Ultralong‐Range Energy Transport in a Disordered Organic Semiconductor at Room Temperature Via Coherent Exciton‐Polariton Propagation

Abstract

Amorphous molecular solids are inherently disordered, exhibiting strong exciton localization. Optical microcavities containing such disordered excitonic materials have been theoretically shown to support both propagating and localized exciton‐polariton modes. Here, the ultrastrong coupling of a Bloch surface wave photon and molecular excitons in a disordered organic thin film at room temperature is demonstrated, where the major fraction of the polaritons are propagating states. The delocalized exciton‐polariton has a group velocity as high as 3 × 107 m s–1 and a lifetime of 500 fs, leading to propagation distances of over 100 µm from the excitation source. The polariton intensity shows a halo‐like pattern that is due to self‐interference of the polariton mode, from which a coherence length of 20 µm is derived and is correlated with phase breaking by polariton scattering. The demonstration of ultralong‐range exciton‐polariton transport at room temperature promises new photonic and optoelectronic applications such as efficient energy transfer in disordered condensed matter systems.

Polariton-assisted excitation energy channeling in organic heterojunctions

“…Polaritons are inherently delocalized with a delocalization volume covering the whole cavity, and when two different excitons couple simultaneously to the same resonant mode of an optical cavity, the length scale of energy transfer has been shown to dramatically increase compared to that of typical Förster resonance energy transfer distances (<10 nm) 25 – 31 . Moreover, organic polaritonic photodiodes and OCSs have recently been made 32 , 33 , and in-plane exciton transport at micrometer length scale in disordered organic materials in the strong coupling regime was recently observed 34 , 35 . Organic polaritons can increase the overall efficiency of exciton harvesting in planar HJs in two different ways, by their delocalized nature and by long-range energy transfer, thus potentially enabling efficient OPVs from simple planar HJ structures.…”

Microcavity-Like Exciton-Polaritons can be the Primary Photoexcitation in Bare Organic Semiconductors

“…The specular reflection hence suggests that these films support EPs, despite the absence of any cavities or nanophotonic systems. The angle-dependent measurements which allow us to distinguish lower and upper EP states, provide further strong proof of EP formation 36 . We hence propose that the exciton states in the organics are admixed with mildly confined photonic modes supported by the dielectric constant mismatch between the molecules and the surrounding environment.…”

“…These systems show EP lasing 34 , but because only photonic information is propagated, they have not been widely developed for optoelectronics such as LEDs or photovoltaics. A similar situation holds for the case of excitons coupled to Bloch surface waves (Bloch-wave polaritons), where ultralong range transport of predominantly light-like states has been achieved, but not matter-like states crucial for many devices 35,36 .…”

Ultralong‐Range Polariton‐Assisted Energy Transfer in Organic Microcavities

“…[13] Energy transfer between donor-acceptor species,e xciton harvesting and polariton propagation has also been reported in other structural configurations in which organic excitons are coupled to either surface plasmons or Bloch surface waves. [14][15][16] Vibron hybridisation effects have also been observed at room temperature, by coupling two molecular vibrational modes to the same IR electromagnetic field. [17][18][19] Other work has demonstrated hybridisation of excitons in a2 Dm aterial and an organic semiconductor,w ith the degree of hybridisation being tuneable via application of an electric field.…”