Towards quasi-solid state Dye-sensitized Solar Cells: effect of Al2O3 nanoparticle inclusion in liquid electrolyte

One of the main hindrances towards Dye-sensitized Solar Cell (DSC) industrialization is the presence of liquid electrolyte, which limits the long-term stability of these devices. Different ways for the fabrication of solid or quasi-solid cells were explored, for example by using ionic liquids, or quasi-solid and solid state electrolytes, such as gel-polymer electrolytes. All these solutions effectively improve the long term-stability of the cell, but usually reduce the performances with respect to liquid electrolyte-based device. Recently, some example of oxide nanoparticle (NP) inclusion (such as silica, titania or alumina) into ionic liquid electrolyte, ionic gel electrolyte, polymer electrolyte and gel-polymer electrolyte have been reported, and generally an improvement of the cell performance with respect to the same electrolyte without NPs have been obtained. In the present work we present the gelation of a liquid electrolyte by means of the addiction of alumina nanoparticles. We report on the synthesis of γ-Al2O2 NPs and on their morphological and compositional characterization, exploiting Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). Quasi-solid state electrolytes were prepared by dispersing different concentrations of NPs into commercial liquid electrolyte. Their properties were studied through FESEM investigation, viscosity measurements, ultraviolet (UV)-visible absorbance and reflectance spectroscopy, and electrochemical impedance spectroscopy (EIS) analysis. The increase of viscosity of the gelled electrolyte was accompanied by an improved scattering of light while increasing NP concentration values. Moreover, the EIS analysis showed an enhancement of the redox couple diffusion coefficient and a simultaneous decrement of the Pt/electrolyte charge transfer time. Quasi-solid state DSCs were fabricated using such electrolyte solutions and characterized in terms of photovoltaic performances through current-voltage (I-V), incident photon-to-electron conversion efficiency (IPCE), open circuit voltage decay (OCVD) and electrochemical impedance spectroscopy measurements, also studying the effect of the electrolyte volume. The results show an increase of the photocurrent due to NP inclusion and a corresponding decrease of the fill factor, leading to no appreciable difference in photovoltaic performance with respect to liquid electrolyte-based cells. Interestingly, an enhancement of the photovoltage was evidenced while increasing the cell thickness for the quasi-solid electrolyte based cells. Finally, the electrolyte gelation leads to an extended stability over time, with photoconversion efficiency remaining unchanged after more than 5 months.