Facile formation of a micro-crater structure for light scattering in quasi-solid state dye-sensitized solar cells

Abstract

Herein, we introduce a hierarchical micro-porous TiO₂ photo-anode with optimal morphology tailored for gel-type electrolyte DSSCs. To prepare a micro-crater structure, acetylene-black was blended into the TiO₂ paste, and thereafter evaporated at high temperature in a sintering process. The formation of a micro-crater structure with optimal morphology improved the light scattering effect without an additional light scattering layer. In addition, it provided a wide entrance for a fast and complete electrolyte infiltration into the TiO₂ film. Full contact between the TiO₂ film and the gel-type electrolyte by the micro-crater structure improved the performance of the DSSCs compared to that of the DSSCs without a micro-crater structure. Photo-conversion efficiency of the DSSC using this method was enhanced from 7.9% to 8.7%, while maintaining a high stable efficiency in a 300 hours stability test.

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Introduction

Dye-sensitized solar cells (DSSCs) are attracting widespread interests due to the structure and materials not being hazardous for the environment, the possibility of aesthetic design for differentiation, the facile fabrication process, and the high photoelectric conversion efficiency among organic solar cells.¹ A typical DSSC consists of a dye-adsorbed TiO₂ photo-anode and a platinum counter electrode with a liquid-type electrolyte containing an I⁻/I₃⁻ redox couple, which is used to fill in between the two electrodes.² Many efforts have been devoted to improving the performance of DSSCs by increasing the energy conversion efficiency with advanced dyes and improving the light harvesting efficiency with more charge transport in the TiO₂ photo-anode films or less hole–electron recombination at the interface between the photo-anode and the electrolyte. Recently reported advanced sensitized dyes have shown that the porphyrin dye and organic–inorganic hybrid perovskites can be better alternatives for a commercialized ruthenium dye.^3,4 With new research on the TiO₂ photo-anode films, the advantages of the doping method, such as K-doping, Ta-doping, and Sn-doping, have been introduced.^5–7 Diverse hard-template, soft-templating synthesis, solvothermal treatment and rapid microwave synthesis methods were reported to enhance the light harvesting efficiency by the modification of TiO₂ film morphology.^8–14

Meanwhile, as a crucial part of DSSCs, liquid-type electrolytes are conventionally used by many researchers as ionic conducting layer material. The high photoelectric conversion efficiency of liquid-type electrolytes is attributed to the high ionic conductivity which comes from: (1) high mobility of redox couple in solvent medium and (2) low contact resistance between the TiO₂ photo-anode film and electrolyte by a fast and complete infiltration process. On the other hand, liquid-type electrolytes cannot avoid the problem of leakage from the cell by incomplete sealing or by the saturation vapor pressure at high temperature even with complete sealing.¹⁵ Furthermore, the high fluidity of liquid-type electrolytes induces an easy segregation of sensitizing dye from the TiO₂ nanoparticles to the liquid-type electrolyte medium.¹⁶

UV-cured polymer gel electrolytes have more stability and less charge recombination than liquid-type electrolytes at room temperature in a cross-linked polymer network. DSSCs using UV-cured polymer gel electrolytes have shown an even better performance than DSSCs with liquid-type electrolytes at high temperatures due to the improved ionic mobility.¹⁷ There was an effort to enhance the performance of DSSCs using UV-cured polymer gel electrolytes at room temperature by the introduction of an additional nano-gel type electrolyte between the electrolyte and the counter electrode as a high charge transport layer.¹⁸ Otherwise, there has been less effort to tailor the morphology of the TiO₂ photo-anode than the efforts on gel-type electrolyte DSSCs. Normally, additional light scattering layers are used on dense TiO₂ nanoparticle photo-anode films to get a higher light harvesting efficiency. However, despite the cumbersome additional process of a double layer TiO₂ film, the light harvesting efficiency is less than that in a hierarchical micro-porous structure or multi-layer film. Besides, the infiltration process of gel-type electrolyte into a complex TiO₂ film takes a long time and gives rise to a high contact resistance in DSSC fabrication.

For effective light harvesting in DSSCs, hierarchical micro-porous nanocrystalline TiO₂ films were introduced via hard- and soft-templating routes.¹⁹ The hard-templating synthesis is commonly used to fabricate ordered three-dimensional (3D) TiO₂ porous structures with a crystalline framework, high specific surface area, and tailored pore structure. This templating route involves the use of colloidal particles, polymeric beads, and anodic alumina membranes; thereafter, these templates would be removed to generate unique porous materials. The soft-templating synthesis involves the use of organic or polymeric surfactants which self-assemble into a diversity of supermolecular structures, including spherical micelles, hexagonal rods, lamellar, and other structures in solution, which are used as soft templates to tune the structure and size of the pores in TiO₂ materials.¹³ Use of the hierarchical micro-porous TiO₂ films had the merits of an improved light scattering effect for enhanced performance of DSSCs. In addition, the large pin holes on the surface improved the infiltration process of the gel-type electrolyte into the TiO₂ films. However, achieving both purposes simultaneously with a simple fabrication method to facilitate a micro-crater structure for the wide channel of a gel-type electrolyte and nano-pores for the light scattering effect have been great challenges.

In the present study, we exerted ourselves to find an optimal hierarchical TiO₂ photo-anode morphology tailored for the gel-type electrolyte DSSCs by simply adding high-temperature volatile materials in the TiO₂ paste without using a cumbersome hard or soft template. To prepare a micro-crater structure, acetylene-black (hereafter referred to as AB) was blended into the TiO₂ paste, and thereafter evaporated at high temperature in a sintering process. Sensitized dye was adhered onto the TiO₂ surface, and then the gel-type electrolyte was infiltrated into the micro-crater shaped pores in the TiO₂ film. (Scheme 1) To find the optimal light scattering condition in the micro-crater structures, the changes of optical properties of the TiO₂ photo-anode films according to the formation of the hierarchical micro-crater morphology by the addition and evaporation of AB at high temperature were analyzed using a spectrophotometer. Also, the photovoltaic performance and long-term stability of the prepared DSSCs incorporating a double layer gel-type electrolyte were studied using a solar simulator and IPCE measurement system. This method demonstrated a photo-conversion efficiency of 8.7% without using an additional light scattering layer, which is around 10% higher than the method of fabricating the TiO₂ film without adding AB in the TiO₂ paste.

Scheme 1 Novel fabrication process using a TiO₂ photo-anode film with acetylene-black for the double layer gel-type electrolyte DSSC. (a) TiO₂ film on FTO glass (b) evaporation of acetylene-black at a high temperature treatment (c) hierarchical micro-porous structure formation and (d) fabrication of a double layer gel-type electrolyte between the two electrode glasses.

Experimental

Synthesis of TiO₂ nanoparticles and pastes

TiO₂ nanoparticles were synthesized by using the hydrothermal method.^6,7 Tetrabutyl titanate (10 mL), butanol (60 mL), acetone (10 mL), and acetic acid (10 mL) were mixed. A mixture of butanol (40 mL) and distilled water (3 mL) was then added to the above solution, followed by stirring for 1 h and the hydrothermal process at 240 °C for 6 hours in an autoclave. The mixture was cooled to room temperature and enriched using a rotary-evaporator in a 13% TiO₂ colloid.

TiO₂ pastes (hereafter referred to as normal TiO₂ pastes) were prepared by the combustion process using the TiO₂ colloid for the starting material. Ethyl cellulose was added for the binder, and terpineol for the solvent of the TiO₂ colloidal solution, followed by a mixing process using a paste blender. Then, to facilitate the formation of a micro-crater structure on the TiO₂ photo-anode films, TiO₂ pastes containing AB (hereafter referred to as AB TiO₂ pastes) were prepared using a paste-blending method in the ref. 20.

Preparation of TiO₂ photo-anode films

To compare the optical properties between the TiO₂ films with AB (referred to as AB TiO₂ film) and without AB (referred to as normal TiO₂ film), both TiO₂ pastes were screen printed onto the FTO glass (TEC 8/2.3 mm, 8 Ω per sq, Pilkington) in a total area of 100 × 100 mm² and then sintered at temperatures of 200 °C, 450 °C, and 550 °C, respectively, in ambient air using a muffle furnace.

The transmittance, opacity, and haze of the TiO₂ photo-anodes were measured by a transmittance mode, and the diffuse reflectance and whiteness index of the TiO₂ photo-anodes were measured by a reflection mode using a UV-vis spectrophotometer (CM-5 Konica Minolta, Japan) and analyzed using a color data software (SpectraMagic NX Ver.2.1 Konica Minolta, Japan).

Synthesis of electrolytes

Liquid-type electrolytes were prepared by dissolving 0.05 M I₂, 0.1 M LiI, 0.48 M TBP, 0.12 M NaSCN, and 0.6 M BMII in methoxypropionitrile.

UV-cured polymer gel type electrolytes were prepared by the addition of UV-curable polyurethane acrylate in the liquid-type electrolyte at a 12 : 10 weight ratio with 5 wt% photo initiator in the total solution.

Nano-gel type electrolytes were prepared by the addition of 7 wt% fumed silica nanoparticles in the liquid-type electrolyte as previously reported.¹⁸

Fabrication of the DSSCs

For the fabrication of the DSSCs, TiO₂ photo-anode films were prepared by the screen printing method with an aperture area of 0.2 cm² and then sintered at 550 °C for 30 minutes. Dye adsorption was carried out by dipping the TiO₂ photo-anode films into a 4 × 10⁻⁴ M t-butanol–acetonitrile (Merck, 1 : 1) solution of the standard ruthenium dye (N719, Solaronix) for 48 hours at 25 °C. In the case of the double layer gel-type electrolyte, firstly the UV-cured polymer gel-type electrolyte was coated on the TiO₂ photo-anode films and then the nano-gel type electrolyte was coated later. Counter electrodes were prepared by dropping the 10 mM hydrogen hexachloroplatinate (IV) hydrate (99.9%, Aldrich) in a 2-propanol solution onto the transparent FTO glass and sintering at 400 °C for 30 minutes. The two electrodes were assembled and separated using a thermal adhesive polymer film (Surlyn, thickness 60 μm).

Photovoltaic characterization of the DSSCs

The photovoltaic properties of the prepared DSSCs were measured using a lamp with an air mass 1.5G filter as the light source in a solar simulator and an exposure control instrument (Newport). The light intensity was adjusted with a reference Si cell (Fraunhofer Institute for Solar Energy System). Photovoltaic performance was characterized by V_oc, J_sc, and FF (fill factor), and the overall photo-conversion efficiency was characterized using the current density–voltage (J–V) curve.

Incident-photo-to-current efficiencies (IPCE) were measured by the illumination of the prepared DSSCs with a xenon arc light source through a filter monochromator and optical chopper at 2 Hz under bias light (Newport).

For the stability test under continuous light illumination, the prepared DSSCs were irradiated in the open circuit state using a xenon arc lamp at 1 sun condition in an ambient temperature (McScience). Then, the photo-conversion efficiency was characterized periodically.

Results and discussion

To find the optimal heat treatment condition of the AB TiO₂ film prepared as in the above procedure, optical properties were measured by a spectrophotometer. Fig. 1(a) shows the transmittance and opacity of the TiO₂ photo-anode films, including 1.0 wt% AB. These TiO₂ photo-anode films were heat treated at 200 and 450 °C for 60 minutes, and at 550 °C for 5 minutes. Transmittance was observed to be near zero at 200 °C, indicating no evaporation of AB. Transmittance of 35% at 450 °C was attributed to the partial evaporation of AB. The maximum transmittance of 67% at 550 °C indicates that a nearly pure white color was obtained from the almost full decomposition of AB.

Fig. 1 Optical properties of the TiO₂ photo-anode films containing 1 wt% of acetylene-black measured by a spectrophotometer according to (a) the heat treatment conditions of 200 °C for 60 minutes, 450 °C for 60 minutes, and 550 °C for 5 minutes, and (b) the heat treatment times of 5 minutes, 30 minutes, and 50 minutes at 550 °C. Each photograph images indicated in the inset correspond to each measured point.

In this research, opacity was measured as a parameter for the scattering efficiency of visible light due to the formation of hierarchical micro-porous structures in the TiO₂ photo-anode films. The difference between the refractive index of the anatase crystalline TiO₂ films (R.I. 2.55) and the air (R.I. 1) is large enough to provide a diffuse reflectance and opacity by the light-scattering. Opacity in optics is the measure of impenetrability to electromagnetic radiation, especially visible light, also termed as a mass attenuation coefficient or a mass absorption coefficient. In a more special condition, if a beam of light with frequency, ν, travels through a medium with constant opacity value, Kν, and constant mass density value, ρ, then the intensity will be reduced with distance, x, according to the formula

I(x) = I₀e^−Kvρx

where, x is the distance of the light which has travelled through the medium, l(x) is the intensity of light remaining at distance x, and l₀ is the initial intensity of light at x = 0.

Opacities of the AB TiO₂ photo-anodes were measured after different heat treatment conditions. The opacity value could not be measured after 200 °C heat treatment due to no visible light transmission by the remaining organic materials and AB. The opacity value of 85% after 450 °C heat treatment was attributed to partial evaporation of AB resulting in partial absorption and partial scattering of visible light. The maximum opacity value of 98% after 550 °C heat treatment was attributed to nearly full decomposition of AB which facilitated the formation of a hierarchical micro-crater structure on the TiO₂ photo-anode film.

In order to find an optimized heat treatment time at selected temperatures, AB TiO₂ photo-anode films were prepared using different heat treatment times of 30 and 50 minutes at the optimal heat treatment temperature of 550 °C. The maximum transmittance value of 68% was obtained at 30 minutes heat treatment by the more complete evaporation of AB. This result also confirmed that the more complete evaporation of AB has facilitated a more effective light scattering structure. On the other hand, excess heat treatment up to 50 minutes has been proven too long to obtain an effective structure, while exhibiting the lower transmittance of 66% and the lower opacity value of 95% due to the excess heat treatment and loss of light scattering effect in the TiO₂ photo-anode film. Generally, the thickness of the TiO₂ film decreases according to heat treatment time, improving the connectivity between the TiO₂ nanoparticles. However, this increase of density in AB TiO₂ photo-anode films induces a decrease of nano- and micro-pores facilitated by the evaporation of AB.

The transmittance and the opacity values shown above supported that AB evaporated completely under the optimized heat treatment conditions, facilitating the formation of effective hierarchical micro-crater TiO₂ photo-anode films for a light scattering effect. To compare the optical property variable to the wavelengths of the visible light, transmittance and diffuse reflectance were measured between the TiO₂ photo-anode films without and with AB to understand the wavelength range of the light scattered by the effective formation of a hierarchical micro-crater structure on the TiO₂ nano-crystalline frame.

Fig. 2(a) shows the light transmittances in accordance with a wavelength of the bare FTO glass, normal TiO₂ films, and AB TiO₂ films. Light transmittances at around 360 nm wavelength are near zero in the case of both normal and AB TiO₂ films due to the ultraviolet light region absorbance of the TiO₂ materials, while bare FTO glasses without TiO₂ films exhibit rather higher light transmittances. Light transmittances are recovered between 400 nm and the visible light regions depending on the thickness and light scattering effect of the normal TiO₂ films; however, the transmittance of the AB films does not reach the analogous level of normal films even with only one period of coating thickness. Fig. 2(b) shows the diffuse reflectance in accordance with a wavelength of the bare FTO glass, normal TiO₂ films, and AB TiO₂ films. Diffuse reflectance at around 360 nm wavelength is near zero in case of the both normal and AB TiO₂ films, which confirms that ultraviolet light region absorptions by TiO₂ materials are dominant in that wavelength region. Peak diffuse reflectance wavelengths were found at around 400 nm in both TiO₂ films, and meanwhile bare FTO glasses exhibited no diffuse reflections. Furthermore, AB films have shown a rather higher diffuse reflectance between 400 nm and the visible wavelength region than that of normal films, which also confirms that the recycled lights at visible wavelength region are not absorbed by the TiO₂ nanoparticle materials, but are diffuse, as reflected by the micro-crater structure.

Fig. 2 Comparison of the (a) transmittance and (b) reflectance of the TiO₂ photo-anode films prepared as bare FTO glass, normal TiO₂ film screen printed once or three times, AB TiO₂ film screen printed once or three times, respectively. Here, AB denotes the acetylene-black.

In order to investigate the effect of the thickness of the TiO₂ films against the light scattering properties, different thickness of TiO₂ films with and without AB were prepared using a screen printing method by coating once or three times repeatedly. The thickness were 8.2 μm and 19.3 μm for the once only and three times coatings in normal TiO₂ films, and 8.3 μm and 19.6 μm by the once only or three times coatings in AB TiO₂ films, respectively. Micro-pores of AB based TiO₂ anodes are seen clearly between the TiO₂ nanoparticles in the SEM images in Fig. 3. These pores were formed by the evaporation of AB at high temperature on a TiO₂ nanoparticle film. The blending ratio of AB to TiO₂ paste before heat treatment was 1 wt%. Micro-pores with mixed sizes ranging from several nanometers to several hundred nanometers were formed hierarchically on TiO₂ films along the channel established by the evaporation of AB at a high temperature treatment.

Fig. 3 Scanning electron micrograph images of (a) normal TiO₂ photo-anode film and (b) AB TiO₂ photo-anode film. Inset images are cross-section views.

Fig. 4(a) shows the light transmittance, opacity, and haze values of the bare FTO glass, normal TiO₂ film and AB TiO₂ film measured by the transmittance mode of the spectrophotometer. The light transmittances of the films decreased in proportion to the thickness or coating numbers, while exhibiting lower values in AB films compared to the normal films. This phenomenon can be explained directly by the increase of diffuse reflectance of the normal and AB TiO₂ films, and the diffuse reflectance and whiteness index values measured by the reflectance mode of the spectrophotometer are shown in Fig. 4(b). From the data of the bare FTO glass, the opacity value by the transmittance mode of 14% and the diffuse reflectance value by the reflectance mode of 27%, we can understand that the a metal oxide layer on the transparent glass scatters or reflects the visible light fundamentally. In the case of the normal TiO₂ films, the thickness of the film was a very important parameter for the light scattering effect, exhibiting higher opacity and haze values in the transmittance mode and higher diffuse reflectance and whiteness index values in the reflectance mode. However, the thick films with nanoparticles limit an easy infiltration of the gel-type electrolyte. In the case of the AB TiO₂ films, the thickness was not a major parameter for the light scattering effect, because the formation of the hierarchical micro-crater structure was a more effective parameter. By increasing the thickness of the AB films, the opacity and haze values were not changed much and the whiteness index value increased slightly. Therefore, it is clear that the AB TiO₂ films are more effective for light recycling with a thin layer than are normal TiO₂ films and the hierarchical micro-crater structure can provide wide channels for easy infiltration of the electrolyte. The easy infiltration of the electrolyte is more important for the gel-type electrolyte having higher viscosity, which is essential to enhance the long-term stability of the DSSCs.

Fig. 4 Comparison of the various optical properties according to the thickness measured by the (a) transmittance mode and (b) diffuse reflectance mode of the bare FTO glass, normal TiO₂ films, and AB TiO₂ films, which were screen printed once or three times, respectively.

Photovoltaic evaluation results of the DSSCs using different types of electrolytes with AB TiO₂ photo-anode films are shown in Fig. 5, and the photovoltaic parameters of each are indicated in Table 1. (See Fig. 2 and Table 1 of ref. 18 for the performance of DSSCs using normal TiO₂ photo-anode films for different electrolyte types.) The enhancement of photo-conversion efficiency by using the AB process was confirmed by all the different electrolyte types. When the photovoltaic parameters were compared with a DSSC using a conventional liquid-type electrolyte, the short-circuit current density of a DSSC using a UV cured polymer gel type electrolyte is lower, due to the low ionic mobility in a viscous medium at room temperature, despite its merit of the long-term stability and rather higher photovoltaic performance at high temperature.¹⁷ However, the DSSCs using the double layer gel type electrolyte with an additional nano-gel type electrolyte on the UV cured polymer gel type electrolyte have shown an analogous level of photo-conversion efficiency to that of the DSSC using a high performance liquid-type electrolyte with a similar short-circuit current density and a higher open circuit voltage while maintaining the long-term stability of the UV cured gel type electrolyte. This is because of the improved charge transfer rate between the electrolyte and counter electrode, although the thickness of the gel-type electrolyte has become even thicker.

Fig. 5 Current density–voltage (J–V) curves of the DSSCs fabricated using different types of electrolytes: liquid-type, UV-cured polymer gel type, and double layer gel-type on the AB TiO₂ photo-anode films, measured by solar simulator under 1 sun, AM 1.5G condition.

Table 1 Photovoltaic parameters of the DSSCs fabricated using different types of electrolytes as liquid type, UV-cured polymer gel type, and double layer gel-type on the AB TiO₂ films, and measured by solar simulator under 1 sun, AM 1.5G condition

Electrolytes	JSC (mA/cm^2)	VOC (V)	Fill factor	η (%)
Liquid type	20.673	0.636	0.645	8.492
UV type	17.385	0.645	0.616	6.918
UV + nano-gel	19.242	0.714	0.634	8.721

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Incident-photo-to-current efficiencies (IPCEs) were compared between the DSSCs with normal films and AB films commonly a using double layer gel-type electrolyte and the results are shown in Fig. 6. The IPCEs of the DSSCs using AB TiO₂ photo-anode films were increased in the longer visible wavelength regions from 500 nm to 750 nm due to the effect of light recycling in the TiO₂ photo-anode films compared to the DSSCs using normal TiO₂ photo-anode films without a light scattering layer. This is a typical result indicating that the improved photo-conversion efficiency comes from a light scattering effect and have the merit of the utilization of longer visible wavelength regions.

Fig. 6 Incident-photo-to-current efficiencies (IPCEs) measurement of the DSSCs using normal TiO₂ photo-anode films or AB TiO₂ photo-anode films commonly with a double layer gel-type electrolyte.

The stabilities of the DSSCs incorporating the AB TiO₂ photo-anode film and a double layer gel-type electrolyte under continuous light illumination conditions were tested and the results are shown in Fig. 7. As a representative parameter indicating the long-term stability, photo-conversion efficiencies of the DSSCs using a double layer gel-type electrolyte were maintained well until the end of 300 hours testing. This result means that the contacts between the TiO₂ films and the electrolytes are maintained and sensitized dyes are not segregated in the measured time.

Fig. 7 Stability test results of the DSSC using a double layer gel-type electrolyte with an AB TiO₂ photo-anode film compared with DSSCs using a liquid type or UV type electrolyte with a normal TiO₂ photo-anode film measured by a solar simulator under continuous light illumination at ambient temperature.

Conclusions

In summary, we demonstrated the control of morphology and optical properties of the TiO₂ photo-anode films by simply adding high temperature volatile material in the TiO₂ paste without using a cumbersome hard or soft template. As a result, the light scattering effect and the infiltration process of a gel-type electrolyte could be improved. We investigated the optimal heat treatment condition in which complete evaporation of AB was facilitated for the formation of a micro-crater structure, and improved the optical properties such as opacity, haze and whiteness index values. On the contrary, the excess heat treatment condition resulted in the shrinkage of the micro-porous morphology. In this study, the performance of the DSSCs with a double layer gel-type electrolyte was enhanced to the level of 8.7% from 7.9% by using an AB TiO₂ film through the optimization of optical properties without using any additional light scattering layer. Furthermore, the facile process of electrolyte infiltration by this method is considered to be a great prospect for large scale commercialization of quasi-solid state DSSCs in the near future.

Acknowledgements

This work was supported by the Basic Research Program (2011-0018113), the Global Frontier Research Center for Advanced Soft Electronics (2011-0031635) of the National Research Fund (NRF) funded by the Korea government (MEST), and the International Collaborative R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy of the Korea government (2011-8520010020).

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