Dual-functional semiconductor-decorated upconversion hollow spheres for high efficiency dye-sensitized solar cells

Abstract

Upconversion/semiconductor submicron hollow spheres composed of inner Na_xGdF_yO_z:Yb/Er shell and outer TiO₂ shell (denoted Na_xGdF_yO_z:Yb/Er@TiO₂) were, for the first time, crafted by exploiting colloidal carbon spheres as the scaffold. The hollow spheres were then incorporated into the TiO₂ nanoparticle film photoanode to yield dye-sensitized solar cells (DSSCs) with improved performance. The implementation of Na_xGdF_yO_z:Yb/Er@TiO₂ 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 J_sc, 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 revealed that when Na_xGdF_yO_z:Yb/Er hollow spheres without the deposition of TiO₂ shell were integrated in the photoanode, a high charge transfer resistance was found. In stark contrast, the judicious decoration of Na_xGdF_yO_z:Yb/Er hollow spheres with a thin layer of TiO₂ shell markedly improved the contact between the resulting Na_xGdF_yO_z:Yb/Er@TiO₂ shell/shell hollow spheres and the TiO₂ 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_xGdF_yO_z:Yb/Er@TiO₂ 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.