Chapter 2 Literature Review
2.2 Components of DSSCs
2.2-1 Substrate
The most used substrates for DSSC are transparent conducting oxide (TOC), the coated glass substrates. The choice of TCO coated glass is usually a compromise between transmittance and conductance. Fluorine-doped tin oxide, the SenO2．F or FTO and indium tin oxide, or ITO are the most commonly used TCOs for thin film photovoltaic cells.[17] The reason choosing TCO coated glass substrates is for the procedure of TiO2 electrode including sintering and deposited film at 400- 500℃.
FTO coated glass is the best choice for such high temperature process, although ITO can be more easily produced and is more inexpensive.
For meeting the trend of consumer electronic devices, recently a new focus of DSSC technology is directed to the realization of lightweight plastic film-type cells.
For this purpose, replacement of TCO coated solid glass substrates with flexible plastic substrates has been the subject of intense study. The use of flexible substrates also brings about a significant merit for drastic cost reduction by manufacturing the entire cell through roll-to roll assembling. [18]
2.2-2 Nanocrystalline Photo-anode
For adsorption more dye molecules to increase the cell efficiency, high surface mesoporous semiconductor for DSSC become very important. Titanium dioxide is the fundamental semiconductor for DSSC because of its non-toxic properties, easy produce process, high stability and low coast. The surface is composed of 15-20nm-sized particles, and about 100 times the geometric area occupied for each micrometer of thickness. A roughness factor, defined as the ratio of the real surface area to the projected area, is at least 1000 to ensure efficient solar light harvesting by
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the currently used sensitizers.[19] The thickness of TiO2 film is typically 5-20μm and analysis of the layer morphology shows the porosity to be ~50-65%.
There are two main ways to prepare TiO2 photoanode. One is the “sol-gel method”[20, 21], by which TiO2is prepared from hydrolysis of Ti-alkoxides and addition of binder. Narrow particle size distribution and fine-ordered crystal structure can be obtained by carefully controlling every preparing step. However, this method is limited to small scale for laboratory only although these properties are desired in
The above two well-mixed paste is applied to TCO substrate by (1) Doctor Blade Method, (2) Screen Printing and (3) Spin Coating techniques. After coating and air-drying, the film is then sintered at 450-500 ℃ for 30 minutes in order to decompose organic binders and surfactants and to improve electrical contact between adjacent TiO2 particles in the porous layer as well as between the TiO2 particles and substrate at the same time.
There are still other ways for preparing nanostructured TiO2 electrodes, including electrodeposition [24-26], evaporation [27], sputter deposition[28-32], chemical vapor deposition [33, 34], ect. Other methods use highly-ordered TiO2 nanotube arrays [35]
and mixture of TiO2 nanowires and nanoparticles [36] as photoelectrode. Both front-side and back-side illumination were applied when using the TiO2 nanotube arrays electrode, indicating that electron transport in the nanotube is faster than between nanopaticles.
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2.2-3 The Sensitizer: Organic dye
As mentioned before, the organic dye becomes a sensitizer which absorbs most of the incident light and increases the cell efficiency. Organic dye used in photoelectrochemical cells should meet the follow needs:
1. Absorption: With good absorption in visible light region up to wavelength 920nm, almost the incident light from sun.
2. Energetic: With sufficient electrons on excited state providing the driving force to make electrons inject to conduction band of TiO2 thin film. Organic dye should also have relative low ground state for reduction by the redox couple in electrolyte.
3. Kinetics: The rate of electron injection should be high, and the lifetime of excited electrons should be long enough.
4. Stability: The organic dye can be operated under normal environment for more than 108 times of the redox cycle reaction and can be operated for more 20 years.
5. Interfacial Properties: It can attach on TiO2 surface and not easily desorption from TIO2 electrode.
A breakthrough of organic sensitized dye is accomplished by Gratzel’s group at EPFL in Switzerland by using metallo-organic ruthenium complex as the “dye” along with nanostructured TiO2 electrode.[4] The dye have the general structure ML2(X)2, where L stands for 2,2’-bipyridyl-4,4’-dicarboxylic acid, M for ruthenium or osimium and X for halide, cyanide, thiocyanate. The cis-RuL2 (NSC)2, also called N3 dye has shown impressive performance and has been wide used in DSSC research. Fig. 2-3 compares the spectral response of the photocurrent observed with two sensitizers. The incident photo to current conversion efficiency (IPCE) of DSSC is plotted as a function of excitation wavelength. Both chromophores show high IPCE values in the
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visible range. However, the response of the black dye extends 100nm further into the IR than of N3. The photocurrent onset is close to 920nm, i.e. near the optimal threshold for single junction converters. Recently there is a credible challenger identified to the “black dye” (tri(isothiocyanate)-2,2’,2”-terpyridyl-4,4’4”
-tricarboxylate)Ru (Ⅱ).
Fig. 2-3 Spectral response curve of the photocurrent for the DSSC sensitized by N3 and the black dye. The chemical structure of N3 dye and black dye. [17]
Lately several studies showed up by modifying the function groups to improve excitation lifetime and increase the open circuit voltage of the cell which are called N719, N749 (black dye)…ect. (Fig.2-4). Recent work has focused on the molecular engineering of suitable ruthenium compounds, which are known for their excellent stability. Cis-Di-(thiocyanato)bis(2,2’-bipyridyl-4,4’-dicarboxylate)ruthenium( Ⅱ ),
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coded as N3 or N719 dye depending on whether it contains four or two protons, was found to be an outstanding solar light absorber and charge-transfer sensitizer. Fig. 2-5 shows the structures of three ruthenium complexes with different colors that have been widely employed as sensitizers for the DSSC. This feature can be applied to design multicolor DSSC in art and architecture.
Fig. 2-4 The chemical structure of N719 dye and N749 (black dye). [37]
Fig. 2-5 Structure of the ruthenium sensitizers RuL3 (yellow) cis-RuL2(NCS)2 (red) and RuL (NCS)3 (green) where L) 2,2’-bipyridyl-4,4’-dicarboxylic acid and L) 2,2’,2”-terpyridyl -4,4’,4”-tricarboxylic acid. The lower part of the picture shows nanocrystalline TiO2 films loaded with a monolayer of the respective sensitizer. [38]
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Another important issue being raised up lately is the dye adsorption process, in which the sintered TiO2 electrode is immersed into a dye solution, usually 2×10-4M in solvent traditionally. The dye adsorption should be done immediately after high temperature sintered process for TiO2 electrode for not letting water content in the reductant providing electrons to redox the oxidized dye molecules at photoelectrode and as oxidant receiving from counter electrode.
Redox Couples
Requirements and properties of redox couple in electrolyte should be defined:
1. Redox potential
The redox couple reversible potential has to be equal to the negative of dye reversible potential. The more negative the potential, the large the thermodynamic driving force for the dye regeneration. However the potential request should make the balance between the driving and the open circuit potential hence the cell performance in order to avoid unnecessary loss of usable energy.
2. High solubility
To make sure sufficient supply of the redox mediator and to minimize the possibility of diffusion-limited situations, an adequate concentration of redox couple is needed. Because diffusion-limited would result in an undesirable lifetime of the oxidized dye and consequently would increase the possibility of
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dye decomposition. Concentration are commonly used at 0.1-0.5M 3. High diffusion coefficient
A high diffusion coefficient is needed because the mass transport of the redox couple in a solar cell (through solution and TiO2 network) occurs solely by diffusion.
4. No significant spectral characteristics in the visible region
In order to prevent the situation of less light being available for the light-to-electricity conversion and thus low energy conversion, the redox couples should not able to have absorbance in the visible light region.
5. High stability of both the reduced and oxidized forms of couple
For efficient redox “shuttling” in solar cells, both the oxidized and reduced forms of the couple need to be present in solution and both forms must have high stability. In the case of iodide/triiodide system, the reduce form is in excess. [39, 40]
6. Highly reversible couple
The oxidation of the reduce form and reduction of the oxidized form of the redox couple must be electrochemically and chemically reversible to ensure the fast electron transfer and avoid unwanted side reactions.
7. Chemically inert system
The components of the redox couple system must be chemically inert to avoid the side reaction, e.g., no chemical reactivity with TiO2, no surface activity, ect.
Based on the requirements listed above, many redox couples have been tested for DSSC systems. Now the I－/I3－ redox couple still remains the best choice because of its kinetics and suitable redox potential for TiO2 electrode.
In practical use, the redox couple is prepared by dissolving I2 and some iodine
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salt such as KI, LiI, alkyl methylmidazolium iodide, ect. to form I－/I3－ couple. The triiodide would form instantaneously when iodide is added into iodide via this equation:
I－+ I2 → I3
－ (2-13)
Ionic liquid utilizing iodide as anion like DMPII (diemethyl-propyldazium iodide) has also been introduced to be iodide source in DSSC systems. It is believed that those “liquid-like” salts have higher dissociation rate than tradition iodide salt.
Solvents used for electrolyte
Some criteria are made for a suitable solvent for liquid-type electrolytes as below:
1. The solvents must be liquid and have low volatility at the operation temperature (usually 40-80℃) to avoid freezing or expansion of the electrolyte which would damage the whole cells.
2. They have low viscosity for rapid diffusion of carriers.
3. The chosen redox couple should have high dielectric constant to promote dissolution in solvent.
4. The solvent should not make desorption or dissolution of sensitized dye.
5. The solvent should not decompose under illumination or after long time use.
6. The solvent should better be low coast for large scale production, and have low toxicity.
Typical liquid solvents are acetonitrile (ACN) [41], methoxyactonitrile, 3-methoxypropionitrile (3-MPN), ethylene carbonate (EC) [42], propylene carbonate[37], ect. and their mixture[43-46]. ACN has performed the best among these solvents, but it is still unwelcoming due to two reasons: (1) highly volatile with low boiling point (82℃) and (2) it is a carcinogenic chemical.
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There are also new conceptions for the electrolyte of DSSC: Quasi-solid state polymer electrolyte, using ionic liquid as solvent for electrolytes containing an I－/I3
－
redox couple.[47] The request for long-operation stability of DSSC is a driving force of to substitute liquid electrolyte by solid or quasi-solid state electrolyte.[48-52]
However, the mass transport of the triiodide has been considered as a limiting factor for ionic liquids due to their low diffusion coefficient and lower concentration in these electrolytes versus iodide. A high concentration of redox couple is needed to achieve a domination of the exchange-reaction based fast charge transport process between I－ and I3
－ in viscous electrolytes. On the other hand, the absorption in visible light by I－ competes with the absorption of the dye and high concentration of I－ promotes thr back electron transfer from conduction band of the photoanode to the triiodide[53].
2.2-5 Counter electrode
The reaction on counter electrode is rtiiodide reduction:
I3
－ + 2e－ → I－ (2-14)
This reaction plays an important role in the overall DSSC system because it is responsible for the regeneration of oxidized dye molecules. The conversion efficiency of DSSC might be lowered if the speed of dye regeneration is lower than the dye oxidization by photo injection.
Science the ITO or FTO shows slow kinetics of triiodide reduction in organic solvents [54 55], catalytic material is coated in order to accelerate the reduction reaction. Platinum (Pt) has been almost exclusively as the catalytic material. However, different methods preparing thin Pt film lead to different efficiency and coast. Fang et al. used sputtered Pt layer and they found out that the sputtered Pt layers on different type of substrates (steel sheet, nickel sheet, polyester film, and conducting plastic film) have slightly different cell efficiencies in comparison with that based on a conducting
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glass. [56] In recent reports, Kim et al. have demonstrated the preparation of a new counter electrode consisting of Pt nanosized phase in NiO or TiO2 porous phase using a RF co-sputtering system. They indicated that by applying Pt in a metal oxide biphase electrode, the short circuit current density and cell efficiency were increased due to the increased active surface area of the nanosixed Pt.
Nonetheless, sputtering system is not proper for mass production considering the coast and the environment request of ultra-high vacuum. Papageorgiou et al. [57]
developed a method called “thermal cluster platinum catalyst” (TCP). Counter electrode made by this method has low Pt loading (around 2-10g/cm3), superior kinetic performance, and mechanical stability comparing with other deposition methods like sputtering, spin coating, thermal[5, 58], and electrochemical deposition.
Wei et al. [59, 60] developed a simple technique called “two-step dip coating”
for preparing a Pt nanoclusters counter electrode for DSSC system. With an appropriate surface conditioner, the adhesion of PVP-capped Pt nanoclusters on ITO glass becomes satisfactory. Electrodes employing this method exhibit not only ultra low Pt usage but also good catalytic performance.
There are still other materials used for counter electrode, such as conducting polymer [61, 62], nanocarbon [63], carbon black [64, 65] and carbon nanotubes [66], some of them even use polymer-catalyst composites[67-69]. These new material used as counter electrode usually requires porous film on the substrate to obtain acceptable catalytic reduction efficiency.
2.2-6 Sealant and Spacer
Sealing is a very important process in DSSC system to avoid the humid environment and to prevent the decomposition of dye molecules. The thickness of spacer is also having dilemma between making lower IR drop and the risk of short
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circuit.
Surlyn, a thermoplastic resin which has good toughness, becomes a good sealant used in DSSC. It is inert to electrolyte and shows great tightness. However, there are still other types of resin which also used in DSSC system, and optimal one is not decided.
2.2-7 Post-treatment/Pretreatments/Underlayer
Recent paper [70] revealed that a post-treatment of the TiO2 film can efficiently improve the performance of DSSC based on the fact that an extra layer of TiO2 is grown onto the TiO2 nanoparticles constituting the film. There are many hypotheses concerning the following aspects:
These post-treatments have been carried out by TiCl3 electrodeposition, Ti-isopropoxide and, the best, titanium tetrachloride (TiCl4) post-treatment [70]. The effect of these post-treatment is believed to increase the dye loading making more efficient photon-current conversion which affect the short circuit current density (JSC), and the current conversion efficiency (IPCE). It is also important to note that the TiCl4 treatment would not be beneficial if a given TiO2 nanoparticle film is already at the correct potential to reach the maximum efficiency of the electron injection, which depends on the history and source of TiO2 to be made.
In addition, another report indicates that the TiCl4 pretreatment to ITO or FTO can certainly enhances the suppression of dark current leading an increase in VOC and
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enlarge the surface area of the mesoscopic film leading the improvement of JSC [73].
Xia et al. also present a new FTO/TiOX/mesoscopic TiO2 electrode which can be applied to be the blocking layer of DSSC [74]. According to their study, the blocking layer improved reduction of the electron loss at the FTO/mesoscopic TiO2 and FTO/electrolyte interface by a TiO2 compact layer between the FTO and mesoscopic TiO2 layer which made by RF sputtering system. This is also another discovery finding out a layer made by TiO2 can improve the total performance of DSSC due to various functions.
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