5. Electrolyte additives

Electrolyte is an integral component of DSCs, and its composition has huge effect on performance, and long term stability. It consist of redox active species such as iodide/triiodide, Co (III)/Co(II), Fc (I)/Fc (0), and Cu (II)/Cu(I), etc., and certain additives which are known to adsorb on TiO2 surface such as lithium cation (Li<sup>+</sup> ), 4-ter butylpyridine (4-tBP) and guanidium thiocyanate (GuNCS) and others as shown in Table 1 [160–162]. Source of Li+ is mostly LiI for iodide/triiodide mediator and LiTFI or LiClO4 for cobalt and copper based redox shuttles. Two widely studied redox systems for DSCs are iodide/triiodide and Co(III)/Co(II) with most recent as Cu(II)/Cu(I) [160–162]. Iodide/triiodide redox shuttle has been the favorable choice historically, but it results in lower photovoltage due to higher (less positive) redox potential, higher dye regeneration overpotenial due to complex two step chemistry and corrosion of the DSCs components [160]. On the other hand, one electron redox shuttles such as cobalt and copper offer higher photovoltage, tunability, and less dye regeneration overpotential making them

popular for recent studies [52, 163]. For iodide/triiodide most commonly employed additives

No. Additive Conc. (M) Electrolyte/dye CB effect Electron lifetime Jsc Voc PCE 1. Li<sup>+</sup> [146] 0.05–0.5 I/Co Down Inc. Inc. Dec. Inc. 2 CDCA [147] 0.1 Co/Ru (II) Inc. Inc. Inc. Inc. Inc. 3 Li2CO3 [148] 0.0025 I/Ru (II) Up Inc. Inc. Inc. Inc. 4 K2CO3 [148] 0.05 I/Ru (II) Up Inc. Inc. Inc. Inc. 5 GuNCS [149] 0.1 I Down Dec. Inc. Dec. Inc. 6 GuNO3 [150] 0.1 I Up Inc. Inc. Inc. Inc.

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1 I/Ru (II) Down Dec. Inc. Dec. Inc.

0.25–0.5 I/Co Up Inc. Dec. Inc. Inc.

0.5 I/Ru (II) Up Inc. Dec. Inc. Dec.

0.5 Co Up Inc. Dec. Inc. Inc.

0.1 Co Up Inc. Inc. Inc. Inc.

0.5 I/Ru (II) Up Inc. Dec. Inc. Inc.

0.5 I/Ru (II) Up Inc. Dec. Inc. Dec.

only. Generally speaking, cationic additives charge the TiO2 surface positively thus lowering the conduction band (Table 1, entry 1) [56, 146]. Electron rich or nitrogen containing additives on the other hand charge the TiO2 surface negatively or increase electron density thus raising the conduction band, blocking the recombination and resulting in higher Voc. An important factor is the concentration which is commonly optimized empirically such as for Li2CO3, GuNO3, etc., additives [148, 150]. Electrolyte additives and their known effect in terms of

TiO2 modification and subsequent DSC device parameters are shown in Table 1.

, GuNCS and 4-tBP, whereas one electron redox shuttles mainly employ Li+ and 4-tBP

are Li<sup>+</sup>

7

8

9

10

11

12

13

[151]

[152, 153]

[154–156]

[157]

[159]

[159]

[158]

Up = upward shift, Down = downward shift, Inc. = increase, and Dec. = decrease.

Table 1. Summarizing the effect of electrolyte additives effect on TiO2 and DSC parameters.

Titanium Dioxide Modifications for Energy Conversion: Learnings from Dye-Sensitized Solar Cells http://dx.doi.org/10.5772/intechopen.74565 405


Up = upward shift, Down = downward shift, Inc. = increase, and Dec. = decrease.

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 fluorocar-

Electrolyte is an integral component of DSCs, and its composition has huge effect on performance, and long term stability. It consist of redox active species such as iodide/triiodide, Co (III)/Co(II), Fc (I)/Fc (0), and Cu (II)/Cu(I), etc., and certain additives which are known to

thiocyanate (GuNCS) and others as shown in Table 1 [160–162]. Source of Li+ is mostly LiI for iodide/triiodide mediator and LiTFI or LiClO4 for cobalt and copper based redox shuttles. Two widely studied redox systems for DSCs are iodide/triiodide and Co(III)/Co(II) with most recent as Cu(II)/Cu(I) [160–162]. Iodide/triiodide redox shuttle has been the favorable choice historically, but it results in lower photovoltage due to higher (less positive) redox potential, higher dye regeneration overpotenial due to complex two step chemistry and corrosion of the DSCs components [160]. On the other hand, one electron redox shuttles such as cobalt and copper offer higher photovoltage, tunability, and less dye regeneration overpotential making them

), 4-ter butylpyridine (4-tBP) and guanidium

bon chains for modifying TiO2 are rare in literature at this stage.

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

404 Titanium Dioxide - Material for a Sustainable Environment

adsorb on TiO2 surface such as lithium cation (Li<sup>+</sup>

5. Electrolyte additives

popular for recent studies [52, 163]. For iodide/triiodide most commonly employed additives are Li<sup>+</sup> , GuNCS and 4-tBP, whereas one electron redox shuttles mainly employ Li+ and 4-tBP only. Generally speaking, cationic additives charge the TiO2 surface positively thus lowering the conduction band (Table 1, entry 1) [56, 146]. Electron rich or nitrogen containing additives on the other hand charge the TiO2 surface negatively or increase electron density thus raising the conduction band, blocking the recombination and resulting in higher Voc. An important factor is the concentration which is commonly optimized empirically such as for Li2CO3, GuNO3, etc., additives [148, 150]. Electrolyte additives and their known effect in terms of TiO2 modification and subsequent DSC device parameters are shown in Table 1.

Table 1. Summarizing the effect of electrolyte additives effect on TiO2 and DSC parameters.

Since NCS containing Ru (II) sensitizers are incompatible with cobalt, inclusion of CDCA (Table 1, entry 2) substantially lowered the recombination losses and increased the PCE from 1.9 to 5.7% [147]. An interesting study, was the inclusion of Li2CO3 and K2CO3 (Table 1, entries 3 and 4) as a source of Li<sup>+</sup> , where former outperformed latter [148]. Li2CO3 enhanced the device performance (6.5–7.6%) without lowering Voc, presumably due to formation of carbonate layer on TiO2, as evidenced by FT-IR. In a comparative study, GuNO3 showed overall better performance compared to well-known GuNCS, without negative effect on Voc [150]. It was supported by the favorable effect of NO3 on TiO2 CB (upward shift), which was not observed for NCS without affecting diffusion negatively. Thiophene (Table 1, entry 7) when added in 1 M concentration had Li<sup>+</sup> like effect to enhance the Jsc [151]. 4-tBP (Table 1, entries 8–10) and its derivatives such as methyl pyridine, pyrimidine, pyrazole, triazole, thiazole and quinolone has been extensively explored by Arakawa et al. [154, 164–167]. Out of these, 4-trimethylsilylpyridine (Table 1, entry 10), have particularly shown better overall performance due to its bulkiness to block recombination reaction at interface, and better electron donating ability without negatively effecting the electron injection [157]. In a recent study, tris(4-methoxyphenyl)amine (TPAA, Table 1 entry 11) as an electron donor was explored by Boschloo et al. [158].

to achieve required functionality. On the other hand, soft modification (simple rinse and dry) post sintering surface treatment with additives, co-adsorbents, and electrolyte additives is rather simple to apply. With the discussion and literature provided in the chapter we hope the state of knowledge learned from dye-sensitized solar cells will benefit the scientific community to expand on the functionality of TiO2 as it is being applied and explored in the fields

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2 Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde,

3 Department of Chemistry, University of Engineering and Technology (UET), Lahore, Punjab,

[1] Joya KS, Joya YF, Ocakoglu K, van de Krol R. Water-splitting catalysis and solar fuel devices: Artificial leaves on the move. Angewandte Chemie, International Edition. 2013;

[2] Outlook WE. World energy outlook 2015. International Energy Agency. 2015

[6] Turner JA. Sustainable hydrogen production. Science. 2004;305(5686):972-974

[3] Colton W. The Outlook for Energy: A View to 2040. Exxon Mobil Corporation; 2011

[4] Olah GA. Beyond oil and gas: The methanol economy. Angewandte Chemie, Interna-

[5] Chu S, Majumdar A. Opportunities and challenges for a sustainable energy future.

[7] Joya KS, Morlanes N, Maloney E, Rodionov V, Takanabe K. Immobilization of a molecular cobalt electrocatalyst by hydrophobic interaction with a hematite photoanode for highly stable oxygen evolution. Chemical Communications. 2015;51(70):13481-13484 [8] Ocakoglu K, Joya KS, Harputlu E, Tarnowska A, Gryko DT. A nanoscale bio-inspired lightharvesting system developed from self-assembled alkyl-functionalized metallochlorin

of energy storage (batteries, super capacitors), photocatalysis, PVs, and sensors.

\* and Khurram S. Joya2,3

1 Chemistry Department, University of Mississippi, MS, USA

\*Address all correspondence to: hac@go.olemiss.edu

Author details

Hammad Cheema<sup>1</sup>

Denmark

Pakistan

References

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Nature. 2012;488(7411):294-303

The inclusion of TPAA in cobalt electrolyte particularly blocked the recombination with oxidized sensitizer which lead to 26% increase in the DSC performance. 2-ethylimidazole and benzimidazole (Table 1, entries 12 and 13) due to labile proton and lone pairs on electron were expected to be good coordinating candidates to modify TiO2 as studied by Wei et al. [159]. Benzimidazole and 2-ehtylimidazole were found to perform best when employed in the molar ratio of 9.5/0.5 respectively (7.93% PCE compared to 6.8%). These additives showed pyridine type effect in modifying TiO2. To this point, only few reports are available on the long term stability effect of these additives on TiO2 properties and DSC device performance [168, 169].

In this chapter, DSC electrolyte additives are discussed with respect to liquid based systems, whereas liquid in these electrolytes eventually has to be replaced for long term stability either by solid or semi-solid (gel type) systems. Reader are kindly referred to the published literature for semisolid gel type electrolyte which generally apply similar additives and offer better long term stability [162, 170–172].
