Preparation And Photoelectric Properties Of Hierarchical Porous TiO₂Photoelectrode

With the growing demand for clean and renewable energy, the generation of electricityfrom solar conversion by photovoltaic devices has attracted much attention. Among variousphotovoltaic devices, dye-sensitized solar cells (DSSCs) and quantum-dot-sensitized solar cells(QDSSCs) have been investigated extensively because of their relatively high power conversionefficiency (PCE), ease of fabrication, and low production costs. Porous TiO2films are widelyused in photoanodes of DSSCs and QDSSCs. It serves as a high surface area substrate forsensitizer absorption and a conductor of photo-generated electrons from the sensitizers to theanode. An effective light harvesting and a good charge transport in TiO2photoanode arenecessary for obtaining high PCE for DSSCs and QDSSCs. In this project, hierarchical porousTiO2films that composed of TiO2hollow spheres or Ag/TiO2/rGO composites have been studiedto tailor the light harvesting and the electron transport in the TiO2photoanodes. Some interestingexperimental results are summarized as below:1. TiO2hollow spheres (THS) of400nm in diameter are mixed into TiO2nanoparticle filmas light scattering centers to improve the light harvesting and photovoltaic performance of solarcells based on the CdS quantum-dot sensitized TiO2(TiO2/CdS), N719dye-sensitized TiO2(TiO2/N719) and CdS/N719co-sensitized TiO2(TiO2/CdS/N719) photoanodes, respectively. Thelight harvesting and photovoltaic performance of the three types of solar cells are highlydependent on the THS content in TiO2films due to:(1) the significant light scattering among orwithin THS,(2) the extended pore size distribution, and (3) the modulated electron transport inthe TiO2film. The optimal content of THS is around10%by weight for TiO2/CdS andTiO2/CdS/N719solar cells, and is around15%for TiO2/N719solar cell. It is suggested that, inTiO2/CdS and TiO2/CdS/N719solar cells, high content THS can facilitate the aggregation oflarge CdS nanocrystals, which limits the injection of photo-generated electrons from CdS to TiO2due to serious internal recombination loss, and thus decreases the PCE. The co-sensitized solarcell with the optimal THS content exhibits higher PCE compared to the single-componentsensitized solar cells.2. Highly uniform hierarchical THS with diameters from400nm to950nm were preparedand blended into TiO2nanoparticle film (15%by weight) as light scattering centers in N719dye-sensitized TiO2photoanodes. It is found that the light harvesting and thephotoelectrochemical performance of the TiO2photoanodes are highly dependent on the THSsize. It is suggested that the hierarchical THS significantly enhance the light harvesting by scattering. However, the adding of THS with size above600nm results in a decrease of thephotocurrent, which is attributed to the high charge recombination loss in the loose TiO2structure. In this experiment, the optimal size of THS is around600nm.3. A three-component Ag/TiO2/rGO nanocomposite was used as photoanodic material inN719DSSCs. The Ag nanocrystals exhibited localized surface plasmon resonance (LSPR) effectand increased the light absorption at short-wavelengths(350-600nm), whereas the rGO improvedthe light absorption at long-wavelengths. The photovoltaic performance of the DSSCs dependson the content of Ag nanocrystals. The optimal sample is0.1M Ag/TiO2/rGO which is40%higher than the TiO2/rGO. Higher content of Ag nanocrystals would cause electron trapping andspecular reflection that will limit the injection of photo-generated electrons and light absorptionand thus decrease the PCE of DSSCs.