Facile one-step fabrication of CdS0.12Se0.88 quantum dots with a ZnSe/ZnS-passivation layer for highly efficient quantum dot sensitized solar cells

Bandgap-tunable alloyed CdS0.12Se0.88 quantum dot (QD) sensitizers on a TiO2 photoanode with a ZnSe/ZnS passivation layer (denoted as CSS(ZSS)) were successfully synthesized for the first time by a facile one-pot successive ionic layer adsorption and reaction (SILAR) process from a cationic solution containing Zn2+ and Cd2+, and an anionic solution containing Se2− and S2−. A high power conversion efficiency (PCE) up to 6.14% (Jsc = 20.4 mA cm−2, Voc = 578 mV, FF = 0.52) was achieved, which is almost double the efficiency of 3.40% for the quantum dot sensitized solar cells (QDSCs) without Zn2+ feeding in the cationic solution. The results indicated that the light absorbance enhancement as well as the optical bandgap variation (from 2.13 eV to 1.89 eV) effectively promoted the light harvesting, leading to an increased photocurrent density. A careful control of the molar ratio of Zn/Cd by the SILAR cycles played a vital role in determining Jsc and Voc, and the possible explanation and mechanisms are discussed. The charge recombination at the interface between the QDs and electrolyte was also elaborated.