The effect of Li+ intercalation on different sized TiO2 nanoparticles and the performance of dye-sensitized solar cells

The effect of Li+ insertion into different sized TiO2 nanoparticles and their influences on the photoconversion efficiency of dye-sensitized solar cells (DSSC) were investigated. TiO2 nanoparticles with different particle sizes (22 nm, 14 nm and 6 nm) doped with Li+ were employed to form thin film electrodes and their properties were characterized by X-ray diffraction (XRD) and electrochemical impedance spectroscopy analysis. XRD evidenced the presence of anatase as the main phase. From the XRD analysis, it was observed that the Li+ ions could be inserted into both the surface and bulk of the TiO2 nanoparticles. In the larger particle size, the Li+ ions are inserted into the bulk anatase where as Li+ ions bounded on the TiO2 surface for the smaller crystallite size. The photovoltaic properties were measured by a current–voltage meter under AM1.5 simulated light radiation. It exhibited that the overall photoconversion efficiency of DSSC was decreased in the larger particles while it was enhanced in the smaller nanoparticles when Li+ was doped into the TiO2 nanoparticles. A nearly 40% decrease in the efficiency (η) of DSSC was observed upon intercalation of Li+ ions into 22 nm sized TiO2 nanoparticles (P25). The 14 nm sized TiO2 nanoparticles (P90) showed slightly less efficiency (η) upon Li+ doping than that of the undoped sample. However, the smallest sized TiO2 nanoparticles (6 nm) showed higher efficiency than that of the undoped one. This phenomenon is explained based on electron trapping and charge recombination due to lithium doping.

► Lithium was doped into different sized TiO2 nanoparticles.
► The dye-sensitized solar cell efficiency was decreased in the larger particles.
► The cell efficiency was enhanced in the smaller particles.