application for photoanodes of dye-sensitized solar cells

Plasma-etched nanoporous TiO2 using Ag nanoparticle masks: application for photoanodes of dye-sensitized solar cells Hsin-Han Huang1, Haoming Chang1, Hsiao-Wei Liu2, Ching-Wen Hsu2, I-Chung Chiu2, Mao-Ying Teng3, Hong-Jen Lai3, I-Chun Cheng2,4 and Jian-Zhang Chen1 1

Institute of Applied Mechanics, National Taiwan University, Taipei 10617, Taiwan Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan 3 Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu County 31040, Taiwan 4 Department of Electrical Engineering, National Taiwan University, Taipei 10617, Taiwan E-mail: [email protected] 2

Received 3 March 2014, revised 14 April 2014 Accepted for publication 16 April 2014 Published 20 May 2014 Materials Research Express 1 (2014) 025505 doi:10.1088/2053-1591/1/2/025505

Abstract

We investigate dye-sensitized solar cells (DSSCs) with nanoporous TiO2 photoanodes etched by inductively coupled plasmas (ICPs). Thermally Shrunk Ag nanoparticles are used as the etching masks during the ICP etching procedure. The efficiency of the assembled DSSC increases first and then decreases as the ICP etching time increases. The enhancement of light trapping/scattering is observed after ICP etching with Ag nanoparticle masks, however, over-etching may mitigate the effect. Interfacial charge transfer impedance of TiO2/dye/ electrolyte also decreases first and then increases as the etching time increases, a trend highly correlated to the variation of the cell efficiency. Our experimental results indicate that the enhancement of light trapping (which leads to the increase of photocurrent), increase of open circuit voltage and reduction of charge transport resistance lead to the improvement of cell efficiency. The optimized etching time is around 30 s to 1 min. In comparison to the counterpart experiment, the DSSC with TiO2 photoanode etched by ICP without Ag nanoparticle shadow masks, the cell reveals monotonic decreases of photocurrent level, open circuit voltage, and efficiency with the ICP etching time. Keywords: TiO2, nanoporous, dye-sensitized solar cells

Materials Research Express 1 (2014) 025505 2053-1591/14/025505+11$33.00

© 2014 IOP Publishing Ltd

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Mater. Res. Express 1 (2014) 025505

1. Introduction

Dye-sensitized solar cells (DSSCs) are a promising technology in renewable energy harvesting [1–5]. The photoelectron generation and transport are separated into two distinct materials. The electrons are optically excited in dyes and then inject into semiconductor nanoporous photoanodes. In economic aspect, the costs can be merely a fraction of Si-based photovoltaic devices because the semiconductor photoanodes are usually made of TiO2 nanoparticles that have been already in a mass-production process such as flame synthesis [6–8]. DSSCs have thus attracted considerable research interest in both academia and industry; there have been tremendous research efforts focusing on the developments of dyes [9–14], counter electrodes [5, 15–19], and semiconductor photoanodes [3, 7, 20–32]. Semiconductor photoanodes are important in both electrical and optical aspects. In conventional DSSCs, light passes through the glass substrates and semiconductor photoanodes to excite dyes optically. Photoelectrons transfer into the photoanodes from the dyes and transport through the semiconductor to the external loading circuits. The morphology of semiconductor layers is therefore important because it influences both the electrical impedance for electron transport and the light-trapping for photon passage. The light absorption can be enhanced by the light scattering or trapping in the photoanodes through the morphology manipulations of the photoanodes [20–22, 24, 33–35]. Plasma technology has been applied extensively to the fabrication processes of TiO2 photoanodes for DSSCs. Plasma surface treatments on TiO2 photoanodes were shown to enhance the performance of DSSCs [36–39]. Surface treatment of TiO2 films using atmospheric-pressure non-equilibrium dc pulse discharge plasma jet also increased the efficiency of DSSCs [40, 41]. One-minute atmospheric-pressure-plasma-jet (APPJ) TiO2 sintering process has been developed to replace the furnace-sintering process that takes a longer processing time and a larger thermal budget [3, 28]. APPJs processes involve reaction chemistry in non-thermal plasmas; the participation of oxygen atoms in the plasmas can effectively remove organics by oxidation, rendering rapid processing capability [3, 28, 42]. TiO2 prepared by modified dielectric barrier discharge (DBD) jet with elevated substrate temperatures were successfully used for DSSCs [43, 44]. Post DBD treatments on TiO2 improved the cell efficiencies of DSSCs [45]. In addition, cyclonic atmospheric-pressure plasma was used for the surface treatment on electrospun poly(vinylidenefluoride-co-hexafluoropropylene) (PVDFHFP) microfibrous membrane of DSSCs [46]. In this paper, we use inductively coupled plasmas (ICPs) to fabricate nanopillar porous TiO2 photoanodes of DSSCs. Thermally shrunk Ag nanoparticles are used as the etching mask for the nanopillar fabrication. The cell efficiency increases first then decreases as the nanopillar height (etching time) increases. ICP etching creates nanopillar structure to increase the light scattering and trapping in photoanodes, however, it also reduces the TiO2 total volume and total surface area for dye-anchoring. The DSSC performance and the related material properties are also discussed.

2. Experimental details

Fluorine-doped tin oxide (FTO) coated glass slides (FTO thickness: 40 nm; sheet resistance: 8 Ω/□) were used as the substrates for DSSCs. The TiO2 pastes (E-solar P300, Everlight Chemical Industrial Co.) were first screen-printed onto the FTO glass, and subsequently 2

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Figure 1. Fabrication procedure of nanopillar porous TiO2 structure.

calcined at 510 °C for 15 min. This coating procedure was repeated three times for the TiO2 thickness to reach 11 μm. Afterwards, a 10 nm thick Ag layer was deposited on the nanoporous TiO2 layer by e-beam evaporation followed by annealing process at 500 °C for 30 min in a forming gas (N2/H2 ratio = 9:1) atmosphere. During the annealing process, the Ag films shrank into nanospheres which were used as the etching masks for the ICP etching. The ICP condition of the etching process was as follows: CHF3 flow rate, 50 sccm; RF power, 300 W; substrate bias, 100 W; working pressure, 0.6 Pa. Four different etching durations, 30 s, 1 min, 5 min, and 10 min were used to obtain nanopillars of various depths. The Ag nanoparticles were then removed using an etching solution of diluted HNO3 (HNO3/H2O = 1:10). The fabrication 3

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Figure 2. SEM images (45° top view) of porous TiO2 with various etching times: (a)

0 min; (b) 0.5 min; (c) 1 min; (d) 5 min; (e) 10 min.

procedure of nanopillar porous TiO2 structure is illustrated in figure 1. An experimental counterpart with TiO2 etched without Ag nanoparticle shadow masks was also performed for comparison. The fabricated nanoporous TiO2 photoanodes were then immersed in a mixed solution of acetonitrile (99.9%, J. T. Baker) and tertiary butyl alcohol (99.9%, J. T. Baker) containing 3 × 10−4 M of N719 dye (Solaronix) for 24 h. The dye-absorbed porous TiO2 photoanodes were rinsed with ethanol and dried at room temperature. A 10 nm Pt was dc-sputtered onto an FTO glass substrate as the counter electrode, which was assembled with the dye-absorbed TiO2 photoanode with spacers. Thereafter, the liquid electrolyte (E-Solar EL 100, Everlight Chemical Industrial Co.) was injected into the assembled cells. An AM1.5 solar simulator (WACOM, WXS-155S-L2) equipped with a Keithley 2400 electrometer was used to measure the IV characteristics of the cells. The cells are tested with light shining through the FTO substrates. The illuminated cell area was 0.22 cm2. The morphology of TiO2 films was examined by scanning electron microscopy (SEM, Hitachi S800). A UV-Vis-NIR spectrophotometer (JASCO V-670) with integration sphere was used to measure the transmission (T) and reflection (R) of the pure and dye-anchored porous TiO2 coated FTO glass substrates. The absorption (A) was then calculated using the formula A = 1-TR. Electrochemical impedance spectroscopy (EIS) analysis is performed using an electrochemical workstation (Zahner Zennium). The EIS spectra were obtained by applying sinusoidal perturbations of ±10 mV with frequencies ranged from 0.1 to 105 Hz at open circuit voltage (Voc) to the cell under an illumination of a solar simulator.

3. Results and discussion

Figure 2 shows the TiO2 surface morphologies with various etching times. It can be clearly observed that the nanopillar structure is deepened with the etching time. As the etching time

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Figure 3. Light absorption spectra of TiO2 layers with various etching times. (a) TiO2

etched with silver nanoparticle masks; (b) TiO2 etched without silver nanoparticle masks.

reaches 10 min, the nanopillar structure becomes less distinct, probably due to the damage of Ag nanosphere shadow masks by ICPs. Figure 3 plots the absorption spectra of pure TiO2 layers with various etching durations. The reference TiO2 without any ICP etching is also plotted for comparison. For the TiO2 layers etched with Ag nanoparticle shadow masks, the absorption significantly increases after 30 s ICP etching. For the etching time between 30 s and 5 min, no apparent difference is observed. The absorption drops tremendously as the ICP etching time reaches 10 min. The increase of absorption is attributed to the increase of light scattering/trapping due to the roughened structure of the nanopillar TiO2 layer. In comparison to TiO2 etched without shrunk Ag nanoparticles, the absorption spectra show no clear alteration with the etching time. Figure 4 shows the IV characteristics of assembled DSSCs. For the DSSCs with nanopillar photoanodes, the cell efficiency increases initially and then decreases with the etching time, as shown in figure 4(a). On the other hand, in the case of DSSCs with photoanodes purely etched by ICP, the cell efficiency monotonically decreases with the etching time. The corresponding cell performance parameters are listed in tables 1 and 2, respectively. The cell efficiency is strongly correlated with the photocurrent levels and open circuit voltages. Similar to cell

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Figure 4. IV characteristics for (a) DSSCs with nanopillar photoanodes; (b) DSSCs with

photoanodes etched without Ag nanoparticle masks. Table 1. Performance parameters for DSSCs with nanopillar TiO2 photoanodes.

Ref (no ICP etching) ICP etching 30 sec ICP etching 1 min ICP etching 5 min ICP etching 10 min

Voc (V)

Jsc (mA cm−2)

F.F (%)

η(%)

0.734 0.749 0.741 0.732 0.684

9.25 9.81 9.96 9.75 7.33

68.69 69.93 68.84 66.17 66.22

4.67 5.14 5.08 4.72 3.32

efficiency, photocurrent level and open circuit voltage increases first and then decreases with the increase of the etching time. The initial etching creates the nanopillar surface to enhance the light trapping, causing the increase of photocurrent levels. As the etching time increases, the total volume of TiO2 and the total amount of adsorbed dyes are reduced, leading to the decrease of photocurrent levels. These two competing mechanisms result in the occurrence of optimized photocurrent level for DSSCs with photoanodes etched for around 30 s to 1 min. Open circuit voltage increases with the decrease of surface recombination centers [47–49]. The ICP etching may influence the open circuit voltage in two aspects. The etching process microscopically

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(a) -12 -10

-Z''(Ω)

-8 -6 -4

REF ICP etching 30 sec ICP etching 1 min ICP etching 5 min ICP etching 10 min

-2 0 10

15

20

30

25

35

40

45

Z'(Ω) (b) -12 -10

-Z''(Ω)

-8 -6 -4

REF ICP etching 30 sec ICP etching 1 min ICP etching 5 min ICP etching 10 min

-2 0 20

25

30

35

40

45

50

55

Z'(Ω)) (c)

Rct1

Rct2

Zw

Rs

CPE1

CPE2

Figure 5. Nyquist plots for (a) DSSCs with nanopillar photoanodes; (b) DSSCs with photoanodes etched without Ag nanoparticles. (c) Equivalent circuit for EIS analysis.

roughens the surface, thereby increasing the density of surface recombination centers. This should generally reduce the open circuit voltage. On the other hand, the etching chemicals may passivate the surface recombination centers [50–52], leading to an increase in open circuit voltage. The former mechanism together with the thinning of TiO2 layer should be responsible for the monotonic decrease of open circuit voltage with the etching time in regard to the DSSCs with photoanodes etched without Ag nanoparticle masks. In the case of DSSCs with nanopillar photoanodes, the open circuit voltage rises for the early stage of etching but decreases for longer etching time. We speculate that the presence of Ag nanosphere may confine the etching chemical species to facilitate the passivation of the surface recombination centers, leading to the increase of open circuit voltage at the early stage of ICP etching. The longer etching time may damage the Ag nanospheres, as evidenced by figure 2(e) in which the resultant nanopillar structures are not clearly distinguished. This makes etching chemicals less confined such

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Table 2. Performance parameters for DSSCs with TiO2 etched without Ag nanoparticle

masks. Ref (no ICP etching) ICP etching 30 sec ICP etching 1 min ICP etching 5 min ICP etching 10 min

Voc (V)

Jsc (mA cm−2)

F.F(%)

η(%)

0.754 0.712 0.712 0.682 0.667

9.95 9.32 8.87 7.85 6.48

63.84 68.04 68.40 65.74 67.79

4.79 4.52 4.32 3.52 2.93

that the recombination center creation mechanism dominates to decrease the open circuit voltage. The modification of TiO2 surface by ICP etching is also evidenced by the EIS measurement. Figure 5(a), (b) show EIS Nyquist plots for DSSCs and figure 5(c) illustrates the model equivalent circuit. Two semicircircles are clearly identified for each measurement. The most significant change occurs in the right semicircle whose diameter represents the interfacial charge transfer impedance of TiO2/dye/electrolyte [53]. For the case of nanopillar photoanode DSSCs, this charge transfer impedance decreases first and then increases as the etching time increases, as shown in figure 5(a). On the other hand, the impedance monotonously increases with the etching time for the DSSCs with photoanodes etched without Ag nanosphere shadow masks, as shown in figure 5(b). The reduction of TiO2 volume by ICP etching should result in the decrease of total surface area. This will increase the interfacial charge transfer impedance of TiO2/dye/electrolyte to obstacle the charge transfer, resulting in the lower cell efficiency. The creation of surface recombination centers can also increase the interfacial change transfer impedance. However, in the case of TiO2 photoanodes etched with Ag nanosphere shadow masks, the interfacial TiO2/dye/electrolyte impedance decreases initially during the early stage of etching. This may also be attributed to the passivation of surface recombination centers, thereby facilitating the charge transfer and reducing the interfacial impedance.

4. Conclusion

DSSCs with nanopillar porous TiO2 photoanodes are successfully fabricated. The nanopillar structures are created by ICP etching with thermally shrunk Ag nanoparticles as the etching masks. The efficiency of the assembled DSSC increases initially and then decreases with the ICP etching time. The cell efficiency is strongly correlated with the photocurrent level and open circuit voltage. The interfacial charge transfer impedance of TiO2/dye/electrolyte also decreases first and increases with the etching time. Our experimental results infer that the enhancement of light trapping, increase of photocurrent, increase of open circuit voltage, and reduction of the charge transport impedance lead to the increase of cell efficiency. The increase of etching time also decreases the total volume of TiO2 and total amount of absorbed dyes. This competing mechanism leads to the optimized etching time of ∼30 s to 1 min A counterpart experiment for DSSCs with TiO2 photoanodes etched without Ag nanoparticle masks exhibits a monotonic decrease of efficiency as the ICP etching time increases, owing to the thinning of TiO2 layer.

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Acknowledgments

The authors gratefully acknowledge the funding support from the National Science Council of Taiwan under grant nos. NSC102-2221-E-002-060 (JZC); NSC100-2221-E-002-151-MY3, NSC101-2628-E-002-020-MY3 and NSC102-3113-P-002-027 (ICC). The authors thank Material and Chemical Research Laboratories, Industrial Technology Research Institute for the early stage funding support.

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