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Organic Solar Cells – A New Application Field for Buchwald Ligands

Ligands that are best known for their important role in organic synthesis, helps Organic Photovoltaics to reach a record conversion efficiency

Despite the low-cost, light-weight, and mechanical properties of active layers, organic photovoltaic cells were for many years, regarded as less favorable than the traditional inorganic photovoltaics, such as silicon solar cells because of low efficiency, stability and low strength [1, 2]. However, this situation has changed recently when Chinese researchers made a big step forward in the development of a new generation of organic solar cells (OSC) that reached the conversion efficacies of up to 11.7%. [3, 4]. They invented a conducting donor polymer, PffBT4T-C9C13, a key component of OSC, with longer C9 and C13 alkyl side chains (fig. 1) which help to promote better solubility and film morphology [3]. Before this innovation, all top-performance OSC devices were using halogenated solvents, which are environmentally hazardous. On the other hand, organic solar cells from non-halogenated solvents lead to inferior performance.

Fig. 1. Chemical structure of PffBT4T-C9C13 polymer

To overcome this problem, the authors applied a palladium-complex catalyzed direct arylation polymerization reaction in halogen free hydrocarbon-based media using Buchwald ligands. Buchwald ligands are best known for their role in arylation coupling reactions for organic synthesis [5], however, application of Buchwald ligands for fabrication of organic photovoltaics is relatively new. In 2012, Horie et al reported successful Pd-catalyzed synthesis of low-bandgap conjugated polymers via arylation [6].

The synthetic route of PffBT4T-C9C13 is a complex, microwave assessed procedure, and consists of a few steps that can be found in the supplementary information of ref [3]. In fig. 2 we would like to highlight a key step in the polymer synthesis. The typical polycondensation reaction of the boronic acid, pinacol ester derivative with fluorinated di-bromo benzothiadiazole is proceeding via the direct arylation using [Pd(dba)₂] (46-0210) with the SPhos ligand (15-1143) as a catalyst. The resulting product subsequently undergoes bromination with further polymerization in a microwave reactor at 140°C.

Fig. 2. The role of Pd/SPhos catalyst in the synthetic route of PffBT4T-C9C13 polymer

For the further optimization of the polymerization conditions, we offer a wide range of Buchwald Catalysts and Ligands. We believe that the implementation of these ligands will greatly improve the scope of palladium catalyzed cross-coupling reactions in the fabrication of new polymers.

References:

Chem. Rev. 2009, 109, 5868.
Chem. Rev. 2010, 110, 6736.
Nat. Energy 2016, 1, 15027.
J. Am. Chem. Soc. 2017, 139, 2387.
Chem. Sci., 2013, 4, 916.
Macromol. Rapid Commun. 2012, 33, 1927.

Products Mentioned:
46-0210 Bis(dibenzylideneacetone)palladium(0) [Pd(dba)2] (32005-36-0)
15-1143 2-Dicyclohexylphosphino-2',6'-dimethoxy-1,1'-biphenyl, min. 98% SPhos (657408-07-6)

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