Efficient solar cells sensitized by a promising new type of porphyrin: dye-aggregation suppressed by double strapping

Porphyrin sensitizers play essential roles in the development of efficient dye-sensitized solar cells (DSSCs). To further improve power conversion efficiency (PCE), it is vital to reduce undesirable dye aggregation that causes serious charge recombination and lowered open-circuit voltages (V _oc). To this end, we herein report a new class of porphyrin-based dyes XW40 and XW41, with the porphyrin cores strapped with two circle chains. Compared with the reference sensitizer XW10 which contains a porphyrin core wrapped in four dodecoxyl chains, the double strapping in XW40 not only effectively suppresses the dye aggregation but also improves the dye loading amount. As a result, the V _oc and photocurrent (J _sc) were improved by 19 mV and 0.8 mA cm^-2, respectively, compared with the corresponding values of XW10, and the efficiency was improved from 8.6% obtained for XW10 to 9.3% for XW40. To further extend the spectral response, an electron-withdrawing benzothiadiazole (BTD) unit was introduced as an auxiliary acceptor in XW41. Impressively, the onset wavelength of its IPCE spectrum was dramatically red-shifted to 830 nm. However, the extended π-conjugation framework results in aggravated dye aggregation, and thus a lowered efficiency of 8.2% was obtained for XW41. Through a combined approach of coadsorption and cosensitization, the efficiencies were dramatically enhanced to 10.6% and 10.2% for XW40 and XW41, respectively, as a result of simultaneously enhanced V _oc and J _sc. The results of this work provide a novel strategy for developing efficient DSSCs by employing strapped porphyrin dyes.