Exploitation of Ionic Liquid Electrolyte for Dye-Sensitized Solar Cells by Molecular Modification of Organic-Dye Sensitizers

When an alkyl-functionalized metal-free organic dye (MK-2) was employed as a sensitizer, dyesensitized TiO2 solar cells (DSSCs) fabricated with an ionic liquid (IL) electrolyte showed an energy conversion efficiency of 7.6%, which was comparable to that with a volatile liquid electrolyte. This contrasts with the case of DSSCs employing conventional Ru complex sensitizer (N719), whose efficiency was decreased markedly when the electrolyte was replaced from file volatile to IL. As it result, with the IL electrolyte, the conversion efficiency with MK-2 was higher than that with N719. The result was mainly attributed to longer electron lifetime in the DSSCs with MK-2. The longer electron lifetime is remarkable because with conventional volatile electrolyte, the lifetimes with many organic dyes including MK-2 were shorter than that with N719. This high compatibility of MK-2 with the IL electrolyte shows that the molecular modification of organic dyes can overcome the both disadvantages of ILs and organic dyes, mostly originated from their high viscosity and from the short electron lifetime observed with volatile electrolyte. respectively. Long-term stability of the DSSCs with MK-2 and the ionic electrolyte was also investigated, showing the efficiency of more than 7% for 2000 h under 1 sun conditions.