Dye/Semiconductor-Sensitized Solar Cells Based On TiO₂Nanowire Array

1D TiO₂nanostructures have been extensively studied in solar energy conversionfield, for the excellent photogenerated charge separation and transport properties andgood chemical stability of TiO₂nanostructures. Especially,1D single-crystalline TiO₂nanowire arrays as one of the most potential photoanode materials have attractedimmense scientific research interests, focusing on the preparation and solar energyconversion application of TiO₂nanowire arrays. In this thesis, concentrated on theTiO₂nanowire array based dye/semiconductor-sensitized solar cells, we carried outour research on the controllable synthesis of TiO₂nanowires, facilitatinglight-harvesting and charge separation employing nanowire-nanoparticle compositestructure, enhancing light-harvesting via disordered hollow scattering layer and theall-solid-state semiconductor-sensitized solar cells, searching for approaches inachieving highly efficient TiO₂nanowire based solar cells. Details are listed asfollows:By using titanate nanosheet （TN） film as a seed layer,1D single-crystalline rutileTiO₂nanowire array has been successfully prepared on SnO₂:F （FTO） conductiveglass and other solid substrates via a facile hydrothermal process, solving the problemconcerning the growth dependence of rutile nanowires on the FTO substrate. Detailedstudy revealed that the TN film can suppress the aggragation of nanowires duringgrowth period, thus increasing the dye-loading amount of nanowire arrays; In addition,the TN film can also form a blocking layer which covers the bare FTO surfaceunoccupied by rutile nanowires, suppressing the charge recombination atFTO/electrolyte interface. Under AM1.5G simulated solar irradiation, rutilenanowires grown on a TN film performed better in DSSCs than those on bare FTOconductive glass in terms of all cell parameters including short-circuit current,open-circuit voltage and fill factor, thus giving an improvement of energy conversionefficiency of about3.4times compared with the one without the TN film.In order to enhance the light harvesting efficiency, rutile TiO₂nanowires （NWs）array-anatase nanoparticles （NPs） photoanode system is chosen and fabricated via asimple two-step synthesis process. As an example, a dye-sensitized TiO₂NW-NPcomposite photoanode, only1.4μm thick, exhibits a solar energy conversionefficiency of3.8%under AM1.5G simulated solar irradiation, which is2.2times and 1.5times higher than that of the NW array and NP photoanodes, respectively.Kinetics study in terms of stationary/transient obsorption and open-circuit decaytechnique, demonstrated the synergistic effect of rutile NWs and anatase NPs forphotoelectrochemical solar energy conversion. On one hand, the introduce of anatasenanoparticles improved the light harvesting efficiency of composite structure; On theother hand, the presence of a rutile-anatase heterojunction at the interface reducedcharge recombination at both the NP/electrolyte and NP/dye interfaces, thusimproving the charge collecting efficiency of composite structure.On the purpose of further increasing the light harvesting property of TiO₂NW-NP photoanode, disordered hollow structure （pore size²00nm,450nm） isintroduced as TiO₂nanoparticle scattering layer. Under AM1.5G simulated solarirradiation, energy conversion efficiency of3.47%（pore size²00nm） and4.07%（pore size⁴50nm） are achieved, compared with2.62%of DSSC without the hollowstructure. Optical transmittance and reflectance spectrum and incidentphoton-to-current conversion efficiency study revealed the effective light-scatteringproperties of hollow structures in TiO₂nanoparticle scattering layer, especially overthe wavelength range of580-700nm, thus improved the light harvesting efficiency ofTiO₂NW-NP photoanode. Hence, the photocurrent and overall energy conversionefficiency for the DSSC based on the light-scattering layer is remarkably improved.In order to overcome the inefficient light harvesting problem of monolayereddye-sensitized solar cells, and simultaneously avoiding the solvent leakage andvolatility of liquid electrolyte, all solid-state solar cell device of TiO₂NWs/Sb2S3/CuI/Au was prepared based on the as-synthesized TiO₂NWs, employingdirect bandgap semiconductor Sb2S3as sensitizer, and p-type CuI as hole-transportingmaterial. The device exhibits a solar energy conversion efficiency of1.2%under AM1.5G simulated solar irradiation. The IPCE measurement indicates that thephotocurrent onset is at750nm, which is consistant with the bandgap of Sb2S3, andthe peak efficiency reached64%, which is superior to dye-sensitized cells. But due tothe serious interfacial charge recombination process in solid-statesemiconductor-sensitized solar cells, the dark current of the device is obviously higherthan dye-sensitized solar cells, which lead to lower device open-circuit voltage （0.3V）and fill factor （42%） and finally poor device performance. Therefore, suppressinginterfacial recombination by interfacial engineering is the keypoint for improving theperformance of solid-state semiconductor-sensitized solar cells.