Solar cells sensitized with porphyrin dyes with a carbazole donor: The effects of an auxiliary benzothiadiazole acceptor and bulky substituents on the donor

摘要

Three porphyrin sensitizers XW54-XW56 containing a carbazole donor have been designed and synthesized by introducing a benzothiadiazole (BTD) unit as the auxiliary electron acceptor to extend the absorption spectra and/or bulky dihexyloxyphenyl groups into the carbazole unit to suppress dye aggregation and improve the photovoltage (V-OC). The BTD unit incorporated in XW54 obviously broadens and red-shifts the absorption threshold to ca. 700 nm, as compared with that of 650 nm observed for XW1. Thus, XW54 exhibits a much broader monochromatic photon-to-electron conversion efficiency (IPCE) spectrum with an extremely red-shifted onset wavelength of 780 nm, resulting in a photocurrent density (J(SC)) of 11.60 mA cm(-2), higher than that of XW1. Unfortunately, the V-OC value was decreased owing to the more severe dye aggregation caused by the large conjugation framework induced by the presence of the BTD unit. As a result, XW54 shows an efficiency of 6.26%, slightly higher than that of 6.11% obtained for XW1. On the other hand, with the bulky dihexyloxyphenyl donor groups introduced to XW55, a highest V-OC, of 860 mV was achieved, which can be ascribed to the efficient prevention of charge recombination and suppression of dye aggregation. Thus, XW55-based cells exhibit an improved efficiency of 6.60%. On the basis of XW54 and XW55, two bulky dihexyloxyphenyl groups and a BTD unit were simultaneously introduced to XW56, affording a highest efficiency of 7.03%, with the J(SC )and V-OC, values of 12.5 mA cm(-2) and 785 mV, respectively. These results compose a novel approach for developing efficient dye-sensitized solar cells (DSSCs) by simultaneously introducing bulky dihexyloxyphenyl groups and a benzothiadiazole unit, which may synergistically broaden the absorption spectra and suppress the dye aggregation, resulting in improved photocurrent and photovoltage.