J Mater Sci: Mater Electron DOI 10.1007/s10854-016-5007-2

Structural, morphological, optical and photoelectrochemical cell properties of copper oxide using modified SILAR method A. S. Patil1 • G. M. Lohar1 • Vijay J. Fulari1

Received: 21 March 2016 / Accepted: 18 May 2016 Ó Springer Science+Business Media New York 2016

Abstract The copper oxide thin films have been synthesized using modified successive ionic layer adsorption and reaction (modified SILAR) method for photoelectrochemical cell application. Prepared thin films have been studied for their structural, morphological, optical and photoelectrochemical cell behaviour with the variation of SILAR cycles. X-ray diffraction analysis shows CuO thin films exhibit polycrystalline nature with monoclinic structure. The surface morphology shows formation of bunch of copper oxide nanorods. The optical properties of copper oxide thin films have been studied using the UV–Vis spectrophotometer. The room temperature photoluminescence spectra shows strong emission centred at 465 and 516 nm. Also, the photoelectrochemical cell properties of CuO show 0.26 % efficiency for 100 SILAR cycles.

1 Introduction Solar energy is considered to be the most capable renewable energy source because conventional fossil fuels are limited in earth crust also they causes green-house effect due to CO2 emission [1]. To make solar electricity cost effective, reduction of the cost of source materials and simplification of the fabrication process needed. Thin film solar cells can decrease the cost of photovoltaic cells and these thin films can be deposited on flexible substrates which make it one of the promising candidates for future development in photovoltaic devices [2]. Cupric oxide & Vijay J. Fulari [email protected] 1

Holography and Materials Research Laboratory, Department of Physics, Shivaji University, Kolhapur, MH 416 004, India

(CuO) is p-type transition semiconductor with narrow band gap (*1.3–2.1 eV) and it has significant applications such as solar energy transformation [3], biosensors [4], batteries [5], catalysis [6], magnetic storage media [7], super capacitors [8] and gas sensors [9] etc. The CuO exhibits properties such as non toxicity, great chemical stability, abundance in nature, cost effective synthesis, high surface area, good electrochemical activity, suitable redox potential and stability in various solutions as well as in air [10, 11]. The optical properties of CuO show that, it is a suitable candidate for solar cell fabrication, because it has high solar absorbency, better optical band gap and low thermal emittance [12]. It has band gap energy which is closer to ideal energy gap for solar cells which allows good solar spectral absorption, which further fascinate more solar cell efficiency [13]. CuO have been deposited using various physical or chemical methods such as electrodeposition [14], sputtering [15], sol–gel method [16], chemical vapor deposition (CVD) [17], spray pyrolysis [18], thermal oxidation [19], and successive ionic layer adsorption and reaction method (SILAR) [20]. Among all these methods SILAR method has various benefits such as suitable mass production, low temperature, cost effective solution method, good control over composition, thickness and phase purity. Also by changing deposition parameters like deposition cycles, dipping and rinsing time we can modify the properties of copper oxide [21]. In the normal SILAR method rinsing step is used which is avoided in modified SILAR method. Avoiding rinsing step increases rate of deposition and reduces deposition time. Mageshwari and Sathyamoorthy [22] prepared CuO by SILAR method, they have reported that band gap energy of CuO decreases from 2.48 to 2.31 eV with increasing SILAR cycles and they found PL emission wavelength at 468 nm. Akaltun [10] showed band

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gap energy of CuO decreases from 2.03 to 1.75 eV with increase in film thickness due to increase in SILAR deposition cycles. Bayansal et al. [23] studied effect of coumarin as an additive on CuO by SILAR method. They found that band gap energy decreases with increase in content of coumarin. Dagher et al. [24] synthesized CuO by microwave thermal method. In this study they obtained blue shift in band gap energy as well as in photoluminescence study with decrease in nanoparticle size. Anandan et al. [25] used CuO nanaorod array as a cathode in dye synthesized solar cell. They have been obtained energy conversion efficiency up to 0.29 %. Bhaumik et al. [26] synthesized copper oxide nanostructures by hydrothermal process also they fabricated ZnO/CuO thin film hybrid solar cell device. The solar cell efficiency of these thin film devices is found to be 2.88 %. Also by observing extensive literature survey, it is seen that very less work is carried out using modified SILAR method for photoelectrochemical cell study. In the present study, pure CuO is deposited by modified SILAR method with variation of deposition cycles and reported its structural properties using X-ray diffraction (XRD), morphological properties using field emission scanning electron microscopy (FE-SEM), compositional analysis using energy dispersive X-ray spectroscopy (EDS), optical properties using UV–Vis spectroscope and spectrofluorometer. The photoluminescence study of CuO thin films have been studied at room temperature. The aim of our work is, to synthesis the CuO thin films using modified SILAR method and to study structural, morphological, compositional and optical and photoelectrochemical cell properties of CuO for solar cell fabrication.

2 Experimental 2.1 Materials Chemicals used in this experiment are of AR grade. The copper sulphate pentahydrate (CuSO45H2O), sodium thiosulphate (Na2S2O3), sodium hydroxide (NaOH) are used for deposition of CuO thin films. Also, all analytical reagents are used without further purification for this experiment. CuO thin films were deposited on glass substrates by using modified SILAR method. For deposition of CuO, 0.1 M sodium thiosulphate (Na2S2O3) solution was added to 0.1 M copper sulphate pentahydrate (CuSO45H2O) solution till copper sulphate solution becomes colourless (complex solution) which was used as cationic solution. 0.1 M sodium hydroxide (NaOH) solution was maintained at 70 °C temperature which was used as anionic solution. Ultrasonically cleaned glass substrates were first dipped in

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anionic solution for 5 s and then in cationic solution for 5 s, this completes one SILAR cycle. The CuO thin films were prepared for 90, 100, 110, 120 cycles. The schematic representation of modified SILAR method is shown in Fig. 1. As number of SILAR cycle increases thickness of CuO thin film is increased. But after 120 cycles CuO films are pill out and thickness is found to be deceased at 130 cycles which is shown in Fig. 2. So, in present investigation only four (90, 100, 110, 120 cycles) thin films are studied. 2.2 Characterization techniques The structural properties of CuO thin films deposited by SILAR method are investigated using X-ray diffraction ˚ ) radiation from a study by employing Cu-Ka (k = 1.54 A Bruker D2 phaser table top model. The morphological and compositional study has been carried out using field emission scanning electron microscopy (FE-SEM) and EDS of Mira-3, Tescan Pvt, Brno-Czech Republic. The optical properties were studied by using UV–Vis spectrophotometer (UV-1800 Model Shimadzu, Japan). For the study of photoluminescence (PL) Bruker Spectrofluorometer (Flouromax 4) were used.

3 Results and discussion 3.1 X-ray diffraction (XRD) spectroscopic study Figure 3 shows XRD patterns of CuO films grown by modified SILAR method with variation of deposition cycles on glass substrate. The diffracting angle varies from 30° to 90°. The diffraction pattern of the films shows polycrystalline nature with monoclinic structure. The intense peaks observed at diffraction angle (2h) near about 35° and 38° shows (-111) and (111) strong reflections [20]. The observe ‘d’ values are well agreed with standard ‘d’ values shown in Table 1 and JCPDS card no. 00-001˚, 1117. The observed lattice constants are a = 4.65 A ˚ ˚ b = 3.41 A, c = 5.10 A. The average crystallite size (D) is calculated using Scherrer formula [27, 28] D¼

kk b cos h

ð1Þ

where, k is a shape factor (0.9), k is wavelength of X-ray ˚ ) and h is Bragg’s angle of the peaks, b is radiation (1.54 A broadening of diffraction line measured at full width of half maximum (FWHM) of the peak. From the Fig. 3 it is observed that the changes are observed in full width at half maximum (FWHM) and peak intensity. Resulting, change has been observed in the crystallite size. The crystallite size increases with increase in number of deposition cycles. In

J Mater Sci: Mater Electron Fig. 1 Schematic representation of two step SILAR method

Fig. 2 Variation of thickness of CuO films with SILAR deposition cycles

SILAR method layer by layer deposition of CuO occurs. Hence, as numbers of deposition cycles increases number of anions and cations increase these are responsible for increase in film thickness and crystallite size [10, 29, 30]. 3.2 Field emission scanning electron microscopic study (FE-SEM) Figure 4 shows the field emission scanning electron microscopic (FE-SEM) images of CuO thin films deposited using modified SILAR method. The FE-SEM studies have been made for annealed samples (350 °C). Figure 4a shows FE-SEM images of CuO thin film deposited at 90

Fig. 3 XRD patterns of CuO thin film with variation of SILAR cycle

SILAR cycles. The deposited CuO thin films show bunch of nanorods. The average width and length of bunch is near about 300 and 500 nm, respectively. Along with nanorods, small spheres are also seen. Figure 4b shows FE-SEM images of CuO deposited with 100 SILAR cycles. Here observed that average width of bunch of nanorods is 400 nm and length is 800 nm. Figure 4c shows FE-SEM images of CuO deposited with 110 SILAR cycles and the observed average width and length of bunch is near about 500 and 1000 nm, respectively. Figure 4d FE-SEM images of CuO deposited with 120 SILAR cycles. In this case observed average width and length of bunch is near about 350 and 1100 nm, respectively. From FE-SEM study, it is

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J Mater Sci: Mater Electron Table 1 Observed and calculated parameters from XRD Sr. no. 1.

Cycles 90

2.

100

3.

110

4.

120

2h (°)

Observed ‘‘d’’

Standard ‘‘d’’

D (nm) 15

35.5341

2.52

2.51

38.9275

2.31

2.31

35.6599

2.51

2.51

38.7273

2.32

2.31

35.5788

2.52

2.51

39.0139

2.30

2.31

35.5277

2.52

2.51

38.8165

2.32

2.31

49.2215

1.84

1.85

Fig. 5. These results are supported by XRD study and well agreed with the results obtained by Aklatun [10]. 3.3 Energy-dispersive X-ray spectroscopic study (EDS)

26 30 35

observed that the grain size increases with increase in deposition cycles. And the bunch of CuO nanorods look like flower and which is porous in nature [4]. The increase in grain size is due to increase in deposition cycles. The ion by ion deposition leads to create the nanoparticles, these nanoparticles are agglomerated and these agglomerated nanoparticles are initially creates nanorods and as deposition cycles increases these nanorods are chemically attached to each other and bundle of nanorods are formed with increase in size of bunch of nanorods. This is shown in

For the study of chemical stoichiometry quantitative analysis technique EDS is used. Figure 6 shows EDS spectrum recorded on selected area of CuO with variation of SILAR cycles. Figure 6a–d shows the EDS spectra for 90, 100, 110 and 120 SILAR cycles respectively. Table 2 shows compositional analysis (weight and atomic percentage) of CuO with respect to SILAR cycles. Present study show that prepared CuO consists of Cu and O with proportion 1:1 approximately within experimental error. This study has good agreement with results reported by Zhang et al. [31]. 3.4 Absorption To study the optical properties of materials for solar cell application is a important task [32]. The optical absorption study of deposited CuO is carried out using UV–Vis spectrophotometer. Figure 7 shows UV–Vis absorption

Fig. 4 FE-SEM images of CuO with variation of SILAR cycles, a 90 SILAR cycles, b 100 SILAR cycles, c 110 SILAR cycles, d 120 SILAR cycles

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Fig. 5 Growth mechanism of surface morphology

spectra of synthesized CuO by modified SILAR method. From Fig. 7 it is seen that absorption increases with increase in SILAR cycles. This is due to increase in thickness [21]. Figure 8 shows the variation of (ahm)2 with incident photon energy (hm) which gives bandgap energy. Present study reveals that observed band gap energy is 2.29, 2.17, 2.07 and 2.24 eV with respect to 90, 100, 110 and 120 SILAR cycles shown in Table 3. The band gap energy is observed to be decreases with increase in SILAR deposition cycles. The observed band gap energy is blue shifted from its bulk band gap (Eg = 1.85 eV) [33] this is due to quantum confinement effect. But as deposition cycle increases thickness of thin films increases and this is responsible for red shift. These results are well agreed with earlier literature [10]. The decrease in the band gap energy with deposition cycles can be recognized to the improvement in the morphological changes of the films, crystallinity, grain size, changes of atomic distances and structural defects in the films etc. [34, 35]. Also, as thickness of film increases the film becomes more homogeneous and the number of defects and disorder are reduced which decreases the density of localized states in the band structure and these localized states may combine with the band edges, resulting in the reduction of the band

gap energy [36]. At 120 cycles band gap energy is increased due to over growth of CuO and which consistent with FE-SEM results. Figure 9 shows the transmittance of CuO thin films with deposition cycles, from Fig. 9 it is observed that transmittance of the thin film is observed to be decreases with increases in deposition cycles. This is due to increase in thickness with increase in deposition cycles. The maximum transmittance is observed for 90 cycles which is near about 92 % at 1100 nm and minimum transmittance is observed for 120 cycles which is near about 65 % at 1100 nm ( Table 3). 3.5 Photoluminescence spectroscopic study (PL) PL is important tool to investigate optical properties as well as electronic transitions in materials including band edge transitions. Using spectrofluorometer, the emission properties of CuO have been studied. Figure 10 shows photoluminescence spectra of CuO thin films with variation of SILAR deposition cycles. The deposited CuO thin films are excited at 385 nm and recorded in the range 400–600 nm. The strong emission peak observed at 516 nm is due to band to band transitions of CuO. Also, another shoulder peak is observed at 465 nm. The emission at 465 nm is due

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Fig. 6 EDS of CuO with variation of SILAR cycles, a 90 SILAR cycles, b 100 SILAR cycles, c 110 SILAR cycles, d 120 SILAR cycles Table 2 Compositional analysis from EDS Sr. no.

No. of cycles

Weight%

Atomic%

Copper

Oxygen

Copper

Oxygen

1

90

49.45

50.55

19.76

80.24

2

100

54.95

45.05

23.49

76.51

3

110

55.23

44.77

23.70

76.30

4

120

56.64

43.36

24.75

75.25

to defect related emission or surface impurities like intrinsic defects, interstitial metal ions in the oxide, oxygen vacancies and surface states present in the CuO crystal which are occurred during growth process [22, 37–39]. These results are in well agreed with earlier literature [17, 40]. 3.6 Photoelectrochemical cell performance (J–V measurement) The PEC performance of CuO thin films on ITO substrates is determined with two electrode configuration comprising CuO thin film as a working electrode and as a counter electrode. An electrolyte used for this study was 0.5 M Na2SO4. The J–V curves of CuO thin solar cells with

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Fig. 7 Optical absorption spectra of CuO with variation of SILAR cycles

variation of SILAR cycles under illumination are shown in Fig. 11. The CuO thin films with 100 SILAR cycle shows an active area efficiency of 0.26 % with an open circuit voltage (Voc) of 530.81 mV, a short circuit current density (Jsc) of 185.27 lA/cm2 and a fill factor (FF) of 0.53 %. The low Jsc and FF values arise from a high series resistance. The J–V characteristics of the CuO are recorded using CH

J Mater Sci: Mater Electron

Fig. 8 Variation of bandgap energy with variation of SILAR cycles Fig. 10 Photoluminescence spectra of CuO with variation of SILAR cycles

Table 3 Variation of bandgap with deposition cycles Sr. no.

Cycles

Band gap energy (eV)

1.

90

2.29

2.

100

2.17

3.

110

2.07

4.

120

2.24

Fig. 11 J–V characteristics of CuO films with SILAR deposition cycles

Fig. 9 Variation of transmittance with respect to deposition cycles

instrument 608E model with a two electrode configuration using a tungsten lamp (250 W/m2). The following cell configuration is used to record J–V plots, Glass/ITO/CuO/ Na2SO4/Pt/ITO/Glass. Imax Vmax FF ¼  Isc Voc g¼

Isc  Voc FF Pin

ð2Þ ð3Þ

where, the FF is fill factor which is calculated by using Eq. (2) and power conversion efficiency (g) values of solar

cells are calculated using Eq. (3) [41, 42] Jsc is a short circuit current density, Voc open circuit voltage, Pin power density of the incident light. The observed and calculated parameters from J–V curve are shown in Table 4. The CuO thin film deposited at 100 cycles shows the better photoelectrochemical cell performance due to the less crystallite size, the relatively better surface morphology and relatively better optical band gap compared to other deposited thin films.

4 Conclusion The CuO is successfully synthesized using modified SILAR method with variation of deposition cycles. The X-ray diffraction spectroscopic study show that deposited CuO is polycrystalline in nature with monoclinic structure. Also it has flower like morphology which is porous in nature. The optical band gap energy varies from 2 to

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J Mater Sci: Mater Electron Table 4 Observed and calculated parameters from J–V measurement

Number of SILAR cycles

Jsc (lA/cm2)

Jmax (lA/cm2)

Vmax (mV)

FF

g (%)

90

50.32

584.69

307.74

30.13

0.31

0.04

100

185.27

530.81

371.89

140.29

0.53

0.26

110

41.00

644.51

430.10

18.25

0.25

0.03

120

56.01

549.73

352.80

30.13

0.35

0.05

2.1 eV with respect to deposition cycles. The photoluminescence show strong emission at 465 and 516 nm. The optical and morphological properties of CuO thin films deposited using modified SILAR method show that CuO is useful for solar cell application. The maximum photoelectrochemical cell efficiency is 0.26 % for 100 SILAR cycles. Acknowledgments The authors are thankful to the DST (DST-FIST, DST-PURSE) India for providing instrumental facilities at department of physics Shivaji University, Kolhapur. One of the authors (A. S. Patil) is thankful to UGC New Delhi, for awarding fellowship through UGC-BSR scheme.

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