ZnO Photoanode Microstructure Design And Charge Recombination Mechanism On Interface

Due to its simple process and low cost, the dye sensitized solar cell （DSSC） as a newgeneration of solar cell has been widely studied in the past twenty years. In this paper, the dyesensitized solar cell based on ZnO material was studied in details by analyzing morphology,optical properties and photoelectric performance. The influence of charge recombinationmechanisms on ZnO bilayer nanostructure, TiO₂decoration and thiol treatment on ZnOnanorod/Au nanoparticle photoanode has been studied.Fristly, by controlling low temperature hydrothermal growth condition, bilayer ZnOstructures such as nanodendrites over nanorod array, nanoflower layer over nanorod array anddouble nanorods layers were fabricated. The shape evolution of nanodendrite-nanorod bilayerZnO structure was elucidated by growing-renucleating mechanism. Comparing to thenanoflower-nanorod structure, the dye sensitized solar cell with nanodendrite-nanorod structurehas lower charge recombination rate, prolonged electron lifetime and enhanced photocurrent,mainly ascribing to its fewer defects. The research of ZnO nanostructure on chargerecombination provides a new point of view for the study of ZnO DSSC.Secondly, we have demonstrated a simple method to tune the TiO₂shell structure over ZnOnanocrystal aggregates for DSSC application by taking the advantage of the hydrolysis andcondensation differential of different TiO₂precursors. By immersing ZnO film into tetrabutyltitanate （TBOT） of butyl alcohol solution or titanium isopropoxide （TTIP） of butyl alcoholsolution, ZnO nanocrystal aggregates decorated with TiO₂nanoparticles or TiO₂thin film can beprepared. The dye sensitized solar cell with TiO₂nanoparticles decorated ZnO nanocrystalaggregates has better performance due to the significant increase of photocurrent and the betterelectron collection mechanism comparing to that of ZnO nanocrystal aggregates surroundingwith TiO₂thin layer. By adjusting the concentration of TBOT and the circles of decoration, theperformance of ZnO nanocrystal aggregates device decorated with TiO₂nanoparticles has beenoptimized. The simple method allows an easy control over the modification of interface and optimizing device performance.Finally, molecular surface chemical treatment was introduced into the plasmon-enhanceddye sensitized solar cell to suppress the charge recombination between metal nanoparticle andelectrolyte. Dodecanethiol molecules as surface treatment agent were successfully anchored tothe exposed Au nanoparticles sites and formed a fence-like protection on ZnO nanorods/Aunanoparticles/N719photoanode. The plasmon-enhanced light absorption due to the presence ofAu nanoparticles was not affected by the dodecanethiol surface treatment. The chargerecombination on photoanode/dye/electrolyte interface was substantially retarded by theinsulating of exposed Au nanoparticles sites from the oxidized form of electrolyte （I-3） viadodecanethiol molecules. The strategy of molecular surface chemical treatment on thephotoanode of dye sensitized solar cell with metal nanoparticles fully exploits the surfaceplasmon resonance effect, and explores new way of protecting metal nanoparticles for theplasmon-enhanced dye sensitized solar cell.