Herein we report a novel method for fabricating highly efficient dye-sensitized solar cells (DSCs) at low temperature (<150 °C). The ionic polymers of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were layer-by-layer deposited as an interfacial layer (IL) on the FTO substrate, before the nanoporous TiO2 layer was coated by a typical doctor-blade method. The presence of ultrathin (PAH/PSS)n IL created excellent adhesion of the TiO2 layer to the FTO substrate, leading to efficient transport of photo-injected electrons from TiO2 to FTO, as well as blocking the back-transport reaction from FTO to I3−. As a result, the fill factor (FF) was remarkably increased from 0.709 to 0.783, with a significant increase of the open circuit voltage (Voc) from 760 to 803 mV and the short circuit current (Jsc) from 8.078 to 8.768 mA cm−2, leading to the improvement of the photovoltaic conversion efficiency (η) from 4.41 to 5.52%. With optimization of the TiO2 electrode structure, η of the DSC fabricated at 140 °C was enhanced to 7.14%.