16 p-Type Dye Sensitized Solar Cells …
329
the presence of a broad metal-to-ligand charge transfer (MLCT) band of significant
intensity. Not like the other classes of sensitizers, cyclometalated [Ru(NˆN) 2 (CˆN)]
+
provide flexibility to be used in both n- and p-type DSCs by varying the functional
group. When it is used in a n-DSCs, the CˆN ligand bears an ancillary role (similar to
electron pushing in a ‘push-pull’ dye architecture) but when it comes to a carboxylic
or phosphonic acid group in p-DSC it acts as the anchoring unit (Marinakis et al.
2017). Through functionalization of the C,N-ligand in [Ru(NˆN) 2 (CˆN)]
+ complex
(NˆN ¼ bidentate N,N
0 -ligand, CˆN4 ¼ cyclometalated C,N-ligand), the electronic
properties can be changed due to the localization of the highest-occupied molecular
orbital (HOMO) on the Ru/CˆN domain (Bomben et al. 2011, 2012; Ji et al. 2012,
43; He et al. 2014; Marinakis et al. 2017).
The first application of cyclometalated ruthenium complexes of the type Ru[(NˆN) 2 (CˆN)]
+ , as sensitizers for NiO p-DSCs, was reported by Ji et al. (2012).
These dyes demonstrated broad absorption in the visible region. Furthermore, hole
injection to the VB of the semiconductor is promoted by the carboxylic anchoring
group attached to the phenylpyridine ligand. Rigid phenyl linkers have been inserted
to systematically vary the distance between the Ru-[(NˆN) 2 (CˆN)]
+ core and the
carboxylic anchoring group. Increasing the number of phenylene linkers depressed
charge recombination losses and thereby, improved the photogenerated hole lifetime.
Therefore, the devices fabricated by incorporating dyes with longer spacer groups
reached higher efficiencies as well as higher J SC and V OC (With O12 dye, V OC =
82 mV, J SC = 1.84 mA cm
−2 , FF = 0.34 and η = 0.015%) (Ji et al. 2012).
Zhiqiang Ji et al. have also studied the behavior of the triphenylamino linker
attached to the para position of the ruthenium–carbon bond of the [NˆC] ligand. This
could improve the electronic coupling for hole injection and thereby generate comparatively higher J SC values than the prior sensitizers with phenylene linkers. They have
synthesized the sensitizers using 2,2
-bipyridine (O3), 1,10-phenanthroline (O13),
and bathophenanthroline (O17), in a manner that increases the conjugation of NˆN
ligand, leading to intensification of the extinction coefficient, and a concurrent redshift in absorption. Nevertheless, the O3 sensitizer exhibits comparatively higher Jsc
of 3.04 mA cm
−2 . But the V OC is 93 mV which is low yielding a lower efficiency
of 0.099% and fill factor is 0.35. However, the results suggest that O3 can perform
efficient dye regeneration and slow charge recombination (Ji et al. 2013).
Another panchromatic cyclometalated Ru(II) complex (O18) is reported by He
et al. (2014). which has the capability of absorbing photons from visible to near-IR
region of the electromagnetic spectrum extending up to 800 nm. O18 has an improved
molar extinction coefficient (ε = 1.9 × 104 M
−1 cm
−1 at 593 nm in solution) that
could stabilize the LUMO level of the dye. Due to the use of a π-conjugated system of
2,2
-bipyridyl (bpy) ligands they have increased the absorption cross section. Overall
performance is relatively good (Jsc = 3.43 mA cm
−2 V OC = 93 mV; FF = 0.33; η
= 0.104%) (He et al. 2014).
In addition to the ruthenium dyes, iridium complexes are also being used as
cyclometalated sensitizers for p-DSCs. Marcello Gennari and coworkers synthesised
three new cyclometalated iridium dyes and surprisingly they have revealed slow
charge recombination and a long-lived charge separation which is found to be unique
329
the presence of a broad metal-to-ligand charge transfer (MLCT) band of significant
intensity. Not like the other classes of sensitizers, cyclometalated [Ru(NˆN) 2 (CˆN)]
+
provide flexibility to be used in both n- and p-type DSCs by varying the functional
group. When it is used in a n-DSCs, the CˆN ligand bears an ancillary role (similar to
electron pushing in a ‘push-pull’ dye architecture) but when it comes to a carboxylic
or phosphonic acid group in p-DSC it acts as the anchoring unit (Marinakis et al.
2017). Through functionalization of the C,N-ligand in [Ru(NˆN) 2 (CˆN)]
+ complex
(NˆN ¼ bidentate N,N
0 -ligand, CˆN4 ¼ cyclometalated C,N-ligand), the electronic
properties can be changed due to the localization of the highest-occupied molecular
orbital (HOMO) on the Ru/CˆN domain (Bomben et al. 2011, 2012; Ji et al. 2012,
43; He et al. 2014; Marinakis et al. 2017).
The first application of cyclometalated ruthenium complexes of the type Ru[(NˆN) 2 (CˆN)]
+ , as sensitizers for NiO p-DSCs, was reported by Ji et al. (2012).
These dyes demonstrated broad absorption in the visible region. Furthermore, hole
injection to the VB of the semiconductor is promoted by the carboxylic anchoring
group attached to the phenylpyridine ligand. Rigid phenyl linkers have been inserted
to systematically vary the distance between the Ru-[(NˆN) 2 (CˆN)]
+ core and the
carboxylic anchoring group. Increasing the number of phenylene linkers depressed
charge recombination losses and thereby, improved the photogenerated hole lifetime.
Therefore, the devices fabricated by incorporating dyes with longer spacer groups
reached higher efficiencies as well as higher J SC and V OC (With O12 dye, V OC =
82 mV, J SC = 1.84 mA cm
−2 , FF = 0.34 and η = 0.015%) (Ji et al. 2012).
Zhiqiang Ji et al. have also studied the behavior of the triphenylamino linker
attached to the para position of the ruthenium–carbon bond of the [NˆC] ligand. This
could improve the electronic coupling for hole injection and thereby generate comparatively higher J SC values than the prior sensitizers with phenylene linkers. They have
synthesized the sensitizers using 2,2
-bipyridine (O3), 1,10-phenanthroline (O13),
and bathophenanthroline (O17), in a manner that increases the conjugation of NˆN
ligand, leading to intensification of the extinction coefficient, and a concurrent redshift in absorption. Nevertheless, the O3 sensitizer exhibits comparatively higher Jsc
of 3.04 mA cm
−2 . But the V OC is 93 mV which is low yielding a lower efficiency
of 0.099% and fill factor is 0.35. However, the results suggest that O3 can perform
efficient dye regeneration and slow charge recombination (Ji et al. 2013).
Another panchromatic cyclometalated Ru(II) complex (O18) is reported by He
et al. (2014). which has the capability of absorbing photons from visible to near-IR
region of the electromagnetic spectrum extending up to 800 nm. O18 has an improved
molar extinction coefficient (ε = 1.9 × 104 M
−1 cm
−1 at 593 nm in solution) that
could stabilize the LUMO level of the dye. Due to the use of a π-conjugated system of
2,2
-bipyridyl (bpy) ligands they have increased the absorption cross section. Overall
performance is relatively good (Jsc = 3.43 mA cm
−2 V OC = 93 mV; FF = 0.33; η
= 0.104%) (He et al. 2014).
In addition to the ruthenium dyes, iridium complexes are also being used as
cyclometalated sensitizers for p-DSCs. Marcello Gennari and coworkers synthesised
three new cyclometalated iridium dyes and surprisingly they have revealed slow
charge recombination and a long-lived charge separation which is found to be unique