Enhanced Solar Light Harvesting with Q-CdTe/Se Sensitized Inverse Opal TiO₂

This study reports enhanced light harvesting in an ultrathin Q-CdTe/Se quantum dot layer sensitized TiO2 photonic crystal that is enabled by light trapping which increases light absorbance in the photonic crystal structure. We fabricated TiO2 inverse opals (i-TiO2-o) with stop bands centered at ca. 550 nm (550-i-TiO2-o) sensitized with 2–4 nm Q-CdTe with absorption edges tuned to the red, within, and to the blue of the stop band, followed by surface modification with Se. The photoelectrode behavior was examined in the presence of an alkaline selenide electrolyte which enhances stability and efficiency, relative to the usual sulfide electrolyte. Modification with Se resulted in type-II Q-CdTe/CdSe characteristics of enhanced red-shifted absorbance and quenched emission, and a major increase in the light-to-current conversion was measured on both nc-TiO2 and i-TiO2-o after annealing attributed to improved charge separation across the Q-CdTe/CdSe interface. A 4-fold amplification was measured on average for i-TiO2-o sensitized with green-emitting 2.3 nm Q-CdTe to both the blue and red of the stop band relative to similarly sensitized nonscattering nc-TiO2, before and after Se-modification, which is attributed to light trapping amplifying absorbance. The gain is shown to depend on the absorption edge relative to the stop band; a smaller gain was measured for i-TiO2-o sensitized with 3 nm yellow-emitting Q-CdTe with an absorbance edge within the stop band. An enhancement by 4–5 fold in the overall visible photon-to-current-conversion efficiency resulted at i-TiO2-o sensitized with green-Q-CdTe/CdSe relative to similarly modified nc-TiO2: about 2-fold is attributed to the type-II structure and 2-fold to light trapping in the photonic crystal enhancing light absorbance by the layer of quantum dots.