**3. Results and discussion**

Figure 2 shows the PA spectra of the nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption times for CdS QDs were fixed at 40 min (average diameter: ~ 4.2 nm).

Optical Absorption and Photocurrent Spectra of CdSe Quantum

CdS quantum dots only (modulation frequency: 33 Hz).

(modulation frequency: 33 Hz).

Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 481

Fig. 2. Photoacoustic spectra of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe quantum dots for different adsorption times together with that adsorbed wth

Fig. 3. Photoacoustic spectra of nanostructured TiO2 electrodes adsorbed with CdSe

quantum dots without a preadsorbed CdS quantum dot layer for different adsorption times

The spectra were normalized to the photon energy of 4.0 eV. With increasing adsorption time, the red-shift of optical absorption at the shoulder point (indicated by arrows) can be clearly observed, implying the growth of CdSe QDs. Also, the comparison between the adsorption of CdSe QDs on the nanostructured TiO2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in PA spectra. For that, Figure 3 shows the PA spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs without a preadsorbed CdS QD layer for different adsorption times. The spectra are also normalized to the photon energy of 4.0 eV. As the PA spectra below the CdSe QDs adsorption time of 8 h agree with that of pure nanostructured TiO2 electrode within the experimental accuracy, the CdSe QDs average size is very small less than 1 nm or no adsorption. Optical absorption in the visible light region due to the adsorbed CdSe QDs can be also observed both in Figs. 2 and 3. With increasing adsorption time, the red-shift of optical absorption at the shoulder point (indicated by arrows) can be clearly observed in Fig.3, also implying the growth of CdSe QDs. The exponential slopes at the fundamental absorption edges in combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes in Fig. 2 are higher than those of CdSe QDs adsorbed on nanostructured TiO2 electrodes without a pre-adsorbed CdS QD layer in Fig. 3, indicating that the uniformity of the average sizes or crystal quality of CdSe QDs in the former is better than that of the latter. Relative to the band-gap energy of 1.73 eV for bulk CdSe, the shoulder points in PA spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs shown in Figs. 2 and 3 exhibit blue-shifts, which is indicative of the quantum confinement effect. This fact implies that the radii of the CdSe QDs are smaller than the Bohr radius of bulk CdSe (~5.6 nm). We assume that the photon energy at the shoulder point corresp**o**nds to the lowest excitation energy of the CdSe QDs (Shen & Toyoda, 2004). The average diameter of the CdSe QDs for each adsorption time both in combined CdS/CdSe and CdSe without a preadsorbed CdS layer adsorbed on nanostructured TiO2 electrodes can be estimated with the effective mass approximation (Shen et al., 2008; Bawendi et al., 1989). The dependence of the average diameter on the adsorption time is shown in Fig. 4, both of combined CdS/CdSe (●) and CdSe without apre-adsorbed CdS layer (○). It can be observed that the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS QD layer grow rapidly during the initial adsorption process (less than 2 h adsorption). After a certain time, the crystal growth rate is slowed down, which is similar to the reference reported (Toyoda et al., 2007). When the adsorption time is sufficiently long (in this case ~ 24 h), the average diameter of CdSe QDs becomes constant at about 7.1 nm. On the other hand, it can be observed that the CdSe QDs adsorbed on the nanostructured TiO2 electrodes without a pre-adsorbed CdS layer show very slow growth or no growth during the initial adsorption process (less than 8 hours) due to the fact that the PA spectra below the CdSe adsorption time below 8 hours agree with that of the pure nanostructured TiO2 electrode within the experimental accuracy. Therefore, the crystal growth rate of CdSe QDs adsorbed on TiO2 electrode with a pre-adsorbed CdS QD layer is faster than that of CdSe QDs adsorbed on the TiO2 electrodes without a pre-adsorbed CdS QD layer. There are several possibilities for the faster crystal growth rate of CdSe QDs adsorbed on the nanostructured TiO2 electrode with a pre-adsorbed CdS QD layer. First, it is possible that the active CdSe QDs form from the excess Cd remaining after CdS adsorption directly on the nanostructured TiO2 electrode (Niitsoo et al., 2006). This is suggested that CdS QDs act as seed layer for subsequent CdSe growth (Sudhager et al., 2009). Second one is the passivation effect of CdS QDs on the nanostrctured TiO2 surface to reduce defects or dislocations. Third, CdSe QDs are grown on the assembly of CdS QDs, indicating the growth on the similar crystal structure (or close lattice constant) different from the growth on TiO2.

The spectra were normalized to the photon energy of 4.0 eV. With increasing adsorption time, the red-shift of optical absorption at the shoulder point (indicated by arrows) can be clearly observed, implying the growth of CdSe QDs. Also, the comparison between the adsorption of CdSe QDs on the nanostructured TiO2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in PA spectra. For that, Figure 3 shows the PA spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs without a preadsorbed CdS QD layer for different adsorption times. The spectra are also normalized to the photon energy of 4.0 eV. As the PA spectra below the CdSe QDs adsorption time of 8 h agree with that of pure nanostructured TiO2 electrode within the experimental accuracy, the CdSe QDs average size is very small less than 1 nm or no adsorption. Optical absorption in the visible light region due to the adsorbed CdSe QDs can be also observed both in Figs. 2 and 3. With increasing adsorption time, the red-shift of optical absorption at the shoulder point (indicated by arrows) can be clearly observed in Fig.3, also implying the growth of CdSe QDs. The exponential slopes at the fundamental absorption edges in combined CdS/CdSe QDs adsorbed on nanostructured TiO2 electrodes in Fig. 2 are higher than those of CdSe QDs adsorbed on nanostructured TiO2 electrodes without a pre-adsorbed CdS QD layer in Fig. 3, indicating that the uniformity of the average sizes or crystal quality of CdSe QDs in the former is better than that of the latter. Relative to the band-gap energy of 1.73 eV for bulk CdSe, the shoulder points in PA spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs shown in Figs. 2 and 3 exhibit blue-shifts, which is indicative of the quantum confinement effect. This fact implies that the radii of the CdSe QDs are smaller than the Bohr radius of bulk CdSe (~5.6 nm). We assume that the photon energy at the shoulder point corresp**o**nds to the lowest excitation energy of the CdSe QDs (Shen & Toyoda, 2004). The average diameter of the CdSe QDs for each adsorption time both in combined CdS/CdSe and CdSe without a preadsorbed CdS layer adsorbed on nanostructured TiO2 electrodes can be estimated with the effective mass approximation (Shen et al., 2008; Bawendi et al., 1989). The dependence of the average diameter on the adsorption time is shown in Fig. 4, both of combined CdS/CdSe (●) and CdSe without apre-adsorbed CdS layer (○). It can be observed that the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS QD layer grow rapidly during the initial adsorption process (less than 2 h adsorption). After a certain time, the crystal growth rate is slowed down, which is similar to the reference reported (Toyoda et al., 2007). When the adsorption time is sufficiently long (in this case ~ 24 h), the average diameter of CdSe QDs becomes constant at about 7.1 nm. On the other hand, it can be observed that the CdSe QDs adsorbed on the nanostructured TiO2 electrodes without a pre-adsorbed CdS layer show very slow growth or no growth during the initial adsorption process (less than 8 hours) due to the fact that the PA spectra below the CdSe adsorption time below 8 hours agree with that of the pure nanostructured TiO2 electrode within the experimental accuracy. Therefore, the crystal growth rate of CdSe QDs adsorbed on TiO2 electrode with a pre-adsorbed CdS QD layer is faster than that of CdSe QDs adsorbed on the TiO2 electrodes without a pre-adsorbed CdS QD layer. There are several possibilities for the faster crystal growth rate of CdSe QDs adsorbed on the nanostructured TiO2 electrode with a pre-adsorbed CdS QD layer. First, it is possible that the active CdSe QDs form from the excess Cd remaining after CdS adsorption directly on the nanostructured TiO2 electrode (Niitsoo et al., 2006). This is suggested that CdS QDs act as seed layer for subsequent CdSe growth (Sudhager et al., 2009). Second one is the passivation effect of CdS QDs on the nanostrctured TiO2 surface to reduce defects or dislocations. Third, CdSe QDs are grown on the assembly of CdS QDs, indicating the growth on the similar crystal

structure (or close lattice constant) different from the growth on TiO2.

Fig. 2. Photoacoustic spectra of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe quantum dots for different adsorption times together with that adsorbed wth CdS quantum dots only (modulation frequency: 33 Hz).

Fig. 3. Photoacoustic spectra of nanostructured TiO2 electrodes adsorbed with CdSe quantum dots without a preadsorbed CdS quantum dot layer for different adsorption times (modulation frequency: 33 Hz).

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Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 483

Fig. 5. IPCE spectra of nanostructured TiO2 electrodes adsorbed with CdS/CdSe quantum dots for different adsorption times together with that adsorbed with CdS quantum dots only.

Fig. 6. IPCE spectra of nanostructured TiO2 electrodes adsorbed with CdSe quantum dots

The photocurrent-voltage cureves of (a) combined CdS/CdSe QD- and (b) CdSe QDsensitized solar cells are shown in Fig. 7 (a) and (b), respectively, for different adsorption times, together with that obtained with cells adsorbed with CdS only. However, the

without a preadsorbed CdS QD layer for different adsorption times.

Fig. 4. Dependence of the average diameter on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS layer (○).

Figure 5 shows the IPCE spectra of the nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption time of CdS QD layer is fixed at 40 min. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. With increasing adsorption time, the redshift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. The IPCE peak value increases with the increase of adsorption time up to 8 hours (~ 75%), then decreases until 24 h adsorption owing to the increase of recombination centers or interface states, together with the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO2. Also, the comparison between the adsorption of CdSe QDs on the TiO2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in IPCE spectra. For that, Figure 5 shows the IPCE spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs without a pre-adsorbed layer of CdS QD layer for different adsorption times. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed in Fig. 6, also indicating the photosensitization by CdSe QDs. With increasing adsorption time, the red-shift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. However, the appearance of the spectrum in Fig. 6 is different from that of combined CdS/CdSe QDs, namely in the reduction of maximum IPCE value (~ 60%) and the adsorption time dependence of the spectrum shape. Also, the IPCE spectra below the CdSe QDs adsorption time of 8 h agree with that of pure nanostructured TiO2 electrode within the experimental accuracy, indicating that the CdSe QDs adsorbed on the nanostructured TiO2 electrode without a pre-adsorbed CdS layer show very slow growth or no growth similar to the results of PA characterization in Fig. 3. These results demonstrate that the spectral response of IPCE is enhanced upon combined CdS/CdSe sensitization rather than single CdSe QDs sensitization, indicating the possibility of the reduction in recombination centers and interface states owing to the possibilities of active CdSe QDs by the excess Cd remaining after CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO2 surface.

Fig. 4. Dependence of the average diameter on the adsorption time, both of combined

CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO2 surface.

Figure 5 shows the IPCE spectra of the nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs for different adsorption times, together with that adsorbed with CdS QDs only. The pre-adsorption time of CdS QD layer is fixed at 40 min. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. With increasing adsorption time, the redshift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. The IPCE peak value increases with the increase of adsorption time up to 8 hours (~ 75%), then decreases until 24 h adsorption owing to the increase of recombination centers or interface states, together with the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO2. Also, the comparison between the adsorption of CdSe QDs on the TiO2 electrodes with and without a pre-adsorbed CdS QD layer was carried out to evaluate the difference in IPCE spectra. For that, Figure 5 shows the IPCE spectra of the nanostructured TiO2 electrodes adsorbed with CdSe QDs without a pre-adsorbed layer of CdS QD layer for different adsorption times. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed in Fig. 6, also indicating the photosensitization by CdSe QDs. With increasing adsorption time, the red-shift of photoelectrochemical current can be clearly observed, implying the growth of CdSe QDs. However, the appearance of the spectrum in Fig. 6 is different from that of combined CdS/CdSe QDs, namely in the reduction of maximum IPCE value (~ 60%) and the adsorption time dependence of the spectrum shape. Also, the IPCE spectra below the CdSe QDs adsorption time of 8 h agree with that of pure nanostructured TiO2 electrode within the experimental accuracy, indicating that the CdSe QDs adsorbed on the nanostructured TiO2 electrode without a pre-adsorbed CdS layer show very slow growth or no growth similar to the results of PA characterization in Fig. 3. These results demonstrate that the spectral response of IPCE is enhanced upon combined CdS/CdSe sensitization rather than single CdSe QDs sensitization, indicating the possibility of the reduction in recombination centers and interface states owing to the possibilities of active CdSe QDs by the excess Cd remaining after

CdS/CdSe (●) and CdSe without a preadsorbed CdS layer (○).

Fig. 5. IPCE spectra of nanostructured TiO2 electrodes adsorbed with CdS/CdSe quantum dots for different adsorption times together with that adsorbed with CdS quantum dots only.

Fig. 6. IPCE spectra of nanostructured TiO2 electrodes adsorbed with CdSe quantum dots without a preadsorbed CdS QD layer for different adsorption times.

The photocurrent-voltage cureves of (a) combined CdS/CdSe QD- and (b) CdSe QDsensitized solar cells are shown in Fig. 7 (a) and (b), respectively, for different adsorption times, together with that obtained with cells adsorbed with CdS only. However, the

Optical Absorption and Photocurrent Spectra of CdSe Quantum

Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 485

an increase in adsorption time up to 8 h due, mainly, not only to the increase of the amount of CdSe QDs but the improvement in crystal quality and decrease of interface states. However, the increase in adsorption times after more than 8 h leads to deterioration in Jsc and Voc. High adsorption time of CdSe QDs might cause an increase in recombination centers, poor penetration of CdSe QDs, and the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO2. Therefore, η of the combined CdS/CdSe QDsensitized solar cell shows a maximum of 3.5% at 8 h adsorption times. On the other hand, Jsc and η below the CdSe QDs adsorption time of 8 h without a pre-adsorbed CdS layer show very small values close to zero, indicating the very small amount of CdSe QDs adsorption similar to the results of PA and IPCE characterization. We can observe that Jsc, Voc, FF, and η in

Fig. 8. Dependence of the photovoltaic parameters ((a) Jsc and (b) Voc ) on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○).

appearance of the current-voltage curves of combined CdS/CdSe QD-sensitized solar cells is different from those of CdSe QD-sensitized solar cells. Figure 8 and 9 illustrates the photovoltaic parameters ((a) Jsc; (b) Voc; (c) FF; (d) η) of combined CdS/CdSe QD-sensitized (●) and CdSe QD-sensitized (○) solar cells as a function of CdSe QDs adsorption times.

Fig. 7. Photocurrent-voltage curves of (a) combined CdS/CdSe quantm dot- and (b) CdSe quantum dot-sensitized solar cells for different adsorption times together with that adsorbed with CdS quantum dots only.

We observe that the parameter of Jsc in combined CdS/CdSe QD-sensitized solar cells increases with the increase of CdSe QDs adsorption times up to 8 h. On the other hand, Voc and FF are independent of adsorption times. The performance of solar cells improved with

appearance of the current-voltage curves of combined CdS/CdSe QD-sensitized solar cells is different from those of CdSe QD-sensitized solar cells. Figure 8 and 9 illustrates the photovoltaic parameters ((a) Jsc; (b) Voc; (c) FF; (d) η) of combined CdS/CdSe QD-sensitized (●) and CdSe QD-sensitized (○) solar cells as a function of CdSe QDs adsorption times.

Fig. 7. Photocurrent-voltage curves of (a) combined CdS/CdSe quantm dot- and (b) CdSe quantum dot-sensitized solar cells for different adsorption times together with that

We observe that the parameter of Jsc in combined CdS/CdSe QD-sensitized solar cells increases with the increase of CdSe QDs adsorption times up to 8 h. On the other hand, Voc and FF are independent of adsorption times. The performance of solar cells improved with

adsorbed with CdS quantum dots only.

an increase in adsorption time up to 8 h due, mainly, not only to the increase of the amount of CdSe QDs but the improvement in crystal quality and decrease of interface states. However, the increase in adsorption times after more than 8 h leads to deterioration in Jsc and Voc. High adsorption time of CdSe QDs might cause an increase in recombination centers, poor penetration of CdSe QDs, and the decrease of energy difference between LUMO in CdSe QDs and the bottom of conduction band of TiO2. Therefore, η of the combined CdS/CdSe QDsensitized solar cell shows a maximum of 3.5% at 8 h adsorption times. On the other hand, Jsc and η below the CdSe QDs adsorption time of 8 h without a pre-adsorbed CdS layer show very small values close to zero, indicating the very small amount of CdSe QDs adsorption similar to the results of PA and IPCE characterization. We can observe that Jsc, Voc, FF, and η in

Fig. 8. Dependence of the photovoltaic parameters ((a) Jsc and (b) Voc ) on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○).

Optical Absorption and Photocurrent Spectra of CdSe Quantum

Dots Adsorbed on Nanocrystalline TiO2 Electrode Together with Photovoltaic Properties 487

diffusion, acoustic velocity and so on. Improved TG technique features very simple and compact optical setup, and is applicable for samples with rough surfaces. Comparing with transient absorption (TA) technique, improved TG method has higher sensitivity sue to its zero background in TG signals, which avoids the nonlinear effect and sample damage. Figure 9 shows that the hole and electron relaxation times of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs are faster about twice than those with CdSe QDs

without a pre-adsorve CdS layer, indicating the decreases in recombination centers.

Fig. 10. Ultrafast carrier dynamics of combined CdS/CdSe and CdS without preadsorbed

We have described the performance of quantum dot-sensitized solar cells (QDSCs) based on CdSe QD sensitizer on a pre-adsorbed CdS layer (combined CdS/CdSe QDs) together with the basic studies of optical absorption and photoelectrochemical current characteristics. It can be observed from optical absorption measurements using photoacoustic (PA) spectroscopy that the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS layer grow more rapidly during the initial adsorption process than those without a pre-adsorbed CdS layer. Photoelectrochemical current in the visible light region due to the adsorbed CdSe QDs can be observed, indicating the photosensitization by combined CdS/CdSe QDs. The maximum IPCE value (~ 75%) of the CdSe QDs on the nanostructured TiO2 electrodes with a pre-adsorbed CdS QD layer is 30% greater than that without a pre-adsorbed CdS layer. It indicates the possibilities of a decrease in recombination centers, interface states, and inverse transfer rate that is suggested by the preliminary ultrafast photoexcited carrier carrier dynamics characterization owing to the possibilities of active CdSe QDs by the excess Cd remaining after CdS adsorption and passivation effect of CdS QDs on the nanostructured TiO2 surface. The short-circuit current (Jsc) in combined CdS/CdSe QD-sensitized solar cells shows

CdS quantum dots layer with a transient grating (TG) technique.

**4. Conclusion** 

Fig. 9. Dependence of the photovoltaic parameters( (c) FF and (d) η ) on the adsorption time, both of combined CdS/CdSe (●) and CdSe without a preadsorbed CdS QD layer (○).

CdSe QD-sensitized solar cells without a pre-adsorbed CdS QD layer increase with the increase of adsorption times up to 24 h, indicating the difference of the crystal growth and the formation of recombination centers in combined CdS/CdSe and CdSe QDs.

Figure 9 shows the preliminary ultrafast photoexcited carrier dynamics characterization of combined CdS/CdSe and CdSe without a pre-adsorbed CdS layer (average diameters of both CdSe QDs are ~ 6 nm) using a improved transient grating (TG) technique (Katayama et al., 2003; Yamaguchi et al., 2003; Shen et al., 2010). TG signal is proportional to the change in the refractive index of the sample due to photoexcited carriers (electrons and holes). TG method is a powerful time-resolved optical technique for the measurements of various kinds of dynamics, such as carrier population dynamics, excited carrier diffusion, thermal diffusion, acoustic velocity and so on. Improved TG technique features very simple and compact optical setup, and is applicable for samples with rough surfaces. Comparing with transient absorption (TA) technique, improved TG method has higher sensitivity sue to its zero background in TG signals, which avoids the nonlinear effect and sample damage. Figure 9 shows that the hole and electron relaxation times of nanostructured TiO2 electrodes adsorbed with combined CdS/CdSe QDs are faster about twice than those with CdSe QDs without a pre-adsorve CdS layer, indicating the decreases in recombination centers.

Fig. 10. Ultrafast carrier dynamics of combined CdS/CdSe and CdS without preadsorbed CdS quantum dots layer with a transient grating (TG) technique.
