Combined computational and experimental study of carbazole dyes for iodide- and cobalt-based ZnO DSSCs

A combined computational and experimental study has been performed to investigate the best carbazole dyes for iodide- and cobalt-based DSSCs. TDDFT calculations have been implemented to figure out the electronic properties of three kinds of carbazole dyes, namely SD4, JM1 and JM2. These carbazole dyes have similar structures and their main difference lies on the position of their alkyl chain substituent. The calculated absorbance-spectra of the three dyes in the UV–visible range are in good agreement with the experimental results. The HOMO–LUMO analysis reveals that the SD4 dye has a better charge separation and a LUMO position slightly higher compared to the others. Calculations have been carried out to quantify the intramolecular charge-transfer (ICT). The result shows that JM2 has the highest q^CT but unfortunately accompanied by a short d^CT which indicates that the opportunity of electron recombination is high in JM2. The computational results also indicate that SD4 is the most promising candidate as a sensitizer in DSSC compared to JM1 and JM2. These results are in agreement with our experimental test using iodide-based electrolyte. On the other hand, the experiments show that JM2, which has two hexyl chains located in the edge of molecule, has the best performance when coupled with cobalt-based electrolyte.