4. Post-staining surface treatment additives

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,

Figure 13. Post staining surface treatment example "alkyl-thicket" layer formation, adopted from Ref. [127], with permission from The Royal Society of Chemistry.

dipping time, etc. Below section highlights the most successful strategies categorized on the basis of anchoring groups.

and worked collegially with common electrolyte additives such as Li+

resulted in both Jsc and Voc enhancements.

Figure 15. Alkoxysilyl-based alkyl chain additives to modify TiO2.

Figure 14. Additives used to modify TiO2 in post-staining fashion.

4.2. Alkoxysilyl-based anchoring additives

sensitizers [134].

butylpyridine). Increase in electron lifetime, decrease in dark current, and entrapment of Li+

Titanium Dioxide Modifications for Energy Conversion: Learnings from Dye-Sensitized Solar Cells

http://dx.doi.org/10.5772/intechopen.74565

403

In addition to "alkyl thicket" barrier layer for modifying TiO2 surface, Hanaya et al. also introduced and studied the silanol-based sensitizer and additives for TiO2 anchoring (Figures 13 and 15). It was found that titano-siloxane bonds are stronger and are more resistant to detachment from TiO2 compared to carboxylic acid anchors [127, 133]. However, it should be noted, other groups reported on the synthetic challenges related to the inclusion of siloxanes for DSC

An interesting and effective evolution is the replacement of hydrocarbon chain by fluorocarbon chain while keeping alkoxysilyl anchoring groups the same (Figure 16) [135–138]. In the regime of organic PV's (OPVs) fluorinated alkyl chains has been effectively employed to result in surface segregated monolayer to achieve better charge transfer, polymer alignment and

, and tBP (4-ter

#### 4.1. Carboxylic/phosphonic acid anchoring additives

Similar to dyes and co-adsorbing additives carboxylic/phosphonic acid anchoring groups are widely applied for surface treatment additives as well (Figure 14). Effect of long alkyl chain on enhancing electron lifetime in TiO2 was already known in conjunction with DSC sensitizers [129–131]. However, Hanaya et al. popularized the concept of "alkyl thicket" as an insulating barrier layer to block unwanted electron recombinations at TiO2. As the result, overall device PCE increased impressively ~20% (9.44–11.3%), with increase in Jsc from 15.1 to 15.8 mA/cm<sup>2</sup> and increase in Voc from 0.826 to 0.958 V (16% increase). It is interesting to note, the presence of long alkyl chains was not found to impede the charge transfer and diffusion with potentially favorable effect on long term stability as well [47]. These additives are usually applied in a hierarchical (stepwise) way with dipping in 0.1 M conc. (empirical) solution with the longest chain additives followed by small chain additives which can penetrate well in smaller cavities. In a very interesting study, a multifunctional methoxy-terminated oligomeric poly(ethylene glycol) (PEG) chain containing a carboxylic acid at one chain end (Mw - 2000) (m-PEG-succinic acid, Figure 14) is employed for post staining surface treatment [132]. The presence of electron rich oxygen atoms in m-PEG was claimed to favorably co-ordinate with vacant sites on TiO2 Titanium Dioxide Modifications for Energy Conversion: Learnings from Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.74565 403

Figure 14. Additives used to modify TiO2 in post-staining fashion.

Figure 15. Alkoxysilyl-based alkyl chain additives to modify TiO2.

and worked collegially with common electrolyte additives such as Li+ , and tBP (4-ter butylpyridine). Increase in electron lifetime, decrease in dark current, and entrapment of Li+ resulted in both Jsc and Voc enhancements.

#### 4.2. Alkoxysilyl-based anchoring additives

dipping time, etc. Below section highlights the most successful strategies categorized on the

Figure 13. Post staining surface treatment example "alkyl-thicket" layer formation, adopted from Ref. [127], with per-

Similar to dyes and co-adsorbing additives carboxylic/phosphonic acid anchoring groups are widely applied for surface treatment additives as well (Figure 14). Effect of long alkyl chain on enhancing electron lifetime in TiO2 was already known in conjunction with DSC sensitizers [129–131]. However, Hanaya et al. popularized the concept of "alkyl thicket" as an insulating barrier layer to block unwanted electron recombinations at TiO2. As the result, overall device PCE increased impressively ~20% (9.44–11.3%), with increase in Jsc from 15.1 to 15.8 mA/cm<sup>2</sup> and increase in Voc from 0.826 to 0.958 V (16% increase). It is interesting to note, the presence of long alkyl chains was not found to impede the charge transfer and diffusion with potentially favorable effect on long term stability as well [47]. These additives are usually applied in a hierarchical (stepwise) way with dipping in 0.1 M conc. (empirical) solution with the longest chain additives followed by small chain additives which can penetrate well in smaller cavities. In a very interesting study, a multifunctional methoxy-terminated oligomeric poly(ethylene glycol) (PEG) chain containing a carboxylic acid at one chain end (Mw - 2000) (m-PEG-succinic acid, Figure 14) is employed for post staining surface treatment [132]. The presence of electron rich oxygen atoms in m-PEG was claimed to favorably co-ordinate with vacant sites on TiO2

basis of anchoring groups.

mission from The Royal Society of Chemistry.

402 Titanium Dioxide - Material for a Sustainable Environment

4.1. Carboxylic/phosphonic acid anchoring additives

In addition to "alkyl thicket" barrier layer for modifying TiO2 surface, Hanaya et al. also introduced and studied the silanol-based sensitizer and additives for TiO2 anchoring (Figures 13 and 15). It was found that titano-siloxane bonds are stronger and are more resistant to detachment from TiO2 compared to carboxylic acid anchors [127, 133]. However, it should be noted, other groups reported on the synthetic challenges related to the inclusion of siloxanes for DSC sensitizers [134].

An interesting and effective evolution is the replacement of hydrocarbon chain by fluorocarbon chain while keeping alkoxysilyl anchoring groups the same (Figure 16) [135–138]. In the regime of organic PV's (OPVs) fluorinated alkyl chains has been effectively employed to result in surface segregated monolayer to achieve better charge transfer, polymer alignment and

Figure 16. Alkoxysilyl and fluorocarbon based additives to modify TiO2.

direction of dipole moment at the interface [139–141]. For TiO2 modification, detailed studies focused on unrevealing the impressive effect of fluorinated alkyl chains evidenced, enhanced electron lifetime in TiO2, de-aggregating behavior for organic dyes, negative (upward) conduction band shift of TiO2 with metal complex dye, hydrophobicity and overall PCE enhancements presumably due to fluorinated self-assembled monolayer formation (FSAM) [53, 94, 142–145]. Interestingly, in one study, cationically charged TMEA-TMOS (Figure 15) outperformed C16 based alkyl chain analog when used with Ru (II) dye and cobalt redox shuttle. Detailed studies on unrevealing the structure–property relationship of such fluorocarbon chains for modifying TiO2 are rare in literature at this stage.
