Synthesis Of Gold Nanocrystals And Its Applications In Quantum-dots Sensitized Solar Cells

Recently, quantum dot sensitized solar cells (QDSSCs) are attracting more and more attentiondue to the rapid evolution in power conversion efficiency in the past few years. An impressiveefficiency as high as5.4%has been reported for the Mn-doped CdS/CdSe co-sensitized QDSSCs.It is well known that the QDs have the ability to efficiently convert the energy of single photoninto multiple electron-hole pairs via multiple exciton generation (MEG). If the carriers generatedby the MEG effect can be effectively extracted from QDs, the theoretical photovoltaic conversionefficiency of QDSSCs can reach up44%, which is much higher than the Schockley-Queisser limitof31%.Despite much efforts devoted to the development of QDSSCs, up to now, the efficiency ofQDSSCs (~6%) is still lag far behind that of conventional DSSCs with a record of~11.5%. Thereason is that the optimal QDSSCs configuration has not been obtained. Further improvements areneeded to enhance the optical absorption and reduce the charge recombination. Recently, strategiesfor increasing the values of have focused on using1D nanomaterials and nanostructures fortheir unique optoelectronic properties. The conventional photoanodes used in QDSSCs are madefrom the mesoporous nanoparticles (NPs) film. When the photogenerated electrons transportedthrough the particle-to-particle hopping, undesirable loss of electrons at grain boundaries viarecombination couldn’t be avoided. And this would lower the performances of QDSSCs.One-dimensional nanostructures, have been considered to be advantageous over the traditionalnanoparticle films for its ability to provide direct and efficient pathways for photogeneratedcarriers and then improve the performances of QDSSCs. However, the performances of most1Dnanostructure based QDSSCs were not as high as expected due to their low surface area, and thenthe low QDs-loading capacity. As we know, the QDs are an important part of the QDSSCs,playing a significant role in absorbing light, generating photo-stimulated carriers.1Dnanostructure based QDSSCs with the low QDs-loading capacity can not compare with theQDSSCs based on mesoporous nanoparticles film. Therefore, enhancing the light absorption should be another effective way to increase the conversion efficiency of1D nanostructure basedQDSSCs.Recently, the noble metal nanoparticles, such as gold, silver and copper, have attracted muchattention, and have been introduced into the photoelectronic devices to enhance the lightharvesting and photocurrent generation through their scattering or localized surface plasmonresonance (LSPR) effects. In one route, light scattering off the plasmonic metal NPs can beemployed to redirect light into the solar cell, which will increase the optical path-length and thenthe absorption of solar cells. In another route, the metal NPs harvest light and the electrons in thevicinity of metal/semiconductor Schottky junction can be excited. And then, the excited electronscan inject to the semiconductor before cooling via tunneling through the Schottky barrier. In fact,such plasmon-induced charge injection can be recognized as one of the sensitization effects in thephotocatalysis and polymer film solar cells to increase the photocatalytic activity and theconversion efficiency. To the best of our knowledge, there are few reports documenting theincorporation of plasmonic materials into the1D nanostructure based QDSSCs.In order to solve the weaker optical absorption of1D nanostructure based QDSSCs due to thelow QDs-loading capacity. In this work, we prepared a good dispersion of gold nanocrystals, andintroduced into the1D TiO2nanorods QDSSCs for the plasmon resonance effect to some extentcould increase the photoelectric conversion efficiency. And we observed that the "hot electron"injected into the semiconductor due to the plasma resonance effect in Au/TiO2composite system. Specific research work carried out in the following three aspects:(1) Synthesis of gold nanocrystals: The monodisperse Au nanocrystals with well-definedquasi-spherical structure were synthesized using the sodium citrate reduction method. Thegold nanocrystals had relatively uniform size with a diameter of about20nm. Furthermore,the gold nanorods with a length of55nm and a width of about20nm were also prepared.using the seed crystal growth method(2) The applications of gold nanocrystals in quantum dot sensitized solar cells: The rutileTiO2nanorod arrays were prepared directly on the F-doped SnO2(FTO) glass substrate byusing a facile hydrothermal growth at180oC. The QDSSCs based on theTiO2/CdS/CdSe/ZnS nanorod arrays photoanode were prepared using the successive ion layer adsorption reaction (SILAR) and chemical bath deposition (CBD). The optimizedTiO2/15CdS/20CdSe/5ZnS photoanode was obtained by adjusting the cycles of the reaction.The conversion efficiency reached to1.66%. When the gold nanocrystals were introducedto the solar cells, the was enhanced by~4.2%from1.66%to1.73%.(3) The photoelectric properties of composite Au/TiO2nanorod arrays: The goldnanocrystals were deposited on TiO2nanorods through the in situ growth method. XPSresults indicated that the electrons can transfer from TiO2to Au through the Au/TiO2interfaces. Under the visible light radiation (≥420nm), a clear photocurrent could beobserved from the I-t curves. The experimental results indicated that the SPR excitedelectrons in the gold NPs could inject to the conduction band of TiO2nanorods efficientlyvia tunneling through the Schottky barrier.