If one lives where the wind blows, the hail falls and the home insurance underwriter is up to speed – the savings a residential photovoltaic solar cell system offers is consumed by the risk insurance premium. Spending say $20K to save $2K annually with risk insurance at $6K per year is a nonstarter. Weather risk to solar installations plays a huge role across much of the world.
Thus when research turns up ways to build panels at lower cost one perks up; there is 1.5 of an order of magnitude reduction to cover to get realistic pricing practicality. That $20K has to get under $1K for the at risk part of the system – the panels themselves.
A Cornell University team led by William Dichtel, assistant professor of chemistry and chemical biology, has discovered a simple process for building an organic molecular framework that could pave the way for the development of more economical, flexible and versatile solar cells. The work is described in the current issue of Nature Chemistry.
Dichtel’s strategy uses organic dye molecules assembled into a structure known as a covalent organic framework (COF). Organic materials have long been recognized as having potential to create thin, flexible and low-cost photovoltaic devices, but it has been proven difficult to organize their component molecules reliably into ordered structures likely to maximize device performance.
COFs are a class of materials first reported in 2005 and offer a new way to address this long-range ordering problem. Until now, the known methods for creating them had significant limitations.
Dichtel explains where his team began, “We had to develop a completely new way of making the materials in general.” The strategy they chose 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 a 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 Cornell team used X-ray diffraction to confirm the material’s molecular structure and surface area measurements to determine its porosity.
Once the framework is assembled, the pores between the molecular latticework could potentially be filled with another organic material to form a light, flexible, highly efficient and easy-to-manufacture solar cell. The next step is to begin testing ways of filling in the gaps with complementary molecules.
At the core of the structure are molecules called phthalocyanines, a class of common industrial dyes used in products from blue jeans to ink pens. Phthalocyanines are also closely related in structure to chlorophyll, the compound in plants that absorbs sunlight for photosynthesis. The compounds absorb almost the entire solar spectrum – a rare property for a single organic material. This makes the Cornell effort extraordinary in scope.
Dichtel explains more with, “For most organic materials used for electronics, there’s a combination of some design to get the materials to perform well enough, and there’s a little bit of an element of luck. We’re trying to remove as much of that element of luck as we can.”
The structure by itself is not a solar cell just yet, but it is a model that will significantly broaden the scope of materials that can be used in COFs, Dichtel said. “We also hope to take advantage of their structural precision to answer fundamental scientific questions about moving electrons through organic materials.”
“This is the very beginning of our work,” Dichtel said.
Just so, but this work looks very good, very good indeed.