Effect of Ligand Structures of Copper Redox Shuttles on Photovoltaic Performance of Dye-Sensitized Solar Cells.

In recent years, copper(I/II) complexes have emerged as alternative redox shuttles in dye-sensitized solar cells (DSSCs), exhibiting more positive redox potential than iodine- and cobalt-based redox shuttles. In particular, copper(I/II) complexes with 1,10-phenanthroline- or 2,2'-bipyridyl-based ligands attained moderate to high power conversion efficiencies (6-11%) with a high open-circuit voltage (VOC) over 1.0 V due to the positive potentials. Although copper(I/II) complexes with 1,10-phenanthroline-based ligands with 2,9-substituents have been developed, the effect of their ligand structures on the photovoltaic performance of DSSCs have not been fully addressed due to limited synthetic access to 1,10-phenanthroline derivatives. In this study, we designed and synthesized a series of copper(I/II) complexes with 1,10-phenanthroline ligands with different substituents at the 2,9-positions: bis(2-n-butyl-1,10-phenanthroline)copper(I/II) ([Cu(bp)2]1+/2+), bis(2-ethyl-9-methyl-1,10-phenanthroline)copper(I/II) ([Cu(emp)2]1+/2+), bis(2,9-diethyl-1,10-phenanthroline)copper(I/II) ([Cu(dep)2]1+/2+), and bis(2,9-diphenyl-1,10-phenanthroline)copper(I/II) ([Cu(dpp)2]1+/2+). The more positive redox potentials of [Cu(emp)2]1+/2+ and [Cu(dep)2]1+/2+, compared to that of bis(2,9-dimethyl-1,10-phenanthroline)copper(I/II) ([Cu(dmp)2]1+/2+), originate from the larger steric hindrance of the ethyl group instead of the methyl group, whereas the redox potential of [Cu(bp)2]1+/2+ is significantly shifted to the negative direction because of the lower steric hindrance of the 2-monosubstituted 1,10-phenanthroline ligands. The efficiency of the DSSC with [Cu(bp)2]1+/2+ (5.90%) is almost comparable to the DSSC with [Cu(dmp)2]1+/2+ (6.29%). In contrast, the DSSCs with [Cu(emp)2]1+/2+ (3.25%), [Cu(dep)2]1+/2+ (2.56%), and [Cu(dpp)2]1+/2+ (2.21%) exhibited lower efficiencies than those with [Cu(dmp)2]1+/2+ and [Cu(bp)2]1+/2+. The difference can be rationalized by the electron collection efficiencies. Considering the similar photovoltaic properties of the DSSCs with [Cu(bp)2]1+/2+ and [Cu(dmp)2]1+/2+, the use of copper(I/II) complexes with 2-monosubstituted 1,10-phenanthroline ligands as the redox shuttle may be useful to improve the short-circuit current density while retaining the rather high VOC value when dyes with a smaller bandgap (i.e., better light harvesting) are developed.

Renaissance of Fused Porphyrins: Substituted Methylene-Bridged Thiophene-Fused Strategy for High-Performance Dye-Sensitized Solar Cells.

Over the last decades, porphyrin sensitizers have made a remarkable contribution to performance improvement in dye-sensitized solar cells (DSSCs). In particular, versatile push-pull-type porphyrin sensitizers have achieved power conversion efficiencies (η) over 10% as a result of their improved light-harvesting abilities. Meanwhile, aromatic ring fusion to a porphyrin core is an attractive option for highly efficient DSSCs because of its expanded π-conjugation and resultant red-shifted absorption. Nevertheless, aromatic-fused porphyrin sensitizers have suffered rather low cell performances due to their mismatch of HOMO-LUMO levels, high aggregation tendency, and short lifetime of the excited states. Bearing these in mind, we envisioned that the fusion of substituted methylene-bridged small aromatic ring to a porphyrin core would overcome these drawbacks, boosting the cell performance. Herein, we report a series of substituted methylene-bridged thiophene-fused porphyrins, AfZnP, DfZnP, and DfZnP- iPr. After optimization, DSSC with the donor-side thiophene-fused DfZnP- iPr achieved an η-value of 10.1%, which is comparable to that of DSSC with GY50 (10.0%), a representative high-performance push-pull-type porphyrin sensitizer. More importantly, cosensitization of DfZnP- iPr with a complementary sensitizer LEG4 further led to an η-value of 10.7%, which is the highest value ever reported for DSSCs with fused porphyrin sensitizers. Therefore, our strategy will reboot the exploration of aromatic-fused porphyrin sensitizers for high-performance DSSCs.

A Hydroxamic Acid Anchoring Group for Durable Dye-Sensitized Solar Cells Incorporating a Cobalt Redox Shuttle.

A hydroxamic acid group has been employed for the first time as an anchoring group for cobalt-based dye-sensitized solar cells (DSSCs). The porphyrin dye YD2-o-C8HA including a hydroxamic acid anchoring group exhibited a power conversion efficiency (η) of 6.4 %, which is close to that of YD2-o-C8, a representative porphyrin dye incorporating a conventional carboxylic acid. More importantly, YD2-o-C8HA was found to be superior to YD2-o-C8 in terms of both binding ability to TiO2 and durability of cobalt-based DSSCs. Notably, YD2-o-C8HA photocells revealed a higher η-value (4.1 %) than YD2-o-C8 (2.8 %) after 500 h illumination. These results suggest that the hydroxamic acid can be used for DSSCs with other transition-metal-based redox shuttle to ensure high cell durability as well as excellent photovoltaic performance.

Photovoltaic Properties and Long-Term Durability of Porphyrin-Sensitized Solar Cells with Silicon-Based Anchoring Groups.

Anchoring groups for dye-sensitized solar cells (DSSCs) play a decisive role in high-power conversion efficiency (η) and long-term cell durability. To date, a carboxylic acid is the most widely used anchoring group for DSSCs. However, the carboxylic acid tends to dissociate from a TiO2 surface during the cell operation as well as in the presence of water. Considering that the dye dissociation from TiO2 leads to a decrease in the cell performance, stable anchoring groups are highly desirable to achieve long-term durability of DSSCs toward their practical application. In this study, we designed and synthesized a series of porphyrin dyes with the triethoxysilyl anchoring groups, ZnPSi1, ZnPSi2, and ZnPSi3, to evaluate the effects of the silicon-based anchoring group on cell durability and photovoltaic properties. The DSSCs based on ZnPSi1, ZnPSi2, and ZnPSi3 exhibited moderate η-values of 2.2, 4.7, and 2.3%, respectively. It is noteworthy that the η-value of the DSSC based on ZnPSi2 (4.7%) is the highest among DSSCs based on porphyrin dyes with silicon-based anchoring groups. The moderate η-values are mainly attributed to the low charge collection efficiency originating from the low surface coverage and plausible tilted geometry of the dyes on TiO2. More importantly, we demonstrated that the DSSC based on ZnPSi2 revealed higher long-term cell durability under illumination than that based on reference porphyrin YD2 -o -C8 having a conventional carboxylic acid anchoring group.