Enhanced light harvesting of CdSe quantum dot sensitized bilayered ZnO nanostar/TiO2 nanotubes

Abstract

Bilayered ZnO nanostar/TiO2 nanotubes (TNTs) were developed by electrochemical anodization, followed by the hydrothermal method. A ZnO nanostar film composed of ZnO nanorods (NRs) was steadily changed depending on the hydrothermal time (1, 2, and 6 h). In the case of ZnO(1 h)/TNTs, the nanostar-shaped ZnO film with a length of 1.2–1.8 μm and a width of 40–60 nm consisting of conventional NRs was formed. On the other hand, with increasing hydrothermal time, a thicker pyramidal-shaped ZnO NRs film was formed with a length of 2.8–3.4 μm and a width of 50–80 nm resulting from a limited mass transport of the reacting precursor. Then, it was remarkable that ZnO NRs formed omnidirectional nanostar particles without any directionality. On the basis of this architecture, the photoelectrochemical properties were assessed using CdSe quantum dots (QDs) as photosensitizers. The ZnO/TNTs film showed enhanced light absorption due to the high deposition of CdSe QDs resulting from the increase of specific surface area. In particular, the extent of light absorption was steadily increased with the hydrothermal time of ZnO, indirectly demonstrating that longer hydrothermal time can make a thicker ZnO layer on the TNTs. Also, it was confirmed that the light scattering effect from micrometer-sized ZnO particles was achieved to improve light absorption in the long wavelength range (600–670 nm). This point was again confirm by the finding that the wavelength showing the maximum efficiency in the incident photon to current conversion efficiency (IPCE) was shifted from 555 nm to 585 nm (2 h synthesis) and 590 nm (6 h synthesis), which is indicative of the light scattering effect from ZnO particles on CdSe QD-sensitized solar cells (QDSSCs). Accordingly, the dual effects (specific surface area plus light scattering effect) gave rise to the abrupt increase of short-circuit photocurrent (Jsc) in bilayered ZnO/TNTs based CdSe QDSSCs in the photocurrent–voltage (J–V) characteristics. In particular, the ZnO(2h)/TNTs electrode exhibited the best enhancement (0.94%), showing conversion efficiency with a Jsc of 6.13 mA/cm2 and a moderately increased open-circuit voltage (Voc) attributed from the upper ZnO layer in the total film (ZnO + TNTs), compared to that (0.49%) of TNTs-based QDSSCs.