Poly(butyl acrylate)/NaI/I2 electrolytes for dye-sensitized nanocrystalline TiO2 solar cells

Solid polymer electrolytes consisting of NaI and I2 dissolved in poly(butyl acrylate) (PBA, Mw=99,000 g/mol) were prepared and applied to dye-sensitized solar cells (DSSC). Upon incorporation of salt, the free ester carbonyl stretching bands of PBA in FT-IR spectra shift to a lower wave number, confirming the complex formation between sodium ions and carbonyl oxygens. Coordinative interactions and structural changes of the PBA/NaI/I2 electrolytes have been also characterized by X-ray photoelectron spectroscopy (XPS), wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). The conversion efficiency of DSSC employing PBA/NaI/I2 electrolytes (1.66% at 10 mW/cm2) is unexpectedly much higher than that employing poly(ethylene oxide) (PEO, Mw=1,000,000 g/mol)/NaI/I2 (0.07%), although the ionic conductivity of the former (2.1×10−6 S/cm at room temperature) is not greatly different from that of the latter (1.6×10−6 S/cm). The significantly improved solar cell performance is mostly attributed to (1) the easier penetration of PBA through TiO2 nanoporous layer due to its lower molecular weight, and (2) the sticky property of PBA to allow a better adhesion with the electrodes.