Review on nanostructured semiconductors for dye sensitized solar cells

Electronic Materials Letters, Vol. 8, No. 3 (2012), pp. 231-243 DOI: 10.1007/s13391-012-1038-x

Review Paper

Review on Nanostructured Semiconductors for Dye Sensitized Solar Cells T. Prakash* Department of Medical Bionanotechnology, Chettinad University, Kelambakkam, Tamil Nadu, 603103, India

Nanostructured semiconductors with different morphologies are used widely in various applications in order to enhance their technological advancements compared with the bulk sample. This flourishing nanoscience field has enabled rapid developments that have created numerous opportunities for scienctific advancements with various devices. Considering large environmental impacts such as global warming, problems of nuclear waste storage and nuclear accidents, there is an urgent need for environmentally sustainable energy technologies such as solar cells and fuel cells. In the present paper, the role of nanostructured semiconductors in dyesensitized solar cells (DSSCs) is reviewed entensively. The review discusses the present developmental prospects of DSSCs and the problems associated with its layer materials and propose a method of overcoming these problems. Keywords: dye-sensitized solar cells, nanostructured semiconductors, heterojunctions, hole transporters

1. HISTORICAL PERSPECTIVE OF NANOCRYSTALLINE MATERIALS Materials with crystallite dimensions in the nanometer length scale (typically less than 100 nm) have significant fundamental and technological importance due to their modified properties that are vastly different to, and sometimes superior than, those of the bulk materials. Because of such tunable functional properties there are an increased numbers of publications and patents in this field, but there is really nothing new about nanoscience. The earliest civilizations used nanoscale materials for a variety of applications; in particular, they used metal nanoparticles as decorative pigments (although perhaps not deliberately) since the time of the Ancient Romans, such as those contained in the glass of the famous Lycurgus Cup (fourth century AD). The Cup is shown in Fig. 1(a) can be seen at the British Museum[1] with different coloration depending on the transmitted and reflected light in which it is viewed. An analysis of the glass reveals that it contains a very small amount of tiny metal crystals (~70 nm) containing Ag and Au with a molar ratio of approximately 14:1. Another famous example of the use of nanoscale materials from ancient times is Maya blue, which is an azure blue dye that was used by the Mayas and Aztecs around the seventh century AD. Recently, it has been shown that Maya blue consists of metal and oxide nanocrys*Corresponding author: [email protected] ©KIM and Springer

tals in addition to indigo and silica.[2] In the seventeenth century, the glass manufacturing processes induced outstanding advances on stained glasses fabrication procedures developed by Antonio Neri, a Florentine glass maker and priest, used to produce variety of metallic colloids for glass staining increased substantially of church windows that are shown in Fig. 1(b). In 1689, John Kunckel translated Neri's work into German; Kunckel is often credited with the discovery that transparent glass can be stained with metallic colloids.[3] Then, in early 1857, Michael Faraday[4,5] undertook groundbreaking work on colloidal metals as shown in Fig. 1(c). In 1898, Bredig[6] prepared gold colloids by striking arcs between gold electrodes immersed in dilute alkali. Donau[7] continued this research in 1905 by suggesting that passing CO through a solution of chloroauric acid can provide a colloidal gold sol. Later, Zsigmondy[8] discovered the seeding method and popularised the use of formaldehyde in mild alkali as a method for producing gold collids from salts, for this invention he got Nobel Prize award in 1925. During a similar period, Mie[9] and Gans[10] proposed a theoretical basis for the optical properties of such metal colloids, and this basis continues to be used widely today. Despite of these early advances, the excitement caused by Nobel Laureate Richard Feynman[11] lecture, titled “There’s Plenty of Room at the Bottom,” was delivered on December 29, 1959, at the annual American Physical Society meeting on the campus of Caltech. Although Feynman certainly popularized nanotechnology, his influence did not directly lead to the design of nanoscale materials.

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T. Prakash: Review on Nanostructured Semiconductors for Dye Sensitized Solar Cells

Fig. 1. (a) View of a fourth century AD Lycurgus cup in (left) reflected and (right) transmitted light, (b) a seventeenth century cathedral stained glass window, and (c) Faraday’s colloidal gold.

Rapid progress in nanotechnology could only occur after the arrival of sophisticated instrumentation that was capable of viewing and manipulating materials in nanoscale dimensions. In particular, the inventions of the scanning tunneling microscopy[12] (STM) in 1982 and atomic force microscopy[13] (AFM) in 1986 have allowed scientists to fulfil Feynman’s vision. The 1986 Nobel Prize in Physics was awarded to Gerd Binnig and Heinrich Rohrer to honour their design of the STM. They shared the Prize with Ernst Ruska, who was the inventor of the first electron microscope, which is another essential tool for modern nanomaterial researchers. During a similar period, Eric Drexler (1980) published first paper on nanotechnology,[14] and Gleiter (1981) proposed[15] many fundamental notions on the generation of nanoscale materials. In 1986, Eric Drexler published[16] a book titled “Engine of Creation” which disseminates his provocative ideas on molecular nanotechnology to a general audience outside the scientific community. These initiatives have created a revolution in science and the growth of the nanoscience field. Now, it is possible to prepare and study nanocrystals using various methods, tools, and techniques. Advances in both experimental and theoretical methods have led to an increased understanding of nanocrystal properties.

2. RECENT PERSPECTIVES ON NANOCRYSTALLINE MATERIALS In nanocrystalline materials, due to size reduction, two types of atomic arrangements appear to co-exist: the crystalline component formed by atoms located in the lattice of the crystallite (grain) and the interfacial components comprising atoms that are in the interface (grain boundaries). High-resolution transmission electron microscope (HRTEM) images of a nanocrystalline material are shown in Fig. 2(a): the fringes with different orientations confirm the presence of the crystalline component ‘grain’. The simulated schematic atomic structure using a two-dimensional hard sphere model[17] for grains and grain boundaries of nanocrystalline materials is shown in Fig. 2(b). By varying the diameter of the grain dimension, the percentage of atoms residing in the

Fig. 2. (a) HRTEM image of a nanocrystalline material and (b) the stimulated atomic structure for grains and grain boundaries of nanocrystalline material.

grain boundaries of nanocrystalline materials can be varied. Assuming the shape of the grain is spherical, the volume fraction of the nanocrystalline material associated with the boundaries can be calculated using C = 3∆/d, where ∆ is the average grain boundary thickness (~ 1 nm) and d is the average grain diameter. Thus, the volume fraction of atoms in the grain boundaries can be as much as 50% for 5 nm grains and this decreases to approximately 30% for 10 nm grains and 3% for 100 nm grains. In contrast, for coarse-grained bulk materials, the volume fraction of atoms in the grain boundaries is negligible.[18] Numerous physical, chemical, and mechanical methods have been developed in the past few decades for the preparation of nanocrystalline materials, and these methods have stimulated extensive research focused on controlling the particle size, shape, and composition under mild conditions. The synthesis of nanoscale materials has generally been grouped into two broad categories: “bottom-up” and “topdown.” The bottom-up approach is based on building from atoms and molecules to crystallites whose properties vary discretely with the number of constituent entities. Conversely, when the crystallite size is reduced from macroscopic towards the nanometre scale, it is the “top-down” approach. The “bottom-up” approaches are interesting due to the possibilities of manipulating the final product properties (e.g. size, shape, stoichiometry, surface area, pore size, surface decoration, etc.). The combustion technique, laser ablation, and ball milling are “top-down” approaches; electro-

Electron. Mater. Lett. Vol. 8, No. 3 (2012)


deposition, co-precipitation, spray pyrolysis, sol-gel process, reverse micelles, sonochemical, hydrothermal, and chemical vapour deposition are comes under “bottom-up” approaches. The principles that underlie the mechanical alloying/milling, sol-gel process and co-precipitation methods are discussed in detail below since these three methods are most widely used to synthesis nanocrystals. 2.1 Sol-gel process Sol-gel refers to solution-gelation and is a widely used method for producing ceramic materials with high purity and homogeneity. This approach offers control of the composition and structure at the molecular level. The process involves the generation of a colloidal suspension, which is subsequently converted to gel. In the process, reactive metal precursors are initially hydrolized, followed by condensation and polymerization reactions. The metal alkoxides are organometalic compounds with an organic ligand attached to a metal or metalloid atom. These alkoxides are the result of direct or indirect reactions between a metal (M) and an alcohol (ROH); typical examples include methoxide (OMe; MOCH3) and ethoxide (OEt; MOC2H5). During hydrolysis, the alkoxy groups (OR) are replaced by hydroxy ligands (OH), i.e. M(OR)Z + H2O → M(OH)(OR)Z + ROH where R is the alkyl group (CnH2n+1). The mechanism of this reaction involves the addition of a negatively charged HOd− group to the positively charged metal center (Md+) followed by the removal of ROH.[19] Several factors are known to affect the hydrolysis reaction[20]: (a) the nature of the alkyl group, (b) the nature of the solvent, (c) the concentration of each species in the solvent, (d) temperature, (e) the water to alkoxide molar ratio, and (f) the presence of acid or base catalysts. Subsequent condensation eliminates either water or alcohol in order to produce metal oxide or hydroxide linkages. In this process, the two mononuclear complexes of M, each comprising only one metal M, can react with each another to form a polynuclear complex consisting of two metal atoms. Condensation only occurs when at least one hydroxo ligand is bonded to the cation M; this is designated as M-OH for simplicity. Condensation can proceed via olation and oxolation reactions. Olation is a reaction in which a hydroxo or an “ol” bridge M-OH-M bond is formed between two cations whereas oxalation involves the formation of oxo bridges M-O-M between to metal cations.[21] The “ol” or “oxo” bridges between the two metal atoms lead to the formation of condensed oxide or hydroxide species. Under acid conditions, three-dimensional solid phase networks consisting of extended linear M-O-M chain polymers are fabricated.[22] Inorganic polymerization is believed to occur in three stages during the acid catalyzed condensation[23]: (a)

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polymerization of monomer units to form particles, (b) particle growth, and (c) particles linking to form chains then solid networks that extend throughout the liquid medium, which thickens it to a gel. The removal of the solvents and appropriate drying techniques are important steps to achieve gel densification. When a solvent evaporates from the gel under atmospheric conditions, the capillary pressure due to the interfacial tension of the solvent puts high stress on the gel network. This leads to considerable shrinkage and fracture of the gel during the drying process. The resulting hard, glassy, and porous product is called a xerogel. When the liquid within the gel is removed above its critical temperature and pressure (hypercritical) in an autoclave, the capillary pressure can be eliminated. The product obtained from this process is referred to as an aerogel.[24] The crystalline nature of the amorphous aerogel was achieved after heat treatment. This transition exhibits crystallites with high surface areas and unique properties. 2.2 Co-precipitation technique The co-precipitation technique is used widely in industry and research laboratories to synthesize complex nanocrystalline materials through a simple process and the chemistry involved is relatively straightforward. The atomic mixing of the constituents using chemical co-precipitation yields a final product of near perfect stoichiometry without high temperature treatments. In this process, a solution of metal chloride, oxychloride, nitride, sulphide or acetate can be used as the precursor. The solution is then neutralised with an “acidic” solution, such as oxalic acid, ammonium oxalic or ammonium hydroxide, in order to precipitate the mixed oxalate or hydroxide from the solution. The final crystalline oxide is obtained through the heat treatment of the precipitates. For example, if the starting precursor is metal chloride, then the corresponding metal hydroxides will precipitate with the addition of a base solution such as sodium hydroxide (NaOH) or ammonium hydroxide (NH4OH). The resulting chloride salts (NaCl or NH4Cl, respectively) are then washed off and the hydroxide is calcined after filtration and washing in order to obtain the final oxide powder. M(Cl)n + n(XOH) + H2O → M(OH)n + n(XCl) M(OH)n

heat treatment

→ MO + n(H2O)

In this process, controlling various factors such as the solubility product of various metal ion compounds, pH, concentration, mixing or stirring rate, drop wise addition rate, temperature, washing mode, and drying temperature assist with the production of satisfactory mono-distributed crystallites. Alternatively, surface-controlling agents (surfactants), e.g. cetyltrimethylammonium bromide (CTAB), can be used to control the size and shape of the nanocrystals.[25]


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balls or rods), milling atmosphere (e.g. air, nitrogen, inert gas, etc.), milling environment (e.g. dry milling, wet milling), milling media-to-powder weight ratio, temperature, and duration. It is well known that the properties of the milled powders of the final product, such as the particle size distribution, degree of disorder or amorphization, and final stoichiometry, depend on the milling conditions.

3. INFLUENCE OF THE CRYSTALLITE SIZE ON THE PHYSICAL PROPERTIES OF SEMICONDUCTORS

Fig. 3. Photograph of a planetary ball mill: (a) commercial instrument, (b) a tungsten carbide ball, and (c) stainless steel vial with tungsten carbide inner wall.

2.3 Ball milling method Ball milling is commonly used to prepare samples via mechanical alloying (MA) and mechanical milling (MM). The MA process is a unique process for the fabrication of several alloys and advanced materials at room temperature. There are numerous articles that discuss the various features of the mechanical alloying technique.[26,27] In essence, the term milling refers to the breaking down of relatively coarse materials to their ultimate fineness. This process can be successfully performed in both high-energy mills (attritor, planetary, and vibratory ball mills), and low-energy tumbling mills. Photographs of a planetary ball mill instrument, tungsten carbide ball, and stainless steel vial with a tungsten carbide inner wall are shown in Fig. 3. Compared with other mills, the planetary ball mill is used widely for quick dry or wet grinding of inorganic and organic samples. In this method, the size reduction by milling or alloying is achieved by imparting mechanical energy to the sample. The material is crushed in a vial using grinding balls. The grinding vial and supporting disc are rotating in opposite directions, so that the centrifugal forces alternate between moving in the same and opposite directions. Thus, centrifugal forces act on the grinding balls and material in the grinding vial due to the rotation of the grinding bowl around its own axis and the rotation of the supporting disc. This results in the grinding balls moving along the inner wall of the vial as a frictional effect, and the balls impacting the opposite wall of the grinding vial as an impact effect; the impact effect is further enhanced by the grinding balls also hitting each another. The amount of energy imparted on the sample by the refractory or metallic balls depends on a number of parameters[28] including the mill type (e.g. high-energy mills and lowenergy mills), the milling tool materials (e.g. ceramics, stainless steel, tungsten carbide, etc.), types of milling media (e.g.

Semiconductor nanocrystals offer the opportunity to explore the evolution of the electrical and optical properties as the size of the crystallite decreases from the bulk to the nanometre scale. In addition, the nanocrystals strongly sizedependent optical and electrical properties render them attractive in optoelectronics. 3.1 Optical properties The optical properties of semiconductors, such as the absorbance and emission, result from the excitonic transitions. The influence of size on these properties occurs when the diameter is comparable to or smaller than the exciton diameter; the consequent changes in the electronic structure are called the confinement effect. Excitons are imaginary quasiparticles produced by the pairing of an electron (from the conduction band) and a hole (from the valence band) resulting from the Coloumb interaction. Such crystallite size in semiconductor nanocrystals is understandable from the work of Burs,[29] and his effective mass approximation theory. According to the theory,

Fig. 4. Crystallite size effect on optical properties of nanocrystals; For ZnO: me* = 10, mh* = 0.8, Eg = 3.2 eV, and ε = 86; for TiO2: me* = 10, mh* = 0.8, Eg = 3.2 eV, and ε = 86; and for SnO2: me* = 0.3, mh* = 0.8, Eg = 3.6 eV and ε = 12.


T. Prakash: Review on Nanostructured Semiconductors for Dye Sensitized Solar Cells 2 2

2

πh 1 1 1.786e * Eg ≈ Eg + ---------2- ------ + ------ – -----------------(1) m m εR e h 2R where Eg and Eg* are the bandgap value of the bulk and confined crystallite, respectively; ε is the dielectric constant; R is the radius of the crystallite; me is the effective mass of electron; and mh is the effective mass of the hole. This theory has also been applied in order to determine the bandgap widening (∆Eg = Eg* − Eg) in ZnO, SnO2, and TiO2 nanocrystals. A plot between the crystallite diameter and bandgap widening is presented in Fig. 4. As a result of the confinement effect, the shifts in the absorption onset position to shorter wavelengths are evident. 3.2 Electrical properties The electrical resistivity of nanocrystalline materials is significantly influenced by the grain boundary scattering. As a result of the increased volume fraction of the atoms lying at the grain boundaries, the electrical resistivity was found to be higher than that in a coarse-grained material with the same chemical composition. Table 1 shows the influence of the grain size on the electrical resitisivity of nanocrystalline Ni-P and (Fe99Cu1)78Si9B13 samples. These experimental results confirm that if the crystallite size is smaller than the electron mean free path, then the grain boundary sattering dominates; hence, the electrical resitivity and temperature coefficient of the resistivity is also expected to increase. Ramasamy et al. recently reviewed[30] the electrical properties of nanocrystalline materials; they stated that the electrical resistivity of nanocrystalline materials is sensitive not only to the grain boundaries but also to other types of imperfections in solids such as vacancies and dislocations. Tschöpe et al. listed[31] three reasons that govern the electrical activity of the grain boundary in polycrystalline semiconductors: (a) the formation of interfacial states as a result of broken symmetry, (b) the altered defect thermodynamics at grain boundaries, and (c) the inhomogeneous distribution of charged defects resulting in a current blocking space charge region. The increased grain boundary resistivity of nanocrystalline electroceramics may lead to a wide range of device Table 1. Influence of crystallite size on electrical resistivity of nanocrystalline materials.[34]

Sample

Room Temperature Grain temperature coefficient of size resistivity resistivity (nm) (µΩ cm) (K−1)

Ref.

Ni-P

11 51 102

360 220 200

1.26 × 10−3 2.00 × 10−3 Lu et al.[35] 2.35 × 10−3

(Fe99Cu1)78Si9B13

30 50 90

126 60 44

2.01 × 10−3 1.06 × 10−3 Liu et al.[36] 1.06 × 10−3

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applications. It has been observed that the doping of elements such as B, Bi, Co, Cu, Sb, and Sn in nanocrystalline ZnO results in breakdown voltage increases[32] to the value 30 kV/cm from its bulk value[33] of 1.3 kV/cm.

4. APPLICATION OF SEMICONDUCTOR NANOCRYSTALS IN SOLAR ENERGY CONVERSION The supply of energy from the Sun is enormous: 3 × 1024 joules per year, or approximately 10,000 times more than the global energy consumption by the current population. That is, covering 0.1% of the Earth’s surface with solar cells with an efficiency of 10% could satisfy the population’s present energy consumption needs.[37] Conventional photovoltaic cells are based on the concept of charge separation at the interface of two materials with different conduction mechanisms and are currently providing an efficiency of 42.8% in standard terrestrial conditions.[38] In order to fabricate these cells, materials with high purity and ultra high vacuum based multilayer thin film deposition facilities are required; thus, solar energy is often more expensive than energy generated using other conventional sources. For commercial and domestic use, solar cells require not only high efficiency approaches to energy consumption, but also the prospect of low fabrication costs. Capturing the energy freely available from the Sun using nanostructured semiconductors is exhibiting strong potential and has created new opportunities for scientists to fabricate low cost solar cells such as dye-sensitized solar cells (DSSCs), extremely thin absorbers (ETAs), and hybrid solar cells. These devices are attracting significant attention due to their high light-to-electricity conversion efficiencies that are achievable using low cost and simple fabrication techniques. 4.1 Dye-sensitized solar cells Titanium dioxide (TiO2) is a wide bandgap semiconductor (Eg (anatase) ~ 3.2 eV) that can absorb only UV light, which accounts for approximately 4% of the total sunlight that can generate charge carriers to promote surface redox reactions. This inherent property of TiO2 restricts its practical applications. Thus, in order to effectively harvest the visible light that occupies approximately 43% of the total sunlight, fabricating a light-harvesting device with highly efficient visible light absorbance is essential. Since the late 1980s, much research has focused on developing TiO2 that can absorb both UV and visible light by sensitizing it with suitable dyes. In this case, the sensitizer dyes act as visible light harvesters. In their seminal 1991 Nature paper, O'Regan and Grätzel reported[39] a dye sensitized solar cell (DSSC) with a solar power conversion efficiency of 7% at AM 1.5 G. This breakthrough was achieved using high surface area nanocrystalline anatase TiO2 coated with an adsorbed dye in order to


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Fig. 5. The doctor blading method; (a) schematic illustration, (b) manual spreading of paste, and (c) deposition of films on top of the substrate in a commercial instrument.

Fig. 7. Schematic representation of the DSSC working principle: (1) the injection of an electron from the dye to the conduction band of TiO2 and (2) the interception process. Fig. 6. Conduction and valence bands positions (vs. NHE (Normal hydrogen electrode), pH = 7) of Ta2O5, Nb2O5, ZnO, TiO2, SnO2, and CdS.

maximise the light harvesting. The coating on the transparent conducting substrate is undertaken using a doctor blading technique (see Fig. 5 for its principle by schematic illustration and instrumentation). Since this first report, rapid advances have been made in the field of DSSCs, both in terms of fundamental understanding and cell design. Recently, the Grätzel group has reported a cell with 11.4% efficiency. In the DSSC, a porous high surface area oxide (typically a 10 µm thick layer genereally grown using TiO2 nanocrystals with average particle sizes of 5 - 20 nm) as a first layer. Alternatively, wide bandgap semiconductors such as SnO2 (Eg = 3.6 eV),[38] ZnO (Eg = 3.2 eV),[40] CdO (Eg = 2.5 eV), [41] and Nb2O5 (Eg = 3.4 eV)[42] are alreternative material for TiO2 (see Fig. 6, for the band edge positions[43] of these semiconductors). A dye (normally an organometallic rutheniumbipyridyl based charge transfer complex), a redox couple (usually I3−/I−), and a counter FTO electrode are coated with a few atomic layers of platinum or carbon in order to catalyze the redox reaction with the electrolyte. The working principle of DSSCs is shown in Fig. 7. Under sunlight irradiation, the dye molecules become photoexcited and quickly inject an electron into the conduction

band of the semiconductor; then, the original state of the dye is subsequently restored by electron donation from the electrolyte, which is usually the solution of an organic solvent or ionic liquid solvent containing the I3−/I− redox system. The sensitizer regeneration using iodide intercepts the recapture of the conduction band electron by the oxidized dye. The iodide is regenerated, in turn, by the reduction of triiodide at the counter electrode, with the circuit being completed through the external load. The voltage generated under illumination corresponds to the difference between the Fermi level of the electron in the semiconductor electrode and the redox potential of the electrolyte. Along with these processes, the electrons in the conduction band of the semiconductor may be recombined with the oxidised dye sensitizers or electron acceptor species in the electrolyte solution. In a typical device fabrication, the dye stained TiO2 plate is washed with ethanol, then dried and placed such that the TiO2 side is facing upwards. The counter electrode is placed on top of the plate, facing the conductive side with the TiO2 film. The plates are held together with two binder clips with an offset of 4 mm to create a strip of uncoated TiO2. The contacts are made using crocodile clips on the uncoated portions of the glass plates. Then, one or two drops of electrolyte solution are introduced from the edges of the plates between the two electrodes. Next, the solar cell device is exposed to natural sunlight or simulated AM (Air Mass) 1.5


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solar light in a laboratory using xenon or tungsten halogen lamp (100 mWcm−2) through the TiO2 coated plate. AM 1.5 light is the average solar light that reaches the Earth's atmosphere at an angle of 48.2° (1/cos 48.2 = 1.5). Here, the negative electrode is TiO2 coated FTO and the positive electrode is catalyst coated FTO. The resulting I-V characteristics of the device were measured using the circuit and its output curves are shown in Figs. 8(a) and 8(b). The efficiency of the solar cell is directly related to the fill factor (FF) that is calculated using the I-V curve. The voltage (Vmax) and current (Imax) at the maximum power point can be determined by computing the product of I and V at various points along the curve and selecting the point where the product is maximum. Thus, the maximum power (Pmax) and fill factor (FF) can be obtained using the following equations: Pmax = ( Vmax ) ⋅ ( Imax ) and ( Pmax ) FF = -----------------------( Voc ) ⋅ ( Isc ) where Isc is the short circuit current and Voc is the open circuit photovoltage. The overall efficiency (η) is calculated from the graph using the equation: ( Voc ) ⋅ ( Isc ) ⋅ ( FF ) - × ( 100 ) η = --------------------------------------Is where Is is intensity of the incident light. The performance of the DSSC can be quantified not only using the efficiency and FF, but also using the incident photon-to-current efficiency (IPCE). IPCE is a product of the quantum yield for charge injection, which is the efficiency of collecting electrons in the external circuit and the fraction of radiant powder absorbed

Fig. 8. (a) Typical experimental setup used to measure the currentvoltage (I-V) characteristics of DSSCs and (b) circuit output curve and its respective power curve for DSSCs.

by the device. In practice, the IPCE measurement is performed using light in a monochromatic scan, so it is plotted as a function of wavelength. It is calculated using the following equation: 3

–2

( 1.23 × 10 )(Photocurrent density, µAcm ) IPIE = ------------------------------------------------------------------------------------------------------------------------------------–2 (wavelength, nm )(Intensity of incident light, mW·cm )

Grätzel recently reported[38] the nanocrystalline effect on the IPCE of DSSCs. The IPCE value obtained with the single crystal electrode is only 0.13% near 530 nm, where the sensitizer has its maximum absorption, whereas it reaches 88% with the nanocrystalline electrode that is more than 600 times higher. The photocurrent in standard sunlight is augmented by approximately 103 - 104 fold when passing from a single crystal to a nanocrystalline electrode (standard, or full, sunlight is defined as having a global intensity (Is) of 1,000 Wm−2, AM 1.5). This striking improvement primarily results from the increased light harvesting of the dye-sensitized nanocrystalline film compared with a flat single crystal electrode; however, it is also partly a result of the mesoscopic film texture favouring photogeneration and collection of charge carriers. Over the past ten years, different efficiency records have been announced for DSSCs with different active areas: efficiencies of more than 11% have been achieved by EPFL and Sharp Corporation in small area DSSCs. Furthermore, conversion efficiency records of 8.12%, 10.1%, 10.4%, and 9.9% have been announced respectively by ECN, EPFL, Sharp Corporation, and Tokyo University of Science for strip solar cells with aperture areas of