Enhanced charge collection efficiency of dye-sensitized solar cells based on size-tunable hierarchically structured TiO2 beads

Hierarchically structured mesoporous TiO2 beads (HS-TBs), which are used as photoelectrodes in highly efficient dye-sensitized solar cells (DSCs), were prepared by an electrostatic spray (e-spray) technique. To prepare different sized HS-TBs, the electric field and the concentration of TiO2 particles were carefully controlled, because they are critical factors in preparing size-controlled TiO2 beads. Four different HS-TBs were formulated with average diameters of 250, 450, 700, and 1200 nm as high-quality photoelectrodes for use in DSCs. In this study, we found that the zero-dimensional HS-TBs were the most promising photoelectrode for DSCs due to enhanced charge collection efficiency and better penetration of electrolytes through relatively large pores among the HS-TBs. The HS-TBs were characterized by intensity modulated photocurrent spectroscopy (IMPS), the scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) method, Barrett-Joyner-Halenda (BJH) method, and electrochemical analysis. The conversion efficiency of HS-TB photoelectrodes improved with increasing bead size due to the enhanced electron transport through electrodes. The present HS-TB cells exhibit a noticeable improvement in the overall efficiency: maximum 9.54% (1200 nm) versus 5.83% for the reference cell made of a TiO2 nanocrystalline film.