Improved Preparation of Benzothiazolium Iodides and Their Application in Dye Sensitized Solar Cells

We prepared 3-methyl benzothiazolium iodide (MBTI) and 2,3-dimethyl benzothiazolium iodide (DMBTI) by a quaternization reaction of benzothiazole/2-methyl benzothiazole and iodomethane using Teflon-lined,stainless steel autoclaves.The influence of the 2-position methyl group of the benzothiazolium heterocycle on the thermal stability and melting point of benzothiazolium iodides was studied by thermogravimetric analysis and differential scanning colorimetry.The relationship between I3-/I-redox behavior in solution and the Pt counter electrode/solution interface with the benzothiazolium structure was investigated by cyclic voltammetry using a Pt disk ultramicroelectrode and electrochemical impedance spectroscopy (EIS).Dye sensitized solar cells (DSCs) with MBTI and DMBTI were then assembled.The preparation of the benzothiazolium iodides is simple with short reaction time,easy purification,and high yields.Compared with MBTI,DMBTI has a higher melting point and excellent thermal stability.DSCs with an electrolyte solution composed of 0.1 mol·L-1 I2,0.1 mol·L-1 LiI,0.6 mol·L-1 1-methyl-3-propylimidazolium iodide (MPII),and 0.3 mol·L-1 DMBTI in γ-butyrolactone gave a short circuit photocurrent density of 16.91 mA·cm-2,an open circuit voltage of 0.65 V and a fill factor of 0.57.This corresponds to a photoelectric conversion efficiency of 6.28% at the illumination (air mass 1.5,100 mW·cm-2).