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, MK-2. Two dyes, MKZ-35 and -36, were synthesized with glycine and beta-alanine, respectively. Dye-sensitized TiO2 solar cells (DSSCs) with MKZ-35 and -36 showed smaller values in the short-circuit current (J(sc)) and higher values in open-circuit voltage (V-oc) compared with the values with MK-2. The lower J(sc) was due to less injection efficiency and fast geminate recombination while the higher V-oc was attributed to longer lifetime of the injected electrons in the DSSCs. DFT calculations showed that MKZ-35 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 MKZ-36 than that with MK-2 seems to be due to the longer distance between the TiO2 surface and conjugated framework of the dye.