3. Dye solution co-adsorbing additives

"Soft modification," simple rinse and dry to alter TiO2/dye/electrolyte interface favorably is to add additives along with the dye adsorption solution known as "co-adsorbents" [95–98]. These additives are also known as de-aggregating agents which aid in favorable dye orientation on TiO2 and increased electron injection through minimization of dye–dye intermolecular charge transfer and π-π stacking [99]. These mainly insulating additives are known to occupy the vacant spaces among dye molecules which prevents the diffusion of oxidized redox species (e.g., I3 ) to TiO2 (Figure 7) [100, 101]. This approach has been widely explored for DSCs since the first report in 1993, though such modifications of TiO2 surface were already explored by Miayasak et al. in 1978 [95, 96]. These co-adsorbents are amphiphilic in nature and consists of an anchoring group (Figure 4) such as carboxylic or phoshonic/phosphinic moiety and a long alkyl chain or three dimensional aromatic and alkyl components acting as a buffer between TiO2 and electrolyte. These co-additives can be divided based on the chemical identity of anchoring group and their influence on the interface can be best studied through EIS and Titanium Dioxide Modifications for Energy Conversion: Learnings from Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.74565 397

Figure 7. Illustration of dye on TiO2 surface with (right) and without co-additive, I3 is the oxidized redox component.

Figure 8. Representative co-additives.

2.3. Metals and metal oxides for TiO2 modifications

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structure of TiO2 (hard modification).

in-depth analysis of such modifications [75, 89].

3. Dye solution co-adsorbing additives

(e.g., I3

Doping or intentional addition of impurities while synthesis of TiO2 can have substantial effect on band structure and surface states which dictate charge transport and dye/TiO2 interface in DSCs [88]. The purpose of doping is to achieve higher conductivity and minimized recombinations. In the regime of DSCs, doping has been studied with metals (lithium, magnesium or calcium), metalloids (boron, silicon, germanium, antimony), non-metals (carbon, nitrogen, sulfur, fluorine and iodine), transition metals, post transition metals and lanthanides. For a detailed analysis on choosing the dopant and its effects interested readers are referred to previously published review on the topic [88]. These dopants are generally added during the synthesis stage and require subsequent sintering step to be integrated as the part of crystalline

Additionally, to enhance the interface properties of DSCs at TiO2/dye/electrolyte for efficient charge transfer wide bandgap and electronically insulating metal oxides has been widely studied [75, 89]. Wide band gap metal oxides such as ZnO, Nb2O5, and WO3 and electronically insulating oxides such as SrO, Al2O3, ZrO2 and SiO2 are known to form barrier layer at the interface which impedes back electron transfer at the interface boundary thus lowering recombination losses [90]. Most recently, MgO have been studied for TiO2 modification during synthesis, leading to negative shift of up to 200 mV of TiO2 owing to its more basic nature [91, 92]. An alternative approach is to surface treat the TiO2 film with Mg+2 precursor such as (Mg (OC2H5) or Mg (NO3)2) followed by high temperature sintering, however, concentration control becomes very critical for final performance [90, 93, 94]. Albeit, these studies report higher Voc for DSCs employing Mg+2 and reduced recombination losses. However, negatively shifted conduction band can also lower the electron injection if the sensitizer-excited state potential gets very close to CB energy. Interested readers are referred to relevant reviews for

"Soft modification," simple rinse and dry to alter TiO2/dye/electrolyte interface favorably is to add additives along with the dye adsorption solution known as "co-adsorbents" [95–98]. These additives are also known as de-aggregating agents which aid in favorable dye orientation on TiO2 and increased electron injection through minimization of dye–dye intermolecular charge transfer and π-π stacking [99]. These mainly insulating additives are known to occupy the vacant spaces among dye molecules which prevents the diffusion of oxidized redox species

) to TiO2 (Figure 7) [100, 101]. This approach has been widely explored for DSCs since the first report in 1993, though such modifications of TiO2 surface were already explored by Miayasak et al. in 1978 [95, 96]. These co-adsorbents are amphiphilic in nature and consists of an anchoring group (Figure 4) such as carboxylic or phoshonic/phosphinic moiety and a long alkyl chain or three dimensional aromatic and alkyl components acting as a buffer between TiO2 and electrolyte. These co-additives can be divided based on the chemical identity of anchoring group and their influence on the interface can be best studied through EIS and electron lifetime measurements. Since aggregation is a common phenomenon for mostly planar organic sensitizer including phthalocyanine and porphyrin, the effect of co-additives on aggregation can be studied by simple current dynamic measurements at different light intensities [102, 103].

#### 3.1. Carboxylic acid based anchoring co-additives

Since the first report on deoxycholic acid (DCA) and chenodeoxycholic acid (CDCA, Figure 8) in 1993 by Grätzel et al. in ethanolic dye solution along with the sensitizers, CDCA has become the most widely used co-additive [96]. CDCA adsorption for pre-stained and post stained TiO2 films was found less effective in terms of enhancing Jsc and Voc [104]. Generally, the optimum concentration of CDCA in the dye solution depends on the nature of the dye and study of several concentrations (such as 2, 10, 20, and 40 times, etc., of the dye) is a normal routine [105, 106]. It should be kept in mind that excess of CDCA or any other co-adsorbent can adversely affect the dye loading as well. Under optimum conditions, CDCA is generally known to positively (downward) shift the conduction band of TiO2, increasing electron injection along with lower recombination losses thus enhancing both Jsc and Voc. It should be noted that CDCA mainly serves the role of de-aggregating agent for organic, porphyrin and phthalocyanine sensitizers and a recombination blocking agent for Ru (II) sensitizers since latter dyes do not aggregate on TiO2 [102]. In a recent study for ladder-like carbazole donor and cyanoacrylic acid (CA) anchor based D-A-π-A type dye, CDCA resulted in up to 9% enhancement in PCE when co-adsorbed with the dye [107]. Concentration of CDCA was 5 mM compared to 0.3 mM of the dye. The effect for the presence of CDCA was analyzed through EIS measurements which confirmed higher recombination resistance leading to 8% increase in Jsc and 2% increase in Voc for most efficient dye in the series (C1). In a similar study for an organic phenothiazine based dye (P2), effect of different concentrations of CDCA was studied in detail [106]. CDCA concentration of 10 mM was found to result in optimum improvement in DSCs performance compared to 0.3 mM concentration of the dye.

with 7% increase in PCE due to DPA (4:1 dye/co-additive concentration was employed). Later on in a unique example octydecyl phosphonic acid (OPA) was also characterized for Z-907 and

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OPA (18C) was found more effective in blocking recombination compared to DPA (10C) because of longer alkyl chain with overall 20% increase in PCE (8.4% versus 7% no OPA, Figure 10) with 18:1 (dye:OPA) concentration ratio. This is one of the highest efficiency reported so far for a Ru (II) dye containing NCS with cobalt redox shuttles, owing to inherently

In this class of co-additives, dineohexyl phosphinic acid (DINHOP, Figure 11) is known as an efficient molecular insulator to electronically passivate the oxide junctions such as TiO2, even outperforming CDCA in some comparative studies vide infra [105, 116]. DINHOP particularly benefits from the three dimensional orientation leading to better surface coverage [116]. Increase in PCE of 9% was realized for Z-907 and DINHOP with 1:1 dye concentration (PCE

In a comparative study for different small molecules (Figure 9 containing phenylphosphonic acid (PPA), diphenylphosphinic acid (DPPA), phosphoric acid triethyl ester (PATEE), and phosphoric acid tributylester (PATBE) were employed as co-adsorbents [117]. It was found that alkyl chains performed better than the aromatic containing co-additives with ~12% enhancement in device PCE with N719 sensitizer. The observed effect was established by EIS measurements (Nyquist plot), showing larger semicircle for high performing PATBE, pointing

Figure 10. Effect of OPA on Z907 performance, taken from Ref. [66], with permission from The Royal Society of

cobalt redox shuttle [112].

7.9% versus 7.24%).

Chemistry.

higher recombination losses [113–115].

Figure 8 shows the CDCA alternatives such as pivalic acid (PVA), 3,4,5-Tris(dodecyloxy) benzoic acid (DOBA) and EG1 (an amidoamine dendritic molecule) [108–110]. PVA in a comparative study approach showed enhanced electron lifetime and negative shift in the conduction band of TiO2. This lead to 53 mV increase in Voc and 8% increase in PCE. Adsorption of PVA before staining was found to be more effective, compared to co-adsorption with the dye. In an example with ss-DSCs (solid state-DSCs), DOBA and Z907 sensitizer resulted in negative shift of the TiO2 CB, decreased charge recombination, higher hydrophilicity and enhanced PCE as evidenced by EIS, and IMVS (intensity modulated photovoltage decay spectroscopy) measurements. In another example, strategically designed amidoaminedendron based molecules (EG0–2) were studied as the co-additives and compared with CDCA. This study showed that superior surface blocking, higher electron injection, minimized intermolecular energy transfer and higher PCE can be tailored with increasing size of the dendritic molecules. In spite of co-additives examples (Figure 8), CDCA is mostly widely employed co-additive to modify the interface on TiO2, however, it should be employed with caution particularly in terms of its co-adsorbing concentration.

#### 3.2. Phosphonic/phosphinic acid anchoring and zwitterion-based co-additives

Co-additives with phosphorous containing anchoring groups are generally believed to anchor strongly compared to carboxylic acid anchors. First example of such an additive was appeared in 2003 by Zakeerudin et al. when 1-decyl phosphonic acid (DPA, Figure 9) was used with a Z-907 (a benchmark hydrophobic dye historically popular for stable devices) [111]. It was claimed as the first example of stable DSCs under heat stress and continuous illumination,

Figure 9. Representative additives.

with 7% increase in PCE due to DPA (4:1 dye/co-additive concentration was employed). Later on in a unique example octydecyl phosphonic acid (OPA) was also characterized for Z-907 and cobalt redox shuttle [112].

sensitizers and a recombination blocking agent for Ru (II) sensitizers since latter dyes do not aggregate on TiO2 [102]. In a recent study for ladder-like carbazole donor and cyanoacrylic acid (CA) anchor based D-A-π-A type dye, CDCA resulted in up to 9% enhancement in PCE when co-adsorbed with the dye [107]. Concentration of CDCA was 5 mM compared to 0.3 mM of the dye. The effect for the presence of CDCA was analyzed through EIS measurements which confirmed higher recombination resistance leading to 8% increase in Jsc and 2% increase in Voc for most efficient dye in the series (C1). In a similar study for an organic phenothiazine based dye (P2), effect of different concentrations of CDCA was studied in detail [106]. CDCA concentration of 10 mM was found to result in optimum improvement in DSCs performance

Figure 8 shows the CDCA alternatives such as pivalic acid (PVA), 3,4,5-Tris(dodecyloxy) benzoic acid (DOBA) and EG1 (an amidoamine dendritic molecule) [108–110]. PVA in a comparative study approach showed enhanced electron lifetime and negative shift in the conduction band of TiO2. This lead to 53 mV increase in Voc and 8% increase in PCE. Adsorption of PVA before staining was found to be more effective, compared to co-adsorption with the dye. In an example with ss-DSCs (solid state-DSCs), DOBA and Z907 sensitizer resulted in negative shift of the TiO2 CB, decreased charge recombination, higher hydrophilicity and enhanced PCE as evidenced by EIS, and IMVS (intensity modulated photovoltage decay spectroscopy) measurements. In another example, strategically designed amidoaminedendron based molecules (EG0–2) were studied as the co-additives and compared with CDCA. This study showed that superior surface blocking, higher electron injection, minimized intermolecular energy transfer and higher PCE can be tailored with increasing size of the dendritic molecules. In spite of co-additives examples (Figure 8), CDCA is mostly widely employed co-additive to modify the interface on TiO2, however, it should be employed with

compared to 0.3 mM concentration of the dye.

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caution particularly in terms of its co-adsorbing concentration.

Figure 9. Representative additives.

3.2. Phosphonic/phosphinic acid anchoring and zwitterion-based co-additives

Co-additives with phosphorous containing anchoring groups are generally believed to anchor strongly compared to carboxylic acid anchors. First example of such an additive was appeared in 2003 by Zakeerudin et al. when 1-decyl phosphonic acid (DPA, Figure 9) was used with a Z-907 (a benchmark hydrophobic dye historically popular for stable devices) [111]. It was claimed as the first example of stable DSCs under heat stress and continuous illumination, OPA (18C) was found more effective in blocking recombination compared to DPA (10C) because of longer alkyl chain with overall 20% increase in PCE (8.4% versus 7% no OPA, Figure 10) with 18:1 (dye:OPA) concentration ratio. This is one of the highest efficiency reported so far for a Ru (II) dye containing NCS with cobalt redox shuttles, owing to inherently higher recombination losses [113–115].

In this class of co-additives, dineohexyl phosphinic acid (DINHOP, Figure 11) is known as an efficient molecular insulator to electronically passivate the oxide junctions such as TiO2, even outperforming CDCA in some comparative studies vide infra [105, 116]. DINHOP particularly benefits from the three dimensional orientation leading to better surface coverage [116]. Increase in PCE of 9% was realized for Z-907 and DINHOP with 1:1 dye concentration (PCE 7.9% versus 7.24%).

In a comparative study for different small molecules (Figure 9 containing phenylphosphonic acid (PPA), diphenylphosphinic acid (DPPA), phosphoric acid triethyl ester (PATEE), and phosphoric acid tributylester (PATBE) were employed as co-adsorbents [117]. It was found that alkyl chains performed better than the aromatic containing co-additives with ~12% enhancement in device PCE with N719 sensitizer. The observed effect was established by EIS measurements (Nyquist plot), showing larger semicircle for high performing PATBE, pointing

Figure 10. Effect of OPA on Z907 performance, taken from Ref. [66], with permission from The Royal Society of Chemistry.

Figure 11. Co-additives employed in the comparative study.

to higher recombination resistance as the result of co-additive pretreatment of TiO2 film before dipping in dye solution.

4-guanidino butyric acid (GBA, Figure 8) was first time employed in 2005 by Grätzel group with a Ru (II) sensitizer K19 [118]. In that detailed study, cyclic voltammetry was employed to determine the density of states (DOS), EIS to analyze interface charge transfer properties and photovoltage decay measurements for the effect of GAB on electron life time and capacitance of TiO2. GBA was found to have similar kind of effect as t-butyl pyridine on TiO2 conduction band with negative shift in the quasi-fermi level of TiO2. Additionally, increase in Voc did not come as the result of decreased Jsc, thus leading to higher overall PCE (~9% increase with 1:1 concentration with the dye). In 2009, same group studied and compared GBA and 4 aminobutyric acid (ABA) for solid state DSCs and additives effect on long term stability (Figure 11) [101]. GBA outperformed ABA with about 37% increase in PCE compared to 16% decrease with ABA at 1:1 concentration. This was caused presumably due to more effective barrier formation to recombination and upward shift in the conduction band of TiO2 by GBA. Enhancement in device performance due to GBA was caused by 15% increase (113 mV) in Voc and 18.5% increase in Jsc. GBA was also found to enhance the long term stability.

potentially play multiple roles when co-adsorbed along with another organic dye (NKX2677) [119, 120]. IPCE confirmed the increased photocurrent response, EIS was used to rationalize the higher Voc due to increased electron lifetime, and transient absorption studies showed the carbazole cation formation favorable for hole conduction. In a detailed study on similar lines with different molecules (HC3–5, Figure 12), black dye (BD) was optimized from 10.3% PCE to 11.3% for BD + HC-5, with 1:1 dye solution concentration [121]. As the result of cosensitization, BD + HC-5 mainly realized enhancement in Jsc (8.5%). It should be noted that CDA (chenodeoxycholic acid) as a co-adsorbent was also added in 100 times excess in this study. In a recent study, similar effects were claimed with LD03 and LD04 when co-sensitized along with N719. N719 + LD04 showed highest enhancement of PCE from 7.896% to 8.955% (13.4% increase) due to better light harvesting, decrease in aggregation and higher electron recombination resistance [122]. BPHA (2-(4-butoxyphenyl)-N-hydroxyacetamide), Figure 12), was recently applied for chemical modification of TiO2 before dye adsorption [123]. Though BPHA, was found to lower the adsorbed dye concentration, however, faster regeneration was reasoned for improved device performance (10–20%). Co-sensitizing approach of adsorbing multiple dyes on TiO2 for enhanced light harvestings works on the same principles, in addition to order of staining, and dyes ratio, etc., interested readers are referred to the cited work [124–126].

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To minimize the non-productive electron recombination pathways at the interface, chemical bath surface treatment of stained or dyed TiO2 is a very effective approach (soft modification). Such that the highest reported efficiency DSCs (12–14.3%), applied the most extensive surface treatments known (Figure 13) [25, 53, 127, 128]. It should be noted that post staining surface treatment additive need to be inert towards the sensitizer, i.e., it should not impede light harvesting and detach it from the surface of TiO2. Surface treatment is less complex than coadditive approach and offer better control on treatment parameters such as concentration,

4. Post-staining surface treatment additives

Figure 12. Small molecule co-sensitizing dyes as co-additives to modify TiO2.

In a recent study based on C106 dye, Chandiran et al. studied four different additives (Figure 11) for their potential effect on the TiO2 interface and device performance with different concentrations [105]. For C106, 4-guanidino butyric acid (GBA) resulted in 11% PCE, compared to 10.8% with CDCA at 0.5:1 (dye:CDCA) ratio compared to 6:1 for GBA (dye: GBA). In the same study, dineohexyl phosphinic acid (DINHOP) also showed slightly better results compared to CDCA (11% versus 10.8%), whereas dodecyl phosphonic acid (DPA) at 6:1 ratio marginally improved to 9.7% PCE. The device PCE without additives was reported to be 10.6%.

#### 3.3. Dual function dyes as co-additives

An interesting approach to achieve multiple functionality such as light absorption and aggregation/recombination blocking at TiO2 surface is to employ small molecule organic light absorbing dyes along with main dye [119–122]. Few successful example of such yellow to orange dyes which can be termed as co-sensitizers and co-additives are shown in Figure 12. In a detailed study in 2011, it was shown that Y-shaped molecules such as HC-A, can

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Figure 12. Small molecule co-sensitizing dyes as co-additives to modify TiO2.

to higher recombination resistance as the result of co-additive pretreatment of TiO2 film before

4-guanidino butyric acid (GBA, Figure 8) was first time employed in 2005 by Grätzel group with a Ru (II) sensitizer K19 [118]. In that detailed study, cyclic voltammetry was employed to determine the density of states (DOS), EIS to analyze interface charge transfer properties and photovoltage decay measurements for the effect of GAB on electron life time and capacitance of TiO2. GBA was found to have similar kind of effect as t-butyl pyridine on TiO2 conduction band with negative shift in the quasi-fermi level of TiO2. Additionally, increase in Voc did not come as the result of decreased Jsc, thus leading to higher overall PCE (~9% increase with 1:1 concentration with the dye). In 2009, same group studied and compared GBA and 4 aminobutyric acid (ABA) for solid state DSCs and additives effect on long term stability (Figure 11) [101]. GBA outperformed ABA with about 37% increase in PCE compared to 16% decrease with ABA at 1:1 concentration. This was caused presumably due to more effective barrier formation to recombination and upward shift in the conduction band of TiO2 by GBA. Enhancement in device performance due to GBA was caused by 15% increase (113 mV) in Voc

and 18.5% increase in Jsc. GBA was also found to enhance the long term stability.

In a recent study based on C106 dye, Chandiran et al. studied four different additives (Figure 11) for their potential effect on the TiO2 interface and device performance with different concentrations [105]. For C106, 4-guanidino butyric acid (GBA) resulted in 11% PCE, compared to 10.8% with CDCA at 0.5:1 (dye:CDCA) ratio compared to 6:1 for GBA (dye: GBA). In the same study, dineohexyl phosphinic acid (DINHOP) also showed slightly better results compared to CDCA (11% versus 10.8%), whereas dodecyl phosphonic acid (DPA) at 6:1 ratio marginally improved to 9.7% PCE. The device PCE without additives was reported to be

An interesting approach to achieve multiple functionality such as light absorption and aggregation/recombination blocking at TiO2 surface is to employ small molecule organic light absorbing dyes along with main dye [119–122]. Few successful example of such yellow to orange dyes which can be termed as co-sensitizers and co-additives are shown in Figure 12. In a detailed study in 2011, it was shown that Y-shaped molecules such as HC-A, can

dipping in dye solution.

Figure 11. Co-additives employed in the comparative study.

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10.6%.

3.3. Dual function dyes as co-additives

potentially play multiple roles when co-adsorbed along with another organic dye (NKX2677) [119, 120]. IPCE confirmed the increased photocurrent response, EIS was used to rationalize the higher Voc due to increased electron lifetime, and transient absorption studies showed the carbazole cation formation favorable for hole conduction. In a detailed study on similar lines with different molecules (HC3–5, Figure 12), black dye (BD) was optimized from 10.3% PCE to 11.3% for BD + HC-5, with 1:1 dye solution concentration [121]. As the result of cosensitization, BD + HC-5 mainly realized enhancement in Jsc (8.5%). It should be noted that CDA (chenodeoxycholic acid) as a co-adsorbent was also added in 100 times excess in this study. In a recent study, similar effects were claimed with LD03 and LD04 when co-sensitized along with N719. N719 + LD04 showed highest enhancement of PCE from 7.896% to 8.955% (13.4% increase) due to better light harvesting, decrease in aggregation and higher electron recombination resistance [122]. BPHA (2-(4-butoxyphenyl)-N-hydroxyacetamide), Figure 12), was recently applied for chemical modification of TiO2 before dye adsorption [123]. Though BPHA, was found to lower the adsorbed dye concentration, however, faster regeneration was reasoned for improved device performance (10–20%). Co-sensitizing approach of adsorbing multiple dyes on TiO2 for enhanced light harvestings works on the same principles, in addition to order of staining, and dyes ratio, etc., interested readers are referred to the cited work [124–126].
