Systematic optimization of the substituents on the phenothiazine donor of doubly strapped porphyrin sensitizers: an efficiency over 11% unassisted by any cosensitizer or coadsorbent

Four novel porphyrin dyes XW48–XW51 with a doubly strapped porphyrin framework and judicious optimization of the substituents of the phenothiazine donor have been synthesized and utilized as DSSC dyes. On the basis of XW40, bulky 2,6-bis(hexyloxy)phenyl groups were incorporated into the phenothiazine donor by replacing the hexyl, 4-hexyloxyphenyl and both of them, to afford XW48, XW49 and XW50, respectively. On the basis of XW50, XW51 was synthesized by using 2,4-bis(hexyloxy)phenyl in place of the 2,6-bis(hexyloxy)phenyl units. As a result, the Voc values were enhanced stepwisely from 730 mV (XW40) to ca. 750 mV (XW48/XW49) and then to 761 mV (XW50). Compared with XW50, XW51 contains a bulky donor with more evenly distributed alkoxyl chains, which tend to form a more compact self-assembled monolayer to impede the penetration of I3−. Hence, an extraordinarily high Voc of 781 mV was achieved for XW51. As a result, the efficiency was dramatically improved to 11.1%, which is the highest efficiency for DSSCs based on an individual sensitizer using the iodine electrolyte, to the best of our knowledge. Furthermore, high efficiencies of 10.1% and 9.7% were also obtained for XW50 and XW51, respectively, based on the cobalt electrolyte.