2.3. Metals and metal oxides for TiO2 modifications

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 structure of TiO2 (hard modification).

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

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 inten-

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

is the oxidized redox component.

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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

sities [102, 103].

Figure 8. Representative co-additives.

3.1. Carboxylic acid based anchoring co-additives

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