The Future of Organometallic Chemistry

By Andrea d’Aquino

Meeting the world’s increasing annual energy demand in a sustainable way is one of the greatest challenges facing humanity. Growing concerns about limited fossil fuels, and the accumulation of CO2 in the atmosphere from burning those fuels, have stimulated tremendous interest in developing affordable and efficient renewable energy technologies. Among renewable energy sources, solar energy is the most promising; the Sun has the potential to meet energy demand on a global scale, as the Earth receives more energy from the Sun in an hour than is consumed by all of humanity in an entire year. Photovoltaic technologies, which harvest sunlight to generate electricity without producing CO2, are the world’s fastest-growing energy technology.

Currently, inorganic semiconductors (e.g. crystalline and amorphous Si) dominate the solar cell production market owing to their power conversion efficiencies, that is, the proportion of incident sunlight that is converted into electrical energy. However, these materials are themselves extremely energy intensive to make, and require temperatures exceeding 1500 °C to purify and manufacture. In addition, purely inorganic semiconductors are rigid, heavy, and opaque, further limiting their application. Ultimately, for photovoltaics to supersede fossil fuels as the dominant energy technology, large area panels need to be mass produced inexpensively so that enough solar energy can be captured to power the world. Complementary to the inorganic counterparts, organic solar cells, especially those employing semiconducting polymers, have attracted interest due to their amenability to low cost, solution-based fabrication techniques such as roll-to-roll printing, and due to their light weight and flexibility. Although organic polymers are still dominating the field, they suffer from limitations in power conversion efficiency. Organometallic semiconductors, therefore, offer a potential solution as the introduction of metal ions into conjugated polymers offers many advantages: (1) the metal ions act as architectural templates in the assembly of the organic subunits allowing for high molecular weight polymers to be synthesized in a reproducible fashion and  without the generation of toxic metal wastes (which are generated during the synthesis of organic polymers); (2) they can provide redox-active and paramagnetic centers that facilitate charge generation and transport; (3) metal centers can be used to easily tune the electronic and optical properties of organic π-systems; (4) they allow fine- tuning of the HOMO−LUMO gap through the interaction of the metal d-orbitals with the ligand orbitals; (5) there is a  diversity of molecular frameworks based on coordination number, geometry and valence shells of different metal atoms, allowing for a broad range of accessible polymer film morphologies, which is critical for efficient charge generation and extraction. Metalated conjugated polymers have shown exceptional promise as p-type  semiconductors in photovoltaic cells and are emerging as viable alternatives to the all-organic congeners currently used. Metalated conjugated polymers have the potential to drive the development of photovoltaic technologies that can be made inexpensively through solution-based fabrication techniques and with favorable physical properties, such as flexibility, semi-transparency, and lightweight design. Additionally, these materials offer more opportunities for application in electronics, smart cars, power-generating windows and appliances. These hybrid materials are chemically versatile and can be synthesized in large quantities under ambient conditions without producing toxic byproducts, enabling the fabrication of inexpensive, large-area photovoltaic devices.

Challenges posed by the increasing demands of a growing population on a shrinking pool of natural resources will require solutions grounded in chemistry. In particular, the increasingly urgent need for renewable energy will require the development of high-performing, inexpensive, and environmentally benign semiconducting materials to efficiently harness solar energy. In order to overcome this challenge interdisciplinary solutions are required, and we must look beyond the traditional organometallic complexes and catalysts and, instead, pursue next-generation organometallic materials.

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