| As human society is in great demand for energy, highly efficient use of solarenergy has become an inevitable choice of the development of human societycivilization in modern times. As a representative of the rising third-generation solarcells, quantum dots sensitized solar cells (QDSCs) have received more and moreresearchers‘extensive concerning because of their low production cost and hightheoretical light-to-energy conversion efficiency. Compared with the traditionaldye-sensitized solar cells, quantum dots (QDs) have significant advantages, such astunable band gap, high extinction coefficients and multiple exciton generation,making the highest theoretical photoelectric conversion efficiency of QDSCs to be44%, which is much higher than the Shockley-Queisser ultimate value of singlecrystal silicon solar cells (~31%). Thus, QDSCs possess a very considerable researchand application prospect.Up to now, the highest photoelectric conversion efficiency of QDSCs is less than7%, which is far behind the conversion efficiency of dye-sensitized solar cells. Howto further improve the photoelectric conversion efficiency have been becoming thecurrent hot research hotspots. As the core component part of QDSCs, the photoanodescomposing of semiconductor oxide and QDSCs directly influence the absorption ofsunlight utilization and transmission property of photo-induced electrons, and alsodetermines the final photoelectric conversion efficiency. The traditional TiO2nanocrystallines possess a large number of surface defect state and grain boundarybarriers, which count agaist the transport of photo-induced electrons. Onedimensional (1D) structures, especially vertically aligned nanowires/nanorods on top of a substrate have been widely investigated as photoelectrodes in solar cells becausethey provide a direct conduction pathway for photo-induced electrons to rapidlycollect and transport, which have been widely used in sensitized solar cells. On theother hand, the absorption edge of tttraditional CdS and CdSe quantum dots withwider band gap is less than750nm, leading to the useless of near-infrared light in thewhole sunlight. Therefore, developing new type of narrow band gap quantum dots isan effective method to improve the light harvesting efficiency and final photoelectricconversion efficiency.In our paper, we have constructed novel photoanode structure in three ways toimprove the light harvesting efficiency and interface transfer of photo-inducedelectrons, and finally enhance the photoelectric conversion efficiency of QDSCs. Inthe meantime, we have utilized surface photovoltage and transient photovoltagetechniques as well as other photoelectrochemistry measurements to study the transferbehavior of photo-induced charge carries, and finally discuss the relationship betweencharge behavior and photovoltaic performance. Specific work includes below aspects:1. We have employed a facile two-step hydrothermal process to fabricate the3Dbranched ZnO/TiO2heterojunction nanorod arrays on FTO substrates and appliedthem as photoanodes in QDSCs. The ZnO nanorod branches on the rutile trunks offerincreased QD loading and high light scattering property. Moreover, the presence ofTiO2/ZnO heterojunction is favorable for electron transfer from ZnO to TiO2,resulting in the enhancement of electron collection efficiency. At the same time, theTiO2/ZnO heterojunction could also reduce the electron recombination at theelectrode/electrolyte interface and increase the electron lifetime. As a result, QDSCsbased on B-ZnO/TiO2NRAs with the thickness of only1μm, exhibited an overall0.73%energy conversion effciency, which was55%improvement over the QDSCsbased on TiO2NRAs, indicating its application potential in the field of photovoltaicsolar energy conversion.2. Hierarchical anatase TiO2nanotube branches have been successfully assembledonto the primary rutile TiO2nanorod arrays through a liquid-phase conversionprocess using ZnO nanorods as a template. The newly designed H-TiO2nanorod arrayelectrodes offered large surface area for high QD loading with high light scatteringproperty. In the meantime, the presence of a rutile–anatase heterojunction at the interface helped the rutile nanorods to efficiently collect photo-injected electronsfrom anatase nanotubes, reducing charge recombination with electrolyte and QDs. Asa result, the QD-sensitized H-TiO2NRA photoanode, only1μm, exhibited a maximalsolar energy conversion efficiency of1.04%, which was2.7times higher than that ofthe TiO2NRA photoanode.3. A facile one-step hydrothermal method was carried out to prepare efficientCdS-decorated TiO2nanorod photoanodes with the aid of glutathione (GSH). In thehydrothermal process, the CdS QDs are covalently linked to the surface of TiO2nanorods through the GSH bifunctional molecule which also acts as stabilizer andsulfur source inthis one-step fabrication. The results from FESEM and HRTEMimages reveal that the CdS QDs are well dispersed on the surfaces of TiO2nanorods.Surface photovoltage (SPV) measurements demonstrate that the CdS QDssensitization enhances the visible spectral absorption of TiO2nanorod arrays, as wellas their separation efficiency of photo-induced electron–hole pairs. As a result, theCdS-decorated TiO2nanorod heterostructure shows a maximum power conversionefficiency (PCE) of0.278%under illumination at100mW cm2by employingpolysulfide electrolyte. In addition, the preferable and stable photocurrents ofTiO2-CdS composite electrodes confirm that CdS QDs could be anchored onto thesurface of TiO2nanorods effectively and stably via bifunctional groups of GSH.4. Vertically aligned TiO2nanorod arrays (NRAs) modified with Ag2S QDs havebeen successfully prepared via a successive ionic layer adsorption and reaction(SILAR) process. We have studied the process of separation of photo-induced chargecarriers by SPV measurement. By controlling the amount of attached Ag2S QDs, wefound that the photovoltaic characteristics change significantly. After the fabricationof QDSCs, the Ag2S QDs/TiO2nanorod composite film exhibited a best powerconversion efficiency of0.148%when illuminated with light intensity100mW cm2.Finally, the relation between the performance of QDSCs and the photo-inducedcharge separation behavior was also discussed. |
PDF Full Text Request