The enhanced light harvesting performance of dye-sensitized solar cells based on ZnO nanorod-TiO2 nanotube hybrid photoanodes

Journal Pre-proof The enhanced light harvesting performance of dye-sensitized solar cells based on ZnO nanorod-TiO2 nanotube hybrid photoanodes ˘ ˘ Burcu Bozkurt C ¸ ırak, C ¸ igdem Eden, Yas¸ar Erdogan, Zeynep Demir, ¨ K. Volkan Ozdokur, Bulent Caglar, Sibel Morkoc¸ Karadeniz, Tuba ˘ C Kılınc¸, Ali Ercan Ekinci, C ¸ agrı ¸ ırak

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S0030-4026(19)31861-3

DOI:

https://doi.org/10.1016/j.ijleo.2019.163963

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IJLEO 163963

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Optik

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¨ ˘ Please cite this article as: Bozkurt C ¸ ırak B, Eden C ¸ , Erdogan Y, Demir Z, Ozdokur KV, Caglar B, Morkoc¸ Karadeniz S, Kılınc¸ T, Ercan Ekinci A, C ¸ ırak C ¸ ;, The enhanced light harvesting performance of dye-sensitized solar cells based on ZnO nanorod-TiO2 nanotube hybrid photoanodes, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163963

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The enhanced light harvesting performance of dye-sensitized solar cells based on ZnO nanorod-TiO2 nanotube hybrid photoanodes

Burcu Bozkurt Çıraka, Çiğdem Edenb, Yaşar Erdoğanb, Zeynep Demirb, K. Volkan Özdokurc, Bulent Caglarc, Sibel Morkoç Karadenizd, Tuba Kılınçd, Ali Ercan Ekincid and Çağrı Çırakd,* a

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Department of Alternative Energy Sources, Vocational School, Erzincan Binali Yıldırım University, 24100, Erzincan, Turkey. b Institute of Science and Technology, Erzincan Binali Yıldırım University, 24100, Erzincan, Turkey. c Department of Chemistry, Art & Science Faculty, Erzincan Binali Yıldırım University, 24100, Erzincan, Turkey. d Department of Physics, Art & Science Faculty, Erzincan Binali Yıldırım University, 24100, Erzincan, Turkey. * Corresponding author: E-mail address: [email protected] Tel:+90 446 224 3032/40042,

Abstract

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In this study, TiO2/ZnO hybrid nanocomposites (TZ) were fabricated by two step synthesis route for potential applications as photoanode in dye sensitized solar cells (DSSC). Titania

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nanotubes were grown on titanium sheets by electrochemical oxidation. ZnO nanorods were decorated with hydrothermal method by equimolar (0.1M) precursors on TiO2 nanotubes. Hydrothermal temperatures values were changed to find the optimal ZnO

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decoration density to increase photo conversion efficiency. The characterization of the photoanodes was made using FE-SEM, XRD and XPS technics. The results show that,

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hydrothermal reaction temperature can dramatically enhance the light harvesting performance of TZ photoanodes. The photo conversation efficiency of DSSC by TZ

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photoanode was reached 1.67% for hydrothermal temperature of 130 C which is twofold higher than that of DSSC by TiO2 nanotubes (0.81%). The improvement in photo conversation conversion efficiency can be assigned to synergetic effect in TZ photoanode, lower recombination rate and charge transfer resistance. Keywords: DSSC, TiO2 nanotube array, Zinc oxide, hydrothermal method

1. Introduction In recent years, investigation of clean and secure energy sources have been received tremendous of attention because of the lack of fossil based sources and their hazards to environment [1]. Solar energy as a renewable source is taken into account as one of the most reliable and endless energy source for coping with the world's energy problem [2] .

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Dye sensitive solar cells (DSSC), in particular, have gained great importance due to their low

cost and simple fabrication as compared with conventional silicon solar cells [3–8]. DSSC are photonic devices based on oxide semiconductor films sensitized by dye molecules.

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Furthermore, the overall efficiency of DSSC’s is rather low for commercialization, many

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studies have been dedicated to improve efficiency since their discovery in 1991 by Michael Gratzel et al. [3]. One approach is inhibiting recombination of electron and holes which

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separated when the dye absorbed light. Among the many materials were utilized for improving the photo-anode efficiency such as; zinc oxide (ZnO) [9] and niobium oxide

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(Nb2O5) [10], Titanium dioxide (TiO2) is the generally materials of choice as a semiconductors for DSSCs, probably due to its unique properties, such as cost effective and

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easy preparation, biocompatibility and non-toxic behavior [11–15]. Although, TiO2 can be

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found in its rutil, anatase and brookite form, anatase gives the highest efficiency in terms of porosity, transparency, surface area, film thickness and pore diameter [4]. On the other hand, various types TiO2 were prepared for photoanode material such as; TiO2 nanoparticles [3], mesoporous TiO2 [16], TiO2 nanotubes [17], TiO2 nanorods [18]. However, TiO2 nanotubes (TNA) showed higher efficiency compared to other TiO2 structures [19]. TNA are generally prepared with the aid of sol-gel technique[20], hydrothermal method

[21] and electrochemical anodization of titanium [22]. It has been shown that under optimized anodic conditions an oxide containing very regular pore sequences can be produced [23]. Further increase in photo conversion efficiency can be obtained by preparation of nanocomposite photoanode with other metal chalcogenides. This process not only reduce the recombination rate, but also increase the surface area of photoanodes. Many metal oxides have been utilized for the preparation of composite structure such as; ZnO [24], aluminum oxide (Al2O3) [25], tin oxide (SnO2) [26] and magnesium oxide (MgO)

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[7]. ZnO, in particular, is one of the most studied multifunctional semiconductor, due to its wide band gap and high stimulant binding energy [27]. In addition, many studies have

shown that nanostructured TiO2-ZnO composites have excellent photo conversion

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performance compared to bare and bulk forms due to enhanced charge transport and

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lifetime [17,28–31]. Various fabrication methods have been used for preparation of ZnO

hydrothermal method [34].

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nanorods including, chemical bath method [32], chemical vapor method [33] and

In this study, TiO2 nanotube / ZnO nanorod hybrid nanocomposites (TZ) were

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fabricated by anodic oxidation and hydrothermal method, respectively. TZ photoanodes

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were characterized by field emission scanning electron microscopy (FESEM), X-ray diffractometer (XRD), X-ray photoelectron spectrometry (XPS). The effect of different

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hydrothermal temperatures on DSSC performance was also investigated.

2. Experimental

2.1. Preparation of TNAs by Anodic Oxidation Method Titanium foils (1.3x2.5 cm) (0.25 thickness, 99.7% purity, Sigma Aldrich) were cleaned in an ultrasonic bath with acetone, 2-propanol and deionized water for thirty minutes,

respectively and dried with nitrogen gas. Then, TiO2 nanotubes were synthesized by anodic oxidation on titanium foils according to our previous study[35] where solution of 0.4 wt% NH4F (98% purity, Sigma-Aldrich), 5% (wt) deionized water and ethylene glycol (99.8% purity, Sigma-Aldrich) was used as an electrolyte. Platinum mesh (Pt) (99.9% purity, SigmaAldrich) and titanium foil were used as counter and working electrode, respectively. DC potential of 30 V, was applied for 3 h at room temperature under mild stirring. The prepared TNAs were cleaned with methanol in ultrasonic bath for 2 min. and dried under

anatase phase.

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2.2. Preparation of TZ photoanodes by Hydrothermal Method

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nitrogen gas. The amorphous TNAs were annealed at 450 ° C for 1 hour to convert into the

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The hydrothermal method was applied to form ZnO nanorods on the surface of the TNA in

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the anatase phase. 20 mL aqueous solution of 0.1 M zinc nitrate hexahydrate (98% purity, Sigma Aldrich) and 20 mL of 0.1 M hexamethylenetetramine (99.7% purity, Sigma Aldrich) solution were prepared and mixed. TNAs along with the solutions were placed in a Teflon-

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coated (autoclave) stainless steel hydrothermal reactor (DAB-2, Berghof GmbH)[17].

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Hydrothermal processes were carried out at different hydrothermal temperatures (50 ° C, 70 ° C, 90 ° C, 110 ° C, 130 ° C) for 4 hours. After hydrothermal process, TZ structures were

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washed with deionized water and dried with nitrogen gas.

2.4. Assembling and Performance Tests of DSSCs The DSSC was assembled as described in the previous study [35] where TZ, Pt coated FTO, I-/I3- were used as dye sensitized working electrode, counter electrode and electrolyte, respectively. TZ photoanodes were immersed into 0.05 mM ethanolic solution of Ru-based

commercial N719 (Sigma Aldrich) dye for 12 hours in the dark. Then, TZ photoanodes were rinsed with ethanol to remove free dye molecules. Platisol T (Solaronix) was used for the modification of FTO glass (Sigma Aldrich, ~ 13 Ω / sq) and modified FTO (Pt/FTO) was dried at 450 °C for 10 minutes. A 60 μm thick filler (Solaronix) were placed between TZ photoanodes and Pt/FTO. Finally, the I‾ / I3‾ redox pair (Hi-30, Solaronix) was injected into the cell.

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The performance tests of the DSSC were performed with the current-voltage measuring device (Keithley 2400), the potentiostat / galvanostat (Gamry-Interface 1000) used for EIS

measurements and the solar simulator (ABET 10500) used as the light source. The solar cell

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J-V curves were obtained using a solar simulator at 100 mW / cm2 illumination conditions

using the AM 1.5 G standard filter. Pmax, Voc, Isc, FF and ƞ values were calculated by using J-V

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curves. Finally the solar cell was connected to the Gamry-Interface 1000 potentiostat and

[17].

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2.6. Characterization details

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EIS measurements were performed by using 10 mV AC in the frequency range 10-2 - 105 Hz

Phase analyzes of the generated TZ photoanodes were performed by XRD (PANanalytical,

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Empyrean) between 10°- 90° angles at 2° rotation speed using Cu-Kα radiation (λ = 1.5406

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A°, 45 mV and 40 mA). Surface morphology of TZ photoanodes formed at different hydrothermal temperatures was analyzed by FESEM (FEI Quanta 450). Chemical composition of photoanodes were investigated with the X-Ray Photoelectron Spectroscopy (XPS) (Thermo Scientific, K-Alpha) using a monochromic Al anode X-ray gun (1486.6 eV). The binding energies of the spectrum were calibrated by adjusting the core level peak position of C1 to 284.8 eV [17].

3. Results and Discussions 3.1 Characterization of TZ photoanodes Initial studies were devoted to investigate surface morphology of photoanodes and given in

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Figure 1. Homogenously orientated TNA with 90 nm inner diameter and ~ 4 μm length were shown in Fig.1. The needle-like ZnO nanorods were clearly seen in Fig.1 b-f. It can be concluded from the figure, arising temperature lead an increase in the amount and size of

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ZnO nanorods at the temperature ranges between 50 to 110 oC. However, the amount and size of ZnO nanorods were decreased for 130 oC and this situation is in good agreement

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with literature [36].

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Fig 1. FESEM images of photoanodes a)TNA, b)TZ50, c) TZ70, d) TZ90, e)TZ110, f) TZ130

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Fig 2 shows the XRD patterns of TZ photoanodes. Characteristic anatase peaks were seen for (101), (004), (200), (105), and (204) diffractions at 25.35 °, 37.92 °, 48.12 ° and 63.15 ° (2θ) for anatase TiO2 (JCPDS No. 21-1272). Other diffraction peaks were came from titanium substrate (T). As shown in Fig 2, anatase (A) and both wurtzite (W) diffraction peaks were measured in the XRD pattern of TZ photoanodes fabricated by different hydrothermal temperatures. In the XRD spectra of TZ photoanodes, the diffraction angles

of 31,93°, 34,48°, 36,42°, 47,24° and 56,81° (2θ) were corresponded to the hexagonal wurtzite ZnO (100), (002), (101), (102) and (110) (JCPDS No. 36-1451) [17]. The intensity of wurtzite peaks of ZnO gradually increased up to 110° C and decreased at 130 °C and these

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results are consistent with the FESEM results.

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Fig 2. XRD patterns of TZ and bare-TNA (A: Anatase, W: Wurtzite, T: Titanium)

Fig 3 shows the XPS spectrum of the TZ130. The survey scan spectrum was shown in Figure 3a and proving the presence of Ti, Zn and O elements in the TZ photoanodes. High resolution XPS spectrum of Ti was given in Figure 3b. The binding energies at 548.5 eV at 464.3 eV can be attributed to Ti 2p3p/2 and Ti 2p1/2 peak indicating form of Ti is Ti+4 in photoanode [37,38]. In Figure 3c, Zn 2p3/2 and Zn 2p1/2 peaks were clearly seen at binding

energy of 1021.3 eV and 1044.2 eV, respectively. Since the peak separation of Zn 2p peaks was 22.9 eV, it is understood that Zn was in the form of ZnO on the surface of TZ photoanodes [39]. In Figure 3d, the O 1s peaks were fitted using gauss functions. The first peak of at the binding energy of 529.9 eV is corresponded to the oxygen atoms of TiO 2

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while the shoulder peak at 531.5 eV can be attributed O atom in ZnO structure [17,40].

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Fig 3. XPS spectra of TZ130 a) survey scan, b) Ti 2p scan, c) 2p scan and O 1s scan

The EIS technique is a practical tool to examine the kinetics of DSSCs. A higher frequency intersection on the actual axis corresponds to the series resistance at which the left semicircle at a higher frequency represents the charge transfer resistance for redox

reaction at the electrolyte / CE interface. Fig 4 shows Nyquist curves of DSSCs assembled from TNA and TZ photoanodes. The counter relation between electron transfer and recombination is a decisive factor in the solar cell performance and lower charge transfer resistance indicating the faster electron transfer [17,27]. The increasing hydrothermal

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temperature has led decline in the semi-circle radii of the Nyquist plots [17,41–43].

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Fig 4. Nyquist curves of a)TNA, b) TZ50, TZ70, TZ90, TZ110 and TZ130

3.2 Solar Cell Performance of DSSCs The photocurrent density-voltage (J - V) curve indicates the performance of the DSSCs and was shown in Figure 7. The performance parameters (Jsc, Voc, FF and ƞ) were calculated and

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given in Table 1.

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Fig 5. J-V curves of TNAs and TZ photoanodes prepared at different temperatures

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Table 1. The photo conversation parameters of DSSCs under AM-1.5G illumination. JSC (mA/cm2) 2.9

VOC (V) 0.69

FF (%) 53

 (%) 0.81

TZ50

3.1

0.68

52

0.87

TZ70

4.5

0.65

53

1.31

TZ90

4.9

0.61

54

1.37

TZ110

4.9

0.63

55

1.40

TZ130

5.3

0.68

56

1.67

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TNA

According to the Table 1, the Jsc, VOC and ƞ values of TZ photoanodes were obtained as 5.3 mA.cm-2, 0.68 V and 1.67% for TZ130. The enhancement in current density and ƞ values is probably due to the decoration of TNA with ZnO nanorods at different hydrotermal temperatures. The efficiency of the DSSC’s were increased with the arising hydrothermal temperature while, VOC potential was fluctuating. The efficiency obtained for TZ130 was doubled according to the efficiency obtained for TNA. This efficiency enhancement can be attributed to enhanced electron transport and suppression of the recombination rate

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between photoanode and electrolyte [17,27]. Among the temperature studied, the photoanode TZ130 gave the best efficiency probably due to the synergistic effect between

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the TiO2 nanotubes and the ZnO nanorods. 4. Conclusions

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In summary, TZs were fabricated with two step method as photoanodes for DSSC. Anodic

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oxidation method was used for the preparation of TNA and TZ were prepared by hydrothermal method. The effect of hydrothermal temperature on the morphology, crystal

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and electronic structure, solar conversion performance was investigated. Then, the effect of TZ photoanodes fabricated by applying different hydrothermal temperatures on DSSC

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performance was determined by J-V and EIS measurements. The photovoltaic efficiency of the DSSCs enhanced, while the charge transfer resistance decreased to 110 °C and

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increased at 130 °C. The highest current density and photovoltaic efficiency values for TZ130 were reached as 5.3 mA / cm2, 0.68 V and 1.67%, respectively. The photovoltaic performance of TZ130 photoanode enhanced more than two times, as compared to TNA photoanodes. This enhancement can be attributed to the synergetic interaction of the TiO2 nanotube and ZnO nanorods that resulted a decline recombination rate thus, increase in electron transfer.

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 authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Acknowledgements This paper is produced from MSc dissertation of Çiğdem Eden which was supported by the

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Fig 1. FESEM images of photoanodes a)TNA, b)TZ50, c) TZ70, d) TZ90, e)TZ110, f) TZ130 Fig 2. XRD patterns of TZ and bare-TNA (A: Anatase, W: Wurtzite, T: Titanium)

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Fig 3. XPS spectra of TZ130 a) survey scan, b) Ti 2p scan, c) 2p scan and O 1s scan

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Fig 4. Nyquist curves of a)TNA, b) TZ50, TZ70, TZ90, TZ110 and TZ130

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Fig. 5. J-V curves of TNAs and TZ photoanodes prepared at different temperatures