to the ITO coating on glass substrate. Even though the presence of Zn and O is evident from the EDX spectrum of the film deposited at 250 °C, the absence of peaks corresponding to ZnO in XRD pattern indicate that the deposited film is in amorphous nature. The surface topography of the films is also studied by AFM. 2D (two-dimensional) and 3D (three-dimensional) AFM images of ZnO films prepared at various substrate temperatures are shown Fig. 4. The sporadic distribution of grains with different sizes for the films at 250 °C is an indication of the amorphous nature of the film which is in agreement with XRD results. From the AFM images, it is observed that the grain size becomes larger and the crystallinity is improved with the increase in the substrate temperature. When the substrate temperature is increased from 300 to 400 °C, the distributions of grains are uniform with grain size of 60–70 nm. The root mean square (RMS) value of surface roughness is found to be 8.2–12.4 nm in the films. The higher roughness and the porous nature which are established from AFM is a useful parameter for dye absorption.
Optical studies The transmittance spectra of the grown films in the wavelength range of 350–1200 nm are shown in Fig. 5. The films are highly transparent in the visible region with an average transmittance reaching up to 85% and present a sharp ultraviolet cut-off at approximately 380 nm. The transmittance of the sample increases with the increase of substrate temperature. The best transparencies found for Ts = 350 °C are in good agreement with XRD results which describe the best arrangement with a preferred orientation along (0 0 2). An increase in the transmittance of ZnO films can also be attributed to the removal of organic species on the film at higher temperature. Scattering is less for c-axis oriented film, which is the reason for the increased transmittance value of the films with substrate temperature 300 and 350 °C. The decrease in transmittance value for further increase of temperature is due to the strong scattering and absorption [15]. The higher transparency with an optical transmittance value of 60–85% in the visible region is attributed to the higher crystalline nature of the film with preferential orientation along c-axis. This result shows that the transmittance of the c-axis oriented film is suitable for the fabrication of dye-sensitized solar cell [21]. Using the absorption data, the band gap energy is estimated using Tauc relationship [22] as follows
ahm ¼ Aðhm Eg Þn
ð2Þ
where hm is the photon energy, Eg is the optical band gap of the film material, A is a constant and n is 1/2, for direct band gap semiconductor. An extrapolation of the linear region of (ahm)2 on the y-axis versus photon energy (hm) on the x-axis give the value of the optical band gap Eg (Fig. 6). The calculated band gap (Eg) are found to be 2.8, 2.9, 3.2 and 2.8 eV, for the films deposited at 250, 300, 350, and 400 °C, respectively. It is evident from the figure that increase of band gap with substrate temperature up to 350 °C shows the improvement of crystallinity with lesser defects [2]. The extinction coefficients (k) of ZnO thin film are determined using the formula
k¼
ak 4p
ð3Þ
The variations of k with the wavelengths are shown in Fig. 7. It has been found that k values vary in the range of 0.15–0.09. The observed low extinction coefficient value is a qualitative indication of the homogeneity of the films. The refractive index of ZnO thin film is determined using the formula
n¼
Fig. 3. EDX spectrum of the ZnO films deposited on ITO glass at, (a) 250 °C, (b) 350 °C.
ð1 þ RÞ1=2 ð1 RÞ1=2
ð4Þ
The variation of refractive index with wavelength for the ZnO thin films deposited on ITO glass substrate are shown in Fig. 8. The increase in refractive index is attributed to improvement in film crystallinity. Fig. 9 shows the room temperature PL spectra of the ZnO thin films. In PL spectra of the film with substrate temperature 250 °C, the peak observed at 418 and 437 nm are ascribed to radiactive defects related to the interface traps exciting at grain boundaries and electron transition from zinc interstitial energy level to valence band respectively [23]. A strong UV emission is observed at around 398 nm corresponding to the near band edge emission of ZnO material for the film deposited with substrate temperature above 250 °C and this peak is an indicator of the good crystallinity [15]. The near band edge emission (NBE) has been attributed to free exciton annihilation from the position peak energy [14]. The absence of peak at 520 nm (which is due to
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Fig. 4. AFM image of the ZnO films deposited on ITO glass at different substrate temperatures. (a) 250 °C, (b) 300 °C, (c) 350 °C, (d) 400 °C.
oxygen vacancy in ZnO) indicates that it is free from such defects [15]. The intensity of the UV emission of ZnO films is dependent on the microcrystalline structure and stoichiometry [15]. Proper substrate temperature is favourable for the migration of atoms to favourable positions and the enhancement of crystallinity takes place, and this will decrease the concentration of intrinsic defects
and improves the NBE luminescence efficiency. Hence, the peak intensity of the UV emission increases with increase of substrate temperature up to 350 °C and then decreases for higher temperature at 400 °C. At high growth temperature (400 °C) zinc acetylacetonate decomposes and evaporation of Zn occurs before reaching the substrate, which degrade the stoichiometry of ZnO films. The
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Fig. 8. Refractive index of the ZnO films deposited on ITO glass at different substrate temperatures. Fig. 5. Transmission spectra of the ZnO films deposited on ITO glass at different substrate temperatures as a function of wavelength.
Fig. 9. PL spectra of the ZnO films deposited on ITO glass at different substrate temperatures. Fig. 6. Band gap of the ZnO films deposited on ITO glass at different substrate temperatures.
degradation of stoichiometry leads to the decrease in intensity of near band edge emission [15]. From PL spectra, it is concluded that the films prepared with substrate temperature 350 °C has good crystalline quality and less defect density. Such spray deposited ZnO thin films are one of the promising candidates for the UV optical devices. Electrical properties
Fig. 7. Extinction Coefficient of the ZnO films deposited on ITO glass at different substrate temperatures.
Hall – effect measurements are performed in order to investigate the electrical properties of the sprayed ZnO thin films on ITO glass substrate with different substrate temperatures. The measured values of resistivity (q), Hall mobility (l), carrier concentration (n) and conductivity (r) are shown in Table 2. The results show that the prepared film is an n-type semiconductor material with an resistivity of 3.52–6.22 104 X cm. It is observed that the resistivity decreases with increase in substrate temperature up to 350 °C whereas carrier concentration increases with it. The enhancement of carrier concentration leads to increase in conductivity up to 350 °C and then decreases. An increased resistivity and decreased carrier concentration are observed for the film deposited at 400 °C is due to structural changes taken place [23].
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Resistivity (q) X cm 4
300 350 400
4.38 10 3.52 104 6.22 104
Carrier concentration (n) cm3
Hall mobility (l) cm2/Vs
Conductivity (r) 1/X cm
5.01 6.59 2.68
29.22 36.99 26.89
1.98 103 2.83 103 1.58 103
Photovoltaic characterization Optical absorption peak of Hibiscus rosasinensis extract and Hibiscus rosasinensis absorbed ZnO deposited ITO coated glass substrate is shown in Fig. 10. The extract of Hibiscus rosasinensis exhibits peak at 551 nm. The appearance of 545 nm peak in the dye-sensitized ZnO confirms the incorporation of dye into the ZnO mesoporous film [24]. The photocurrent voltage (J–V) characteristics of the DSSCs with sprayed ZnO on ITO coated glass substrate as photoanode is shown in Fig. 11. It is observed that a maximum conversion efficiency of 0.42% with short circuit density 1.4 mA/cm2, open circuit voltage of 400 mV and a fill factor of 0.75. The observed low conversion
efficiency may be due to low light harvesting nature of the natural dye (Hibiscus rosasinensis) used, as well as smaller absorption of dye molecules by ZnO which inject only few photo-electrons at ZnO photoanode. The UV–visible spectra shows the low level absorption of dye molecules by ZnO photoanode. An efficiency of 0.3% is reported for the ZnO nanowires on ITO substrate and 0.2% for the fabricated hybrid polymer/ZnO photovoltaic devices with vertically oriented nanorods. The observed efficiency of 0.42% for the DSSCs prepared using Hibiscus rosasinensis as natural dye with ZnO coated on ITO glass substrate (spray pyrolysis technique) as photoanode is reasonably in agreement with the reported values [24–26] . Conclusions Wurtzite ZnO thin films with porous structure were obtained by spray pyrolysis using Zinc acetylacetonate as precursor. In this study, annealing of film was not required for crystallization of ZnO as better crystalline structure was obtained with preferential orientation along c-axis for the deposited film. The observed petal shaped grains with porous nature, increased surface roughness, higher transmittance value and n-type conductivity of the film made it as a promising candidate for DSSC. The NBE observed in PL showed better crystallinity with lesser defects. The absorbance of dye in ZnO mesoporous structure was confirmed from the UV analysis, which played a key factor in DSSC. An efficiency of 0.42% was observed for the prepared DSSC using the extract of Hibiscus rosasinensis as dye. To enhance the efficiency of DSSC, the growth of nanorods from the seed layer formed by spray pyrolysis is the future plan of work. Acknowledgement
Fig. 10. Optical absorption spectra of H.R extract and H.R extract absorbed ZnO thin film.
The authors thank the UGC Major Project, New Delhi, for the financial support through research Grant No. 42-860/2013(SR). References
Fig. 11. J–V characteristics of DSSC in tests ZnO thin film.
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