Controlled Sulfur Incorporation in SILAR-Deposited CdS Quantum Dots for Suppression of Interfacial Recombination in TiO₂ Quantum Dot-Sensitized Solar Cells

Abstract

Quantum dot-sensitized solar cells (QDSSCs) are limited by severe interfacial recombination and inefficient electron transport at the TiO₂/CdS junction. In this work, the interfacial charge dynamics were controlled by regulating sulfur availability during successive ionic layer adsorption and reaction (SILAR) growth of CdS quantum dots on mesoporous TiO₂ photoanodes. Sulfur was introduced in controlled molar ratios relative to the Cd²⁺ precursor to modify defect density and band alignment.

An optimum sulfur level produced a marked improvement in photovoltaic performance, where power conversion efficiency increased from 0.262% (undoped CdS) to 0.519%, accompanied by higher photocurrent density and open-circuit voltage. Electrochemical impedance analysis showed a strong increase in recombination resistance and a prolonged electron lifetime (0.017 s → 0.159 s), while Mott–Schottky measurements revealed a positive flat-band shift (−0.747 V → −0.588 V vs Ag/AgCl), indicating improved interfacial energetics. The enhancement originates from sulfur-induced passivation of surface trap states in CdS, which suppresses back electron transfer to the electrolyte and facilitates forward electron transport through the TiO₂ network. Consequently, performance enhancement is dominated by interfacial recombination suppression.

These results demonstrate that tuning sulfur incorporation during CdS growth provides a simple and scalable strategy to engineer interfacial charge transport and improve the efficiency and stability of TiO₂/CdS QDSSCs.