Poly(ethylene-co-acrylic acid)-g-poly(ethylene glycol) graft copolymer templated synthesis of mesoporous TiO₂ thin films for quasi-solid-state dye sensitized solar cells

An amphiphilic graft copolymer, poly(ethylene-co-acrylic acid)-graft-poly(ethylene glycol) (PEAA-g-PEG), consisting of a PEAA backbone and PEG side chains was synthesized via an esterification reaction. ¹H nuclear magnetic resonance and Fourier-transformed infrared analysis demonstrated esterification between carboxylic acid of PEAA and hydroxyl group of PEG. Small angle X-ray scattering results revealed that the crystalline domain spacing of PEAA increased from 11.3 to 12.8 nm upon using a more polar solvent with a higher affinity for poly(acrylic acid), while the crystalline domain spacing of PEAA disappeared with PEG grafting, indicating structural change to an amorphous state. Mesoporous TiO₂ thin films were synthesized via a sol-gel reaction using PEAA-g-PEG graft copolymer as a structure-directing agent. The hydrophilically-preformed TiO₂ nanoparticles were selectively confined in the hydrophilic PEG domains of the graft copolymer, and mesoporous TiO₂ thin films were formed, as confirmed by scanning electron microscopy. The morphology of TiO₂ films was tunable by varying the concentrations of polymer solutions and the amount of preformed TiO₂. A quasi-solid-state dye-sensitized solar cell fabricated with PEAA-g-PEG templated TiO₂ film exhibited an energy conversion efficiency of 3.8% at 100 mW/cm², which was greater than that of commercially-available paste (2.6%) at a similar film thickness (3 μm). The improved performance was due to the larger surface area for high dye loading and organized structure with good interconnectivity.