w-ZnO nanostructures with distinct morphologies: Properties and integration into dye sensitized solar cells

Abstract Herein, we report the cost-effective and single-step reaction strategies for the synthesis of distinct morphology of ZnO namely, nanoparticles, nanotubes and nanowires involving co-precipitation and hydrothermal methods. Powder X-Ray diffraction analysis indexed nine distinct peaks which correspond to the w-ZnO for all the synthesized nanostructures. The formation mechanism of nanostructures are proposed, in which the role of surfactants (polyvinylpyrrolidone and sodium dodecyl sulfate) and the oriented self-assembly are taken into account. The Raman spectral analysis of the nanostructures confirms the presence of highly intense E2low and E2high phonon vibrational modes corresponding to the movement of O-atoms and Zn sub-lattices of w-ZnO respectively. The prepared yellowish ZnO nanowires reveal intensification in optical absorption and shrinkage in band gap due to increased oxygen vacancies in the sample. The quenching of green luminescence in the photoluminescence spectrum of the nanowire further confirms the oxygen vacancies. All the nanostructures were integrated into dye sensitized solar cells as photoanode materials. The photovoltaic parameters and electrochemical charge transfer properties of the fabricated dye sensitized solar cells (DSSCs) were evaluated by using photocurrent density-photovoltage curves and Nyquist plots of electrochemical impedance spectroscopic (EIS) studies. Due to the presence of large pore size in nanowires and nanotubes, the dye infiltration and electrolyte diffusion rates are high and cause the highest photocurrent densities of 1.714 mA cm−2 and 1.813 mA cm−2 respectively. The specific hollow tube like nanostructures is channelizing the photo-injected electrons directly to the collector electrode and results in high photovoltaic conversion efficiency of 1.109%. High value of charge transfer resistance across the ZnO/dye/electrolyte interfaces in nanocapsule and nanoparticle based DSSCs enforce low conversion efficiency to the devices.