Three-dimensional ZnO nanodendrite/nanoparticle composite solar cells

Three-dimensional (3D) ZnO nanodendrite (ND)/nanoparticle (NP) composite films have been synthesized on fluorine-doped tin oxide (FTO) substrates using a wet chemical route. Aligned and high-density ZnO nanowires (NWs) are synthesized on the FTO substrate to be the trunks of the ND array by chemical bath deposition (CBD) in an aqueous solution of zinc acetate and hexamethylenetetramine. The branches of the NDs are subsequently formed on the surface of NWs using another CBD in an aqueous solution of zinc acetate and NaOH without any organic structure-directing agent and seed layer. ZnO NPs are further bottom-up grown within the interstices of the ZnO NDs using a base-free CBD method to form the 3D ZnO ND/NP composite film. Transmission electron microscopy (TEM) characterization and intensity modulated photocurrent spectroscopy (IMPS) measurements indicate that the 3D and quasi-signal-crystalline framework for fast electron transport are successfully constructed in the ZnO ND array. With an anode thickness of 3.5 mm, the efficiency of the D149-sensitized ZnO ND/NP composite DSSC is 3.74%, which is superior to that of the N719-sensitized TiO2 NP DSSC. IMPS measurements indicate that the dynamics of electron transport in the ZnO ND/NP composite DSSC are insensitive to light intensity and the electron transport rate in the ZnO ND/NP composite anode is enhanced by thirty-fold compared to that in the TiO2 NP anode. The superior electron transport properties in the ZnO ND/NP composite DSSC are ascribed to the 3D quasi-signal-crystalline framework of the ND array constructed in the composite anode.