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New Organic Framework for More Efficient Graetzel Solar Cells Discovered

Dichtel's organic dyes organized into stacked, porous two-dimensional sheets

Scientists from Cornell University have unveiled a basic process that could give solar cells a boost in efficiency and a drop in price. The process employs molecules typically used in coloring blue jeans and in ink dyes. Their purpose if to build an organic framework that could serve as a building block for cheaper and flexible solar cells.

William Dichtel, an assistant professor of chemistry and chemical Biology is the man behind the discovery, and he reports it in the journal Nature Chemistry. Aided by a team of other researchers, he assembled dye molecules into a structure known as a covalent organic framework (COF), which had only been discovered in 2005. Organic materials have the potential to create thin, flexible and low-cost solar cells, such as Graetzel’s, but they lack in efficiency, because it’s difficult to organize their molecules into ordered structures reliably.

“We had to develop a completely new way of making the materials in general,” Dichtel said. The strategy uses a simple acid catalyst and relatively stable molecules called protected catechols to assemble key organic molecules into a neatly ordered two-dimensional sheet. These sheets stack on top of one another to form a lattice that provides pathways for charge to move through the material.

The reaction is also reversible, allowing for errors in the process to be undone and corrected.

“The whole system is constantly forming wrong structures alongside the correct one,” Dichtel said, “but the correct structure is the most stable, so eventually, the more perfect structures end up dominating.” The result is a structure with high surface area that maintains its precise and predictable molecular ordering over large areas.

The material’s internal structure has been confirmed by using X-ray diffraction. The molecules at the core of this framework are called phtalocyanines (the dyes mentioned earlier). They have a structure similar to chlorophyll, the substance that gives plants their green color and that has the role of photosynthesis, the process that feeds the plants with solar energy.

By combining organic materials with electronics, the Cornell researchers hope to get some design that allows the hybridization to work with solar cells more efficiently or at least as efficient as silicon. The framework can be combined with other organic materials to form more efficient Graetzel cells, for example.

“This is the very beginning of our work,” says Dichtel, outlining that their discovery is not a complete solar cell, but merely a scientific tool on which solar cells can be built. That could take longer, though, but what’s most important is that this is a new discovery in the field and the base has been set for future experimentation.

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