Oligomer approaches for solid-state dye-sensitized solar cells employing polymer electrolytes

Three different approaches utilizing oligomers, termed the "oligomer approaches", followed by in situ self-solidification, were applied successfully to prepare solid-state dye-sensitized solar cells. The oligomer approach employs (1) supramolecules containing quadruple hydrogen-bonding sites at both chain ends, (2) oligomer blends with high-molecular-weight polymers, and (3) nanocomposites of oligomer with SiO₂ nanoparticles. The overall energy conversion efficiency was as high as 4.5% at 1 sun condition, which may be primarily due to both the increased ionic conductivity and the better interfacial contact between the TiO₂ layer and the electrolyte due to the deep penetration of the small-sized liquid oligomer electrolyte. A threshold ionic conductivity of ∼1 × 10^-4 S cm^-1 was also observed, below which the ionic conductivity may play a key role in determining the photocurrent and the energy conversion efficiency. Above the threshold ionic conductivity, the ionic conductivity may play a less-important role and other factors such as the recombination of the electrons on the surface of the TiO₂ layer with I₃^- in the electrolyte and the charge-transfer resistance at the interface between the counter electrode and the electrolyte may be critical. The effect of chemical properties of the oligomer terminal groups was also investigated to determine that the methyl terminal group is the best among -OH, -NH₂, and -COOH primarily due to the least amount of change in the flat band potential.