*2.3.1. Ruthenium sensitizer*

Desilvestro et al. [5] was the first to report the use of ruthenium complex tris(2,2′-bipyridyl-4,4′-di-carboxylate)ruthenium(II) dichloride dye in DSSC. The percentage conversion of absorbed incident photons to current (IPCE) for this DSSC was 44%. In 1991, O'Regan and Grätzel, reported IPCE of more than 80% from a DSSC using [Ru(2,2′-bipyridine-4,4′-dicarboxylicacid)2(µ-(CN)Ru(CN) (2,2′-bipyridine)2)2] dye adsorbed on a mesoporous, nanocrystalline TiO2 surface. The electrolyte contained I −/I3 − and the counter electrode was platinum [6]. The efficiency of the DSSC was more than 7%. Nazeeruddin et al. [7] have prepared several ruthenium(II) complexes. These sensitizers are cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) dye sensitizers. X comprises halide anions, CN− and SCN− . The cisdi(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) dye has been coded as N3. Among all the ruthenium complexes, N3 is a better sensitizer for charge transfer. N3 absorbs a wide wavelength range in the visible light. It has four carboxyl groups that strongly adsorb on the TiO2 surface and has a long excited state lifetime. The IPCE value exhibits more than 80% between 480 and 600 nm. The electrons are injected into the TiO2 CB via a metal-to-ligand charge transfer (MLCT) route as shown in **Figure 4**. According to Bryant et al. [8], the carboxylated complexes exhibit two t2 → π\* MLCT bands in the near UV and visible region. The absorbance of Ru(2,2′-bipyridine-4,4′-dicarboxylicacid)2(NCS)2, i.e. N3 dye at visible region, t2 → π\* is higher than other dihalogeno derivative dyes [7].

**Figure 4.** Charge transfer route from dye to TiO2.

The N3 dye was almost no match in terms of charge transfer ability until Nazeeruddin et al. [9] developed the triisothiocyanato-(2,2′:6′,6″-terpyridyl-4,4′,4″-tricarboxylato) Ru(II) tris(tetrabutylammonium) or 'black dye' and coded as N749. The DSSC with black dye showed a broader IPCE spectrum in the visible region compared to N3. The overall efficiency obtained for this DSSC with black dye was 10.4% under 1 Sun illumination [10].

The substitution of two protons in the carboxyl group of N3 dye with tetrabutylammonium cations resulted in [Bu4N]2[Ru(4-carboxy-4-carboxylate-2,2′bipyridine)2(NCS)2] or N719 dye. This dye exhibits a higher efficiency than N3 dye [11]. The higher efficiency is related to the higher Voc that resulted from the upshift of the TiO2 Fermi level. However, the performance of DSSC using N719 dye is still lower than the N749 since N719 does not absorb in the red. To extend the EM absorption region, the dye can be tuned. This can be accomplished by introducing a π\* molecular orbital ligand and by using a strong donor ligand to destabilize the metal t2g orbital [12]. By achieving this, the absorption range can be stretched from visible to the near infrared region. Islam et al. [12] have synthesized ruthenium complexes containing 2,2′-biquinoline-4,4′-dicarboxylic acid where the π\* orbital is lower or at a more positive potential than that containing 2,2′-bipyridine-4,4′-dicarboxylic acid. The DSSC using this sensitizer exhibited lower efficiency due to the dye excited state being at a more positive potential than the CB of TiO2. This led to reduced electron injection driving force and lowered the photocurrent. The nanocrystalline TiO2 soaked in [Bu4N]2[cis-Ru(4-carboxy-2-[2′-(4′ carboxypyridyl)]quinoline)2(NCS)2] has been investigated by Yanagida et al. [13]. They found that the IPCE spectrum extended up to 900 nm. Unfortunately, the maximum IPCE value obtained for this dye is lower (~40%) compared to the N719 (~80%). This is due to the lower LUMO which is 0.24 V below that of N719.

#### *2.3.2. Porphyrin sensitizer*

**3.** In order for the electrons to be transferred to the oxidized dye molecules efficiently for dye regeneration, the redox level has to be at more negative potential than the HOMO potential of the dye. The LUMO has to be less positive compared to the TiO2 CB for electron

Desilvestro et al. [5] was the first to report the use of ruthenium complex tris(2,2′-bipyridyl-4,4′-di-carboxylate)ruthenium(II) dichloride dye in DSSC. The percentage conversion of absorbed incident photons to current (IPCE) for this DSSC was 44%. In 1991, O'Regan and Grätzel, reported IPCE of more than 80% from a DSSC using [Ru(2,2′-bipyridine-4,4′-dicarboxylicacid)2(µ-(CN)Ru(CN) (2,2′-bipyridine)2)2] dye adsorbed on a mesoporous, nanocrys-

−/I3

[6]. The efficiency of the DSSC was more than 7%. Nazeeruddin et al. [7] have prepared several ruthenium(II) complexes. These sensitizers are cis-X2bis(2,2′-bipyridyl-4,4′-dicarbox-

di(thiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylate) ruthenium(II) dye has been coded as N3. Among all the ruthenium complexes, N3 is a better sensitizer for charge transfer. N3 absorbs a wide wavelength range in the visible light. It has four carboxyl groups that strongly adsorb on the TiO2 surface and has a long excited state lifetime. The IPCE value exhibits more than 80% between 480 and 600 nm. The electrons are injected into the TiO2 CB via a metal-to-ligand charge transfer (MLCT) route as shown in **Figure 4**. According to Bryant et al. [8], the carboxylated complexes exhibit two t2 → π\* MLCT bands in the near UV and visible region. The absorbance of Ru(2,2′-bipyridine-4,4′-dicarboxylicacid)2(NCS)2, i.e.

N3 dye at visible region, t2 → π\* is higher than other dihalogeno derivative dyes [7].

− and the counter electrode was platinum

and SCN−

. The cis-

**4.** The dye covering the TiO2 surface should not stack on each other.

ylate)ruthenium(II) dye sensitizers. X comprises halide anions, CN−

injection.

12 Nanostructured Solar Cells

*2.3.1. Ruthenium sensitizer*

talline TiO2 surface. The electrolyte contained I

**Figure 4.** Charge transfer route from dye to TiO2.

The porphyrin sensitizer also requires a binding group such as carboxylic acid and 8 hydroxylquinoline (HQ) to adsorb efficiently the TiO2 semiconductor [14]. The linkers containing carboxylic acid or HQ can be located at *β*-positions or *meso*-positions or both (shown in **Figure 5**).

**Figure 5.** Basic porphyrin structure. The mesoposition is at C─CH═C and β-position is at C─CH═CH─C. The hydrogen at meso- and β-positions will be substituted by functional groups such as diarylamino, fluorene, etc.

Kay and Grätzel were the first to report on DSSC using copper porphyrin [15]. The overall efficiency was 2.6%. The development of porphyrin sensitizers for SSCs gained more attention when Wang et al. [16] reported an efficiency of 5.6% under AM 1.5 illumination using zincporphyrin as the sensitizer with the co-adsorbent chenodeoxycholic acid (CDCA). The efficiency was increased to 7.1% reported by the same group for the zinc-porphyrin sensitizer with the aryl group as the electron donor and malonic acid as the acceptor ,which is shown in **Figure 6** [17]. Since then, the research on development of the porphyrin sensitizer increased rapidly. Park et al. [18] have shown that electron injection can be enhanced using two equivalent π-conjugated malonic acid linkers at the *β*-position. This led to higher *J*sc.

**Figure 6.** Structure of malonic acid porphyrin substituted at the β-position.

The serious dye aggregation problem for porphyrins on TiO2 films compared with the ruthenium complexes led to poor DSSC efficiency. The problem was solved by introducing long alkyl chains and 3,5-di-tert-butylphenyl groups to the porphyrin ring at the *meso*position [19]. By attaching the diarylamino group to the porphyrin ring, the DSSC exhibits an efficiency of 6.0% [20]. The efficiency was further enhanced to 6.8% by attaching two *tert*-butyl groups in the diarylamino group instead of two long alkyl chains (C6H13) coded as YD2 and co-adsorbed with CDCA. Bessho et al. [21] reported that the efficiency increased up to 11% when a thin reflecting layer of 5 µm thickness was coated on the TiO2 and sensitized with the YD2 sensitizer.

To further improve the performance of porphyrin based DSSC, light harvesting has to be enhanced which means the HOMO and LUMO energy gap must be decreased. There are two approaches: (1) to fuse or dimerize porphyrins and (2) by coupling a chromophore to the porphyrin ring. Eu et al. [22] have fused two quinoxaline derivatives to the zinc porphyrin to form 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6′-carboxyquinoxalino[2, 3-*β*] porphyrinatozinc (II) or ZnQMA and 5,10,15, 20-tetrakis(2,4,6-trimethylphenyl)-6′,7′-dicarboxyquinoxalino[2, 3-*β*]porphyrinatozinc (II) or ZnQDA. ZnQMA and ZnQDA based DSSCs exhibit the efficiencies of 5.2% and 4.0% respectively. The IPCE spectrum for both porphyrin sensitizers extended only up to ~700 nm. The fused porphyrin approach has successfully extended the light absorption to wavelengths longer than that in the visible region (~1000 nm) for nickel porphyrins fused with perylene anhydride as reported by Jiao et al. [23]. Unfortunately, the overall efficiency obtained was only 1.36%. The reason for low performance is the dye aggregation that resulted in the LUMO energy to be very close to the TiO2 CB edge and the short lifespan of the dye excited state.

The introduction of a highly conjugated π-extended chromophore at the *meso*-position can enhance light harvesting of the porphyrin dye. Wu et al. [24] has modified porphyrin by attaching fluorene, acenaphthene and biphenyl to one of the *meso*-positions. A broad IPCE spectrum near 800 nm with stronger response in the 400–500 and 550–750 nm regions were observed for DSSC using these three dyes. They observed that fluorenyl substituents showed the highest efficiency (8.1%). A year before, the same group [25] showed that pyrene-functionalized porphyrin exhibited an efficiency of 10.06% superior to N719 (9.3%). Dye aggregate formation significantly limited the performance of the porphyrin based solar cell. In order to further suppress dye aggregation, a long alkoxy chain zinc porphyrin was employed for protection of the porphyrin core. In 2014, Mathew et al. [26] reported an efficiency as high as 13% for porphyrin-sensitized DSSC. The porphyrin was coded SM315. The mediator used for this DSSC was Co(II/III).

## *2.3.3. Non-metallic organic dyes*

Kay and Grätzel were the first to report on DSSC using copper porphyrin [15]. The overall efficiency was 2.6%. The development of porphyrin sensitizers for SSCs gained more attention when Wang et al. [16] reported an efficiency of 5.6% under AM 1.5 illumination using zincporphyrin as the sensitizer with the co-adsorbent chenodeoxycholic acid (CDCA). The efficiency was increased to 7.1% reported by the same group for the zinc-porphyrin sensitizer with the aryl group as the electron donor and malonic acid as the acceptor ,which is shown in **Figure 6** [17]. Since then, the research on development of the porphyrin sensitizer increased rapidly. Park et al. [18] have shown that electron injection can be enhanced using two equivalent

The serious dye aggregation problem for porphyrins on TiO2 films compared with the ruthenium complexes led to poor DSSC efficiency. The problem was solved by introducing long alkyl chains and 3,5-di-tert-butylphenyl groups to the porphyrin ring at the *meso*position [19]. By attaching the diarylamino group to the porphyrin ring, the DSSC exhibits an efficiency of 6.0% [20]. The efficiency was further enhanced to 6.8% by attaching two *tert*-butyl groups in the diarylamino group instead of two long alkyl chains (C6H13) coded as YD2 and co-adsorbed with CDCA. Bessho et al. [21] reported that the efficiency increased up to 11% when a thin reflecting layer of 5 µm thickness was coated on the TiO2 and sensitized with the

To further improve the performance of porphyrin based DSSC, light harvesting has to be enhanced which means the HOMO and LUMO energy gap must be decreased. There are two approaches: (1) to fuse or dimerize porphyrins and (2) by coupling a chromophore to the porphyrin ring. Eu et al. [22] have fused two quinoxaline derivatives to the zinc porphyrin to form 5,10,15,20-tetrakis(2,4,6-trimethylphenyl)-6′-carboxyquinoxalino[2, 3-*β*] porphyrinatozinc (II) or ZnQMA and 5,10,15, 20-tetrakis(2,4,6-trimethylphenyl)-6′,7′-dicarboxyquinoxali-

π-conjugated malonic acid linkers at the *β*-position. This led to higher *J*sc.

**Figure 6.** Structure of malonic acid porphyrin substituted at the β-position.

YD2 sensitizer.

14 Nanostructured Solar Cells

Metal free or non-metallic organic dyes have been studied intensively to replace rutheniumbased sensitizers in DSSC. The metal free organic dyes have a molar extinction coefficient that is usually higher than Ru complexes [27–29]. Metal free dyes have opto-electronic properties that are easily tuned and they are cheaper to produce [30]. The general design principle for dye sensitizer is shown in **Figure 7**.

**Figure 7.** Design structure for non-metallic dye. The electrons from the donor will be transferred to TiO2 through the π - bridge and the acceptor.

In general, organic dyes can be grouped as neutral and ionic organic dyes. Examples of neutral organic dyes are coumarins, triphenylamine, phenothiazine and indoline. Examples of ionic organic dyes are squarylium, cyanine, hemicyanine and merocyanine.

Tian et al. [31] have synthesized organic dyes with phenothiazine (PTZ) as the electron donor and rhodamine-3-acetic acid or cyanoacrylic acid as the electron acceptor. The DSSC utilizing the dye with cyanoacrylic acid as the anchoring acceptor exhibited 5.5% efficiency. Marszalek et al. [32] reported two novel organic dyes. The dyes comprised of electron donating 10-butyl- (2-methylthio)-10*H*-phenothiazine with and without the vinyl thiophene group (VTP) as the *π*-bridge. The acceptor used is cyanoacrylic acid. With VTP, the IPCE value observed was up to 80% in the wavelength range between 380 and 750 nm, whereas without VTP, the range was between 380 and 650 nm. This results in higher *J*sc and efficiency for the DSSC using the VTP attached dye. The photocurrent density enhanced from 11.2 to 15.2 mA/cm2 and the efficiency reached 7.4%.

Coumarin-based dye is a promising sensitizer for DSSC because it has good photoelectric conversion properties [33]. Wang et al. [33] reported that a DSSC using coumarin dye, 2 cyano-3-(5-{2-[5-(1,1,6,6-tetramethyl-10-oxo-2,3,5,6-tetrahydro-*1H, 4H, 10H*-11-oxa-3a-azabenzo[*de*] anthracen-9-yl)-thiophen-2-yl]-vinyl}, -thiophen-2-yl)-acrylic acid exhibited an efficiency of 8.2%.
