Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 108

Figure 13 b clearly explains the advantage of tree-like photoanodes compared to
conventional nanowire electrodes. From Fig. 13 b the photocurrent of the DSSC
has monotonically increased by extending the length of branches in the electrode.
The similar photocurrent enhancement in hierarchical 3-D tree-like photoanodes
has been widely found in the DSSCs literature [ 120 - 122 ]. The efficiency
enhancement of 3-D tree-like electrodes is mainly due to enhanced surface area
enabling higher sensitizer loading and light harvesting and also is due to reduced
charge recombination by direct conduction along the stem of the nano-tree [ 123 ].
The hierarchical branches in tree-like electrodes has been grown either by seed
layer-assisted method, [ 124 - 126 ] or self-catalyst-based mechanism [ 127 ]. The
stem of the electrode (back bone) may be nanowire or nanotube. However, the
overall photoconversion efficiency of the 3D tree-like photoanode strongly
depends on their growth mechanism. Wu et al. compared the performance of
hierarchical TiO 2 nanowire array photoanodes (hydrothermally grown without
seed layer) with P25 nanoparticle under similar dye uptaking condition [ 127 ]. The
J sc and V oc of P25-based DSSC is 12.0 mA cm -2 and 794 mV, respectively, which
is much lower than those of 3hr grown hierarchically TiO 2 nanowire arrays
(13.9 mA cm -2 and 826 mV). By increasing the branch length through increasing
hydrothermal duration time to 9 h, the efficiency of the device promotes to 7.34 %.
Further extending nanowire branch growth duration to *12 h the conversion
efficiency found to be decreasing to 6.35 %, which may be ascribed to the for-
mation of more recombination pathways at nanowire/electrolyte interfaces which
lower the conversion efficiency of the device. Interestingly, diffusion coefficient of
iodine redox shuttle is *2 orders higher in hierarchical nanoforest electrodes (D
(I 3 - ) = 3.3 9 10 -8 cm 2 /s) than NP electrode (D (I 3 - )=5.4 9 10 -10 cm 2 /s) [ 128 ].
It is understood that the enhanced mass transport of the redox shuttle anticipated to
result high V oc in the device and is realized in many tree-like anode-based DSSCs.
Recently, we demonstrated the QDSSCs with high open-circuit voltage as high
as 0.77 V in ZnO nanowires array electrodes [ 70 ]. The performance of the cell can
even be increased to a promising 3 %, using a novel photoanode architecture of
''pine tree'' ZnO nanorods (NRs) on Si NWs hierarchical branched structure. The
different stages of fabricating hierarchical ZnO nanorods on Si backbone nano-
wires are schematically shown in Fig. 14 .
The typical experimental procedure of pine tree-like Si/ZnO electrodes is as
follows: in first stage, the backbone Si nanowires were fabricated via the vapor-
liquid-solid (VLS) process on the Au nanoparticle decorated FTO glass by using a
chemical vapor deposition (CVD) system. By introducing a 10 % silane (SiH 4 )in
Hydrogen (H 2 ) mixture in a tube furnace, Si nanowires were synthesized under the
reactor temperature and pressure were kept at 480-520 C and 40 torr for 3 min.
This Si NW thickness is assumed as total thickness of the photoanode. The n-type
doping of Si NWs was achieved by using PH 3 gaseous precursor as in situ doping
sources. A more detailed description of the Si NWs growth conditions and their
electrical characteristics can be found elsewhere [ 70 ]. At second stage, ZnO
hierarchical rods were grown on Si NWs. The 25 nm thick ZnO film, as a seed
layer, was deposited on FTO glass by using magnetron sputtering. The sample was