22-05-2012, 05:20 PM
A dye-sensitized nano-porous I solid-state photovoltaic cell
A dye-sensitized nano-porous I solid-state photovoltaic cell.pdf (Size: 401.98 KB / Downloads: 30)
introductian
The fascinating phenomenon of dye-sensitization (DSN)
of the semiconductor surface (injection of caniers into
the bands by photoexcited dye molecules adsorbed at the
surface) was first observed more t_han a centuq ago and
continues to arouse the interest of physicists and chemists.
In 1873 Vogel 111 made the important discovery that dyecoated
silver halide grains sensitize the photographic film
to the absorption spectrum of the dye. The mechanism of
DSN was explained by Mott and Gurney [Z] as originating
from transfer of an electron by the excited dye molecule
to the conduction band of the silver halide. Subsequent
studies [MI confmned that anodic (cathodic) sensitization
occurs when excited dye molecules adsorbed at the surface
of the semiconductor inject electrons (holes) into the
conduction (valence) band and the type of sensitization
(anodic or cathodic) depends on the band positions of the
semiconductor and the energy levels of the dye.
Experiment
Nano-porous layers of Ti02 were coated on fluorine-doped
conducting tin oxide (CTO) glass (1.5 cm x 2 cm, sheet
resistance about 40 s2 0 - I ) by the following method.
Titanium isopropoxide (1 ml) and glacial acetic acid
(5 E!) ze add4 to isopiqi;ano! (I5 GI) afid 5 iii! wa:a
is added drop by drop to the mixture which is kept
stirred. Hydrolysis of titanium isopropoxide produces fine
crystallites of Ti02 and the above procedure prevents their
agglomeration. The CTO glass plate was placed on the
surface of a hot plate (surface temperature about 125'C)
and the solution is evenly spread using a glass rod and
allowed to dry. The plate is then sintered at 450°C for
20 min and the process is repeated until a fully covered
semitransparent film has been deposited.
Results and discussion
The construction of the nano-porous n-TiOz/cyanidin/CuI
photovoltaic cell is shown in figure l(a). The schematic
diagram of figure l(b) illustrates the mechanism of
photogeneration of the carriers. A dye molecule excited
from the ground level SO to S* injects an electron into the
conduction band of TiO2. The unoccupied ground level
is filled by transfer of an electron from the valence band
of CUI. (Alternatively, a hole is injected into the valence
band.) Figure 2 shows the photocurrent action spectrum
of the cell which is peaked at about 571 nm. (The peak
at about 360 nm in figure 2 originates from absorption in
TiOz.) The absorption spectrum of an aqueous solution of
cyanidin is peaked at about 530 nm (figure 3 curve (a)),
whereas in a cyanidin solution mixed with Ti02 sol, the
absorption peak is at about 572 nm (figure 3, curve (b))
which is almost the same as the peak spectral response
of the cell.
Conclusion
We believe that the efficiency of the cell can be increased
by improving the CUI coating technique and finding other
sensitizers. The present coating method undoubtedly
leaves some interctystallite pores of the Ti02 film unfilled.
Prevention of the formation of voids and parallel shortcircuiting
will enhance the short-circuit photocurrent, open
circuit voltage and long-term stability. A suitable coating
on the outer glass surface would readily filter out W
radiation-that would degrade the dye and the cell can be
protected from moisture by an effective sealing. Although
cyanidin-type pigments are relatively unstable compounds,
the Ti4+-cyanidin complex on the surface of 'IiOz is highly
stable, although the stability may not be sufficient for a
practical device.