Researchers have made significant progress in polymer solar cell technology by developing a method that uses sidechain engineering to improve molecular interactions. This method eliminates the need for toxic halogenation processing solvents, which improves the efficiency and stability of the battery. The study highlights the advantages of oligoethylene glycol (OEG)-based sidechains, marking a key step towards greener, more efficient, and more wearable-friendly solar cells.
Polymer solar cells are known for their light weight and flexibility, making them ideal for wearable devices. However, the toxic halogenated solvents required in the production process hinder their widespread application. These solvents pose environmental and health risks that limit the attractiveness of these solar cells. Unfortunately, less toxic alternative solvents lack the same solubility and therefore require higher temperatures and longer processing times.
This inefficiency further hinders the application of polymer solar cells. The development of a method that eliminates the use of halogenated solvents can significantly improve the efficiency of organic solar cells, making them more suitable for wearable technology.
In a recently published article**, the researchers outline how side-chain engineering can be used to improve molecular interactions between polymer donors and small molecule acceptors, thereby reducing the need for halogenated processing solvents.
*Recently published in Nano Research Energy.
The hybrid morphology of polymer donors and small molecule acceptors is strongly influenced by their molecular interactions, which can be determined by the interfacial energies between the donor and acceptor materials. When their surface tension values are similar, the interfacial energy and molecular interactions between donor and acceptor are expected to be more favorable"Yun-Hi Kim, a professor at Kyungsang National University in South Korea, said. "To enhance the hydrophilicity of polymer donors and reduce molecular deimpurities, side-chain engineering may be a viable approach. "
The role of sidechain engineering
Sidechain engineering refers to the addition of a chemical group called a sidechain to the backbone of a molecule. The chemical groups in the side chain affect the properties of the macromolecule. The researchers speculate that the addition of oligoethylene glycol (OEG)-based side chains will improve the hydrophilicity of the polymer donor, thanks to the oxygen atoms in the side chains. Molecules that are hydrophilic are attracted to water.
Schematic diagram of the overall performance of polymer solar cells and the thermal stability of hydrophilic side chain molecules in polymer solar cells According to the overall performance and thermal stability, a mixture of hydrocarbons and hydrophilic oligoethylene glycol (2eg) performs better than standard solvents when manufacturing PSCs. Source**: Tsinghua University Press, Nano-Research Energy
Differences in the hydrophilicity of polymer donors and small molecule acceptors affect their interactions. With the increase in the hydrophilicity of polymer donors and the improvement in their interactions with small molecule acceptors, non-halogenated processing solvents can be used without affecting the performance of solar cells. In fact, a polymer solar cell made with an OEG side chain attached to a benzodithiophene polymer donor has a power conversion efficiency of 177%, up from 156%。
Improve efficiency and stability
To compare the results, the investigators designed benzodithiophene-based polymer donors with OEG side chains, hydrocarbon side chains, or 50% hydrocarbon side chains and 50% OEG side chains. Kim said"This elucidates the effect of side-chain engineering on the hybrid morphology and performance of polymer solar cells processed with non-halogenated solvents. Our results suggest that polymers with hydrophilic OEG side chains can improve miscibility with small molecule acceptors and improve the power conversion efficiency and device stability of polymer solar cells during non-halogenation processing. "
In addition to improved power conversion efficiency, polymer solar cells with OEG side chains also have higher thermal stability. Thermal stability is critical for the scaling of polymer solar cells, so the researchers heated them to 120 degrees Celsius and then compared the power conversion efficiency. After 120 hours of heating, the polymer with hydrocarbon side chains had a power conversion efficiency of only 60% of the original and surface irregularities, while the mixture of hydrocarbons and OEGs maintained 84% of the initial power conversion efficiency.
Kim said"Our findings can provide useful guidance for the design of polymer donors for the production of efficient and stable polymer solar cells using non-halogenated solvent processing. "
References: Soodook SEO, Jun-Young Park, Jin Su Park, Seungjin Lee, Do-Yeong Choi, Yun-Hi Kim, and Bumjoon JPublished by Kim on July 24, 2023 in Nano Research Energy**:"Hydrophilic side-chain polymer donors can be treated with non-halogenated solvents to achieve efficient, thermally stable polymer solar cells"。
doi: 10.26599/nre.2023.9120088
Compilation**: scitechdaily