Room Temperature Solid-State Synthesis of a Conductive Polymer for Applications in Stable I2-Free Dye-Sensitized Solar Cells

A solid-state polymerizable monomer, 2,5-dibromo-3,4-propylenedioxythiophene (DBProDOT), was synthesized at 25 degrees C to produce a conducting polymer, poly(3,4-propylenedioxythiophene) (PProDOT). Crystallographic studies revealed a short interplane distance between DBProDOT molecules, which was responsible for polymerization at low temperature with a lower activation energy and higher exothermic reaction than 2,5-dibromo-3,4-ethylenedioxythiophene (DBEDOT) or its derivatives. Upon solid-state polymerization (SSP) of DBProDOT at 25 degrees C, PProDOT was obtained in a self-doped state with tribromide ions and an electrical conductivity of 0.05 Scm(-1), which is considerably higher than that of chemically-polymerized PProDOT (2x10(-6) Scm(-1)). Solid-state C-13 NMR spectroscopy and DFT calculations revealed polarons in PProDOT and a strong perturbation of carbon nuclei in thiophenes as a result of paramagnetic broadening. DBProDOT molecules deeply penetrated and polymerized to fill nanocrystalline TiO2 pores with PProDOT, which functioned as a hole-transporting material (HTM) for I-2-free solid-state dye-sensitized solar cells (ssDSSCs). With the introduction of an organized mesoporous TiO2 (OM-TiO2) layer, the energy conversion efficiency reached 3.5% at 100 mWcm(-2), which was quite stable up to at least 1500 h. The cell performance and stability was attributed to the high stability of PProDOT, with the high conductivity and improved interfacial contact of the electrode/HTM resulting in reduced interfacial resistance and enhanced electron lifetime.