**3. Conclusion**

302 Solar Cells – New Aspects and Solutions

 ZnS Zn2+ S2-

without

0 50 100 150 200

**Time (ps)**

/CdSe

/CdSe/ZnS

(b)

TiO2

1.0 TiO2

Fig. 8. Photocurrent density-photovoltage characteristics of CdSe QD-sensitized solar cells

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Voltage (V)

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

Fig. 9. Transient grating kinetics of CdSe QD adsorbed TiO2 electrode before and after the

0.0 0.2 0.4 0.6 0.8

**Norm. TG signal (arb.units)**

Sample Jsc (mA/cm2) Voc (V) FF η (%) TiO2/CdSe 4.2 0.43 0.39 0.69 TiO2/CdSe/ZnS 8.2 0.49 0.53 2.1 TiO2/CdSe/Zn2+ 7.6 0.50 0.53 2.1 TiO2/CdSe/S2- 3.8 0.44 0.43 0.82

(a)

voltage, FF is the fill factor, and η is the energy conversion efficiency.

without and with surface modifications.

1.0 TiO2

0

2

4

Current density (mA/cm2

6

8

10

)

surface modification with Zn2+ (a) and ZnS (b).

**Time (ps)**

0.0 0.2 0.4 0.6 0.8

**Norm. TG signal (arb.units)**

0 50 100 150 200

/CdSe TiO2 /CdSe/Zn2+ 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.
