Energy-efficient, microwave-assisted hydro/solvothermal synthesis of hierarchical flowers and rice grain-like ZnO nanocrystals as photoanodes for high performance dye-sensitized solar cells

ZnO nanoparticles with different morphologies including marigold flower-like (MGFL), multipod jasmine flower-like (MPJF), sea urchin-rod flower-like (URFL), calendula flower-like (CDFL) and rice grain shape-like (RGSL) were successfully synthesized by decomposing either the Zn(OH)42− or Zn(NH3)42+ precursor in different solvents such as H2O, ethylene glycol (EG) and ethanol (EtOH) via one-pot rapid microwave-assisted hydro/solvothermal (MW-HT/ST) methods. From the obtained ZnO, we have developed two types of innovative photoanode configurations such as the “nano-hybrid architecture” and the “bi-layer architecture” via integration and layer-by-layer coating of flower-like URFL-ZnO and RGSL-ZnO nanoparticles, respectively, for dye-sensitized solar cells (DSSCs). Interestingly, the URFL/RGSL-ZnO nano-hybrid architecture photoanode-based DSSCs showed remarkably enhanced power conversion efficiency (PCE) as high as 5.64% compared to DSSCs based on their individual components such as flower-like URFL-ZnO and RGSL-ZnO nanoparticles which exhibited PCEs of 2.05% and 0.95%, respectively. In contrast, two types of “bi-layer architecture” photoanodes which were composed of an RGSL-ZnO layer on top of URFL-ZnO and vice versa exhibited PCEs of 1.74% and 2.26%, respectively. It is revealed that the “hybrid architecture” exhibits superior enhancement in PCE when compared to the “bi-layer architecture” assembly and their respective individual bare ZnO components, which was mainly attributed to the synergistic effect of the two different morphologies when blended together at a “nanoscale” level. Indeed, superior light-scattering ability and anchoring of more dye molecules were provided by URFL-ZnO. The fast electron transport through better inter-particle and electronic contacts with the fluorine-doped SnO2 glass (FTO) substrate was facilitated by RGSL-ZnO nanoparticles. Hence, the present investigation facilitates a promising way to enhance the efficiency of ZnO-based DSSCs by tuning different morphologies with innovative device architecture.