Photoanode electrode

ZnO photoanode electrode

Both for tetrapod-like ZnO (T-ZnO) and commercial ZnO (C-ZnO), the ZnO pastes for screen-printing were prepared typically by mixing ZnO powder, ethyl cellulose (EC), and terpineol, and the detailed procedure is as follows. EC (5–15 mPa s,#46070, Fluka) and EC (30–70 mPa s, #46080, Fluka) were dissolved in ethanol to yield 10 wt% solution, individually. 12g EC (5–15) and 12g EC (30–70) were added to a round bottomed rotavap flask containing 12g ZnO powders, and 25g terpineol. The mixture paste was dispersed in an ultrasonic bath. And a rotary-evaporator (BUCHI V850) is used to remove the ethanol and water in the mixture paste. Finally, the paste was made with a three-roll mill (EXAKT E50).

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To prepare the ZnO photoanode electrode (PE), the NSG FTO glass used as current collector was first cleaned in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with water and ethanol. After UV-O3 treatment for 20 min, a layer of ZnO paste with 0.238cm³ area was coated on the FTO glass by screen-printing without blocking layer on the substrate. And then, the substrate was kept in a clean box for 10 min so that the paste can relax to reduce the surface irregularity and then dried for 10 min at 90 °C. This screen-printing procedure with ZnO paste (coating, storing and drying) was repeated to get an appropriate thickness of 5 to 42 μm for the PE. The electrodes coated with the ZnO pastes were gradually heated under O2 flow as the temperature evolution shown in Fig. 3-3, in order to remove the organic EC in the paste and neck the NPs.

Fig. 3-3. Heating curve for ZnO photoanode electrode.

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After cooling to room temperature (RT), the ZnO PE was immersed into a solution made of 0.5 mM D149 organic sensitizer and 1 mM CDCA in AN/t-BuOH mixture (v/v = 1 : 1) at 65 °C for 30 min, and the PE was then rinsed with AN to remove excess dye molecular on it.

Flexible TiO₂ photoanode electrode

Fig. 3-4. Sketch schemes of the electrophoretic cell, the preparation of TiO₂ electrodes, and the significant parameters in EPD process.

ITO/PEN substrates were cleaned with mild soap and ethanol, thoroughly rinsed with deionized water (18.2 MΩ), then dried by a clean air stream. In this part,

electrophoretic deposition (EPD) was used to fabricate the flexible TiO₂ photoanode film on ITO/PEN substrate. The TiO₂ suspension for EPD consisted of 0.25g TiO₂ in 100mL IPA, and stirred with a magnetic stirrer overnight; it was ultrasonically

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dispersed for 1.5 hrs before adding into the electrophoretic cell as shown in Fig. 3-4 (a).

The two electrophoretic electrodes of fluorine-doped tine oxide (F:SnO2, FTO) conductive glass, and ITO/PEN film were separated by 1.5 cm, and served as the cathode and anode electrode, respectively. The dispersed TiO2 NPs, that have positive electric surface charge, are forced by an external electric field to overcome the gravity and the friction force, and deposited onto the ITO/PEN substrate. A Keithley 2400 Source Meter was applied as a power supply for different currents and deposition durations at the constant current mode which was more effective and controllable than the constant voltage mode [68]. After drying TiO2-deposited ITO/PEN substrate at RT and one atmospheric pressure, high pressure treatment enhanced the photovoltaic performance of the device.

The EPD TiO₂ PE was immersed in a solution of 0.5 mM N719 dye solution in AN/t-BuOH (v/v=1:1) binary solvent at 40 ^oC for 4 hrs to adsorb sufficient N719 dye for light harvesting. To remove the remaining dye, the dye-sensitized photoanode was rinsed with AN, and dried under atmosphere condition at RT.

3.2.2 Electrolyte

The electrolyte plays an important role in the process of dye regeneration or the so-called hole-transport material (HTM). For different functions or optimization for

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different cells, the composition of the electrolyte will be quite different. In this research, we used several electrolytes listed in Table 3-1.

Table 3-1. The compositions of the electrolytes used in different chapter of this thesis.

Electrolyte # Compositions Used in

EL 1 0.6 M PMII, 0.05 M I², and 0.5 M TBP in AN 4-3 ~ 4-4 EL 2 0.2 M I2, 0.5M TBP in PMII/C8MImPF6 mixture

(v/v=35:65) 4-5

EL 3 0.4 M LiI, 0.4 M TBAI, 0.04M I2 and 0.5 M NMBI in

AN/MPN mixture (v/v=1:1) 5-1 ~ 5-3

EL 4 0.8 M TBAI, 0.1 M I2, and 0.5 M NMBI in MPN 5-4 EL 5 0.8 M PMII, 0.1 M I2, and 0.5 M NMBI in MPN 5-4

EL 6 0.5 M PMII, 0.05 M I₂ in MPN 5-4

EL 7 0.5 M LiI, 0.05 M I2 in MPN 5-4

3.2.3 Counter electrode

High-temperature process

To prepare the counter Pt-electrode, two holes (< 1 mm diameter) were drilled in the FTO glass by a drilling machine. The perforated substrate was washed in a detergent solution using an ultrasonic bath for 15 min, and then rinsed with water and ethanol. After removing residual organic contaminants by heating in air for 20 min at 400 °C, the Pt catalyst was deposited on the FTO glass by dip-coating with the H2PtCl6

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solution (2 mg Pt in 1 ml IPA) with repetition of the heat treatment at 400 °C for 20 min.

Low-temperature process

The screen-printing paste prepared by dissolving H2PtCl6·6H2O (0.6, 1.2, and 1.8 w.t.%) in terpineol solution were screen-printed on the substrate using a 250 mesh screen and then dried at 60 °C for 20 min. Then the Pt ions on the electrodes were reduced by immersing it in 10mM NaBH4 aqueous solution at 40 °C. And the reduced reaction is shown below. After 2 h, the electrodes were rinsed with distilled water and then dried at 60 °C for 20 min. Then, the UV-O3 post-treatment were applied for 20 min:

. (3-1)

3.2.3 Sealing and additional materials

A sealing material prevents the leakage and the evaporation of the electrolyte solvent. Chemical stability and mechanical strength of the sealing material against the chemical components in the electrolyte and the cell-broken is required. Surlyn (Du Pont), a copolymer of ethylene and acrylic acid, meets the chemical requirements.

For mechanical strength requirements, the additional UV-gel is used in this study.

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Fig. 3-5. The transmittance spectra of the substrates used in this study.

In order to avoid the degradation of dye in long-term stability test, it is necessary to use UV-cut filters in DSC with FTO/glass substrates to cut the photon with energy 4.13eV (300 nm) to 3.1eV (400 nm) shown in Fig. 3-5. However, the DSC made by polymer ITO/PEN substrate in this study didn‟t use any UV-cut filter due to the cut-off wavelength of ITO/PEN is about 380 nm.

3.3 Device measurement

To establish the characteristics of the DSC, solar cells and its components will go through the testing measurements applied below:

The morphologies and dimensions of the samples were characterized using a

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JEOL-6500 field emission scanning electron microscope (FESEM) operated at 10 KeV.

The advanced structures were analyzed using a JEOL JEM-2100F field emission transmission electron microscope (FETEM) operated at 200 KeV. The surface area of the nanoparticles (NPs) was achieved by Micromeritics ASAP (Accelerated Surface

Area and Porosimetry System) 2010. The phases were characterized using an X-ray diffractometer (XRD, Philip PW1700) operated at 40 keV and 40 mA with Cu Kα

radiation. The scanning step size and the collection time for each step were set at 0.02° and 5 s, respectively. The film thickness of the photoanode was measured by Surfcorder ET-3000 (Kosaka Laboratory Ltd.). The macro-structure of the sample was observed by an optical microscopy (Nikon). The UV-visible spectra were measured on a Hitachi U-2800 spectrophotometer.

For current–voltage characteristics, the white light source (Yamashita Denso, YSS-100A) was used to give an irradiance of 100 mWcm^-2 (the equivalent of one sun at AM1.5) on the surface of the solar cell, and the data was collected by an electrochemical analyzer (Autolab, PGSTAT30) at a temperature of 25±2 ^oC. The scan rates of AN-based and IL-based DSC were 0.05 and 0.005 Vs^-1, respectively.

The light power was calibrated by using a reference cell of silicon photodiode (BS-520, Bunko Keiki) before each measurement. The spatial uniformity, spectral content and temporal stability testing, which were classified every month, are shown in Figs. 3-6,

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3-7, and 3-8. The calibration certificate validates Class AAA performance for ASTM E927-05 standard. The white light source is certified to Class AAA performance for all 3 standards shown in Table 3-2 in order to ensure the accuracy of the measurement.

Fig. 3-6. The uniformity of the white light source (Yamashita Denso, YSS-100A).

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Fig. 3-7. The standard AM 1.5 spectrum compared with the spectrums from the white light source (Yamashita Denso, YSS-100A).

Fig. 3-8. The output variation of the white light source (Yamashita Denso, YSS-100A) utilizing a 1 second data acquisition time and after 15 minutes lamp warm-up.

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Table 3-2. Class AAA Standards and Specifications

Organization IEC JIS ASTM

Performance Parameter 60904-9-2007 C 8912 E92-05 Spectral Mismatch 0.75% - 1.25% 0.75% - 1.25% 0.75% - 1.25%

Non-Uniformity of Irradiance 2.0% <±2% 2%

Temporal Instability

0.5% STI

<±1% 2%

<2.0% LTI

The impedance measurements were carried out applying a bias of the open circuit voltage (Voc), by using an electrochemical analyzer (Autolab, PGSTAT30) at a temperature of 25±2 ^oC. The EIS measurements of AN-based DSC were recorded in a frequency range from 50 mHz to 1 MHz with AC amplitude of 10 mV, and the frequency range was set from 1 mHz to 1 MHz for the IL-based DSC.

Fig. 3-9. Emission spectrum of the white LED array.

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An IPCE measurement system (C-995, PV-measurement Inc.) measured the action spectra of the incident monochromatic photon-to-current conversion efficiency (IPCE) for solar cells. A white LED array, whose emission spectrum is shown in Fig. 3-9 and covers the range of dye absorption, serves as an additional light bias source in IPCE measurement. In order to minimize the capacitance effect of the DSC [69], the chopper frequency was set to 4 Hz.

Fig. 3-10. Measured spectral transmittance for ND filters used in this study.

All photocurrent transient experiments were performed at RT by illumination with the white light source (Yamashita Denso, YSS-100A) and series neutral density (ND) filter (Edmund Optics). In Fig. 3-10, the transmittance spectra of ND filter are uniformly attenuated in the 400-700nm wavelength range, which is also the mainly response range for dye absorber. The time evolution of the photocurrent was

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collected with the electrochemical analyzer (Autolab, PGSTAT30).

The durability test in this study including visible light irradiation (100 mW/cm²) and thermal stress (60 ^oC) aging of hermetically sealed cells with UV-cut filter was performed with a sun test xenon arc lamp (ATLAS Ci3000 xenon Fadeometer).

Following a period of continuous light irradiation, PV measurements were taken after the cells cooled to RT.

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Chapter 4 Towards efficient