Polymer Gel Electrolytes Based on PEG-Functionalized ABA Triblock Copolymers for Quasi-Solid-State Dye-Sensitized Solar Cells: Molecular Engineering and Key Factors

Abstract

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 degrees 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 degrees 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.