Ultrarapid Sonochemical Synthesis of ZnO Hierarchical Structures: From Fundamental Research to High Efficiencies up to 6.42% for Quasi-Solid Dye-Sensitized Solar Cells

Zinc oxide (ZnO) hierarchical structures (HSs) have recently demonstrated notable photochem. and photovoltaic performances attributed to their nano/micro combined architectures. In this study, ZnO HSs were synthesized at room temp. using ultrarapid sonochem. This novel approach can effectively overcome deficiencies in the synthesis via traditional direct pptn. by promoting nucleation and accelerating diffusion. Only 15 min was needed to complete the formation of highly crystd. and uniformed HSs consisting of interconnected monocryst. nanosheets using sonochem. The formation of HSs through in situ observations was interpreted using a new mechanism based on oriented attachment and reconstruction. In the nonequil. synthesis system, thicker, porous, and coarse crystd. ZnO sheets were first constructed via oriented attachment of small-sized nanocrystals. After reconstruction, ultrathin, integrated, and monocryst. nanosheets were obtained. According to the two-dimensional nanosheets to three-dimensional HSs, the formation was much more sophisticated because repeated and parallel heterogeneous oriented attachments with reconstructions dominated the final morphologies of the HSs. The relationships between synthetic conditions and HSs structures were established. Based on the photoanodes in dye-sensitized solar cells (DSCs), the performances of these differently structured HSs were tested. HSs with densely assembled nanosheets exhibited better performances in photoelec. conversions. Systematic investigations were also carried out by selecting two representative HSs to demonstrate the crit. factors governing the optical and elec. properties of photoanodes. Finally, under AM 1.5 and 100 mW.cm-2 light irradn., high photoelec. conversion efficiencies of up to 6.42% were achieved. These results established a new record for quasi-solid ZnO-based DSCs.