Suppression of charge recombination by application of Cu2ZnSnS4-graphene counter electrode to thin dye-sensitized solar cells

This paper proposes a new mechanism to explain the performance of thin dye-sensitized solar cells (DSSC). Near-stoichiometric flower-like Cu2ZnSnS4 (CZTS) microspheres with a high specific surface area was fabricated for use as the photocathode in a DSSC. To improve the extraction and transfer of electrons, graphene was added to the CZTS. A DSSC with a 10-μm TiO2 photoanode layer exhibited a slightly degraded efficiency with a CZTS-graphene photocathode, relative to a Pt counter electrode (CE). Nevertheless, when the thickness of the TiO2 photoanode was reduced to 2 μm, the efficiency of a DSSC with a CZTS-graphene photocathode was greater than that of a Pt-DSSC. It is speculated that, unlike the Pt CE, a CZTS-graphene photocathode not only collects electrons from an external circuit and catalyzes the reduction of the triiodide ions in the electrolyte, but also utilizes unabsorbed photons to produce photo-excited electrons and suppresses charge recombination, thus enhancing the performance of the cell. The use of narrow band gap p-type semiconductors as photocathodes offers a new means of fabricating thin dye-sensitized solar cells and effectively improving the cell performance.