Building high-efficiency CdS/CdSe-sensitized solar cells with a hierarchically branched double-layer architecture

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO₂. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (V_oc) and short-circuit current density (J_sc). Also, when the double layer was subjected to further branching, a large increase in J_sc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance.