Quantum-dot-sensitized solar cells (QDSSCs) represent a promising technology for efficient solar energy conversion, benefiting from multiple exciton generation and solution-based fabrication. However, interface recombination remains a major barrier to performance enhancement, particularly the back transfer of photo-generated electrons to the electrolyte. In this study, we identify that charge transfer at the TiO₂/CdS interface does not occur through the conduction band but rather via the faradaic layer of TiO₂, which plays a pivotal role in back electron transfer. To address this issue, we introduce a uniform Gd(OH)₃ thin film on the surface of TiO₂ by a successive ionic layer adsorption and reaction (SILAR) method, which effectively inhibits back transfer by reducing polysulfides (S_n²⁻) reduction, leading to a substantial improvement in the short-circuit current density (J_SC) and open-circuit voltage (V_OC). Using techniques such as transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), in situ Raman and in situ ultraviolet–visible (UV–vis), we clarify the interfacial charge transfer mechanism, demonstrating how the Gd³⁺ treatment effectively suppresses the back reaction. This work provides a simple approach to improve QDSSCs performance and offers valuable insights for optimizing other photoelectrode materials.