Arrays of CdSe sensitized ZnO/ZnSe nanocables for efficient solar cells with high open-circuit voltage

Abstract

Highly ordered arrays of CdSe coated ZnO/ZnSe core–shell nanocables on FTO (SnO₂ : F) glass substrates have been synthesized using ZnO nanowires as precursors via in situ successive ion exchanges without any organic ligands involved. While the low open-circuit voltage (V_OC) (typically below 0.72 V) is a main factor limiting the power conversion efficiency (PCE) of quantum dot sensitized solar cells (QDSSCs), we design and exploit the arrays of ZnO/ZnSe/CdSe nanocables as efficient photoelectrodes for photoelectrochemical (PEC) solar cells, achieving a PCE of 4.54% and a V_OC as high as 0.836 V by using a nanostructured Cu₂S counter-electrode under AM 1.5G illumination with an intensity of 100 mW cm⁻². The high photovoltage is attributed to the ZnSe layer with a high conduction band edge, which reduces carrier recombination by passivizing the surface of ZnO nanowires and upwardly shifts the conduction band of ZnO in the heterojunction. A V_OC up to 0.855 V is achieved for the same cell using a typical platinized FTO (Pt/FTO) counter-electrode. However, the Cu₂S counter-electrode, which is demonstrated to have higher catalytic activity, contributes to improvements in the fill factor (FF) and short-circuit current density (J_SC) and consequently results in a 55% improvement in PCE.