**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.

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


Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 291

In 2003, Katayama and co-workers (Katayama et al., 2003; Yamaguchi et al., 2003) proposed an improved TG technique (it was also called a lens-free heterodyne TG (LF-HD-TG) or a near field heterodyne TG (NF-HD-TG) technique in some papers), which overcomes the difficulties that exist in the conventional TG technique. The improved TG technique features (1) simple and compact optical equipment and easy optical alignment and (2) high stability of phase due to the short optical path length of the probe and reference beams. This method is thought to be versatile with applicability to many kinds of sample states, namely opaque solids, scattering solids with rough surfaces, transparent solids, and liquids, because it is applicable to transmission and reflection-type measurements. The principle of the improved TG technique has been explained in detail in the previous papers (Katayama et al., 2003; Yamaguchi et al., 2003) and is only described briefly here. Unlike the conventional TG technique, only one pump beam and one probe beam without focusing are needed in the improved TG technique. The pump beam is incident on the transmission grating. Then, the spatial intensity profile of the pump beam is known to have an interference pattern in the vicinity of the other side of the transmission grating, and the interference pattern has a grating spacing that is similar to that of the transmission grating. When a sample is brought near the transmission-grating surface, it can be excited by the optical interference pattern. The refractive index of the sample changes according to the intensity profile of the pump light and the induced refractive index profile functions as a different type of transiently generated grating. When the probe beam is incident in a manner similar to that of the pump beam, it is diffracted both by the transmission grating (called a reference light) and the transiently generated grating (called a signal light). In principle, the two diffractions progress along the same direction; therefore, these two diffractions interfere, which is

detected by a detector positioned at a visible diffraction spot of the reference beam.

**2.3 Power dependence of carrier dynamics in CdSe QDs adsorbed onto TiO2**

the TG measurement. The probe pulse wavelength was 775 nm.

TiO2, i. e., Ne,TiO2) , as follows (Guijarro et al., 2010a, 2020b):

 

**electrodes** 

*n t*

In the improved TG technique used for studying the ultrafast carrier dynaimcs of semiconductor QDs, the laser source was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The light was separated into two parts. One of them was used as a probe pulse. The other was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm used as pump light in

As described in depth in previous papers (Katayama et al., 2003; Shen et al., 2005, 2007), the TG signal is directly proportional to the change in the refractive index occurring in the sample (n(t)) upon photoexcitation. In the timescale of these experiments less than 1 ns, assuming Drude's model, the refractive index change, i.e., the TG signal intensity for CdSe QD adsorbed samples, will be a linear function of the concentration of photogenerated carriers (electrons and holes in CdSe QDs, i.e., Ne,CdSe and Nh,CdSe, and injected electrons in

<sup>2</sup>

where *<sup>p</sup>* is the radial probe frequency, *e* is the elementary charge, 0 is free-space permittivity, and n0,CdSe (2.7) and n0,TiO2 (2.5) are the refractive indices of CdSe and TiO2, respectively. The important feature of the TG signal is that both the photoexcited electron

*nnn mmm*

111 222

, , ,

*e CdSe h CdSe e TiO CdSe e CdSe p CdSe h CdSe p TiO e TiO p*

*N te N te N te*

0, , 0 0, , 0 0, , 0

2 2 2

222

 

2 2

  (1)

Aqueous solutions were employed in all cases. A 0.5 M (CH3COO)2Cd (98.0%, Sigma-Aldrich) solution was used as the cadmium source, while a sodium selenosulfate (Na2SeSO3) solution was used as the selenium precursor. The sodium selenosulfate solution was prepared by heating under reflux for 1h a mixture of 1.6 g of Se powder, 40 mL of 1 M Na2SO3 (98.0%, Alfa Aesar) and 10 mL of 1 M NaOH. The resulting solution was filtered and mixed with 40 mL of 1M CH3COONa (99.0+%, Fluka) solution, and finally was stored in the dark. The pH of the sodium selenosulfate solution was optimized to improve the QD deposition rate by using 0.25 M H2SO4 and/or 0.1 M NaOH stock solutions.


After the CdSe QD adsorption, the samples were coated with ZnS by alternately dipping them three times in 0.1 M Zn(CH2COO) and 0.1 M Na2S aqueous solutions for 1 minute for each dip (Yang et al., 2003; Diguna, et al., 2007a; Shen et al., 2008).

### **2.2 An improved transient grating (TG) technique**

The transient grating (TG) method is a well-established laser spectroscopic technique of four wave mixing (Eichler et al., 1986; Harata et al., 1999). In the TG method, two time-coincident short laser pulses (pump beams) with the same wavelength and intensity intersect with an angle in a sample to generate an optical interference pattern at the intersection. Interaction between the light field and the material results in a spatially periodic modulation of the complex refractive index, which works like a transient diffraction grating for a third laser pulse (probe beam) incident to the photoexcited region. Then, by measuring the time dependence of the diffraction light of the probe beam, dynamics of the transient grating produced in the sample can be monitored. The TG technique is a powerful tool for detecting population dynamics (Rajesh et al., 2002), thermal diffusion (Glorieux et al., 2002), diffusion of photoexcited species (Terazima et al., 2000), energy transfer from photoexcited species to liquids (Miyata et al, 2002), structural or orientational relaxation (Glorieux et al., 2002), the sound velocity of liquids (Ohmor et al., 2001), and so on. Although this technique provides valuable information, it presents some technical difficulties for general researchers (Harata et al., 1999). First, the three beams must overlap on a small spot, typically within a spot diameter of less than 100 μm on a sample, and each beam must be temporally controlled. This is very difficult for pulsed laser beams, especially for those with a pulse width of ~100 femtoseconds. Secondly, since the diffraction of the probe beam, namely the signal, is quite weak, it is difficult to find the diffraction beam during the measurements. Thirdly, for a solid sample, the surface must be optically smooth. It is almost impossible to measure a sample with a rough surface by using the conventional TG technique.

3. Direct adsorption (DA) of previously synthesized QDs (Guijarro et al., 2010b)

using soaking times ranging from 1 h to 1 week.

each dip (Yang et al., 2003; Diguna, et al., 2007a; Shen et al., 2008).

sample with a rough surface by using the conventional TG technique.

been reported (Guijarro et al., 2009).

**2.2 An improved transient grating (TG) technique** 

Aqueous solutions were employed in all cases. A 0.5 M (CH3COO)2Cd (98.0%, Sigma-Aldrich) solution was used as the cadmium source, while a sodium selenosulfate (Na2SeSO3) solution was used as the selenium precursor. The sodium selenosulfate solution was prepared by heating under reflux for 1h a mixture of 1.6 g of Se powder, 40 mL of 1 M Na2SO3 (98.0%, Alfa Aesar) and 10 mL of 1 M NaOH. The resulting solution was filtered and mixed with 40 mL of 1M CH3COONa (99.0+%, Fluka) solution, and finally was stored in the dark. The pH of the sodium selenosulfate solution was optimized to improve the QD deposition rate by using 0.25 M H2SO4 and/or 0.1 M NaOH stock solutions.

Colloidal dispersions of CdSe QDs capped with trioctylphosphine (TOP) were prepared by a solvothermal route which permits size control. DA of CdSe QDs was achieved by immersion of TiO2 electrodes in a CH2Cl2 (99.6%, Sigma Aldrich) CdSe QD dispersion,

4. Linker assisted adsorption (LA) of previously synthesized QDs (Guijarro et al., 2010b) LA was performed employing p-mercaptobenzoic acid (MBA; 90%, Aldrich), cysteine (97%, Aldrich), and mercaptopropionic acid (MPA; 99%, Aldrich) as molecular wires. First, the linker was anchored to the TiO2 surface by immersion in saturated toluene solutions of cysteine (5 mM) or MBA (10 mM) for 24 h. Secondly, these electrodes were washed with pure toluene for ½ h to remove the excess of the linker. Finally, the modified electrodes were transferred to a toluene CdSe QD dispersion for 3 days, to ensure QD saturation. The procedure for modification of TiO2 with MPA has previously

After the CdSe QD adsorption, the samples were coated with ZnS by alternately dipping them three times in 0.1 M Zn(CH2COO) and 0.1 M Na2S aqueous solutions for 1 minute for

The transient grating (TG) method is a well-established laser spectroscopic technique of four wave mixing (Eichler et al., 1986; Harata et al., 1999). In the TG method, two time-coincident short laser pulses (pump beams) with the same wavelength and intensity intersect with an angle in a sample to generate an optical interference pattern at the intersection. Interaction between the light field and the material results in a spatially periodic modulation of the complex refractive index, which works like a transient diffraction grating for a third laser pulse (probe beam) incident to the photoexcited region. Then, by measuring the time dependence of the diffraction light of the probe beam, dynamics of the transient grating produced in the sample can be monitored. The TG technique is a powerful tool for detecting population dynamics (Rajesh et al., 2002), thermal diffusion (Glorieux et al., 2002), diffusion of photoexcited species (Terazima et al., 2000), energy transfer from photoexcited species to liquids (Miyata et al, 2002), structural or orientational relaxation (Glorieux et al., 2002), the sound velocity of liquids (Ohmor et al., 2001), and so on. Although this technique provides valuable information, it presents some technical difficulties for general researchers (Harata et al., 1999). First, the three beams must overlap on a small spot, typically within a spot diameter of less than 100 μm on a sample, and each beam must be temporally controlled. This is very difficult for pulsed laser beams, especially for those with a pulse width of ~100 femtoseconds. Secondly, since the diffraction of the probe beam, namely the signal, is quite weak, it is difficult to find the diffraction beam during the measurements. Thirdly, for a solid sample, the surface must be optically smooth. It is almost impossible to measure a In 2003, Katayama and co-workers (Katayama et al., 2003; Yamaguchi et al., 2003) proposed an improved TG technique (it was also called a lens-free heterodyne TG (LF-HD-TG) or a near field heterodyne TG (NF-HD-TG) technique in some papers), which overcomes the difficulties that exist in the conventional TG technique. The improved TG technique features (1) simple and compact optical equipment and easy optical alignment and (2) high stability of phase due to the short optical path length of the probe and reference beams. This method is thought to be versatile with applicability to many kinds of sample states, namely opaque solids, scattering solids with rough surfaces, transparent solids, and liquids, because it is applicable to transmission and reflection-type measurements. The principle of the improved TG technique has been explained in detail in the previous papers (Katayama et al., 2003; Yamaguchi et al., 2003) and is only described briefly here. Unlike the conventional TG technique, only one pump beam and one probe beam without focusing are needed in the improved TG technique. The pump beam is incident on the transmission grating. Then, the spatial intensity profile of the pump beam is known to have an interference pattern in the vicinity of the other side of the transmission grating, and the interference pattern has a grating spacing that is similar to that of the transmission grating. When a sample is brought near the transmission-grating surface, it can be excited by the optical interference pattern. The refractive index of the sample changes according to the intensity profile of the pump light and the induced refractive index profile functions as a different type of transiently generated grating. When the probe beam is incident in a manner similar to that of the pump beam, it is diffracted both by the transmission grating (called a reference light) and the transiently generated grating (called a signal light). In principle, the two diffractions progress along the same direction; therefore, these two diffractions interfere, which is

detected by a detector positioned at a visible diffraction spot of the reference beam. In the improved TG technique used for studying the ultrafast carrier dynaimcs of semiconductor QDs, the laser source was a titanium/sapphire laser (CPA-2010, Clark-MXR Inc.) with a wavelength of 775 nm, a repetition rate of 1 kHz, and a pulse width of 150 fs. The light was separated into two parts. One of them was used as a probe pulse. The other was used to pump an optical parametric amplifier (OPA) (a TOAPS from Quantronix) to generate light pulses with a wavelength tunable from 290 nm to 3 µm used as pump light in the TG measurement. The probe pulse wavelength was 775 nm.

### **2.3 Power dependence of carrier dynamics in CdSe QDs adsorbed onto TiO2 electrodes**

As described in depth in previous papers (Katayama et al., 2003; Shen et al., 2005, 2007), the TG signal is directly proportional to the change in the refractive index occurring in the sample (n(t)) upon photoexcitation. In the timescale of these experiments less than 1 ns, assuming Drude's model, the refractive index change, i.e., the TG signal intensity for CdSe QD adsorbed samples, will be a linear function of the concentration of photogenerated carriers (electrons and holes in CdSe QDs, i.e., Ne,CdSe and Nh,CdSe, and injected electrons in TiO2, i. e., Ne,TiO2) , as follows (Guijarro et al., 2010a, 2020b):

$$\Delta n(t) = \frac{1}{2n\_{0, \text{CdSe}}} \left( \frac{-N\_{e, \text{CdSe}}(t) \varepsilon^2}{m\_{e, \text{CdSe}} \alpha\_p^2 \varepsilon\_0} \right) + \frac{1}{2n\_{0, \text{CdSe}}} \left( \frac{-N\_{h, \text{CdSe}}(t) \varepsilon^2}{m\_{h, \text{CdSe}} \alpha\_p^2 \varepsilon\_0} \right) + \frac{1}{2n\_{0, \text{TiO}\_2}} \left( \frac{-N\_{e, \text{TiO}\_2}(t) \varepsilon^2}{m\_{e, \text{TiO}\_2} \alpha\_p^2 \varepsilon\_0} \right) \tag{1}$$

where *<sup>p</sup>* is the radial probe frequency, *e* is the elementary charge, 0 is free-space permittivity, and n0,CdSe (2.7) and n0,TiO2 (2.5) are the refractive indices of CdSe and TiO2, respectively. The important feature of the TG signal is that both the photoexcited electron

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 293

process existed or not under our experimental conditions. For this purpose, we measured the dependence of the TG kinetics on the pump intensity (from 2.5 to 16.5 μJ/pulse) for CdSe QD adosrobed TiO2 electrodes (Figure 1) (Shen et al., 2010a). As shown in Fig. 1, we found that the dependence of the maximum signal intensity on the pump intensity was linear, and that the waveforms of the responses overlapped each other very well when they were normalized at the peak intensity. These results mean that the decay processes measured in the TG kinetics were independent of the pump intensity under our experimental conditions, and many-body recombination processes could be neglected. Therefore, it is reasonable to assume that the decay processes of photoexcited electrons and holes in the CdSe QDs are due to one-body recombination processes such as trapping and

**2.4 Separation of the ultrafast electron and hole dynamics in the CdSe QDs adsorbed** 

Figure 2 shows a typical kinetic trace of the TG signal of CdSe QDs adsorbed onto a TiO2 nanostructured film (prepared by CBD method for 24 h adsorption) measured in air. The vertical axis was plotted on a logarithmic scale. Three decay processes (indicated as A, B, and C in Fig. 2) can be clearly observed. We found that the TG kinetics shown in Fig. 2 could be fitted very well with a double exponential decay plus an offset, as shown in eq. (2):

> / / 1 2 1 20 *t t y Ae Ae y*

where A1, A2 and y0 are constants, and τ1 and τ2 are the time constants of the two decay processes (A and B in Fig. 2). Here, the constant term y0 corresponds to the slowest decay process (C in Fig. 2), in which the decay time (in the order of ns) is much larger than the time scale of 100 ps measured in this study. The time constants of the fast (τ1) and slow (τ2) decay processes of photoexcited carriers in air are 6.3 ps and 82 ps, respectively (Table 1). As mentioned above, τ1 and τ2 are independent of the pump intensity under our experimental conditions, so the three decay processes are mostly due to one-body recombination such as

Fig. 2. TG kinetics of CdSe QDs adsorbed onto a nanostructured TiO2 film measured in air. The vertical axis is plotted on a logarithmic scale. Three decay processes A, B and C can be

(2)

transfer under our experimental conditions.

**onto TiO2 nanostructured electrodes** 

carrier trapping and carrier transfer.

clearly observed (Shen et al., 2010a).

and hole carrier densities contribute to the signal. In principle, the exact contribution on Δn(t) by each carrier depends inversely on its carrier effective mass. According to the Drude theory (Kashiski et al., 1989; Kim et al., 2009), it can be considered that only free photoexcited electrons and holes are responsible for the population grating signals. For CdSe, the effective masses of electrons and holes are 0.13*m0* and 0.44*m0* (*m0* is the electron rest mass), respectively (Bawendi et al., 1989), so both the photoexcited electron and hole carrier densities in the CdSe QDs contribute to the signal. It is known that the effective mass of electrons for TiO2 is about 30 *m0*, which is about two orders larger than that for CdSe. Therefore, the TG signal due to the injected electrons in TiO2 (no holes injected into TiO2) can be ignored (Shen et al., 2005, 2006a, 2007, 2008a, 2010a).

Fig. 1. Dependence of the TG kinetics of CdSe QDs adsorbed onto nanostructured TiO2 films (CBD method and the CdSe adsorption time was 24 h) on the pump light intensity (a); Dependence of the TG peak intensity on the pump light intensity (b); Normalized TG kinetics measured with different pump light intensities (c) (Shen et al., 2010a).

For a semiconductor material, usually, there are three kinds of photoexcited carrier relaxation dynamics. The first one is a one-body recombination, which is trapping and/or transfer of photoexcited electrons and holes. In this case, the lifetimes of the photoexcited electrons and holes are independent of the pump light intensity. The second one is a twobody recombination, which is a radiative recombination via electron and hole pairs. The third one is a three-body recombination, which is an Auger recombination via two electrons and one hole, or via one electron and two holes. In the latter two cases, the lifetimes of photoexcited electrons and holes are dependent on the pump light intensity. In order to determine what kinds of photoexcited carrier dynamics are reflected in the TG kinetics, we first confirmed many-body recombination processes such as the Auger recombination

and hole carrier densities contribute to the signal. In principle, the exact contribution on Δn(t) by each carrier depends inversely on its carrier effective mass. According to the Drude theory (Kashiski et al., 1989; Kim et al., 2009), it can be considered that only free photoexcited electrons and holes are responsible for the population grating signals. For CdSe, the effective masses of electrons and holes are 0.13*m0* and 0.44*m0* (*m0* is the electron rest mass), respectively (Bawendi et al., 1989), so both the photoexcited electron and hole carrier densities in the CdSe QDs contribute to the signal. It is known that the effective mass of electrons for TiO2 is about 30 *m0*, which is about two orders larger than that for CdSe. Therefore, the TG signal due to the injected electrons in TiO2 (no holes injected into TiO2)

Fig. 1. Dependence of the TG kinetics of CdSe QDs adsorbed onto nanostructured TiO2 films (CBD method and the CdSe adsorption time was 24 h) on the pump light intensity (a); Dependence of the TG peak intensity on the pump light intensity (b); Normalized TG kinetics measured with different pump light intensities (c) (Shen et al., 2010a).

<sup>0</sup> <sup>20</sup> <sup>40</sup> <sup>60</sup> <sup>80</sup> -0.2

Time (ps)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

**(b)**

Signal

intensity (arb. units)

0 10 20 30 40 50 60 70

(c)

Relative pump intensity (arb. units)

For a semiconductor material, usually, there are three kinds of photoexcited carrier relaxation dynamics. The first one is a one-body recombination, which is trapping and/or transfer of photoexcited electrons and holes. In this case, the lifetimes of the photoexcited electrons and holes are independent of the pump light intensity. The second one is a twobody recombination, which is a radiative recombination via electron and hole pairs. The third one is a three-body recombination, which is an Auger recombination via two electrons and one hole, or via one electron and two holes. In the latter two cases, the lifetimes of photoexcited electrons and holes are dependent on the pump light intensity. In order to determine what kinds of photoexcited carrier dynamics are reflected in the TG kinetics, we first confirmed many-body recombination processes such as the Auger recombination

can be ignored (Shen et al., 2005, 2006a, 2007, 2008a, 2010a).

Pump intensity

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Normalized T

G signal intensity

**(a)**

0 20 40 60 80 100

Time (ps)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Signal

intensity (arb. units)

process existed or not under our experimental conditions. For this purpose, we measured the dependence of the TG kinetics on the pump intensity (from 2.5 to 16.5 μJ/pulse) for CdSe QD adosrobed TiO2 electrodes (Figure 1) (Shen et al., 2010a). As shown in Fig. 1, we found that the dependence of the maximum signal intensity on the pump intensity was linear, and that the waveforms of the responses overlapped each other very well when they were normalized at the peak intensity. These results mean that the decay processes measured in the TG kinetics were independent of the pump intensity under our experimental conditions, and many-body recombination processes could be neglected. Therefore, it is reasonable to assume that the decay processes of photoexcited electrons and holes in the CdSe QDs are due to one-body recombination processes such as trapping and transfer under our experimental conditions.

### **2.4 Separation of the ultrafast electron and hole dynamics in the CdSe QDs adsorbed onto TiO2 nanostructured electrodes**

Figure 2 shows a typical kinetic trace of the TG signal of CdSe QDs adsorbed onto a TiO2 nanostructured film (prepared by CBD method for 24 h adsorption) measured in air. The vertical axis was plotted on a logarithmic scale. Three decay processes (indicated as A, B, and C in Fig. 2) can be clearly observed. We found that the TG kinetics shown in Fig. 2 could be fitted very well with a double exponential decay plus an offset, as shown in eq. (2):

$$y = A\_1 e^{-t/\tau\_1} + A\_2 e^{-t/\tau\_2} + y\_0 \tag{2}$$

where A1, A2 and y0 are constants, and τ1 and τ2 are the time constants of the two decay processes (A and B in Fig. 2). Here, the constant term y0 corresponds to the slowest decay process (C in Fig. 2), in which the decay time (in the order of ns) is much larger than the time scale of 100 ps measured in this study. The time constants of the fast (τ1) and slow (τ2) decay processes of photoexcited carriers in air are 6.3 ps and 82 ps, respectively (Table 1). As mentioned above, τ1 and τ2 are independent of the pump intensity under our experimental conditions, so the three decay processes are mostly due to one-body recombination such as carrier trapping and carrier transfer.

Fig. 2. TG kinetics of CdSe QDs adsorbed onto a nanostructured TiO2 film measured in air. The vertical axis is plotted on a logarithmic scale. Three decay processes A, B and C can be clearly observed (Shen et al., 2010a).

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 295

response measured in the Na2S solution. Such a faster decay process with a characteristic time of a few picoseconds in the TG response measured in the Na2S solution was considered to correspond to electron transfer from the QDs in direct contact with the TiO2 (first layer of deposited QDs) (Guijarro et al., 2010a, 2010b). It is worth noting that the relative intensity A1 (0.07) measured in the Na2S solution is much smaller compared to the A1 (0.39) measured in air and it could be ignored here. The slower relaxation process in the TG response was not influenced by the presence of the Na2S solution, as shown in Fig. 3. The decay time τ2 (85 ps) and the relative intensity A2 (0.31) for the slower decay process in the TG response measured in the Na2S solution are almost the same as those measured in air (Table 1). The slower electron relaxation mostly corresponds to electron transfer from the CdSe QDs to TiO2 and trapping at the QD surface states, in which the decay time depends to a great extent on the size of the QDs and the adsorption method that is used (Guijarro et al., 2010a, 2010b; Shen et al., 2006, 2007; Diguna et al., 2007b). The slowest decay process (with a time scale of ns) may mostly result from the non-radiative recombination of photoexcited electrons with defects that exist at the CdSe QD surfaces and at the CdSe/CdSe interfaces. The difference between the two TG responses measured in air and in Na2S solution (normalized for the longer time), which was termed the "difference response", is believed to correspond to the photoexcited hole dynamics in the CdSe QDs measured in air. As shown in Fig. 3, the difference response decays very fast and disappears around 10 ps and can be fitted very well with a one-exponential decay function with a decay time of 5 ps (Table 1). Thus, we did well in separating the dynamics of photoexcited electrons and holes in the CdSe QDs and found that the hole dynamics were much faster than those of electrons. Some papers have also reported that the hole relaxation time is much faster than the electron relaxation time in CdS and CdSe QDs (Underwood et al., 2001; Braun et al., 2002). In air, the fast hole decay process with a time scale of about 5 ps can be considered as the trapping of holes by the CdSe QD surface states. This result is in good agreement with the experimental results obtained by a

femtosecond fluorescence "up-conversion" technique (Underwood et al., 2001).

are time constants; A1, A2 and y0 are constants (Shen et al., 2010a).

**2.5 Electron injection from CdSe QDs to TiO2 nanostructured electrode** 

ions to a few ps in air via hole trapping.

Thus, by comparing the TG responses measured in air and in a Na2S solution (hole acceptor), we succeeded in separating the dynamic characteristics of photoexcited electrons and holes in the CdSe QDs. We found that charge separation in the CdSe QDs occurred over a very fast time scale from a few hundreds of fs in the Na2S solution via hole transfer to S2-

TG kinetics A1 τ1 (ps) A2 τ2 (ps) y0

In air 0.39± 0.01 6.3±0.4 0.29 ±0.01 82 ±7 0.27± 0.01 In Na2S 0.07± 0.01 9 ±1 0.31± 0.01 85 ±1 0.25± 0.01 Difference 0.33± 0.01 5.0± 0.3 - - - Table 1. Fitting results of the TG responses of CdSe QDs adsorbed onto nanostructured TiO2 films measured in air and in Na2S solution (hole acceptor) as well as their "difference response" as shown in Fig. 3 with a double exponential decay equation (eq. (2)). τ1 and τ<sup>2</sup>

To investigate the electron transfer rate from CdSe QDs to TiO2 electrodes, Shen and coworkers measured the TG kinetics of CdSe QDs adsorbed on both nanostructured TiO2 electrodes and glass substrates under the same deposition conditions (Shen et al., 2008).

In order to separate the photoexcited electron dynamics and hole dynamics that make up the TG kinetics, the TG kinetics of the same sample was measured both in air and in a Na2S aqueous solution (hole acceptor) (1 M) (Shen et al., 2010a). As shown in Fig. 3, a large difference can be clearly observed between the TG responses measured in air and in the Na2S solution. By normalizing the two TG responses at the signal intensity of 90 ps, we found that they overlapped with each other very well for time periods of longer than 15 ps, but the fast decay process apparently disappeared when the time period was less than 10 ps in the TG kinetics measured in the Na2S solution (hole acceptor). This great difference can be explained as follows. In air, both hole and electron dynamics in the CdSe QDs could be measured in the TG kinetics. In the Na2S solution, however, photoexcited holes in the CdSe QDs will transfer quickly to the electrolyte and only electron dynamics should be measured in the TG kinetics. Therefore, the "apparent disappearance" of the fast decay process in the Na2S solution implies that the hole transfer to S2- ions, which are supposed to be strongly adsorbed onto the CdSe QD surface, can be too fast in these circumstances as indicated by Hodes (Hodes, 2008) and therefore could not be observed under the temporal resolution (about 300 fs) of our TG technique. This observation is particularly important, because the result directly demonstrated that the transfer of holes to sulfur hole acceptors that are strongly adsorbed on the QD surface could approach a few hundreds of fs. An earlier study on the dynamics of photogenerated electron-hole pair separation in surface-space-charge fields at GaAs(100) crystal/oxide interfaces using a reflective electro-optic sampling method

Fig. 3. TG responses of the CdSe QDs adsorbed onto nanostructured TiO2 films measured in air (-) and in Na2S solution (□) as well as their "difference response" (○) (Shen et al., 2010a).

showed that the hole carrier transit time was faster than 500 fs (Min et al., 1990). We believe an ultrafast hole transfer time from the QDs to hole acceptors that are strongly adsorbed on the QD surface is a more feasible and reasonable explanation, since photoexcited holes can more easily reach the surface of QDs with diameters of a few nm. The TG response measured in the Na2S solution, which is considered to only relate to electron dynamics as mentioned above, can be fitted well with eq. (2). As shown in Table 1, besides the slower decay process, a faster decay process with a decay time of 9 ps was also detected in the TG

In order to separate the photoexcited electron dynamics and hole dynamics that make up the TG kinetics, the TG kinetics of the same sample was measured both in air and in a Na2S aqueous solution (hole acceptor) (1 M) (Shen et al., 2010a). As shown in Fig. 3, a large difference can be clearly observed between the TG responses measured in air and in the Na2S solution. By normalizing the two TG responses at the signal intensity of 90 ps, we found that they overlapped with each other very well for time periods of longer than 15 ps, but the fast decay process apparently disappeared when the time period was less than 10 ps in the TG kinetics measured in the Na2S solution (hole acceptor). This great difference can be explained as follows. In air, both hole and electron dynamics in the CdSe QDs could be measured in the TG kinetics. In the Na2S solution, however, photoexcited holes in the CdSe QDs will transfer quickly to the electrolyte and only electron dynamics should be measured in the TG kinetics. Therefore, the "apparent disappearance" of the fast decay process in the Na2S solution implies that the hole transfer to S2- ions, which are supposed to be strongly adsorbed onto the CdSe QD surface, can be too fast in these circumstances as indicated by Hodes (Hodes, 2008) and therefore could not be observed under the temporal resolution (about 300 fs) of our TG technique. This observation is particularly important, because the result directly demonstrated that the transfer of holes to sulfur hole acceptors that are strongly adsorbed on the QD surface could approach a few hundreds of fs. An earlier study on the dynamics of photogenerated electron-hole pair separation in surface-space-charge fields at GaAs(100) crystal/oxide interfaces using a reflective electro-optic sampling method

Fig. 3. TG responses of the CdSe QDs adsorbed onto nanostructured TiO2 films measured in air (-) and in Na2S solution (□) as well as their "difference response" (○) (Shen et al., 2010a). showed that the hole carrier transit time was faster than 500 fs (Min et al., 1990). We believe an ultrafast hole transfer time from the QDs to hole acceptors that are strongly adsorbed on the QD surface is a more feasible and reasonable explanation, since photoexcited holes can more easily reach the surface of QDs with diameters of a few nm. The TG response measured in the Na2S solution, which is considered to only relate to electron dynamics as mentioned above, can be fitted well with eq. (2). As shown in Table 1, besides the slower decay process, a faster decay process with a decay time of 9 ps was also detected in the TG

Time (ps)

 in air in Na2

S Difference Fitting results


0.0

N

o

r

m

aliz

e

d

T

G

sig

n

al inte

nsity

0.2

0.4

0.6

0.8

1.0

response measured in the Na2S solution. Such a faster decay process with a characteristic time of a few picoseconds in the TG response measured in the Na2S solution was considered to correspond to electron transfer from the QDs in direct contact with the TiO2 (first layer of deposited QDs) (Guijarro et al., 2010a, 2010b). It is worth noting that the relative intensity A1 (0.07) measured in the Na2S solution is much smaller compared to the A1 (0.39) measured in air and it could be ignored here. The slower relaxation process in the TG response was not influenced by the presence of the Na2S solution, as shown in Fig. 3. The decay time τ2 (85 ps) and the relative intensity A2 (0.31) for the slower decay process in the TG response measured in the Na2S solution are almost the same as those measured in air (Table 1). The slower electron relaxation mostly corresponds to electron transfer from the CdSe QDs to TiO2 and trapping at the QD surface states, in which the decay time depends to a great extent on the size of the QDs and the adsorption method that is used (Guijarro et al., 2010a, 2010b; Shen et al., 2006, 2007; Diguna et al., 2007b). The slowest decay process (with a time scale of ns) may mostly result from the non-radiative recombination of photoexcited electrons with defects that exist at the CdSe QD surfaces and at the CdSe/CdSe interfaces.

The difference between the two TG responses measured in air and in Na2S solution (normalized for the longer time), which was termed the "difference response", is believed to correspond to the photoexcited hole dynamics in the CdSe QDs measured in air. As shown in Fig. 3, the difference response decays very fast and disappears around 10 ps and can be fitted very well with a one-exponential decay function with a decay time of 5 ps (Table 1). Thus, we did well in separating the dynamics of photoexcited electrons and holes in the CdSe QDs and found that the hole dynamics were much faster than those of electrons. Some papers have also reported that the hole relaxation time is much faster than the electron relaxation time in CdS and CdSe QDs (Underwood et al., 2001; Braun et al., 2002). In air, the fast hole decay process with a time scale of about 5 ps can be considered as the trapping of holes by the CdSe QD surface states. This result is in good agreement with the experimental results obtained by a femtosecond fluorescence "up-conversion" technique (Underwood et al., 2001).

Thus, by comparing the TG responses measured in air and in a Na2S solution (hole acceptor), we succeeded in separating the dynamic characteristics of photoexcited electrons and holes in the CdSe QDs. We found that charge separation in the CdSe QDs occurred over a very fast time scale from a few hundreds of fs in the Na2S solution via hole transfer to S2 ions to a few ps in air via hole trapping.


Table 1. Fitting results of the TG responses of CdSe QDs adsorbed onto nanostructured TiO2 films measured in air and in Na2S solution (hole acceptor) as well as their "difference response" as shown in Fig. 3 with a double exponential decay equation (eq. (2)). τ1 and τ<sup>2</sup> are time constants; A1, A2 and y0 are constants (Shen et al., 2010a).

### **2.5 Electron injection from CdSe QDs to TiO2 nanostructured electrode**

To investigate the electron transfer rate from CdSe QDs to TiO2 electrodes, Shen and coworkers measured the TG kinetics of CdSe QDs adsorbed on both nanostructured TiO2 electrodes and glass substrates under the same deposition conditions (Shen et al., 2008).

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 297

This result is in good agreement with the experimental results obtained with a femtosecond fluorescence up-conversion technique and transient absorption measurements (Underwood et al., 2001). The slow decay process was considered to reflect the photoexcited electron relaxation processes. For the glass substrate, no electron transfer from the QDs to the glass substrate could occur. Thus, the slow decay process only corresponded to the relaxation of photoexcited electrons in the CdSe QDs, which was mostly due to trapping at the CdSe QD surfaces. For the nanostructured TiO2 substrate, the CdSe QDs were adsorbed onto the surfaces of the TiO2 nanoparticles and electron transfer from the CdSe QDs to the TiO2 nanoparticles could occur, which had been confirmed by the photocurrent measurements (Shen et al., 2004a, 2004b). As mentioned above, the TG signal due to the change in the refractive index *n t*( ) of the TiO2 nanoparticles resulted from the injected free electrons from the CdSe QDs would be very small and can be ignored. Therefore, the slow decay process of the CdSe QDs adsorbed on the nanostructrued TiO2 film mostly reflected the decrease of the photoexcited electron densities in the CdSe QDs due to both electron trapping at the CdSe QD surfaces/interfaces and electron transfer from the CdSe QDs to the TiO2 nanoparticles. This is the reason why the τ2 obtained for the CdSe QDs adsorbed on the nanostructured TiO2 electrode was much smaller than that obtained for the CdSe QDs adsorbed on the glass substrate. For the CdSe QDs on the nanostructured TiO2 electrode, the

where *kr* is the intrinsic decay rate (mostly trapping) in CdSe QDs and *ket* is the electrontransfer rate from CdSe QDs to TiO2. The intrinsic decay time of electrons in the CdSe QDs adsorbed on the nanostructured TiO2 electrode was assumed to be the same as that in the CdSe QDs adsorbed on the glass substrate. In this way, we can estimate the electron transfer rate from the CdSe QDs into the nanostructured TiO2 electrode to be approximately 5.6 x 10 9 s-1 using the values of τ2 obtained from the CdSe QDs adsorbed on these two kinds of substrates. This result is very close to the electron transfer rates from CdSe QDs to TiO2 substrates measured recently using a femtosecond transient absorption (TA) technique by

**2.6 Changes of carrier dynamics in CdSe QDs adsorbed onto TiO2 nanostructured** 

The photoexcited carrier dynamics of CdSe QDs, including the electron injection rates to the TiO2 electrodes, depends greatly on the size of the CdSe QDs, the adsorption method (mode of attachment), the adsorption conditons such as adsorption time, and the properties of the TiO2 electrodes such as crystal structure. In the following, two examples will be

The first example is the carrier dynamics of CdSe QDs adsorbed onto TiO2 electrodes with SILAR method (Guijarro et al., 2010a). Figure 5 shows the normalized TG kinetics of CdSe adsorbed TiO2 electrodes with different SILAR cycles and an example of the fitting result of the TG kinetics with eq. (3) to determine the parameters A1, A2, τ1 and τ2 shown in Fig. 5. Figure 6 shows the dependence of τ1 and τ2 on the number of SILAR cycles. As shown in Fig. 6, τ1 increases from 2.8 to 6.3 ps and τ2 increases from 83 to 320 ps upon increasing the number of SILAR cycles from 2 to 9-15. With the SILAR cycle increasing, both the average QD size and the number of QDs increase. Therefore, for a low number of cycles, all the CdSe QDs would be in direct contact with the TiO2 electrode, favouring electron injection.

<sup>2</sup> *r et k kk* (4)

decay rate *k2* (*k2* = 1/τ2) can be expressed as

Kamat and co-workers (Robel et al., 2007).

**electrodes versus adsorption conditions** 

introduced.

Figure 4 shows the TG kintics (pump beam wavelength: 388 nm) of CdSe QDs adsorbed on a TiO2 nanostructured electrode and on a glass substrate (CBD method). The average diameter of the CdSe QDs was about 5.5 nm for both kinds of substrates. The TG kinetics were fitted to a double exponential decay function (Eq. (3)) using a least-squares fitting method, convoluting with a 1ps Gaussian representing the laser pulse.

$$y = A\_1 e^{-t/\tau\_1} + A\_2 e^{-t/\tau\_2} \tag{3}$$

where *A*1 and *A*2 are constants, and τ1 and τ2 are the time constants of the two decay processes, respectively.

The fast decay time constants τ1 were obtained to be 2.3 ps for both the two kinds of substrates. The slow decay time constants τ2 were obtained to be approximately 140 ps and 570 ps for the CdSe QDs adsorbed on the TiO2 electrode and on the glass substrate, respectively. It was found that τ1 was the same for both kinds of substrates. However, the value of τ2 for the CdSe QDs adsorbed on the nanostructured TiO2 electrode was much smaller than that on the glass substrate. Under the experimental conditions (pump intensity < 2 μJ/pulse in this case), the waveforms of the responses overlapped each other very well when they were normalized at the peak intensity, as mentioned above. Therefore, it is reasonable to assume that the two decay processes in the CdSe QDs adsorbed on these two kinds of electrodes were due to one-body recombination, such as trapping, decay into intrinsic states and/or a transfer process. Because the fast decay time constant is independent of the substrates and it is known that hole trapping time is much faster than that of electron relaxation (Underwood et al., 2001; Braun et al., 2002), we believe that the fast decay process with a time scale of 2-3 ps mainly corresponded to a decrease in the photoexcited hole carrier density due to trapping at the CdSe QD surface/interface states or relaxation into the intrinsic QD states, as mentioned earlier.

Fig. 4. TG kinetics of the CdSe QDs (average diameter: 5.5 nm) adsorbed on a nanostructured TiO2 electrode and on a glass substrate under same adsorption conditions (Shen et al., 2008).

Figure 4 shows the TG kintics (pump beam wavelength: 388 nm) of CdSe QDs adsorbed on a TiO2 nanostructured electrode and on a glass substrate (CBD method). The average diameter of the CdSe QDs was about 5.5 nm for both kinds of substrates. The TG kinetics were fitted to a double exponential decay function (Eq. (3)) using a least-squares fitting

where *A*1 and *A*2 are constants, and τ1 and τ2 are the time constants of the two decay

The fast decay time constants τ1 were obtained to be 2.3 ps for both the two kinds of substrates. The slow decay time constants τ2 were obtained to be approximately 140 ps and 570 ps for the CdSe QDs adsorbed on the TiO2 electrode and on the glass substrate, respectively. It was found that τ1 was the same for both kinds of substrates. However, the value of τ2 for the CdSe QDs adsorbed on the nanostructured TiO2 electrode was much smaller than that on the glass substrate. Under the experimental conditions (pump intensity < 2 μJ/pulse in this case), the waveforms of the responses overlapped each other very well when they were normalized at the peak intensity, as mentioned above. Therefore, it is reasonable to assume that the two decay processes in the CdSe QDs adsorbed on these two kinds of electrodes were due to one-body recombination, such as trapping, decay into intrinsic states and/or a transfer process. Because the fast decay time constant is independent of the substrates and it is known that hole trapping time is much faster than that of electron relaxation (Underwood et al., 2001; Braun et al., 2002), we believe that the fast decay process with a time scale of 2-3 ps mainly corresponded to a decrease in the photoexcited hole carrier density due to trapping at the CdSe QD surface/interface states or relaxation into the intrinsic QD states, as mentioned earlier.

/ / 1 2 1 2 *t t y Ae Ae* 

(3)

method, convoluting with a 1ps Gaussian representing the laser pulse.

Fig. 4. TG kinetics of the CdSe QDs (average diameter: 5.5 nm) adsorbed on a

nanostructured TiO2 electrode and on a glass substrate under same adsorption conditions

processes, respectively.

(Shen et al., 2008).

This result is in good agreement with the experimental results obtained with a femtosecond fluorescence up-conversion technique and transient absorption measurements (Underwood et al., 2001). The slow decay process was considered to reflect the photoexcited electron relaxation processes. For the glass substrate, no electron transfer from the QDs to the glass substrate could occur. Thus, the slow decay process only corresponded to the relaxation of photoexcited electrons in the CdSe QDs, which was mostly due to trapping at the CdSe QD surfaces. For the nanostructured TiO2 substrate, the CdSe QDs were adsorbed onto the surfaces of the TiO2 nanoparticles and electron transfer from the CdSe QDs to the TiO2 nanoparticles could occur, which had been confirmed by the photocurrent measurements (Shen et al., 2004a, 2004b). As mentioned above, the TG signal due to the change in the refractive index *n t*( ) of the TiO2 nanoparticles resulted from the injected free electrons from the CdSe QDs would be very small and can be ignored. Therefore, the slow decay process of the CdSe QDs adsorbed on the nanostructrued TiO2 film mostly reflected the decrease of the photoexcited electron densities in the CdSe QDs due to both electron trapping at the CdSe QD surfaces/interfaces and electron transfer from the CdSe QDs to the TiO2 nanoparticles. This is the reason why the τ2 obtained for the CdSe QDs adsorbed on the nanostructured TiO2 electrode was much smaller than that obtained for the CdSe QDs adsorbed on the glass substrate. For the CdSe QDs on the nanostructured TiO2 electrode, the decay rate *k2* (*k2* = 1/τ2) can be expressed as

$$k\_2 = k\_r + k\_{et} \tag{4}$$

where *kr* is the intrinsic decay rate (mostly trapping) in CdSe QDs and *ket* is the electrontransfer rate from CdSe QDs to TiO2. The intrinsic decay time of electrons in the CdSe QDs adsorbed on the nanostructured TiO2 electrode was assumed to be the same as that in the CdSe QDs adsorbed on the glass substrate. In this way, we can estimate the electron transfer rate from the CdSe QDs into the nanostructured TiO2 electrode to be approximately 5.6 x 10 9 s-1 using the values of τ2 obtained from the CdSe QDs adsorbed on these two kinds of substrates. This result is very close to the electron transfer rates from CdSe QDs to TiO2 substrates measured recently using a femtosecond transient absorption (TA) technique by Kamat and co-workers (Robel et al., 2007).

### **2.6 Changes of carrier dynamics in CdSe QDs adsorbed onto TiO2 nanostructured electrodes versus adsorption conditions**

The photoexcited carrier dynamics of CdSe QDs, including the electron injection rates to the TiO2 electrodes, depends greatly on the size of the CdSe QDs, the adsorption method (mode of attachment), the adsorption conditons such as adsorption time, and the properties of the TiO2 electrodes such as crystal structure. In the following, two examples will be introduced.

The first example is the carrier dynamics of CdSe QDs adsorbed onto TiO2 electrodes with SILAR method (Guijarro et al., 2010a). Figure 5 shows the normalized TG kinetics of CdSe adsorbed TiO2 electrodes with different SILAR cycles and an example of the fitting result of the TG kinetics with eq. (3) to determine the parameters A1, A2, τ1 and τ2 shown in Fig. 5. Figure 6 shows the dependence of τ1 and τ2 on the number of SILAR cycles. As shown in Fig. 6, τ1 increases from 2.8 to 6.3 ps and τ2 increases from 83 to 320 ps upon increasing the number of SILAR cycles from 2 to 9-15. With the SILAR cycle increasing, both the average QD size and the number of QDs increase. Therefore, for a low number of cycles, all the CdSe QDs would be in direct contact with the TiO2 electrode, favouring electron injection.

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 299

0.84 to 0.41 while A2 increases from 0.22 to 0.54 as the SILAR cycle increases from 2 to 9-15. For a low number of SILAR cycles, A1 is large, indicating the predominance of electron injection from QDs in direct contact with the TiO2. As the SILAR cycle increases, A2 increases because the contribution of the slower electron transfer through CdSe/CdSe

Fig. 6. Dependence of τ1 (A) and τ2 (B) on the number of SILAR cycles. Dashed lines correspond to linear fittings including all data, whereas the solid line shows a linear fitting forced to pass through the origin without considering the point corresponding to 15 SILAR

0

100

200

ps

B.

300

2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0 6,5

0 2 4 6 8 10 12 14 16

number of cycles

The second example is the carrier dynamics of CdSe QDs adsorbed onto TiO2 electrodes with linker-assisted (LA) method (Guijarro et al., 2010b). Figure 7 shows the TG kinetics for TiO2 nanostrucuted electrodes modified by linker-adsorbed (using cysteine, pmercaptobenzoic acid (MBA) and mercaptopropionic acid (MPA)) CdSe QDs (average size: 3.5 nm), together with the TG kinetics of the CdSe QD solution used for the adsorption. The TG kinetics of the LA adsorption samples were not sensitive to adsorption time. It indicates that the specific interaction between the thiol group (-SH) of the three kinds of linkers and the QD leads to homogeneous adsorption, in which all the QDs are adsorbed directly on the TiO2 surface with no aggregation. As shown in Fig. 7, strong dependence of the decay of the TG kinetics on the linker nature was observed. This result suggests that, not only the length, but also other factors such as the dipole moment, the redox properties or the electronic structure of the linkers may play a role in the carrier dynamics. The TG kinetics of the LA adosprtion samples were fitted very well to eq. (3),

interfaces becomes more and more important.

A.

1 / ps

cycles (Guijarro et al., 2010a).

Fig. 5. Transient grating kinetics of CdSe QD adsorbed TiO2 electrodes with different SILAR cycles (a) and an example of the fitting of the TG kinetics (15 cycles) to a double exponential decay (eq. (3)) (b) (Guijarro et al., 2010a).

However, for a large number of deposition cycles, a large fraction of CdSe QDs would not be in direct contact with the TiO2 electrode, resulting in an increase in electron injection time. As discussed in depth by Guijarro and co-workers (Guijarro et al., 2010a), in this case, the fast decay component was attributed to direct electron transfer from the first layer of deposited CdSe QDs besides the hole trapping, and the slow one to electron injection into the TiO2 from CdSe QDs in the outer layers (and trapping at surface and interfacial QD/QD states). For the fast decay component, the increase in QD average sizes resulted from the increase of SILAR cycles likely plays a key role in determining the values of τ1. The results were in qualitative agreement with those reported by Kamat and co-workers (Robel et al., 2007) showing that electron injection into TiO2 from CdSe QDs becomes faster as the QD size gets smaller. It was considered to be due to an increase in the driving force as the CdSe conduction band shifts toward higher energies. A plot of τ1 versus the number of SILAR cycle numbers (Fig. 6(a)) shows approximately a linear dependence. For the slow decay component, the electron relaxation time τ2 also increases upon increasing the number of SILAR cycles (Fig. 6(b)), which can be explained as follows. The increase in the number of SILAR cycles results in not only an average increase of the CdSe quantum dot size but also an increase in the average distance of the CdSe QDs to the oxide particle. Correspondingly, the concentration of QDs not in direct contact with the TiO2 would also grow. This leads to an increase in the average time needed for transferring the photoexcited electrons from the QDs to the oxide. Therefore, the values of A1 and A2 mainly reflected the relative contribution of the fast electron transfer (from the first QD layer in direct contact with the TiO2) and the slow one (from the QDs in the outer layers), respectively. A1 decreases from

Fig. 5. Transient grating kinetics of CdSe QD adsorbed TiO2 electrodes with different SILAR cycles (a) and an example of the fitting of the TG kinetics (15 cycles) to a double exponential

1.0 Data: SILAR15Norm\_TGNorm

increasing number of cycles

> Model: ExpDecay2 Chi^2 = 0.00055 y0 0 }0 x0 0 }0 A1 0.41003 }0.01051 t1 6.28445 }0.3352 A2 0.53775 }0.00824 t2 314.21743 }22.45739

 2 cy cles 3 cy cles 4 cy cles 5 cy cles 7 cy cles 9 cy cles 15 cycles

0 20 40 60 80 100 120

time / ps

However, for a large number of deposition cycles, a large fraction of CdSe QDs would not be in direct contact with the TiO2 electrode, resulting in an increase in electron injection time. As discussed in depth by Guijarro and co-workers (Guijarro et al., 2010a), in this case, the fast decay component was attributed to direct electron transfer from the first layer of deposited CdSe QDs besides the hole trapping, and the slow one to electron injection into the TiO2 from CdSe QDs in the outer layers (and trapping at surface and interfacial QD/QD states). For the fast decay component, the increase in QD average sizes resulted from the increase of SILAR cycles likely plays a key role in determining the values of τ1. The results were in qualitative agreement with those reported by Kamat and co-workers (Robel et al., 2007) showing that electron injection into TiO2 from CdSe QDs becomes faster as the QD size gets smaller. It was considered to be due to an increase in the driving force as the CdSe conduction band shifts toward higher energies. A plot of τ1 versus the number of SILAR cycle numbers (Fig. 6(a)) shows approximately a linear dependence. For the slow decay component, the electron relaxation time τ2 also increases upon increasing the number of SILAR cycles (Fig. 6(b)), which can be explained as follows. The increase in the number of SILAR cycles results in not only an average increase of the CdSe quantum dot size but also an increase in the average distance of the CdSe QDs to the oxide particle. Correspondingly, the concentration of QDs not in direct contact with the TiO2 would also grow. This leads to an increase in the average time needed for transferring the photoexcited electrons from the QDs to the oxide. Therefore, the values of A1 and A2 mainly reflected the relative contribution of the fast electron transfer (from the first QD layer in direct contact with the TiO2) and the slow one (from the QDs in the outer layers), respectively. A1 decreases from

decay (eq. (3)) (b) (Guijarro et al., 2010a).

0.0 0.2 0.4

0.0

0.2 0.4 0.6 0.8

TG Signal / A.u.

0.6 0.8 1.0

TG Signal / A.u.

0.84 to 0.41 while A2 increases from 0.22 to 0.54 as the SILAR cycle increases from 2 to 9-15. For a low number of SILAR cycles, A1 is large, indicating the predominance of electron injection from QDs in direct contact with the TiO2. As the SILAR cycle increases, A2 increases because the contribution of the slower electron transfer through CdSe/CdSe interfaces becomes more and more important.

Fig. 6. Dependence of τ1 (A) and τ2 (B) on the number of SILAR cycles. Dashed lines correspond to linear fittings including all data, whereas the solid line shows a linear fitting forced to pass through the origin without considering the point corresponding to 15 SILAR cycles (Guijarro et al., 2010a).

The second example is the carrier dynamics of CdSe QDs adsorbed onto TiO2 electrodes with linker-assisted (LA) method (Guijarro et al., 2010b). Figure 7 shows the TG kinetics for TiO2 nanostrucuted electrodes modified by linker-adsorbed (using cysteine, pmercaptobenzoic acid (MBA) and mercaptopropionic acid (MPA)) CdSe QDs (average size: 3.5 nm), together with the TG kinetics of the CdSe QD solution used for the adsorption. The TG kinetics of the LA adsorption samples were not sensitive to adsorption time. It indicates that the specific interaction between the thiol group (-SH) of the three kinds of linkers and the QD leads to homogeneous adsorption, in which all the QDs are adsorbed directly on the TiO2 surface with no aggregation. As shown in Fig. 7, strong dependence of the decay of the TG kinetics on the linker nature was observed. This result suggests that, not only the length, but also other factors such as the dipole moment, the redox properties or the electronic structure of the linkers may play a role in the carrier dynamics. The TG kinetics of the LA adosprtion samples were fitted very well to eq. (3),

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 301

the recombination of photoexcited electrons in the QDs with electrolyte redox species. The third one is the recombination of photoexcited electrons and holes through surface and/or interface defects. To improve the photovoltaic performance, surface passivation should be carried out. In the previous work (Shen et al., 2008; Diguna et al., 2007a), it was found that ZnS surface coating on the CdSe QDs could imporve the photovoltaic properties of CdSe QDSCs significantly. In addition to ZnS, the authors also tried surface mosifications with Zn2+ and S2- surface adsorption. The effects of the three kinds of surface modification (ZnS, Zn2+, and S2-) on the photovoltaic performances and ultrafast carrier dynamics of CdSe

The three kinds of surface modifications for CdSe QD adsorbed TiO2 nanostructured electrodes (24 h adsorption at 10 ℃ using CBD method) were carried out as follows: (1) the samples were coated with ZnS by alternately dipping them two times in 0.1 M Zn(CH2COO) and 0.1 M Na2S aqueous solutions for 1 minute for each dip; (2) the samples were adsorbed with Zn2+ by dipping them in 0.1 M Zn(CH2COO) for 6 min; (3) the samples were adsorbed with S2- by dipping them in 0.1 M Na2S for 6 min. For characterization of the photovoltaic performances, solar cells were assembled using a CdSe QD sensitized TiO2 electrode as the working electrode and a Cu2S film as the counter electrode (Hodes et al., 1980). Polysulfide electrolyte (1 M S and 1 M Na2S solution) was used as the regenerative redox couple (Shen et al., 2008b). The active area of the cells was 0.25 cm2. The photovoltaic characteristics of the solar cells were measured using a solar simulator (Peccell Technologies, Inc.) at one sun (AM1.5, 100 mW/cm2). For the TG measurements, the pump and the probe wavelengths are

Figure 8 shows the photocurrent density-photovoltage curves of CdSe QDSCs, in which the QD surfaces were not modified and modified with Zn2+, S2-, and ZnS, respectively. The photovoltaic properties of short circuit current density, open circuit voltage and fill factor (FF) were improved after modifying the surface with ZnS or Zn2+, especially the short circuit current density was improved significantly (Table 3). As a result, the energy conversion efficiencies of the samples with ZnS or Zn2+ surface modification increased as many as 3 times compared to the sample without surface modification. The improvement of the photovoltaic performances by means of the ZnS or Zn2+ surface modification may be due to (1) the decrease of surface states of TiO2 and CdSe QDs and (2) the formation of a potential barrier at the CdSe QD/electrolyte and TiO2/electrolyte interfaces. However, there is little effect of the modification with S2- on the photovoltaic properties. So, maybe Se2- is rich on

Figure 9 shows the TG kinetics of the CdSe QD adsorbed TiO2 electrodes before and after Zn2+ and ZnS surface modification. From the TG kinetics, two decay processes were observed. The fast and slow decay processes were mostly attributed to photoexcited hole trapping and electron transfer dynamics, as discussed earlier. For the sample before and after Zn2+ surface modification, the fast decay times were 4.5 and 2 ps, and the slow decay times were 53 ps and 25 ps, respectively. It indicates that the hole and electron relaxation became faster after the surface modification. Similar results were also obtained after the ZnS and S2- surface modifications as shown in Fig. 10. The faster electron transfer due to the surface modification may result from the decrease of the surface trapping states. Further studies on the mechanism of the effects of the surface modifications on the photovoltaic

performances and the carrier dynamics are in progress now.

QDSCs have been investigated.

530 nm and 775 nm, respectively.

the CdSe QD surfaces.

and electron transfer rate constants were calculated according to eq. (4) by assuming that the intrinsic decay rate constant in adsorbed TiO2 electrodes is the same as that in the CdSe colloidal dispersion (Table 2). The values of the electron transfer rate constant from CdSe to TiO2 via MPA are in good agreement with the results obtained by Kamat's group (Robel et al., 2007), ranging from 1.0 x 109 to 2 x 1011 s -1. With respect to the effect of QD size, it was found that the smaller the QD, the faster the electron injection. Very interestingly, a direct correalation between the ultrafast carrier dynamics and the incident photon current conversion efficiency (IPCE) values measured in the absence of electron acceptors in solution for CdSe QDSCs was found. (Guijarro et al., 2010b).

Fig. 7. TG kinetics for CdSe-sensitized TiO2 electrodes using cysteine, MPA and MBA as linkers. All the curves are normalized to the maximum value. The TG kinetics corresponding to the CdSe solution used for LA adsorption is also given for comparison (Guijarro et al., 2010b).


Table 2. Fitting parameters of TG kinetics (Fig. 7) of CdSe solution and LA adsorption samples (eq. (3)) and calculated electron transfer rate constants (eq. (4)) (Guijarro et al., 2010b).

### **2.7 Effect of surface modification on the ultrafast carrier dynamics and photovoltaic properties of CdSe QD sensitized TiO2 solar cells**

As mentioned above, the energy conversion efficiency of QDSCs is still less than 5% at present (Mora-Sero', et al., 2010; Zhang et al., 2011) and more studies are necessary for improving the photovoltaic properties of QDSCs. The poor photovoltaic performance of QDSCs usually results from three kinds of recombination. The first one is the recombination of the injected electrons in TiO2 electrodes with electrolyte redox species. The second one is

and electron transfer rate constants were calculated according to eq. (4) by assuming that the intrinsic decay rate constant in adsorbed TiO2 electrodes is the same as that in the CdSe colloidal dispersion (Table 2). The values of the electron transfer rate constant from CdSe to TiO2 via MPA are in good agreement with the results obtained by Kamat's group (Robel et al., 2007), ranging from 1.0 x 109 to 2 x 1011 s -1. With respect to the effect of QD size, it was found that the smaller the QD, the faster the electron injection. Very interestingly, a direct correalation between the ultrafast carrier dynamics and the incident photon current conversion efficiency (IPCE) values measured in the absence of electron

Fig. 7. TG kinetics for CdSe-sensitized TiO2 electrodes using cysteine, MPA and MBA as linkers. All the curves are normalized to the maximum value. The TG kinetics corresponding to the CdSe solution used for LA adsorption is also given for comparison (Guijarro et al., 2010b).

0 20 40 60 80 100 120

 Cysteine MPA

MBA CdSe Solution

time / ps

Sample A1 τ1 (ps) A2 τ2 (ps) k1,et x 10-11 (s-1) k2,et x 10-9 (s-1)

**2.7 Effect of surface modification on the ultrafast carrier dynamics and photovoltaic** 

As mentioned above, the energy conversion efficiency of QDSCs is still less than 5% at present (Mora-Sero', et al., 2010; Zhang et al., 2011) and more studies are necessary for improving the photovoltaic properties of QDSCs. The poor photovoltaic performance of QDSCs usually results from three kinds of recombination. The first one is the recombination of the injected electrons in TiO2 electrodes with electrolyte redox species. The second one is

LA (cysteine) 0.46 ± 0.01 3.9 ± 0.3 0.41 ± 0.01 127 ± 7 1.4 ± 0.3 5.9 ± 0.6 LA (MPA) 0.50 ± 0.01 4.2 ± 0.2 0.46 ± 0.01 204 ± 13 1.2 ± 0.2 2.9 ± 0.5 LA (MBA) 0.38 ± 0.02 7.7 ± 0.3 0.77 ± 0.01 245 ± 24 0.2 ± 0.1 2.1 ± 0.6 Table 2. Fitting parameters of TG kinetics (Fig. 7) of CdSe solution and LA adsorption samples (eq. (3)) and calculated electron transfer rate constants (eq. (4)) (Guijarro et al.,

CdSe solution 0.18 ± 0.01 8.8 ± 0.8 0.77 ± 0.01 506 ± 45

0.0

0.2

0.4

0.6

TG Signal/ A.U.

0.8

1.0

**properties of CdSe QD sensitized TiO2 solar cells** 

2010b).

acceptors in solution for CdSe QDSCs was found. (Guijarro et al., 2010b).

the recombination of photoexcited electrons in the QDs with electrolyte redox species. The third one is the recombination of photoexcited electrons and holes through surface and/or interface defects. To improve the photovoltaic performance, surface passivation should be carried out. In the previous work (Shen et al., 2008; Diguna et al., 2007a), it was found that ZnS surface coating on the CdSe QDs could imporve the photovoltaic properties of CdSe QDSCs significantly. In addition to ZnS, the authors also tried surface mosifications with Zn2+ and S2- surface adsorption. The effects of the three kinds of surface modification (ZnS, Zn2+, and S2-) on the photovoltaic performances and ultrafast carrier dynamics of CdSe QDSCs have been investigated.

The three kinds of surface modifications for CdSe QD adsorbed TiO2 nanostructured electrodes (24 h adsorption at 10 ℃ using CBD method) were carried out as follows: (1) the samples were coated with ZnS by alternately dipping them two times in 0.1 M Zn(CH2COO) and 0.1 M Na2S aqueous solutions for 1 minute for each dip; (2) the samples were adsorbed with Zn2+ by dipping them in 0.1 M Zn(CH2COO) for 6 min; (3) the samples were adsorbed with S2- by dipping them in 0.1 M Na2S for 6 min. For characterization of the photovoltaic performances, solar cells were assembled using a CdSe QD sensitized TiO2 electrode as the working electrode and a Cu2S film as the counter electrode (Hodes et al., 1980). Polysulfide electrolyte (1 M S and 1 M Na2S solution) was used as the regenerative redox couple (Shen et al., 2008b). The active area of the cells was 0.25 cm2. The photovoltaic characteristics of the solar cells were measured using a solar simulator (Peccell Technologies, Inc.) at one sun (AM1.5, 100 mW/cm2). For the TG measurements, the pump and the probe wavelengths are 530 nm and 775 nm, respectively.

Figure 8 shows the photocurrent density-photovoltage curves of CdSe QDSCs, in which the QD surfaces were not modified and modified with Zn2+, S2-, and ZnS, respectively. The photovoltaic properties of short circuit current density, open circuit voltage and fill factor (FF) were improved after modifying the surface with ZnS or Zn2+, especially the short circuit current density was improved significantly (Table 3). As a result, the energy conversion efficiencies of the samples with ZnS or Zn2+ surface modification increased as many as 3 times compared to the sample without surface modification. The improvement of the photovoltaic performances by means of the ZnS or Zn2+ surface modification may be due to (1) the decrease of surface states of TiO2 and CdSe QDs and (2) the formation of a potential barrier at the CdSe QD/electrolyte and TiO2/electrolyte interfaces. However, there is little effect of the modification with S2- on the photovoltaic properties. So, maybe Se2- is rich on the CdSe QD surfaces.

Figure 9 shows the TG kinetics of the CdSe QD adsorbed TiO2 electrodes before and after Zn2+ and ZnS surface modification. From the TG kinetics, two decay processes were observed. The fast and slow decay processes were mostly attributed to photoexcited hole trapping and electron transfer dynamics, as discussed earlier. For the sample before and after Zn2+ surface modification, the fast decay times were 4.5 and 2 ps, and the slow decay times were 53 ps and 25 ps, respectively. It indicates that the hole and electron relaxation became faster after the surface modification. Similar results were also obtained after the ZnS and S2- surface modifications as shown in Fig. 10. The faster electron transfer due to the surface modification may result from the decrease of the surface trapping states. Further studies on the mechanism of the effects of the surface modifications on the photovoltaic performances and the carrier dynamics are in progress now.

Ultrafast Electron and Hole Dynamics in CdSe Quantum Dot Sensitized Solar Cells 303

Fig. 10. Fast (τ1) (a) and slow (τ2) (b) decay times of the transient grating kinetics for CdSe QD adsorbed TiO2 electrodes before and after the surface modifications with ZnS, Zn2+ and S2-.

S2-

ZnS

without

Zn2+

**(b)**

S2-

τ**2 (ps)**

In summary, the photoexcited carrier dynamics in CdSe QDs adsorbed onto TiO2 nanostructured electrodes have been characterized by using the improved TG technique. It has been demonstrated that both photoexcited electron and hole dynamics can be detected by using this improved TG technique. By comparing the TG responses measured in air and in a Na2S solution (hole acceptor), the dynamics of photoexcited electrons and holes in the CdSe QDs has been sucssesfully separated from each other. It was found that charge separation in the CdSe QDs occurred over a very fast time scale from a few hundreds of fs in the Na2S solution via hole transfer to S2- ions to a few ps in air via hole trapping. On the other hand, the electron dynamics in the CdSe QDs, including trapping and injection to the metal oxide electrodes, depends greatly on the QD size, and adsorption methods (such as CBD, SILAR, DA and LA adsorption methods) and conditons (such as adsorption time and SIALR cycle number). In addition, surface modifications such as ZnS coating and adsorption with Zn2+ have been demonstrated to improve the photovoltaic properties and have a great influence on the ultrafast carrier dynamics of CdSe QDSCs. Detailed studies on the correlations between the

carrier dynamics and the photovoltaic properties in QDSCs are in progress now.

Part of this research was supported by JST PRESTO program, Grant in Aid for Scientific Research (No. 21310073) from the Ministry of Education, Sports, Science and Technology of the Japanese Government. The authors would like to thank Dr. Kenji Katayama and Dr. Tsuguo Sawada for their help in the setup of TG equipment and also thank Mr. Yasumasa Ayuzawa for his help in some experiments. The authors are grateful to Prof. Juan Bisquert, Prof. Roberto Go´mez, Mr. Ne´stor Guijarro, Dr. Sixto Gime´nez, and Dr. Iva´n Mora-Sero´

Adachi, M.; Murata, Y.; Okada, I.; &Yoshikawa, S. (2003), *J. Electrochem. Soc.*, Vol. 150, G488.

Bawendi, M. G.; Kortan, A. R.; Steigerwals, M.; & Brus, L. E. (1989), *J. Chem. Phys*.,

**3. Conclusion** 

1

2

3

ZnS

4

τ1 (ps)

5

without **(a)**

Zn2+

6

**4. Acknowledgment** 

**5. References** 

for their collaboration and help in the research.

Vol. 91, p. 7282.

Fig. 8. Photocurrent density-photovoltage characteristics of CdSe QD-sensitized solar cells without and with surface modifications.


Table 3. Photovoltaic properties of CdSe QDSCs before and after surface modifications with Zn2+, S2-, and ZnS, respectively. Jsc is the short circuit current density, Voc is the open circuit voltage, FF is the fill factor, and η is the energy conversion efficiency.

Fig. 9. Transient grating kinetics of CdSe QD adsorbed TiO2 electrode before and after the surface modification with Zn2+ (a) and ZnS (b).

Fig. 10. Fast (τ1) (a) and slow (τ2) (b) decay times of the transient grating kinetics for CdSe QD adsorbed TiO2 electrodes before and after the surface modifications with ZnS, Zn2+ and S2-.
