With the purpose of achieving highly stable and efficient polymer electrolytes for dye-sensitized solar cell (DSSC) applications, a series of new poly(ionic liquids) (PILs), namely, poly(1-N-methylimidazolium-pentylpolydimethylsiloxane)iodide, with different ratios of pentylimidazolium iodide moieties, have been studied and employed to prepare solid-state electrolytes for DSSCs. PILs were further mixed with two types of ionic liquids (ILs), 1-methyl-3-propylimidazolium iodide (MPII) and 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide (MPITFSI), and a plasticizer, such as ethylene carbonate (EC), to lower their viscosity and to increase the diffusion coefficient and ionic conductivity. The assembled devices prepared, using the quasi-solid-state electrolytes showed, light-to-electricity conversion efficiencies up to 6% for an active area of 0.2064 cm2. After 250 days, PIL-based cells retained 84% of their initial efficiency. These new findings encourage worldwide practical applications of DSSCs.