TiO₂ compact layers prepared for high performance dye-sensitized solar cells

Graphical Abstract: Display Omitted -- Highlights: • Higher short-circuit photo current density can be obtained using TiO₂ compact layers. • Investigation of the effects of AZO annealing temperature on cell efficiency. • Comparison of TiO₂ porous sintered at different temperatures. • Grey Taguchi analysis offers an efficient way to increase experimental efficiency. -- Abstract: This work presents the optimal deposition parameters for TiO₂ compact layers and low cost Al₂O₃-doped zinc oxide (ZnO:Al₂O₃ = 97wt%:3wt%, AZO) transparent conducting films, using radio frequency (rf) magnetron sputtering to improve the performance of dye-sensitized solar cells (DSSCs). The structure, morphology and photocatalytic performance of the TiO₂ compact layer (blocking layer) and the effect of AZO films on the DSSC's conversion efficiency are studied. The grey-based Taguchi method is used to determine the processing parameters for the deposition of the TiO₂ compact layer (rf power, substrate temperature, O₂/(Ar+O₂) flow ratios and sputtering pressure) by considering multiple performance characteristics. These films are analyzed using scanning electron microscopy, X-ray diffraction and UV-vis-NIR spectroscopy. The results show that the TiO₂ compact layer prevents charge recombination, which results in a greater short-circuit photocurrent density. The intensity of the X-ray peaks for the AZO films also increases with annealing treatment, which gives improved crystallinity for the films. This study further determines the effect of the AZO annealing temperature on cell conversion efficiency. If the AZO is annealed at 500 °C and the TiO₂ porous layer is sintered at 450 °C with a holding time of 30 min, the experimental results demonstrate that the efficiency of the DSSCs with TiO₂ compact layer are increased from 2.24 to 5.69%