Preparation And Photoelectric Properties Of Quantum Dots Sensitized TiO₂Photoelectrode

Quantum dot-sensitized solar cells (QDSSCs) are considered as promising new low-cost and high-efficiency solar cells, which exhibit advantages over dye-sensitized solar cells (DSSCs) due to the tunable band gap, high extinction coefficients and large intrinsic dipole moment of quantum dot (QDs). However, as limited by the immature study of photoelectrochemical mechanism, materials and preparation method, QDSSCs exhibit much lower photoelectric conversion efficiency than DSSCs. The highest photoelectric conversion efficiency of QDSSCs is about5.4%. In this thesis, we studied several kinds of photoelectron aiming to optimize the light harvesting and electron transport of QDSSCs.1. Sized controlled TiO2hollow spheres were synthesized using template method. Then different content of TiCh hollow spheres were mixed with TiO2nanoparticles to fabricate mesoporous TiO2films on conductive FTO glass using doctor-blade method. The influence of TiO2hollow spheres on the photoelectrochemical properties of TiO2film photoelectrode was studied. The results show that TiO2hollow spheres improve the light absorbance of TiO2films due to the strong light scattering ability. Further more, the introduce of TiO2hollow spheres into TiO2film decreases the surface charge trap-sites of TiO2nanocrystals, which act as charge recombination centers, and hence enhances the photoelectron lifetime. The electrochemical properties of the TiO2film is dependent on the content of TiO2hollow spheres within the TiO2films, and the TiO2film containing10%TiO2hollow spheres exhibits the highest photocurrent and open-circuit voltage among the tested samples. After sensitized with CdS using the successive ionic layer adsorption and reaction (SILAR) method, the photoelectrochemical measurement reveals that the TiO2/CdS photoanodes with10%TiO2hollow spheres shows the highest photocurrent (～2.4mA/cm2) among the test samples, which is ascribed to the high utilization of light, and more deposition of CdS. The lowering of the photocurrent in TiO2/CdS photoanode with higher TiO2hollow spheres content may be mainly attributed to the poor carriers transfer between TiO2particles.Two kinds of TiO2hollow spheres, with diameters of350nm and700nm were coated on TiO2nanoparticals photoanodes as light scattering layers, respectively. The optical and IPCE (Incident Photon To Current Efficiency) measurement results show that both kinds of TiO2hollow spheres exhibit improvement on the light harvesting of CdS sensitized QDSSCs. And the light scattering performance of the700nm TiO2hollow spheres is better than that of the350nm TiO2hollow spheres. As a result, the improvement of IPCE by700nm TiO2hollow spheres coating was more evident, due to the higher light scattering and weaker hindering of electrolyte from penetrating into the TiO2film.2. Oleic Acid (OA) capped CdSxSe1-x (x=1,0.75,0.5,0.25,0) colloidal QDs were synthesized using a non-injection method and applied as sensitizer in QDSSC. It is found that the photoelectrochemical response range of the QDSSC was varied by changing the Se content of CdSxSe1-x QDs. This confirmed that the CdSxSe1-x QDs have potential to construct a "rainbow solar cell". During the IPCE measurement, we find that the IPCE value increased with the time of light illumination. We considered that partial OA capped on the CdSxSe1-x colloidal QDs were degradated by TiO2during the light illumination. As a result, the electron transfer between the TiO2and CdSxSe1-xQDs was improved.3. CdS QDs were deposited in TiO2nanoparticles films by SILAR method, and then N719dye were absorbed in TiO2/CdS film to fabricate CdS and N719co-sensitized TiO2solar cell. The deposition of CdS sensitizer on TiO2surface could efficiently suppress the recombination of photogenerated electrons and redox couples in electrolyte, and hence increase the open-circuit voltage of the QDSSC. It is also found that the deposition of CdS influence the short-circuit current. On one hand, the CdS deposition on TiO2surface could decrease the absorption of N719dye and hence decrease the short-circuit current. On the other hand, the co-sensitization of CdS and N719improves the separation photoelectrons and holes in co-sensitized TiO2solar cell, which increases the short-circuit current. The experiment result shows that the co-sensitized TiO2solar cell with6cycles of CdS exhibited the highest photoelectric conversion efficiency, which increased by18.4%compared with N719dye sensitized TiO2solar cell.