Effects of RF and pulsed DC sputtered TiO2 compact layer on the performance dye-sensitized solar cells

Surface & Coatings Technology 231 (2013) 126–130

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Effects of RF and pulsed DC sputtered TiO2 compact layer on the performance dye-sensitized solar cells Q.Q. Liu a, D.W. Zhang a, J. Shen a, Z.Q. Li a, J.H. Shi a, Y.W. Chen a, Z. Sun a, Z. Yang b, S.M. Huang a,⁎ a Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, Department of Physics, East China Normal University, North Zhongshan Rd. 3663, Shanghai 200062, PR China b National Key Laboratory of Nano/Micro Fabrication Technology, Key Laboratory for Thin Film and Microfabrication of the Ministry of Education, Research Institute of Micro/Nano Science and Technology, Shanghai Jiao Tong University, Shanghai 200240, PR China

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Available online 8 February 2012 Keywords: Dye-sensitized solar cell TiO2 compact layer Magnetron sputtering

a b s t r a c t We investigated the characteristics of TiO2 compact layers grown by RF and pulsed DC magnetron sputtering on F-doped SnO2 (FTO) electrodes from a TiO2 ceramic target. The morphological and microstructural properties of the formed TiO2 layers were characterized by scanning electronic microscopy and X-ray diffraction. The deposition rate of the compact TiO2 layer by pulsed DC sputtering was much higher than that of RF deposition under the same sputter power and deposition pressure. Moreover, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSCs) is strongly dependent on the thickness of both RF and DC sputtered TiO2 layer inserted between FTO electrode and nanoporous TiO2 layer. The thickness of the sputtered TiO2 layer was changed from 0 to 200 nm. The electrochemical impedance spectroscopy (EIS) technique was employed to evaluate the recombination resistance and electron lifetime in DSCs with differently thick TiO2 passivating films. The DSC fabricated on 160 nm thick TiO2 passivating FTO electrode showed the maximum power conversion efficiency of 7.90% due to highly effective prevention of the electron transfer to electrolyte. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Dye-sensitized solar cells (DSCs) have attracted much attention as the next-generation solar cells with low production costs and high efficiency for energy conversion, reaching 11% and a module efficiency of 7% [1]. The working electrode of DSC is typically composed of a thick TiO2 mesoporous film on flurorine-doped SnO2 transparent conducting oxide (FTO) glass. The TiO2 mesoporous film provides a large surface area for anchoring the light-harvesting dye molecules [2,3]. To obtain high performance DSCs, it is desirable to prevent the carrier leakage at mesoporous FTO/TiO2 interface by electron transfer to redox ions in the electrolyte [4]. Thin and compact TiO2 film between the FTO and mesoporous TiO2 layer has been suggested as effective passivating layer (or hole blocking layer) in the DSCs due to its high transparency, chemical stability, and suitable work function with FTO and nanocrystalline TiO2 layer [5–8]. TiO2 with thin and compact structures can be deposited by various techniques such as using spray pyrolysis [5], spin coating [6], dip coating [7], and sputtering [8]. The growth of a thin film employing vacuum technology, such as sputtering and evaporation, has a variety of advantages, i.e., easy control, clean process, reproducibility, high adhesion, and large scale-up. It was found that the structure, composition, and photocatalytic properties of sputtered TiO2

⁎ Corresponding author. E-mail address: [email protected] (S.M. Huang). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.01.064

films are influenced by various deposition conditions such as target substrate distance, sputtering power and working pressure [9–12]. The formation of a thin and compact TiO2 film on a FTO substrate has a significant effect on both the conversion efficiency and the commercialization of DSC. Hattori and Goto [13] reported that RF sputtered TiO2 film with rutile structure acted as an effective short circuit preventive layer (blocking layer) between FTO and porous TiO2 films. Very recently, Jin-A Jeong and Han-Ki Kim [14] also reported that the DSCs fabricated on 50 nm-thick RF sputtered TiO2 passivating FTO electrode using a TiO2 ceramic target showed the highest power conversion efficiency of 4.42% due to effective passivating properties of the dense TiO2 layer. Although the passivation or blocking properties of the RF sputtering TiO2 film have been reported in DSCs, the deposition rate is very low [13,14]. Moreover, the optical, structural, surface properties and thickness of the TiO2 passivating layer play important roles in blocking the direct contact between the redox electrolyte and the FTO surface, reducing the electron recombination and raising the cell efficiency. Their roles have not been understood well up to now. In this work, the TiO2 passivating layer was grown by DC magnetron sputtering on the FTO electrodes at room temperature using a TiO2 ceramic target. From a practical and industrial point of view, DC sputtering is desirable for the large area coating because it is much simpler than RF sputtering and it gives a higher deposition rate. We studied the thickness effect of passivating TiO2 layer on the performance of the DSCs. The photoelectrochemical properties of the DSCs

Q.Q. Liu et al. / Surface & Coatings Technology 231 (2013) 126–130

2. Experimental A homemade magnetron sputtering system was applied. The system was equipped with six 3 in. magnetron guns (US' Gun) separately powered by DC power supply (Advanced Energy, MDX DC 500 W) and an rf source (Advanced Energy, RFX 600A). MDX LOW-POWER 500 W is a leading performer in basic magnetron sputtering. The pulsed DC power delivered in MDX drive provides capability enabling dielectric sputtering applications due to its ability to reduce or even eliminate arcing during the deposition process. The fundamental behavior of reverse-voltage pulsing and its ability to reducing arcing has been studied in early times [15], but very little work has been published with detailed processing information so far. In this work, a titanium dioxide ceramic target was used. The substrates were mounted on a 150 mm diameter holder. The target-substrate distance was about 8 cm. The substrate holder was water cooled. The flurorinedoped SnO2 transparent conducting oxide (FTO) glass (15Ω/sq, 13 × 14 mm2) was ultrasonically cleaned in ethanol, acetone, and deionized water, respectively. The thickness of the FTO film on the substrate is 400 nm. Prior to the deposition, the chamber was evacuated to a background pressure of 1 × 10 − 4 Pa, and the target was pre-sputtered for 10 min to remove the impurities on the target surface. The working pressure during the film deposition was controlled at 0.6 Pa by applying pure Ar gas. The discharge power was between 30 and 120 W. TiO2 films with different thicknesses were deposited by RF and DCmagnetron sputtering on FTO glass substrates at room temperature. Arc suppression was found to be effectively addressed during DC sputtering of a TiO2 target by MDX 500. DSCs were prepared by modifying the fabrication procedures reported in our previous work [16,17]. The TiO2 working electrodes were prepared by the screen-printing technology on variously thick sputtered TiO2/FTO substrates. The thickness of the film was controlled by the printing times. The screen-printed layer was dried in air at room temperature for 15 min and then kept at 100 °C for 10 min. Then the film was fired at 500 °C in air for 30 min. After sintering at 500 °C for 15 min and cooling down to 80 °C, the nanostructured TiO2 films were immersed into the dye solution (0.5 mM N719 [18] (Solaronix) in acetonitrile and tert-butyl alcohol (volume ratio of 1:1)) at room temperature for 20 h. After rinsed with acetonitrile, the TiO2 anodes were assembled with the prepared Pt counter-electrodes. The cells were sealed with Surlyn 1702 (Dupont) gasket with a thickness of 60 μm. A drop of electrolyte solution (0.1 M Guanidine Thiocyanate (GuSCN), 0.03 M I2, 1.0 M 1-methyl-3-propylimidazolium iodide (MPII) and 0.5 M tert-buthylpyridine in acetonitrile) was introduced into the cell by capillarity. Finally, the holes were sealed using the same Surlyn film and a cover glass with a thickness of 0.7 mm. The morphology of DC and RF sputtered TiO2 films was examined by field emission scanning electronic microscopy (FE-SEM). The transmittance of the compact films on FTO was measured with an UV–VIS scanning spectrophotometer (Hitachi, U-3010) from 300 to 800 nm. The crystallographic orientation of the sputtered TiO2 films was determined by an X-ray diffractometer (XRD, Dandong DX2700) using Cu Kα irradiation. The thickness of sputtered TiO2 films was measured with surface profiler (Dektak 6M, Veeco). Photoelectrochemical characteristics of the DSCs were analyzed by electrochemical impedance spectroscopy (EIS) [16,17]. The EIS spectra were measured with a potentiostat/falvanostat (PG30.FRA2, Autolab) in the dark. The frequency range was 0.1 Hz to 100 kHz. The applied bias and

ac amplitude were set at open-circuit voltage (Voc) of the DSCs and 10 mV between the Pt counter electrode and the TiO2 working electrode, respectively. The obtained spectra were fitted with Z-View software (V3.10) in terms of appropriate equivalent circuits. The photovoltaic (PV) performance parameters of DSCs were measured under a simulated illumination (Solar simulator: Newport Oriel 93194A) with a light intensity of 100 mW cm − 2. Quantum efficiency measurements (QE) (74125, Oriel, USA) were carried out for these cells. 3. Results and discussions Fig. 1 shows the growth rate of the TiO2 compact layer on the FTO glass as a function of RF and DC power, respectively. The deposition rate of films deposited at 0.6 Pa increases with the increasing of RF and DC sputter power. The RF sputtering rate of TiO2 film is about 0.16–1.8 nm/min at 40–120 W RF power. This result is well consisted with that reported in Ref. [19]. In contrast, DC sputtering produced a much higher deposition rate. The DC sputtering rate is about 2.6– 8.5 nm/min at the same discharge power range. Hossain et al. [20] reported deposition of TiO2 film by DC reactively sputtering of two facing Ti targets at a DC input power of 500 W for a fixed gas ratio of Ar to O2 of 7:3. The DC sputtering rate of TiO2 obtained in this work is also much larger than theirs. Fig. 2 shows the optical transmittance spectra of DC sputtered TiO2 compact layers on FTO glass substrates as a function of the compact TiO2 thickness. The bare FTO glass substrate shows a transmission of about 80% in the wavelength range of 400–800 nm. The FTO glass sample coated with DC sputtered TiO2 passivating layer below 200 nm thickness showed a transmittance about 70% in the range of 400–800 nm, which was a little lower than that of the bare FTO glass substrate. It was also found that at the same thickness, DC sputtered TiO2 passivating layer covered FTO glass showed a higher transmittance than that of RF sputtered TiO2 covered sample. Fig. 3 shows the optical transmittance spectra of the bare FTO glass and 100 nm-thick DC and RF deposited TiO2 films on FTO glass substrates. The RF deposited TiO2 film on FTO glass substrate shows a transmittance about 67% in the range of 400–800 nm. Thus, the RF deposited TiO2 film may be more compact than DC deposited TiO2 film. To investigate the morphology of the TiO2 compact layer grown on the FTO glass substrate, FESEM analysis was employed. Figs. 4 and 5 show FESEM images and XRD patterns of DC and RF sputtered TiO2 layers on the FTO glass. The films were deposited at the same power (100 W). Their thicknesses are 200 nm. Fig. 3 shows the SEM images of the as-sputtered TiO2 layers. Both films display a rough and densely packed morphology, presenting particles about 100 nm in diameter. The rough structure of the compact TiO2 layer can be expected to improve the adhesion between the FTO and mesoporous 9

Deposition rate (nm/min)

with differently thick TiO2 passivating films were analyzed by electrochemical impedance spectroscopy (EIS). It was found that the power conversion efficiency of DSCs critically depended on the thickness of the TiO2 passivating layer. The DSCs with a 160 nm-thick DC sputtered TiO2 passivating layer showed the highest power conversion efficiency of 7.90% due to effective passivating properties of the compact TiO2 layer.

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Discharge Power (W) Fig. 1. Deposition rate of the TiO2 compact layer on the FTO electrode as a function of DC power (a) and RF power (b).

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Wavelength (nm) Fig. 2. Optical transmittance spectra of DC sputtered TiO2 compact layer on FTO glass substrates as a function of the compact TiO2 thickness.

TiO2 layer and enhance the mechanical strength of the porous TiO2 films to the glass substrates. Moreover, by comparing Fig. 4 (a) and (b), we found that the morphology of the RF sputtered film showed particles with clearer crystalline faces or edges. The RF sputtered film is also more compact than the DC sputtered layer, which is consisted with the optical measurement result shown in Fig. 3. Fig. 5 shows XRD patterns of bare FTO glass substrate and DC and RF deposited TiO2 compact films on FTO glass substrates. The XRD patterns of the bare FTO glass sample exhibited polycrystalline peaks at 2θ = 27.161° (110), 38.421° (200) and 52.11° (211). From Fig. 5, it can be seen that both DC and RF sputtered TiO2 layers on FTO glass substrates show identical peaks with the XRD plot of the bare FTO glass sample. Therefore, the structure of the TiO2 passivating layer on the FTO electrode could be amorphous during the DC and RF sputtering process. On the other hand, from FESEM measurement result shown in Fig. 4(a) and (b), the RF sputtered TiO2 film displays particles with clearer crystalline faces or edges than the DC sputtered layer. No rutile or anatase related peaks of TiO2 were detected. Probably, the deposited compact TiO2 film may be not thick enough. One of the intrinsic characteristics of the classical DC magnetron sputter process is the relatively high processing voltage necessary to sustain the discharge by secondary electron emission from the target surface. In RF processes, on the other hand, ionization is mainly driven by oscillating electrons in the bulk plasma [21], which leads to substantially lower discharge voltages. The higher processing and discharge voltage from the TiO2 target surface resulted in the higher sputter rate shown in Fig. 1. Besides, in RF processes there is an enhanced substrate bombardment by plasma ions of moderate energy. This is due to the

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Fig. 4. FESEM images of TiO2 compact layer by DC (a) and RF sputtering (b) on FTO glass substrates.

higher difference between plasma and floating potential Vp–Vf in an RF discharge [22]. Such moderate energy ion bombardment assisted the TiO2 compact film growth during the RF deposition and led to better and denser films shown in Figs. 3 and 4. EIS has been widely used to investigate the interfacial charge transfer processes occurring in DSCs [17,23–28]. In order to understand the difference in performance of DSCs formed on DC sputtered

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2 Theta (degree) Fig. 5. XRD patterns of bare FTO glass substrate (a) and RF (b) and DC (c) deposited TiO2 compact films on FTO glass substrates.

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Frequency (Hz) Fig. 7. Bode phase plots of DSCs with differently thick DC sputtered TiO2 compact layers on FTO glass substrates under forward bias 0.8 V in the dark.

thickness of the RF sputtered TiO2 film, but the related efficiency is smaller and within the range of 4.14–5.02%. It is well known that the Voc value is strongly related to the DSC structure [29,30]. From Fig. 8, the insertion of the thin (0–190 nm) and compact DC sputtered TiO2 film between mesoporous TiO2 layer and FTO has a slight influence on the Voc. The addition of compact TiO2 films enlarged the effective adhesion area of dye molecular to TiO2 particle/film surfaces to some extent. Therefore the contact interface was improved and consequently the FF was enhanced. From Fig. 8, the DSC fabricated on the bare FTO/glass substrate showed Voc of 0.79 V, Jsc of 11.4 mA cm− 2, FF of 0.52, and η of 4.6%. It was observed that the efficiency of the DSC fabricated on the 70–190 nm thick DC sputtered TiO2/FTO/glass substrate is higher than that of the DSC on the bare FTO/glass substrate. The higher efficiency could be attributed

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TiO2 compact layers with different thicknesses, EIS measurement was carried out under forward bias 0.8 V in the dark. Fig. 6 shows the impedance spectra of DSCs fabricated with differently thick TiO2 compact layers under forward bias 0.8 V in the dark. Resistances of RS, RCT1 and RCT2 were estimated by fitting the data with the equivalent circuit shown in the inset of Fig. 6, where RS, RCT1 and RCT2 mainly represent sheet resistance of the electrolyte and the transparent conductive oxide, charge transfer resistance at Pt counter electrode and mesoporous TiO2/dye/electrolyte charge transfer resistance, respectively [17,27,28]. From Fig. 6, it can be found that the recombination resistance at the TiO2/dye/electrolyte interface (RCT2) increased with the increasing of the thickness of compact layer from 0 to 160 nm, and then decreased slightly with the further increasing of the thickness. The RCT2 value of the cell with 160 nm compact layer is the largest. The increase of the RCT2 with the increase of the thickness of TiO2 compact layer from 0 to 160 nm indicates reduction of electron recombination and more efficiency of electron transport [17]. The simulated RCT2 of the cell with 160 nm compact layer is about 68 Ω, much larger than the one of the cell with 70 nm compact layer (about 47 Ω). As shown in Fig. 7, the characteristic frequency peak of porous TiO2 at intermediate frequency region in corresponding Bode phase plot does shift a little with increasing the compact TiO2 film thickness, which indicates that the electron lifetime in porous TiO2 layer is effected by the insertion of compact TiO2 film. Fig. 8 shows the photovoltaic performance of DSCs formed on DC sputtered TiO2 compact layers with different thicknesses under AM1.5 illumination (100 mW/cm2). The open-circuit voltage (Voc) is almost kept at about 0.8 V when the compact layer thickness is changed from 0 to 190 nm. The fill factor (FF) of the DSC, however, varies with the changing of the thickness of the compact layer. The DSC directly formed on FTO without TiO2 compact layer only showed a FF value of 0.52. The FF value increased to 0.66 with a 70 nm-thick compact layer, and achieved a maximal FF of 0.67 at 160 nm thickness. Moreover, it was worthy of note that the insertion of this DC sputtered TiO2 layer between microporous TiO2 film and the FTO electrode leads to the increasing of the photocurrent density (Jsc) and efficiency (η) simultaneously. The Jsc and η values increase with the increase of the TiO2 compact layer thickness from 0 to 160 nm and then decrease with the further increase in thickness from 160 to 190 nm. The Jsc and η enhancement is a result of unfailing improvement of QE as shown in Fig. 9. The maximum QE occurs at the absorption maximum of the dye [18], 540 nm, with the highest QE being 54% for 160 nm-thick compact layer. Compared to the case of the DSC on the bare FTO, the QE of the DSC with a 100–190 nm-thick TiO2 compact layer was enhanced in the 300−800 nm wavelength region. In addition, the performance of DSC with a RF sputtered TiO2 compact film also changed with the

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The study demonstrates that the DC sputtered TiO2 as dense and compact layers on FTO glass substrates are a promising electrode scheme for highly efficient DSCs, due to its effective prevention of leakage current from FTO to electrolyte as well as its high deposition rate and good transparency.

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Acknowledgments 20

This work was supported by National Natural Science Foundation of China (No. 10774046) and Shanghai Municipal Science and Technology Committee (No. 09JC1404600).

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Wavelength (nm) Fig. 9. Dependence of QE spectra on DC sputtered TiO2 compact layer thickness which changes in the range from 0 to 190 nm.

to the effective passivation of the FTO electrode by the dense TiO2 layer, and prevention of the electron transportation to the electrolyte. The DSC fabricated on the 160 nm thick DC sputtered TiO2/FTO/glass substrate showed the highest η (7.90%) due to the largest photocurrent density. From Fig. 8, the optimized passivating thickness of the TiO2 layer is 160 nm. However, further increase in thickness of the TiO2 passivating layer up to 190 nm resulted in a decrease of η value (6.54%). The reason is due to the decreased substrates transmission and decreased conductivity for electron transfer from nanoporous TiO2 layer to the FTO electrode. Therefore, thin and compact DC sputtered TiO2 film between the FTO and mesoporous TiO2 layer has been demonstrated as effective passivating layer (or hole blocking layer) in the DSCs. 4. Conclusions The characteristics of DC and RF sputtered TiO2 passivating layers with various thicknesses on the FTO electrode were investigated for its application in DSCs. The DC sputtered TiO2 passivating layer showed the better transparency than the RF sputtered TiO2 film. The deposition rate of the compact TiO2 layer by DC sputtering was much higher than that of RF deposition under the same deposition conditions. It was found that the energy conversion efficiency of the DSC is critically dependent on the thickness of DC sputtered TiO2 compact layer inserted between FTO electrode and microporous TiO2 layer. The DSC fabricated on 160 nm-thick compact TiO2 covered FTO electrode achieved the highest energy conversion efficiency of 7.90% due to effective prevention of electron transfer to electrolyte.

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