Quasi-Solid-State Dye-Sensitized Solar Cells with over 36% Efficiency Achieved by a Direct-Contact Cell Structure and In Situ Gelation of Copper-Based Electrolytes

A novel polymer gel electrolyte (PGE) based on a copper redox system was developed for the first time to fabricate quasi-solid-state dye-sensitized solar cells (QS-DSSCs). Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) was used as the gelator to prepare the PGE, which can be injected into the cells at low viscosity and subsequently undergo in situ gelation within the cells. These PGEs were applied to DSSCs with direct-contact and sandwich structures for comparative analysis. The optimal PVDF-HFP concentration in the PGE was found to be 7 wt %. Electrochemical analysis indicated that the ion mobility and conductivity of the PGEs were comparable to those of liquid electrolytes, though charge transfer at the polymer gel/PEDOT interface was slightly reduced, resulting in a marginally lower power conversion efficiency (PCE) for QS-DSSCs compared with liquid cells. Under 1-sun conditions, direct-contact cells exhibited a higher PCE of 10.45%, surpassing the 9.54% of sandwich-structured cells. Electrochemical impedance spectroscopy revealed that direct-contact QS-DSSCs had higher capacitance and elevated Fermi levels compared to those of sandwich structures. Under room light conditions, sandwich-structured QS-DSSCs achieved a higher PCE of 22.67% compared to 19.02% for liquid cells at 200 lx. In contrast, direct-contact QS-DSSCs achieved an outstanding PCE of 36.06% and demonstrated superior stability relative to that of sandwich-structured QS-DSSCs.