Cost‐Effective Dyes Based on Low‐Cost Donors and Pd‐Free Synthesis for Dye‐Sensitized Solar Cells

Abstract

Renewable solar energy has attracted much attention and is an essential alternative energy supply to address the increasing energy demand and environmental concerns. In this aspect, photovoltaic (PV) technology is a good candidate for utilization of solar energy. One of the promising emerging PV technologies is dye-sensitized solar cells (DSSCs), where dye molecules, socalled photosensitizers, are used as the light absorber. In this regard, dyes can be considered among the most important components in DSSCs. The very first dyes used for high-performance DSSCs were based on Ru-polypyridyl complexes. [1][2][3][4] However, they usually show relatively low extinction coefficient and involve difficult synthesis and purification steps. [5] Thus, a new group of dyes called metal-free organic dyes subsequently emerged for DSSC applications. The organic dyes provide high extinction coefficient and flexibility in design and tuning of their electronic properties. [6] The structures of organic dyes are usually based on the state-of-the-art donor-π-acceptor (D-π-A) design. Unlike Ru-polypyridyl complexes whose excitation process is based on metal-to-ligand charge transfer (MLCT), the relevant electronic transition upon photoexcitation in the organic dyes is intramolecular charge transfer (ICT). [7] The electron density moves from the donor part to the acceptor part, leading to efficient charge separation.A great deal of research in DSSCs has focused on the dye design and modification to tune the dye properties. In general, the structures of organic dyes usually contain a large extended conjugated system from the donor and π-spacer units to give broad and intense spectral absorption in visible light. The molecular design for the organic dyes normally relies on linking conjugated units via C─C or C─N bond formation. The reactions involved inevitably are Pd cross-coupling reactions, such as the Suzuki-Miyaura, Stille, and Buchwald-Hartwig reactions. These reactions involve the use of Pd-based catalysts including a specific coupling partner, for example, organoborons for the Suzuki-Miyaura reaction and organostannanes for the Stille reaction. Notwithstanding their versatility, the Pd catalysts are normally expensive as Pd is a rare metal and they need additional ligands added to perform the catalysis. The high cost of the catalysts can increase the total cost of dye synthesis, making the dyes more expensive. Moreover, some starting materials, such as organostannanes, cause toxicity concerns and possible tin contamination in the products. [8] The organostannanes have been acknowledged to cause toxicity to a range of neurological and liver systems. [9] Therefore, more sustainable alternative reactions must be sought to address the toxicity concerns and lower the cost of dye production. One of the promising alternative reactions is the Wittig or Horner-Wadsworth-Emmons (HWE) reaction.As mentioned earlier, the organic dye architecture is based on D-π-A design and the modification of the dye structure is normally carried out by changi...