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Research Theme I: Towards Commercialization of Conjugated Polymers: Low Cost, Easy Synthesis, and Green Processing
Organic Solar Cells (OSCs) have shown promise compared to other photovoltaic technologies due to their potential for low cost, easy processing, light weight, and flexibility. The rapid growth in efficiency of OSCs (~20%) demonstrate that this technology is close to commercial levels; however, many of these conjugated polymers have increasingly long and complex synthetic routes. For a conjugated polymer to be realized in a commercially viable OSC, it should have (1) solution processability, (2) high efficiency, (3) sufficient stability, and (4) low cost.
Krebs and co-workers have calculated that in order match current inorganic solar cells, the polymer donor in an OSC would needs to have a materials only cost (MOC) less than $12/g at 10% efficiency. For refence, most record holding efficiency polymers are far too expensive, such as the popular PM6 ($184.20/g) and D18 ($179.76/g). With this in mind, we have previously created a new synthetic route to a polymer called PTQ10, which can achieve OSC efficiencies over 15%. This material was previously reported but via an unruly synthetic route which led to unacceptably high MOC of $214.18/g. Our new synthetic approach was able drastically reduce the cost to $30.29/g, but more work is needed to achieve that milestone.
I. Tin Free Polymerization Methodology: The current approach to polymerize PTQ10 is through a Stille polycondensation, and while this polymerization approach can yield nice polymers, the use of organotin is problematic. Aside from the toxicity, cost analysis reveals that approximately 40% of the overall cost of PTQ10 through our new synthetic approach comes from the trimethyltin chloride reagent used to make the stannylated thiophene monomer. Therefore, finding effective polymerization approach that avoids the Stille coupling can be an important area of further investigation. Two potential routes include direct arylation polymerization (DArP) and Suzuki polymerization approach. Both of these avoid the dangerous and expensive organotin compounds and continue to push PTQ10 to the commercialization milestone.
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II. Green Solvent Processability: Most OSCs reported in literature use halogenated and/or aromatic solvents such as chlorobenzene, dichlorobenzene, chloroform, toluene, among others to apply a line layer of polymer film on the appropriate device. While these solvents can be effective in making an OSC, they have various hazards that are detrimental for large scale-up of manufacturing such OSCs. For these reasons, We want to design conjugated polymers that are processable in green solvents (defined “green” as non-halogenated and non-aromatic solvents) such as ethanol, methanol, and water. One common approach to add green solvent processability to a conjugated polymer is to replace the alkyl side chains with oligio(ethylene glycol) (OEG) variants. We have previously demonstrated this strategy with both a variety of FTAZ and PTQ10 derivatives . While notable gains were achieved in these works, an ultimate green solvent was not achieved. This creates an opportunity to design a low cost and environmentally-friendly processable polymer for high efficiency solar cells.
III. New Simple Building Blocks: Over the past 30 years, there has been substantial gains in understanding the structure-property relationships in conjugated polymers. We have developed a strong understanding of the impact of functional group, such as fluorine and cyano, on the optoelectronic, photovoltaic, and morphological properties of the polymers. With that suite of knowledge, we can now engineer brand new chemical building blocks which are can enjoy both straightforward synthetic processes and also high efficiency. Furthermore, reducing the synthetic complexity of conjugated polymers down from 15-30 steps to 3-8 simple reactions will make way for the scale up of these new materials.
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Research Theme II: Designing Conjugated Polymers which are Recyclable
Learning from the shortcomings of traditional polymer/plastic chemistry, ideal conjugated polymers should be recyclable to avoid further plastic pollution. However, while there are numerous research groups designing new polymers, only a few study the degradation or recyclability of such materials. Nearly all high-performance conjugated polymers are NOT recyclable and massive commercialization of such plastics (such as in flexible solar panels, rollable displays, foldable phones) poise to further exacerbate the problem of plastics in the environment.
Of the research into recyclable conjugated polymers, the main theme is to impart chemical moieties, primarily imine bonds, which can be cleaved under acidic aqueous conditions. While this approach has shown promise, the current high-performance polymers would need to be completely redesigned to contain these new imine bonds. Additionally, because of the difficulty of polymerization via imine condensation, many of these materials are only degradable and not actually recyclable (i.e. the original monomers cannot be recovered). Along with significant synthetic complexity of this approach, the new polymers have been reported to have remarkably different properties – lower reactivity resulting in lower molecular weight and inferior mechanical properties, changes in optical and electrochemical properties impacting the performance, etc.
Rather than designing new polymers or redesigning current polymers with new cleavable moieties, we plan on converting one of the biggest problems which still plagues organic electronics into a major advantage: stability. While impressive device results have been shown throughout literature, many devices have serious issues with stability resulting in short lifetimes. There are a variety of everyday factors which can limit the long-term stability of a polymer/device, include mechanical stress, heat, oxygen, water, and light. To combat these stressors, the most common approach is to introduce engineering controls, such as encapsulating the device. We envision a reusable lifecycle where a conjugated polymer will be quasi-stable via encapsulation during the lifetime of the device, then upon removal of the engineering control, be reliably degraded back to small molecule precursors which can be recycled to make a new batch of conjugated polymers.
Research Theme III: Collaborative Efforts
Organic photovoltaics is an incredibly multidisciplinary field which achieves its greatest success when working with the large and diverse fraction of researchers, including chemists, material scientists and theorists, engineers/fabrication specialists, device physicists, computer scientists, and so much more. While we might do amazing work on designing new conjugated polymer donors, we are still dependent on the work of many others to truly realize an organic solar panel.
We have been fortunate to work with some of the absolute best scientists across the world. Here is a current map of all the different labs which Dr. Rech has collaborated with thus far:
Here are some previews for our current collaborations:
I. Life Cycle Analysis and Synthetic Complexity:
II. Machine Learning to Discover New Building Blocks:
III. Greenhouse Integration:
IV. Roll-to-roll Fabrication: