**1. Introduction**

286 Solar Cells – New Aspects and Solutions

Zhou, Y.; Zhang, F.; Tvingstedt, K.; Barrau, S.; Li, F.; Tian, W. & Inganas, O. (2008).

Zhou, W.; Nie, W. ; Li, Y.; Liu, J. & Carroll, D. L. (2009). Fabrication considerations in fiber

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Zou, D.; Wang, D.; Chu, Z.; Lv, Z.; & Fan, X. (2010). Fiber-shaped flexible solar cells. *Coordination Chemistry Reviews*, Vol. 254, pp.1169–1178, ISSN 0010-8545. Zou, J.; Yip, H.-L.; Hau, S. K. & Jen, A. K.-Y. (2010). Metal grid*/*conducting polymer hybrid

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Vol.96, pp.203301-1- 203301-3, ISSN: 1077-3118.

ISBN: 9780819477064.

378, ISSN 0927-0248.

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based organic photovoltaics. Proceedings of SPIE Vol. 7416, pp. 741613-1– 741613-5,

(2007). ITO-free wrap through organic solar cells–A module concept for cost efficient reel-to-reel production. *Sol. Energy Mater. Sol. Cells,* Vol. 91, No.5, pp.374–

transparent electrode for inverted polymer solar cells. *Applied Physics* Letters,

A potential candidate for next-generation solar cells is dye-sensitized solar cells (DSSCs). Much attention has been directed toward DSSCs employing nanostructured TiO2 electrodes and organic-ruthenium dye molecules as the light-harvesting media. The high porosity of nanostructured TiO2 film enables a large concentration of the sensitizing dye molecules to be adsorbed. The attached dye molecules absorb light and inject electrons into the TiO2 conduction band upon excitation. The electrons are then collected at a back conducting electrode, generating a photocurrent. DSSCs exhibit high photovoltaic conversion efficiencies of about 11% and good long-term stability. In addition, they are relatively simple to assemble and are low-cost (O'Regan & Grätzel, 1991; Grätzel, 2003; Chiba et al., 2006). However, in order to replace conventional Si-based solar cells in practical applications, further effort is needed to improve the efficiency of DSSCs. A great amount of work has been done on controlling the morphology of the TiO2 electrodes by employing ordered arrays of nanotubes, nanowires, nanorods and inverse opal structures (Adachi et al., 2003; Paulose et al., 2006; Law et al., 2005; Song et al., 2005; Nishimura et al., 2003) in order to improve the electron transport and collection throughout the device. Another important factor in improving the performance of DSSCs is the design of the photosensitizer. The ideal dye photosensitizer for DSSCs should be highly absorbing across the entire solar light spectrum, bind strongly to the TiO2 surface and inject photoexcited electrons into the TiO2 conduction band efficiently. Many different dye compounds have been designed and synthesized to fulfill the above requirements. It is likely that the ideal photosensitizer for DSSCs will only be realized by co-adsorption of a few different dyes, for absorption of visible light, near infrared (NIR) light, and/or infrared (IR) light (Polo et al., 2004; Park et al., 2011). However, attempts to sensitize electrodes with multiple dyes have achieved only limited success to date.

Narrow-band-gap semiconductor quantum dots (QDs), such as CdS, CdSe, PbS, and InAs, have also been the subject of considerable interest as promising candidates for replacing the sensitizer dyes in DSSCs (Vogel et al., 1990, 1994; Toyoda et al., 1999, 2003; Peter et al., 2002; Plass et al., 2002; Shen et al., 2004a, 2004b, 2006a, 2006b, 2008a, 2008b, 2010a, 2010b; Yu et al., 2006; Robel et al., 2006; Niitsoo et al., 2006; Diguna, et al., 2007a , 2007b; Kamat, 2008, 2010; Gimenez et. al., 2009; Mora-Sero et al., 2009, 2010). These devices are called QD-sensitized solar cells (QDSCs) (Nozik, 2002, 2008; Kamat, 2008). The use of semiconductor QDs as sensitizers

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 289

Scheme 1. Electron- hole pairs are generated in semiconductor QDs after light absorption. Then photoexctied electrons in the semiconductor QDs are injected to the conduction band of TiO2 and/or trapped by surface or interface states. The photoexcited holes are scavenged by reducing species in the electrolyte and/or trapped by surface or interface states. The nanostructured TiO2 is employed as an electron conductor and electrons transport in TiO2 to a transparent conductive oxide (TCO) substrate, while the electrolyte is used as a hole

**2. Electron and hole dynamics in CdSe QDs adsorbed onto TiO2 electrodes** 

**2.1 Preparation methods of CdSe QD adsorbed TiO2 nanostructured electrodes**  The method for preparing the TiO2 electrodes has been reported in a previous paper (Shen et al., 2003). A TiO2 paste was prepared by mixing 15 nm TiO2 nanocrystalline particles (Super Titania, Showa Denko; anatase structure) and polyethylene glycol (PEG) (molecular weight (MW): 500,000) in pure water. The resultant paste was then deposited onto transparent conducting substrates (F-doped SnO2 (FTO), sheet resistance = 10 Ω/sq). The TiO2 films were then sintered in air at 450 ºC for 30 min to obtain good necking. The highly porous nanostructure of the TiO2 films (the pore sizes are of the order of a few tens of

nanometers) was confirmed through scanning electron microscopy (SEM) images.

1. Chemical bath deposition (CBD) method (Hodes et al., 1994; Shen et al., 2008)

2. Successive ionic layer adsorption-reaction (SILAR) method (Guijarro et al., 2010a)

CdSe QDs can be adsorbed onto the TiO2 nanostructured electrodes by using the following

Firstly, for the Se source, an 80 mM sodium selenosulphate (Na2SeSO3) solution was prepared by dissolving elemental Se powder in a 200 mM Na2SO3 solution. Secondly, 80 mM CdSO4 and 120 mM of the trisodium salt of nitrilotriacetic acid (N(CH2COONa)3) were mixed with the 80 mM Na2SeSO3 solution in a volume ratio of 1:1:1. The TiO2 films were placed in a glass container filled with the final solution at 10 ºC in the dark

In situ growth of CdSe QDs using the SILAR method was carried out by successive immersion of TiO2 electrodes in ionic precursor solutions of cadmium and selenium.

transporter and hoels are transported to a counter electrode.

for various times to promote CdSe adsorption.

methods:

has some unique advantages over the use of dye molecules in solar cell applications (Nozik, 2002, 2008). First, the energy gaps of the QDs can be tuned by controlling their size, and therefore the absorption spectra of the QDs can be tuned to match the spectral distribution of sunlight. Secondly, semiconductor QDs have large extinction coefficients due to the quantum confinement effect. Thirdly, these QDs have large intrinsic dipole moments, which may lead to rapid charge separation. Finally, semiconductor QDs have potential to generate multiple electron-hole pairs with one single photon absorption (Nozik, 2002; Schaller, 2004), which can improve the maximum theoretical thermodynamic efficiency for photovoltaic devices with a single sensitizer up to 44% (Hanna et al., 2006). However, at present, the conversion efficiency of QDSCs is still less than 5% (Mora-Sero´, et al., 2010; Zhang, et al., 2011). So, fundamental studies on the mechanism and preparation of QDSCs are still necessary and very important. In a semiconductor quantum dot-sensitized solar cell (QDSC), as the first step of photosensitization, a photoexcited electron in the QD should rapidly transfer to the conduction band of TiO2 electrode and a photoexcited hole should transfer to the electrolyte (Scheme 1). Thus charge separation of the photoexcited electrons and holes in the semiconductor QDs and the electron injection process are key factors for the improvement of the photocurrents in the QDSCs. In this sense, the study on the photoexcited carrier dynamics in the QDs is very important for improving the conversion efficiency of the solar cell. To date, the information on the carrier dynamics of semiconductor QDs adsorbed on TiO2 electrode is limited, although a few studies have been carried out for CdS, CdSe and InP QDs using a transient absorption (TA) technique (Robel et al., 2006, 2007; Tvrdy et al., 2011; Blackburn et al., 2003, 2005). Most of them focused on the electron transfer process and the measurements mostly were carried out in either dispersed colloidal systems or dry electrodes. In recent years, the authors' group has been applying an improved transient grating technique (Katayama et al., 2003) to study the photoexcited carrier dynamics of semiconductor nanomaterials, such as TiO2 nanoparticles with different crystal structures and CdSe QDs absorbed onto TiO2 and SnO2 nanostructured electrodes (Shen et al., 2005, 2006, 2007, 2008, 2010). The improved TG technique is a simple and highly sensitive time-resolved optical technique and has proved to be powerful for measuring various kinds of dynamics, such as population dynamics and excited carrier diffusion. Comparing to the TA technique, the improved TG technique has a higher sensitivity and measurements under lower light intensity are possible (Katayama et al., 2003; Shen et al., 2007, 2010a). This fact is very important for studying the carrier dynamics of QDs used in QDSCs under the conditions of lower light intensity similar to sun light illumination. The improved TG technique is also applicable to samples with rough surfaces, like the samples used in this study.

This chapter will focus on the ultrafast photoexcited electron and hole dynamics in CdSe QD adsorbed TiO2 electrodes employed in QDSCs characterized by using the improved TG technique. CdSe QDs were adsorbed on TiO2 nanostructured electrodes with different adsorption methods. The following issues will be discussed:


has some unique advantages over the use of dye molecules in solar cell applications (Nozik, 2002, 2008). First, the energy gaps of the QDs can be tuned by controlling their size, and therefore the absorption spectra of the QDs can be tuned to match the spectral distribution of sunlight. Secondly, semiconductor QDs have large extinction coefficients due to the quantum confinement effect. Thirdly, these QDs have large intrinsic dipole moments, which may lead to rapid charge separation. Finally, semiconductor QDs have potential to generate multiple electron-hole pairs with one single photon absorption (Nozik, 2002; Schaller, 2004), which can improve the maximum theoretical thermodynamic efficiency for photovoltaic devices with a single sensitizer up to 44% (Hanna et al., 2006). However, at present, the conversion efficiency of QDSCs is still less than 5% (Mora-Sero´, et al., 2010; Zhang, et al., 2011). So, fundamental studies on the mechanism and preparation of QDSCs are still necessary and very important. In a semiconductor quantum dot-sensitized solar cell (QDSC), as the first step of photosensitization, a photoexcited electron in the QD should rapidly transfer to the conduction band of TiO2 electrode and a photoexcited hole should transfer to the electrolyte (Scheme 1). Thus charge separation of the photoexcited electrons and holes in the semiconductor QDs and the electron injection process are key factors for the improvement of the photocurrents in the QDSCs. In this sense, the study on the photoexcited carrier dynamics in the QDs is very important for improving the conversion efficiency of the solar cell. To date, the information on the carrier dynamics of semiconductor QDs adsorbed on TiO2 electrode is limited, although a few studies have been carried out for CdS, CdSe and InP QDs using a transient absorption (TA) technique (Robel et al., 2006, 2007; Tvrdy et al., 2011; Blackburn et al., 2003, 2005). Most of them focused on the electron transfer process and the measurements mostly were carried out in either dispersed colloidal systems or dry electrodes. In recent years, the authors' group has been applying an improved transient grating technique (Katayama et al., 2003) to study the photoexcited carrier dynamics of semiconductor nanomaterials, such as TiO2 nanoparticles with different crystal structures and CdSe QDs absorbed onto TiO2 and SnO2 nanostructured electrodes (Shen et al., 2005, 2006, 2007, 2008, 2010). The improved TG technique is a simple and highly sensitive time-resolved optical technique and has proved to be powerful for measuring various kinds of dynamics, such as population dynamics and excited carrier diffusion. Comparing to the TA technique, the improved TG technique has a higher sensitivity and measurements under lower light intensity are possible (Katayama et al., 2003; Shen et al., 2007, 2010a). This fact is very important for studying the carrier dynamics of QDs used in QDSCs under the conditions of lower light intensity similar to sun light illumination. The improved TG technique is also applicable to samples with rough surfaces, like the samples

This chapter will focus on the ultrafast photoexcited electron and hole dynamics in CdSe QD adsorbed TiO2 electrodes employed in QDSCs characterized by using the improved TG technique. CdSe QDs were adsorbed on TiO2 nanostructured electrodes with different

1. Pump light intensity dependence of the ultrafast electron and hole dynamics in the

2. Separation of the ultrafast electron and hole dynamics in the CdSe QDs adsorbed onto

4. Changes of carrier dynamics in CdSe QDs adsorbed onto TiO2 electrodes versus

5. Effect of surface modification on the ultrafast carrier dynamics and photovoltaic

adsorption methods. The following issues will be discussed:

properties of CdSe QD sensitized TiO2 electrodes.

TiO2 nanostructured electrodes;

adsorption conditions;

CdSe QDs adsorbed onto TiO2 nanostructured electrodes;

3. Electron injection from CdSe QDs to TiO2 nanostructured electrode;

used in this study.

Scheme 1. Electron- hole pairs are generated in semiconductor QDs after light absorption. Then photoexctied electrons in the semiconductor QDs are injected to the conduction band of TiO2 and/or trapped by surface or interface states. The photoexcited holes are scavenged by reducing species in the electrolyte and/or trapped by surface or interface states. The nanostructured TiO2 is employed as an electron conductor and electrons transport in TiO2 to a transparent conductive oxide (TCO) substrate, while the electrolyte is used as a hole transporter and hoels are transported to a counter electrode.
