Operation Principles of Dye-sensitized Solar Cells

reverse saturation current, which can be measured in reasonably large reverse voltage.

There are two main characteristic quantities, the open circuit voltage (V_OC), which represents the voltage produce in the absence of any current, and short circuit current ( I_SC)_, which stands for the current with no voltage across the cell.

The open circuit voltage is determined by the energy difference between the Fermi level of the semiconductor under illumination and the Nernst potential of the redox couple in the electrolyte. However, the experimentally observed open circuit voltage for various sensitizers is smaller than the difference between the conduction band edge and the redox couple. This is generally due to the competition between electron transfer and charge recombination. [15]

When photovoltaic devices are in dark, they should obey the ideal diode equation:

(2-10)

This indicates that a positive applied voltage can make current flow easy.

2. Incident Photons to Current Conversion Efficiency (IPCE)

IPCE is an important parameter when determining the performance of a photovoltaic device. IPCE is defined as the number of electrons flowing through the external circuit over the number of photons incident on the cell surface at a particular

wavelength, which means IPCE the ratio of the observed photocurrent over the incident photon flux.

3. Cell Efficiency (η)

We called VOC as the maximum voltage at photovoltaic device, and ISC as the maximum short circuit current under illumination. The IV-curve yielding the maximum power is called PMAX. In addition, another important parameter of cell performance is the Fill Factor (FF), which is defined as follows:

(2-11)

FF is an efficiency factor, used for checking whether the PMAX is ideally equal to V_OC × I_SC or not, because there are many types of impedances, including the contact resistance, electrolyte resistance, and charge transfer resistance, etc. inside the cell, may cause potential drop.

The overall energy conversion efficiency (η) is defined to be the maximum power generated by DSSC under illumination: (TOC) coated glass substrates. For meeting the trend of consumer electronic devices, a new focus of DSSC technology is directed to the realization of light weight film-type cells. Electrochemical anodic oxidization of titanium metal foil could fabricate highly

ordered TiO2 array. Therefore, TCO coated solid glass substrate is replaced with a TiO2 nanotube arrays and TiO₂ nanowires/TNA hybrid on a Ti foil in this study.

2.2.2 Nanocrystalline Photo-anode

Titanium dioxide is a fundamental semiconductor for DSSC because of its non-toxic properties, easy produce process, high stability and low cost. Energy conversion in a DSSC is based on the injection of an electron from a photo-excited state of the sensitizer dye into the conduction band of semiconductor.

For nanoparticle TiO2 film, there are two methods to prepare photoanode. One is the “sol-gel method” [16,17], by which TiO2 is prepared from hydrolysis of Ti-alkoxides and addition of a 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 standard electrode.

Nanoparticle efficiency was limited in the random walking of electron transport and recombination. To improve the efficiency of charge collection and reduce the recombination, different TiO2 morphologies, such as vertically TiO2 nanotube arrays, nanorods, and nanowires, have been investigated. Recently, TiO2 nanotubes were applied in solar cells. [1] The conversion efficiency was reported to increase from 1.6% to 1.9% under various tube length and morphology. [1]

There are many methods for preparing nanostructured TiO2 electrodes, including electrodeposition [18,19,20], evaporation[21], sputter deposition [22,23,24,25,26], and chemical vapor deposition [27,28]. The highly-ordered TiO2 nanotube arrays [29] and mixture of TiO2 nanowires and nanoparticles [ 30 ] are typically used as the photo-electrode.

The formation mechanism of TiO2 nanotube arrays (TNAs) and mixture of TiO2

which is the result of competition between field-assisted anodic oxidation and electrochemical etching. “Electrochemical anodization method” which Zwilling and co-worker published the first report on anodized TiO2 nanotube in 1999. [11]The typical porous structure observed only in sufficient HF was added to the electrolyte mixture called the first generation. In subsequent work, the smooth and long tube has been controlled in various electrolytes due to the different ion diffusion and the amount of water. [31,32]

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 photoelectron chemical 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 than 20 years.

5. Interfacial properties: It can attach on TiO2 surface and cannot be easily desorbed from TIO2 electrode.

A breakthrough of organic sensitized dye is accomplished by Grätzel’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 osmium and X for halide, cyanide, thiocyanate. The cis-RuL2 (NSC)2, also called N3 dye has shown impressive performance. Figure 2.2 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 visible range. However, the response of the black dye extends 100nm further into the IR than of N₃. The photocurrent onset is close to 920 nm, 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).

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)…etc. (see Figure 2.3).

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), coded as N3 or N719 dye depending on whether it contains four or two protons. N719 dye was found to be an outstanding solar light absorber and charge-transfer sensitizer. Figure 2.4 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.

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 sintering process for TiO2 electrode for not letting water content in the pores of electrode react with the excited dye molecules to reduce any impact on the long-term stability. In practice, we keep the photoelectrode in anhydrous condition before and after dye adsorption. The overall dye adsorption process should be stored in a moisture-free environment.

2.2-4 Electrolyte

Electrolyte systems consist of redox couple and solvent, which works as reducing agent 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 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. [33,34]

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, etc.

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 salt such as KI, LiI, alkyl methylmidazolium iodide, etc. to form I^－/I3

－ couple. The triiodide would form instantaneously when iodide is added into iodide via this equation:

I

^－

+ I

2

→ I

3

－

(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 cost for large scale production, and have low toxicity.

Typical liquid solvents are acetonitrile (ACN) [ 35 ], methoxyactonitrile, methoxypropionitrile (MPN), ethylene carbonate (EC) [36], propylene carbonate [37], etc. and their mixture [38,39,40,41]. ACN has performed the best among these solvents, but it is still not well-received due to two reasons: (1) highly volatile with low boiling point (82℃) and (2) it is carcinogenic.

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. [42] The request for long-operation stability of DSSC is a driving force of to substitute liquid electrolyte by solid or quasi-solid state electrolyte. [43,44,45,46,47]

However, the mass transport of the triiodide ion 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 the back electron transfer from conduction band of the photoanode to the triiodide [48].

2.2.5 Counter electrode

The reaction on counter electrode is a triiodide reduction:

I

3

－

+ 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.

Since the ITO or FTO shows slow kinetics of triiodide reduction in organic solvents [49,50], 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 cost. 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 glass.

[51] In recent reports, Kim et al. [52] 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 nanosized Pt.

cost and the environment request of ultra-high vacuum. N. Papageorgiou [53] 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, 54], and electrochemical deposition.

Wei et al. [55,56] 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 ultralow Pt usage but also good catalytic performance.

Other materials can be used as the counter electrode, such as conducting polymer [57,58], nanocarbon [59], carbon black [60,61] and carbon nanotubes [62]. Some even use polymer-catalyst composites [63,64,65]. These new material used as counter electrode usually requires porous film on the substrate to obtain acceptable catalytic reduction efficiency.

Surlyn® (SX1170-60), 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.

2.2.7 Post-treatment/Pretreatments/Underlayer

Recent paper [66] 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

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 enlarge the surface area of the mesoscopic film leading the improvement of J_SC [71].

Xia et al. reported a new FTO/ TiOX/ mesoscopic TiO2 electrode was used as the blocking layer of DSSC [72]. According to their study, the blocking layer reduced 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.

2.3 Anodic oxidization technique

2.3.1 The development of anodized TiO

₂

nanotubes

First generation of TiO

₂

nanotube

In 1999, Zwilling and co-workers anodizes Ti and Ti-6Al-4V (TA6V) alloy in an electrolyte containing 0.5 M chromic acid and 0.095 M HF [73], while the first report on anodized TiO₂ nanotubes (called first generation). The nanoporous structure observed only under a sufficiency HF was added to the electrolyte mixture, as pure chromic acid (CA) was leading to the formation of a thin but stable oxide layer with no apparent pore structure. Otherwise, the length of this kind method could enhance for a long processing time. TiO₂ nanotubes reach a steady state length when anodized. After typically 10 to 20 minutes of anodization, the etching rate equals the dissolution rate and it causes the length of nanotube stop increasing with additional anodizing time.

Second generation of TiO

₂

nanotubes

Grimes and co-workers [74]overcome the limitation described before since they knew other fluorine salts (as fluorine ion source besides HF) and combined buffers, bases and milder acids to adjust the pH and fluorine ion content. Salts like KF, NH4F, or NaF totally dissociate in aqueous solution and then hydrolyze with water to form HF [75]. Moreover, HF is a relatively mild acid in acidic solution (pH<3.45) more than 50% of the fluorine exist in the form of HF. As a result pH and fluorine ion concentration are closely related (and solutions with KF, NaF, and NH4F and no additional acid are basic.) The experiments found that they could grow nanotubes up to 4.4 µm (Figure 2.5) using a solution of 0.1 M KF as fluorine source, 1 M H2SO4 as acid, 0.2 M citric acid presumably serving as buffer and NaOH as base to be added until the desired pH of 4.5 was obtained [72]. Later in 2005, Grimes and co-workers reported even longer nanotubes of up to 6 µm, over 17 to 20 h of anodization using the

electrolyte as before [76].

Third generation of TiO

2

nanotubes

The third generation of nanotubes refer to smooth tubes (no ripples along the wall),

prepared by using organic electrolytes (some almost water-free) to minimize the dissolution rate of formed oxides.

As illustrated in Figure 2.6 [77], it can be observed that tubes obtained in water are much rougher and irregular than the other. The reason was to use a viscous electrolyte, where ion diffusion is slower than in water. Meanwhile, the pH gradient between the bottom and top of the tubes increased. This led to the formation of TiO2 nanotubes up to 7 µm thick (compared with the first generation). It implied that smoothness and regular morphology of the tube walls to the lower diffusion coefficient of the electrolyte which suppresses pH burst at the pore bottom which occur when working in aqueous media.

TiO2 nanotube arrays with lengths of up to 1000 µm were achieved using a non-aqueous, polar organic electrolyte such as formamide, dimethyl sulfoxide, ethylene glycol or diethylene glycol [78,79,80].

In 2007, Grimes and co-workers published the synthesis of 0.36 mm long nanotube [76], practically demonstrating that the nanotube lengths was only limited by the initial titanium foil thickness. The following section will discuss the fundamental aspects and chemistry of the growth of TiO2 by anodization.

2.3.2 The growth of TiO

2

nanotube: fundamental aspects

The formation mechanism of TNAs nanostructure is similar to anodic aluminum oxide (AAO), which is the result of competition between field-assisted anodic oxidation, defined as the formation of the anodic layer under a applied electric field by Eq.(2-15)-(2-17) and chemical/field assisted dissolution of the forming oxide by Eq.

(2-18), which can be regarded as dissolution promoted by the presence of fluoride ions

(chemical dissolution) and by the electric field weakening the bond between Ti and O schematically illustrated by Figures 2.7(a)-(d). Initially, field-enhanced oxidation occurs at the Ti/Ti oxide interface by Eq. (2-15)-(2-17) when oxygen ions diffusion to the Ti

(chemical dissolution) and by the electric field weakening the bond between Ti and O schematically illustrated by Figures 2.7(a)-(d). Initially, field-enhanced oxidation occurs at the Ti/Ti oxide interface by Eq. (2-15)-(2-17) when oxygen ions diffusion to the Ti

在文檔中 二氧化鈦奈米管陣列與複合結構在染料敏化太陽能電池之應用 (頁 20-0)