In view of the ongoing climate change, it is imperative to shift energy production to renewable and sustainable energy sources. Here, photovoltaics proves to be an attractive alternative to fossil fuels, not least due to steadily increasing external quantum efficiencies of solar cells. However, conventional improvements approach a physical barrier: The Shockley‑Queisser limit of 34%. It is dictated by the interplay of semiconductor bandgaps and the width of the solar spectrum (Figure 1). If the band gap is too high, low energy photons cannot promote electrons from the valence to the conduction band. A narrow bandgap in turn leads to high energy photons generating surplus energy, which is lost as heat. A new approach to significantly reduce these thermalization losses are charge-carrier multiplication processes. Here, high energy photons are absorbed and converted into two charge carriers of lower energy, which are injected into the conduction band. This opens up a route to efficiently harvest the blue part of the solar spectrum, potentially enhancing the conversion efficiency limit by one third.
Figure 1: In high bandgap semiconductors (left) low-energy photons are not able to promote electrons from the valence to the conduction band. In low bandgap semiconductors (centre), high-energy photons result in energy wasted as heat. Charge-carrier multiplication processes like Singlet Fission (right) convert one high-energy photon to two low-energy charge carriers which can be injected into the semiconductor.
One charge-carrier multiplication process is singlet fission. Here, a singlet exciton interacts with a ground state chromophore to yield two triplet excitons. This process is usually only observed in solid state or concentrated solutions. In our work, we present a two‑step sequential mechanism, which allows for singlet fission to occur even in dilute solutions.
A first indication for this process can be found in transient absorption experiments. Here, it is observed that the initial singlet exciton is converted to a triplet species on the order of a few nanoseconds. Interestingly, the amplitude of the latter continues increasing even after the singlet signature has vanished. This leads to the conclusion that a third species has to be involved as an intermediate. However, there are no indications for any further components in the experiments. So what could this elusive species be? The answer is surprisingly simple: Molecular oxygen. Its comparatively long lifetime allows it to act as a catalyst, temporarily storing part of the energy of the initial excitation before transferring it to a second chromophore (Figure 2).
Figure 2: In the sequential Singlet Fission process, the electronic configuration of molecular oxygen allows it to act as a catalyst.
In order to understand the intricacies of the underlying mechanism, a look at the electronic configuration of molecular oxygen is necessary. In its ground state, it has two unpaired electrons which constitutes a triplet state (3O2). Its first excited singlet state (1O2) is located only 0.97 eV higher in energy. Thus, an organic chromophore with a sufficient S1 ‑ T1 energy gap is able to sensitise oxygen, resulting in a triplet exciton and 1O2. Subsequently, the latter interacts with a ground state chromophore. This second step regenerates 3O2 and produces an additional triplet exciton. In total, the reaction balance is exactly the same as for a bimolecular singlet fission process. This sequential singlet fission mechanism can be applied to a variety of chromophores, as long as they possess suitable S1 and T1 energy levels.
For a more detailed explanation of sequential singlet fission, visit: https://www.nature.com/articles/s41467-019-13202-5
*This post was co-authored by Klaus Wollscheid and Dr. Tiago Buckup.