Liquid carbon dioxide coating of CdS quantum-dots on mesoporous TiO ₂ film for sensitized solar cell applications

CdS quantum dots (QDs) were uniformly deposited on mesoporous TiO ₂ films by high-pressure coating with liquid carbon dioxide (l-CO ₂) as a coating solvent. The size and coverage of CdS QDs over the TiO ₂ surface were controlled by the number of l-CO ₂ coating cycles applied at a fixed precursor concentration. The formation of CdS QDs on the mesoporous TiO ₂ film was confirmed by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). In addition, depth profiling investigation revealed that CdS QDs were uniformly deposited throughout the ∼15 μm-thick mesoporous TiO ₂ film. The energy conversion efficiency of a CdS QD-sensitized solar cell (QDSSC) was observed to increase up to ∼1.33% under standard conditions (1.5 AM, 100 mW cm ^-2) with an increase in the number of coating cycles to six. Meanwhile, a further increase in the number of coating cycles to ten caused a decrease in the cell performance, which may be attributed to the hindrance of electrolyte infiltration into the mesoporous TiO ₂ electrode caused by the blocking of the pores with the coated CdS. The cell performance of the l-CO ₂-coated CdS-QDSSC is slightly higher than that of the l-CO ₂-coated CdS-QDSSC prepared by a conventional successive ionic layer absorption and reaction (SILAR) method with ethanol as the solvent (1.16%).