Highly efficient dye-sensitized solar cell with a ZnO nanosheet-based photoanode

Zinc oxide (ZnO) nanosheets (ZnO-NS) were prepared for the photoanode of a dye-sensitized solar cell (DSSC), first by directly growing layered hydroxide zinc carbonate (LHZC) on an FTO substrate using a chemical bath deposition (CBD) method and then by transforming the LHZC into ZnO through pyrolysis at 300 °C. A light-to-electricity conversion efficiency (η) of 6.06% was achieved for the DSSC with ZnO-NS as its photoanode, under 100 mW cm−2 illumination, and this (η) was found to be much higher than that of the DSSC with ZnO nanoparticles (ZnO-NP) as the photoanode (2.92%). The far superior performance of the DSSC with ZnO-NS is essentially attributed to (i) higher effective electron diffusion coefficient of ZnO-NS (3.59 × 10−3 cm2s−1) than that of ZnO-NP (1.12 × 10−3 cm2s−1), and to (ii) higher dye loading on ZnO-NS (2.66 × 10−7 mol cm−2) than that on ZnO-NP (1.99 × 10−7 mol cm−2); this higher electron diffusion coefficient and dye-loading are attributed to the specific morphology of the ZnO-NS. A further improvement in the efficiency of the DSSC with ZnO-NS could be achieved through the electrophoretic deposition (EPD) of a very thin layer (3 μm) of titanium dioxide nanoparticles (TiO2-NPs of average size 14 nm) onto the ZnO-NS layer (12 μm). Notwithstanding a decrease in the effective electron diffusion coefficient (3.07 × 10−3 cm2s−1) in the TiO2-NP/ZnO-NS film, with reference to that in the ZnO-NS film (3.59 × 10−3 cm2s−1), a far higher cell efficiency was obtained in favor of the cell with TiO2-NP/ZnO-NS (7.07%), compared to that of the cell with bare ZnO-NS (6.06%); this enhancement in the η of the cell with TiO2-NP/ZnO-NS is ascribed to an increased dye-loading in favor of its cell (3.92 × 10−7 mol cm−2), with reference to that in the case of the cell with bare ZnO-NS (2.66 × 10−7 mol cm−2). As against the common ruthenium dyes, such as N3 and N-719, a metal-free dye, coded as D149, was used in this research. The efficiency achieved for the best DSSC in this work is the highest ever reported for a DSSC with ZnO as the main semiconductor material.