Cu 2ZnSnS 4 hierarchical microspheres as an effective counter electrode material for quantum dot sensitized solar cells

We explore the application of Cu ₂ZnSnS ₄ (CZTS) microspheres as an effective counter electrode material for low-cost and high-efficiency quantum dot sensitized solar cells (QDSSCs). Nearly monodisperse Cu ₂ZnSnS ₄ (CZTS) hierarchical microspheres with diameters of ∼2 μm built from nanoflakes have been synthesized via a facile solvothermal approach. The nanoflakes are assembled from CZTS quantum dots with 3-5 nm, showing a three-tiered organization of hierarchical microspheres. The morphology, crystal structure, composition, and optical properties of the CZTS microspheres are characterized by SEM, HRTEM, XRD, XPS, EDS, EELS, Raman, and UV-vis analysis. Chemical conversion and phase transformation from hexagonal CuS to tetragonal CZTS have been systematically investigated to reveal formation mechanism of the CZTS microspheres. These CZTS microspheres are used as an effective counter electrode material in QDSSCs for the first time to show high electrocatalytic activity for catalyzing reduction of polysulfide (S ^2-/S _n ^2-) electrolyte, contributing to significant improvement in short current density (J _SC) and fill factor (FF). A solar cell using the CZTS microspheres-coated FTO (SnO ₂:F) glass substrate as a counter electrode achieves a power conversion efficiency (PCE) of 3.73% under AM 1.5G illumination with an intensity of 100 mW cm ^-2, which is much higher than that (0.33%) of the cell using the bare FTO glass substrate as a counter electrode and is also higher than that (2.27%) of the cell using the noble Pt-coated FTO glass substrate as a counter electrode. © 2012 American Chemical Society.