332
S. Peiris et al.
More recently Marinakis and coworkers have investigated performance and
long-term stability of p-DSCs with a cycloruthenated dye through electrolyte solvent tuning. Fabrication of p-DSC using NiO as the semiconductor material along
with the zwitterionic cyclometalated ruthenium dye [Ru(bpy)2(H1)](H31¼(4-(2phenylpyridin-4-yl)phenyl)phosphonic acid and I 3
− /I
− /acetonitrile (AN) electrolyte,
produced a device with V OC = 95 mV, J SC = 4.06 mA cm
−2 ; and η = 0.139%. With
the use of propionitrile (PN), valeronitrile (VN) and 3-methoxypropionitrile (MPN)
as electrolyte solvents, the trend of J SC was observed as AN > PN > MPN > VN >
NMP and V OC follows the trend VN > PN > MPN > AN > NMP. This means there
is an effect created by the electrolyte solvent on the efficiency of a DSC (Marinakis
et al. 2017).
Effort to enhance the photoconversion efficiency of p-DSCs is mainly directed
towards improving performance of individual components. Currently, numerous
researches based on novel p-type semiconductor materials, sensitizers and electrolytes are underway. Though NiO is the primarily used semiconductor material
in p-DSCs, there are few drawbacks that have been encountered when it is utilized.
The VB of NiO lies at +0.50 V versus the NHE and if the typically used I
− /I 3
−
redox mediator (+0.35 V vs. NHE) is used as the electrolyte, the V OC will be limited
to about 150 mV leading to a poor overall performance of the device (Zhang et al.
2016). On the other hand, NiO is a dark coloured compound which absorbs a significant amount of solar radiation. It has been reported that nearly 30–40% of incident
photons are absorbed by a NiO film of thickness 2.3 μm (Yu et al. 2012). Furthermore, fast recombination reactions between the reduced sensitizers and the holes in
NiO, cause a significant loss in the number of holes injected into the semiconductor
(Ji et al. 2013), since the electronic conductivity of porous NiO is relatively low
compared to TiO 2 . The magnitude of hole diffusion coefficient in NiO (10
−8 to 10
−7
cm
2 s
−1 ) is three orders lower than that of the electron diffusion in TiO 2 . As a result,
the cathodic current is slowed down due to the delay of injection of holes and their
collection back into the electrolyte. NiO also possesses lower dielectric constant
and poor chromophore surface loading (Odobel and Pellegrin 2013). Scientists seek
novel approaches to overcome these issues related with NiO and on the other hand,
introduce different types of semiconductor material that can replace it. Flynn et al.
have taken attempts to control the morphology and microstructure of NiO and bring
about a methodology to synthesise high quality NiO hexagonal plates, which are
ultra-thin and composed of a high density of nanoscale pores. They have observed
more than a 10-fold improvement in the electrical mobility compared to conventional
nanoparticles. Further, they revealed an increment of 20–30% of the performance
of p-DSCs fabricated using these NiO plates. Although it is commonly suggested
in literature that the black colour of NiO is due to mixed valence Ni
2+ /Ni
3+ , a study
conducted by Adèle Renaud and coworkers show that it is due to the presence of
Ni
0 and progressive oxidation could completely change the colour to pale green, and
also create larger particle sizes that could lower the efficiency. According to their
findings, the presence of elemental Ni does not seem to significantly affect the photovoltaic performances of p-DSCs (Renaud et al. 2013b). Zannotti et al. have tried
using Ni/Mg mixed metal oxides for p-DSCs. When the concentration of MgO is
S. Peiris et al.
More recently Marinakis and coworkers have investigated performance and
long-term stability of p-DSCs with a cycloruthenated dye through electrolyte solvent tuning. Fabrication of p-DSC using NiO as the semiconductor material along
with the zwitterionic cyclometalated ruthenium dye [Ru(bpy)2(H1)](H31¼(4-(2phenylpyridin-4-yl)phenyl)phosphonic acid and I 3
− /I
− /acetonitrile (AN) electrolyte,
produced a device with V OC = 95 mV, J SC = 4.06 mA cm
−2 ; and η = 0.139%. With
the use of propionitrile (PN), valeronitrile (VN) and 3-methoxypropionitrile (MPN)
as electrolyte solvents, the trend of J SC was observed as AN > PN > MPN > VN >
NMP and V OC follows the trend VN > PN > MPN > AN > NMP. This means there
is an effect created by the electrolyte solvent on the efficiency of a DSC (Marinakis
et al. 2017).
Effort to enhance the photoconversion efficiency of p-DSCs is mainly directed
towards improving performance of individual components. Currently, numerous
researches based on novel p-type semiconductor materials, sensitizers and electrolytes are underway. Though NiO is the primarily used semiconductor material
in p-DSCs, there are few drawbacks that have been encountered when it is utilized.
The VB of NiO lies at +0.50 V versus the NHE and if the typically used I
− /I 3
−
redox mediator (+0.35 V vs. NHE) is used as the electrolyte, the V OC will be limited
to about 150 mV leading to a poor overall performance of the device (Zhang et al.
2016). On the other hand, NiO is a dark coloured compound which absorbs a significant amount of solar radiation. It has been reported that nearly 30–40% of incident
photons are absorbed by a NiO film of thickness 2.3 μm (Yu et al. 2012). Furthermore, fast recombination reactions between the reduced sensitizers and the holes in
NiO, cause a significant loss in the number of holes injected into the semiconductor
(Ji et al. 2013), since the electronic conductivity of porous NiO is relatively low
compared to TiO 2 . The magnitude of hole diffusion coefficient in NiO (10
−8 to 10
−7
cm
2 s
−1 ) is three orders lower than that of the electron diffusion in TiO 2 . As a result,
the cathodic current is slowed down due to the delay of injection of holes and their
collection back into the electrolyte. NiO also possesses lower dielectric constant
and poor chromophore surface loading (Odobel and Pellegrin 2013). Scientists seek
novel approaches to overcome these issues related with NiO and on the other hand,
introduce different types of semiconductor material that can replace it. Flynn et al.
have taken attempts to control the morphology and microstructure of NiO and bring
about a methodology to synthesise high quality NiO hexagonal plates, which are
ultra-thin and composed of a high density of nanoscale pores. They have observed
more than a 10-fold improvement in the electrical mobility compared to conventional
nanoparticles. Further, they revealed an increment of 20–30% of the performance
of p-DSCs fabricated using these NiO plates. Although it is commonly suggested
in literature that the black colour of NiO is due to mixed valence Ni
2+ /Ni
3+ , a study
conducted by Adèle Renaud and coworkers show that it is due to the presence of
Ni
0 and progressive oxidation could completely change the colour to pale green, and
also create larger particle sizes that could lower the efficiency. According to their
findings, the presence of elemental Ni does not seem to significantly affect the photovoltaic performances of p-DSCs (Renaud et al. 2013b). Zannotti et al. have tried
using Ni/Mg mixed metal oxides for p-DSCs. When the concentration of MgO is