Harvesting light with organic radical semiconductors

Among organic radicals, chlorinated trityl radicals (right diagram) have attracted significant interest due to their exceptional performance in organic light-emitting diodes. They have special open-shell structures where not every electron is paired up, with the highest occupied molecular orbital having only one electron.

Generally, for all other closed-shell molecules, each occupied molecular orbital has two electrons with opposite spins. Upon the electrical excitation of closed-shell molecules, either a spin-0 singlet state where the two electron spins point in opposite directions, or a spin-1 triplet state where both electron spins are aligned, is generated, but only the singlet state emits light.

The generation of the ‘dark’ triplet state is thus a significant disadvantage when producing light-emitting diodes.

However, in radicals only a ‘bright’ doublet-excited state is formed, avoiding the formation of the non-light-emitting triplet state upon electrical excitation, which improves the internal quantum efficiency (the fraction of electrons injected that actually produce photons). In addition, trityl compounds stand out for their remarkable stability and high photoluminescence efficiency, which is the ratio of the number of photons
emitted by a material to the number of photons it absorbs, in the red or near-infrared region.

The trityl radical itself is inherently non-emissive due to its alternant hydrocarbon structure and molecular energy level degeneracy. In alternant hydrocarbons, the molecular framework forces the unpaired electron to be distributed in a symmetrical way across the molecule. This symmetry causes the excited states of the radical to relax primarily without emitting photons.
However, when trityl radicals bond with the electron-donating or electron-withdrawing groups, a non-zero transition dipole moment of photoluminescence is produced, making the trityl radicals emissive. The inversion of molecular energy levels contributes to the high stability of trityl radicals, making them ideal candidates for organic light-emitting diodes.

Our group has also reported a series of tris(2,4,6-trichlorophenyl) methyl (TTM) radical derivatives, in which the para-chlorine atoms are substituted with phenyl-based groups to tune the emission colour and efficiency by altering the spatial distributions of the atoms in the molecule. This structural modification provides a versatile platform for the development of red and near-infrared organic light-emitting diodes.

Extensive research has been conducted on the
electroluminescent properties of TTM radicals;
however, their potential to generate electric
charge when absorbing photons remains largely
unexplored. To investigate the possibility of
employing TTM for light harvesting applications, P3TTM is selected as the model system. The para-
chlorine atoms of TTM are replaced by the phenylring to enhance the intermolecular interaction. We first studied the P3TTM photo-physics in toluene solution with different concentrations.

In highly diluted solutions, P3TTM behaves as an isolated molecule, exhibiting red emission at 645nm with a mono-exponential decay lifetime. However, as the concentration increases, the photoluminescence is partially quenched, and a new broad emission band appears in the near-infrared region, suggesting possible intermolecular interactions.

A similar phenomenon is observed in solid-state doped thin films, with P3TTM radical dopants. In thin films, red-shifted emission bands are observed in addition to the molecular emission. Broad red-emission bands are often attributed to the formation of short lived excited states known as ‘excimers’, a common ph enomenon. However, excimer formation requires strong non-covalent ‘π–π’ interactions between relatively planar molecules. Interestingly, the propeller-shaped geometry of the TTM radical should prevent such strong π–π interactions. Here, we provide evidence that the red-shifted photoluminescence originates from a fully charge-separated anion–cation pair rather than from a conventional excimer state.

To understand what causes the unusual red-shifted emission we observed, we used time-resolved absorption spectroscopy, which lets us watch how excited states evolve in the first few billionths of a second after a material absorbs light. As can be seen in the thin-film absorption spectra, at the earliest times, we see two clear absorption features that come from ordinary molecular excitations. As time passes, these features gradually change into two new absorption peaks.

To identify these new late-time absorption peaks, we compared the measurements with spectroelectrochemistry, where we generate positively and negatively charged versions of the molecule and record how they absorb light. The later-time spectra in our time-resolved measurements match almost perfectly with the spectral fingerprints of these radical cations and anions. This tells us that, after absorbing light, an excited P3TTM molecule can pass an electron to a neighbour, creating a pair of oppositely charged
radicals.

When these charges later recombine, they emit the broad, red-shifted light observed earlier – explaining why we see emission that cannot be due to conventional excimers. Crucially, these charge pairs can also be pulled apart by applying an external electric field. To test this, we fabricated standard multilayer diode structures using P3TTM as the only light-absorbing material. In the dark the current is essentially zero, but under blue illumination the current increases by a factor of about 1,000 at –10 V, showing that P3TTM can indeed generate photocurrent. When the applied voltage is large enough to fully separate the charges, the device collects nearly all the generated electrons and holes.

What makes this particularly exciting is that, unlike traditional organic solar cells – which require two different materials to provide an energetic “push” for charge separation – P3TTM can achieve this on its own. The energy stored in the initial excited state is high enough that forming a separated electron–hole pair is actually favourable. This means we can build a single-component light-harvesting device, dramatically simplifying the architecture of future organic electronic technologies. While complete charge separation is common in inorganic semiconductors like silicon, achieving the same behaviour in molecular materials is extremely rare.