17 Conducting Polymers as Cost Effective Counter Electrode …
349
valence band and conduction band with a band gap having energy in the range of UV
region in the electromagnetic spectrum, hence a photon with energy that matches the
band gap can cause the excitation of an electron leaving behind a hole in the valence
band. These charges are been migrated to the external circuit to provide electrical
current. TiO 2 is the semiconductor material most commonly used and it is been used
either in the form of rutile or anatase or a mixed composition of both these natural
forms. The morphology and network geometry has a significant impact on the efficiency and photoelectrochemical performance of DSCs. A multilayered structure of
mesoporous network of TiO 2 has yielded high cell efficiency (Wang et al. 2004).
Highly ordered nano structure of TiO 2 such as network structure of TiO 2 crystal like
nanowires (Adachi et al. 2004), structure consisting of submicrometer-sized mesoporous TiO 2 beads (Chen et al. 2009) and hollow TiO 2 hemisphere films deposited
on substrate material (Yang et al. 2008) have been developed with high photovoltaic
performances. In addition plasma enhanced polymerized aniline/TiO 2 DSCs with
favourable photovoltaic performances have been developed and investigated (Ameen
et al. 2009).
There are specific criteria to be satisfied by the dye sensitizer material. Most
importantly the LUMO level of the sensitizer material must be closer (about 150 mV)
in energy and must match the conduction band edge of the semiconductor in order
to decrease energetic potential losses associated with the transfer of electrons. The
HOMO level of the sensitizer material should be at a sufficient low level to enable
acceptance of electrons from an electrolyte or hole conducting material. Additionally
it needs to absorb all incident light below the near IR wavelength. The sensitizer
material should be composed of carboxylate/phosphate group for anchorage onto the
semiconductor oxide surface. Furthermore the sensitizer material needs to possess
the endurance to withstand multiple turnovers and prolonged exposure to sunlight
without degradation (Grätzel 2001). Initially ruthenium(II) based dyes such as the
N3 dye (Nazeeruddin et al. 1993) and the black dye (Nazeeruddin et al. 1997) had
been commonly used as the sensitizer material with considerable efficiency but their
usage had been restricted due to the high cost and limited availability of Ru metal.
Afterwards metal free organic dyes had been used (Mishra et al. 2009) and currently
the use of natural dyes (Zhou et al. 2011) with structure modification is investigated as
a substitute material for the sensitizer material with considerable efficiencies (Gong
et al. 2012).
The electrolyte which is responsible for the electron collection at the CE and electron transportation back to the oxidized dye molecule needs to satisfy specific criteria
such as rapid electron transfer kinetics, sufficient driving force to regenerate the dye,
low light absorption and ability to regenerate at minimum overpotential at the CE.
The most commonly used electrolyte is the iodide/triiodide (Boschloo et al. 2009)
redox couple in an organic matrix usually acetonitrile. Alternative redox couples
such a Br
− /Br
−
3 , SCN
− /(SCN) 2 , SeCN
− /(SeCN)
−
3 , [Fe(CN) 6 ]
3−/4− , Co(II)/Co(III)
(Daeneke et al. 2012), [Mn(acac) 3 ]
0/1+ (Perera et al. 2014) and disulfide/thiolate
are used (Wang et al. 2010). Although liquid electrolytes as mentioned above offer
favourable kinetics there are challenges in its usage in terms of long term durability
349
valence band and conduction band with a band gap having energy in the range of UV
region in the electromagnetic spectrum, hence a photon with energy that matches the
band gap can cause the excitation of an electron leaving behind a hole in the valence
band. These charges are been migrated to the external circuit to provide electrical
current. TiO 2 is the semiconductor material most commonly used and it is been used
either in the form of rutile or anatase or a mixed composition of both these natural
forms. The morphology and network geometry has a significant impact on the efficiency and photoelectrochemical performance of DSCs. A multilayered structure of
mesoporous network of TiO 2 has yielded high cell efficiency (Wang et al. 2004).
Highly ordered nano structure of TiO 2 such as network structure of TiO 2 crystal like
nanowires (Adachi et al. 2004), structure consisting of submicrometer-sized mesoporous TiO 2 beads (Chen et al. 2009) and hollow TiO 2 hemisphere films deposited
on substrate material (Yang et al. 2008) have been developed with high photovoltaic
performances. In addition plasma enhanced polymerized aniline/TiO 2 DSCs with
favourable photovoltaic performances have been developed and investigated (Ameen
et al. 2009).
There are specific criteria to be satisfied by the dye sensitizer material. Most
importantly the LUMO level of the sensitizer material must be closer (about 150 mV)
in energy and must match the conduction band edge of the semiconductor in order
to decrease energetic potential losses associated with the transfer of electrons. The
HOMO level of the sensitizer material should be at a sufficient low level to enable
acceptance of electrons from an electrolyte or hole conducting material. Additionally
it needs to absorb all incident light below the near IR wavelength. The sensitizer
material should be composed of carboxylate/phosphate group for anchorage onto the
semiconductor oxide surface. Furthermore the sensitizer material needs to possess
the endurance to withstand multiple turnovers and prolonged exposure to sunlight
without degradation (Grätzel 2001). Initially ruthenium(II) based dyes such as the
N3 dye (Nazeeruddin et al. 1993) and the black dye (Nazeeruddin et al. 1997) had
been commonly used as the sensitizer material with considerable efficiency but their
usage had been restricted due to the high cost and limited availability of Ru metal.
Afterwards metal free organic dyes had been used (Mishra et al. 2009) and currently
the use of natural dyes (Zhou et al. 2011) with structure modification is investigated as
a substitute material for the sensitizer material with considerable efficiencies (Gong
et al. 2012).
The electrolyte which is responsible for the electron collection at the CE and electron transportation back to the oxidized dye molecule needs to satisfy specific criteria
such as rapid electron transfer kinetics, sufficient driving force to regenerate the dye,
low light absorption and ability to regenerate at minimum overpotential at the CE.
The most commonly used electrolyte is the iodide/triiodide (Boschloo et al. 2009)
redox couple in an organic matrix usually acetonitrile. Alternative redox couples
such a Br
− /Br
−
3 , SCN
− /(SCN) 2 , SeCN
− /(SeCN)
−
3 , [Fe(CN) 6 ]
3−/4− , Co(II)/Co(III)
(Daeneke et al. 2012), [Mn(acac) 3 ]
0/1+ (Perera et al. 2014) and disulfide/thiolate
are used (Wang et al. 2010). Although liquid electrolytes as mentioned above offer
favourable kinetics there are challenges in its usage in terms of long term durability