Enhanced Conversion of Light at TiO₂ Photonic Crystals to the Blue of a Stop Band and at TiO₂ Random Films Sensitized with Q-CdS: Order and Disorder

Abstract

Significant enhancements in the conversion of light to current were observed at the blue edge of TiO2 inverse opals (i-TiO2-o) and at highly disordered TiO2 films (i-TiO2-d) sensitized with Q-CdS in sulfide electrolyte. i-TiO2-o with stop bands centered at 390 and 450 nm were modified with mercaptopropionic acid−Q-CdS with absorption edges tuned to the red or to the blue of the stop-band edges. A 4.7 average enhancement factor was measured at the blue edge of the stop band when it coincided with low Q-CdS absorption, while a 1.4−1.8 average gain was measured at the red edge. The blue-edge gain can be ascribed to localized or slowed light in the low refractive index medium and was found to extend 30−70 nm to the blue of the stop-band center. Light localization effects were suppressed when the stop-band edges overlapped with appreciable absorption. A highly disordered TiO2 film fabricated by replicating a template from 150, 190, and 243 nm diameter polystyrene spheres exhibited a similar gain per adsorbed Q-CdS in the same spectral window as the blue edge of the photonic crystal when quantum dot absorption was low. This gain is ascribed to slowed light resulting from the interference of multiple internal scattering events in the disordered medium.