Polariton-assisted excitation energy channeling in organic heterojunctions

Exciton-polaritons are hybrid light-matter states resulting from strong exciton-photon coupling. The wave function of the polariton is a mixture of light and matter, enabling long-range energy transfer between spatially separated chromophores. Moreover, their delocalized nature, inherited from the photon component, has been predicted to enhance exciton transport. Here, we strongly couple an organic heterojunction consisting of energy/electron donor and acceptor materials to the same cavity mode. Using time-resolved spectroscopy and optoelectrical characterization, we show that the rate of exciton harvesting is enhanced with one order of magnitude and the rate of energy transfer in the system is increased two- to threefold in the strong coupling regime. Our results exemplify two means of efficiently channeling excitation energy to a heterojunction interface, where charge separation can occur. This study opens a new door to increase the overall efficiency of light harvesting systems using the tool of strong light-matter interactions.

where V is the electronic coupling matrix element between the two states, h is Planck's constant, k B is the Boltzmann constant and is the temperature. However, it is challenging to calculate the electron transfer rate from this equation for complicated systems such as organic heterojunctions. Forrest et al.
have investigated a large family of typical donor and acceptor combinations, and the results provide a reference to estimate the electron transfer rate in organic heterojunctions using the offset energy as an input. 5 The offset energy (∆ ) for excited state electron transfer from DPA to PTCDA is 1.6~1.7

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Supplementary Note 2: Optical properties of PTCDA thin film PTCDA has been extensively studied due to its promising properties in applications such as optoelectronic devices. Thin PTCDA films prepared by vacuum deposition are known to crystallize into two polymorphic forms, α and β, depending on the growth conditions. 6 Here the thin-film X-ray diffraction of the films used in our experiments show a peak at 27.8º (see Supplementary Figure 2), indicating the existence of only the α-form. 7 It agrees with recent studies showing that films grown on quartz substrates at the room temperature consist only of the α-form. 8  Purcell's effect: The Purcell effect is the enhancement of a quantum system's spontaneous rate of emission by its environment. In our study, the lifetime of the reference PTCDA film in and outside of cavity is the same (3.7±0.25 ns, see Table 1 in the main text). It indicates that the Purcell effect caused by the cavity here is negligible. The Purcell effect can be interpreted to be caused by a change of the density of final states during the emission process. As the optical density at 720 nm between the reference and planar cavity is almost the same (Supplementary Figure 5), the Purcell effect caused by the difference of the density of states is negligible. Thus, the difference in the rate of emission between reference cavity and the HJ cavity cannot be attributed to the Purcell effect.

Supplementary Note 4: Calculation of the exciton diffusion length
It is generally known that excimeric states in organic thin films can act as energy traps for singlet exciton transport, resulting in much smaller diffusion lengths than expected. In PTCDA thin films, the energy relaxation from the Frenkel states to the low energy excimeric states is ultrafast (400-500 fs) compared to the typical hopping rates (10 3 to 10 6 fs). 9 The energy of the excimer and polariton are similar in our system; an exchange of energy between these states is therefore expected. Thus, we can use the exciton diffusion model based on decay at 720 nm (from the excimeric state) to determine the relevant diffusion constant and length. The PL decay dynamics were modeled by calculating the number and distribution of the excitations in the film n(x, t) according to the 1-D diffusion equation 19,20 : where D is the diffusion coefficient, and k is the PL decay rate of the sample without a quenching film.
The exciton diffusion model assumes that the unquenched PL decay is mono-exponential. The PL of PTCDA films doesn't follow a single exponential decay due to strong interaction between molecules and the disorder of molecular arrangement. However, in our system, the main effect of the quenching film (DPA) on the decay of PTCDA occurs at early times. The long emission tail almost keeps the same with and without DPA ( Figure 3). Therefore, we use k calculated from = 1⁄ , where τ e is the time taken for the PL to fall to 1/e of its initial value for a PTCDA film without a quenching layer. In the