Fast dye coating

Chapter 3. High performance dye sensitized solar cells by adding titanate co-adsorbant

4.4 Result and Discussion

4.4.1 Fast dye coating

The viscosity of EG at room temperature is relatively high (15.1 MPas), but it is reduced to 0.5–1.0 MPas at 80–100 ℃. Therefore, hydrophilic EG can easily penetrate and deliver dyes to the TiO₂ nanoparticle layer. However, the viscosity of Gly is 1.4 MPas at 20 ℃ and 21.3 MPas at 90 ℃.¹¹⁷ This high viscosity impedes Gly penetration of the nanoparticle layer and therefore dye coating was observed only at the top surface of the electrode in the figure 4.3 ( ~2 mm coating for 3–5 min). The EG/Gly system can take advantage of a synergistic effect of both solvents in comparison to the individual solvents.

Figure 4.2 Performance of solar cells based on (a) mixtures of EG and Gly and (b) comparison of the EG/Gly mixture with the pure solvents at a dipping time of 5 min. A reference cell is prepared by a

conventional method.

Figure 4.3 SEM photographs of porous TiO2 film showing the cross section of dye adsorption (a), and EDAX graphs by mapping of the distribution of the different chemical elements constituting the specimen can be obtained for the Ru element (b)and the graph of viscosity by controlling temperature

with EG, Gly, and EG/Gly mixed solvent (c).

Figure 4.4 Cell performance for samples with dipping times of 1–10 min. The concentration of dye and ratio of solvents is fixed at 30 mm and 1:1, respectively.

To find the optimum conditions, solvent mixtures with various volume ratios have been investigated.

Dye (d) was added to EG/Gly (d⁺EG/Gly), and the resulting solution was kept at >40 ℃ until use.

Figure 4.2 shows the performance of solar cells based on the EG/Gly solvent system under various conditions. Figure 4.2 a shows the variation of performance with solvent ratio when the reaction was run for 5 min; EG/Gly ratios were 1:1, 1:2, 1:3, 2:1, and 3:1 (w/w). Maximum performance was obtained with an EG/Gly ratio of 1:1. This mixture of solvents gives a performance of 9.1% with a short-circuit current density (Jsc) of 15.8 mAcm^-2, an open-circuit voltage (Voc) of 0.81 V, and a fill factor (FF) of 0.71. These values are comparable to reference cell performances, and even higher (Figure 4.2 b). Cell-performance changes are mainly due to current-density variation, which supports the idea that the variable in this experiment is the amount of coated dyes without damages (possibly caused by high-temperature) in the Table 4.1. Use of other EG/Gly ratios showed noticeable lowering in efficiency. When the amount of Gly is increased, most of d⁺Gly remains on top of the electrode and only small amounts of dyes in EG is delivered for coating. On the other hand, a larger amount of EG leaves only a small amount of Gly on top, which may cause exposure of dyes/TiO2 to air or moisture and which may result in dye degradation.

Figure 4.4 shows the variation of performance with adsorption time. Reaction time is varied from 1 to 10 min, whereas the concentration of dye and ratio of solvents is fixed at 30 mm and 1:1, respectively. Use of pure EG as solvent may give fast dye-coating times of 3 min.

Table 4.1 The performance of solar cells based on the EG and glycerol ratio.

That system can be run only for around 3 min because dye degradation was observed shortly after 3 min. Often, 3 min was already enough time to see the start of degradation. However, the d⁺EG/Gly system enables samples to survive in ambient conditions for > 3 min. Therefore, we could increase reaction times to achieve optimum conditions. The mixture does not penetrate all the way through to the TiO2 nanoparticle layer and the new system operated optimally at reaction times of 5 min.

Table 4.2 The performance of solar cells based on the adsorption time.

Figure 4.5 DRIFT compares conformation of dyes depending on coating methods: (a) DRIFT spectra for a reference cell by a conventional method and a cell made by mixed solvent, and (b) detail DRIFT

depending on running time.

Once d⁺EG/Gly is on top of the electrode, mainly EG with dye penetrates easily to the nanoporous layer, whereas most of d⁺Gly stays on top; Voc increases as adsorption time increases, whereas Jsc rises only for the first 5 min in the Table 4.2. Consequently, we observed maximum performance after 5 min. There might still be a small amount of oxygen or moisture in the solvents and the TiO2 working electrode before placing the mixture; these may cause a loss of efficiency after 5 min.

To confirm that dye saturation on the TiO2 surface is comparable to that of conventional methods, dye-coated working electrodes were immersed into aq. NaOH and the amount of loaded dye on the TiO2 surface was measured. The NaOH solution removed all molecules on the surface of TiO2 in the same area and solutions were analyzed by UV/Vis spectroscopy.^{115, 118} The average amounts of dye

molecules were (0.0224 ± 0.001) and (0.0198 ± 0.001) mmolg^-1 for the reference and d⁺EG/Gly, respectively. Interestingly, the amount of dyes in a reference electrode made by the dipping method was higher than that in d⁺EG/Gly dyes made by the simple method. That means that the adsorption of dye molecules reached about 90% of saturation on the TiO2 layer. Although we observed a lower amount of dyes in d⁺EG/Gly, cell performances were still comparable to the reference cells manufactured by the conventional method.

Diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy was employed to investigate molecules on the TiO2 surface. DRIFT analysis gives conformation differences of dyes depending on coating methods. DRIFT spectra for a reference cell and a cell made by d⁺EG/Gly are given in Figure 4.5 (a). Briefly, signal positions and intensities are almost identical. The main signals of interest appear at 2100, 1726, 1626, 1541, and 1368 cm^-1, which correspond to NC(S), carbon double bonds (C=O), carbonyl group asymmetric vibrations (COO^-), aromatic carbon double bonds (C=C), and carbonyl group symmetric vibrations (COO^-), respectively.88-91, 109

A detailed DRIFT analysis depending on running time is provided in Figure 4.5 (b). As time increases, the intensities for major signals increase for 5 min and then decrease. A similar trend was observed for the pure EG system after 3 min. Although the peak decrease is less drastic relative to the pure EG system, still small decreases, possibly due to dye oxidation, were observed.

Figure 4.6 EIS analyses of DSSCs with reference and EG/Gly (v/v=1:1) cells. The Rct is 1.12 and 3.06 Wcm2 and Rc is 1.11 and 3.33 Wcm2 for the reference and the EG/Gly ratio of 1, respectively.

Figure 4.7 Long-term stability of cells prepared by a new solvent system with a reference cell (conventional method).

Electrochemical impedance spectroscopy (EIS, Figure 4.6) of the cells revealed that the EG/Gly solvent mixture affects the second arc from the left, which is related to dye–TiO2/electrolyte interface resistance. The first and third arcs correspond to the Pt electrode and electrolyte diffusion, respectively.

The increase in the second arc is normally observed when electron recombination at the dye–

TiO2/electrolyte interface is retarded.86, 92-95, 119

As described above, the amount of dye molecules in the EG/Gly solvent system was lower than that of the conventionally prepared electrode. However, efficiency is quite comparable and even higher. That could be explained by EIS analysis. Although values for the first arc are almost identical, the value for the second arc is increased by 10% in the case of a cell that is coated fast. Alcohol functional group attachment on metal oxides is already known, thus Gly may be attached on the surface between dyes.

Because in those experiments high-concentration and temperature kinetics were utilized, it was necessary to observe long-term stabilities to verify whether dyes were properly adsorbed. Notably, FTIR studies show that dyes on TiO2 by this method show no differences relative to the conventional method. As shown in Figure 4.7, no noticeable efficiency change was observed. This may support the assertion that the presented dye system could be adoptable for DSSC coating. It is also believed that a high concentration of dyes may cause dimerization and low performance. However, the new solvent system utilizes a highly viscous solvent, which may prevent dye aggregation as diffusion of dye molecules in the EG/Gly mixture is limited in the Figure 4.8.

Figure 4.8 Dye mixture diffusion in the EG/gly 1:1 solvent. A small drop of 30 mM d⁺EG/gly was intentionally dropped in EG/gly solvent @ around 30 ℃. As observed, dye mixing occurs very slowly.

Only surface which may be highly hydrated or EG rich area showed weak dye diffusion.

((a) Before dropping of highly concentrated dye mixture (b) right after dropping, (c) 5 minutes after, (d) 1 hour later, (e) 17 hours later.)

4.4.2 Application of dual dye bands on working electrodes

As shown in figure 4.1 (b), the fast dye-coating method was applied to prepare dual-dye-layer working electrodes. A TiO2 coated fluorine-doped tin oxide (FTO) working electrode (14 mm in thickness) was placed on top of a hot plate, which was set at 90℃ at ambient conditions. The temperature of the TiO2 surface was expected to be < 90℃; d⁺EG/Gly was poured on the substrate and kept for variable lengths of time (on a minute scale) to obtain partially dye-coated working electrodes;

d⁺EG/Gly slowly penetrated the nanoporous TiO2 layer and delivered dyes to the TiO2 surface. By controlling penetration time, it was possible to coat dyes to the desired depth. After coating of the upper layer, the electrode was immersed in a different dye solution for the rest of the electrode.

Coating depth depended on penetration time and was measured by electron-probe X-ray microanalyses (EPMA) for Ru. We focused on Ru, which should be detected only for the coated depth.

Figure 4.9 shows EPMA data for working electrodes with variable preparation times (1–3 min). The EG/Gly mixture was quite viscous at room temperature (15.1 MPas), and the lukewarm d⁺EG/Gly (6.0–3.3 MPas, 40–60 ℃) was still viscous enough to slowly move down to the porous film.

Figure 4.9 EPMA analysis of Ru for dye-coated samples with controlled time from 1 to 3 min.

According to the data, d⁺EG/Gly delivered dyes in the nanoporous TiO2 film to a depth of about 3, 7, and 9–10 mm depending on running time (1, 2, and 3 min, respectively). Because the temperature of d⁺EG/Gly was lower than that of the working electrode on a hot plate, the first minute resulted in a Ru gradient. This trend was not observed from the samples run for > 2 min. Of course, the moving speed of d⁺EG/Gly may depend on the porosity of the TiO2 layer; 3–4 min was enough to deliver d⁺EG/Gly to the bottom of the whole layer. Modifying the EG/Gly ratio did not result in significant change in penetration depth, irrespective of the amount of dye coating. That supported the idea that EG mainly delivers dye molecules to the TiO2 layer. If the whole phase delivers dyes to such depth, higher viscosities (higher ratio for Gly) may retard dye penetration depth significantly.

To coat only half of the TiO2 working electrode in depth, we chose a coating time of 2 min using black dyes. Because black dyes have extended absorption spectra (to 700 nm), they must be located on the second part of the layer from the light-entering side. Once the electrode was half-coated with black dye, the rest of the working electrode was coated with N719 dye by soaking it in a solution;

d⁺EG/Gly still surrounded TiO2 nanoparticles even when using the new solution for the second dye coating, which could retard desorption of black dye and cocktailing of the second dye on the first layer at low temperatures. The working electrode for the double-dye structure was completed after a 1 h coating of the rest of the working electrode with a solution of N719 in ethanol (3 mm) at 60℃.

Figure 4.10 Performance of solar cells based on the N719 dye cell, the N749 dye cell, and the dual-dye cell.

Table 4.3 Performance of solar cells based on N719, N749, and the half-adsorption cell.

Figure 4.10 and Table 4.3 show a solar cell performance comparison. When the N719 and N749 dyes are coated on working electrodes separately, their efficiencies are near 8.1% and 4.7 %, respectively. After building a double layer with the N719 and N749 dyes, efficiency increases to 9.4%.

We did not observe a drastic increase by making a double layer. Although we could observe a current- density increase for the double-layer solar cells, a Voc drop is a main factor for cancelling the current- density increase; the Voc value for the N719-only cell is around 0.77 V, whereas that of the N749-only cell is about 0.75 V. The final Voc value is 0.76 V, which is closer to the second dye value. In addition, the current-density increases were confirmed by incident photon-to-current efficiency (IPCE) measurements in the Figure 4.11. IPCE values are higher for double-dye-layer-coated cells over the whole wavelength range than other cells. Clearly, the photocurrent increase stems from the observed spectra extensions. Because the high DSSC efficiency could originate from the photovoltage increase, matching dyes with similar open circuit voltages would be a desired set for applications.

Figure 4.11 The values of IPCE spectra for N719 dye cell, N749 dye cell and the half adsorption cell.

4.5 Conclusion

Herein, we presented a simple dye coating method using a mixture of dye (d), ethylene glycol (EG), and glycerol (Gly): d⁺EG/Gly. At high temperatures, Gly is still viscous, but EG viscosity is reduced.

At temperatures near 90 ℃, EG becomes a mobile vehicle to deliver dyes to the TiO₂ surface and the relatively viscous Gly keeps the surface protected from air and moisture. As a result, dye coating can be easily controlled and fast (on a minute scale). Because d⁺EG/Gly can control coating depth for ruthenium-based dyes on nanoporous TiO2 electrodes, it enabled the facile preparation of dual-dye- band working electrodes. The efficiency of working electrodes increased to 9.4% by applying the double-dye-layer electrode with N719 and N749 dyes.

Chapter 5. Electrocatalytic Activity of NiO on Silicon Nanowires with a Carbon Shell and its Application to DSSC Counter Electrodes

Chapter I is reproduced in part with permission of C. Jung et al, 2015

5.1 Research overview

In order to improve the catalytic activity of a material, it is critical to maximize the effective surface area directly contacting the electrolyte. Nanowires can be a promising building block for catalysts in electrochemical applications because of their huge surface area. Nickel oxide (NiO) decoration was achieved by drop-casting a nickel-dissolved solution onto vertically aligned silicon nanowire arrays with a carbon shell (SiNW/C). Via the hybridization of the NiO and silicon nanowire arrays with a carbon shell, this study aimed to achieve a synergic effect for the catalytic activity performance. This study demonstrated that the resulting nanomaterial exhibits excellent electrocatalytic activity and performs well as a counter electrode for dye-sensitized solar cells (DSSCs). The compositions of the materials were studied with X-ray diffraction, X-ray photoelectron spectroscopy, and energy dispersive spectroscopy. Their micro- and nanostructures were investigated with scanning electron microscopy and transmission electron microscopy. The electrochemical activity towards I⁻/I3

− was examined by cyclic voltammetry and electrochemical impedance spectroscopy. The obtained peak power conversion efficiency of the DSSC based on the NiO@SiNW/C counter electrode was 9.1%; this was greater than that of the DSSC based on the Pt counter electrode.

5.2 Background and Introduction

Carbon based nanomaterials have received much attention and have been actively studied by many research groups for energy harvesting and storage applications. This is because of their natural- abundance, eco-friendliness, electrical conductivity, and catalytic activity with high stability. For example, Dai et al. developed sodium ion batteries based on a three-dimensional anode consisting of N-doped graphene foam, which delivered a high initial reversible capacity and long-term retention.¹²⁰ Kim et al. also demonstrated three-dimensional gel structures based on reduced graphene oxide which was employed for high performance electrical double layer supercapacitor electrodes.¹²¹

Recently, many research groups have focused intensive efforts into replacing noble metal catalysts within dye-sensitized solar cells (DSSCs) with carbon-based nanomaterials, such as carbon nanotubes,^122-123 graphene nanoribbons,¹²⁴ and graphene nanoplatelets.^125-126 Defect-free graphene

nanosheets have received much attention because of their unique properties such as a high surface area, efficient electrocatalytic activity, and good electrical conductivity. However, the performance of applications based on these nanosheets is still unsatisfactory and this is because there are insufficient active sites for the reduction of I⁻/I3

−.^127-128 Kavan et al. reported that the device performance is dependent on the concentration of both defects and oxygen-containing functional groups.¹²⁹ In addition to the graphitic domain, the defect sites may be important for any applications that require catalysts, since charge carriers are required to transfer from the conducting carbon backbone to an electrolyte or any contacting medium. However, the existence of more defect sites could result in a lower grade of conductivity if a limited amount of graphitic carbon exists. Therefore, the presence of controlled reactive charge carrier sites may increase the catalytic activity without impairing the properties of the carbon. The use of metal, metal oxides, or metal complexes with carbon nanomaterials can synergically improve the catalytic performance, and there have been many previous reports on this subject. Du et al. reported that PtNi-MWCNT hybrid nanostructures functioned as a high performance and durable electrocatalyst for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs).¹³⁰ Dao et al. introduced graphene-NiO nanohybrids as a counter electrode for DSSCs, and the power conversion efficiency increased from 5.2 to 7.4%, both with and without NiO immobilization on the graphene.¹³¹

We report the synthesis of nickel oxide-decorated silicon nanowires with carbon shells (NiO@SiNW/C) and their application as counter electrodes for DSSCs. A defective carbon shell on the large surface area of the silicon nanowires offers a huge amount of electrocatalytic sites. In addition, NiO is well-known as a p-type semiconductor with a wide bandgap (3.5 eV), and it exhibits superior electron blocking properties in organic bulk-heterojunction solar cells.¹³² In the same manner, the NiO that is decorated on the carbon shell functions as an electron blocking layer in this research.

Here, we demonstrate that the NiO@SiNW/C exhibits sufficient electrocatalytic activity for the regeneration of I⁻/I3

−.

5.3 Experimental

5.3.1. Fabrication of SiNWs

Vertically aligned SiNWs were fabricated by the metal assisted chemical etching (MACE) method, as reported in previous literature.^133-134 Commercially available, heavily doped n-Si wafer (0.001–

0.003 Ω • cm, Global wafer) was cut into 1.5 ⅹ 1.5 cm² pieces. The Si pieces were cleaned with ethanol, acetone, and 2-propanol. This was followed by UVO treatment for 30 min. The Si pieces

(AgNO3). Following etching for 15 min, the silver dendrites that were present on the Si surface were removed in a diluted nitric acid (HNO3) solution for a period of 15 min. Finally, the SiNW samples were rinsed with deionized water and acetone, and subsequently dried in a flow of N2 gas.

5.3.2. Fabrication of NiO@SiNW/C

Carbon shell coating: The carbon shell coating on the surface of the SiNW arrays was fabricated using the thermal CVD method. Once the temperature had attained approximately 1075 ℃, 100 sccm of hydrogen (H2) and 100 sccm of methane (CH4) were introduced into the reactor for 5 min. This was followed by cooling to room temperature under an argon (Ar) atmosphere.

NiO decoration: The NiO was decorated onto the surface of the carbon shell by the drop-casting method. 20 μL of 15 mM nickel nitrate (Ni(NO3)2) solution in ethanol was dropped onto the sample and naturally dried. The sample was subsequently heated to a temperature of 450 ℃ for 30 min.

5.3.3Fabrication of DSSCs

The FTO substrate was cleaned with a detergent solution, deionized water, a mixture of ethanol and acetone [1/1 (v/v)], and 2-propanol in an ultrasonic bath. This was conducted for a period of 10 min for each fluid, followed by UVO treatment for 15 min. Nanocrystalline TiO2 paste (20 nm, ENB- Korea) was doctor-bladed onto the substrate and it was subsequently annealed at 500 ℃ for 1 h.

Following annealing, the TiO2-coated substrate was immersed in 0.3 mM (Bu4N)2[Ru(dcbpyH)2(NCS)2] (termed N719) dye in a solution consisting of a mixture of acetonitrile and tert-butanol [1/1 (v/v)] for 12 h at room temperature. A platinized counter electrode was used as a reference electrode and it was prepared by coating a 10 mM H2PtCl6 solution in ethanol onto a clean FTO substrate. It was subsequently annealed at 450 ℃ for 30 min. The as-prepared working electrode and the counter electrode were sandwiched together and sealed with a 50 μm thick Surlyn film (DuPont). An electrolyte with the composition of 50 mM 4-tert-butylpyridine, 50 mM 1-hexyl-2, 3-dimethylimidazolium iodide, 5 mM lithium iodide, and 2 mM iodine in acetonitrile was introduced into the cell.

5.3.4Characterization and Measurements

Both the micro- and nanostructures, in addition to the compositional information of the SiNW, SiNW/C, and NiO@SiNW/C were characterized using SEM (Hitachi S-4700) and TEM (JEOL, JEM- 2100F) with the use of EDS, XPS (ULVAC-PHI X-TOOL), and XRD (Rigaku, SmartLab) equipment.

The J-V curves were measured under 100 mW/cm² AM 1.5G (ABET Technology, LS 150 simulator).