Size and shape fine-tuning of SnO2 nanoparticles for highly efficient and stable dye-sensitized solar cells

Abstract

Innovative solution routes led to two types of tin dioxide nanocrystals, i.e. 10–15 nm spheroid cassiterite nanoparticles and 50–150 nm anisotropic cassiterite particles showing octahedral facets. Nanoporous SnO₂ electrodes of various architectures (mono- or bilayered) were then processed by the screen-printing method using suitable combinations of these SnO₂ particles; the final texture, composition and morphology of the photoanodes obtained depending upon the nature of the post-treatment (with or without TiCl₄). After sensitization by the ruthenium dye N719, ATR-FTIR studies revealed that chemisorption of the dye onto porous cassiterite SnO₂ layers took place through a bridging coordination mode. As-prepared dye-sensitized photoanodes, when embedded in DSC devices containing a liquid electrolyte, led to a record overall power conversion efficiency (PCE) of 3.2% for pure SnO₂ composed of both kinds of particles and to very promising PCE above 4% for photoanodes post-treated with TiCl₄. The remarkable photovoltaic performances of the photoanodes including both kinds of particles, associated or not with a TiCl₄ post-treatment, were due to improved V_oc and FF, and were related to: (i) lower charge transfer resistance at the SnO₂–N719–electrolyte interface; (ii) onset of dark current occurring at higher potential; (iii) enhanced electron lifetimes as determined by transient V_oc decay measurements. Finally, the most striking feature of this study concerns the improvement of the power conversion efficiency upon aging under ambient conditions and the amazing long-term stability of DSCs fabricated from different SnO₂-based photoanodes since standard devices built from N719 dye and I₃⁻/I⁻ electrolytes usually show fast decrease of efficiency.