Can we Tame Highly Reactive Radicals into Highly Efficient Solar Cells?
It is clear. We are facing the tightest race in human history, the climate crisis. The winds of change are, however, already upon us, with solar cells being omnipresent. They are on top of our homes, in our newspapers, and in governmental policies. Of course, behind the scenes, they appear in thousands of scientific articles trying to outcompete the efficiency of all others.
When we think about solar cells, we envisage the blue silicon ones. Silicon is a key element in the current technological revolution, and this makes us vulnerable to the geopolitics of its production and distribution. Organic photovoltaics (OPVs) break this dependency issue, and can lead to a more equitable, global innovative landscape. These OPVs use organic molecules instead of silicon to harvest solar radiation. Conjugated polymers, our fancy name for the plastics used in electronics, have dominated the field of OPVs. And just like your day-to-day plastic objects, they are flexible, light, and easily processable. These conjugated polymers have a fundamental flaw, though, the existence of dark triplet states which decrease the efficiency of the solar cells. My project will tackle this fundamental issue with radical molecules.
What even are radicals? A chemist, (or Wikipedia, or ChatGPT…), would tell you they are highly reactive molecules. You have synthesised them as well, by cutting a plastic eraser in two. In doing so, you cleaved some carbon chain bonds, forming radicals onto each half. However, they are so reactive that you were never able to stick the halves back together: these radicals recombined almost instantaneously with neighbouring radicals on their side. We want our molecules to be stable for decades, not for microseconds. Even more so, their traditional role in spectroscopy was quenching the fluorescence of surrounding molecules. Since good solar cell materials need to be good emitters, and these radicals killed any emission, they were disregarder altogether. Both arguments made them clearly undesirable for any long-term, efficient optoelectronic applications.
So, where is the catch? Given enough steric bulk and conjugation, they become remarkably stable, and that is exactly what conjugated polymers offer structurally. The steric bulk prevents them from recombining like they did in our erasers. The conjugation allows the radicals to hop between thousands of atoms, rather than being blocked on a few, so they can survive for as long as any normal compound. This conveniently solves the first important issue.
The answer to the second limitation lies within their electronic structure. Analogous to a ball on a staircase, a molecule can sit only on certain energy levels. When a traditional material is excited with a photon (a brick of light energy) it gets excited from a ground energy singlet level S0 to an excited energy singlet level S1. This can then fall and get trapped in the undesired dark low-lying energy triplet level T1. Radicals avoid this problem by simply not having the bad triplet levels! They go from doublets to doublets, all being emissive. A tweak in their symmetry is enough to transform them into highly luminescent molecules, and the backbone of the polymers offer exactly that.
This project lies at the intersection of organic synthesis, spectroscopy, and device fabrication. A wholistic approach is needed to tackle such a multifaceted problem as global warming. And our radical conjugated polymers might play a decisive role in this, harvesting one photon at a time!
Sergiu Petrușca
NanoDTC PhD Student, c2023