Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 109

Fig. 14 a Fabrication scheme of Si/ZnO hierarchical structure b SEM image of n-Si-NW/ZnO-NR,
and c schematic Si/ZnO hierarchical structure for photoanode d HRTEM images of ZnO seed-
coated Si Nanowire, and e and f their corresponding high magnification TEM images. (Sudhagar
et al. [ 70 ] Reprint permission from ACS Publishers)
transferred to a beaker containing 25 mM zinc nitrate and 25 mM methenamine in
water for the growth of ZnO NWs and was kept in the oven at 85 C. After 12 h,
the electrodes were removed from the beaker and rinsed with water then dried by
N 2 . The ZnO NWs present an average length and diameter of 2.2-2.5 lm and
100-150 nm, respectively. Figure 14 d, e shows the TEM images of bare and ZnO
seed particle coated Si nanowire, respectively.
Figure 15 a clearly depicted that the branched ZnO NRs directly grown on the
Si-NWs forming a pine tree structure. HR-TEM image (Fig. 15 b) at the top of ZnO
NRs reveals that the ZnO NRs are single crystalline and were grown along the [001]
direction. The porosity of the electrode has been maintained even after QDs coating
(Fig. 15 c), which is beneficial for effective electrolyte penetration through entire
device. The optical reflectance property of ZnO-NR and 3-D n-Si-NW/ZnO-NR
HBS has been studied by diffuse reflectance (Fig. 15 e). Both sensitized electrodes
present a decrease in the reflectance for the wavelength shorter than 620 nm due to
light absorption of CdS/CdSe sensitizer. Analyzing the reflectance values at longer
wavelength, before the absorption threshold, the improved scattering properties of
the pine tree HBS, in comparison with ZnO NWs, are manifested in the higher
reflection values at these wavelengths. The IPCE presents (Fig. 15 f) a similar
threshold to reflectance. It is important to highlight that no significant IPCE is
observed at wavelengths longer than 700 nm, indicating a negligible contribution
of the Si-NWs to QDSC performance. This fact also suggests that the light absorbed
by Si-NWs is negligible and consequently the differences observed in reflectance
values at longer wavelength in (Fig. 15 e) are in fact due to the scattering properties.
The enhanced light scattering is also responsible to the higher IPCE obtained
for pine tree HBS in the wavelength range of 500-650 nm. By sensitization with
CdS/CdSe QDs, the devices yielded an overall efficiency of *3 %, which was
higher than that of ZnO nanowire arrays due to increased QDs loading and
enhanced light scattering by the 3D geometry. It was also noted that the doping in