Engineering the interfaces of ITO@Cu₂S nanowire arrays toward efficient and stable counter electrodes for quantum-dot-sensitized solar cells

Among the issues that restrict the power conversion efficiency (PCE) of quantum-dot-sensitized solar cells (QDSSCs), insufficient catalytic activity and stability of counter electrodes (CEs) are critical but challenging ones. The state-of-the-art Cu/Cu₂S CEs still suffer from mechanical instability and uncertainty due to the reaction of copper and electrolyte. Herein, ITO@Cu₂S core-shell nanowire arrays were developed to fabricate CEs for QDSSCs, which have no such issues in Cu/Cu₂S CEs. These nanowire arrays exhibited small charge transfer resistance and sheet resistance, and provided more active catalytic sites and easy accessibility for electrolyte due to the three-dimensional structure upon use as CEs. More interestingly, it was found that the interface of ITO/Cu₂S significantly affected the performance of ITO@Cu₂S nanowire array CEs. By varying synthetic methods, a series of ITO@Cu₂S nanowire arrays were prepared to investigate the influence of ITO/Cu₂S interface on their performance. The results showed that ITO@Cu₂S nanowire array CEs with a continuous Cu₂S nanocrystal shell fabricated via an improved cation exchange route exhibited excellent and thickness-dependent performance. The PCE of corresponding QDSSCs increased by 11.6 and 16.5% compared to that with the discrete Cu₂S nanocrystal and the classic Cu/Cu₂S CE, respectively, indicating its promising potential as a new type of CE for QDSSCs.