Redox Shuttle-Based Electrolytes for Dye-Sensitized Solar Cells: Comprehensive Guidance, Recent Progress, and Future Perspective

Abstract

A redox electrolyte is a crucial part of dye-sensitized solar cells (DSSCs), which plays a significant role in the photovoltage and photocurrent of the DSSCs through efficient dye regeneration and minimization of charge recombination. An I^-/I₃ ^- redox shuttle has been mostly utilized, but it limits the open-circuit voltage (V _oc) to 0.7-0.8 V. To improve the V _oc value, an alternative redox shuttle with more positive redox potential is required. Thus, by utilizing cobalt complexes with polypyridyl ligands, a significant power conversion efficiency (PCE) of above 14% with a high V _oc of up to 1 V under 1-sun illumination was achieved. Recently, the V _oc of a DSSC has exceeded 1 V with a PCE of around 15% by using Cu-complex-based redox shuttles. The PCE of over 34% in DSSCs under ambient light by using these Cu-complex-based redox shuttles also proves the potential for the commercialization of DSSCs in indoor applications. However, most of the developed highly efficient porphyrin and organic dyes cannot be used for the Cu-complex-based redox shuttles due to their higher positive redox potentials. Therefore, the replacement of suitable ligands in Cu complexes or an alternative redox shuttle with a redox potential of 0.45-0.65 V has been required to utilize the highly efficient porphyrin and organic dyes. As a consequence, for the first time, the proposed strategy for a PCE enhancement of over 16% in DSSCs with a suitable redox shuttle is made by finding a superior counter electrode to enhance the fill factor and a suitable near-infrared (NIR)-absorbing dye for cosensitization with the existing dyes to further broaden the light absorption and enhance the short-circuit current density (J _sc) value. This review comprehensively analyzes the redox shuttles and redox-shuttle-based liquid electrolytes for DSSCs and gives recent progress and perspectives.

Figure 1

DSSC device structure with an n-type TiO₂ semiconductor and its working principle.

Figure 2

Modified diode equivalent circuit for the dye-sensitized solar cell.

Figure 3

J–V curve of the DSSC.

Figure 4

(a) Charge transfer resistance of electrode/electrolytes determined from Nyquist plots in EIS measurement of a symmetrical dummy cell. Reprinted with permission from ref (60). Copyright 2014 Royal Society of Chemistry. (b) Apparent diffusion of I₃^– determined from the cathodic limiting current of J–V curves in LSV measurements (scan rate 10 mV s^–1) of a symmetrical dummy cell. Reprinted with permission from ref(62). Copyright 2017 Royal Society of Chemistry.

Scheme 1. Structures of Some Important Redox Couples (R⁺/R) Used in DSSC Electrolytes as Redox Mediators and Their Standard Redox Potentials

Figure 5

(a) Strategy for enhancing the maximum theoretical photovoltage (V_oc) by simultaneously lowering the redox shuttle redox potential and the ground-state oxidation potential of the dye toward a more positive potential to ensure the optimum driving force for fast dye regeneration. (b) Comparison of the redox potentials and possible maximum open-circuit voltages among three of the most efficient redox couples used in the DSSC.

Figure 6

Electronic configurations of Co(II) species in doublet and quartet states and Co(III) species in singlet and triplet states.

Figure 7

Chemical structure of dyes and plots of current transients for various Co complexes with two different dyes: (a) D29; (b) D35. Mass transport can be avoided by using a less bulky Co(bpy)₂ complex redox shuttle with the sterically bulky D35 dye. Reprinted with permission from ref (15). Copyright 2010 American Chemical Society.

Figure 8

(a) Energy levels in DSSC for Y123 dye and Cu complexes, the chemical structures of (b) various Cu complexes and (c) Y123 dye, and (d) the minimum-energy structures of Cu(II) complexes. Reprinted with permission from ref (16). Copyright 2016 American Chemical Society.

Figure 9

Modification of DSSC device structure by direct contact between the dye-sensitized TiO₂ photoanode and the electrically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) in the FTO. Reprinted with permission from ref (12). Copyright 2018 Elsevier.

Figure 10

Effect of tris(4-alkoxyphenyl)amine (TPA) mediator on TPA-Co mixed redox shuttles. The recombination of TiO₂ (e^–) was retarded by the steric effect of the TPA mediator. Reprinted with permission from the graphical abstract of ref (114). Copyright 2018 American Chemical Society.

Figure 11

p-Type DSSC device structure with a p-type semiconductor and its working principle.

Scheme 2. Structures of the Most Effective Additives in DSSC Redox Electrolytes

Figure 12

Effect of the coordination of an additional Lewis base (LB) on the oxidation and reduction in copper complexes, with the most favorable paths for oxidation and reduction being ACD and DBA, respectively, and the possible mechanism of dye regeneration and electrolyte regeneration by copper complexes in the presence of LB. Reprinted with permission from ref (164). Copyright 2020 Wiley.

Figure 13

Normalized emission spectra of different ambient light bulbs and AM 1.5G standard. Reprinted with permission from ref (172). Copyright 2021 Royal Society of Chemistry.

Scheme 3. Proposed Strategy for the PCE Enhancement of over 16% in DSSCs under 1-Sun Illumination