Influence of Nb-doped TiO ₂ blocking layers as a cascading band structure for enhanced photovoltaic properties

Nb-doped TiO ₂ (Nb-TiO ₂ ) blocking layers (BLs) were developed using horizontal ultrasonic spray pyrolysis deposition (HUSPD). In order to improve the photovoltaic properties of the dye-sensitized solar cells (DSSCs), we optimized the Nb doping level of the Nb-TiO ₂ BLs by controlling the Nb/Ti molar ratio (0, 5, 6, and 7) of the precursor solution for HUSPD. Compared to bare TiO ₂ BLs, the Nb-TiO ₂ BLs formed a cascading band structure using the positive shift of the conduction band minimum of the Nb-TiO ₂ positioned between fluorine-doped tin oxide (FTO) and TiO ₂ . This results in the increase of the potential current and the suppression of the electron recombination. Hence, it led to the improvement of the electrical conductivity, due to the increased electron concentration by the Nb doping into TiO ₂ . Therefore, the DSSC fabricated with the Nb-TiO ₂ BLs at a Nb/Ti molar ratio of 6 showed superior photoconversion efficiency (∼7.50 ± 0.20%) as a result of the improved short-circuit current density. This is higher than those with the other Nb-TiO ₂ BLs and without BL. This improvement of the photovoltaic properties for the DSSCs can be attributed to the synergistic effects of uniform and compact BL relative to the prevention of the backward electron transport at the FTO/electrolyte interface, efficient electron transport at interfaces relative to a cascading band structure of FTO/Nb-TiO ₂ /TiO ₂ multilayers and the facilitated electron transport at the BLs relative to the increased electrical conductivity of the optimized Nb-TiO ₂ BLs.