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Peachlike rutile TiO 2 microsphere films were successfully produced on transparent conducting fluorinedoped tin oxide substrate via a facile, one-pot chemical bath route at low temperature (T = 80−85°C) by introducing polyethylene glycol (PEG) as steric dispersant. The formation of TiO 2 microspheres composed of nanoneedles was attributed to the acidic medium for the growth of 1D needle-shaped building blocks where the steric interaction of PEG reduced the aggregation of TiO 2 nanoneedles and the Ostwald ripening process. Dye-sensitized solar cells (DSSCs) assembled by employing these complex rutile TiO 2 microspheres as photoanodes exhibited a light-to-electricity conversion efficiency of 2.55%. It was further improved to a considerably high efficiency of 5.25% upon a series of post-treatments (i.e., calcination, TiCl 4 treatment, and O 2 plasma exposure) as a direct consequence of the well-crystallized TiO 2 for fast electron transport, the enhanced capacity of dye loading, the effective light scattering, and trapping from microstructures.

Upconversion/semiconductor submicron hollow spheres composed of inner Na x GdF y O z :Yb/Er shell and outer TiO 2 shell (denoted Na x GdF y O z :Yb/Er@TiO 2 ) were, for the first time, crafted by exploiting colloidal carbon spheres as the scaffold. The hollow spheres were then incorporated into the TiO 2 nanoparticle film photoanode to yield dye-sensitized solar cells (DSSCs) with improved performance. The implementation of Na x GdF y O z :Yb/Er@TiO 2 hollow spheres in DSSCs imparted the light trapping due to the light scattering from submicron hollow spheres, and the harvesting of near infrared solar photons by the upconversion material (i.e., dual functionalities), thereby resulting in an increased short-circuit current density Jsc, and thus an improved power conversion efficiency PCE. The electrochemical impedance spectroscopy measurements were performed to scrutinize the interfacial charge transfer characteristics of DSSCs. The measurements 2 revealed that when Na x GdF y O z :Yb/Er hollow spheres without the deposition of TiO 2 shell were integrated in the photoanode, a high charge transfer resistance was found. In stark contrast, the judicious decoration of Na x GdF y O z :Yb/Er hollow spheres with a thin layer of TiO 2 shell markedly improved the contact between the resulting Na x GdF y O z :Yb/Er@TiO 2 shell/shell hollow spheres and the TiO 2 nanoparticle film photoanode, leading to a much decreased charge transfer resistance. Taken together, compared to the PCE of 6.81% for the pristine device, the DSSC assembled with the introduction of 8 wt% Na x GdF y O z :Yb/Er@TiO 2 hollow spheres in the photoanode exhibited an optimal PCE of 7.58% and a maximum short-circuit current density J sc of 18.72 mA/cm 2 under AM 1.5G one sun illumination, corresponding to 11.31% performance enhancement. As such, the implementation of upconversion submicron hollow materials in photoanode may stand out as an intriguing strategy to improve the device performance of DSSCs. Figure 2. Digital images of (a) the red-emitting Gd 2 O 3 :Yb(20%)/Er(2%) and (b) the green emitting Na x GdF y O z :Yb(20%)/Er(2%) under the 980-nm near infrared (NIR) irradiation. (c)The energy level diagram for the upconversion emission from Na x GdF y O z :Yb/Er under the 980n-m NIR irradiation. (d) The emission spectra of Gd 2 O 3 :Yb(20%)/Er(2%) and Na x GdF y O z :Yb(20%)/Er(2%) exposed to a 980-nm NIR diode laser operating at 2 W. 13 of semiconductor-decorated upconversion hollow spheres on the device performance. In order to reduce the contact resistance between the Na x GdF y O z :Yb/Er hollow spheres and the TiO 2 nanoparticle film, a layer of amorphous TiO 2 was coated on the surface of Na x GdF y O z :Yb/Er hollow spheres to produce the shell/shell hollow spherical materials. It is worth noting that the resulting Na x GdF y O z :Yb/Er@TiO 2 shell/shell hollow spheres still exhibited the green emission. The elemental mapping of Na x GdF y O z :Yb/Er@TiO 2 hollow spheres revealed that all elements were uniformly distributed in the material, includ...