Efficient Solar Cells Based on Porphyrin Dyes with Flexible Chains Attached to the Auxiliary Benzothiadiazole Acceptor: Suppression of Dye Aggregation and the Effect of Distortion

Donor−π–acceptor-type porphyrin dyes have been widely used for the fabrication of efficient dye-sensitized solar cells (DSSCs) owing to their strong absorption in the visible region and the ease of modifying their chemical structures and photovoltaic behavior. On the basis of our previously reported efficient porphyrin dye <b>XW11</b>, which contains a phenothiazine-based electron donor, a π-extending ethynylene unit, and an auxiliary benzothiadiazole acceptor, we herein report the syntheses of novel porphyrin dyes <b>XW26</b>–<b>XW28</b> by introducing one or two alkyl/alkoxy chains into the auxiliary acceptor. The introduced chains can effectively suppress dye aggregation. As a result, <b>XW26</b>–<b>XW28</b> show excellent photovoltages of 700, 701, and 711 mV, respectively, obviously higher than 645 mV obtained for <b>XW11</b>. Nevertheless, the optimized structures of <b>XW26</b> and <b>XW27</b> exhibit severe distortion, showing large dihedral angles of 57.2° and 44.0°, respectively, between the benzothiadiazole and benzoic acid units, resulting from the steric hindrance between the benzoic acid unit and the neighboring alkyl/alkoxy chain on the benzothiadiazole unit, and thus blue-shifted absorption, decreased photocurrents. and low efficiencies of 5.19% and 6.42% were observed for <b>XW26</b> and <b>XW27</b>, respectively. Interestingly, <b>XW26</b> exhibits a more blue-shifted absorption spectrum relative to <b>XW27</b>, indicating that the steric hindrance of the alkyl/alkoxy chains has a more pronounced effect than the electronic effect. Different from <b>XW26</b> and <b>XW27</b>, <b>XW28</b> contains only one alkyl chain neighboring the ethynylene unit, which does not induce obvious steric hindrance with the benzoic acid unit, and thus distortion of the molecule is not seriously aggravated compared with <b>XW11</b>. Hence, its absorption spectrum and photocurrent are similar to those of <b>XW11</b>. As a result, a higher efficiency of 9.12% was achieved for <b>XW28</b> because of its suppressed dye aggregation and higher photovoltage. It is worth noting that a high efficiency of 10.14% was successfully achieved for <b>XW28</b> upon coadsorption with CDCA, which is also higher than the corresponding efficiency obtained for <b>XW11</b>. These results provide a novel approach for developing efficient porphyrin dyes by introducing chains into the suitable position of the auxiliary benzothiadiazolyl moiety to suppress dye aggregation, without seriously aggravating distortion of the dye molecules