The influence of using pyridinium molten salts as co-adsorbents to modify the monolayer of a TiO2 semiconductor on the performance of a dye-sensitized solar cell is studied. The current–voltage characteristics are measured under AM 1.5 (100 mW cm−2). The pyridinium molten salts significantly enhance the open-circuit photovoltage (Voc), the short circuit photocurrent density (Jsc) as well as the solar energy conversion efficiency (η). 1-Ethyl-3-carboxypyridinium iodide ([ECP][I]) is applied successfully to prepare an insulating molecular layer with N719, and achieve high energy conversion efficiency as high as 4.49% at 100 mW cm−2 and AM 1.5. The resulting efficiency is 20% higher than that of a non-additive device. This enhancement of conversion efficiency is attributed to the negative shift of the conduction band (CB) edge and the abundant concentration of I− on the surface of the electrode when using [ECP][I] as the co-adsorbent.