Highly efficient solid-state solar cells based on composite CdS-ZnS quantum dots

Quantum dot sensitized solar cells (QDSSCs) are of great interest for solar energy conversion because of their tunable band gap and promise of stable, low-cost performance. Herein, we provide a new method to suppress charge recombination in solid-state QDSSCs. The composites of ZnS and CdS QDs with different ratios are adsorbed on nanostructured TiO₂ surfaces (denoted as CdS-ZnS), in which CdS and ZnS act as QDs sensitizer and blocking material, respectively. A combination of 83% CdS-17% ZnS gives the highest efficiency (η = 0.68%), which is significantly higher than the case of pure CdS (η = 0.33%). For a relatively low percentage of ZnS, the recombination impedance at the TiO₂/HTM interface could be enhanced, leading to a longer electron lifetime (τ_n), higher short current (J_sc) and power conversion efficiency (η). At a higher proportion of ZnS, however, losses in photocurrent cause net decrease in η. Compared with the conventional recombination barrier structure, in which ZnS blocking layers are deposited after CdS QDs (denoted as CdS/ZnS), the CdS-ZnS based device is more advantageous to suppress the electrons-hole recombination and increase τ_n, resulting in a higher J_sc, fill factor (FF) and open-circuit voltage (V_oc) and thus higher η.