Alternation of Charge Injection and Recombination in Dye-Sensitized Solar Cells by the Addition of Nonconjugated Bridge to Organic Dyes

Most metal-free organic dyes for dye-sensitized solar cells are designed by following a donor conjugated-bridge acceptor structure with a carboxyl acid as an anchoring unit. In this work, we examined the influence of a nonconjugated methylene unit between the cyano group and carboxyl acid by applying it to a previously reported carbazole dye, <b>MK-2</b>. Two dyes, <b>MKZ-35</b> and <b>-36</b>, were synthesized with glycine and β-alanine, respectively. Dye-sensitized TiO₂ solar cells (DSSCs) with <b>MKZ-35</b> and <b>-36</b> showed smaller values in the short-circuit current (<i>J</i>_sc) and higher values in open-circuit voltage (<i>V</i>_oc) compared with the values with <b>MK-2</b>. The lower <i>J</i>_sc was due to less injection efficiency and fast geminate recombination while the higher <i>V</i>_oc was attributed to longer lifetime of the injected electrons in the DSSCs. DFT calculations showed that <b>MKZ-35 </b>dyes interact with each other. One possible explanation for the longer electron lifetime is that the interacted molecules may act as a 3D enlarged dimer molecule or form an induced quadrupole, reducing the interaction between the dyes and acceptor species. On the other hand, the longer electron lifetime with <b>MKZ-36 </b>than that with <b>MK-2</b> seems to be due to the longer distance between the TiO₂ surface and conjugated framework of the dye