Exploratory synthesis and photovoltaic performance comparison of D-pi-A structured Zn-porphyrins for dye-sensitized solar cells

Abstract

The design and synthesis of D-pi-A structured Zn(II)-porphyrin sensitizers with extended pi-conjugation, coded as SGT-012, SGT-016, SGT-052 and SGT-053, were explored. The key schematic concept for the molecular design and synthesis of porphyrin sensitizers, with the target of modulation of donor groups by embedding an electron donor into the skeleton of two typical D-pi-A porphyrin models, such as D-porphyrin-A sensitizers (SGT-012 and SGT-016) and D-triple bond-porphyrin-triple bond-BTD-acceptor sensitizers (SGT-052 and SGT-053), was proposed investigate the influence of the donor ability and the bulk of donor groups on the photophysical properties and cell performance of dye-sensitized solar cells (DSSCs). Also, based on the photophysical properties and cell performances, the co-sensitisation strategy was conducted to further enhance the cell performances. SGT-012 and SGT-052 porphyrins, containing a strong donor unit, exhibited similar S-band absorption and a slightly red-shifted Q-band absorption compared to SGT-016 and SGT-053 porphyrins containing a weak bulky donor unit, respectively. To further extend the pi-conjugation and absorption to a longer wavelength range, the triple bond at two meso-positions of the porphyrin core and a benzothiadiazole (BTD) strong electron acceptor was introduced to yield SGT-052 and SGT-053, resulting in a red-shift and broad visible region absorption ability. It was indicated that these modifications lead to the formation of a stronger intramolecular charge transfer complex, which is favourable for harvesting sunlight, than those of SGT-012 and SGT-016 porphyrins. To prevent undesirably reduced V-oc caused by charge recombination processes and dye aggregation from porphyrin-sensitized solar cells, HC-Al of co-adsorbent was adopted to fabricate SGT-052- and SGT-053-based solar cells. The DSSCs with SGT-052 and SGT-053 exhibit better light harvesting ability than the DSSCs with SGT-012 and SGT-016 porphyrins, due to the formation of the stronger intramolecular charge transfer complex. Thus, the incident photon-to-current conversion efficiency (IPCE) of SGT-052 and SGT-053-based DSSCs was extremely red-shifted to a wavelength of 800 nm, resulting in higher J(sc) values of 15.3 and 14.6 mA cm(-2), respectively. The DSSC utilising SGT-052 and HC-Al exhibited a higher photovoltaic performance (eta(eff) similar to 9.6%) than did other sensitizers. On the basis of SGT-052-based DSSC, its DSSCs co-sensitized with SGT-012 were prepared to improve the J(sc), V-oc and power conversion efficiency (10.2%).