Effects of the number of chromophores and the bulkiness of a nonconjugated spacer in a dye molecule on the performance of dye-sensitized solar cells

Three organic photosensitizers, Single-, Double- and Triple-CBZ, containing one, two or three carbazole-based chromophores, respectively, and a nonconjugated spacer in each molecule, were designed and successfully synthesized. The electro-optical and photovoltaic properties of the photosensitizers were investigated in terms of molecular microstructure, i.e., the number of the chromophores and the bulkiness of the nonconjugated spacer. A dye-sensitized solar cell (DSSC) based on Triple-CBZ containing three carbazole chromophores and a bulky tris(hexyloxyphenyl)ethane (THPE) spacer showed a considerably improved power conversion efficiency (PCE) of over 55%, mainly due to the enhanced open-circuit voltage (Voc) and short circuit current, when compared with that of the DSSC with Double-CBZ (or Single-CBZ) containing two (or one) carbazole chromophores and a hexyl spacer. More chromophores in the dye molecule increased the molar extinction coefficient, thereby enhancing the energy harvesting efficiency. In addition, a bulkier spacer was favorable for effectively protecting the charge recombination between TiO2 and electrolytes, thereby improving Voc by the longer electron lifetime.

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► Organic dyes with both chromophores and a non-conjugated spacer were synthesized.
► The dye with multi-chromophores increased short-circuit current of DSSCs.
► The dye with bulkier spacer increased open-circuit voltage of DSSCs.
► The bulkier spacer of the dye acted as energy barrier to charge recombination.