The molecular weights and structural properties of polymers play key roles in the efficiency of gelators in polymer gel electrolytes (PGEs) for quasi-solid-state dye-sensitized solar cells (QSS-DSSCs). To find an appropriate gelator, we synthesized well-defined poly­(acrylonitrile-co-N,N-dimethylacrylamide)-block-poly­(ethylene glycol)-block-poly­(acrylonitrile-co-N,N-dimethylacrylamide) ABA triblock copolymers with various molecular weights and copolymer compositions by reversible addition–fragmentation chain-transfer polymerization. The ratio of acrylonitrile (AN)/N,N-dimethylacrylamide (DMAA) in the triblock copolymers influences their solubility in liquid electrolytes (LEs) and thermal stability. The highest thermal stability was up to 360 °C, and this was achieved by the polymer with an AN/DMAA ratio of ≤4. The thermal stability was related to excessive randomness in the P­(AN-co-DMAA) block that hinders cyclization among nitrile groups. Both the molecular weights and the AN/DMAA ratios enabled gel formation by controlling the amount of the polymer, and hence, they influence the ionic conductivity and diffusion as well. Based on the electrochemical properties, polymers with molecular weights above 100 kg/mole were efficient as PGEs in QSS-DSSCs. The overall power conversion efficiency (PCE) of 14 wt % SGT-626 PGE-based QSS-DSSCs was 9.72% under AM 1.5G solar illumination, comparable with an overall PCE of 9.79% for LE DSSCs. The overall PCE of the QSS-DSSCs further increased to 10.02% by incorporating 3 wt % TiO2 nanoparticles in the 14 wt % SGT-626 PGE. The SGT-626 PGE-based QSS-DSSC was also tested under indoor light conditions, and the best PCE of 21.26% was achieved under a white LED light of 1000 lux, which is higher than the PCE of 19.94% for the LE DSSC. The long-term device stability test under adverse conditions (50 °C and 1 sun illumination) reveals the improved stability of PGE-based QSS-DSSCs over LE DSSCs. In terms of PCE and long-term device stability, our PGE QSS-DSSCs have great potential over LE DSSCs for future indoor and outdoor applications.

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.

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For efficient polymer gel electrolytes (PGEs) in quasi-solid-state dye-sensitized solar cells (QSS-DSSCs), six ABA triblock copolymers based on (poly­(acrylonitrile-co-N-(isobutoxymethyl)­acrylamide)-block-poly­(ethylene glycol)-poly­(acrylonitrile-co-N-(isobuto-xymethyl)­acrylamide)) (P­(AN-co-BMAAm)-b-PEG-b-P­(AN-co-BMAAm)) with various copolymer compositions and molecular weights, coded as SGT-605, SGT-606, SGT-608, SGT-609, SGT-611, and SGT-612, have been synthesized by reversible addition–fragmentation chain transfer (RAFT) polymerization using PEG-functionalized macro-RAFT agents. The effects of copolymer compositions and molecular weights in P­(AN-co-BMAAm)-b-PEG-b-P­(AN-co-BMAAm) triblock copolymers were investigated in terms of electrochemical properties and photovoltaic performance as PGEs. The ionic conductivity was increased with N-(isobutoxymethyl)­acrylamide (BMAAm) composition of these triblock copolymers, which is attributed to the availability of free iodide ions by complex formation among acrylamide groups with Li+ ions. However, polymer gel electrolytes with high molecular weights enhance ionic conductivity due to the lower amount of polymers required for the gel formation. Thus, the photovoltaic performances of PGE-based QSS-DSSCs improved along with the increase in the molecular weight of the triblock copolymer. The addition of 7 wt % TiO2 nanofiller into PGEs produced a higher ionic conductivity and diffusion of I3 – than the corresponding PGEs. The resulting power conversion efficiency (PCE) of QSS-DSSCs using SGT-612/TiO2 composite PGEs under simulated 1-sun condition was 9.83% (V oc, 792.8 mV; J sc, 16.65 mA/cm2; FF, 74.52%), which was higher than that of liquid electrolyte DSSCs (PCE 9.53%; V oc, 743.8 mV; J sc, 17.16 mA/cm2; FF, 74.64%). The long-term device stability of PGE-based QSS-DSSCs was better than the liquid-state DSSCs.