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

Abstract

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 β-alanine, respectively. Dye-sensitized TiO2 solar cells (DSSCs) with MKZ-35 and -36 showed smaller values in the short-circuit current (Jsc) and higher values in open-circuit voltage (Voc) compared with the values with MK-2. The lower Jsc was due to less injection efficiency and fast geminate recombination while the higher Voc 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.