14 Photoenergy Conversion (Dye-Sensitized Solar Cells)
473
(more negative) than the redox potential of the I 3
− /I
− ion couple (0.4 V vs. NHE),
in order to achieve efficient dye regeneration by electron transfer from the reduced
dye to the I 3
− ion in the redox mediator. An energy gap of over 0.5 V is necessary
for efficient regeneration, according to a few previous reports. Clearly, the LUMO
energy level must be sufficiently lower than the CB energy level of the NiO electrode
(E cb , −3.0 V vs. NHE). However, there are also electron-loss processes, which lower
the photovoltaic performances of DSSCs: recombination of the holes injected into
the VB of the NiO electrode with the reduced dyes (geminate recombination) or with
the I
− ions at the NiO surface (dark current). The geminate recombination process
almost always competes with the oxidation of the reduced dyes by the I 3
− ions. Thus,
for the generation of greater photocurrents and photovoltages, the hole injection and
dye regeneration processes must be kinetically more favorable than the electron-loss
geminate recombination and dark current processes.
Consequently, innovative ideas for molecular design of efficient dye sensitizers are
required to develop high-performance DSSCs. In this context, a variety of ruthenium
(Ru) complex dyes and organic dyes have been designed and developed. To date, the
solar energy-to-electricity conversion yield (η) of up to 13% has been achieved in
DSSCs (O’Regan and Grätzel 1991; Hagfeldt et al. 2010; Nazeeruddin et al. 1993,
1997, 1999, 2001, 2003; Pashaei et al. 2016; Robertson 2006; Chen et al. 2007; Xie
and Guo 2007; Imahori et al. 2009; Ooyama and Harima 2009; Ning and Tian 2009;
Mishra et al. 2009; Ning et al. 2010; Yen et al. 2012; Ooyama et al. 2012; Ooyama
and Harima 2012; Odobel et al. 2010; Yum et al. 2011; Nikolaou et al. 2017; Li and
Diau 2013; Ladomenou et al. 2014; Manfredi et al. 2014; Zhang and Cole 2015; Lee
et al. 2015; Li et al. 2017; Urbani et al. 2014; Higashino 2015; Brogdon et al. 2018;
Ji et al. 2018; Mathew et al. 2014; Yella et al. 2014).
14.2 Dye Sensitizers for N-TiO 2 -Based Type-I DSSCs
Most of the conventional dye sensitizers developed for n-TiO 2 -based type-I DSSCs
have at least one carboxyl group (–COOH) as anchoring groups to bind to the TiO 2
electrode by the formation of bidentate linkage between the carboxyl group of the dye
and the Brønsted acid site, hydroxyl group (Ti–OH), on the TiO 2 surface (Fig. 14.3).
Ru complexes (O’Regan and Grätzel 1991; Hagfeldt et al. 2010; Nazeeruddin et al.
1993, 1997, 1999, 2001, 2003; Pashaei et al. 2016), porphyrin dyes (Imahori et al.
2009; Li and Diau 2013; Urbani et al. 2014; Higashino 2015; Brogdon et al. 2018;
Ji et al. 2018; Mathew et al. 2014; Yella et al. 2014; Wang et al. 2005a; Tanaka et al.
2007; Hayashi et al. 2008; Eu et al. 2008; Campbell et al. 2007; Lee et al. 2009; Lu
et al. 2009; Hsieh et al. 2010; Bessho et al. 2010; Yella et al. 2011; Higashino et al.
2015; Liu et al. 2016), phthalocyanine dyes (He et al. 2001; Aranyos et al. 2001;
Nazeeruddin et al. 1999a; He et al. 2002; Reddy et al. 2007a, b; Giribabu et al. 2007;
Mori et al. 2010; Kimura et al. 2012; Cid et al. 2007, 2009; Yum et al. 2008; GarcíaIglesias et al. 2011; Ragoussi et al. 2012; Ince et al. 2012; Yamamoto et al. 2016),
and organic dyes (Hagfeldt et al. 2010; Robertson 2006; Chen et al. 2007; Xie and
473
(more negative) than the redox potential of the I 3
− /I
− ion couple (0.4 V vs. NHE),
in order to achieve efficient dye regeneration by electron transfer from the reduced
dye to the I 3
− ion in the redox mediator. An energy gap of over 0.5 V is necessary
for efficient regeneration, according to a few previous reports. Clearly, the LUMO
energy level must be sufficiently lower than the CB energy level of the NiO electrode
(E cb , −3.0 V vs. NHE). However, there are also electron-loss processes, which lower
the photovoltaic performances of DSSCs: recombination of the holes injected into
the VB of the NiO electrode with the reduced dyes (geminate recombination) or with
the I
− ions at the NiO surface (dark current). The geminate recombination process
almost always competes with the oxidation of the reduced dyes by the I 3
− ions. Thus,
for the generation of greater photocurrents and photovoltages, the hole injection and
dye regeneration processes must be kinetically more favorable than the electron-loss
geminate recombination and dark current processes.
Consequently, innovative ideas for molecular design of efficient dye sensitizers are
required to develop high-performance DSSCs. In this context, a variety of ruthenium
(Ru) complex dyes and organic dyes have been designed and developed. To date, the
solar energy-to-electricity conversion yield (η) of up to 13% has been achieved in
DSSCs (O’Regan and Grätzel 1991; Hagfeldt et al. 2010; Nazeeruddin et al. 1993,
1997, 1999, 2001, 2003; Pashaei et al. 2016; Robertson 2006; Chen et al. 2007; Xie
and Guo 2007; Imahori et al. 2009; Ooyama and Harima 2009; Ning and Tian 2009;
Mishra et al. 2009; Ning et al. 2010; Yen et al. 2012; Ooyama et al. 2012; Ooyama
and Harima 2012; Odobel et al. 2010; Yum et al. 2011; Nikolaou et al. 2017; Li and
Diau 2013; Ladomenou et al. 2014; Manfredi et al. 2014; Zhang and Cole 2015; Lee
et al. 2015; Li et al. 2017; Urbani et al. 2014; Higashino 2015; Brogdon et al. 2018;
Ji et al. 2018; Mathew et al. 2014; Yella et al. 2014).
14.2 Dye Sensitizers for N-TiO 2 -Based Type-I DSSCs
Most of the conventional dye sensitizers developed for n-TiO 2 -based type-I DSSCs
have at least one carboxyl group (–COOH) as anchoring groups to bind to the TiO 2
electrode by the formation of bidentate linkage between the carboxyl group of the dye
and the Brønsted acid site, hydroxyl group (Ti–OH), on the TiO 2 surface (Fig. 14.3).
Ru complexes (O’Regan and Grätzel 1991; Hagfeldt et al. 2010; Nazeeruddin et al.
1993, 1997, 1999, 2001, 2003; Pashaei et al. 2016), porphyrin dyes (Imahori et al.
2009; Li and Diau 2013; Urbani et al. 2014; Higashino 2015; Brogdon et al. 2018;
Ji et al. 2018; Mathew et al. 2014; Yella et al. 2014; Wang et al. 2005a; Tanaka et al.
2007; Hayashi et al. 2008; Eu et al. 2008; Campbell et al. 2007; Lee et al. 2009; Lu
et al. 2009; Hsieh et al. 2010; Bessho et al. 2010; Yella et al. 2011; Higashino et al.
2015; Liu et al. 2016), phthalocyanine dyes (He et al. 2001; Aranyos et al. 2001;
Nazeeruddin et al. 1999a; He et al. 2002; Reddy et al. 2007a, b; Giribabu et al. 2007;
Mori et al. 2010; Kimura et al. 2012; Cid et al. 2007, 2009; Yum et al. 2008; GarcíaIglesias et al. 2011; Ragoussi et al. 2012; Ince et al. 2012; Yamamoto et al. 2016),
and organic dyes (Hagfeldt et al. 2010; Robertson 2006; Chen et al. 2007; Xie and